Homo and Hetero-Assembly of Inorganic Nanoparticles

by

Cristina Resetco

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Chemistry University of Toronto

© Copyright by Cristina Resetco 2012

ii

Homo and Hetero-Assembly of Inorganic Nanoparticles

Cristina Resetco

Master of Science

Department of Chemistry

University of Toronto

2012

Abstract

This thesis describes the synthesis and assembly of metal and semiconductor

nanoparticles (NPs). The two research topics include i) hetero-assembly of metal and

semiconductor NPs, ii) effect of ionic strength on homo-assembly of gold nanorods

(GNRs). First, we present hetero-assembly of GNRs and semiconductor quantum dots

(QDs) in a chain using biotin-streptavidin interaction. We synthesized alloyed CdTeSe

QDs and modified them with mercaptoundecanoic acid to render them water-soluble and

to attach streptavidin. We synthesized GNRs by a seed-mediated method and selectively

modified the ends with biotin. Hetero-assembly of QDs and GNRs depended on the size,

ligands, and ratio of QDs and GNRs. Second, we controlled the rate of homo-assembly of

GNRs by varying the ionic strength of the DMF/water solution. The solubility of

polystyrene on the ends of GNRs depended on the ionic strength of the solution, which

correlated with the rate of assembly of GNRs into chains.

Key words: nanoparticles, self-assembly, alloyed quantum dots, gold nanorods, ionic

strength, plasmon, zeta potential

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Acknowledgements

I am deeply grateful to my supervisor, Prof. Eugenia Kumacheva, for her

inspiration, guidance, and support. I am fortunate to have had the opportunity to conduct

research in the group of Eugenia Kumacheva and to build upon previous work that has

been fundamental in the field of assembly of nanoparticles. I would like to especially

thank Kun Liu for his collaboration, advice, and encouragement. I acknowledge the

support of the University of Toronto and the Province of Ontario for the funding, which

enabled me to pursue my graduate research.

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Table of Contents

Chapter 1 ............................................................................................................................. 1 Self-Assembly of Inorganic Nanoparticles ......................................................................... 1

1.1 Introduction ......................................................................................................... 1 1.2 Methods of nanoparticle assembly ............................................................................ 3

1.2.1 Assembly of nanoparticles in solution ............................................................... 3 1.2.2 Assembly with Bifunctional Linkers ................................................................. 5 1.2.3 Assembly of nanoparticles by biomolecular recognition ................................... 7 1.2.4 Template-assisted self-assembly ........................................................................ 9

1.3 Properties of nanoparticle assemblies ..................................................................... 11 1.3.1 Interactions between plasmons......................................................................... 11 1.3.2 Interactions between plasmons and excitons ................................................... 13

1.4 Applications of nanoparticle assemblies ................................................................. 15 1.4.1 Sensing ............................................................................................................. 15 1.4.2 Biomedical applications of nanoparticles ........................................................ 20 1.4.3 Nanoelectronic Devices .................................................................................... 20 1.4.4 Catalysis ........................................................................................................... 21

References ..................................................................................................................... 23 Chapter 2 ........................................................................................................................... 26 Materials and Methods ...................................................................................................... 26

2.1 Materials .................................................................................................................. 26 2.1.1 Quantum dot synthesis and surface modification ............................................ 26 2.1.2 Gold nanorod synthesis and surface modification ........................................... 26

2.2 Methods ................................................................................................................... 27 2.2.1 Synthesis of CdTeSe Quantum Dots ................................................................ 27 2.2.2 Surface Modification of Quantum Dots ........................................................... 27 2.3.1 Synthesis of gold nanorods with longitudinal surface plasmon bands less than 800 nm ....................................................................................................................... 28 2.3.2.1 Synthesis of gold nanorods with longitudinal surface plasmon bands greater than 800 nm ............................................................................................................... 28 2.3.3 Determination of nanorod concentration.......................................................... 30 2.4.1 Self-assembly of gold nanorods with polystyrene ........................................... 31 2.4.2 Self-assembly of gold nanorods functionalized with biotin ............................. 31 2.4.3 Self-assembly of gold nanorods with 11-mercaptoundecanoic acid ................ 31

2.5 Phase separation experiments of polystyrene......................................................... 32 2.6 Characterization ...................................................................................................... 32

2.6.1 UV-VIS spectrometry ...................................................................................... 32 2.6.2 Fluorescence Spectroscopy .............................................................................. 32 2.6.3 Scanning Transmission Electron Microscopy (STEM) imaging .............. 33 2.5.4 Electrokinetic potential measurement .............................................................. 33

References ..................................................................................................................... 34 Chapter 3 ........................................................................................................................... 35 Hetero-Assembly of Metal and Semiconductor Nanoparticles ......................................... 35

3.1 Motivation for co-assembly of quantum dots and gold nanorods ........................... 35 3.2.1 Synthesis of Quantum Dots .............................................................................. 37

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3.2.2 Optical properties of CdTeSe quantum dots .................................................... 40 3.2.3 Surface Modification of Quantum Dots ........................................................... 44 3.3.1 Synthesis of Gold Nanorods ............................................................................. 47 3.3.2 Surface modification of gold nanorods ............................................................ 50

3.4 Assembly of gold nanorods into chains using biotin and streptavidin .................... 52 3.5 Self-Assembly of gold nanorods and quantum dots using biotin and streptavidin . 56 Conclusion ..................................................................................................................... 58 References ..................................................................................................................... 60

Chapter 5 ........................................................................................................................... 81 5.1 Summary ................................................................................................................. 81 5.2 Future Perspectives and Challenges ........................................................................ 83 References ..................................................................................................................... 85

vi

Abbreviations

BDAC Benzyldimethylhexadecylammonium chloride Biotin-HPDP Biotin disulfide N-hydroxy-succinimide ester Cd Cadmium CTAB Cetyl trimethylammonium bromide DMF Dimethyl formamide DMSO Dimethyl sulfoxide GNR Gold nanorod HPA n-hexylphosphonic acid LSPR Longitudinal surface plasmon resonance MUA Mercaptoundecanoic acid NP Nanoparticle NR Nanorod PS Polystyrene QD Quantum dot Se Selenium STEM Scanning tunneling electron microscopy Te Tellurium TEM Transmission electron microscopy THF Tetrahydrofuran TOP Tri-n-octylphosphine TOPO Tri-n-octylphosphine oxide

1

Chapter 1

Self-Assembly of Inorganic Nanoparticles

1.1 Introduction

Nanoparticles (NPs) with dimensions in the range between 1 and 100 nm differ

significantly from bulk counterparts and open a new frontier in the design of

nanomaterials with new optical, electrical, and magnetic properties. The dimensions of

NPs are comparable to the wavelength of light, which results in fundamentally different

properties that depend on size, shape, and inter-particle interactions. Both semiconductor

and metal nanomaterials exhibit new size-dependent properties at the nanoscale that can

be tuned during synthesis. Semiconductor NP, or quantum dots, have discrete electron

energy states and a size-dependent tunable band gap between the conduction and valence

bands. Metal NPs exhibit surface plasmon resonance produced by collective oscillation of

conduction-band electrons induced by the electric field of incident light. The high

proportion of surface atoms on NPs results in different reactivity and inter-particle

interactions at the nanoscale. The difference in reactivity of NPs has been exploited in the

fabrication of catalysts, since gold NPs are highly active for multiple catalytic reactions,

while bulk gold metal is practically inert.33

Self-assembly is controlled organization of NP building blocks into higher order

structures. The two major approaches to self-assembly are top-down and bottom-up. In

the top-down method, such as photolithography, sections are etched from a large-scale

substrate to produce a specific patterned structure.1 Some of the limitations of top-down

microfabrication involve physical resolution limits, heat dissipation, and cost. Bottom-up

assembly uses NP building blocks to form hierarchical structures based on interactions

2

between individual components. The driving forces for NP assembly can be electrostatic,

van der Waals, capillary, hydrophilic or hydrophobic.1 Bottom-up assembly methods can

be devised to mimic biological structures, such as protein folding into helixes.10

Currently, studies of collective properties of NPs organized into hierarchical

structures are at the forefront of nanotechnology. Self-assembled NPs exhibit new

properties as a result of their interactions, which depend on inter-particle distance and

orientation. Geometric alignment of component building blocks in a nanostructure has an

effect on the interactions of optical and electric fields with the material. For example,

interaction of metal plasmon resonance with semiconductor excitons changes the

radiative and non-radiative decay rates of fluorescence of semiconductors, resulting in

quenching or enhancement of fluorescence.21 Shape anisotropy of NPs can be used to

tune the mode of interaction, such as end-to-end versus side-by-side assembly of gold

nanorods, which results in different optical and electric properties. End-to-end assembly

of nanorods results in a red-shift of surface plasmon resonance (SPR) peak and produces

regions of high-intensity electromagnetic field in the junctions between nanorods.18 Side-

by-side assembly of nanorods produces a blue shift in SPR peak and a decrease in the

intensity of electromagnetic field between nanorods due to destructive interference.18

Versatile techniques for NPs synthesis have been developed and a variety of surface

coatings can be used to impart functionality to NP building blocks and self-assemble

them into complex nanostructures. Nanomaterial synthesis, characterization, and

application is a multidisciplinary field, combining chemistry, biology, physics, and

engineering. Multi-dimensional organized assemblies have potential applications in

3

biosensing, nanoelectronics, optics, catalysis, and surface enhanced Raman spectroscopy

(SERS).

