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with particle shape [913]. In addition to chemical sensing via SERS or

other methods (e.g., plasmon peak shifting as a function of

aggregation state or local refractive index), the elastic light scattering

of gold nanorods makes them visible at the single-particle level in

darkfield optical microscopy, suitable for in vivo tracking [3,12].

Different aspect ratios of gold nanorods scatter different wavelengths

of light, leading in principle to different colored tracking probes. The

size of gold nanorods mimics that of virus particles (for example,

tobacco mosaic virus is 18 nm300 nm) and there are efforts to use

gold nanorods as in vivo reporters and therapeutics [1416]. Finally,

the large extinction coefficients of the plasmon bands of gold

nanorods lead to strong absorption, followed by heat release to the

environment. Because the nanorod plasmon band position can be

tuned to the near-infrared water window of the tissue, photo-

thermal therapy, in which near-infrared light illumination leads to

heat that can destroy diseased cells, is possible [9

17]. For true in vivoapplications, issues of toxicity, biodistribution and metabolism must

be addressed; gold nanorods are indeed being explored along these

dimensions [11,12,18].

For the potential of gold nanorods in all of these applications to

be realized, improvements in fundamental understanding of the

crystal growth processes that lead to controllable rods are needed

[19,20]. It is not yet possible, for example, to a priori know what set

of reaction conditions will yield gold nanorods that are 5 nm in

diameter and has an aspect ratio suitable for light absorption at

1200 nm. Therefore, the scope of this article is on synthesis, mecha-

nism, and post-synthetic modifications of gold nanorods that can

chemically tune their surfaces.

2. The seed-mediated growth approach to gold nanorods:

general concepts

The creation of gold nanorods can be accomplished in many ways,

from lithographic deposition of thin goldfilmson a substrate followed

by various chopping procedures [21], to electrochemical deposition in

hard nanotube templates [22,23], to photochemical reactions in

solution [24]. Our approach, seed-mediated growth, is a multistep

controlled redox reaction [46] that is performed in room tempera-

ture aqueous solution, suitable for scaling up; indeed, gold nanorods

based on our method can now be purchased from major chemical

vendors.

Our seed-mediated approach utilizes small seed particles (~1.54 nm

diameter) that arefirst synthesized from~104 M gold salt (HAuCl4) and

excess strong reducing agents to promote isotropic growth overanisotropic growth. These seed particles are then added to a growth

solution that contains additional gold salt, a weak reducing agent, and a

structure-directing agent to control the final particle shape. Ascorbic

acid is an ideal secondary reducing agent because it is too weak to reduce

the additional gold salt in the growth step from Au3+ toAu0 alone, in our

reaction conditions. This allows for the growth to occur over a long time

(minutes to hours), which aids in anisotropic growth. The structure-

directing agent we use is cetyltrimethylammonium bromide (CTAB), a

cationic surfactant well-known to form rodlike micelles on its own in

water [4]. In fact, our original idea behind theuse of CTAB (at0.1 M under

our conditions, far larger than its critical micelle concentration)was that it

would serve as a soft template to promote nanorod formation as the gold

seeds grew upon further metal ion addition [4,25]. We do not consider

CTAB's templating ability as its primary function now, however, althoughwe have found that all chemical components of this molecule, from its

alkyl chain length to its counterion, are critical to retain the ability to

synthesize rods [2628]. Indeed, impurities such as iodide at part-per-

million levels in commercial CTAB batches can ruin the synthesis of gold

nanorods [28,29], although judicious modification of synthetic conditions

with iodide can yield interesting shapes (to be discussed below). The

final gold nanorods obtained all bear a bilayer of CTAB on their surfaces

[3032], and therefore our view, detailed below, is that CTAB's primary

function is to adsorb to the growing seed particles and block crystal

growthalong certain faces [6]. A recentreportfindsCTAB'smicellarnature

to be a key factor in nanorod purification from other shapes due to

depletion attraction forces of surfactant micelles [33]. It is possible to use

other surfactants in the place of CTAB, if conditions are sufficiently altered

(although bromide counterions, thus far, are still present) [34]. We note

Fig. 1. Top: ultravioletvisible spectra of short gold nanorods with aspect ratios(A) 1.420.32, (B) 1.820.49, (C) 2.310.55, (D) 2.650.43, and (E) 2.800.37.

