CHAPTER 1 INTRODUCTION
1.3 Gold Nanorod Assembly
1.3.2 Solution Assembly
The assembly of gold nanorods in solution has received careful scrutiny from the
scientific community. Gold nanorods have been assembled both end-to-end or side-by-side
through both isotropic and anisotropic surface modification with such diverse active
ligands as polymers, short chain molecules, proteins, DNA, organic semiconductors, and
chelating agents both reversibly and irreversibly. This section will elucidate the current
state of the art methods for solution assembly of gold nanorods in the literature and provide
some prospective on what still needs to be done. Specifically, the section will be broken
up into assembly using small molecules (<100s of g/mol) and large molecules (polymers,
both natural and synthetic).
1.3.2.1. Nanorod Assembly with Small Molecules
Gold nanorods can be assembled end-to-end using alkane dithiol molecules. As
mentioned above, molecules with thiol moieties readily bind to the gold nanorods surface.
Therefore, the easiest method to tether two nanorods together would be with a molecule
with one end bound to one nanorod and the other end bound to the other nanorod. Thomas
had done early work(Thomas, Barazzouk et al. 2004) with gold nanorod linking by utilizing
thiol-alkylacids (e.g. 3-mercaptopropionoic acid) characterized by a thiol group on one end
and a carboxylic acid group on the other, which would link gold nanorods through
hydrogen bonding via the carboxylic acid groups. Similar experiments were performed,
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work, alkanedithiol molecules containing or sans a rigid phenyl group were added to
solutions of gold nanorods in 4:1::acetonitrile:water solution. Diluting the water solution
with acetonitrile was necessary in order to weaken the CTAB layer such that the
alkanedithiol could bind. Thomas found that chains of gold nanorods would form with
either linker present, but the absorbance was far more strongly red-shifted in the case of
the phenyl containing dithiol (840 nm compared to 732 nm for alkanedithiol). Furthermore,
Thomas found that linking occurred by one thiol attaching to one nanorod and the other
thiol attaching to the other nanorod, rather than a mechanism that involved daisy-chaining
alkanedithiols together via disulfide bridges. Finally, Thomas found that changing solvents
would not disrupt the linking, making the linking in this case rather robust. Figure 1.8 shows how the absorbance spectra changed as a function of time when gold nanorods were
incubated with alkanedithiol as well as a representative SEM image. Many of the ideas
presented in the Thomas papers are later utilized in work presented in this thesis.
Figure 1.8 a. The absorbance spectra of gold nanorods in the presence of 1,9 nonanedithiol over time. The red shift and broadening of the LSPR peak is consistent with end-to-end
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linking. b. SEM image of gold nanorods linked end-to-end by dithiol molecules. Figures
reproduced from (Shibu Joseph, Ipe et al. 2006).
The small molecule cysteine has also been frequently used to assemble gold nanorods
together end-to-end. Thomas and co-workers first used cysteine to link gold nanorods
together.(Sudeep, Joseph et al. 2005) In this work, Thomas incubates gold nanorods in
solution with cysteine, glutathione, and other alpha-amino acids and studies their assembly.
In this case, only the glutathione and cysteine cause the gold nanorods to assemble, which
Thomas hypothesizes is due to the zwitterionic moieties in conjunction with the thiol
present on these molecules. Essentially, the thiol binds to the gold nanorod surface and the
nanorods are linked together through a “two-point electrostatic interaction” between the
positively charged amino group and the negatively charged carboxylic acid group. Sethi
and co-workers(Sethi, Joung et al. 2009) took this study even further to elucidate the
linking mechanism of the cysteine. Sethi investigated how the molecular moieties present
in the linker affected the assembly of gold nanorods. Specifically, cysteine (thiol,
carboxylic acid, amine), 3-mercaptopropinoic acid (thiol, carboxylic acid), and cysteamine
(thiol, amine) was incubated with gold nanorods at different solution pH and the time
evolution of the absorbance was captured. Here it was determined that cysteine would not
link nanorods while in its zwitterionic form, precluding the hypothesis that a “two-point
electrostatic interaction” would bridge the nanorods. It was suggested, rather, that the
positively charged amine would link the nanorods, resulting in assembly. Finally, using
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by increasing (assembly) and then decreasing (disassembly) the solution pH, as can be seen
in Figure 1.9.
Figure 1.9 a. Cysteine charge group changes as pH is changed. b. SEM image of gold nanorods linked with cysteine. c. Absorbance spectra of gold nanorods linked with cysteine
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at high pH and unlinked at low pH. The LSPR peak broadens and red-shifts when gold
nanorods are linked and blue-shifts when the gold nanorods are unlinked. d. Vials
containing linked (purple) and unlinked (red) gold nanorods in the presence of cysteine.
The picture emphasizes the stark change in color upon linking. Figure a reproduced from
(Sethi, Joung et al. 2009) while the rest were reproduced from (Sun, Ni et al. 2008).
Other small molecules (aside from cysteine and alkane dithiol which are the most
germane to this thesis) have been utilized to assemble gold nanorods. Quan Li and co-
workers(Ma, Urbas et al. 2012) used 4, 15-Bis(4-sulfanatophenyl)porphyrin, an organic
semiconductor, to assemble gold nanorods. The negatively charged sulfate group in the
organic semiconductor bound electrostatically to the CTAB encapsulating the gold
nanorods. The assembly caused a significant change in the fluorescent properties of the
dispersion, which could be tuned through concentration control of the organic
semiconductor. Sreeprasad et al.(Sreeprasad and Pradeep 2011) used
ethylenediaminetetraaceticacid (EDTA), a chelater, to reversibly assemble gold nanorods.