1.2 Methods of nanoparticle assembly

1.2.1 Assembly of nanoparticles in solution

Self-assembly of NPs in solution is a versatile strategy for obtaining

superstructures with various geometries, such as chains, two-dimensional sheets, and

three-dimensional crystals. Colloidal assembly operates under the action of attractive and

repulsive forces between NPs. Attractive forces include dipole-induced van der Waals

forces, hydrogen bonding, π-π stacking, hydrophobic forces, and electrostatic attraction

between oppositely charged NPs.2 Repulsive forces, such as steric hindrance and

electrostatic repulsion are required to balance attractive forces in solution to prevent

uncontrollable aggregation of NPs. The parameters that affect self-assembly of NPs in

solution include the shape, size, monodispersity, surface functional groups, and

concentration of NPs, as well as solvent polarity, pH, ionic strength, and dielectric

constant.

Van der Waals forces induce attraction between particles due to temporary

induced dipoles. Site-specific ligand exchange and appropriate choice of solvent can be

used to guide self-assembly based on van der Waals interactions. Van der Waals forces

depend on the NP separation and include thermally averaged dipole–dipole interactions

(Keesom interaction) ii) dipole-induced dipole interactions (Debye interaction), and iii)

interactions between transient dipoles of polarizable bodies (London dispersion

interactions).2 A broad range of NP assemblies, has been formed due to minimization of

van der Waals energy of the aggregates, including two-dimensional hexagonally packed

4

lattices of Ag NPs,3 three-dimensional face-centered cubic crystals of CdSe NPs,4 and

side-by-side organized gold nanorods forming continuous ‘‘ribbons’’.5 Two-dimensinal

nanoparticle assemblies ordered by van der Waals forces can exhibit a size-selective

sorting effect since the strength of van der Waals forces is proportional to NP size. For a

system with polydisperse NPs, the total potential energy is minimized when largest

particles accumulated in the center are surrounded by smaller particles. 2

NPs with aromatic ligands can be assembled into different structures depending

on solvent polarity. Gold NPs functionalized with hexaalkoxy-substituted triphenylene

(Au–TP) formed one-dimensional chains or two-dimensional hexagonal lattice structures

depending on the ratio of methanol to toluene.6 Solvent polarity increases with a higher

proportion of methanol, which results in stronger interactions between aromatic ligands

of gold NPs. At relatively low solvent polarity, gold NPs form hexagonal close-packed

lattices, due to partial π- π interactions, accompanied by intercalation of only adjacent

pentyloxy groups of the ligands. At high solvent polarity, with the ratio of methanol to

toluene of 2:1, NPs exhibit a 1D arrangement corresponding to full stacking of ligands

interdigitated among adjacent gold particles. The extent of intercalation between aromatic

ligands depends on solvent polarity, which can be used to control interparticle spacing in

the self-assembled structure.

Solution pH influences colloidal stability and interactions between NPs with

hydrogen bonding ligands, such as molecules with carboxylic groups. Nanoparticle

assembly can be reversed by changing solution pH, resulting in strong hydrogen-bond

interactions at low pH and repulsive electrostatic interactions at high pH. Acidic

functional groups are protonated at pH below the pKa, resulting in strong hydrogen-

5

bonding interactions. At pH above the pKa, acidic groups are deprotonated and

negatively charged, resulting in electrostatic repulsion. Reversible assembly and

disassembly of Au nanorods can be achieved by using thiolated bifunctional molecules,

such as 3-mercaptopropionic acid (MPA), 11-mercaptoundecanoic acid (MUA),

glutathione (GSH), and cysteine (CYS).7 Site-specific ligand exchange of gold nanorods

allows one to assemble them in the end-to-end or side-by-side configurations depending

on pH of the solution. Solution pH can be adjusted to assemble and disassemble the

structures, which can be monitored by UV-VIS plasmon peak shift.

1.2.2 Assembly with Bifunctional Linkers

1.2.2.1 Assembly of nanoparticles with synthetic polymers

NPs functionalized with polymers can assemble into superstructures and the

assembly depends on the properties of the solvent, that is, polarity, pH, ionic strength,

and temperature. A polymer ligand can serve as a NP linker or a matrix, which organizes

NPs into ordered structures. Phase separation of block copolymers is a route for

patterning NPs into lamellae, spheres, and branched structures.3 The inter-particle

distance of polymer-nanoparticle composites can be varied with different polymer

molecular weights.

Acharya et al. used a poly(styrene-b-4vinyl pyridine) (PS-b-P4 VP) micelle to

create composite films with gold (Au) and silver (Ag) NPs.8 Solution blending was

followed by spin-coating of the mixture of block copolymer micelles containing Ag NPs

and Au NPs. The Ag NPs were localized in the core of micelles and Au NPs were located

preferentially in the corona regions. In toluene, the polar P4 VP block became insoluble

and led to formation of spherical micelles with P4VP core and a soluble PS corona.

6

Different ratios of Ag and Au NPs in the film allowed to tune of the coupled plasmon

frequency between 450 and 550nm. The characteristic SPR peak exhibited a linear

dependence on the relative concentration of AgNPs and AuNPs in the micelles.

Interparticle spacing was controlled by varying the molecular weight of the copolymer. A

red shift in the SPR peak was observed with at greater interparticle distance, due to

decreased plasmon coupling.

Amphiphilic polymers provide additional control over geometric orientation of

NPs depending on solvent quality. Amphiphilic polymers have been used to assemble

gold NPs in different geometric arrangements depending on solvent polarity. A V-shaped

polystyrene-b-poly(ethylene oxide) amphiphile with a centrally located carboxylic group

was attached to phenol-functionalized gold and silver NPs.3 The polymer-nanoparticle

structures were dispersed in a THF solution and the addition of water triggered formation

of hollow cylindrical tubules that were 20 nm wide and 100 nm long. Addition of

methanol triggered organization of NPs into spherical and short rod-like assemblies.

1.2.2.2 Assembly of nanoparticles with peptides

Peptides and proteins are biomolecules that can form three-dimensional (3-D)

structures, such as α-helixes and β-sheets. There is a wide variety of amino acid building

blocks, which can be synthesized with different functionalities. Peptides can serve

multiple functions for the synthesis and assembly of gold NPs, which can occur as a

multi-step process in solution. NPs functionalized with peptides have been assembled

into 3-D structures governed by the change in peptide conformation.10 A water-soluble

peptide AYSSGAPPMPPF can bind to the surface of gold and mineralize chloroauric

acid to form nanospheres in the presence of HEPES buffer. A peptide amphiphile, C12-

7

PEPAu was formed by functionalizing the peptide N-terminal with a hydrophobic

aliphatic tail. In HEPES buffer, C12-PEPAu self-assembled into a left-handed twisted

nanoribbon configuration due to hydrophobic/hydrophilic interactions. The self-assembly

was combined with mineralization by introducing chloroauric acid to form gold NPs

inside the double helixes.

1.2.3 Assembly of nanoparticles by biomolecular recognition

Biomolecules have structural and functional properties that can be utilized for the

controlled organization of NPs by using on biorecognition of complementary moities. For

example, proteins and nucleic acids can be chemically and genetically engineered with

specific binding behaviour and multifunctional properties.

1.2.3.1 Assembly of nanoparticles with DNA

DNA is a versatile molecular linker that can be used to organize NPs in a

controlled manner. DNA can be precisely “programmed” to form complementary strands

that can be attached to certain facets of NPs, resulting in site-specific interactions

between different NPs. 11 The length of DNA strands can be synthetically controlled,

which provides a route for tuning interparticle separation in superstructures. During the

synthesis, multiple functional groups can be appended to DNA, which makes it a

versatile ligand for different types of NPs. Physical stability and biocompatibility of

DNA contribute to the widespread applications of DNA-NP composite structures in

biomedicine and nanotechnology.

Three-dimensional assembly of superlattices of gold nanorods, triangular

nanoprisms, and rhombic dodecahedra was achieved using DNA of different lengths.11

8

DNA linkers bound to NPs directed the crystallization process and controlled the

separation distance between NPs. Nanoparticle anisotropy dictated the structure of the

assembled superlattice, yielding NP arrangements that favoured the maximum number of

DNA linker interactions. Cooperative melting transitions, where `melting' refers to the

dehybridization of DNA bases linking the particles, have been analyzed to determine the

surface coverage of DNA. Melting phase transitions of DNA–gold nanoparticle

assemblies varied with NP size, DNA sequence, DNA grafting density, DNA linker

length, and interparticle distance. Gold nanorods preferentially assembled with long axes

parallel to each other resulting in superlattices with long-range hexagonal symmetry.

Face-to-face interactions dominated crystallization of triangular prisms resulting in

columnar 1D arrangement. The relative thinness of prisms (7nm) and relative rigidity of

double-stranded DNA resulted in low DNA density along the side of a column and no 3D

ordering. Maximum DNA interactions of rhombic dodecahedra occured in a face-

centered cubic (fcc) lattice. The combination of NP anisotropy and functionality of DNA

as a linker is a powerful approach for constructing highly-ordered assemblies of NPs.

1.2.3.2 Assembly of nanoparticles using biotin-avidin interactions

High affinity of biotin for avidin and streptavidin proteins has been used for

organizing NPs in different arrangements. Selective attachment of functional molecules

to anisotropic NPs is a key requirement for controlled self-assembly with defined

orientation. Modification with bulky linking molecules could be more sight-specific and

selective, especially for nanorods with diameters ranging 20-30 nm and an additional

surfactant bilayer. Solution conditions including relative concentrations of reagents, ionic

strength, and pH have an impact on nanoparticle stability, interactions, and self-assembly.

