ABSTRACT 
 
 
 
 
Title of Dissertation: SYNTHESIS AND POLYMER-MEDIATED 
REGIOSELECTIVE SELF-ASSEMBLY OF 
SHAPED PLASMONIC NANOPARTICLES 
  
 Xiaoying Lin, Doctor of Philosophy, 2021 
  
Dissertation directed by: Professor John T. Fourkas, 
Department of Chemistry and Biochemistry 
Professor Zhihong Nie, 
Fudan University 
 
Plasmonic nanoparticles with collective excitations of the conduction-band 
electrons have many potential applications in surface-enhanced spectroscopies, 
photocatalysis, photovoltaics, and biomedicine. The plasmonic properties are highly 
tunable via the size, shape, chemical composition, and surrounding media of individual 
nanoparticles, as well as by the interactions with other nanoparticles in close proximity. 
Fabricating plasmonic nanostructures with precisely controlled size, shape, and 
interparticle distance is critical for harnessing desirable properties.  Although top-down 
lithographic techniques are widely used for fabricating shaped plasmonic nanoparticles 
and ensembles, the products are limited to 2D planar structures and the cost can be 
prohibitive. As low-cost and versatile alternatives, colloidal synthesis and self-
assembly have been explored to fabricate complex plasmonic nanostructures. 
In this dissertation, wet-chemistry synthetic and self-assembly approaches are 
explored for fabricating well-defined plasmonic nanostructures with high structural 
 
 
complexity. Chapter 2 describes a facile synthetic method for circular and triangular 
gold nanorings with tunable diameters, ring thicknesses, surface roughness, and hence 
the plasmonic response. The gold nanorings with rough surface show 100-fold higher 
enhancement factor than solid gold nanoparticles as substrate for surface-enhanced 
Raman spectroscopy. In chapter 3, we demonstrate regioselective bonding between 
nanospheres and nanoplates originating from the steric hindrance of polymeric ligand 
brushes and the anisotropy of nanoparticles. The regioselectivity enables a self-
assembly system with precise control over the relative orientation of Au nanospheres 
on Ag nanoplates and the stoichiometry of reactive groups of copolymeric ligands 
dictates the number of nanoparticles in one nanocluster. The yield of each assembly 
was ~70% without further purification. Optical study reveals that different bonding 
modes affect the plasmonic coupling of assembled structures. In chapter 4, the 
regioselective bonding is applied to fabricate complex plasmonic nanocluster, 
nanoflowers and nanobuds, with distinct bonding modes. Compared with nanobuds, 
nanoflowers with the same number of petals show stronger electric field enhancement 
and further localized surface plasmon resonance peak shifts.   
The synthetic and self-assembly methods demonstrated in this dissertation have 
great potentials and versatility in designing not only plasmonic nanoclusters, but also 
other inorganic nanoparticle-based functional structures with high complexity. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
SYNTHESIS AND POLYMER-MEDIATED REGIOSELECTIVE SELF-
ASSEMBLY OF SHAPED PLASMONIC NANOPARTICLES 
 
 
 
by 
 
 
Xiaoying Lin 
 
 
 
 
 
Dissertation submitted to the Faculty of the Graduate School of the  
University of Maryland, College Park, in partial fulfillment 
of the requirements for the degree of 
Doctor of Philosophy 
2021 
 
 
 
 
 
 
 
 
 
 
Advisory Committee: 
Professor John T. Fourkas, Chair 
Professor Zhihong Nie (Fudan University) 
Professor Sang Bok Lee 
Professor Efrain Rodriguez 
Professor Mohamad Al-Sheikhly, Dean?s Representative 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
? Copyright by 
Xiaoying Lin 
2021 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Acknowledgements 
I would like to thank my advisors, Dr. John T. Fourkas and Dr. Zhihong Nie, for 
their outstanding guidance throughout my PhD study. Thank you to Dr. Nie for leading 
me into and guiding me through my PhD research with interesting projects and brilliant 
ideas. It has been such good experience that past years just seem too short. Thank you 
to Dr. Fourkas for mentoring me with patience and kindness, especially during my 
preparation for this dissertation. It would be hard for me to finish without his 
constructive suggestions and efficient revisions.  
I am grateful to my dissertation committee members, Dr. Sang Bok Lee, Dr. Efrain 
Rodriguez, and Dr. Mohamad Al-Sheikhly for your valuable time and suggestions. In 
my first semester as a graduate student, I had a short interview with Dr. Sang Bok Lee 
where he shared some helpful tips that I still benefit from in my everyday research. I 
took Advanced Inorganic Chemistry with Dr. Rodriguez and Polymer Physics with Dr. 
Al-Sheikhly in the same semester, probably the busiest but happiest one. I learnt a lot 
from their great lectures and instructions, as well as their enthusiastic attitude towards 
science and research. 
I am thankful to all my previous and current labmates. Dr. Yi Liu, Dr. Meihua Lin, 
Dr. Chuncai Kong, Dr. Shaoyi Zhang, and Dr. Chenglin Yi have helped me profoundly 
at the early stage of my research, without whom I cannot build up my projects easily. 
They are not just good mentors and collaborators, but also great friends in life. My 
biggest thanks go to my current labmates, Qian Zhang, Kyle Webb, and Chelsey Lamar. 
We have done great jobs to keep it going together, and we will all make it to the end. I 
ii 
 
 
am thankful to all the Fourkas group members for sharing the wonderful atmosphere in 
research and daily conversations, as well as kind attention and interest to my research.  
I appreciate the chance to work with and get help from great people here in the 
University of Maryland. I am thankful to Dr. Andrei Vedernikov for his help and 
guidance, as the program director and course instructor, in my first-year study. Thanks 
to Dr. YuHuang Wang for insightful advises and permission on using the UV-vis-NIR 
spectrometer in his lab. I am grateful to Dr. Sz-Chian Liou from Nanocenter for 
teaching and assisting me with electron microscopes through the past five years. I 
would like to thank Diane Canter for handling my questions and paperwork.  
My special thanks go to the Department of Chemistry and Biochemistry for 
supporting my study and funding my research under special circumstances. The 
department chair, Dr. Janice Reutt-Robey, and the directors of chemistry graduate 
program, Dr. Efrain Rodriguez and Dr. Amy Mullin, are the guardian angels for 
graduate students. I feel secure and not on edge in our department with supportive 
environment despite of the pandemic era and uncertainties in policy for international 
students in the past year.  
Finally, I would like to thank my former advisor and mentor in my undergraduate 
study, Dr. Hao Zhang and Dr. Xue Zhang, respectively. It is because of their excellent 
mentorship that I decided to pursue a PhD degree and a career in chemistry. I am not 
sure where my next step goes, but I will always remember where and how I started. 
iii 
 
 
Table of Contents 
Acknowledgements ....................................................................................................... ii 
Table of Contents ......................................................................................................... iv 
List of Tables ............................................................................................................... vi 
List of Figures ............................................................................................................. vii 
List of Abbreviations .................................................................................................. xii 
Chapter 1: Introduction ................................................................................................. 1 
1.1 Plasmonic nanoparticles and localized surface plasmon resonance ............. 1 
1.1.1 Chemical composition ................................................................................. 2 
1.1.2 Size or particle volume ................................................................................ 3 
1.1.3 Geometric morphology ................................................................................ 4 
1.1.4 Surrounding medium ................................................................................... 6 
1.2 Plasmon Coupling in nanoensembles ........................................................... 7 
1.3 Applications of plasmonic nanoparticles and ensembles.................................... 9 
1.4 Fabrication of plasmonic ensembles ................................................................. 10 
1.4.1 Top-down lithographic techniques ............................................................ 11 
1.4.2 Colloidal self-assembly .............................................................................. 12 
1.5 Scope of this dissertation .................................................................................. 15 
Chapter 2: Controllable synthesis of Au nanorings with tunable optical response .... 17 
2.1 Introduction ....................................................................................................... 17 
2.2 Experiments ...................................................................................................... 19 
2.2.1 Materials .................................................................................................... 19 
2.2.2 Synthesis of rounded and triangular Ag nanoplates .................................. 20 
2.2.3 Gold deposition of Ag@Au nanostructures ............................................... 21 
2.2.4 Etching of Ag@Au nanoplates for Au nanorings (thin) ............................ 21 
2.2.5 Multiple metal deposition for thick/rough AuNRs .................................... 21 
2.2.6 Calculation of enhancement factors (EFs) ................................................. 22 
2.27 Sample preparation for characterizations .................................................... 23 
2.3 Results and discussion ...................................................................................... 24 
2.3.1 Seed-mediated growth of Ag nanoplates ................................................... 24 
2.3.2 Templated synthesis of circular and triangular Au nanorings ................... 26 
2.3.3 Thickness and surface roughness of Au nanorings .................................... 30 
2.3.4 Tunable optical properties of Au nanorings............................................... 31 
2.3.5 Surface-enhanced Raman scattering with Au nanorings ........................... 34 
2.4 Conclusion ........................................................................................................ 35 
Chapter 3: Polymeric ligand-mediated regioselective bonding of plasmonic 
nanospheres and nanoplates ........................................................................................ 36 
3.1 Introduction ....................................................................................................... 37 
3.2 Experiments ...................................................................................................... 38 
3.2.1 Materials .................................................................................................... 38 
3.2.2 Synthesis of Au nanospheres and Ag nanoplates ...................................... 39 
3.2.3 Synthesis of thiol-terminated di-block copolymers ................................... 39 
3.2.4 Surface modification of NPs and self-assembly ........................................ 46 
3.2.5 Finite-difference time-domain (FDTD) simulations .................................. 47 
3.2.6 Calculation and grafting-density ................................................................ 48 
3.2.7 Sample preparation for characterizations ................................................... 51 
iv 
 
 
3.3 Results and discussion ...................................................................................... 52 
3.3.1 Self-assembly driven by reactive block copolymers ................................. 52 
3.3.2 Mechanism of regioselective bonding ....................................................... 54 
3.3.3 Edge-type bonding and nanoclusters ......................................................... 56 
3.3.4 Face-type bonding and nanoclusters .......................................................... 60 
3.3.5 Regioselective self-assembly with triangular nanoplates .......................... 62 
3.3.6 Experimental and computational studies on optical properties ................. 64 
3.4 Conclusion ........................................................................................................ 69 
Chapter 4: Regioselective self-assembly of plasmonic nanoflowers and nanobuds ... 71 
4.1 Introduction ....................................................................................................... 72 
4.2 Experiments ...................................................................................................... 73 
4.2.1 Materials .................................................................................................... 73 
4.2.2 Preparation of building blocks ................................................................... 73 
4.2.3 Self-assembly of nanoflowers and nanobuds ............................................. 74 
4.2.4 Finite-difference time-domain (FDTD) simulations .................................. 74 
4.2.5 Sample preparation for characterizations ................................................... 75 
4.3 Results and discussion ...................................................................................... 76 
4.3.1 Polymeric ligand-mediated self-assembly ................................................. 76 
4.3.2 Formation of the plasmonic nanoflowers .................................................. 77 
4.3.3 Formation of the plasmonic nanobuds ....................................................... 80 
4.3.4 Experiments and simulations on extinction spectra of nanoflowers and 
nanobuds ............................................................................................................. 83 
4.3.5 Electric-field enhancement of nanoflowers and nanobuds ........................ 88 
4.4 Conclusion ........................................................................................................ 90 
Chapter 5:  Conclusions and Future Works ................................................................ 91 
5.1 Conclusions ....................................................................................................... 91 
5.2 Future work: Self-assembly of Au nanorings ................................................... 92 
5.2.1 Self-assembled monolayers of Au nanorings ............................................ 93 
5.2.2 Regioselective self-assembly of nanospheres and circular nanoring ......... 95 
References ................................................................................................................... 97 
 
 
 
 
 
 
 
 
 
 
 
v 
 
 
List of Tables 
 
Table 3.1 Characterization of Copolymers. 
Table 3.2 Key parameters of NP-A(B) with copolymer ligands after surface 
modification. 
Table 4.1 Peak wavelength of nanoclusters in experimental and simulated extinction 
spectra. 
 
 
vi 
 
 
List of Figures 
Figure 1.1 Calculated UV-vis extinction, absorption, and scattering spectra of silver 
nanostructures in shapes of (A) a sphere, (B) a cube, (C) a tetrahedron, (D) an 
octahedron, and (E, F) nanoshells with different shell thickness. 
Figure 1.2 Plasmon coupling of AuNS?AuNC dimers according to their nanogap 
morphology. 
Figure 1.3 Programmable assembly of reconfigurable nanoparticle (NP) architectures. 
Figure 2.1 Schematic illustration for (a) the synthesis of circular and triangular gold 
nanorings (AuNRs) with four stages of fabrication. 
Figure 2.2 SEMs of (a) Ag seeds, (b) rounded Ag nanoplates, and triangular Ag 
nanoplates with edge length of (c) 100 nm and (d) 250 nm. 
Figure 2.3 The average diameter/edge length of AgNPs as a function of the amount of 
Ag seed solution added in the seed-mediated growth step. 
Figure 2.4 SEMs and TEMs of circular AuNRs with diameter of 83 ? 12 nm (a) and 
triangular AuNRs with edge length of 115 ? 22 nm (b). (c) EDX mapping of Au and 
Ag in a circular AuNR. (d) The average diameter of AuNRs as a function of the amount 
of Ag seeds added in the template synthesis step. 
Figure 2.5 SEMs of circular and triangular AuNRs with different sizes. 
Figure 2.6 Large-scale SEMs of circular and triangular AuNRs with size distribution. 
Figure 2.7 SEMs of circular AuNRs with different ring thickness and corresponding 
UV-vis-NIR spectra through the post-deposition growth. 
Figure 2.8 SEMs of triangular AuNRs with different ring thickness and corresponding 
UV-vis-NIR spectra through the post-deposition growth. 
vii 
 
 
Figure 2.9 SEMs of triangular AuNRs with different surface roughness and 
corresponding UV-vis-NIR spectra through the post-deposition growth. 
Figure 2.10 Raman spectra of 4-MBA and surface enhanced Raman scattering with 
thin, thick, and rough AuNRs. 
Figure 3.1 The synthetic routes of thiol-terminated copolymers. 
Figure 3.2 1H NMR spectrum of a representative copolymer ligand: PEO45-b-
P(DMAEMA0.40-r-St0.60)419-SH. 
Figure 3.3 1H NMR spectrum of a representative copolymer ligand: PEO45-b-P(AA0.32-
r-St0.68)335-SH.  
Figure 3.4 Representative TGA traces of nanoparticles grafted with copolymers. 
Figure 3.5 Schematic illustration of regioselective bonding of NP-As and NP-Bs into 
edge-type and face-type clusters mediated by the shielding effect of copolymer ligands. 
Figure 3.6 Schematic illustration of (a) a copolymer-grafted nanoplate (NP-A) and (b) 
a copolymer-grafted nanosphere (NP-B).  
Figure 3.7 Schemes, TEMs and SEMs of edge-type NP clusters with different 
configurations. 
Figure 3.8 Energy-dispersive X-ray (EDX) elemental mapping and high-angle annular 
dark field (HAADF) transmission electron micrographs (TEMs, insets) of edge-type (a) 
AB2 and (b) AB3. 
Figure 3.9 Distribution of edge-type ABx clusters. 
Figure 3.10 Distribution of bonding modes in edge-type ABx clusters. 
Figure 3.11 Distribution of the B-A-B angle in edge-type AB2, AB3, and AB4 
structures. 
viii 
 
 
Figure 3.12 Schemes, TEMs and SEMs of (a) face-type AB2 structures and (b) 
alternating chains. 
Figure 3.13 Schemes, TEMs, and SEMs of (a) edge-type AB2 trimers, (b) face-type 
AB2 trimers, and (c) alternating chains. 
Figure 3.14 Distribution of bonding modes with central triangular NP-As in (a) edge-
type AB3 clusters, (b) face-type AB2 clusters, and (c) alternating chains. 
Figure 3.15 Experimental extinction spectra for polymer-grafted Au nanospheres, Ag 
nanoplates, edge-type, and face-type trimers. 
Figure 3.16 Simulated extinction spectra for polymer-grafted Au nanospheres, Ag 
nanoplates, edge-type, and face-type trimers. 
Figure 3.17 Calculated extinction spectra of polymer shell-coated Ag nanoplate in 
THF. 
Figure 3.18 Calculated extinction spectra of edge-type AB2 trimers in different 
polarization and light incidence directions. 
Figure 3.19 Calculated extinction spectra of face-type AB2 trimers in different 
polarization and light incidence directions. 
Figure 3.20 Electric-field intensity enhancement contours of (a, b) edge-type trimers 
and (c, d) face-type trimers with different electric field polarizations. 
Figure 4.1 (a, c) Schematic illustration of polymeric ligand-mediated regioselective 
self-assembly of NP-As and NP-Bs into (b) edge-type and (d) face-type assemblies. 
Figure 4.2 Schematic illustration of (a) a copolymer-grafted Ag nanoplate (NP-A) and 
(b) a copolymer-grafted Au nanosphere (NP-B).  
ix 
 
 
Figure 4.3 Schemes, TEMs, and SEMs of edge-type NP clusters with different 
configurations. 
Figure 4.4 Distribution of e-BAx clusters. 
Figure 4.5 Distribution of bonding modes in e-BAx clusters. 
Figure 4.6 Schemes, TEMs, and SEMs of face-type NP clusters with different 
configurations. 
Figure 4.7 Distribution of f-BAx clusters. 
Figure 4.8 Distribution of bonding modes in f-BAx clusters. 
Figure 4.9 Experimental extinction spectra (normalized) of polymer-grafted Ag 
nanoplates, f-BA, f-BA2, e-BA, and e-BA2 nanoclusters. 
Figure 4.10 Simulated extinction spectra for polymer-grafted Ag nanoplates, f-BA, f-
BA2, e-BA, and e-BA2 nanoclusters. 
Figure 4.11 Calculated extinction spectrum of f-BA2 nanobuds in different polarization 
and light incidence direction. 
Figure 4.12 Calculated extinction spectrum of e-BA nanoflowers in different 
polarization and light incidence direction. 
Figure 4.13 Calculated extinction spectrum of e-BA2 nanoflowers in different 
polarization and light incidence direction. 
Figure 4.14 Electric-field intensity enhancement contours of the f-BA2 nanobud at (a) 
588 nm and (b) 721 nm. 
Figure 4.15 Electric-field intensity enhancement contours of the e-BA2 nanoflower at 
(a, b) 541 nm and (c, d) 766 nm. 
Figure 5.1 Schematic illustration of self-assembled monolayers of AuNRs. 
x 
 