The UVvis spectra were normalized to the same absorbance at ~515 nm for clarity.

Lower panels: transmission electron micrographs of the gold nanorods corresponding

to spectra A, B, C, D, and E; all scale bars are 100 nm.

129C.J. Murphy et al. / Current Opinion in Colloid & Interface Science 16 (2011) 128134


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thata recent exhaustive review article on anisotropic metal nanoparticles

that covers some of the same points we raise here has appeared [35].

3. Two pathways to gold nanorods: with and without additive

impurity ions

The seed-mediated growth approach can be broadly broken down

into two distinct pathways that are distinguished by the presence orthe absence of additive silver ions, at low enough levels that the silver

could be considered an impurity. The addition of silver into the

growth solution likely alters the chemistry at the interface of the

growing particle and the growth solution, which we will discuss

further in the upcoming section on the proposed mechanisms. Other

impurity metal salts can be used, but silver is the most reliable [27].

For high aspect ratio rods (aspect ratio ~20), a three-step seeding

method is used where adjusting the timing of reaction in each step,

rods of increasing aspect ratios (from ~ 4 to 18) can be obtained [4].

The general procedure starts with preformed seed particles, as

described above, and three sets of growth solutions containing

additional gold salt and CTAB. The first growth solution is seeded

with the seed particles; 1015 s later, an aliquot of this solution is

used as the seed for the next growth solution; and so on. After three

steps of seeding, gold nanorods of aspect ratios ~18 can be obtained,

albeit in low absolute yield (~4%). Stopping the seeding process at

intermediate steps generates intermediate aspect-ratio nanorods,

although it is still not possible to reliably generate nanorods of

arbitrary aspect ratio (e.g., aspect ratio 8 as opposed to aspect ratio

12) [4]. While one advantage of this method is the ability to scale up

(compared to nanoporous template, for example), a primary dis-

advantage is the large proportion of gold nanospheres and other

shapes that are formed, greatly reducing the yield of rods. Purification

of the longest rods relies on their slow settling out of solution due to

gravity.

The quality of the seed particles can affect nanorod growth; in

general, smaller seeds lead to more monodisperse nanorods [36,37].

Whether the seeds are single crystalline or twinned also is thought to

influence the type of rods that are formed single crystalline ortwinned, as will be discussed below although for many practical

purposes, the optical spectra and overall dimensions of the final gold

nanorods do not depend on the crystalline nature of the gold core.

The addition of silver was an important breakthrough for the

synthesis of gold nanorods as it allowed for improved shape control

and yield for short aspect ratio rods (aspect ratios 16, nearly tunable

in 0.5 increments) [5,38]. This is the method that has been scaled up

for commercial production. The total amount of silver in the rods is

enough for a few monolayers of silver on the surface of the gold core

[37,39], and XPS data suggest that silver is preferentially localized on

the surface [37]. The preparation of gold nanorods using this silver-

assisted method is not done in the same multi-step seeding format as

for the silverless method above; once seed particles are added to a

growth solution that contains gold salt, CTAB, ascorbic acid, and silvernitrate, the amount of silver nitrate added dictates the final aspect

ratio of the rods [5]. These rods cannot be used as seeds themselves to

grow longer rods, unlike their silverless counterparts (although some

additional sculpting is possible, as will be discussed later); there-

fore, if a higher aspect ratio gold nanorod is desired, the synthesis

must be redone with a higher concentration of silver nitrate. Only

aspect ratios up to ~6 can be prepared in the silver-assisted method.

The yield of particles that are rod-shaped is exceptionally high (~90%)

after two rounds of purification by centrifugation (Fig. 1). However,

the absolute yield in terms of gold ions is less than 15%; that is, less

than 15% of the gold ions that are reagents in the synthesis end up as

part of a gold nanorod [39]. Simply reducing the concentrations of all

reagents in the synthesis decreases nanorod yields, suggesting that

reservoirs of unreduced metal ions, and CTAB, are important to

promote anisotropic growth. Slight changes in growth conditions can

lead to more needle-like nanorod morphologies [40].