9

The variety of biomolecules and ability to modify their properties promoted the

development of nanoparticle assemblies with different geometries and properties arising

from interparticle interactions. Biotin sulfide has been conjugated to Au-tipped CdSe

NRs, resulting in nanorods selectively functionalized at the ends with biotin.12 Dimer

and trimer NR chains and flower-like structures were formed by varying the

concentration of avidin and exploiting the fact that every streptavidin molecule has four

sites for binding biotin.

1.2.3.3 Assembly of nanoparticles by antibody-antigen interactions

Organization of NPs by antigen–antibody interactions in solution was used to

develop immunoassay procedures with optical detection of the association process.

Complementary bioconjugates containing an antibody–antigen pair were attached to

luminescent CdTe quantum dots with antigen bovine serum albumin (BSA) on red-

emitting CdTe, and the anti-BSA antibody (IgG) on green-emitting CdTe.13 The antigen-

antibody complexes with CdTe quantum dots formed a functional nanostructure for

efficient optical detection of antigens or antibodies without multiple binding and washing

steps. Complexation of BSA and IgG resulted in fluorescence resonance energy transfer

(FRET) between different NPs. Emission of the red-emitting NPs was enhanced and

luminescence of the green-emitting NPs was quenched but could be recovered by

addition of the complex to unlabeled antigen.

1.2.4 Template-assisted self-assembly

Templated assembly uses prefabricated structures that induce the organization of

multiple NPs. Templates direct the assembly of NPs so that the resulting nanostructures

10

imitate the geometry or crystallographic arrangement of the template. Templates include

porous membranes, carbon nanotubes, colloidal NPs, chiral lipids, T-shaped dendro-

calixarene amphiphiles, block and dendron rod–coil triblock copolymers.14 Organic

templates consist mainly of carbon-containing compounds, such as carbon nanotubes,

charged surfactant ligands, amphiphilic block copolymers.14 The advantage of using

templates is the ability to produce complex superstructures with long range order. For

particles with ionic surfactants, superstructure formation is mostly determined by

electrostatic interactions with the template surface. NPs with nonionic ligands interact

with templates by hydrogen bonding or dipolar attractive forces.14 The conditions

required to form templated NP assemblies include templates with high degree of order

and stability; incorporation of precursors in the template, and the ability to remove the

original template to obtain a NP superstructure.

Viruses are biological structures that can act as templates for encapsulation of

NPs. Spatially selective assembly of NPs has been achieved by using a common viral

protein tobacco mosaic virus (TMV).15 Controlled assembly of AuNPs or AgNPs on

TMV template has been achieved by varying the solution pH. In acidic solution, AuCl4-

ions preferentially adsorbed on the outer positively charged template surface and a dense

coating of AuNPs was produced upon reduction with hydrazine. In alkaline solution, the

outer surface charge was screened and Ag+ ions migrated into the negatively charged

inner channel, resulting in 1D nanoparticle arrays after photoreduction.

11

1.3 Properties of nanoparticle assemblies

1.3.1 Interactions between plasmons

Surface plasmon resonance (SPR) is the collective oscillation of conduction band

electrons in metal NPs, which is induced by incident light. The frequency of SPR is

determined by the size, shape, and composition of NPs, as well as interparticle

interactions and the dielectric constant of the surrounding medium. 16 Optical extinction

of NPs is composed of absorption and scattering. Plasmon oscillations excited in metal

NPs decay into intra-band type electron-hole excitations inside the metal conduction band

or inter-band type transitions between other bands.16 Absorption of light corresponds to

non-radiative pathway of plasmon decay, while the oscillating electric field can radiate

electromagnetic energy resulting in elastic/Rayleigh scattering. The proportion of light

scattered increases with NPs size, due to greater radiative coupling and a higher

extinction cross-section. Based on Mie theory, for a metal nanosphere with particle size

smaller than the wavelength of incident light λ, the nanoparticle extinction cross-section

(Cext) exhibits a band maximum at the resonance condition if Pr=−2Pm, where P(ω)=Pr

(ω)+iPi(ω) is the complex frequency-dependent dielectric function of the metal, Pm is

the dielectric constant of the surrounding medium, and R is the particle radius.17

Small spherical particles exhibit a single absorption band produced by excitation

of dipole plasmon resonance, with the whole charge distribution oscillating at the same

frequency as the incident electric field. Anisotropic NPs, such as nanorods, have several

SPR modes due to the polarized oscillation of light. According to Gans theory, for

(1)

12

anisotropic particles the surface plasmon resonance condition depends on depolarization

factor L (equation 2).18

For anisotropic particles, the value of L is different for every axis, leading to

unique modes of electron oscillation. For nanorods, the tranverse SPR results from

electron oscillation along the nanorod short axis and a longitudinal SPR results from

electron oscillation along the nanorod long axis. Longitudinal SPR is strongly dependent

on nanorod aspect ratio, which results in a red shift with increasing aspect ratio.

Oscillating electric fields of several metal particles adjacent to each other can

interact and produce new resonances. The electric field E' affecting each particle is

composed of the incident light field E and the perturbation from electric dipole of the

adjacent particle, where ξ is an orientation factor and µ is the dipole moment due to the

particle plasmon (equation 3).19

Plasmon coupling between anisotropic metal NPs depends on their orientation,

which can result in constructive or destructive interference of electric fields. If the

direction of light polarization is parallel to the inter-particle axis, SPR is red-shifted and

if light is polarized orthogonal to the inter-particle axis, the SPR blue shifted relative to

an isolated particle. A red shift in SPR arises from attractive interparticle interaction with

(2)

(3)

13

positive value of ξ, while a blue shift is due to a repulsive interaction between electric

dipoles with negative value of ξ.18 The magnitude of the change in SPR wavelength

increases with the number of particles and a smaller inter-particle distance.17

The junctions between adjacent NPs excited by incident light have strong local

electromagnetic fields and are called “hot spots”.20 The electromagnetic focusing effect

arises from the short-range coupling between neighbouring metallic NPs, instead of long-

range or radiative coupling. Collective plasmon oscillations allow for the manipulation

and confinement of electromagnetic fields at nanometer length scales.

1.3.2 Interactions between plasmons and excitons

The study of metal-semiconductor systems is important for fundamental

understanding of exciton-plasmon interactions, as well as for enhancement of fluorophore

emission for applications in bioassays and optoelectronics. Interactions between adjacent

NPs depend on their composition, geometry, size, surface modification, separation

distance, and the dielectric constant of the medium. Semiconductor nanocrystals

(quantum dots), have a tunable band gap due to electron confinement and quantization of

energy states. The light absorption spectrum of quantum dots corresponds to discrete

exciton resonances, which can be tuned by varying the size of quantum dots.

The main mechanism of coupling between metal and semiconductor NPs is

Coulombic interaction, which is manifested as energy transfer and electromagnetic

enhancement.21 Surface energy transfer between NPs is associated with directional flow

of excitons between semiconductor and metal NPs or dissipation of exciton energy in the

vicinity of metals.21 Electromagnetic enhancement is observed as surface-enhanced

Raman scattering (SERS) and a strong increase in emission intensity.

14

Metal NPs exhibit localized surface plasmon resonances (LSPR), which increase

local electromagnetic field intensity and affect the optical properties of adjacent

fluorophores. Metal nanostructures can act as nanoscale antennas that change the

radiative and nonradiative decay rates of nearby fluorophores resulting in enhancement or

suppression of emission.22 The LSPR-induced electromagnetic field from metallic

nanostructures affects the photoluminescence of adjacent fluorophores by modifying their

radiative and nonradiative decay rates, which determine the emission quantum yield. The

enhancement factor of emission, γem, depends on the enhancement factor of excitation

intensity, γexc; light collection efficiency, κ; quantum yield, η; radiative decay rate, Rrad ;

and nonradiative decay rate, Rnonrad (equation 4). 22

Emission of fluorophores increases with a greater radiative decay rate and

excitation intensity. Enhanced emission of semiconductor NPs originates from the effect

of local electric field produced by metal plasmon resonance. Quenching of emission

occurs due to an increase in nonradiative decay rate, which is a form of surface energy

transfer analogous to Förster resonance energy transfer (FRET). 22 The change in

radiative and non-radiative decay rates is very sensitive to interparticle distance.

Control over separation distance between NPs is essential to tune interparticle

interactions. Surface modification of NPs with polymers, biomolecules, and silica coating

allows to modify interparticle spacing. Spatial control of relative orientation of NPs can

(4)

15

be achieved via biomolecular recognition, such as complementary binding of DNA

strands and biotin-streptavidin binding.

1.4 Applications of nanoparticle assemblies

1.4.1 Sensing

1.4.1.1 Optical Sensing

Strong plasmon absorption and sensitivity make metal NPs suitable for selective

colorimetric sensing of antibodies, DNA, and metal ions. Localized surface plasmon

resonance (LSPR) coupling between metal NPs undergoing assembly leads to long-range

photonic interactions and electromagnetic energy propagation over several hundred

nanometres. Optical sensing with metallic NPs can be based on distance-dependent

coupling between plasmons of adjacent NPs, which results in a shift of their UV-VIS

absorption peak. Metallic NPs are particularly suited for sensing applications due to their

high conductivity, good biocompatibility, and chemical stability.

A self-assembled hybrid structure has been fabricated using CdTe nanowires and

gold NPs connected with a bifunctional poly(ethyleneglycol) (PEG) linker with N-

hydroxy-sulfosuccinimide (NHS) and t-butoxycarbonyl (t-BOC) groups. 22 The hybrid

nanostructure was used for sensing by incorporating antibodies into the PEG chain.