 
Figure 5.2 SEM image of self-assembled monolayer of triangular AuNRs (110 nm in 
edge length, 18 nm in thickness) tethered with PS-SH (50K). 
Figure 5.3 Schematic illustrations of a copolymer-grafted Ag nanoplate, a copolymer-
grafted Au nanosphere, and expected assembled structures. 
  
xi 
 
 
List of Abbreviations 
 
2D                       Two dimensional  
3D                       Three dimensional  
4-MBA                4-mercaptobenzoic acid 
AA                       Acrylic acid 
AgNPs                 Silver nanoplates 
AIBN                   Azobis(isobutyronitrile)  
AuNCs                Gold nanocubes 
AuNRs                Gold nanorings 
AuNSs                Gold nanospheres 
BCPs                   Block copolymers 
CPPA                  4-cyano-4-(phenylcarbonothioylthio) pentanoic acid 
CTAB                 Hexadecyltrimethylammonium bromide 
DEG                   Diethyleneglycol  
DMAEMA         N,N-dimethylaminoethyl methacrylate 
DMF                   N,N-Dimethylformamide 
DMSO                Dimethyl sulfoxide 
DNA                   Deoxyribonucleic acid 
EDX                   Energy-dispersive X-ray spectroscopy 
EF                       Enhancemnet factor 
FDTD                 Finite-difference time-domain 
FEG                    Field emission gun 
GPC                   Gel Permeation Chromatography  
xii 
 
 
HAuCl4              Gold (III) chloride trihydrate  
HyA                   Hydrochloride hydroxylamine 
LSPR                 Localized surface plasmon resonance 
NCs                   Nanocubes 
NIR                   Near-infrared 
NPs                   Nanoparticles 
NRs                   Nanorings 
NSs                   Nanospheres 
PEO                  Poly(ethylene oxide)  
PS                     Polystyrene 
PVP                  Poly(vinylpyrrolidone) 
RAFT               Reversible addition-fragmentation chain transfer 
SAM                 Self-assembled monolayer 
SEM                 Scanning electron microscope 
SERS               Surface-enhanced Raman scattering  
TEM                Transmission electron microscope 
THF                 Tetrahydrofuran 
TGA                Thermogravimetric Analysis 
TSC                 Trisodium citrate 
UV                   Ultraviolet 
vis                    Visible 
xiii 
 
 
Chapter 1: Introduction 
Overview: Plasmonic nanoparticles with collective excitations of the conduction-band 
electrons have many potential applications in surface-enhanced spectroscopies, 
photocatalysis, photovoltaics, and biomedicine. The plasmonic properties are highly 
tunable via the size, shape, chemical composition, and surrounding media of individual 
nanoparticles, and also by interactions with other nanoparticles in close proximity. This 
chapter briefly reviews how these factors determine or affect the optical properties of 
plasmonic nanoparticles and their ensembles, and then discusses applications for and 
current fabrication techniques of assemblies of plasmonic nanoparticles.  
 
1.1 Plasmonic nanoparticles and localized surface plasmon resonance 
Plasmonic nanoparticles are typically conductive nanoparticles that exhibit strong 
interactions with light at a particular frequency due to the collective excitations of the 
conduction-band electrons.1-3 When a bulk metal is excited by light, the induced 
charges can propagate as an electromagnetic wave along the metal?s surface. For 
nanoscale metal particles, however, the oscillating charges are strongly confined and 
so the electric field is enhanced on the surface of a nanoparticle, in a phenomenon 
known as localized surface plasmon resonance (LSPR). The LSPR of metallic 
nanoparticles can generate remarkably enhanced electric fields near the nanoparticles 
and leads to strong extinction at the resonant frequency. The wavelength and strength 
1 
 
 
 
of LSPR can be tuned by the size, shape, chemical composition, and surrounding 
medium of individual nanoparticles.4-10  
1.1.1 Chemical composition 
The LSPR frequency (??????????) of nanoparticles is highly dependent on the plasmon 
frequency (????) of the corresponding bulk material, which is a function of the free 
electron density (????) of the material:4,10  
??????2
??
?? ? ??0?? ?????????? ? =   ,                                     (1.1) 1 + 2???? ?1 + 2????
where ????  is the number density of electrons, e is the electronic charge, m is the 
effective mass of the electron, and ??0 and ???? are the permittivity of free space and of 
the surrounding medium, respectively. 
As the most common plasmonic metals, noble metals (gold, silver, and copper) 
have free-electron densities in the range of 1022 to 1023?cm-3, with corresponding LSPRs 
in the visible region.11 Plasmon resonances are not limited to metals, and occur in 
conducting metal oxides12 as well as in semiconductors with appreciable free carrier 
densities.13,14 For doped semiconductors, a typical value of ???? is in the range of 1016 to 
1021?cm-3, leading to plasmon wavelengths in the microwave and infrared regions.14  
Ag has been widely considered as an optimal plasmonic material that has the 
lowest optical loss in the visible and NIR spectral ranges.15 However, Ag nanomaterials 
suffer from easy oxidation and poor stability. Au is another popular plasmonic material, 
with excellent performance in the visible and NIR spectral ranges, and superior 
chemical stability under ambient conditions. As Ag and Au are relatively expensive, 
2 
 
 
 
Cu and Al have become alternative choices. However, Cu and Al both suffer from 
chemical instability, restricting their further applications under atmospheric 
environments. Alloyed metals have also been broadly considered for plasmonic 
applications. The alloying of different noble metals can be used to tune the LSPR 
frequency or wavelength of the resultant materials.15,16 For example, AuxAg(1?x) alloy 
nanoparticles with varying mole fractions show only one LSPR peak, and the LSPR 
band blueshifts linearly with increasing mole fractions of Ag.16-18  
1.1.2 Size or particle volume 
Mie theory19 has been used to estimate variations in plasmonic response due to 
nanoparticle size, anisotropy, and other extrinsic parameters, independently from 
changes in the chemical composition. The dipolar plasmon response (oscillator strength) 
of the nanoparticle is often defined by its polarizability, which can be expressed by the 
electrostatic Clausius-Mossotti equation (also known as the Lorentz-Lorenz 
equation):20 
3 ?? ? ???? = 4???? ?? ? ??0 ? ,                                               (1.2) ?? + 2????
where R is the radius of the nanoparticle, ??0  and ????  are the dielectric constants of 
vacuum and surrounding medium, respectively, and ?? is the dielectric function of the 
material. 
In the simplified case of a metal nanosphere with a radius R much smaller than 
the incident wavelength of light (the quasi-static approximation), the plasmon response 
is essentially dipolar in nature. The strength and frequency of this resonance are related 
to the total number of electrons in the oscillating dipole (defined by the particle volume, 
3 
 
 
 
which is proportional to R3), the complex dielectric function, and the dielectric constant 
of the surrounding medium.19,20 
In experimental observations, the extinction of plasmonic nanoparticles increases 
as the size of the particle increases.7 Extinction includes both absorption and scattering 
components, which have different scaling relationships with the particle volume. The 
absorption cross section increases linearly with particle volume, whereas the scattering 
cross section is relatively smaller, but scales with the volume squared.20  
1.1.3 Geometric morphology 
Geometric morphology is another important factor determining the plasmonic 
properties of metallic nanoparticles as the dipolar plasmon resonance is highly sensitive 
to shape anisotropy.8 Isolated spherical nanoparticles typically support a single 
resonance frequency, whereas anisotropic nanoparticles will exhibit at least one 
additional plasmon mode, as well as large redshifts in plasmon resonance.8,15 
Figure 1.1 is a typical example showing the strong dependence of plasmonic properties 
on the morphologies of Ag nanoparticles of the same size. The extinction (black), 
absorption (red), and scattering (blue) spectra were calculated for Ag nanoparticles 
with different shapes (i.e., sphere, cube, tetrahedron, and octahedron) and topologies 
(i.e., solid, hollow, and core?shell).  
The spectrum of an isotropic nanosphere (Figure 1.1A) exhibits a single 
resonance peak, whereas the spectra of anisotropic cubes (Figure 1.1B), tetrahedra 
(Figure 1.1C), and octahedra (Figure 1.1D) exhibit multiple, red-shifted resonance 
peaks. The resonance frequency of a nanosphere redshifts when it is made hollow (as 
a nanoshell of 10 nm thickness, Figure 1.1E), and redshifts further if the nanoshell is 
4 
 
 
 
made thinner (5 nm, Figure 1.1F). Surface polarization (i.e., charge separation) is the 
most important factor in determining the frequency and intensity of plasmon resonance 
for a given metal, because it provides the main restoring force for electron 
oscillations.21 Variations in particle size, shape, or dielectric environment will change 
the surface polarization and thus the resonance peak. The calculated spectra of a 40-
nm cube (Figure 1.1B) exhibit more peaks than for a sphere because the cube has 
several distinct symmetries for dipole resonance, compared with only one for the 
sphere.22 The most intense peak is red-shifted compared with that of the sphere, a 
common phenomenon observed for nanostructures with sharp corners.23 Calculations 
of the near fields around nanostructures show that surface charges accumulate at sharp 
corners.24 This segregation of charges into corners increases charge separation, and 
thereby reduces the restoring force for electron oscillation.25 The weaker restoring force 
in turn manifests itself in a red-shift of the resonance peak.  
Similar trends are observed for the tetrahedron (Figure 1.1C) and octahedron 
(Figure 1.1D) as well, although they have reduced scattering cross-sections because of 
their different symmetries and smaller particle volumes. The tetrahedron shows the 
most red-shifted resonance peaks among the three solid nanoparticles due to having the 
sharpest corners. 
5 
 
 
 
 
Figure 1.1 Calculated UV-vis extinction (black), absorption (red), and scattering (blue) 
spectra of silver nanostructures in shapes of (A) a sphere, (B) a cube, (C) a tetrahedron, 
(D) an octahedron, and (E, F) nanoshells with different shell thickness. Adapted from 
Ref. 8. 
1.1.4 Surrounding medium 
As indicated by equations 1.1 and 1.2, the surrounding medium also modulates 
the plasmonic properties of nanoparticles. In experimental observations, polymer-
stabilized Au particles dispersed in organic solvents of varying refractive indices (n = 
6 
 
 
 
1.33-1.60) demonstrated a shift in absorption from red to purple (520 to 545 nm), 
accompanied by an increase in extinction intensity.26 These observations are in 
excellent agreement with Mie theory, based on the changes in dielectric constant of the 
medium. 
Surface ligands can also influence the optical properties of metal nanoparticles by 
changing the mobility of conduction-band electrons. Strongly adsorbing anions and 
surfactants, such as thiols, accelerate plasmon relaxation, resulting in peak broadening. 
This effect has been attributed to a reduction in conduction-band electron mobility, due 
to electronic coupling between the plasmon and charge-transfer bands formed by the 
metal-adsorbate complex.27 
1.2 Plasmon Coupling in nanoensembles 
An important property of plasmonic nanoparticles is the dependence on 
interparticle interactions,28-33 which is the basis for engineering plasmonic 
nanostructures. When plasmonic nanoparticles are within a few nanometers of each 
other, plasmon coupling occurs because of the interacting resonances of the excited 
plasmons. This phenomenon enables the hybridization of the initial plasmon bands, 
leading to a peak shift and near-field enhancement. For example, the plasmon coupling 
between two identical nanospheres along the interparticle axis will create hybridized 
modes, and a new longitudinal LSPR appears in the longer wavelength region 
(redshifted), compared to the wavelengths of the single nanospheres. The LSPR 
redshifts further as the interaction between nanoparticles becomes stronger with a 
decreasing interparticle distance.34-38 The strength of the plasmon coupling can be 
measured by the extent of the shifts, and is dependent on the interparticle gaps, filling 
7 
 
 
 
materials, as well as the plasmonic resonances supported by individual nanoparticles.39-
41 However, the interparticle gaps are not only defined by the interparticle distance 
between coupled nanoparticles. Other factors, including the gap area, gap facets, and 
gap curvature also define the nanogaps, and potentially influence the plasmon coupling, 
especially for ensembles of anisotropic nanoparticles.42-48 Kim et al.48 prepared 
nanogaps with subtle differences in nanogap morphology by attaching a gold 
nanosphere (AuNS) to the face, edge, or vertex of a gold nanocube (AuNC), as 
presented in Figure 1.2a. The experimental results (Figure 1.2b) and simulations 
(Figure 1.2c) indicate that plasmon coupling along the interparticle axis between 
AuNSs and AuNCs lowers in energy as the AuNS adsorbs on increasingly curved 
locations on the AuNC, with the highest energy on the face, followed by the edge, and 
the lowest energy at the vertex. 
The effect of nanoparticle shape anisotropy and assembly orientation on plasmon 
coupling has also been studied in assemblies of Au nanorods.49,50 Whereas the end-to-
end assembly of Au nanorods results in a red shift of the longitudinal SPR band,51 side-
by-side assembly, in strong contrast, shows a blue shift of the longitudinal plasmon 
band.49 The dependence of the plasmonic shift on assembly orientation has been 
explained by Jain et al.49 on the basis of ?selection rules? for plasmon coupling. 
8 
 
 
 
 
Figure 1.2 Plasmon coupling of AuNS?AuNC dimers according to their nanogap 
morphology. (a) Representative SEMs, (b) dark-field scattering spectra, and (c) 
calculated scattering spectra of AuNS?AuNC dimers with a AuNS adsorbed on the 
face, edge, or vertex of the AuNC. The bolder lines in (b) indicate the scattering spectra 
from the AuNS?AuNC dimers having the geometry shown in the SEM images in (a). 
Figure adapted from Ref. 48. 
1.3 Applications of plasmonic nanoparticles and ensembles 
Plasmonic nanostructures with high sensitivity to the surrounding medium and 
enhanced light absorption and scattering are good candidates for chemical sensing,52-58 
bioimaging,59,60 and biomedical therapeutics.61-65 The sensitivity of the SPR to the 
dielectric constant of the medium has been exploited for the optical detection of 
chemical and biological species. For practical applications, such as surface-enhanced 
9 
 
 
 
Raman scattering, strong enhancement of electric field is highly desired for single-
molecule detection.66-69 Strong local field enhancement can be readily found around 
the sharp corners, edges and conical tips of individual plasmonic nanoparticles. 
Appropriate spatial arrangements of plasmonic nanoparticles in ensembles can lead to 
even stronger enhancements.  
Among all plasmonic materials, Au nanoparticles and assemblies with high 
biocompatibility have been extensively studied for biomedical applications such as 
cancer therapy and imaging.70-73 The interaction of Au nanoparticles with light releases 
thermal energy, resulting in a localized temperature rise and generating acoustic waves. 
Thus, Au nanoparticles and assemblies are considered as a promising dual functional 
agent for photothermal therapy and photoacoustic imaging of cancer.74-76 
In plasmonic ensembles, as discussed in Chapter 1.2, the extent of coupling-
induced LSPR shift is dependent on the distance between the interacting nanoparticles. 
By characterizing the distance dependence of the plasmon coupling, Alivisatos et al.77 
demonstrated a strategy using the spectrum of a pair of coupled Au or Ag nanoparticles 
to report the interparticle distance as a ?plasmon ruler? for measuring dynamic 
distances in biomolecular systems.78,79 
1.4 Fabrication of plasmonic ensembles 
Generally, there are two major strategies for fabricating plasmonic nanostructures 
with precise control: top-down lithographic techniques80-82 and bottom-up self-
assembly of nanoparticles.83-85 Sometimes those two methods are combined to create 
desired ensembles,85-87 as each has its own advantages and limitations. 
 
10 
 
 
 
1.4.1 Top-down lithographic techniques 
Top-down lithography has been widely used to fabricate well-defined plasmonic 
arrays with precisely controlled NP shape, size, and interparticle distance.88 With these 
methods, geometries as varied as split rings and coupled nanorods can be patterned 
with precise substrate positioning.81,90 For example, Aizpurua et al.81,82 prepared Au 
nanoring arrays on Si substrate by colloidal lithography with diameter of Au nanorings 
tuned by the size of polystyrene masks. Huang et al.89 reported the fabrication of Au 
nanoring dimers and trimers by electron-beam lithography. Due to the high resolution 
of an electron beam, the spacing of trimers can be controlled precisely within several 
nanometers.  
However, these lithographic techniques have fundamental limitations. First, the 
lithographic techniques suffer from either high cost or low resolution. For example, 
electron beam lithography and focused-ion-beam milling, which enable surface 
patterning with a resolution of nanometers,91 are expensive and cannot be easily used 
to produce nanostructures over large areas. UV lithography suffers from both low 
resolution and time-consuming fabrication.92 The reliance on clean rooms and specific 
instruments also increases the cost of fabrication. In addition, these techniques, 
although powerful for producing planar plasmonic arrays or films on 2D substrates, are 
generally not suitable for complex 3D fabrication. Hence, the products usually show 
limited plasmonic coupling within the same plane, limiting the opportunities to obtain 
strong coupling by vertically stacked plasmonic nanostructure, unless combined with 
other techniques (e.g., in Figure 1.3). 
 