4. Proposed mechanisms of gold nanorod growth in the absence

of silver ions

In the original three-step seeding method, gold nanorods can be

prepared with high aspect ratio (~20), starting from 3.5 nm seeds

prepared by sodium borohydride reduction of gold salt in thepresence of citrate ions, and in the absence of silver ions [4]. High-

resolution transmission electron microscopy and selected area

electron diffraction analysis have shown that the gold nanorods

prepared in the absence of silver ions are pentatetradedral twins,

where the cubic symmetry of the native gold face-centered cubic

lattice is broken by twinning (Fig. 2) [41]. The cross sectional view of

these rods is pentagonal; each endof therod consists offive triangular

facets that are Au{111} faces, and the sides of the rods are either Au

{100} or Au{110} [4]. The surfactant structure (head group, hydro-

carbon chain length, and counterion) is found to play a crucial role to

prepare these high aspect ratio gold nanorods; for example, bromide

is farsuperior to chloride or iodide fornanorod production, even if the

CTA+ cation is intact [27]. Indeed, the high concentrations of CTAB

required for gold nanorod synthesis appear to be a requirement for

high bromide concentrations (as judged by the success of synthesis if

CTAB is lowered by a factor of 10 but the growth solutions are sup-

plemented with sodium bromide) [27]. Derivatives of CTAB with the

same head group and bromide counterion but with different tail

lengths (10, 12, and 14 carbons) produced nanorods with tunable

aspectratios; thelonger tails produced longerrods, andthe 10-carbon

version of CTAB was capable of only producing spheres [26]. Based on

the electron diffraction data of gold nanorods and the accumulative

evidences of the importance of the CTAB structure and concentration,

we postulated a general mechanism for the growth of gold nanorods

in the absence of silver ions [6]. Our mechanism proposes a

differential adsorption of CTAB to different crystal faces followed by

thermodynamically favorable intermolecular interactions of the 16-

carbon cetyl chains to promote surface adhesion (Fig. 2). More

specifically, we postulated that CTAB adsorbs preferentially to theside of rods, Au{100} or Au{110}, via chemisorbed bromide counter-

ions, over the ends of the rods, which display Au{111} faces. This

preferential binding regime agrees with the size of the quaternary

ammonium head group relativeto thelarger binding sites availableon

the {100} and {110} faces of the crystalline rods [6,41]. The binding of

CTAB, ultimately as a bilayer [6], on the side of the rod blocks the

deposition of further gold to the side and promotes the growth from

the ends [6].

Liz-Marzan et al. proposed that the CTAB micellar structure also

promoted the deposition of metal at thetips of gold seed particlesthat

are also surrounded by CTAB [42]. Specifically, they proposed that

AuCl4 ions first displace Br ions and then tightly bind to CTA+

micelles. Addition of ascorbic acid then reduced AuCl4 to AuCl2

at the

micelle surface, and the rate of growth of the different nanorod facetswould be determined by the approach of the micelle and thus gold

species toward the facets of the gold seed particles that are also

covered with CTAB. After calculating the surface potential of metal

ellipsoids in 1 mM NaCl, Liz-Marzan et al. showed that this potential

decays more rapidly at the nanorod tip than along its length, and thus

the micelle can more readily approach the tips of the rods than the

sides and allow deposition of gold. It is noted by this group and ours,

however, that an asymmetry in the seed must be present to create an

asymmetric electric field initially, possibly in the form of a twinning

plane or stacking fault [6,42].

An improved synthesis method of high-aspect-ratio gold nanorods

increased the nanorod yield by adjustment of the pH of the growth

solution [43]. The addition of sodium hydroxide to raise the pH of the

growthsolution from 2.8to 3.5 greatly increases theyield of nanorods

130 C.J. Murphy et al. / Current Opinion in Colloid & Interface Science 16 (2011) 128134


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with the aspect ratio of ~18. To obtain higher aspect ratio nanorods,

the addition of heptane into the growth solution is helpful; in this

way, nanorods with an aspectratio of ~20 have been prepared [25]. By

further increasing the pH to 5.8, nanorods with higher aspect ratio of

~25 canbe prepared,but with much lower shape monodispersity [43].