EDTA was found to bridge gold nanorods by first forming a monolayer on the CTAB, due
to EDTA’s negative charge, and then hydrogen bonding with other adjacent EDTA
molecules, effectively assembling the gold nanorods. Assembly was reversed by adding in
a metal ion, in this case Pb(II) which the EDTA would complex with, disassembling the
gold nanorods. Finally, the authors found that the enhancement of the Raman signal would
decrease as a function of metal ion added. Lastly, Wang et al.(Wang, DePrince et al. 2010)
drove assembly of AuNRs end-capped with undecanethiol by adding increasing amounts
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at various added water compositions with end-linking performed with undecanedithiol and
mercaptoundecanoic acid. They found that assembly with dithiol did not change much,
assembly with mercaptoundecanoic acid was slowed at high water content, and assembly
with the monothiol was accelerated at higher water content.
1.3.2.2. Nanorod Assembly with Large Molecules
Polymers have been used to assemble gold nanorods in solution. The Kumacheva group
has published paper(Nie, Fava et al. 2007), after paper(Nie, Fava et al. 2008), after
paper(Liu, Resetco et al. 2012), after paper(Lee, Andrade et al. 2011), after paper(Liu,
Ahmed et al. 2013) that describe the assembly of gold nanorods typically end-grafted with
polystyrene. The process by which assembly occurs is quite simple and very similar to the
work by Wang et al. with undecanethiol. Kumacheva and co-workers(Nie, Fava et al. 2007,
Nie, Fava et al. 2008) selectively modified gold nanorod ends with polystyrene by placing
a concentrated aliquot of gold nanorods in water into a THF solution containing thiol-
polystyrene. The molecular weight of the polystyrene end-grafts was varied to control
separation distance and assembly type. Gold nanorods were then assembled by titrating in
water, which causes the polystyrene brush to collapse and drives assembly end-to-end, as
shown in Figure 1.10. The exact structure of the assembly was dependent on both the water
concentration and the polystyrene molecular weight where chains, bundles, bundled
chains, and rings were observed, each with distinct optical properties. They extended this
in a later paper(Liu, Ahmed et al. 2013) to be used as a “plasmonic counter” whereby the
“degree of polymerization” of the nanorods could be backed out via the extinction spectra
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that the gold nanorod assemblies could be used in detection of small molecules via solution
surface enhanced Raman spectroscopy.
Figure 1.10 a. TEM image of end-to-end assembled gold nanorods. b. Cartoon depicting PS brush in low (top) and high (bottom) water content. Water is a poor solvent for PS and
so collapses the brush driving end-to-end assembly. c. Absorbance spectra of gold
nanorods at increasing water content. As water content is increased the LSPR red-shifts
indicative of an increasing number of nanorods per chain. Scale bar is 50 nm. Figure
reproduced from reference (Nie, Fava et al. 2007).
DNA has also been utilized to assemble gold nanorods in solution. DNA makes for great
linker material due it its specificity in binding, ease of modification, and reversibility
through denaturation via heat or salt. DNA as a component in assembly schemes for
nanoparticles is not new. This idea was brought to the forefront separately by
Alivisatos(Alivisatos, Johnsson et al. 1996) and Mirkin(Mirkin, Letsinger et al. 1996)
where gold nanoparticles were controllably and reversibly crystalized using grafted DNA
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spurred on by theoretical work on DNA folding(Seeman 1982) leading to DNA
origami(Rothemund 2006), which has been used to create unique structures involving
spherical gold nanoparticles.(Ding, Deng et al. 2010, Hung, Micheel et al. 2010) In the
realm of gold nanorod assembly, DNA has been criminally under-utilized. A 2001
communication by Dujardin et al.(Dujardin, Hsin et al. 2001) described the homogeneous
grafting of DNA to the gold nanorods, followed by addition of a free DNA strand that
bridged two grafted strands, resulting in aggregation. Heating the assembly above 60°C
resulted in the disassembly of the DNA driven agglomeration which was evidenced by the
shift in the absorbance spectrum. More recently, two works have utilized DNA to assemble
gold nanorods very effectively. Oleg Gang and co-workers(Vial, Nykypanchuk et al. 2013)
grafted DNA strands to the sides of gold nanorods and assembled them side-by-side
through the addition of a bridging DNA strand. The length of the bridging DNA was tuned
to control the spacing between the gold nanorods. They studied the dynamics of assembly
of these ribbon like structures using in-situ SAXS and absorbance spectroscopy. In a paper
released the same year, Zhao et al.(Zhao, Zhang et al. 2013) grafted a self-complimentary
disulfide DNA specifically to the ends of gold nanorods. Assembly was then triggered by
changing the solution pH from 8.0 to 5.0, effectively end-linking the gold nanords which
resulted in a bathochromic shift in the LSPR peak. They were then able to de-link the
nanorods by bringing the pH back up to 8.0 and they cycled this multiple times with little
loss in linking efficacy denoted by similar peak shifts.
Peptides and proteins have also been used in the assembly of gold nanorods in solution.
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binding specificity making them attractive as linker molecules. The original protein used
in gold nanorod linking is streptavidin due to its high affinity to bind biotin, better known
as vitamin B7. Murphy and co-workers(Caswell, Wilson et al. 2003) end-grafted biotin to
gold nanorods via a thiol linkage and simply added streptavidin into solution, which
immediately lead to assembly of the nanorods, both side-by-side and end-to-end. The gold
nanorods did not assemble in the absence of either biotin or streptavidin. Jain et al.(Jain,
Roodbeen et al. 2012) employed an oligopeptide to assemble gold nanorods. Here, they
used oligomeric glycine flanked by cysteine molecules to link gold nanorods together.
They also employed the same molecule but with propargyl in the middle such that
modification could take place on the alkyne moiety to further direct properties. They found
that assembly proceeded in a step-wise fashion marked by an increasing red-shift and
broadening in the absorbance.