16

Figure 1. (a) Scheme of 1D Au NP/CdTeNWsuperstructure. (b) Absorption spectra of Au NPs: 1. Au NP; 2. Au NP conjugated with PEG-antibody complexes; 3. Au NP assembled with NW. (c) Reversible shift luminescence wavelength: 1, attachment of a NP to NW; 2, red-shift of luminescence peak after addition of streptavidin; 3, blue-shift of fluorescence peak after addition of Au conjugated with antibodies; 4, red-shift of luminescence peak after addition of streptavidin. (d) Calibration curve for streptavidin based on the shift in NW luminescence peak. Reproduced with permission from ref. 22. Copyright 2007, Nature publishing group.

Upon attachment of gold NPs to CdTe nanowires, the photoluminescence peak of

CdTe NWs blue-shifted 8–10 nm as a result of exciton–plasmon coupling. Streptavidin

added to the solution formed a complex with an antibody, which expanded the PEG chain

and increased the distance between NPs and nanowires and resulted in a red-shift of

photoluminescence peak (Figure 1c). The photoluminescence wavelength of CdTe

nanowires changed almost linearly with streptavidin concentration and the system was

reversible, which led to a promising application as an optical sensor.

17

1.4.1.2 Surface-enhanced Raman scattering (SERS)

Surface-enhanced Raman scattering (SERS) is an analytical technique for

determining chemical information about molecules on metallic substrates based on

inelastic visible light scattered by metal NPs.23 Raman vibrations of molecules can be

enhanced by several orders of magnitude in the presence of metals, such as gold, silver,

and copper. The enhancement of Raman scattering occurs through two different

mechanisms: long range electromagnetic enhancement and short range chemical

enhancement.23 Electromagnetic enhancement originates from increased intensity of the

local electric field due to light absorption by a metal, which affects molecules adsorbed to

the metal surface. Chemical enhancement is produced by electron resonance and charge

transfer between a metal and a molecule, which increases the polarizability of the

molecule.

Most analytes have to be chemisorbed to the substrate for the electron transfer and

the chemical enhancement effect. Molecules with high affinity to gold and silver are

particularly suitable for detection with SERS. The greatest magnitude of surface

enhancement has been observed for molecules adsorbed in the junctions between NPs,

which are referred to as “hot spots”.24

Metallic NPs are ideal substrates for SERS, because they have strong light

scattering and tunable optical properties that allow matching of resonance plasmon

absorption bands with excitation wavelength of a laser source. The curved surface of

anisotropic NPs can further increase the local electric field, which has been called the

“lightning rod” phenomenon.24 The parameters that control the magnitude of SERS

enhancement include the aspect ratio of metallic NPs and the overlap of the excitation

18

wavelength with the nanoparticle plasmon absorption peak. Signals from SERS can be

optimized using anisotropic NPs with high curvature that have absorbance bands in

resonance with the excitation source, which can lead to single-molecule detection

limits.23 Performance of SERS-based sensors can be improved with NPs assembled in

ordered arrays, in a way that maximizes the density of hot spots.

Surface-enhanced Raman spectroscopy has been used for detection of anions,

antibodies, cancer genes and viral DNA. 25 A sensing device using SERS has been

fabricated from anisotropic Ag nanowires arranged in a 2D monolayer for ultrasensitive

detection of 2,4-dinitrotoluene (2,4-DNT) explosive.26

Figure 2. (a) Scanning electron microscopy images of the silver nanowire monolayer on a silicon wafer. (b) SERS spectrum of 2,4-DNT on the thiol-capped Ag nanowire monolayer. Reproduced with permission from ref. 26. Copyright 2003, American Chemical Society

The stretching mode of 2,4-DNT at 1348 cm-1 was clearly distinguishable from

the Raman bands of the matrix resulting a sensitivity of 0.7 pg (Figure 2). Further

developments in SERS sensing applications utilize substrates with an ordered

19

organization of NPs, which produce hot spots for enhanced signal intensity and

sensitivity.

1.4.1.3 Resonant Rayleigh Scattering

Rayleigh scattering is the elastic scattering of electromagnetic radiation by

nanoparticles, which are smaller than the wavelength of incident radiation.1 Elastic light

scattering from metallic NPs is sensitive to nanoparticle size, shape, relative orientation,

and the refractive index of the surrounding medium. Tissues labeled with antibody-

conjugated NPs can be clearly visualized with Rayleigh scattering with monochromatic

light of the scanning laser-confocal reflectance microscope owing to large scattering

cross-sections of gold NPs.1 Unmodified gold nanoparticles can be utilized as probes that

have different Rayleigh scattering depending on the identity and concentration of an

analyte. The proportion of Rayleigh scattering to total extinction of NPs increases with

greater size,26 which allows one to tune nanoparticle properties for specific applications.

Rayleigh scattering can be an efficient and simple analytical technique for the

detection of proteins. Gold NPs stabilized with negatively charged citrate, can bind

proteins with positive charges, such as human serum albumin (HSA), bovine serum

albumin (BSA), and ovalbumin (Ova) through electrostatic attraction, hydrogen bonding,

and hydrophobic effects.27 Association of gold NPs with proteins increases Rayleigh-

scattering intensity and the enhancement is directly correlated with protein concentration

with demonstrated detection limits of 0.38 ng/ml for HSA, 0.45 ng/ml for BSA, and 0.56

ng/ml for Ova. 27

20

1.4.2 Biomedical applications of nanoparticles

Metal NPs have strong light absorption that can be used for localized

photothermal therapy. NPs can be conjugated with antibodies to target cancer tumour

sites for localized photothermal treatment of cancer and drug delivery. Composite NPs

composed of gold sulfide nanoshells incorporated into polymer hydrogels have been used

for drug encapsulation and release.29 The composite particles were loaded with a drug

and then illuminated at the plasmon resonance to collapse the hydrogel.29 Gold

nanoshells converted near-infrared radiation to heat, which activated drug release from a

thermally reversible polymer matrix and allowed to control the rate of drug delivery to

optimize therapeutic efficacy.

Selective photothermal therapy can be administered with gold NPs functionalized

with specific antibodies to target tumour sites. Gold nanorods are particularly suitable for

photothermal therapy because they have tunable absorption maxima in the NIR region

(650-900 nm), where biological tissues have high transmission. Huang et al. have

demonstrated that gold nanorods conjugated with anti-epidermal growth factor receptor

(anti-EGFR) were preferentially bound to cancer cells.30 Cancer cells were

photothermally damaged with half the laser intensity compared to normal cells, which

demonstrates higher nanorod loading on cancer cells due to overexpressed EGFR

antibodies.

1.4.3 Nanoelectronic Devices

Future improvements in the development of electronic devices are aimed at

developing microscale devices. Tunneling of discrete electric charge can be achieved by

Coulomb interactions of electrons which can be controlled by the applied voltage.

21

Conductivity of metallic NPs makes them ideal for production of energy efficient

nanoelectronic devices, such as single electron transistors, which can work with one

electron.31

Metal oxide NPs can act as building blocks for storage devices that function based

on resistance change of metal oxides between the high-resistance state (HRS) and low-

resistance state (LRS) due to an applied voltage.32 Potential advantages of resistive

memory devices include high-speed operation in the order of tens of nanoseconds, good

endurance, and retention properties. The resistive switching phenomenon is attributed to

nanoscale redistribution of charges in metal oxides by formation and splitting of a

filamentary conducting path, or a lower Schottky barrier at the interface due to build-up

of charges or vacancies. 32 Multilevel resistive switching has been demonstrated in

colloidal maghemite (γ -Fe2O3) NPs assembled in a close-packed face-centered cubic

lattice. 32 Five resistant states with discrete resistance values were achieved by varying

the voltage. Multilevel switching was attributed to formation and splitting of many

conducting filaments due to an applied electric field.

1.4.4 Catalysis

Metal NPs have a large surface area, which makes them suitable for catalysis in

hydrogenation, oxidation, Suzuki and Heck coupling. Catalytically active NPs dispersed

in polymer films have additional advantages of better processability, recyclability,

stability, and solubility in different solvents.33 Hybrid nanoparticle catalysts have been

fabricated with carboxylic acid-terminated palladium NPs and silica NPs with an amine-

functionalized random copolymer.34 Assembly of NPs in solution occurred by

electrostatic attraction between acidic functional groups of NPs and basic polymer.

22

Larger silica NPs allowed to control the structure of aggregates in order to increase the

exposed surface area of catalytic metal NPs. The composite SiO2-COOH/poly-NH2

clusters were used as a template to assemble Pd-COOH NPs and calcination produced a

highly porous Pd-SiO2 material. This system exhibited high catalytic activity for

hydrogenation of 9-decen-1-ol with turnover frequencies of 10 100h-1, which was

significantly greater than commercial palladium catalysit with 7200h-1 turnover

frequency.34 Nanoscale catalysts can have a higher activity and selectivity compared to

bulk counterparts, which allows one to develop more efficient and cost-effective

chemical manufacturing processes.

23

References

1. Ozin, G. A., Arsenault, A. C., Cademartiri, L. Nanochemistry. A Chemical Approach

to Nanomaterials. 2nd ed. (Royal Society of Chemistry Publishing, 2009).

2. Kyle J. M. Bishop, Christopher E. Wilmer, Siowling Soh, and Bartosz A. Grzybowski.

Small 2009, 5, No. 14, 1600–1630.

