Functional Gold Nanorods: Synthesis, Self-Assembly, and Sensing Applications
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Functional Gold Nanorods: Synthesis, Self-Assembly,
and Sensing Applications
Leonid Vigderman, Bishnu P. Khanal, and Eugene R. Zubarev*
synthesized in the mid-1990s through an
Gold nanorods have received much attention due to their unique optical and approach based on electrochemical reduc-
electronic properties which are dependent on their shape, size, and aspect tion into rod-shaped templates.[3,4] Due to
ratio. This article covers in detail the synthesis, functionalization, self- the limitations of this technique such as
the low total yield of the procedure, more
assembly, and sensing applications of gold nanorods. The synthesis of three widespread adoption of gold nanorods into
major types of rods is discussed: single-crystalline and pentahedrally-twinned research did not occur until the advent of
rods, which are synthesized by wet chemistry methods, and polycrystalline wet-chemistry synthetic techniques, which
rods, which are synthesized by templated deposition. Functionalization of did not appear until Murphy and cow-
these rods is usually necessary for their applications, but can often be prob- orkers’ seminal work published in 2001.[5]
Continued improvements in synthetic
lematic due to their surfactant coating. Thus, general strategies are provided
methodology have led to better reliability
for the covalent and noncovalent functionalization of gold nanorods. The and have increased the shape-yield of rods
review will then examine the significant progress that has been made in con- to greater than 90 percent.
trollable assembly of nanorods into various arrangements. This assembly can As synthetic capabilities improved, so
have a large effect on measurable properties of rods, making it particularly did the understanding of the physical
applicable towards sensing of a variety of analytes. Other types of sensing not properties of nanorods including their ani-
sotropic optical and electronic properties.
dependent on nanorod assembly, such as refractive-index based sensing, are Excellent reviews have been published
also discussed. that describe the origins and modeling
of the physical properties of gold nano-
rods and so will not be included in this
1. Introduction review in detail.[6–8] Briefly, gold nanorods, like spherical gold
nanoparticles and other noble metal nanoparticles, have the
The fascinating size-dependent properties of noble metal nano-
ability to absorb light of varying wavelength due to the creation
particles have created a great promise for their use in a variety
of plasmon resonances on their surface. These plasmons rep-
of electronic, optical, and biomedical applications. Gold nano-
resent collective oscillations of the electrons surrounding the
rods, specifically, have received a great deal of attention due
nanoparticles and the intensity and wavelength of these sur-
to their unusual physical properties. The nanoscale confine-
face plasmon resonances (SPR) can be highly shape- and size-
ment of electrons on the surface of gold nanoparticles grants
dependent.[6–11] Due to the anisotropic shape of gold nanorods,
them shape- and size-dependent properties not seen in larger
they display two separate SPR bands corresponding to their
particles.[1,2] Initially, spherical or quasi-spherical gold nano-
width and length known as the transverse (TSPR) and longitu-
particles received the most attention due to the ease of syn-
dinal plasmon bands (LSPR). The TSPR is located at just above
thesis of such structures. This is perhaps unsurprising given
500 nm while the LSPR varies widely according to the nanorod
that the spherical shape is often the most thermodynamically
aspect ratio and the overall size. Through careful synthesis, it
and kinetically favorable morphology. In order to access more
is possible to create single crystalline gold nanorods with an
complicated structures, it is necessary to find reaction condi-
LSPR anywhere from the visible (600 nm) all the way into the
tions which can break the propensity towards isotropic growth
near IR (1100+ nm) portion of the electromagnetic spectrum.
