homo and hetero-assembly of inorganic nanoparticles · 1.2.1 assembly of nanoparticles in solution...
TRANSCRIPT
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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
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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
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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
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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
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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
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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
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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-
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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.
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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-
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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
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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.
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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
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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.
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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)
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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)
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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.
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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)
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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.
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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.
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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
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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
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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
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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.
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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.
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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.
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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
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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.
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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).
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2957-2975.
24. Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106.
25. Schatz, G. C. Acc. Chem. Res. 1984, 17, 370-376.
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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.
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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).
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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
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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).
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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.
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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
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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.
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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)
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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
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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.
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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.
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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
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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
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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).
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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,
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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