3. S. A. Harfenist, Z. L. Wang, M. M. Alvarez, I. Vezmar, R. L. Whetten, J. Phys. Chem.

1996, 100, 13904–13910.

4. C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 1995, 270,1335–1338.

5. T. K. Sau, C. J. Murphy, Langmuir 2005, 21, 2923–2929.

6. Yamada, M.; Shen, Z.; Miyake, M. Chem. Commun., 2006, 2569–2571.

7. Shenhar, R.; Norsten, T.; Rotello, V. Adv. Mater. 2005 17, No. 6, March 22.

8. Acharya, H.; Sung, J.; Sohn, B.; Kim, D.; Tamada, K.; Park, C. Chem. Mater., 2009,

21 (18), pp 4248–4255.

9. Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. J. Am. Chem.Soc. 2006, 128, 15098.

10. C. L. Chen, P. J. Zhang, N. L. Rosi, J. Am. Chem. Soc. 2008, 130, 13555.

11. Matthew R. Jones, M.; Macfarlane, R.; Lee, B.; Zhang, J; Young, K.; Senesi, A.;

Mirkin, C. Nat. Mater. 2010, VOL 9.

12. A. Salant, E. Amitay-Sadovsky and U. Banin, J. Am. Chem. Soc., 2006, 128, 10006–

10007.

13. S. Wang, N. Mamedova, N. A. Kotov, W. Chen, J. Studer, NanoLett. 2002, 2, 817 –

822.

14. Jones, M.; Osberg, K.; Macfarlane, R.; Langille,M.; Mirkin, C. Chem. Rev. 2011,

111, 3736–3827

24

15. Dujardin, E.; Peet, C.; Stubbs, G.; Culver, J. N.; Mann, S. Nano Lett. 2003, 3, 413.

16. Jain, P.K.; Huang, X.; El-Sayed, I.H.; El-Sayed, M.A. Plasmonics 2007, 2:107–118.

17. Link S, El-Sayed MA (2003) Annu Rev Phys Chem 54:331.

18. Liz-Marzan L.M.Materials Today 2004, 7:26.

19 Jain, P. K.; Huang, W.; El-Sayed, M. A. Nano Lett. 2007, 7, 2080.

20. Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.;

Aussenegg, F. R. Opt. Commun. 2003, 220, 137.

20. Govorov, A.O; Bryant, G.W.; Zhang,W.; Skeini, T.; Lee, J.; Kotov, N.A.; Slocik,

J.M.; Naik, R.R. Nano Lett., Vol. 6, No. 5, 2006

21. Li, X.; Kao, F.; Chuang, C.; He, S. OPTICS EXPRESS, 2010, 24, Vol. 18, No. 11.

22. Lee, J.; P. Hernandez, J. Lee, A. O. Govorov and N. A. Kotov, Nat. Mater., 2007, 6,

291–295).

23. Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. Rev. 1999, 99,

2957-2975.

24. Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106.

25. Schatz, G. C. Acc. Chem. Res. 1984, 17, 370-376.

26. A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia and P. Yang, Nano

Lett., 2003, 3, 1229–1233.

27. Liu, S.; Yang, Z.; Liu, Z.; Kong, L. Anal. Biochem. 353 (2006) 108–116.

28. Jain, P.; Lee, K.; El-Sayed, I.; El-Sayed, M. J. Phys. Chem. 2006, B 110(14), 7238-

7248.

29. Sershen SR, Westcott SL, Halas NJ J. Biomed. Mater. Res. 2000, 51, 293-298.

25

30. Huang, X.; El-Sa.; El-Sayed, I.; Qian, W.; El-Saian, W.; El-Sayed, M. J. Am. Chem.

Soc. 2006, 128(6), 2115-2120.

31. Grabar. J., Devoret, M. Plenum Press, New York (1992).

32. Hu, Q.; Jung, S.; Huyn, H.; Kim, Y.; Choi, Y.; Kang, D.; Kim, K.; Yoon, T. J. Phys.

D: Appl. Phys. 2011, 44.

33. Cuenya . B.R. Thin Solid Films 2010, 518: 3127–3150.

34. Galow, T.H. Drechsler, U.; Janson, J.A.; Rotello, V.V. Chem Commun. 2002, 1076.

35. Murphy, C.J.; Sau, T.K.; Gole, A.M.; Orendorff, C.J.; Gao, J.; Gou, L.; Hunyadi,

S.E.; Li, T. J. Phys. Chem. B 2005, 109, 13857-13870

26

Chapter 2

Materials and Methods

2.1 Materials

2.1.1 Quantum dot synthesis and surface modification

Cadmium oxide (CdO, 99.99%), selenium powder (Se, 99.999%), tellurium

powder (Te, 99.999%), tri-n-octylphosphine (TOP, 90%), tri-n-octylphosphine oxide

(TOPO, 90%), 11-mercaptoundecanoic acid (MUA, 95%), streptavidin from

Streptomyces avidinii (85% protein), sodium tetraborate 20mM buffer solution pH=9.0

were purchased from Sigma Aldrich and used as received. n-hexylphosphonic acid

(HPA) was obtained from Alfa Aesar (Ward Hill, MA).

2.1.2 Gold nanorod synthesis and surface modification

Gold (III) chloride hydrate (99.999%), hydrogen tetrachloroaurate (III) 30wt.%

solution in dilute hydrochloric acid (99.99%), hexadecyltrimethyl-ammonium bromide

(CTAB, 99.0%), benzyldimethylhexadecylammonium chloride (BDAC), L-ascorbic acid

(99.0%), sodium borohydride (99.0%), biotin disulfide N-hydroxy-succinimide ester

(Biotin-HPDP, 95%) were purchased from Sigma Aldrich.

Carbon-coated copper grids (300 mesh) for transmission electron microscopy

(TEM) samples were purchased from Electron Microscopy Sciences (Fort Washington,

PA).

27

2.2 Methods

2.2.1 Synthesis of CdTeSe Quantum Dots

The synthesis method for alloyed CdTeSe quantum dots (QDs) was based on the

procedure developed by Peng, et al.1 and Jiang, et al.2 All reactions were conducted under

an inert argon atmosphere. TOPO (3.7 g), HPA (0.3 g), and CdO (0.0514 g) were placed

into a 50 mL glass three-necked flask and heated to 150 °C with magnetic stirring. The

temperature was raised to 300 °C to dissolve CdO, which resulted in the formation of a

colourless solution. An injection solution was prepared by dissolving 0.0674 g tellurium

and 0.0214 g selenium in 2 mL TOP. The injection solution was quickly injected at

295 °C and the mixture was stirred for 210 sec. The reaction was quenched by addition of

chloroform and QDs were purified in a mixture of chloroform and methanol by

centrifugation at 7000 rpm using an EPPENDORF centrifuge 5417R.

2.2.2 Surface Modification of Quantum Dots

The original organic passivating layer on QDs was replaced with

mercaptoundecanoic acid according to the method developed by Jiang, et al.3

Mercaptoundecanoic acid (1 g) was added to a three-neck flask and melted at 65 °C

under argon. Approximately 0.6 µmol of

tri-n-octylphosphine oxide (TOPO)-coated QDs was injected and the solution was stirred

for 2 hrs. at 80°C. Dimethyl sulfoxide (DMSO, 5 ml) was injected into the three-neck

flask and stirred for additional 2 hrs. The solution was cooled to room temperature, and

chloroform was added to precipitate out the modified QDs. QDs were purified from

excess ligands by centrifugation at 7000 rpm in a mixture of DMSO and chloroform.

28

QDs were redispersed in 1 mL PBS buffer pH=9.0 and 30 µL of 0.1 mg/mL

streptavidin solution was added. The solution was incubated for 1 hr and excess

streptavidin was removed by 3 cycles of centrifugation at 7000 rpm.

2.3.1 Synthesis of gold nanorods with longitudinal surface plasmon bands less than 800 nm

Gold nanorods (NRs) were synthesized following the procedure developed by

Nikoobakht and El-Sayed.4 For all solutions de-ionized water from Millipore Milli-Q

system was used. Seed nanoparticles were prepared from an aqueous solution of CTAB

(5 mL, 0.20 M) mixed with 5.0 mL of 0.50 mM HAuCl4. The solution was magnetically

stirred and 0.60 mL of ice-cold 0.010 M NaBH4 was added, which resulted in the

formation of a brownish-yellow solution. Vigorous stirring of the seed solution was

continued for 2 min. To prepare the growth solution, CTAB (5 mL, 0.20 M) was added to

0.10 mL of 0.0040 M AgNO3 solution at 25 °C. To this solution, 5.0 mL of 0.0010 M

HAuCl4 and 70 µL of 0.0788 M ascorbic acid were added. Addition of ascorbic acid, a

mild reducing agent, changed the colour of the growth solution from dark yellow to

colorless. Finally, 50 µL of the seed solution was added to the growth solution and the

system was incubated for 24 hrs at 27 °C.

2.3.2.1 Synthesis of gold nanorods with longitudinal surface plasmon bands greater than 800 nm

Gold NRs were synthesized following the procedure developed by Nikoobakht and

El-Sayed.4 The synthesis was scaled up to obtain a 100 mL dispersion of the NRs. Seed

nanoparticles were prepared from a solution of HAuCl4 (0.12 mL, 5 mM) mixed with 2.5

29

mL of an aqueous 0.2 M solution of cetyl trimethylammonium bromide (CTAB). The

solution was magnetically stirred and an ice-cold solution of sodium borohydride (0.5

mL, 10 mM) was added, which resulted in the formation of a brownish-yellow solution.