11 
 
 
 
1.4.2 Colloidal self-assembly 
As a low-cost and versatile alternative, colloidal self-assembly has attracted 
increasing interest for fabricating plasmonic ensembles beyond 2D planes, with 
complex coupling modes and single particle resolution.93 In the past decades, advances 
in the wet-chemical synthesis of nanoparticles have greatly boosted various approaches 
to bottom-up self-assembly. Generally, those assembly approaches can be classified as 
ligand-guided self-assembly,94-96 interfacial self-assembly,97,98 templated 
assembly,99,100 and external-force-directed assembly.101   
Ligand-guided self-assembly is arguably the most versatile and powerful 
approach to fabricate 3D plasmonic assemblies with easily tuned interparticle distance 
by the various surface ligands. Generally, small molecules,102-104 biomolecules (e.g., 
DNA and proteins),105-109 and synthetic polymers110-113 can serve as linkers to guide the 
assembly of nanoparticles by chemical, hydrophobic, or electrostatic interactions. 
Among surface ligands, DNA strands are extremely attractive for the precise 
organization of plasmonic nanoparticles, owing to their programmable base-pairing 
interactions. DNA strands grafted to the surface of nanoparticles guide their assembly 
into discrete nanoclusters or well-defined superlattices via DNA-recognition 
interactions.105-107 Polymeric ligands are drawing increased interest in guiding the 
assembly of plasmonic nanoparticles, as the functionality can be readily tuned by the 
chemical composition (determining the interactions of tethered nanoparticles), 
persistence length (determining the interparticle distance), and conformation of 
polymer species. The scaling and segregation of the polymer can further enrich the 
assembly behavior by producing patchy particles.114-118 Chen et al.117 reported that the 
12 
 
 
 
coverage of polymer on NPs varied when hydrophilic ligands were mixed with 
hydrophobic ligands. The ligand segregation has been demonstrated on both isotropic 
gold nanospheres (AuNSs) and anisotropic gold nanorods to create patchy 
nanoparticles. More discussions and examples of ligand-guided self-assembly of 
plasmonic nanoparticles are presented in Chapter 3.1 and 4.1. 
Interfacial self-assembly uses air/liquid or liquid/liquid interfaces as platforms for 
ordering nanoparticles into planar assemblies.97,98 The organization of nanoparticles 
driven by the reduction in the interfacial energy:119 
????2 2
???? = ? ? ?????/?? ? ?????/?? ? ????/???? ,                                (1.3) ????/??
where r is the radius of the particle and ????/??, ????/??, and ????/?? are the interfacial tension 
between liquid A/liquid B, particle/liquid B and particle/liquid A, respectively. Liquid 
A or B can be change to air in the case of air-liquid interfaces. The fluidity of liquid 
interfaces provides favorable conditions for mass transport, chemical 
interactions/reaction in two dimensions.120 These favorable properties of liquid 
interfaces have been exploited for the self-assembly of plasmonic nanoparticles into 
droplets in Pickering emulsions,121 self-assembled monolayers (SAMs),122 and hybrid 
films or superlattices123-125 with tunable optical properties by altering the size of 
nanoparticles or the spacing between them. Controlled solvent evaporation is 
commonly used to modulate the concentration of nanoparticles and to confine the 
particles within the predetermined phase or area.126 The method is a promising and 
direct alternative to lithography, although transferring the products onto other 
substrates can be challenging based on the mechanical properties of the assembled 
monolayers. 
13 
 
 
 
Templated assembly is an assembly process directed by a predesigned template, 
which can be physical confinement or a soft/chemical template. As a straightforward 
method, the geometry and patterning of templates dictate the spacing and spatial 
arrangement of plasmonic nanoparticles in their assemblies. This fabrication of 
physical confinements or traps to be used as templates can be done by top-down 
lithography. Flauraud et al.90 reported the capillary assembly of Au nanorods into large-
area ordered structures on substrates with predetermined surface patterns. The precise 
control over the organization of nanorods on the substrate was achieved by subtle 
design of the geometry of traps. The templated assembly can also be combined with 
different surface ligands to achieve better site selectivity and higher complexity of 
plasmonic ensembles. Figure 1.3 shows a polymer-pore templated assembly of Au 
nanoparticles with various shapes modified by DNA strands into well-defined stacks.87 
To assemble nanoparticles within a confined environment, 1D pores are fabricated in a 
polymer-coated gold substrate using top-down lithography, and the gold surface at the 
bottom of each pore is densely functionalized with DNA. DNA-functionalized Au 
nanoparticles of controlled size and shape are then assembled in a layer-by-layer 
manner by designing each layer of nanoparticles to have a terminal DNA sequence 
complementary to that of the previous layer. The porous template is removed to 
generate superlattices with 2D periodicity that are composed of oriented plasmonic 
nanoparticles. 
14 
 
 
 
 
Figure 1.3 Programmable assembly of reconfigurable nanoparticle (NP) architectures. 
The bottom images depict cross-sectional views of a single pore. Figure is adapted from 
Ref. 87. 
1.5 Scope of this dissertation 
As discussed in the previous sections, precisely controlled synthesis and self-
assembly of plasmonic nanoparticles are highly desirable for tuning their coupling and 
optical properties for various applications. Although top-down lithographic techniques 
are powerful for fabricating plasmonic ensembles, the cost can be prohibitive. In this 
dissertation, bottom-up techniques are explored for the fabrication of plasmonic 
nanoparticles and assemblies with high geometric complexity. 
The objective of this dissertation is to develop wet-chemistry synthetic and self-
assembly approaches for complex and defined plasmonic nanostructures. In Chapter 2, 
a facile synthetic method is developed for freestanding circular and triangular Au 
nanorings (AuNRs) with tunable sizes, thicknesses, and surface roughness. The in-
plane LSPR peak of AuNRs is highly tunable through the NIR region. In Chapter 3, we 
combine the steric hindrance of polymeric ligand brushes and the anisotropy of 
15 
 
 
 
nanoparticles to create regioselective bonding between Au nanospheres and Ag 
nanoplates. The regioselectivity enables a self-assembly system with precise control 
over the relative orientation of Au nanospheres on Ag nanoplates, and the stoichiometry 
of reactive groups of copolymeric ligands dictates the number of nanoparticles in one 
cluster. Assemblies with different bonding types show different extinction spectra and 
electric field enhancements. In Chapter 4, regioselective self-assembly is applied to 
fabricate complex plasmonic nanoclusters, nanoflowers and nanobuds. The optical 
properties of nanobuds and nanoflowers with the same number of petals are compared 
in experiments and simulations. In Chapter 5, the completed work in this dissertation 
is summarized and future work is proposed. 
 
 
 
 
 
16 
 
 
 
Chapter 2: Controllable synthesis of Au nanorings with tunable 
optical response 
 
Overview: Au nanorings have attracted intensive interests in various studies due to 
their unique physical properties. Synthesizing Au nanorings with controllable geometry 
is highly desirable for tailoring the plasmonic properties for many potential applications. 
This chapter describes a wet-chemistry method for synthesizing circular and triangular 
Au nanorings with tunable diameters, ring thicknesses, surface roughness, and hence 
the plasmonic response.  
 
Part of this chapter is adapted from the article: Lin, X.; Liu, Y.; Lin, M.; Zhang, Q.; 
Nie, Z. Synthesis of Circular and Triangular Gold Nanorings with Tunable Optical 
Properties. Chem Comm. 2017, 53, 78, 10765-10767.  
L. X. and N. Z. conceived and designed the experiments; L. X. performed most of 
experiments; L. Y. and L. M. optimized the synthetic approach; L. M. and Z. Q. helped 
on Raman spectroscopy and contributed in discussions. L. X. wrote the manuscript with 
revisions from N. Z. 
 
2.1 Introduction 
Nanorings are a unique class of nanostructures that have a solid toroidal shape. 
The surface of a nanoring comprises different regions with positive, zero, and negative 
surface curvatures, which are usually not present concurrently in other nanostructures. 
A nanoring also has a higher surface-to-volume ratio than a nanosphere and nanorod 
containing the same number of atoms, which promotes enhanced sensitivity to the 
surrounding medium. Gold nanorings (AuNRs) have diverse applications in biosensors, 
catalysis, bioimaging, and optical and electronic devices, due to their plasmonic nature 
and optical properties.81,82,89,127-129 Compared with solid Au nanoparticles, AuNRs 
17 
 
 
 
exhibit tunable localized surface plasmon resonances (LSPRs) over a broader window 
of wavelengths.81 To date, circular AuNRs have been largely fabricated on substrates 
by top-down lithographic approaches.81,82 However, these methods require special 
equipment and are usually expensive, and cannot easily be used to create free-standing, 
3D structures. Recently, a number of wet-chemical synthetic strategies have been 
developed for the fabrication of ring-like Au nanostructures.130-132 Xue et al.130 reported 
the synthesis of ultrathin triangular Au nanoframes with thicknesss of 1.8 nm to 6 nm 
using silver nanoplates (AgNPs) as templates. Jang et al.132 synthesized nanorings with 
various shapes (e.g., circular, triangular, and hexagonal) that featured a Pt core and Au 
shell by using Au nanoplates as templates. These approaches either involve 
complicated process (i.e., multiple cycles of etching and metal deposition) or show 
limited control over the dimensions and optical properties of the AuNRs. Thus, there 
is still a great need for facile approaches to the synthesis of high-quality AuNRs with 
controlled geometry. 
This chapter describes a wet-chemistry method using AgNPs as templates for 
circular and triangular AuNRs with tunable surface roughness and dimensions, and 
hence tunable optical properties. This method (Figure 2.1) involves epitaxial Au 
deposition on sacrificial templates of shaped Ag nanoplates, chemical etching to 
remove Ag, and further Au deposition to achieve the desired ring thickness.130,133 By 
varying the size of the templates and amount of metal deposition, the diameter and ring 
thickness of AuNRs can be tuned from 50 to 400 nm and from 5 to 30 nm, respectively. 
The surface roughness of the AuNRs can be varied by manipulating the deposition 
rate/amount of Au. As a result, the in-plane LSPR peak of the AuNRs can be controlled 
18 
 
 
 
in the range of ~780-1350 nm. The AuNRs are further demonstrated as substrate for 
surface-enhanced Raman scattering (SERS) analysis of 4-mercaptobenzoic acid (4-
MBA), with an enhancement factor up to 8.6 ? 106, which is approximately 100-fold 
and 10-fold higher than that of Au nanoparticles134 and Au polyhedral nanocrystals135, 
respectively. 
 
Figure 2.1 Schematic illustration for (a) the synthesis of circular and triangular gold 
nanorings (AuNRs) with four stages of fabrication: Ag seeds, Ag templates, Ag@Au 
nanostructures, and Au nanorings; and (b) the preparation of thick and rough AuNRs 
with different precursor infusion rates. 
 
2.2 Experiments 
2.2.1 Materials 
Silver nitrate (AgNO3, 99%), sodium citrate tribasic dihydrate (C6H5Na3O7, 99%), 
L-ascorbic acid (C6H8O6, 99%), tetrachloroauric acid (HAuCl4, 99.9%), 
19 
 
 
 
hydroxylamine hydrochloride (99%), sodium borohydride (NaBH4, 99%), sodium 
hydroxide (99%), ammonium hydroxide solution (28.0-30.0% NH3 basis), 4-
mercaptobenzoic acid (4-MBA, 99%) were purchased from Sigma-Aldrich. Hydrogen 
peroxide (H2O2, 30%) was purchased from EMD Millipore Corporation. Acetonitrile 
(C2H3N, HPLC Grade) was purchased from Fisher Chemical. Ethyl alcohol (absolute, 
200 proof) was received from Pharmco-Aaper. Deionized water (Millipore Milli-Q 
grade) with resistivity of 18.0 M? was used in all the experiments. 
2.2.2 Synthesis of rounded and triangular Ag nanoplates 
Ag seeds were prepared by one-shot injection of 1.2 ml NaBH4 solution into a 
mixture of ultrapure water (200 ml), AgNO3 (200 ?L, 0.1 M), TSC (12 mL, 75 mM), 
and H2O2 (480 ?L). The solution was gently stirred for 5 min before aging for at least 
2 h. The seeds were collected by centrifugation and then dispersed in 6 mL water. 
A growth solution was prepared by adding acetonitrile (20 mL), TSC (200 ?L, 75 
mM), ascorbic acid (300 ?L, 0.1 M), and AgNO3 (240 ?L, 0.1 M) into water (44 mL). 
The shape of the Ag nanoplates was controlled by the stirring method and the stirring 
rate. Magnetic stirring with a moderate rate (~500 rpm) produced rounded Ag 
nanoplates, whereas mechanical stirring with high rate (~1000 rpm) produced 
triangular Ag nanoplates. The diameter/edge length of the Ag nanoplates was 
controlled by varying the ratio of Ag seeds to AgNO3 in the growth solution. For better 
control over the growth kinetics, we varied the dose of Ag seeds while keeping the 
amount of AgNO3 and volume of growth solution as constant. The addition of more 
seeds led to the formation of smaller Ag nanoplates. After 30 min of growth, products 
20 
 
 
 
were collected by centrifugation and then redispersed in 24 mL of 0.3 mM trisodium 
citrate aqueous solution. 
2.2.3 Gold deposition of Ag@Au nanostructures 
A mixture (volume ratio 50:1) of hydroxylamine hydrochloride solution (3 mM) 
to sodium hydroxide (0.2 M) and a HAuCl4 solution (0.30 mM) were infused into a 75 
ml Erlenmeyer flask containing 4 mL of Ag@Au solution via two syringes under 
vigorous stirring. The infusion rate was set as 2 mL/h. After 20 min of infusion, the 
solution was centrifuged, and the products were redispersed into 6 mL of 0.3 mM 
trisodium citrate. 
2.2.4 Etching of Ag@Au nanoplates for Au nanorings (thin) 
The wet etchant solutions (50 mM H2O2 and 50 mM NH3.H2O) was introduced 
into 6 mL of an as-synthesized fresh Ag@Au nanostructure solution in a 50 mL 
Erlenmeyer flask under steady stirring by using a syringe pump (flow rate: 3 mL/h). 
After the solution became transparent, the thin AuNRs were collected by centrifugation 
and redispersed in 1 mL of a 0.3 mM trisodium citrate aqueous solution.   
2.2.5 Multiple metal deposition for thick/rough AuNRs 
Thick AuNRs with smooth surface were synthesized by another round of Au 
deposition onto the thin AuNRs at the same infusion rate (2 mL/h) of HAuCl4 and basic 
HyA solution. After 1.5 h, the solution was centrifuged, and the products were 
redispersed in 1 mL of 0.3 mM trisodium citrate. 
21 
 
 
 
Rough AuNRs were synthesized by the same method, but at a faster infusion rate 
(5 mL/h) of Au precursor and reducing agent. The solution was then centrifuged after 
1 h, and the products were redispersed into 1 mL of 0.3 mM trisodium citrate. 
2.2.6 Calculation of enhancement factors (EFs) 
We estimated the SERS enhancement factors (EFs) of 4-mercaptobenzoic acid (4-
MBA) at 1590 cm-1 using: 
?? ??
???? = ???????? ? ????????  ,                                                  (2.1) 
?????????? ??????????
where ?????????? and ?????????? are the Raman scattering intensities of 4-MBA obtained with Au 
NRs and bulk 4-MBA (without Au NRs), respectively, and ?????????? and ?????????? are the 
numbers of 4-MBA molecules for SERS and bulk measurement, respectively.133,136 
For bulk and SERS samples, 20 ??L 10 mM 4-MBA ethanol solution is cast on 
substrates. We assume that 4-MBA molecules form a thin, even layer on the substrate 
and metal surface with a footprint of ?????????????????? ? 0.54 nm2. The surface occupation 
factor ?? ? 0.32  for Au NRs. The detection spot area is ?? 2?????????? = 1 ? 1 ???? =
10?12 ??2. 
?? ? ?? ? ??
?? = ????
? ????????????
???????? = 3.01 ? 1010,                              (2.2) ????????????????????
?? ? ??
?? ?????????? 5???????? = = 6 ? 10 ,                                                          (2.3) ??????????????????
The Raman scattering intensities for bulk 4-MBA and rough triangular AuNRs were 
50 and 8577, respectively. 
?? ?? 10
???? = ???????? ?  ????????
8577 3.01 ? 10
= ? 5 = 8.6 ? 10
6,         (2.4) 
?????????? ?????????? 50 6 ? 10
22 
 
 
 
EFs for other AuNRs can be calculated in same approach. 
2.27 Sample preparation for characterizations 
Electron microscopy. The AuNRs were imaged using a Hitachi SU-70 Schottky 
field-emission gun scanning electron microscope (FEG-SEM) and a JEOL FEG 
transmission electron microscope (TEM). Samples for SEM were prepared by casting 
a 5 ?L of aqueous solution on silicon wafers and dried at room temperature. TEM 
samples were prepared on 300 mesh copper grids.   
UV-vis absorption spectroscopy. Extinction spectra of samples was measured in 
a range of 400-1350 nm using a PERKIN LAMBDA 35 UV/vis system. 
SERS measurements. For the surface-enhanced Raman scattering (SERS) 
measurement, a silicon wafer was treated in an oxygen plasma cleaner for 60 s. 20 ?L 
of an aqueous solution of the AuNR (3 mg/mL) was cast on a 0.2 ? 0.2 cm2 Si wafer 
and dried at room temperature. 4-MBA was dissolved in ethanol at concentration of 10 
mM. Subsequently, a 20 ?L of the 4-MBA solution was cast on the substrate. The 
samples were then rinsed with pure ethanol and dried. The Raman spectra were 
recorded using a Horiba LabRAM confocal Raman microscope equipped with a He-
Ne laser (633 nm). The laser spot was focused with a spot of 1 ?m2. Raman scattering 
intensity was collected with an accumulation time of 5 s/cycle for 5 cycles. For each 
measurement, three points were selected to average to determine the final Raman 
intensity. 
23 
 
 
 
2.3 Results and discussion 
2.3.1 Seed-mediated growth of Ag nanoplates 
Silver nanoplates (AgNPs), also known as nanoprisms or nanodisks, are 2D 
plasmonic nanostructures with a shape-dependent optical response, which allows them 
to be used in many applications.137-141 In this chapter, we developed a facile wet-
chemistry approach to prepare both triangular and rounded AgNPs with tunable lateral 
dimensions, based on previously reported seed growth methods.142,143 The 
diameter/edge length of AgNPs can be tuned by varying the amount of either the seed 
solution or the AgNO3 precursor.144 The AgNPs showed high tunability of their LSPR 
in the visible and NIR when the lateral dimension was tuned.145 The citrate-stabilized 
AgNPs are ready to be used in templated synthesis for Au nanorings and surface 
modification with polymeric ligands.   
The formation of anisotropic nanoplates is not thermodynamically favored, and 
extra efforts are needed to compete against the minimization of surface area. 
Preferential anisotropic growth is dependent not only upon the selective adhesion of 
capping ligands, but also on the crystal symmetry of the starting nuclei.140,146 In our 
approach, trisodium citrate (TSC)-capped Ag nanoplates (~30 nm in size and ~5 nm in 
thickness) with parallel twin defects were prepared through chemical reduction,145,147 
and then used as the seeds. In the growth stage of nanocrystals, TSC also acts as a face-
blocking agent that selectively binds the {111} facets of the seeds, thus slowing the 
growth rate of {111} and allowing extensive growth along other facets.137,139  
 
24 
 
 
 
In a direct chemical reduction process, Ag seeds were prepared by quickly adding 
NaBH4 (reducing agent) to an aqueous solution of AgNO3 in the presence of TSC and 
H2O2. Under magnetic stirring, the solution changed color from light yellow to dark 
blue in 5 min, indicating the formation of anisotropic Ag seeds, and was then incubated 
for 2h. In the following seed-mediated growth, a growth solution was prepared by 
incorporating acetonitrile, TSC, ascorbic acid, and AgNO3 in water. The last four 
components acted, respectively, as a coordinating ligand, a face-blocking agent, a 
reducing agent, and a Ag(I) precursor. Acetonitrile coordinates with Ag(I), and so was 
added to suppress the self-nucleation of Ag.144 Ag seeds were then added into the 
growth solution with predetermined seed/AgNO3 ratios for different sizes of AgNPs. 
Rounded AgNPs were produced under magnetic stirring (~500 rpm), whereas 
triangular AgNPs were produced under vigorous mechanical stirring (~1000 rpm). 
 