Zubarev has found that Au(III)CTAB complexes are capable of

dissolving gold nanostructures in a shape-dependent manner; this

process can be tuned to purify gold high aspect ratio nanorods and

greatly improve their monodispersity [44].

We note that many other groups are working on different ways to

chemically synthesize gold nanorods, or nanowires; for example,

Giersig and Liz-Marzan report that HAuCl4 can be reduced with

oleylamine, followed with thermal and solvent processing, to yield

1.6 nm diameter single-crystalline gold nanowires that can have

lengths up to 4 m [45]. In this case the control of aspect ratio was

limited, and the growth mechanism was, like for CTAB, attributed to

preferential adsorption of oleylamine to different crystal facets of the

growing nanorod/nanowire [45]. Murray et al. have used a gold(I)

precursor, AuCl, with oleylamine and carbon monoxide as a reducing

agent in a related manner to produce gold nanowires that are 2.5 nm

in diameter and up to several microns long, albeit with limited control

over aspect ratio [46].

5. Proposed mechanisms of gold nanorod growth in the presence

of silver ions

Using a one-step seeding method, which includes AgNO3, one can

prepare low aspect ratio (~16) gold nanorods starting from 1.5 nm

seeds prepared by sodium borohydride reduction of gold salt in the

presence of CTAB [6,38]. While high aspect ratio gold nanorods can be

hundreds of nanometers in length, these lowaspect ratio rods aretens

of nanometers in length, up to about 90 nm. Guyot-Sionnest and

colleagues have discussed the critical nature of the structure of the

seed in determining the final form of gold nanorods grown via the

seed-mediated, silver-assisted synthesis method [47]. Specifically,

they found that single crystalline gold seeds produce single crystalline

gold rods, while multiply twinned gold seeds produce multiplytwinned gold bipyramids [47]. It is suggested that the relatively slow

growth of rods during Ag+ assisted growth uniquely allows gold

atoms to deposit at energetically favorable locations with no defect

character, maintaining the single crystalline or twinned structure of

the seeds.

The critical nature of the silver species during this synthesis has

been even more heavily discussed, and several groups have proposed

different reasons for how the silver species can assist in the

anisotropic growth of gold nanorods. One possibility is that silver

bromide complexes play critical roles during nanorod formation [48].

In this scenario, the deposition of AgBr on a specific facet of gold

nanorods during seed-mediated growth stabilizes the rods and directs

their growth by allowing more rapid incorporation of gold on less

hindered facets. The presence of silver bromide species is further

evidenced by XPS data suggesting the presence of Ag(I) and 1H NMR

spectra for CTAB capped gold nanorods being identical to those of

pure AgBrCTAB [48]. However, the presence of free silver bromide

(as the reaction occurs above the Ksp of AgBr) could lead to spurious

NMR spectra interpretation. Others have mass spectrometric evidence

that AgBr2, as well as AuBr2

, exists at the gold surface for well-

purified nanorods that result from the photochemical synthesis

method [49,50]. Hafner reports Raman evidence of the AuBr bond

formation at 180 cm1, which can be lost if thiols replace adsorbed

bromine on the gold nanorod surface [51].

Others, however, propose that silver bromide complexes may not

explain anisotropic growth, and instead argue that underpotential

deposition of a monolayer or submonolayer of elemental silver on the

gold nanorod surface is of significance [47]. Guyot-Sionnest and

colleagues propose that, during seed-mediated synthesis, under-

potential deposition (UPD) of silver preferentially occurs on the {110}

gold facets compared to the {111} and {100} facets [47]. In this model,

a silver monolayer strongly protects the {110} facet, and, though the

silver may be oxidized and replaced by gold, other facets grow fastest

due to their being less covered with silver.