and instead direct the nanoparticle growth into an anisotropic
The ability of nanorods to absorb near IR light makes them par-
dimension. The first class of anisotropic nanoparticles to gain
ticularly well-suited to biomedical applications since the absorb-
the most popularity has been gold nanorods, which were first
ance of the surrounding tissue in this region is low. This review
will not, however, focus on these applications as several excel-
lent reviews have already been published on the topic.[6,12–15]
L. Vigderman, Dr. B. P. Khanal, Prof. E. R. Zubarev, For nearly all applications, the ability to properly function-
Rice University alize the nanorod surface can determine the success or failure
Department of Chemistry of the project. In general, the functionalization of gold nano-
6100 Main Street, Houston, TX 77005, USA
E-mail: zubarev@rice.edu
rods can be significantly more challenging than the function-
alization of spherical particles, even through the well-known
DOI: 10.1002/adma.201201690 gold-thiol chemistry, due to the unique surfactant capping of
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as-synthesized nanorods. While spherical particles may be
directly thiol-coated during the synthesis or coated only with a Prof. Eugene R. Zubarev
weakly-bound anion, gold nanorods are usually synthesized in received his B.S. in Chemistry
the presence cetyltrimethylammonium bromide (CTAB), which from Moscow State University
binds more strongly to the nanorod surface. Complete or par- and Ph.D. in Chemistry
tial aggregation can easily occur during functionalization if the from Russian Academy of
CTAB structure around the rods is disturbed, leading to loss of Sciences. He then worked
desired optical properties. Thus, general functionalization strat- as a postdoctoral fellow in
egies as well as specific examples pertaining to nanorod appli- the group of Samuel I. Stupp
cations will be discussed. at Northwestern University.
Although the goal of a particular application may be to create In 2005 he moved to Rice
individual nanorods, their assemblies can also be highly desir- University where he is cur-
able due to the modulation of their physical properties as they rently an Associate Professor
are brought close together. This review will cover the significant of Chemistry. His current
progress that has been made on controlling nanorod assembly research is focused on the synthesis and self-assembly of
in the past several years, which has allowed for the production of anisotropic nanocrystals and hybrid nanomaterials.
interesting structures such as nanorod chains, rings, and three-
dimensional supercrystals. Importantly, these assembly tech- Leonid Vigderman received
niques have found significant application in sensing and detec- his B.S. in Chemistry from
tion of a variety of analytes including environmental toxins[16–19] the University of Colorado at
and biomarkers.[20,21] Thus, detection modalities based on the Denver in 2007. He received
anisotropic properties of rods such as their surface-enhanced his M.S. degree from Rice
Raman scattering (SERS) ability will be examined in detail. University in 2009 and is cur-
rently finishing his Ph.D. in
Chemistry at the same institu-
2. Synthesis tion under the supervision
of Prof. Eugene Zubarev. His
There are various methods to produce gold nanorods with dif- current research interests are
ferent structures. The first class of synthetic techniques that will in the morphological control
be discussed are the various aqueous wet-chemical CTAB-medi- and functionalization of gold nanoparticles for biological
ated synthetic procedures which have become the most popular and other applications.
as originated by Murphy et al[5] and El-Sayed et al.[22] While all
of these techniques produce crystalline nanorods, they can be
subdivided into those that lead to rods with single-crystalline Dr. Bishnu P. Khanal was
or pentahedrally-twinned structure. This is an important dis- born in 1978 and grew up in
tinction as the purity, length-scale, and further manipulations Gulariya, Nepal. He received
can depend highly on this difference. The second class of tech- his B.S. and M.S. degree
niques are those based on reduction of gold inside a template in Organic Chemistry from
of some sort, most often an anodized aluminum oxide (AAO) Tribhuvan University, Nepal.
membrane, which produces polycrystalline structures in lim- After receiving his Ph.D.
ited quantities. Finally, several methods exist to synthesize degree in Chemistry from
nanorods in organic solvents which generally lead to much dif- Rice University he worked
ferent morphologies including ultrathin rods and wires. at the Los Alamos National
Laboratory as a Director’s
Postdoctoral Fellow. His
2.1. Silver-Mediated Synthesis of Single-Crystalline Nanorods research mainly involves synthesis, self-assembly and
characterization of metal and semiconductor nanostruc-
tures. He currently works at Intel Corporation as a process
2.1.1. Electrochemical Synthesis
engineer.