Figure 2.2 SEMs of (a) Ag seeds, (b) rounded Ag nanoplates, and triangular Ag 
nanoplates with edge length of (c) 100 nm and (d) 250 nm. Scale bars are 100 nm. 
25 
 
 
 
 
Figure 2.3 The average diameter/edge length of AgNPs as a function of the amount of 
Ag seed solution added in the seed-mediated growth step. 
2.3.2 Templated synthesis of circular and triangular Au nanorings 
The typical synthetic routes for the preparation of AuNRs are illustrated in Figure 
2.1. First, monodisperse Ag nanoplates with rounded and triangular shapes were 
synthesized by the seed-mediated method (Figure 2.2). Two aqueous solutions, 
HAuCl4 (Au precursor) and hydrochloride hydroxylamine (HyA, reducing agent), were 
then simultaneously injected into the solution of Ag nanoplates via two syringes under 
magnetic stirring.148 In this process, Au atoms epitaxially deposited onto the edges of 
Ag templates, that is, the {110} facets, which have the highest surface energy (?{111} < 
? < ? ).149 {100} {110} Subsequent removal of Ag by a mild etchant (a mixture of H2O2 and 
NH3?H2O) produced uniform AuNRs with smooth surface.  
Figure 2.4 shows representative scanning electron micrographs (SEMs) and 
transmission electron microscope micrographs (TEMs) of monodisperse, circular 
26 
 
 
 
AuNRs with diameters of 83 ? 12 nm. The AuNRs have a uniform shape and ring 
thickness. The thickness of the rings, which is roughly equal to the ridge thickness, can 
be measured from SEMs of tilted nanorings. The energy-dispersive X-ray (EDX) 
element mapping in Figure 2.4 shows that the AuNRs are composed of elemental Au, 
although there is a trace amount of Ag background. When triangular Ag nanoplates 
were used as templates, the synthesis produced high-quality, equilateral triangular 
AuNRs with uniform ring thickness and sharp corners (Figure 2.4b and Figures 2.5d-
f). The diameter/edge length of AuNRs can be tuned precisely in the range of 50 to 400 
nm by controlling the amount of Ag seeds, and hence the size of Ag nanoplates. As 
shown in Figure 2.4d, the average diameter/edge length of AuNRs gradually decreased 
with the increase in the amount of Ag seeds added in the growth solution for the 
synthesis of Ag nanoplates. Figure 2.5 shows more SEMs of circular AuNRs with 
different diameters (i.e., 50 nm, 70 nm, and 90 nm) and triangular AuNRs with different 
edge lengths (i.e., 90 nm, 150 nm, and 350 nm). Large-scale SEMs of AuNRs in 
different shapes are presented in Figure 2.6 with size distribution. 
27 
 
 
 
 
Figure 2.4 SEMs and TEMs (inset) of circular AuNRs with diameter of 83 ? 12 nm (a) 
and triangular AuNRs with edge length of 115 ? 22 nm (b). (c) EDX mapping of Au 
and Ag in a circular AuNR. (d) The average diameter of AuNRs as a function of the 
amount of Ag seeds added in the template synthesis step. Scale bars are 300 nm in (a) 
and (b), and 100 nm in (c) and all insets. 
28 
 
 
 
 
Figure 2.5 SEMs of circular and triangular AuNRs with different sizes. (a-c) Circular 
AuNRs with different diameters synthesized with the addition of (a) 4 mL, (b) 3 mL, 
and (c) 2 mL of Ag seed solution. (d-f) Triangular AuNRs with different edge lengths 
synthesized with the addition of (d) 2 mL, (e) 1 mL, and (f) 0.125 mL of Ag seed 
solution. Scale bars are 100 nm. 
 
Figure 2.6 Large-scale SEMs of (a), (b) circular and (c), (d) triangular AuNRs with 
size distribution in (e-h), respectively. Scale bars are 300 nm in (a-c) and 1000 nm in 
(d). 
29 
 
 
 
2.3.3 Thickness and surface roughness of Au nanorings 
It is known that the LSPR of AuNRs is strongly dependent on their ring 
thickness.81 Our synthetic method allows for precise control over the ring thickness 
ranging from 5 to 30 nm by controlling the metal deposition before or/and after the 
removal of Ag templates. Increasing the deposition time from 20 to 45 min led to a 
slight increase in the ring eidth from 2 to 6 nm when Ag templates were not removed.130 
Further increase in the deposition time caused the deposition of Au onto not only the 
{110} facets, but also the {111} facets, and hence the complete coating of Ag 
nanoplates by Au133. This issue can be resolved by multiple Au post-deposition 
processes using thin AuNRs (with Ag nanoplates removed) as seeds. Figure 2.7 shows 
SEMs of circular AuNRs with ring thicknesses in the range of 7 to 22 nm that were 
prepared by varying the amount of Au deposition. The ring thickness grows uniformly 
to form smooth, thick AuNRs. Figure 2.8 shows SEMs of triangular AuNRs with ring 
thicknesses in the range of 10 to 30 nm. The Au deposition occurred on both inner and 
outer surfaces of the ring ridges, as indicated by a slight increase of outer diameter/edge 
length and a clear decrease of the inner diameter/edge length.  
The surface roughness of AuNRs can be tuned by adjusting the infusion rate and 
time of HAuCl4 and HyA solutions. A high infusion rate in combination with a low 
stirring rate facilitates the production of AuNRs with a high surface roughness (Figure 
2.9). We presume that the insufficient diffusive time of precursor towards the seed 
surface leads to the uneven nucleation and deposition of Au on surface of AuNRs. 
30 
 
 
 
2.3.4 Tunable optical properties of Au nanorings 
We monitored the evolution of LSPR spectra over the course of metal deposition 
using UV-vis-NIR spectroscopy. Generally, AuNRs exhibit two distinct peaks: one in 
the visible region, corresponding to the out-of-plane resonance mode, and one in the 
NIR region originating from the in-plane resonance mode.81 As shown in Figure 2.7d, 
the 1050 nm LSPR peak of circular AuNRs blue-shifts to 780 nm with progressive Au 
deposition, whereas the peak at 490 nm red-shifts slightly to 510 nm. This result is in 
good agreement with previous findings, as the decrease of void size and increase of 
ring thickness result in an enhancement of the out-of-plane resonance but a decrease in 
the in-plane resonance.81 
 
Figure 2.7 SEMs of circular AuNRs with an average ring thickness of (a) 7 nm, (b) 15 
nm, (c) 22 nm. (d) Corresponding UV-vis-NIR spectra through the post-deposition 
growth of the samples in (a-c). The samples were obtained by adding different amounts 
of the Au precursor solution: (a) 0 mL, (b) 1.5 mL, and (c) 3.0 mL. 
31 
 
 
 
Triangular AuNRs with ~ 100 nm edge length exhibited an in-plane LSPR peak 
located within the second NIR window (1000 to 1350 nm, Figure 2.8d), which is 
highly desirable for optical theranostics, where light has deeper tissue penetration and 
higher signal-to-noise ratio comparing with light at shorter wavelengths.150 The 
unnormalized peaks also showed decreasing intensity of the LSPR peak corresponding 
to the in-plane polariztion as the ring thickness increases, whereas the out-of-plane peak 
showed increasing intensity as the growth of ring thickness.  
 
Figure 2.8 SEM images of triangular AuNRs with an average thickness of (a) 10 nm, 
(b) 21 nm, and (c) 30 nm. (d) Corresponding UV-vis-NIR spectra through the post-
deposition growth of the samples in (a-c). The samples were obtained by adding 
different amounts of the Au precursor solution: (a) 0 mL, (b) 1.5 mL, and (c) 3.0 mL.  
 
32 
 
 
 
We also monitored growth process of ring surface roughness at high infusion rate 
(4 mL/h) of Au precursor and reducing agent solutions by SEM and UV-vis-NIR 
spectroscopy. The increase in the surface roughness of AuNRs is accompanied by an 
obvious broadening of the out-of-plane LSPR peak and increasing of absorption 
intensity (Figure 2.9d). The nonuniform growth of the irregular tips on the surface of 
the AuNRs results in the peak broadening. And the increased intensity of the out-of-
plane mode is attributed to the decrease of void size and increase of nanoparticle 
volume during the growth process.81 
 
Figure 2.9 TEMs of triangular AuNRs with (a) smooth, (b) rough, and (c) super-rough 
surfaces. (d) Corresponding UV-vis-NIR spectra through the post-deposition growth of 
the samples in (a-c). The samples were obtained by adding different amounts of the Au 
precursor solution: (a) 0 mL, (b) 3.0 mL and (c) 6.0 mL. The scale bars are 150 nm. 
33 
 
 
 
2.3.5 Surface-enhanced Raman scattering with Au nanorings 
Plasmonic nanostructures with rough surface and/or sharp corners are attractive 
for SERS detection, because such structures tend to have a large number of efficient 
electromagnetic enhancement spots.151 As a demonstration, we explored the use of 
circular and triangular AuNRs as substrates for SERS detection (Figure 2.10). The 
SERS substrates were prepared by casting and drying a solution of AuNRs on Si wafer 
to form a thin film. A common Raman probe, 4-mercaptobenzoic acid (4-MBA), which 
has two characteristic bands at about 1590 and 1080 cm?1 associated with the ?8a (a1) 
and ?12 (a1) aromatic ring vibrations,133 was used as a probe molecule. As shown in 
Figure 2.10, our AuNRs exhibit SERS enhancement factors (EFs) that are significantly 
larger than those of Au nanoparticles134 and Au polyhedral nanocrystals.135 The EFs 
were dependent on the shape, ring thickness, and surface roughness of AuNRs. When 
the thickness of AuNRs increased from 10 to 20 nm, the EFs increased from 4.1?106 
to 4.6 ? 106 and from 4.2 ? 106 to 5.1 ? 106 for circular and triangular AuNRs, 
respectively (see 2.2.6 for detailed calculations). Compared with AuNRs with smooth 
surfaces, the EFs of circular and triangular AuNRs (20 nm in ring thickness) with rough 
surfaces increased to 6.5?106 and 8.6?106, respectively. This observation can be 
explained by the presence of more ?hot spots? of the irregular tips on the surface and 
stronger enhancement at sharper tips.152 Overall, triangular AuNRs showed a ~32% 
higher EF than did circular AuNRs, due to a closer edge-to-edge distance and the 
presence of corner-to-edge coupling. For triangular AuNRs with rough surface, the 
close edge-to-edge distance further facilitated the coupling between sharp tips.  
34 
 
 
 
 
Figure 2.10 (a) Raman spectra of 4-MBA (pink) and surface enhanced Raman 
scattering with thin (black), thick (red), and rough (blue) circular AuNRs. (b) Raman 
spectra of 4-MBA (pink) and surface enhanced Raman scattering with thin (black), 
thick (red), and rough (blue) triangular AuNRs. 
2.4 Conclusion 
In summary, we demonstrated a facile wet-chemistry method using Ag nanoplates 
as sacrificial templates for the synthesis of uniform circular and triangular AuNRs with 
tunable size, ring thickness, and surface roughness. These AuNRs exhibit tunable 
LSPR peaks in the visible and NIR regions by tuning the geometric parameters, thus 
enabling potential applications in sensing, catalysis, and optical biomedicine within the 
NIR window. As a demonstration, we showed that triangular rings with rough surfaces 
exhibit an EF of up to 8.6? 106 for SERS analysis of 4-MBA. 
 
   
35 
 
 
 
Chapter 3: Polymeric ligand-mediated regioselective bonding of 
plasmonic nanospheres and nanoplates 
 
Overview: In the design of nanoparticle assemblies, directional interactions play a 
crucial role in controlling the spatial arrangement and orientation of the building blocks. 
Sophisticated surface ligands facilitate the formation of directional bonding via specific 
interactions. Anisotropic building blocks have also attracted much attention, with their 
unique geometries and shape constraints. In this chapter, we combine the steric 
hindrance of polymeric ligand brushes and the anisotropy of nanoparticles to create 
regioselective bonding for a self-assembly system with precise control of the relative 
orientation of gold nanospheres on silver nanoplates. This simple, yet versatile, strategy 
produces assemblies with high regioselectivity and high yields of each assembly. Two 
representative types of assemblies with different bonding modes show different shifts 
in extinction spectra and electric field enhancement patterns. This chapter provides new 
tools for designing plasmonic nanoclusters with increased complexity and optical 
functionality.  
This chapter is adapted from the manuscript published in the following article: Lin, X.; 
Ye, S.; Kong, C.; Webb, K.; Yi, C.; Zhang, S.; Zhang, Q.; Fourkas, J. T.; Nie, Z. 
Polymeric Ligand-Mediated Regioselective Bonding of Plasmonic Nanoplates and 
Nanospheres. J. Am. Chem. Soc. 2020, 142, 41, 17282-17286. 
L. X. and N. Z. conceived and designed the experiments; L. X. performed most of 
experiments; Y. S. performed the simulations of optical properties; W. K., Y. C., and 
Z. S. contributed in the synthesis of polymers and discussion of self-assembly; K. C. 
and Z. Q. helped on electron microscopy and contributed in discussions. L. X. wrote 
the manuscript with revisions from F. J. T., and N. Z. 
 
 
36 
 
 
 
3.1 Introduction 
Inorganic nanoparticles (NPs) often exhibit intriguing optical, electronic and 
magnetic properties that are dependent on size, shape and composition.8 Controllable 
organization of NPs into small clusters allows for modulation of collective physical 
properties that arise from interactions among surface plasmons, excitons, and magnetic 
moments of NPs.153-156 These synergistic properties of assembled NPs have enabled the 
diverse applications of NP clusters in areas including photonics,157 sensing,158 
catalysis,159 energy harvesting,160 and surface-enhanced spectroscopy.161 The coupling 
modes and strengths of NPs are strongly dependent not only on the physical traits of 
constituent NPs, but also on the interparticle distance, and particularly on the spatial 
arrangements of NPs in the clusters.7,48,162,163 Specific and directional bonding between 
individual plasmonic NPs are both highly desirable to achieve precise control over 
spatial arrangements and orientations, and hence the plasmonic coupling. 
Several strategies can be used to introduce directional interactions to assemble 
NPs into defined clusters, including site-specific functionalization of NPs with surface 
patches,117,118,166-168 positioning of NPs on DNA nanostructures with specific binding 
sites,169-171 and confined assembly of NPs in droplets172 or nano-grooves.87,173 In 
particular, assembly of NPs tethered with complementary, single-stranded DNAs at 
specific surface regions has led to a range of small clusters with configurations 
resembling the structure and symmetry of molecules (so-called colloidal molecules). 
However, these assembly approaches either show low yield of desired products or 
require extensive molecular engineering or sophisticated NP functionalization to break 
37 
 
 
 
the symmetry on the surface of NPs.117,118 Programmable assembly of NPs, and 
especially shaped NPs, into precisely controlled clusters remains challenging. 
In this chapter, a polymer-shielding-mediated strategy is described for 
programming the regioselective bonding of non-patchy plasmonic nanoplates (NP-A) 
and nanospheres (NP-B) into discrete clusters with defined geometry and NP 
orientation. The surfaces of the binary NPs are uniformly grafted with block copolymer 
ligands in which the NP-adjacent reactive block carries complementary weak acid or 
base groups for bonding, and the non-reactive block acts as stabilizers for the NPs. The 
steric hindrance of polymeric ligands on anisotropic nanoparticles with different 
surface curvature creates physically distinct regimes, allowing the for the formation of 
two different types of bonding, edge-type and face-type, with precise control of the 
relative orientation of gold nanospheres on silver nanoplates. Two representative types 
of assemblies with different bonding modes are also investigated in extinction spectra 
and electric-field enhancement patterns by experiments and simulations. 
3.2 Experiments 
3.2.1 Materials 
Gold (III) chloride trihydrate (HAuCl4?3H2O, ? 99.9% trace metals basis), sodium 
citrate tribasic dihydrate (? 99%), silver nitrate (AgNO3), L-ascorbic acid, 
tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and dimethyl sulfoxide 
(DMSO) were purchased from Sigma-Aldrich and were used as received. Styrene (St) 
and N, N-dimethylaminoethyl methacrylate (DMAEMA) were passed through an 
Al2O3 column to remove inhibitors. Acrylic acid (AA) was distilled under vacuum 
38 
 