Differing degrees of silver passivation on the {110} facets should

lead to varying ratios of growth on this facet and the nanorod's end

facets, and this is consistent with what is observed for the standardsilver-assisted growth technique (Fig. 3). Inductively coupled atomic

emission spectroscopy has been used to quantify the amount of silver

in rods after synthesis, and it was estimated that four monolayers

worth of silver is present in rods, though the technique does not

differentiate between different Ag(0) and Ag(I) [39]. EXAFS studies

have suggested that Ag0 is the final form of silver for rods grown via a

different photochemical method, consistent with the possibility that

elemental silver is of crucial importance not just for the standard

technique used in our lab but for the variety of synthetic methods

available to produce gold nanorods [52].

It is possible for other surfactants to be used in place of CTAB to

give tunable gold nanorods; for example, Liz-Marzan has shown that

gemini surfactants that form wormlike micelles produce well-con-

trolled nanorods that are sufficiently monodisperse to self-assemble insuperlattices [34]. Even here, however, CTAB-coated gold seed particles,

silver nitrate, and bromide counterions were all present, suggesting

that the growth mechanisms outlined above may be operative [34].

6. Fine-tuning the shape of gold nanorods after synthesis

After nanorod synthesis, rod shapes can be fine-tuned by a variety

of methods. Such methods include etching preferentially at the

nanorod tips to produce shorter rods, or sharper rods; heating or laser

treatment to produce lower-aspect ratio rods; or continued growth

or overgrowth on rods to produce dogbone or dumbbell shaped

nanoparticles, that have new plasmon features, or show different

crystal faces to the solvent that might be useful for catalytic purposes

(especially for more reactive metals).

Fig. 2. A cartoon that demonstrates the proposed mechanism of gold nanorod growth (in the absence of silver). The seed particles have different facets that allow for differential

binding of the surfactant to each face. The preferential binding of the surfactant to the side Au{100} and Au{110} faces, over the {111} faces at the ends, results in blocking the

nanorod growth at the sides and promotion of nanorod growth at the ends.

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Fine-tuning the shape of nanomaterials through chemical etching

after the nanomaterial is synthesized can be accomplished by various

means. In many of these cases, preferential shortening of the rods

occurs, and this is likelydue to a lower degreeof surface passivation of

the rod tips compared to sides by CTAB, which more readily allows tip

etching. FeCl3, for example, has been used at room temperature to

significantly shorten rods, in a controllablemanner,whilehaving little

effect on widths of the rods [53]. Though ferric ions do not normally

have the ability to oxidize gold under the reported reactions, the

authors suggested that the presence of chloride sufficiently alters the

redox potential of gold so that Au(I) and Fe(II) are produced [53].

Bubbling of O2 in thepresence of HClat slightly elevated temperatures

preferentially produces shortened gold nanorods as well, and so does

CuCl2 treatment [54,55]. Gold nanorods can be completely dissolved

with aqueous cyanide treatment in air if sufficient reagents are

present, leading to the complex ion [Au(CN)2]; in this reaction too,

nanorods are preferentially dissolved from the ends first [56]. If a

polymer or silica coating surrounds the rods, hollow tubes of polymer

or silica can be generated by cyanide dissolution of the gold [57]. The

mechanism by which preferential end-reactivity occurs is likely a

composite of improved availability of surface sites (due to less CTAB

on the ends of the rods compared to the sides) and higher surface

energies at the ends of the rods (although one piece of evidence for

higher surface energies on nanoscaleparticles is increased reactivity, asomewhat circular argument).

Heating gold nanorods after synthesis to fine-tune shapes can also

lead to fine changes in the nanorod morphology. Boiling short gold

nanorods, made with silver ion present, in their aqueous reaction

mixture before purification increases both their length and width in

their native growth solution, but they ultimately become fatter and

therefore decrease in aspect ratio [58]. We postulate that boiling

water temperatures are sufficient to destabilize the CTAB bilayer

surrounding the gold nanorods and thus the larger side areas of the

nanorods are available for reaction; in the case of the ascorbic acid-

containing growth solution, gulonic acid (ascorbic acid's oxidation

product) canreduce available metal ion leftover from the synthesis, at

100 C, to add more metal to the nanorod sides and decrease the

aspect ratio [58]. Purification of the nanorods negates these reactions,and purified nanorods appear to be stable to boiling water for at least

1 h [58]. El-Sayed and colleagues showed that gold nanorods grown

by an electrochemical method remain nearly constant in width but

greatly decrease in length upon heating slightly over 100 C [59].