Vigorous stirring of the seed solution was continued for 2 min.

To prepare the growth solution, 2 g CTAB were dissolved in 90 mL of water

followed by addition of 2.97 g BDAC and heating to 60 °C to obtain a clear solution. To

this solution 5 mL of 4 mM AgNO3, 5 mL of HAuCl4 and and 100 µL H2SO4 were

added. Following the addition of 1.24 mL of an aqueous 0.788 M solution of ascorbic

acid, the dark yellow solution turned colorless. Finally, 0.1 mL of the seed solution of

nanoparticles aged for 30 min. was added to the growth solution and placed in a water

bath at 27°C for 24 hours. The NRs were purified by centrifugation cycles at 8,500 rpm

for 30 min. At the end of each centrifugation cycle, the supernatant was removed, and the

precipitated NRs were re-dispersed in deionized water.

2.3.2.2 Purification of gold nanorods by depletion

Gold NRs were purified from nanoparticles and plates by flocculation.7 The

concentration of CTAB in the original solution of NRs was increased to 0.1M by addition

of 0.1645 g CTAB and the solution was warmed to 30 °C to completely dissolve CTAB.

The concentration of BDAC was increased to 0.125 M by addition of 0.198 g BDAC to

gold nanorod solution and the solution was warmed to 60 °C. The solution was left

overnight. The supernatant was removed and the remaining brown film of gold nanorods

was re-dispersed in 2.5 mL water.

30

2.3.2.3 Exchange of BDAC with CTAB on gold nanorods

Gold NR solution was centrifuged at 8500 rpm for 25 min. and re-dispersed in 50

µL of 4 mM AgNO3, 1mL of 0.1 M CTAB, 12.4 µL of 0.0788 M ascorbic acid. The

solution was left for 1 hr. and centrifuged at 8500 rpm for 25 min. The procedure was

repeated 3 times.

2.3.2.4 Polystyrene ligand exchange on gold nanorods

Gold nanorods were re-dispersed in 30 µL of water and injected quickly in a

solution of 1mg/mL thiolated polystyrene Mn=12,000 with the final concentration of gold

nanorods of approximately 2 nM. The solution was sonicated for 30 min. and left for two

days. To remove excess polystyrene, the NR solution in THF was centrifuged at 8500

rpm, for 25 min., for a total of 8 cycles.

2.3.3 Determination of nanorod concentration

The concentration of the NRs was determined by measuring the intensity of

extinction of the NRs at the wavelength corresponding to their longitudinal surface

plasmon resonance. The mole extinction coefficients of the NRs, ε, were obtained using

the approach reported by Orendorrf et al.7 The value of ε at the longitudinal surface

plasmon wavelength of the NRs was determined as a function of absorbance, A; path

length of light, d; Avogadro number, NAV; density of gold, ρ =1.932×107 g/m3;

concentration of gold atoms, c Au; and the volume of a single NR, v0 (equation 1).

(1)

31

2.4.1 Self-assembly of gold nanorods with polystyrene

The assembly of NRs functionalized with polystyrene was triggered by adding

water to a solution of NRs in dimethyl formamide (DMF).6 The total sample volume was

1 ml with gold NR concentration 0.2 nM and a solvent mixture of 15% water in DMF.

Gold NR solution in THF was transferred into a scintillation vial and dried under a

stream of air. Nanorods were re-dispersed in 0.5 g of DMF and 0.5 g of an NaCl solution

in 30% water in DMF was added drop-wise with gentle agitation. The final NaCl

concentration was in the range from 50 to 500µM.

2.4.2 Self-assembly of gold nanorods functionalized with biotin

Gold NRs were functionalized with biotin-HPDP on the ends and assembled into

end-to-end chains by addition of streptavidin.9 Gold NRs were purified after synthesis by

one cycle of centrifugation at 8,500 rpm for 30 min. NRs were redispersed in a solution

of 0.2 nM Biotin-HPDP in ethanol with NR concentration of approximately 0.3 nM. The

solution was incubated for 24 hrs. and purified by 2 cycles of centrifugation at 7,000 rpm

for 30 min and redispersed in water. An aqueous solution of 2.5 µM streptavidin was

added to the solution of biotinylated NRs in deionized water with gentle stirring.

2.4.3 Self-assembly of gold nanorods with 11-mercaptoundecanoic acid

Gold NRs were purified after the synthesis by 3 cycles of centrifugation at 8,500

rpm for 30 min in order to remove excess CTAB. Purification of NRs was required to

promote binding of thiol groups of 11-mercaptoundecanoic acid to the ends of nanorods.

NRs were redispersed in 1:4 water:acetonitrile mixture with 11-mercaptoundecanoic acid

32

concentration of 10µM.10 Assembly of NRs in end-to-end configuration occurred by

hydrogen-bonding between carboxylic acid groups of 11-mercaptoundecanoic acid on

gold nanorod ends, which was facilitated by the aprotic acetonitrile solvent.

2.5 Phase separation experiments of polystyrene

To determine the effect of ionic strength on polystyrene solubility, an aqueous

NaCl solution was added drop-wise into a 2.0 wt% polystyrene (MW 13,000 g/mol)

solution in DMF while sonicating.8 The total sample volume was 5 ml and the solvent

composition was 6% H2O in DMF. The concentration of NaCl was in the range from 100

to 400 µM. After the addition of salt, the solution was sonicated for 30 min. and

incubated for 48 hrs. The supernatant solution was carefully removed and the sediment

was dried for two days in the vacuum oven at 40 oC.

2.6 Characterization

2.6.1 UV-VIS spectrometry

The absorption spectra of gold NRs in water and QDs in chloroform were

recorded at room temperature using a Cary 500 UV/vis/near-IR spectrophotometer.

2.6.2 Fluorescence Spectroscopy

The emission spectra of quantum dots were recorded using an excitation

wavelength of 450 nm and the slit width of 10 nm by using Cary Eclipse fluorescence

spectrophotometer.

33

2.6.3 Scanning Transmission Electron Microscopy (STEM) imaging

Scanning Transmission Electron Microscopy (STEM) images were obtained using a

Hitachi HD-2000 Scanning Transmission Electron Microscopy. Samples for the TEM

imaging were prepared by depositing a droplet of dilute NR solution on a 400 mesh

carbon-coated copper grid and allowing the solvent to evaporate for 30 seconds and then

the solvent was withdrawn.

2.5.4 Electrokinetic potential measurement

Electokinetic potential of gold nanorods was measured using Malvern Zetasizer

Nano ZS. For gold nanorods in water we used the dielectric constant of the medium, ε =

79.7 and viscosity, η=0.89 cP. For 15% H2O in DMF the potential was measured using a

dip cell setup using ε = 48.5, and η=1.6 cP.

34

References

1. Peng, Z.A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183-184.

2. Jiang, W.; Singhal, A.; Zheng, J.; Wang, C.; Chan, W. Chem. Mater. 2006, 18, 4845-

4854.

3. Jiang, W. Mardyani, S.; Fisher, H.; Chan, W. Chem. Mater. 2006, 18, 872-878.

4. Huang, X. H.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006,

128, 2115-2120.

5. Park, K.; Koerner, H.; Vaia, R. Nano Lett., 2010, 10 (4), pp 1433–1439.

6. Liu, K.; Nie, Z.; Zhao, N.; Wei, L.; Rubinstein, M.; Kumacheva, E. Science 2010, 329,

197.

7. Orendorff, C. J. Murphy, C.J. J. Phys. Chem. B 2006, 110, 3990.

8. Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G.; Rubinstein, M. Nat. Mater.,

2007, 6.

9. Caswell, K.K., Wilson, J.N.; Bunz, U.J.; Murphy, C.J. J. Am. Chem. Soc. 2003,125,

13914-13915.

10. Orendorff, C.J.; Hankins, P.L.; Murphy, C.J. Langmuir 2005, 21, 2022-2026.

�

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35

Chapter 3

Hetero-Assembly of Metal and Semiconductor Nanoparticles

3.1 Motivation for co-assembly of quantum dots and gold nanorods

The motivation for constructing hybrid nanostructures originates from the

potential ability to investigate their collective properties and to develop greater insight

into interactions of different types of nanoparticles (NPs), such as metal and

semiconductor NPs. Organized assemblies of plasmonic NPs confine and enhance

electric fields, which has potential applications in biomolecular sensing and high-density

data storage.1 Gold NPs exhibit high stability, tunability of optical absorption, and strong

plasmon resonance that is highly sensitive to the surrounding environment. Gold

nanorods (GNRs) have transverse and longitudinal surface plasmon resonance (SPR) due

to collective oscillations of electrons along the short and long axis, respectively.

Assemblies of GNRs in the end-to-end manner in one dimension produce regions of

enhanced electromagnetic field between the NR ends (hot spots), which affect emission

intensity of adjacent fluorophores.1 Fluorescent semiconductor NPs, quantum dots

(QDs), have tunable emission properties due to the dependence of the electronic band gap

on QD size.

Figure 1. Schematic of the assembly of quantum dots and gold nanorods.