 
 
before use and stored in freezer at ?20 ?. Azobis(isobutyronitrile) (AIBN) was 
recrystallized twice from ethanol. PEO45-CTA, was synthesized by coupling 
poly(ethylene oxide) methyl ether (PEG45-OH, and PEG113-OH, molecular weights 2 
Kg?mol-1 and 5 Kg?mol-1, respectively) with 4-cyano-4-(phenylcarbonothioylthio) 
pentanoic acid (CPPA) through esterification.174 Deionized water (Millipore Milli-Q 
grade) with resistivity of 18.0 M? was used in all of the experiments. 
3.2.2 Synthesis of Au nanospheres and Ag nanoplates 
AuNSs were synthesized using methods previously reported.174 10 mg of 
HAuCl4?3H2O solid was dissolved in 500 mL of water and heated to a boil under 
stirring. 10 mL of sodium citrate aqueous solution (1 wt %) was quickly added into the 
solution, and the solution was kept boiling for 30 min to produce Au nanoparticles (NPs) 
with a diameter of ~15 nm. The as-prepared AuNPs were used as seeds for further 
growth of larger AuNSs in the presence of sodium citrate at 90 ?. The diameter of the 
AuNSs (in the range of ~18-40 nm) was tuned by varying the concentration of the 
HAuCl4 precursor in the growth solution. The resulting AuNSs were collected by 
centrifugation.  
AgNPs were synthesized by seed-mediated growth method described in Chapter 
2.2.2. 
3.2.3 Synthesis of thiol-terminated di-block copolymers 
Thiol-terminated PEO-b-P(DMAEMA-r-St)-SH and PS-b-P(AA-r-St)-SH 
copolymers were synthesized using reversible addition fragmentation chain-transfer 
(RAFT) polymerization. The synthetic procedures are illustrated in Figure 3.1. 
39 
 
 
 
 
Figure 3.1 The synthetic routes of thiol-terminated copolymers: (a) PEOn-b-
P(DMAEMA?-r-St1-?)l-SH and (b) PEOn-b-P(AA?-r-St1-?)m-SH. The non-reactive 
block PEO can also be replaced by PS if the macro-RAFT agent is replaced with PS-
CTA. 
3.2.3.1 Synthesis and characterization of PEO-b-P(DMAEMA-r-St)-SH. 
Figure 3.1a shows the route for the synthesis of thiol-terminated PEO45-b-
P(DMAEMA?-r-St1-?)l [where l and ? are the number of repeat units and the content 
(molar fraction, %) of the DMAEMA monomer in the randomly copolymerized block, 
respectively]. PEO-b-P(DMAEMA-r-St)-CTA was synthesized using PEO-CTA as the 
macroinitiator. Using PEO45-b-P(DMAEMA0.40-r-St0.60)419-SH as an example, 0.2 g of 
PEO-CTA, 7.29 g of styrene, 4.76 g of DMAEMA and 1.64 mg of AIBN were mixed 
and dissolved in 5 mL of anhydrous anisole in a 25 mL Schlenk flask. The reaction 
mixture underwent three freeze-pump-thaw cycles of 10 min each, and then was refilled 
with argon for 30 min. The flask was then placed in a 65 ? oil bath. Polymerization 
proceeded under magnetic stirring for 30 h. To terminate the polymerization, the 
reaction mixture was quickly cooled to room temperature by placing the flask in liquid 
40 
 
 
 
N2. The polymer was purified by three precipitations in hexane and petroleum ether, 
and then was dried under vacuum for 24 h. To obtain thiol-terminated polymers, as-
synthesized dithioester-terminated PEO-b-P(DMAEMA-r-St)-CTA was dissolved in 
DMF with excess n-butylamine (50 equivalents of the amount of the RAFT agent), and 
the mixture was stirred under nitrogen for 2 h. The dithioester end groups were reduced 
to thiols, as confirmed by the color change of the solution from red to light yellow. The 
product was purified to remove excess n-butylamine by three cycles of precipitation in 
cold hexane and re-dissolution in THF and was subsequently dried under vacuum at 
room temperature overnight.   
Based on the 1H NMR spectrum (in CDCl3, see Figure 3.2), the molecular weight 
M -1n(NMR) is 54.5 Kg?mol , as determined by comparing the integrals of the resonance 
peaks of the aromatic ring of PS block (6.4-7.3 ppm), the methylene groups of PEO-
CTA (3.62-3.66 ppm), and methyl groups of DMAEMA block (0-1 ppm). By varying 
the amount of AIBN and the reaction time, PEO-b-P(DMAEMA-r-St)-SH copolymers 
with different lengths and charge ratios were synthesized, as summarized in Table 3.1.  
41 
 
 
 
 
Figure 3.2 1H NMR spectrum of a representative copolymer ligand: PEO45-b-
P(DMAEMA0.40-r-St0.60)419-SH. 
 
3.2.3.2 Synthesis of thiol-terminated PEO-b-P(AA-r-St)-SH 
Figure 3.1b shows the synthetic route for the thiol-terminated block copolymer 
of PEO45-b-P(AA??r-St1-?)m, where m and ? are the number of repeat units and content 
(mole fraction) of AA monomer in the randomly copolymerized block, respectively.  
In the synthesis of PEO45-b-P(AA0.32-r-St0.68)335, PEO45-CTA (0.1 g, 0.05 mmol), 
AIBN (1.62 mg, 0.01 mmol), styrene (1.70 g, 16 mmol) and AA (0.50 g, 7 mmol) were 
42 
 
 
 
dissolved in 3 mL dioxane in a 25 mL Schlenk flask. The mixture was degassed by 
three freeze-pump-thaw cycles. The flask was then placed in a pre-heated oil bath at 
75 ? for the desired time under magnetic stirring. Liquid N2 was used to cool the 
solution quickly to room temperature to terminate the polymerization. The mixture was 
diluted with THF and then precipitated twice in hexane. The obtained pink precipitate 
was dried under vacuum at 40 ? overnight.   
To release the thiol end-group, as-synthesized copolymers were treated with n-
butylamine, with a minor modification of a literature procedure.174 Briefly, 1.0 g of 
PEO-b-P(AA-r-St)-CTA was dissolved in 5 mL DMF. An excess amount of n-
butylamine (~ 0.5 mL) was diluted with 10 mL of DMF, and the mixture was added 
dropwise to the polymer solution under stirring. The reaction proceeded under an argon 
atmosphere for 3 h. With the addition of the n-butylamine solution, the mixture became 
cloudy, due to the formation of insoluble copolymers by the neutralization of carboxyl 
groups of the copolymers with n-butylamine. To avoid gelation and to dissolve the 
copolymers, 10 mL of DI water was added dropwise to the mixture under stirring. After 
the reaction was complete, the solution was neutralized, and the polymer was 
precipitated in excess 0.5 M HCl aqueous solution in an ice-water bath. The precipitate 
was filtered and washed with excess deionized water to remove residual acids and salts. 
Finally, the copolymers were dissolved in 15 mL of THF and precipitated by adding 
25 mL of petroleum ether. The white powder product was dried at 40 ? under vacuum 
for two days.   
1H NMR spectroscopy was used to determine the degree of polymerization and 
composition of the random P(AA?-r-St1-?)m blocks. According to the 1H NMR spectrum 
43 
 
 
 
of the copolymer in DMSO-d6 (Figure 3.3), the number-average molecular weight, 
Mn(NMR) of the product was 33.4 Kg?mol-1 and the mole fraction of -COOH groups, ?, 
was ~0.32, as calculated by using the integrals of the resonance peaks of -CH2CH2O? 
from the PEO block (3.62 ~ 3.66 ppm).  
 
Figure 3.3 1H NMR spectrum of a representative copolymer ligand: PEO45-b-P(AA0.32-
r-St0.68)335-SH.  
 
3.2.3.3 Synthesis of thiol-terminated PS-b-P(AA-r-St)-SH  
Synthesis of macro-RAFT chain transfer agent (PS-CTA): PS-b-P(AA-r-St)-CTA 
was synthesized using PS-CTA as the first block. For PS111-CTA, CPDB (44.0mg, 0.2 
mmol), AIBN (1.62 mg, 0.01 mmol), and styrene (15.6 g, 0.15 mol) were added to a 
44 
 
 
 
50 mL Schlenk flask. The mixture was degassed by three freeze-pump-thaw cycles, 
and then reacted at 85 ?  for 11 hours under magnetic stirring. To terminate the 
polymerization reaction, liquid N2 was used to cool the solution quickly to room 
temperature. The polymer was precipitated in hexane 3 times and dried under vacuum 
for 24 h.   
For PS111-b-P(AA-r-St)-CTA, the as-synthesized macro-RAFT agent PS111-CTA 
(0.95 g, 0.10 mmol), AA (0.97 g, 13.5 mmol), styrene (3.28 g, 31.5 mmol), and AIBN 
(1.64 mg, 10 ?mol) were dissolved in a 6 mL of dioxane in a 25 mL Schlenk flask. The 
mixture was degassed by three freeze-pump-thaw cycles, stirred under argon for 30 
min, and then reacted at 75 ?  for 24 h under magnetic stirring. After the 
polymerization reaction was complete, the solution was cooled down to room 
temperature by using liquid N2. The polymer was precipitated three times in hexane 
and petroleum ether, and then dried under vacuum for 24 h.  
To convert the terminal group from a dithioester to a thiol, PS-b-P(AA-r-St)-
CTA was dissolved in DMF with excess n-butylamine (50 eq.), and the mixture was 
stirred under nitrogen for 2 h. The mixture was then transferred into an ice bath, with a 
dropwise addition of excess 37% HCl to neutralize and precipitate the product. The 
polymer was then washed several times with water, to remove residual acid and salt, 
was precipitated twice in petroleum ether, and then was dried under vacuum for 24 
hours. The parameters of PS-b-P(AA-r-St)-SH and other copolymers used are given in 
Table 3.1. 
 
 
45 
 
 
 
Table 3.1 Characterization of Copolymers* 
Copolymer compositions ?? (??) Mn, (Kg?mol-1) R0 (nm) 
PEO45-b-P(DMAEMA0.32-r-St0.68)98-SH 0.32 13.9 7.9 
PEO45-b-P(DMAEMA0.41-r-St0.59)158-SH 0.41 21.6 9.9 
PEO45-b-P(DMAEMA0.30-r-St0.70)244-SH 0.30 31.3 11.5 
PEO45-b-P(DMAEMA0.40-r-St0.60)419-SH 0.40 54.5 15.7 
PEO113-b-P(AA0.26-r-St0.74)249-SH 0.26 28.8 16.9 
PS111-b-P(AA0.31-r-St0.69)289-SH 0.31 38.8 13.2 
PEO45-b-P(AA0.30 -r-St0.70)185-SH 0.30 19.5 8.9 
PEO45-b-P(AA0.32 -r-St0.68)335-SH 0.32 33.4 11.8 
PEO45-b-P(AA0.36 -r-St0.64)467-SH 0.36 45.2 13.7 
PEO45-b-P(AA0.32 -r-St0.68)550-SH 0.32 53.6 15.2 
* The molecular weight Mn, (Kg?mol-1) was calculated from 1H NMR measurements. 
The root-mean-square end-to-end distance, R0 (nm), was calculated from R = N0.50 b, 
where b is the Kuhn length (b = 1.8 nm for PS, b = 1.1 nm for PEO) and N is the 
number of Kuhn segments calculated from the molar mass of the Kuhn monomer M0 
(M0 = 720 g/mol for PS and M0 = 137 g/mol for PEO). 
 
3.2.4 Surface modification of NPs and self-assembly 
Ag and Au NPs were functionalized using a ligand-exchange method reported 
previously, with minor modifications.174, 115 Taking functionalization of 26 nm Au 
nanospheres with PEO45-b-P(DMAEMA0.32-r-St0.68)335-SH as an example, 15 mg of 
PEO45-b-P(DMAEMA0.32-r-St0.68)335-SH was dissolved in 10 mL of DMF in a 20 mL 
vial. A 2 mL dispersion of Au nanospheres in DMF (~2 mg/mL) was added dropwise 
into the above solution under sonication. The mixture was sonicated for another 1 h 
and incubated overnight to allow for complete ligand exchange. The copolymer-grafted 
NPs were purified to remove untethered copolymers by 10 cycles of centrifugation 
(each for 11,000 rpm for 18 min at 20 ?C, NPs were collected and redispersed in 2 mL 
DMF between cycles). The NPs were re-dispersed in 4 mL of THF at an estimated 
46 
 
 
 
concentration of ~ 3 nM for future use. For Ag nanoplates, the procedure for the surface 
modification was the same as that for Au nanospheres, except that the mixture of Ag 
nanoplates and copolymers was incubated for 1 h after vigorous shaking to allow for 
complete ligand exchange. 
The assembly of NP clusters was triggered by mixing binary building blocks in 
THF, a good solvent for the copolymer ligands, at predetermined concentrations. 
Taking edge-type AB2 structures as an example, 50 ?L of the stock solution of NP-As 
in THF (~ 1.48 nM) was added to a 4 mL glass vial containing 1.0 mL THF, followed 
by the addition of different amounts of the stock solution of NP-Bs under sonication 
for ~ 20 s. The mixture was then sonicated for 3 min and incubated for ~ 2 h.  
       The samples were imaged by scanning electron microscopy and transmission 
electron microscopy for statistical analysis of the positions of Au nanospheres and 
yields of assemblies. 
3.2.5 Finite-difference time-domain (FDTD) simulations 
Simulations of the optical properties of the Au nanospheres, Ag nanoplates, face-
type AB2 trimers, and edge-type AB2 trimers were performed using FDTD solutions 
(Lumerical Solutions, Inc.). Perfectly matched layers were set as the boundary 
conditions. A 3D total-field scattered-field light source with wavelengths ranging from 
400 nm to 1100 nm was introduced into a cuboid containing the target nanostructures. 
Because nanostructures in solvents are randomly oriented, different incident directions 
of the light source were considered in our calculations. Au nanospheres were treated as 
ideal spheres. Therefore, one incident direction and one polarization state were 
considered. For the Ag nanoplates and the face-type AB2 trimers, incident directions 
47 
 
 
 
parallel and perpendicular to the plane of the nanoplates were included in our 
calculations. Moreover, two orthogonal polarization states were considered in each 
incident direction to obtain unpolarized results. We then added up the extinction spectra 
from these two incident directions to produce the UV-vis extinction spectra of Ag 
nanoplates and the face-type AB2 trimers in solvents.  
In the case of edge-type AB2 trimers, we modeled the systems with three incident 
directions, including the directions perpendicular to the nanoplate plane, and parallel 
and perpendicular to the major axis of the AB2 trimer, coplanar with the nanoplate. Two 
orthogonal polarization states were calculated in each incident direction as well. All of 
the extinction spectra from these three incident directions were added. In all cases, the 
background refractive index was set as 1.41. The Au nanosphere was 25 nm in diameter 
and the Ag nanoplate was 85 nm in diameter and 8 nm in thickness. Both the Au 
nanosphere and the Ag nanoplate were coated with 5-nm-thick solvated polymer shells 
whose refractive index was estimated to be 1.45. The gaps between the Ag nanoplate 
and the Au nanosphere were set as 8 nm for the face-type and the edge-type trimer. We 
imposed a uniform mesh grid on the target nanostructure. The maximum mesh step was 
set as 1 nm along all the directions. The permittivity of Au and Ag were adopted from 
the experimental data from Johnson and Christy,175 and Palik,176 respectively. All of 
the electric field profiles were calculated at the wavelength of the plasmon resonance. 
3.2.6 Calculation and grafting-density 
Thermogravimetric analysis (TGA) was performed to quantify the number of 
grafted polymers on NPs (Figure 3.4). Taking PS111-b-P(AA0.31-r-St0.69)289 grafted Ag 
nanoplates as an example, a weight loss was observed at temperatures above 200 ?, 
48 
 
 
 
due to the thermal degradation of copolymers into volatile products. Based on the 
measured weight loss f, the grafting density (??, chains?nm-2, listed in Table 3.2) of 
copolymers on Au nanospheres and Ag nanoplates was estimated by the respective 
equations:123 
?? ?? ????
?? = ? ?? ,                                               (3.1) 
1 ? ?? 6????
and 
?? ?? ????2
?? = ? ?? ,                                              (3.2) 
1 ? ?? 6?????
where f is the weight fraction of the polymeric ligands determined by TGA, NA is 
Avogadro?s constant, ?? is the bulk density of NP?s core material (19.32 g cm-3 for Au 
and 10.49 g cm-3 for Ag), D is the diameter of NPs, and Mn is the number-average 
molecular weight of the copolymer ligands. In the calculation, we assume that the 
density of the NPs is the same as that of the bulk material, and that no free polymer is 
present in the system. For example, the weight losses for PEO45-b-P(AA0.36-r-St0.64)467-
grafted Ag nanoplates (Figure 3.4a, red) and PEO45-b-P(DMAEMA0.41-r-St0.59)158-
grafted Au nanospheres (Figure 3.4b, black) were 8.4% and 4.3%, respectively. 
Following the equations, ? was calculated to be 0.45 chain/nm2 and 0.11 chain/nm2, 
respectively. 
49 
 
 
 
 
Figure 3.4 Representative TGA traces of (a) Ag nanoplates grafted with PEO113-b-
P(AA0.26-r-St0.74)249 (black), PEO45-b-P(AA0.36-r-St0.64)467 (red), and PS111-b-P(AA0.31-
r-St0.69)289 (blue); (b) Au nanospheres grafted with PEO45-b-P(DMAEMA0.41-r-
St0.59)158 (black) and PEO45-b-P(DMAEMA0.30-r-St0.70)244 (red). 
From the calculated ?, the average distance (d) between anchoring points of 
adjacent ligands can be estimated by using equation:123 
2 1.1
?? = ?  ,                                                   (3.3) 
????? ???
where ? is estimated from TGA. The footprint of a single copolymer ligand on the NP 
surface is considered as a circular area with a diameter equal to d. The d values for Ag 
grafted with PEO45-P(A0.36S0.64)467 and Au grafted with PEO45-P(D0.41St0.59)158 were 
estimated to be 1.73 nm and 3.3 nm, respectively. The radius of gyration of the 
corresponding copolymer can be calculated from Rg = (Nb2/6)1/2 (see Table 3.2).177 The 
ratio of Rg/d is estimated to be in the range of 1.6 to 2.0 for NP-Bs and 4.0 to 5.0 for 
NP-As, which suggests that the copolymer chains extended and stretched on both Au 
and Ag surfaces to adopt a polymer brush conformation.178,179 
 
50 
 
 
 
3.2.7 Sample preparation for characterizations  
1H NMR. 1H NMR spectra were recorded using Bruker AV-400 MHz high 
resolution NMR spectrometer. All copolymers were dried under vacuum at 80 ? 
overnight to remove trace amounts of solvent. The NMR samples were prepared by 
dissolving polymers in deuterated reagents, either CDCl3, THF-d4 or DMSO-d6. 
Gel permeation chromatography (GPC). GPC measurements were performed on 
a Waters GPC-a (1515 HPLC pump and Waters 717 Plus autoinjector) equipped with 
a Varian 380-LC evaporative light-scattering detector and a Waters 2487 dual 
absorbance detector. PS standards were used for molecular weight and molecular 
weight distribution calibration, and THF was used as the elution solvent, with a flow 
rate of 2.0 mL?min-1. The data were processed using Empower GPC software (Waters, 
Inc.). 
Thermogravimetric Analysis (TGA). TGA was used to characterize the grafting 
density (?) of copolymers on the surface of the Au and Ag NPs. A 5.0 mL stock solution 
of building blocks was centrifuged to remove the supernatant, hexane was subsequently 
added, and the sample was centrifuged until a pellet formed. The pellets were dried in 
vacuum oven for 24 h to remove the solvent residues before TGA measurement. The 
experiments were carried out under argon flow, with a scan rate of 25 ??min-1 from 25 
? to 600 ?. Before the scan process, the temperature was maintained at 100 ? for 30 
min to remove any residual moisture. The weight fraction of polymer ligands f was 
determined by using the TA Universal Analysis software package.  
UV-vis absorption spectroscopy. Extinction spectra of samples was measured in 
a range of 400-1100 nm using a PERKIN LAMBDA 35 UV-vis system.  
51 
 
 
 
Electron microscopy. The morphologies of assemblies were obtained using a 
Hitachi SU-70 Schottky field-emission-gun scanning electron microscope (SEM) and 
a JEOL FEG transmission electron microscope (TEM). Samples for scanning and 
transmission electron microscopy analysis were prepared by depositing ~2 ?L of 
solution on silicon wafer and 300 mesh copper grids covered with carbon film, 
respectively. The samples were dried at room temperature before imaging measurement. 
3.3 Results and discussion 
3.3.1 Self-assembly driven by reactive block copolymers 
In this chapter, we demonstrate a polymer-shielding-mediated mechanism for 
programming the regioselective bonding of non-patchy plasmonic nanoplates (NP-A) 
and nanospheres (NP-B) into discrete clusters with defined geometry and NP 
orientation (Figure 3.5).  
 