Upon heating the rods to near 600 C, the nanorods not only begin to

change shape but also melt as well [59].

Gold nanorods can furthermore be shape-tuned to a dogbone or

peanut shape.Additionof more ascorbic acid to short gold nanorods,

at room temperature andif the growth solution containing unreduced

metal ions is still present, leads to dogbone-shaped nanoparticles due

to gold overgrowth on the ends of the rods [58]. The relatively large

amounts of unreduced gold salt remaining in solution after synthesis

are reduced by the ascorbic acid, and the reduced metal deposits onto

the less CTAB covered rod tips, which themselves act as seeds to the

newly depositing metal. Dogbone or dumbbell-shaped gold nano-

particles can also be achieved by addition of more HAuCl4 to gold

nanorod solutions, and this likely functions by a similar mechanism

[60]. It is also worthwhile to note that the slow addition of ascorbic

acid over several days to gold nanorod solutions leads to further

growth of nanorods not with a dogbone shape, but with retention of

the original typical nanorod shape [61]. In general, for adding more

gold to gold nanorods, one must be cognizant of the redox-active

species that can still be present in gold nanorod solutions.

Liz-Marzan has shown that if iodide is present during silver-

assisted gold nanorod growth, it is possible, depending on the exact

solution conditions, to grow uniformly fatter nanorods, or to grow

more gold at the ends to form peanut shapes, controlled by local rates

of deposition [37]. These results seemingly contradict those of Korgel

et al., who found that iodide impurities in CTAB prevent nanorod

formation [29]; but relative solution conditions were not identical.

XPS and surface-enhanced Raman data on the nature of the metal and

the halide, in Liz-Marzan's case, support the formation of chemisorbed

halide on both silver and gold (silver being more on the surface than

gold), and preferential binding of iodide to different crystal faces of

the nanorods was invoked to explain the results [37].

Thiols are commonly used to modify gold surfaces, and work along

these lines has been done for gold nanorods as well. Glutathione or

cysteine can bind preferentially to, and hinder the growth of, nanorodtips during overgrowth, leading to a dramatic growth in the width of

rods but little in their length [62]. In this case, these thiolated mole-

cules are added to rod solution, followed by more growth solution.

Rods gain a dogbone or a peanut shape as {111} faces grow outward

from the rod, but, growth at the tips is hindered by the thiolated

molecules, further growth leads to truncated octahedral shaped gold

nanoparticles [62].

Gold nanorods, made with the silver-assisted method, can be

grown into nanoscale octahedral by a combination of polymer coating

and ultrasonication in organic solvent by selective deposition of metal

onto the side facets and the ends [63]. As the nanorods become more

chunky, the plasmon bands shift back towards the blue.

More drastic fine-tuning of nanorod shapes, post-synthesis, can be

achieved, such as the application of laser light at plasmon maximumwavelengths to gold nanorod solutions. With such treatment, rods can

be made shorter or into strange shapes, as well as melted or frag-

mented, depending on laser parameters [64,65].

7. Nanorods grown on surfaces and nanorodnanorod gaps

All of the previous work discussed in this paper has been on the

growth of gold nanorods in colloidal solution from seeds. Yet other

possibilities present themselves: for example, one could immobilize

the seed on a surface and attempt to grow gold nanorods on sub-

strates. Although for practical purposes, electrodeposition of metal

into hollow polycarbonate or aluminum oxide templates might be

easier, the growth of nanorods on surfaces does hold fundamental

interest and may result in aligned nanorods (Fig. 4).

Fig. 3. One of the proposed mechanisms that explain the role of silver in the silver-assisted, seed-mediated method of gold nanorod synthesis. Underpotential deposition of silver

occurs preferentially at the {110} facets of gold, leading to relatively quick growth of other facets until the slower deposition of silver on the end of the rods eventually terminates

growth.