The objective of the present work was to assemble a chain of alternating GNRs.

and QDs. Several design requirements have been considered for the hetero-assembly of

36

GNRs and QDs in a linear chain (Table 1). The method of hetero-assembly was chosen to

favour interactions between GNRs and QDs and prevent aggregation between the same

type of NPs. The self-assembly approach for NPs in solution was based on the surface

modification of NPs with complementary ligands. For linear assembly of hybrid NPs,

GNRs were modified selectively on the ends and QDs were modified in a way that

ensured complete surface passivation and preserved fluorescence. To achieve a linear

assembly with different NPs in alternation, different ligands were used, biotin for GNRs

and streptavidin for QDs. High affinity of biotin for streptavidin was employed for

hetero-assembly of GNRs functionalized with biotin and QDs functionalized with

streptavidin. The optical properties of GNRs and QDs as the building blocks for

assembly have been defined in terms of their absorbance and emission, respectively

(Table 1).

Table 1. Requirements for gold nanorods and quantum dots as the building blocks for their linear assembly.

Gold Nanorod Requirements Quantum Dot Requirements

Longitudinal localized surface plasmon resonance (LSPR) absorption band in the region between 650-700 nm

Emission maximum wavelength between 650-700 nm

Hexadecyltrimethylammonium bromide (CTAB) on the side {100} faces of nanorods

Sufficient surface passivation by ligands before and after modification to preserve fluorescence

Biotin on the end {111} faces of nanorods for conjugation with streptavidin-coated quantum dots

Solubility in water to combine quantum dots with gold nanorods

Diameter below 10 nm to match the hydrodynamic diameter of quantum dots

Sufficient fluorescence quantum yield to observe the effect of gold plasmon resonance on quantum dots

Diameter between 5-10 nm to localize quantum dots between the ends of gold nanorods

37

Spectral overlap of the surface plasmon resonance (SPR) of GNRs with the

emission maximum of QDs is required to maximize interactions between plasmons and

excitons. The longitudinal SPR of GNRs was set to be in the same wavelength region as

the emission maximum of QDs. The change in fluorescence of QDs in hot spots between

GNRs depends on the spectral overlap and the effect of GNRs on the radiative and non-

radiative decay rates of QD fluorescence, which is exhibited as quenching or

enhancement of fluorescence.1

An appropriate choice of solvent for two types of NPs is critical because both

QDs and GNRs have to be colloidally stable. GNRs were synthesized in aqueous

solution, while QDs were synthesized using organometallic reagents and were passivated

by organic ligands. Thus, surface modification of QDs was conducted to render them

water-soluble and combine the two types of NPs in aqueous solution. Selective surface

modification of GNRs with biotin only on the ends was conducted to promote hetero-

assembly in a linear chain and prevent aggregation.

3.2.1 Synthesis of Quantum Dots

Colloidal synthesis of QDs involves the pyrolysis of organometallic precursors

with different concentrations of initial precursors, types of the precursors, solvent

systems, reaction temperatures, and crystal growth time.3 Alloyed CdTeSe QDs were

chosen as the building blocks for hetero-assembly with GNRs due to their tunable

absorption and greater photostability, compared to CdTe. Alloyed CdTeSe were

synthesized using colloidal organometallic pyrolysis with different ratios of precursors.

The synthesis procedure of alloyed CdTeSe QDs has been developed based on the work

of Bailey, et al4 and Jiang, et al5 (Table 2).

38

Table 2. Comparison of synthesis procedures for CdTeSe quantum dots

Procedure by Bailey et al.4

Procedure by Jiang et al.5

Modified procedure

Precursor CdO (CdO, 99.99%) selenium, tellurium

(Cd(CH3)2, 97%) selenium, tellurium

CdO (CdO, 99.99%) selenium, tellurium

Solvent tri-n-octylphosphine oxide (TOPO, 90%) hexadecylamine (HDA, 90%)

tri-n-octylphosphine oxide (TOPO, 90%)

tri-n-octylphosphine oxide (TOPO, 90%) n-hexylphosphonic acid (HPA)

Emission range (nm)

700-850 600-850 650-850

The cadmium precursor used for the synthesis of alloyed CdTeSe QDs was

cadmium oxide, (CdO), instead of dimethyl cadmium, which is toxic and explosive at

high temperatures. A strongly coordinating ligand, n-hexylphosphonic acid (HPA), was

used to form the Cd-HPA complex.11 The synthesis of CdSeTe with HPA was more

reproducible and produced QDs with better stability compared to the synthesis with

hexadecylamine (HDA). In organometallic synthesis of alloyed CdTeSe QDs, tri-n-

octylphosphine oxide (TOPO) was used as the solvent and a diffusion layer controlling

QD growth. The role of TOPO is to direct the growth of QDs, provide electronic

passivation, prevent agglomeration, and render QDs soluble in organic solvents, such as

chloroform and toluene. The formation of QDs began instantly upon injection of Se

and/or Te in trioctylphosphine (TOP) into a colourless solution of Cd precursor dissolved

in a mixture of HPA and TOP.14

The parameters that controlled QD formation were the ratio of total cadmium (Cd)

to total tellurium (Te) and selenium (Se), the ratio of Se to Te, the choice of ligands,

39

concentration of the injection solution, injection temperature, and reaction time. The ratio

of total Cd to Te and Se determined the composition of QDs, which could be

homogeneous or heterogeneous type of alloyed structure. Homogeneous CdTeSe QDs

were formed when Cd was the limiting reagent and the final composition was determined

by the ratio and relative reactivity of Se and Te towards Cd.4 Bailey and co-workers

conducted elemental analysis of CdTeSe QDs using inductively coupled plasma mass

spectrometry (ICP-MS) and verified that CdTeSe QDs synthesized in the presence of

limited Cd have a uniform structure.4 Heterogeneous QDs formed in the presence of

excess Cd, when the higher reactivity of Te resulted in a Te-rich core and Se-rich outer

periphery.

The choice of stabilizing ligand had an effect on the morphology of nanocrystals,

which was wurtzite or branched tetrapod.

Figure 2. CdTeSe QDs (A), CdTeSe tetrapods (B). The scale bar 10 nm.

Figure 2 shows Scanning Tunnelling Electron Microscopy (STEM) images of

semiconductor nanocrystals with wurtzite structure and with branched tetrapod structure,

which were synthesized in the presence of phosphonic acid ligands with different alkyl

chain length. Alloyed CdTeSe NPs with branched structure were synthesized in the

A B

40

presence of octadecylphosphonic acid (ODPA) and an excess of Cd compared to Te and

Se (Fig. 2B). The long alkyl chain of ODPA preferentially binds to the lateral faces of

wurtzite CdTeSe and confines crystal growth to a single dimension.12

3.2.2 Optical properties of CdTeSe quantum dots

The optical properties of alloyed CdTeSe QDs were controlled during synthesis

by varying the ratio of Te and Se in the reaction mixture and changing the reaction time.

During the growth of QDs with increasing reaction time aliquots of QDs were withdrawn

and characterized by ultraviolet-visible spectrophotometry (UV-VIS).

Figure 3. Variation in absorbance of CdTeSe QDs with time 1 (1), 2 (2), 3 (3), 4 (4), 5 (5) min, (A); Variation of the absorbance maxima of CdTeSe QDs with reaction time (B).

B A

600620640660680700720

0 1 2 3 4 5 6Abs

orba

nce

Max

imum

(a.u

.)

Time (min)

B

1 5 A

41

The absorbance peak of CdTeSe QDs shifted to higher wavelengths with

increasing reaction time (Fig. 3). During the course the of the reaction, the diameter of

QDs increased, which corresponded to a smaller separation between electron energy

states of QDs and resulted in a red shift in the absorbance wavelength. Nanoscale

semiconductor NPs have confined electron energy levels and their photoexcitation

produces an electron-hole pair.13 The electron and hole in a QD are characterized by

wave functions with discrete electronic energy levels that depend on the size of QDs.

The growth of QDs during a reaction is described by the La Mer & Dinegar

model.7 Colloidal formation of NPs begins with nucleation followed by slower controlled

growth on the existing nuclei (Fig. 4A).7 Rapid injection of reagents into the reaction

flask increases the precursor concentration over the nucleation threshold, which results in

a short nucleation event that relieves supersaturation.

Figure 4. Schematic of the stages of nucleation and growth of QDs according to La Mer model (A), apparatus for organometallic synthesis of QDs (B).6

The initial NP size distribution is mainly determined by the time during which

nuclei form and start to grow. During later growth stages, Ostwald ripening can occur in

42

which small NPs are dissolved and the resulting material is incorporated on the larger

NPs.8 Ostwald ripening produces NPs with greater size, but with a lower yield.

The optical properties of alloyed CdTeSe QDs have been varied based on the

initial ratio of precursors in the reaction mixture. The ratio of Se to Te was controlled

experimentally in order to tune the absorbance onset and corresponding emission

wavelength of QDs between 600-750 nm. Absorption maxima of CdTeSe QDs shifted to

higher wavelengths with an increasing ratio of Se in the injection solution (Fig. 5).

Higher reactivity of Te led to the predominance of Te in the QD structure upon

nucleation while the final composition reflected the initial ratio of the precursors.

Figure 5. Dependence of absorbance of CdTeSe QDs on the composition CdTe0.63Se0.27 (1), CdTe0.5Se0.5 (2), CdTe0.4Se0.6 (3), CdTe0.3Se0.7 (4), CdTe0.24Se0.76 (5), (A); dependence of the absorbance maximum of CdTeSe QDs on the initial percent of tellurium (%Te) in the reaction mixture (B).

A

B

1

A 5

43

The intensity and stability of fluorescence of QDs was an important requirement

for subsequent ligand exchange and assembly of QDs. Fluorescence of QDs was

described by emission wavelength, full width at half-maximum (FWHM), and stability of

emission. The fluorescence peak wavelength of CdTeSe QDs increased with reaction

time (Fig. 6). The red-shift in QD fluorescence resulted from greater QD size

corresponding to a smaller electron band gap. A smaller difference in energy between the

highest valence band and lowest conduction band resulted in the emission of lower

energy radiation by QDs upon excitation.17

0

10

20

30

40

50

60

70

500 550 600 650 700 750 800 850 900

Fluo

resc

ence

(A

.U.)