Figure 3.5 Schematic illustration of regioselective bonding of NP-As and NP-Bs into 
edge-type and face-type clusters mediated by the shielding effect of copolymer ligands. 
52 
 
 
 
We assumed that the surfaces of the binary NPs are uniformly grafted with block 
copolymer ligands in which the NP-adjacent reactive block carries complementary 
weak acid or base groups for bonding and the non-reactive block acts as stabilizers for 
the NPs (Figure 3.6). When NP-As and NP-Bs are mixed in solution, the neutralization 
reaction between end-tethered copolymer ligands triggers the directional bonding of 
dissimilar NPs to yield uniform clusters with ABx structures (where x is the number of 
NP-Bs in the cluster, 1?x?4). The bonding number x of ABx structures is dictated by 
the stoichiometry of acid/base groups on NP-Bs and NP-A within a cluster. Depending 
on the length of the non-reactive block, the NP-Bs preferentially attach to the edge or 
the face of a central NP-A to produce edge-type or face-type assemblies. The difference 
in bonding direction/position is ascribed to the different steric shielding effect between 
planar and curved polymer brushes on the disc-shaped NP-A. 
 
Figure 3.6 Schematic illustration of (a) a copolymer-grafted nanoplate (NP-A) and (b) 
a copolymer-grafted nanosphere (NP-B).  
As a demonstration, we synthesized circular silver nanoplates180 (diameter, D of 
61 ? 9 nm) as NP-As and Au nanospheres174 (D = 28 ? 3 nm) as NP-Bs using a seed-
mediated growth method. The two types of NPs were modified with two distinct thiol-
terminated block copolymers via ligand exchange method.181, 182 Specifically, a 
copolymer of poly(ethylene oxide)n-b-poly(acrylic acid?-r-styrene1-?)m-SH [PEOn-b-
53 
 
 
 
P(AA?-r-St1-?)m] was used for the NP-As, whereas a copolymer of poly(ethylene 
oxide)k-b-poly(N,N-dimethylaminoethyl methacrylate?-r-styrene1-?)l-SH [PEOk-b-
P(DMAEMA?-r-St1-?)l] was used for the NP-Bs (Figure 3.6 and Table 3.1). Here, n, 
m, k, and l denote the degree of polymerization of the corresponding blocks, whereas ? 
and ? represent the content (%) of each comonomer in the randomly-copolymerized 
blocks. In the copolymer ligands, the PEO block serves as a steric stabilizer for the NPs, 
whereas the random copolymerized block provides complementary reactive groups to 
trigger NP assembly. The copolymer-grafted NP-As and NP-Bs were dispersed in 
tetrahydrofuran (THF), a good solvent for the copolymer ligands. 
3.3.2 Mechanism of regioselective bonding 
The grafting density (?? ) of copolymers on NPs was estimated to be ~0.4 
chains?nm-2 for NP-As and ~0.1 chains?nm-2 for NP-Bs (Table 3.2). In THF, a good 
solvent of the copolymers, the ligands adopted a brush conformation on the surface of 
NPs, as indicated by Rg/d ?4.0 for nanoplates and Rg/d ? 1.6 for nanospheres, where d 
is the average anchoring distance between adjacent polymer chains and Rg is the radius 
of gyration of an unperturbed copolymer chains.178,179,183 The conformations of end-
grafted copolymers on the disc-shaped NP-A exhibited two physically different states: 
(i) a planar brush on each flat face, and (ii) a convex curved brush at the edge of the 
disc. Based on the blob model of polymer chains, the blob size of a planar brush is 
constant from the chain end adjacent to the NP surface to the tip of the brush.179 In 
contrast, the blob size of curved brushes increases as the distance from the convex 
surface increases, thus providing more space for the interpenetration of copolymer 
brushes from interacting NPs.183,184  
54 
 
 
 
Besides the grafting density of copolymer ligands, the conformation of polymer 
brushes may be affected by the quality of solvents, the chemical composition of 
polymers, and osmotic pressure (which is a negligible factor in this system). In our 
experiments, the copolymers were dissolved in good solvents (i.e., THF and DMF) in 
surface-modification and self-assembly process to maintain the stretched chain 
conformation within the polymer brushes. The chemical composition of steric blocks 
may affect the conformation of polymer brushes by interactions like hydrogen bonds. 
In the model system, we used PEO (n=113) as steric blocks for shielding the face-type 
bonding and producing edge-type assemblies. To demonstrate that the regioselectivity 
of NP bonding originates from the steric shielding effect of the non-reactive polymer 
segments, rather than from the chemical composition of copolymer ligands, the PEO 
block was replaced by a steric polystyrene (PS) block of comparable length (n=111) in 
section 3.3.3.  
Table 3.2 Key parameters of NP-A(B) with copolymer ligands after surface 
modification* 
NP-A/NP-B f (%) ?? (chains?nm-2) d (nm) Rg (nm) Rg/d 
Ag-PEO45-P(A0.36S0.64)467 8.6 0.45 1.7 8.8 5.1 
Ag-PEO113-P(A0.26S0.74)249 6.3 0.46 1.7 7.2 4.2 
Ag-PS111-P(A0.31S0.69)289 6.4 0.39 1.9 8.0 4.2 
Au-PEO45-P(D0.41St0.59)158 4.3 0.11 3.3 5.7 1.7 
Au-PEO45-P(D0.30St0.70)244 5.6 0.10 3.5 7.0 2.0 
* f (%) is the weight fraction of copolymer ligands measured by thermogravimetric 
analysis (TGA); Grafting density, ??  (chains?nm-2), and average anchoring distance 
between adjacent chains, d (nm), were calculated and estimated in SI; The radius of 
gyration, Rg (nm), was calculated from R 0.5g = N b/60.5, where b is the Kuhn length (b = 
1.8 nm for PS, b = 1.1 nm for PEO) and N is the number of Kuhn segments calculated 
the molar mass of Kuhn monomer M0 (M0 = 720 g/mol for PS and M0 = 137 g/mol for 
PEO). 
 
55 
 
 
 
When NP-As and NP-Bs in THF were mixed with predetermined feeding-number 
ratios, ?B/A of NP-Bs to NP-As, the neutralization reaction between acid/base groups of 
the end-grafted copolymers triggered the directional bonding of NP-Bs to NP-As to 
form stable clusters. In this process, NP-As and NP-Bs bonded in two distinctive modes, 
edge-type and face-type, depending on the length of PEO block of the copolymer 
ligands on the surface of disc-shaped NP-As (Figure 3.2). 
3.3.3 Edge-type bonding and nanoclusters 
When n of PEOn-b-P(AA?-r-St1-?)m ligands on NP-As is large (n > 100, e.g., n = 
113), the steric effect of the long, non-reactive PEO blocks of the planar copolymer 
brushes on the flat top/bottom faces of the NP-As shields the reactive block adjacent to 
the NP surface from reacting with the copolymers on the NP-Bs. Spherical NP-Bs, thus, 
prefer to attack the NP-As from the edge, where the polymeric segments are less 
crowded, as indicated by larger blob size of curved brushes than that of planar brushes. 
As a result, the assembly process produces edge-type clusters with NP-Bs arranged at 
the edge of a central NP-A (Figure 3.7).  
The control over the coordination number x in ABx clusters was achieved by 
varying the number ratio, ZA/B of acid groups on a single NP-A to base groups on a 
single NP-B, and ?B/A. The ZA/B can be expressed as: 
?? ? ?? ?? ? ? ??
?? ?? ????/?? = ? ? 2  ,                                         (3.4) ?? ? ?? ???? ????
where the subscripts ?? and ?? represent NP-As and NP-Bs, respectively, and ?m, ?l and 
h are the number of acid groups of a PEOn-b-P(AA?-r-St1-?)m chain, the number of base 
groups of a PEOk-b-P(DMAEMA?-r-St1-?)l chain, and the thickness of disc-shaped NP-
56 
 
 
 
As, respectively. In experiments, we tuned the values of ZA/B by varying ?l of PEO-b-
P(DMAEMA-r-St) copolymers on NP-Bs, while keeping ?m of PEO-b-P(AA-r-St) 
copolymers on NP-As constant. As ?l decreased from 168 to 31 (ZA/B increased from 
1.05 to 4.76), the value of x increased from 1 to 4, because more NP-Bs were needed 
to provide base groups for reacting with the acid groups on the central NP-A in a cluster. 
At ?l = 168 (ZA/B = 1.05) and ?B/A = ~1:1, the NPs assembled into AB clusters (Figure 
3.7a). At a reduced ?l of 73 (ZA/B = 2.42), a ~2:1 and ~3:1 of ?B/A yielded AB2 and AB3 
clusters as major products, respectively (Figure 3.7b, c). A high yield of AB4 clusters 
was obtained at ?l =31 (ZA/B = 4.76) and ?B/A = ~4:1 (Figure 3.7d). The heterogeneous 
structures of ABx clusters were confirmed by element mapping under scanning 
transmission electron microscopy (STEM) mode (Figure 3.8).     
The assembly strategy showed high-yield generation of edge-type ABx clusters 
(2?x?4), which we ascribe to the reaction stoichiometry-driven bonding mechanism 
and the steric constrains of the inert PEO blocks. Without any further purification, the 
yields of edge-type AB2, AB3, and AB4 structures were 69%, 72% and 59%, 
respectively (Figure 3.9). The assembly strategy exhibited high regioselectivity in 
positioning the NP-Bs at the edge rather than the faces of a central NP-A. Over 90% 
NP-Bs in ABx structures were bonded to the edge of a NP-A (Figure 3.10). The ABx 
structures showed symmetric arrangements of NP-Bs around the central NP-A. The 
average bond angles between NP-A and NP-Bs in the clusters were 160 ? 20?, 121 ? 
17?, and 91 ? 12? for AB2, AB3, and AB4, respectively (Figure 3.11). The 
regioselectivity of NP bonding originates from the steric shielding effect of the non-
reactive polymer segments, rather than from the chemical composition of copolymer 
57 
 
 
 
ligands. When the PEO block was replaced by a non-reactive polystyrene block of 
comparable length, small clusters with 90% edge-type bonding were also generated. 
 
Figure 3.7 Schemes, TEMs and SEMs of edge-type NP clusters with different 
configurations: (a) AB, (b) AB2, (c) AB3, and (d) AB4. NP-As: PEO113-b-P(AA0.26-r-
St0.74)249-grafted Ag nanoplates;  NP-Bs: Au nanospheres grafted with (a) PEO45-b-
P(DMAEMA0.40-r-St0.60)419, (b, c) PEO45-b-P(DMAEMA0.30-r-St0.70)244, and (d) 
PEO45-b-P(DMAEMA0.32-r-St0.68)98 copolymers. ?B/A: 1:1 (a), 2:1 (b), 3:1 (c) and 4:1 
(d). Au is brighter than Ag under SEM, while darker under TEM. Scale bars: 50 nm. 
 
Figure 3.8 Energy-dispersive X-ray (EDX) elemental mapping and high-angle annular 
dark field (HAADF) transmission electron micrographs (TEMs, insets) of edge-type (a) 
AB2 and (b) AB3.  
58 
 
 
 
 
 
Figure 3.9 Distribution of edge-type ABx clusters. Over 500 clusters were analyzed for 
each plot. 
 
Figure 3.10 Distribution of bonding modes in edge-type ABx clusters. Over 500 
clusters were analyzed for each plot. 
59 
 
 
 
 
Figure 3.11 Distribution of the B-A-B angle in edge-type AB2, AB3, and AB4 
structures. 
3.3.4 Face-type bonding and nanoclusters 
At n = 45 for PEOn-b-P(AA?-r-St1-?)m ligands on NP-As, the steric shielding of 
short PEO blocks is negligible. The carboxyl groups of NP-adjacent block of planar 
brushes on the faces of a NP-A can be readily accessed by the base groups of copolymer 
ligands on a NP-B. Thus, the acid-base neutralization reaction leads to the bonding of 
NP-Bs onto the top and bottom faces of a central disc-shaped NP-A to form face-type 
AB2 structures (Figure 3.12a). In this arrangement, the conformal contact and extent 
of reaction of copolymer ligands on both NP-A and NP-B can be maximized, thus 
minimizing the overall energy of the assembly system. When NP-As and NP-Bs are 
60 
 
 
 
grafted with PEO45-b-P(AA0.32-r-St0.68)335 and PEO45-b-P(DMAEMA0.40-r-St0.60)419 
copolymers, their assembly produces face-type AB2 trimers with one NP-A 
sandwiched between two NP-Bs. The yield of AB2 clusters was 75%, and over 80% of 
the NP-Bs were bonded to the faces of NP-As (Figure 3.12d). When ?m of PEO-b-
P(AA-r-St) copolymers on NP-As decreased from 107 to 56 and ?B/A decreased from 
~2:1 to ~1:1, the assembly process produced linear chains with alternating sequence of 
NP-As and NP-Bs (Figure 3.12b). We attribute the formation of alternating chains, 
rather than discrete AB dimers, to the fact that short PEO-b-P(AA-r-St) copolymers on 
the flat faces of NP-As can only react and consume base groups on one side of NP-Bs, 
thus leaving base groups on the other side to further react with another NP-As. The 
face-type AB2 structure was confirmed by element mapping under STEM mode 
(Figure 3.12c). 
61 
 
 
 
 
Figure 3.12 Schemes, TEMs and SEMs of (a) face-type AB2 structures and (b) 
alternating chains. NP-Bs: PEO45-b-P(DMAEMA0.40-r-St0.60)419-tethered Au 
nanospheres; NP-As: Ag nanoplates grafted with (a) PEO45-b-P(AA0.32-r-St0.68)335 and 
(b) PEO45-b-P(AA0.30-r-St0.70)185 copolymers. (c) EDX elemental mapping and 
HAADF TEM image (inset) of a face-type AB2 structure. (d) Yield (left) and bonding 
mode (right) of clusters in (a). Over 500 clusters were analyzed for each plot in (d). 
3.3.5 Regioselective self-assembly with triangular nanoplates 
Regioselective assembly is applicable to the directional bonding of spherical NPs 
with other shaped nanoplates. Silver triangular nanoplates (edge length: 87 ? 10 nm) 
62 
 
 
 
were grafted with PEOn-b-P(AA?-r-St1-?)m copolymer ligands for use as NP-As. Their 
assembly with NP-Bs produced both edge-type and face-type ABx clusters, following 
the same assembly principle as circular nanoplates. At n = 113, the long PEO blocks 
shielded the acid groups on the flat faces of the NP-As from reacting with the base 
groups on the NP-Bs, thus producing edge-type clusters, e.g., AB2 structures (Figure 
3.13a). The yield of edge-type bonding was ~90% (Figure 3.14). At n = 45, the 
assembly produced face-type AB2 clusters and alternating chains (Figure 3.13b, c), 
with 80% and 86% yield of face-type NP arrangements, respectively (Figure 3.14). 
 