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For instance, several groups have shown that the growth solutions

used to make gold nanorods in the silverless method (e.g., CTAB, gold

salt, and ascorbic acid) can be incubated with gold seed particles that

have been immobilized on a surface, and nanorods do form on the

surface, in spite of the inherent difficulties presented in the surface

itself serving as a nucleation site for growth [66,67]. Astonishingly,

chemical modifications of the surface can produce aligned nanorods,

by a mechanism that is not yet clear [68]. If an electrochemically

active surface is used, one can apply a potential to reduce gold salt toelemental gold on immobilized seed particles in the presence of

growth solutions, rather than a chemical reductant such as ascorbic

acid [69].

Because gold nanorods, upon light irradiation into their plasmon

bands, can create large electric fields at their tips, many workers are

exploring ways to line nanorods up end-to-end to enhance the electric

fields experienced in the gaps for molecular electronics or chemical

sensing (via surface-enhanced Raman scattering) applications. Chemical

modification at the ends of gold nanorods in solution can lead to some

level of alignment, with molecular-scale nanorodnanorod gaps [70,71].

On larger length scales, striped metal nanowires can be prepared by

electrodeposition (e.g., gold/nickel or gold/silver) within hard templates

(diameters ~360 nm) with subsequent chemical etching of the nickel or

silverto producetunable gaps (~5100 nm) betweenthe tipsof verylarge

gold nanorods [72,73]. Gaps may also be cut cleanly through such larger

nanorods by focused ion beam cutting [74]. In an interesting twist,

Bjornholm et al. have linked gold seed particles with dithiol linkers; upon

the addition of growth solution, metal is deposited on the seed particles

away fromthe dithiol linker to generate linked dimers andtrimers of gold

nanorods [75]. Spacings of 12 nm are achievable with this method,

although the nanorods do tend to fuse [75].

8. Conclusions and future work

There appears to be roomfor almost limitless variation in the reagents

that can be used in gold nanorodsyntheses and many minor variations on

the general theme have appeared. Factors to consider, that frequently are

not considered, include the redox potential of agents in the growth

solutions; Ksp's of possible byproducts; temperature or pH-inducedchanges in redox potentials;and activitiesof the reaction byproducts(e.g.,

ascorbic acid's oxidation product, as noted above, can itself still reduce

metal ionsat elevated temperature).Argumentsas to the thermodynamic

stability of different crystal faces, as a mechanism for shape control,

regularly appear in the literature; our own data suggest to us that kinetic

parameters may be more important, and indeed are more consistent with

how delicate the synthesis can be. Far less well-explored is the notion of

doing top-down sculpting chemistry on nanoscale objects themselves.

The old surface science literature from the1940s through the1980s is full

of crystal-face-selective anisotropic etches for many bulk metals and

semiconductors, and could serve as a good source for future work. Y. Xia

has demonstrated the etching concepts thoroughly for silver nanoparti-

cles, and is one of the few people that have explored other precursors for

gold nanoparticles besides HAuCl4 [76

78].Gold nanorods provide a clear visual indicator of their shape

through the color of their solutions that is, the positions of their

plasmon bands. Fine-tuning particle shape can shift the position of

plasmon bands, as can various overcoating procedures (via shifts in

the local refractive index of the medium). We expect that as synthetic

procedures become ever more standardized and understood, designer

nanoparticles with arbitrary plasmon band positions and overall

dimensions can be made at will.

Acknowledgements

We thank our former group members for the excellent work that

has laid the foundations for current thinking. We also thank the

National Science Foundation and the Air Force Office of Scientific

Research for funding.

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91.

Fig. 4. A schematic cartoon illustrating how gold nanorods may be grown on a surface.

Growth on a substrate allows for the possibility of aligned rods with nanoscale gaps

betweenthem. Goldseeds arefirstdepositedon a substrateandthen, upon theaddition

of growth solution, nanorods can be grown from the seeds resulting in arrays of

nanorods on the substrate.



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