Wavelength (nm)

Figure 6. Dependence of fluorescence maximum of CdTeSe QDs on the reaction time 10 (1), 30 (2), 60 (3), 180 (4) sec.

Emission spectra of QDs have a characteristic Gaussian distribution, with the peak

width described by the full width at half-maximum (FWHM).14 The relative

monodispersity of NPs is related to the FWHM and a narrower FWHM indicates a

4 1

44

narrower size distribution. The FWHM of the emission maxima of alloyed CdTeSe

decreased with reaction time, due to focusing of the size distribution (Fig. 6). Focusing of

the size distribution occured when the proportion of NP growth during the nucleation

stage was low compared with subsequent growth and NPs become more monodisperse

with time.8

The intensity of fluorescence of CdTeSe QDs increased with reaction time until it

reached a maximum, which was associated with optimum surface passivation by organic

ligands. A bright point in emission corresponded to the optimal surface structure, which

minimized surface states located in the band gap.16 Emission intensity of QDs, or

quantum yield, refers to the percentage of photons absorbed by QDs to those emitted.

Quantum yield strongly depends on the surface properties of QDs and higher values can

be achieved when most of the surface vacancies and nonradiative recombination sites are

passivated.15 Alloyed QDs synthesized in the presence of excess Te and Se exhibited

better photostability, which may be attributed to stronger ligand interactions with Te and

Se on QD surface compared to Cd.

3.2.3 Surface Modification of Quantum Dots

The choice of ligand was an important factor in achieving control over the

morphology, surface passivation, and stability of QDs. Alloyed CdTeSe QDs were

synthesized with a layer of organic ligands, which required surface modification to render

them water-soluble for assembly with GNRs in aqueous solution. Following synthesis,

CdTeSe QDs were modified by in two stages: modification with a carboxylic acid and

bioconjugation with streptavidin. Mercaptoundecanoic acid (MUA) was used to replace

original organic ligands and form a surface layer with carboxylic acid groups on the

45

periphery that rendered QDs soluble in water. The thiol group of MUA was strongly

coordinated to QD surface and the carboxylic groups on the periphery of QDs imparted

solubility in water. Streptavidin was conjugated to the surface of QDs by coupling

between carboxylic acid groups on QDs and amines on streptavidin.

The factors that affected ligand exchange included the ratio of QDs to ligands,

temperature, rate of agitation, and solvent polarity. We used an excess of MUA to replace

the initial TOPO and HPA ligands on the QDs surface. The temperature of the solution of

QDs in chloroform and MUA was raised to 80 ºC to increase the rate of adsorption and

desorption of ligands. Dimethyl sulfoxide (DMSO) was chosen as a solvent since it has

an intermediate polarity between chloroform and water and facilitated ligand exchange

and transfer of QDs into aqueous solution.

Table 3. Ligand exchange of quantum dots to render them water-soluble

Initial Ligands Final Ligands

Trioctylphosphine oxide (TOPO)

11-Mercaptoundecanoic acid

Hexylphosphonic acid (HPA)

Solvent: Chloroform Solvent: Water

The molar ratio of MUA to QDs required for ligand exchange was estimated

based on the number of surface atoms on QDs. An appropriate ratio of MUA:QD was

necessary to ensure adequate surface passivation and to preserve the optical properties of

QDs. The molar capping ratio (MCR) is the number of functional groups of a ligand

bound to surface atoms of QDs. For CdTeSe QDs, MCR was calculated according to

http://www.sigmaaldrich.com/catalog/ProductDetail.do?lang=en&N4=223301|ALDRICH&N5=SEARCH_CONCAT_PNO|BRAND_KEY&F=SPECi

46

equation 1, where nSH is the number of thiol groups of MUA and nCd, nTe, nSe represent

the number of cadmium, tellurium, and selenium surface atoms on QDs, respectively.19

In order to ensure sufficient stability and preserve fluorescence of QDs, the

optimum MCR was 1.5.19 The optimum MCR depends on the size and surface curvature

of QDs, the size and rigidity of ligands, and the number of available free orbitals on

surface atoms, which is inversely proportional to QD size.19 The effective average

number of surface atoms per QD was determined based on the volume of surface atoms

of QDs assuming a spherical geometry. 19 A surface atom is an atom of Cd2+, Te2-, or Se2-

located on a QD facet with one or more unpassivated orbitals.19 The effective volume of

surface atoms per QD (VSA) was approximated based on spherical geometry, with

interplanar distance d, and nanocrystal radius r, as in equation 2.19

The number of surface atoms per QD, nSA, was determined based on bulk density of QDs,

D, Avogadro’s number, NA, molecular weight of CdTeSe, MW, as in equation 3.19

For wurtzide CdTeSe with radius 2.5 nm and the average inter-planar distance of

0.406 nm, 20 the volume of surface atoms per QD was determined to be 27 nm3. The

calculated number of surface atoms per QD was 595. To achieve sufficient surface

passivation of QDs upon ligand exchange with MUA, there should be 892 molecules of

MCR= nSH nCd + nTe + nSe (1)

VSA= 4/3π[r3-(r-d)3] (2)

nSA = 2(VSA)(D)(NA) MWCdTeSe

(3)

47

MUA per QD. This result is in agreement with the parameters described by Jiang et al.,

who used an 800-fold molar ratio of MUA to QDs. 18

Stability of QDs after ligand exchange with MUA was determined based on the

solubility of QDs in water and fluorescence intensity. After ligand exchange with MUA,

QDs were soluble in water and were transferred to a buffer with pH=9 for bioconjugation

with streptavidin. Bioconjugation with streptavidin was conducted in a basic buffer in

order to deprotonate carboxylic acid groups on QD surface and promote binding with the

amine groups on streptavidin. Fluorescence of QDs was preserved after their surface

modification with MUA. Peak broadening in the fluorescence spectrum may have

resulted from the inhomogeneous surface coverage of QDs with MUA (Fig. 7).

Figure 7. Fluorescence of CdTeSe QDs with n-hexylphosphonic acid and trioctylphosphine oxide (solid line), after ligand exchange with mercaptoundecanoic acid (dashed line).

3.3.1 Synthesis of Gold Nanorods

The synthesis of GNRs was conducted according to a seed-mediated growth

method developed by Nikoobakht, et al.21 The synthesis involved

48

hexadecyltrimethylammonium bromide (CTAB) as a surfactant that directed the

formation of GNRs and imparted colloidal stability in aqueous solution. The

characteristics that make CTAB suitable for GNR synthesis are: good water solubility,

bromide counter ions that can chemisorb on metal surfaces, a sufficiently large head

group to direct crystal growth along particular faces, and a sufficiently long tail to make a

stable bilayer on the metal surface.

The longitudinal SPR of GNRs was tuned between 630-750 nm by varying the

concentration of AgNO3 in the growth solution. According to Jana et al., silver ions

adsorb to the surface of gold NPs and form AgBr by complexing with bromine from the

CTAB surfactant, resulting in restricted growth.22 Higher concentration of AgNO3

promoted the growth of GNRs with greater length, which corresponded to a higher

wavelength of longitudinal SPR. The dimensions of GNRs were 18.8±2.4 nm in diameter

and 37 nm in length.

The original procedure for GNR synthesis was modified in order to reduce the

diameter of GNRs and make it closer to the diameter of QDs of 5 nm. The parameters

that affected the diameter of GNRs were the ratio of the seed to the growth solution, the

concentration of HAuCl4 and AgNO3. The most effective approach for controlling the

diameter of GNRs was the ratio of seed to growth solutions, which was used to reduce

the diameter of GNRs to 13.6±1.6 nm. Figure 8 shows STEM images of GNRs

synthesized according to the original procedure by El-Sayed et al. 21 and according to a

modified procedure, which involved four times the equivalent of the original seed

concentration. A greater proportion of seed in the growth solution resulted in the

formation of GNRs with smaller diameter and a higher aspect ratio. Figure 8a illustrates

49

UV-VIS absorbance spectra of GNRs synthesized in the presence of increasing volumes

of seed in the growth solution.

Figure 8. Variation in absorbance of GNRs synthesized with the volume of the seed solution of 12 (1), 24 (2), 36 (3), 48 (4), 60 (5), 120 (6) µL (A); STEM images of GNRs with diameter 18.8±2.4nm (B) 13.6±1.6nm (C). The scale bar 30 nm.

There was no significant change in the wavelength of the transverse SPR peak of

GNR with different ratios of the seed solution, but the longitudinal SPR peak shifted to

higher wavelengths (Fig. 8A). When a greater seed to growth ratio was used, more

anisotropic growth occurred with a lower diameter and higher length of GNRs since gold

precursor from the growth solution was distributed between a greater number of NPs.

There are two potential mechanisms that account for seed-mediated growth of

GNRs with surfactants. According to one of the mechanisms, the seed is part of a soft

template formed by surfactant molecules and GNR growth occurs by diffusion of gold

atoms into the template.21 According to another mechanism, surfactant-capped seeds

begin to grow and new atoms that are added to NP lattice are stabilized by surfactant

molecules from the solution.21 The dimensions of GNRs were effectively controlled by

adjusting the concentration of AgNO3 to tune the length and changing the ratio of the

seed to the growth solution to control the d