Figure 3.13 (a-c) Schemes, TEMs, and SEMs of (a) edge-type AB2 trimers, (b) face-
type AB2 trimers, and (c) alternating chains. NP-Bs: PEO45-b-P(DMAEMA0.40-r-
St0.60)419-grafted Au nanospheres; NP-As: Ag nanoplates tethered with (a) PEO113-b-
P(AA0.26-r-St0.74)249, (b) PEO45-b-P(AA0.32-r-St0.68)335, and (c) PEO45-b-P(AA0.30-r-
St0.70)185 copolymers. Scale bar: 100 nm. 
63 
 
 
 
 
Figure 3.14 Distribution of bonding modes with central triangular NP-As in (a) edge-
type AB3 clusters, (b) face-type AB2 clusters, and (c) alternating chains. 
3.3.6 Experimental and computational studies on optical properties 
We investigated the optical properties of discrete edge-type and face-type AB2 
trimeric structures assembled from NP-As (D = 83.8 ? 8.0 nm) and NP-Bs (D = 27.5 ? 
3.9 nm). In experiments, the bonding of two NP-Bs with a NP-A in AB2 clusters led to 
a redshift in the localized surface plasmonic resonance (LSPR) bands. The strength of 
redshift was dependent on the relative orientation of NPs. The longitudinal LSPR band 
of face-type AB2 clusters was located at 847 nm, whereas the peak was at 842 nm for 
edge-type trimers (Figure 3.15). The NP orientation-dependent redshift in the 
longitudinal LSPR band was confirmed by finite-difference time-domain (FDTD) 
simulations. When the NP-Bs were arranged at the face of a NP-A in AB2 structures, 
the LSPR band shifted from 827 nm (the in-plane mode of pristine NP-As) to 840 nm 
(Figure 3.16). In contrast, the edge-type AB2 structures displayed a longitudinal LSPR 
peak at 836 nm.  
64 
 
 
 
 
Figure 3.15 Experimental extinction spectra for polymer-grafted Au nanospheres 
(black), Ag nanoplates (red), edge-type (blue), and face-type trimers (olive).  
 
Figure 3.16 Simulated extinction spectra for polymer-grafted Au nanospheres (black), 
Ag nanoplates (red), edge-type (blue), and face-type trimers (olive). 
65 
 
 
 
To investigate the origin of the arrangement-dependent plasmonic coupling, we 
calculated the polarization spectra of Ag nanoplates (Figure 3.17), edge-type trimers 
(Figure 3.18), and face-type trimers (Figure 3.19) with different light incident 
directions: the x, y, and z axes. In all polarization spectra, the out-of-plane mode 
(polarizations along z axis) made a minimal contribution to the overall extinction 
spectra. The major difference in the optical response between the two types of AB2 
trimers originated from different contributions of polarizations along the x and y axes 
(in-plane mode) of the trimers. For the face-type trimer, the peak for polarizations both 
along x and y axes was at 840 nm, and the overall extinction has a peak at the same 
wavelength (Figure 3.19). In contrast, the overall extinction spectrum of the edge-type 
trimer was the sum of spectra with polarizations along x and y axes that had maxima at 
different wavelengths (Figure 3.18). 
 
Figure 3.17 Calculated extinction spectra of polymer shell-coated Ag nanoplate in 
THF. The overall simulated extinction spectrum (magenta curve) was obtained by 
summing the spectra from all contributing incident directions and polarization states. 
66 
 
 
 
 
Figure 3.18 Calculated extinction spectra of edge-type AB2 trimers in different 
polarization and light incidence directions, and the overall simulated extinction 
spectrum obtained from the addition of the all spectra from all contributing incident 
directions and polarization states. For the edge-type trimer, the overall extinction 
spectrum was the sum of spectra at two polarizations, along x and y axes, with maxima 
at different wavelengths, 827 nm and 845 nm, respectively. The overall extinction 
shows a peak at 836 nm.   
67 
 
 
 
 
Figure 3.19 Calculated extinction spectra of face-type AB2 trimers in different 
polarization and light incidence directions, and the overall simulated extinction 
spectrum obtained from the addition of the all spectra from all contributing incident 
directions and polarization states. For the face-type trimer, the maxima for polarizations 
along both the x and y axes were at 840 nm, and the overall extinction therefore exhibits 
a peak at the same wavelength. 
68 
 
 
 
 
Figure 3.20 Electric-field intensity enhancement contours of (a, b) edge-type trimers 
and (c, d) face-type trimers with different electric field polarizations. The incident light 
is (c, e) perpendicular and (d, f) parallel to the plane of nanoplate. Scale bars: 50 nm. 
Both edge-type and face-type trimers exhibit a more significant electric-field 
enhancement at the surface of Ag when the polarization direction is parallel to the plane 
of the nanoplate (Figure 3.20a, c), rather than perpendicular to the plane (Figure 3.20b, 
d). Notably, the edge-type AB2 shows stronger overall electric-field enhancement than 
does the face-type trimer under the same polarization mode. 
3.4 Conclusion 
In summary, we have developed a strategy to control the regioselective bonding 
of plasmonic NPs with distinct shapes to generate discrete nanoclusters. The self-
assembly is driven the acid-base reaction of reactive groups of block copolymer ligands 
on Ag nanoplates and Au nanospheres. The arrangement of nanospheres around a 
nanoplate in the clusters can be modulated by tuning the steric shielding effect of 
copolymer ligands on the surface of nanoplates. The number of nanopsheres in one 
69 
 
 
 
nanoplate-centered cluster is governed by the stoichiometry of the acid-base reaction, 
which can be tuned by feeding ratios of NPs, grafting density of copolymer ligands, 
and the chemical composition of reactive blocks of copolymers.  
This strategy does not require site-specific surface modification of NPs and allows 
assembly of NPs into clusters with high yield and regioselectivity. The different 
arrangements of NPs in clusters resulted in distinctive optical properties of assemblies 
due to different coupling modes of Au nanospheres and Ag nanoplates. This work 
offers great opportunities for designing functional complex nanostructures for different 
applications. 
 
 
 
 
70 
 
 
 
Chapter 4: Regioselective self-assembly of plasmonic 
nanoflowers and nanobuds 
Overview: Controllable organization of plasmonic nanoparticles into uniform and 
well-defined nanoclusters is highly desirable for tailoring interparticle plasmonic 
coupling and electric field confinement. However, many previous reports were limited 
to top-down methods or random clustering of spherical nanoparticles. This chapter 
demonstrates the self-assembly of plasmonic nanoflowers and nanobuds via polymeric-
ligand-mediated regioselective bonding. These assemblies feature a fixed number of 
particles in a controlled spatial arrangement of Au nanospheres and Ag nanoplates. 
Nanoflowers and nanobuds with the same number of petals exhibit localized surface 
plasmon resonance (LSPR) maxima at different wavelengths and have different 
electric-field enhancements. This study demonstrates the feasibility of constructing 
complex plasmonic nanostructures using polymeric ligand-mediated self-assembly and 
offers great opportunities for designing functional nanostructures with high structural 
complexity for diverse applications. 
 
This chapter is adapted from a manuscript to be submitted: Lin, X.; Ye, S.; Kong, C.; 
Webb, K.; Zhang, Q.; Fourkas, J. T.; Nie, Z. Regioselective self-assembly of plasmonic 
nanoflowers and nanobuds.  
L. X. and N. Z. conceived and designed the experiments; L. X. performed most of 
experiments; Y. S. performed the simulations of optical properties; W. K. contributed 
in the synthesis of polymers and discussion of self-assembly; K. C. and Z. Q. helped 
on electron microscopy and contributed in discussions. L. X. wrote the manuscript with 
revisions from F. J. T., and N. Z. 
71 
 
 
 
 
4.1 Introduction 
Assembling plasmonic nanoparticles into well-defined clusters is important for 
tailoring plasmonic coupling and optical properties, allowing for applications in 
sensing, surface-enhanced spectroscopies, and nonlinear optics.185-190 Considerable 
effort has been put into developing precise assembly techniques, as the coupling modes 
and strengths of NPs are dictated by the number, interparticle distance, and spatial 
arrangements of NPs in the clusters.48,191-194 However, most approaches for the 
formation of nanoclusters are limited to single or homogeneous building blocks, with 
little control over spatial arrangements within the clusters.  
In this chapter, we utilize the polymeric ligand-mediated regioselective bonding 
described in Chapter 3 to construct well-defined plasmonic nanoclusters, nanoflowers 
and nanobuds, composed of Ag nanoplates and Au nanospheres. The stoichiometry of 
reactive groups on the polymeric ligands governs the number of particles in one cluster, 
and the regioselective bonding enables precise control over the spatial orientations of 
building blocks. Nanoflowers and nanobuds with the same number of ?petals? exhibit 
distinct plasmonic coupling modes and electric-field enhancements. This study 
demonstrates the feasibility of constructing complex plasmonic nanostructure by 
polymeric ligand-mediated self-assembly and offers great opportunities for designing 
functional nanostructures with high structural complexity for diverse applications. 
72 
 
 
 
4.2 Experiments 
4.2.1 Materials 
Gold (III) chloride trihydrate (HAuCl4?3H2O, ? 99.9% trace metals basis), sodium 
citrate tribasic dihydrate (? 99%), silver nitrate (AgNO3), L-ascorbic acid, 
tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and dimethyl sulfoxide 
(DMSO) were purchased from Sigma-Aldrich and were used as received. Styrene (St) 
and N,N-dimethylaminoethyl methacrylate (DMAEMA) were passed through an Al2O3 
column to remove inhibitors. Acrylic acid (AA) was distilled under vacuum before use 
and stored in freezer at -20 ?. Azobis(isobutyronitrile) (AIBN) was recrystallized 
twice from ethanol. PEO45-CTA, was synthesized by coupling poly(ethylene oxide) 
methyl ether (PEG45-OH, and PEG113-OH, with molecular weights of 2 Kg?mol-1 and 
5 Kg?mol-1, respectively) with 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid 
(CPPA) through esterification. Deionized water (Millipore Milli-Q grade) with a 
resistivity of 18.0 M? was used in all the experiments. 
4.2.2 Preparation of building blocks 
Au nanospheres and Ag nanoplates were synthesized by the seed-growth methods 
described in Chapter 3.2.2 and Chapter 2.2.2, respectively. 
Thiol-terminated di-block copolymers were synthesized by reversible addition 
fragmentation chain-transfer (RAFT) polymerization, as described in Chapter 3.2.3. 
The synthetic routes are shown in Figure 3.2 and details of copolymers are listed in 
Table 3.1. Surface modification of nanoparticles followed the same method as 
described in Chapter 3.2.4. 
73 
 
 
 
4.2.3 Self-assembly of nanoflowers and nanobuds 
The assembly of nanoclusters was triggered by mixing the binary building blocks 
in THF at predetermined concentrations, as described in Chapter 3.2.4. For edge-type 
BAx clusters (nanoflowers), the feeding ratios of NP-A to NP-B (?A/B) were set to be 
equal or very close to the target x values: ?A/B was set to 1:1, 2:1, 3:1, and 4:1 for edge-
type BA, BA2, BA3, and BA4 clusters. respectively.  
For face-type BAx clusters (nanobuds), ?A/B was set to be larger than the target x 
values with excessive NP-As: 1.2:1, 2.5:1, 3.5:1, and 4.2:1 for face-type BA, BA2, BA3, 
and BA4 clusters. respectively. 
4.2.4 Finite-difference time-domain (FDTD) simulations 
Simulations of the optical properties of the AgNPs, f-BA, f-BA2, e-BA, and e-BA2 
clusters were performed by FDTD solutions (Lumerical Solutions, Inc.). Perfectly 
matched layers were set as the boundary conditions. A 3D total-field scattered-field 
light source was introduced into a cuboid containing the target nanostructures with the 
wavelength ranging from 400 nm to 1100 nm. Different incident directions of the light 
source were considered in our calculations to account for the random orientations of 
nanostructures in solution. For AgNPs, f-BA, and f-BA2, incident directions parallel 
and perpendicular to the plane of the nanoplates were included in the calculations, 
respectively. Moreover, two orthogonal polarization states were calculated in each 
incident direction to obtain unpolarized results. The extinction spectra from these two 
incident directions were summed to produce the UV-vis extinction spectra of AgNPs 
and the face-type nanoclusters in solvents. 
For e-BA and e-BA2, three incident directions were calculated, including the 
74 
 
 
 
directions perpendicular to the nanoplate plane, parallel and perpendicular to the major 
axis of the e-BA2 trimer, coplanar with the nanoplate. Two orthogonal polarization 
states were calculated in each incident direction as well. All of the extinction spectra 
from these three incident directions were summed.  
In all cases, the background refractive index was set to 1.41. The Au nanosphere 
was 38 nm in diameter and the Ag nanoplate was 51 nm in diameter and 8 nm in 
thickness. Both the Au nanosphere and the Ag nanoplate were coated with 5-nm-thick 
solvated polymer shells whose refractive index was taken to be 1.45. The gaps between 
the Ag nanoplate and the Au nanosphere were set as 6 nm for both face-type and edge-
type nanoclusters. A uniform mesh grid was imposed on the target nanostructure. The 
maximum mesh step was set as 1 nm along all the directions. The permittivity of Au 
and Ag were adopted from the experimental data from Johnson and Christy,175 and 
Palik,176 respectively. All the electric field profiles were calculated at the wavelengths 
of plasmon resonance. 
4.2.5 Sample preparation for characterizations 
Copolymers and NP building blocks were prepared and characterized using the 
same approaches as described in Chapter 3.2.7. For assemblies: 
UV-vis absorption spectroscopy. Extinction spectra of samples was measured in 
a range of 400-1100 nm using a PERKIN LAMBDA 35 UV/vis system.  
Electron microscopy. The morphologies of assemblies were obtained using a 
Hitachi SU-70 Schottky field-emission-gun scanning electron microscope and a JEOL 
FEG transmission electron microscope. Samples for scanning and transmission 
electron microscopy analysis were prepared by depositing ~2 ?L of solution on silicon 
75 
 
 
 
wafer and 300 mesh copper grids covered with carbon film, respectively. The samples 
were dried at room temperature before imaging measurement. 
4.3 Results and discussion 
4.3.1 Polymeric ligand-mediated self-assembly 
In this chapter, we use the polymeric ligand-mediated regioselective self-assembly 
described in Chapter 3 to fabricate plasmonic nanoclusters with different bonding 
modes (Figure 4.1). With edge-type bonding between the central Au nanosphere (NP-
B) and the surrounding Ag nanoplates (NP-As), the ?nanoflowers? (e-BAx) were 
formed with variable number of petals, x. Similarly, face-type bonding between the 
central NP-B and the surrounding NP-As enables the formation of ?nanobuds? (f-BAx). 
 
Figure 4.1 (a, c) Schematic illustration of polymeric ligand-mediated regioselective 
self-assembly of NP-As and NP-Bs into (b) edge-type and (d) face-type assemblies. 
As described in Chapter 3.3.1 and 3.3.2, NP-As and NP-Bs are uniformly grafted 
with block copolymer ligands carrying complementary weak acid and base groups, 
76 
 
 
 
respectively, for bonding. The outer, non-reactive blocks act as stabilizers for the NPs 
(Figure 4.2). Specifically, NP-As are grafted with a copolymer of poly(ethylene 
oxide)n-b-poly(acrylic acid?-r-styrene1-?)m-SH [PEOn-b-P(AA?-r-St1-?)m], whereas NP-
Bs are grafted with a copolymer of poly(ethylene oxide)k-b-poly(N,N-
dimethylaminoethyl methacrylate?-r-styrene1-?)l-SH [PEOk-b-P(DMAEMA?-r-St1-?)l]. 
The number of petals, x, of BAx structures is dictated by the stoichiometry of the 
acid/base groups on NP-Bs and NP-A within a cluster, and the difference in bonding 
modes is ascribed to the different steric shielding effect between planar and curved 
polymer brushes on the disc-shaped NP-As. 
 
Figure 4.2 Schematic illustration of (a) a copolymer-grafted Ag nanoplate (NP-A) and 
(b) a copolymer-grafted Au nanosphere (NP-B).  
4.3.2 Formation of the plasmonic nanoflowers 
Nanoflowers (e-BAx) are formed by edge-type bonding between the central NP-B 
and the surrounding NP-As (petals), with a variable number of petals, x. When the n of 
PEOn-b-P(AA?-r-St1-?)m ligands on NP-As is large (n > 100, e.g., n = 113), the steric 
effect of the long, non-reactive PEO blocks of the planar copolymer brushes on each 
77 
 
 
 
face of the NP-As shields the reactive block adjacent to the NP surface from reacting 
with the copolymers on the NP-Bs. Spherical NP-Bs, thus, prefer to undergo edge-type 
bonding with NP-As. The control over x is achieved by varying the number ratio, ZB/A, 
of base groups on a single NP-B to acid groups on a single NP-A, and the feeding ratio, 
?A/B. The ZB/A is the reciprocal of the ZA/B (defined in equation 3.4, for NP-A centered 
assemblies), and can be expressed as 
?? ? ?? ?? ?? 2
?? ????/?? = ? ?
??  ,                                            (4.1) 
?? ? ?? ???? ? ? ????
where the subscripts ?? and ?? represent NP-As and NP-Bs, respectively, and ?m, ?l and 
h are the number of acid groups of a PEOn-b-P(AA?-r-St1-?)m chain, the number of base 
groups of a PEOk-b-P(DMAEMA?-r-St1-?)l chain, and the thickness of disc-shaped NP-
As, respectively. In experiments, we tuned the values of ZB/A by varying ?l of PEO-b-
P(DMAEMA-r-St) copolymers on NP-Bs, while keeping ?m of PEO-b-P(AA-r-St) 
copolymers on NP-As constant at 65. As shown in Figure 4.3, when ?l increased from 
65 to 168 (ZB/A increased from 1.36 to 3.79), the number of petals, x, increased from 1 
to 3 for nanoflowers. Without further purification, the yields of e-BA, e-BA2, and e-
BA3 nanoflowers were 68%, 65% and 59%, respectively (Figure 4.4). This assembly 
strategy showed high regioselectivity, with over 90% edge-type bonding in the e-BAx 
clusters (Figure 4.5). 
 
78 
 
 
 
 
Figure 4.3 Schemes (top, left), transmission electron micrographs (TEMs, top, right), 
and scanning electron micrographs (SEMs, bottom) of edge-type NP clusters with 
different configurations: (a) BA, (b) BA2, and (c) BA3. NP-As: PEO113-b-P(AA0.26-r-
St0.74)249-grafted Ag nanoplates; NP-Bs: Au nanospheres grafted with (a) PEO45-b-
P(DMAEMA0.40-r-St0.60)419, (b) PEO45-b-P(DMAEMA0.30-r-St0.70)244, and (c) PEO45-
b-P(DMAEMA0.41-r-St0.59)158 copolymers. ?A/B: (a) 1:1, (b) 2:1, and (c) 3:1. Au is 
brighter than Ag in SEMs but darker in TEMs. Scale bars: 50 nm in TEMs and 200 
nm in SEMs. 
 
Figure 4.4 Distribution of e-BAx clusters. Over 500 clusters were analyzed for each 
plot. 
79 
 
 
 
 
Figure 4.5 Distribution of bonding modes in e-BAx clusters. Over 500 clusters were 
analyzed for each plot. 
4.3.3 Formation of the plasmonic nanobuds 
Nanobuds (f-BAx) are formed by face-type bonding between the central NP-B and 
the surrounding NP-As (petals), with variable number of petals, x. At n = 45 for PEOn-
b-P(AA?-r-St1-?)m-grafted NP-As, the steric shielding of short PEO blocks is negligible. 
NP-As prefer to undergo face-type bonding with an NP-B with maximized conformal 
contact and extent of reaction between copolymer ligands to minimize the overall 
energy of the assembly system. Similar to nanoflowers, the control over x in nanobuds 
is also achieved by varying the number ratio, ZB/A of base groups on a single NP-B to 
acid groups on a single NP-A, and the feeding ratio, ?A/B. Notably, due to the disc-like 
shape of NP-As, although the polymeric ligands on an NP-A are flexible, not all of the 
acid groups of the grafted polymers are accessible to the base groups of a single NP-B 
in the face-type bonding mode (as shown in Figure 4.1c by the red unreacted polymeric 
ligands on one side of Ag nanoplates). In our rough estimation, only ~60% of acid 
groups on one NP-A interact with base groups on a single NP-B in the face-type 
80 
 
 
 
bonding mode. Hence, for nanobuds, a modified number ratio, ?????/??, of base groups on 
a single NP-B to accessible acid groups on a single NP-A in the face-type bonding 
mode, was defined as 
2
???
?? ? ?? ?? ??
??/?? ? 0.6 ? ??
?? ??
????? = 0.6 ? ?  ,                               (4.2) ?? ? ?? ???? ? ? ????
where the subscripts are the same as in equation 4.1.  
In experiments, we tuned the values of ?????/?? by varying ?m of the PEO-b-P(AA-
r-St) copolymers on NP-As, while keeping ?l of the PEO-b-P(DMAEMA-r-St) 
copolymers on NP-Bs constant at 168. As shown in Figure 4.6, when ?m decreased 
from 229 to 107 (ZB/A increased from 1.31 to 3.88), the number of petals, x, increased 
from 1 to 4 for nanobuds. Considering that the unreacted of acid groups on NP-As may 
lead to the formation of NP-A-centered clusters with the presence of extra NP-Bs, we 
introduced excessive NP-As during self-assembly process to ensure the formation of 
nanobuds with NP-Bs in the center of assemblies. Hence, the feeding ratio ?A/B is 
usually larger than the target x value in the formation of nanobuds. Without further 
purification, the yields of f-BA2, f-BA3, and f-BA4 nanobuds were 62%, 65% and 58%, 
respectively (Figure 4.7). The regioselectivity of nanobuds is not as high as that of 
nanoflowers, with over 80% face-type bonding in the f-BAx (x = 1, 2, 3) clusters 
(Figure 4.8). 
 
81 
 
 
 
 
Figure 4.6 Scheme (top, left), TEMs (top, right), and SEMs (bottom) of face-type NP 
clusters with different configurations: (a) BA, (b) BA2, (c) BA3, and (d) BA4. NP-Bs: 
PEO45-b-P(DMAEMA0.40-r-St0.60)419-grafted Au nanospheres; NP-As: Ag nanoplates 
grafted with (a) PEO45-b-P(AA0.30 -r-St0.70)756-SH, (b) PEO45-b-P(AA0.32 -r-St0.68)550-
SH, (c) PEO45-b-P(AA0.36 -r-St0.64)467-SH, and (d) PEO45-b-P(AA0.32 -r-St0.68)335-SH 
copolymers. ?A/B: (a) 1.2:1, (b) 2.5:1, (c) 3.5:1, (d)4.2:1. Au is brighter than Ag in 
SEMs but darker in TEMs. Scale bars: 50 nm in TEMs and 200 nm in SEMs. 
 
Figure 4.7 Distribution of f-BAx clusters. Over 500 clusters were analyzed for each 
plot. 
82 
 
 
 
 
Figure 4.8 Distribution of bonding modes in f-BAx clusters. Over 500 clusters were 
analyzed for each plot. 
4.3.4 Experiments and simulations on extinction spectra of nanoflowers and nanobuds 
We investigated the optical properties of nanobuds (f-BA and f- BA2) and 
nanoflowers (e-BA and e-BA2) using experiments and finite-difference time-
domain (FDTD) simulations (Table 4.1, Figure 4.9 and 4.10). In experiments, the 
bonding of NP-Bs with NP-As in both modes led to redshifts in the LSPR bands. The 
magnitude of the redshift is dependent on the bonding mode, which determines the 
relative orientations of NPs in nanoclusters. Figure 4.9 shows the normalized 
extinction spectra of polymer-grafted Ag nanoplates (NP-As), f-BA, f-BA2, e-BA, and 
e-BA2 nanoclusters in the range of 400-900 nm. The NP-As showed a prominent peak 
at 688 nm originating from the in-plane dipolar resonance of Ag nanoplates. After the 
formation of face-type bonding with NP-Bs, the peak redshifted to 700 nm and 707 nm 
for f-BA and f-BA2 nanobuds, respectively. The nanoflowers showed further redshifts, 
with longitudinal LSPR bands located at 738 nm (e-BA) and 757 nm (e-BA2). The NP 
83 
 
 
 
orientation-dependent redshift in the longitudinal LSPR band was confirmed by FDTD 
simulations, in which the calculated LSPR peaks of nanoflowers redshifted further than 
the nanobuds with the same number of petals (Figure 4.10 and Table 4.1). In simulated 
spectra, notably, the e-BA and e-BA2 nanoflowers showed double peaks at the range of 
700-770 nm, whereas the f-BA2 structures showed a peak with a shoulder at ~580 nm. 
These subtle spectral features were not present in the experimental spectra, which may 
be due to some degree inhomogeneity of the assemblies prepared by the colloidal 
assembly methods. In support of this hypothesis, we observe a single, broader, peak for 
nanoflowers at the same range of wavelengths. 
Table 4.1 Peak wavelength (???????? ) of nanoclusters in experimental and simulated 
extinction spectra. 
 
???????? (nm) AgNP f-BA f-BA2 e-BA e-BA2 
Experiments 688 700  707 738 757 
Simulations 686 715  721 741 766 
 
 
Figure 4.9 (left) Experimental extinction spectra (normalized) of polymer-grafted Ag 
nanoplates, f-BA, f-BA2, e-BA, and e-BA2 nanoclusters and (right) a zoom-in of the 
region in the dashed box.  
84 
 
 
 
 
Figure 4.10 Simulated extinction spectra for polymer-grafted Ag nanoplates, f-BA, f-
BA2, e-BA, and e-BA2 nanoclusters. 
To investigate the origins of the shouldered peak for nanobuds and the double 
peaks for nanoflowers, the extinction spectra with different polarization and light 
incidence directions were calculated, and are presented for f-BA2, e-BA, and e-BA2 in 
Figures 4.11, 4.12, and 4.13, respectively. The polarization spectra of polymer-grafted 
Ag nanoplates were shown in Figure 3.14. The out-of-plane (z axis) polarization had a 
minimal contribution to the overall extinction spectra, whereas the in-plane 
polarizations along the x and y axes contributed equally at the same wavelength to the 
overall extinction. After the formation of nanobuds or nanoflowers, the pristine 
polarizations of Ag nanospheres were changed by the plasmon coupling with Au 
nanospheres along different axes. As shown in Figure 4.11, the shouldered peak of f-
BA2 nanobuds originates from the polarization along z direction, due to the coupling 
85 
 
 
 
of the out-of-plane polarization of Ag nanoplates with the Au nanospheres. In e-BA 
and e-BA2 nanoflowers, the edge-type bonding between Au nanospheres and Ag 
nanoplates is along the y axis, which made the in-plane polarizations along the y axis 
(at 741 nm and 766 nm, respectively) different from those along the x axis (at 688 nm), 
and hence the nanoflowers show double peaks in those ranges. 
 
Figure 4.11 Calculated extinction spectrum of f-BA2 nanobuds in different polarization 
and light incidence direction. The overall simulated extinction spectrum was obtained 
by summing the spectra from all contributing incident directions and polarization states. 
86 
 
 
 
 
Figure 4.12 Calculated extinction spectrum of e-BA nanoflowers in different 
polarization and light incidence direction. The overall simulated extinction spectrum 
was obtained by summing the spectra from all contributing incident directions and 
polarization states. 
87 
 
 
 
 
Figure 4.13 Calculated extinction spectrum of e-BA2 nanoflowers in different 
polarization and light incidence direction. The overall simulated extinction spectrum 
was obtained by summing the spectra from all contributing incident directions and 
polarization states. 
4.3.5 Electric-field enhancement of nanoflowers and nanobuds 
We calculated the electric-field enhancement of f-BA2 and e-BA2 clusters at their 
LSPR peak wavelengths and with the most pronounced polarization directions, as 
shown in Figures 4.14 and 15, respectively. When the electric field was polarized in 
the direction perpendicular to the nanoplates, the f-BA2 structure (Figure 4.14a) 
showed stronger enhancement intensity in comparison with the e-BA2 clusters (Figure 
4.15a, b). When the electric field was polarized in the direction parallel to the 
nanoplates, the e-BA2 structure (Figure 4.15c, d) showed stronger enhancement 
intensity in comparison with the f-BA2 clusters (Figure 4.14b). 
88 
 
 
 
 
Figure 4.14 Electric-field intensity enhancement contours of the f-BA2 nanobud at (a) 
588 nm and (b) 721 nm. The directions of the electric-field polarizations (E) are (a) 
perpendicular and (b) parallel to the plane of nanoplates. The incident light (k) is (a) 
parallel and (b) perpendicular to the plane of nanoplates. Scale bars: 50 nm. 
 
Figure 4.15 Electric-field intensity enhancement contours of the e-BA2 nanoflower at 
(a, b) 541 nm and (c, d) 766 nm. The directions of electric field polarizations (E) are 
(a, b) perpendicular and (c, d) parallel to the plane of nanoplates. The incident light (k) 
is (a, b) parallel and (c, d) perpendicular to the plane of nanoplates. Scale bars: 50 nm. 
89 
 
 
 
4.4 Conclusion 
In this chapter, we used the polymeric ligand-mediated regioselective bonding to 
construct various binary plasmonic nanoclusters, nanoflowers and nanobuds, with 
different spatial arrangements of Au nanospheres and Ag nanoplates. The number of 
petals was tuned by the stoichiometry of the acid-base reaction between the reactive 
blocks of copolymer ligands. The nanoflowers and nanobuds with the same number of 
petals showed different plasmon-coupling modes and electric-field enhancements due 
to the different bonding types between the Ag nanoplates and Au nanospheres. The 
nanoflowers with edge-type bonding better facilitates the coupling of Au nanospheres 
to the in-plane polarization of the Ag nanoplates. Thus, the nanoflowers showed further 
LSPR shift and stronger field enhancement. This study shows the feasibility of 
constructing complex plasmonic nanostructure by polymeric ligand-mediated self-
assembly and offers great opportunities for designing functional nanostructures with 
high structural complexity for diverse applications. 
 
 
 
 
 
 
 
90 
 
 
 
Chapter 5:  Conclusions and Future Works 
5.1 Conclusions 
The objective of this dissertation is to develop wet-chemistry synthesis and self-
assembly approaches for complex and defined plasmonic nanostructures. In chapter 2, 
a facile synthetic method was developed for freestanding circular and triangular Au 
nanorings (AuNRs) with tunable sizes, thicknesses, and surface roughness. The in-
plane LSPR peak of AuNRs is highly tunable through the NIR region by tuning the 
ring thickness through additional Au deposition with controlled infusion rates of 
presursors. In chapter 3, we combined the steric hindrance of polymeric ligand brushes 
and the anisotropy of nanoparticles to create regioselective bonding between gold 
nanospheres on silver nanoplates. The regioselectivity enables a self-assembly system 
with precise control over the relative orientation of Au nanospheres on Ag nanoplates, 
while the stoichiometry of reactive groups of copolymeric ligands dictates the number 
of nanoparticles in one cluster. The yield of each assembly was ~70% without further 
purification. Assemblies with different bonding types showed different LSPR peak 
shifts and electric field enhancements, originating from different bonding types. In 
chapter 4, the regioselective self-assembly was applied to fabricate various plasmonic 
nanocluster, nanoflowers and nanobuds. Edge-type and face-type bonding modes 
dictated the modes of plasmonic coupling between the Ag nanoplate and the Au 
nanospheres within one cluster. Nanoclusters with different bonding types showed 
substantial difference in optical response. Compared with nanobuds, nanoflowers with 
the same number of petals showed stronger electric field enhancement in simulations 
and further LSPR peak shifts in experiments and simulations.   
91 
 
 
 
As discussed in Chapter 3 and 4, the yields of various assemblies were relatively 
high (60 - 75%) without further purifications. However, systematic study is needed in 
the future for the stability of those assemblies as a function of time, temperature, and 
solvent quality, as those factors affect the conformation of polymeric ligands and hence 
the structure of nanoclusters.  
Although there is still plenty of room to optimize the yields and stability of the 
above-mentioned products, the synthetic and self-assembly methods demonstrated in 
this dissertation showed great potentials and versatility in designing not only plasmonic 
nanoclusters, but also other inorganic nanoparticle-based functional structures with 
high complexity.  
5.2 Future work: Self-assembly of Au nanorings  
We have demonstrated a facile synthetic route for freestanding Au nanorings 
(AuNRs) with different dimensions and surface roughness in chapter 2. Similar to Ag 
nanoplates, the as-prepared AuNRs are stabilized by citrate in aqueous solutions and 
can be surface-modified with thiol-terminated polymeric ligands after ligand exchange. 
Grafting polymers onto AuNRs introduces new energetic contributions arising from 
interaction enthalpy and conformational entropy of polymeric ligands. These energetic 
contributions can be tailored by controlling the chemical moieties, persistence lengths, 
and surrounding solvents of polymeric ligands. As plasmonic nanostructures with 
unique morphologies, the AuNRs can be interesting building blocks for various 
explorations in colloidal self-assembly. Here, I propose 2 future works closely relevant 
to this dissertation using Au nanorings as building blocks in interfacial and 
92 
 
 
 
regioselective self-assembly. The plasmonic nature of AuNRs make those assemblies 
good platforms for many potential applications in optical devices and biosensors. 
5.2.1 Self-assembled monolayers of Au nanorings 
Au nanorings with their 2D geometries are perfect building blocks for fabricating 
self-assembled monolayers (Figure 5.1). Interfacial self-assembly, driven by the 
reduction of interfacial energy, is a facile and powerful method for the assembly of 2D 
periodical arrays. After ligand exchange with commercialized thiol-terminated 
polystyrene (PS-SH), the preformed Au nanorings were transferred into toluene, a good 
solvent for PS. 
 
Figure 5.1 Schematic illustration of self-assembled monolayers of AuNRs. 
As a demonstration, a self-assembled array of triangular AuNRs (110 nm in edge 
length, 18 nm in thickness) tethered with PS-SH (50K) was prepared by interfacial self-
assembly (Figure 5.2). In a typical process, 0.2 ml toluene solution containing ~0.1 mg 
polystyrene-tethered AuNRs was drop-cast onto a surface of diethyene glycol (DEG) 
in a Teflon well (1.5?1.5?1.5 cm3). The well is then covered with a glass slide to slow 
93 
 
 
 
down the evaporation rate of toluene. A monolayer of AuNRs formed at the toluene-
DEG interface due to the reduction of interfacial energy. After 5 h, the film was 
transferred onto a piece of Si wafer (1?1 cm2), dried under vacuum for 30 min to 
remove residual solvents, and imaged by SEM. 
 
Figure 5.2 SEM image of self-assembled monolayer of triangular AuNRs (110 nm in 
edge length, 18 nm in thickness) tethered with PS-SH (50K). 
The optical properties of the self-assembled monolayers can be tunabled by the 
geometric parameters of AuNRs and the interparticle distance of adjacent AuNRs. By 
changing the molecular weights of PS ligands (e.g., from 12,000 to 65,000), the 
interparticle distance of AuNRs can be tailored. The self-assembled monolayers with 
tunable optical response have great potential in sensing applications and for fabricating 
optical devices. 
94 
 
 
 
5.2.2 Regioselective self-assembly of nanospheres and circular nanoring 
AuNRs are unique nanostructures with enhanced electric field in their inner cavity 
and outer surface that are both accessible for other nanoparticles to interact with. As 
demonstrated in chapter 3 and 4, the bonding modes of nanoparticles significantly 
affect the optical properties of their assemblies. Regioselective self-assembly of 
AuNRs with other plasmonic nanoparticles is highly desirable for studying their 
plasmon-couplings behaviors. The regioselective bonding is based on the same 
mechanism described in chapter 3.3.1. AuNRs and Au nanospheres (AuNSs) are 
grafted with block copolymer ligands in which the NP-adjacent reactive block carries 
complementary weak acid or base groups for bonding and the non-reactive block acts 
as stabilizers for the NPs (Figure 5.3). The reactive blocks of complementary 
polymeric ligands drive the self-assembly and the nonreactive PEO blocks create 
different level of steric hindrance between the inner cavity and outer surface of the 
AuNRs. Long PEO (n>100) blocks will create a more compact steric brush layer at the 
inner cavity of AuNRs (50 nm in diameter) to block the bonding of AuNSs, and thus 
the AuNSs will prefer to bind to the outer surface of AuNRs to form the diamond ring 
structures. Short PEO (n<50) blocks will not create significant steric layers at the inner 
cavity of AuNRs, and thus the Saturn-like structures will be preferred due to the 
maximum contact area (minimum reaction enthalpy) between the building blocks. The 
diamond ring structure and the Saturn-like structure are expected to show very different 
LSPR peaks and electric field enhancement patterns. This study will enrich our 
understanding of plasmon-coupled system and facilitate the construction of advanced 
plasmonic nanomaterials. 
95 
 
 
 
 
Figure 5.3 Schematic illustrations of a copolymer-grafted Ag nanoplate, a copolymer-
grafted Au nanosphere, and expected assembled structures. 
 
 
96 
 
 
 
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