The first report of reasonably high quality gold nanorods used
an electrochemical approach which was the precursor of the
most common seed-mediated procedure.[23,24] Reported by
Wang and coworkers, this approach utilized a two-electrode
electrochemical cell in which the gold anode provided the gold which was theorized to produce silver ions in solution, led to
source for the reaction while the template for rod-growth was increased rod yield and length.[24] Nanorods were synthesized
a mixed surfactant system of CTAB and tetradodecylammo- with aspect ratios anywhere from 1 to 7 with a corresponding
nium bromide (TDTAB). Small amounts of acetone and hexane longitudinal plasmon as high as 1050 nm with rod diameters of
additives were also present and the entire setup was soni- about 10 nm. Although the exact mechanism was not known,
cated throughout the reaction. The presence of a silver plate, it was theorized that TDTAB was the rod-directing agent and
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that growth may have occurred on the surface of the electrode, aspect ratio rods. However, the shape-yield of nanorods was
with sonication responsible for freeing the rods into solution. still relatively low and a large amount of spheroidal shapes
El-Sayed and coworkers carried out a crystallographic exami- was present. It is possible to estimate the relative abundance
nation of these rods and determined that the majority of the of rods versus other shapes by comparing the the LSPR peak
rods were single-crystalline in nature and grew along the [001] with the low wavelength maximum at around 500 nm, which
direction,[25] the same structure as that created by silver-assisted comes from a combination of the TSPR of nanorods and the
seed-mediated synthesis which will be described next. absorbance of spheroidal particles. A peak closer to 550 nm
usually corresponds to the presence of cubic particles. One
2.1.2. Seed-Mediated Synthesis can estimate that a 10:1 ratio of longitudinal to tranverse peak
corresponds to 90% or higher content of rods of aspect ratio
The next advance from Murphy and coworkers was to replace around 4 (LSPR∼800 nm). In the early work on nanorods syn-
the gold electrode as the source of gold and move to a chem- thesis, this ratio appeared to be closer to 3:1, which indicated a
ical source, chloroauric acid.[5] Electrochemical reduction was relatively high content of spheroidal impurities.[5]
replaced with chemical reduction using a weak reducing agent Significant improvement of this method was achieved by
(ascorbic acid) and silver nitrate. Finally, instead of using a co- El-Sayed et al. who were able to minimize the formation of
surfactant system containing CTAB and TDTAB, only CTAB spheroidal particles and produce rod-like morphology in high
was used during the synthesis, although hexane and acetone yield.[22] This change was achieved by utilizing CTAB-capped
were still added. The ascorbic acid used in this synthesis seeds rather than the citrate-capped seed particles used before.
is unable to reduce gold to the metallic state under the high A TEM image of gold nanorods synthesized by this procedure
CTAB concentration and low pH (∼2.5) growth conditions, and is shown in Figure 1a. Two surfactant systems were explored,
instead reduces it to Au(I) state.[26] However, addition of small including a single CTAB surfactant system and a dual-sur-
seed particles of about 3 nm in diameter into the Au(I) solu- factant system containing CTAB and benzyldimethylhexadecy-
tion resulted in complete reduction to metallic gold, which is lammonium bromide, both of which did not use any organic
catalyzed by the surface of the seeds and leads to the gradual additives such as hexane or acetone. The single-surfactant
change in shape from quasi-spherical to rod-like crystal. It was system coupled with CTAB-capped seed and an appropriate
determined that addition of less seed generally led to higher amount of silver nitrate (∼10–20 mol%) routinely gives greater
Figure 1. Gold nanorod synthesis using the procedure of El-Sayed et al.[22] (a) TEM image of nanorods synthesized in our lab following this procedure.
(b) UV-Vis absorbance spectrum of rods synthesized with increasing amounts of silver nitrate, from left to right, leading to higher LSPR wavelength.
(c) UV-Vis absorbance spectrum demonstrating the increase in LSPR wavelength of rods synthesized in a CTAB/BDAC surfactant mixture with suc-
cessive gold addition, from left to right. (d) Graph showing the dependence of rod LSPR on silver (black) and gold (grey) concentration in the growth
solution. Reproduced with permission.[22] Copyright 2003 American Chemical Society.
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than 95% shape-yield of rods and allows one to reach aspect differences in the final LSPR peak position: an increase in this
ratios up to 4.5 and LSPR peak close to 850 nm (Figure 1b). The ratio was found to generate shorter rods and lower wavelength
use of the dual-surfactant system can generate nanorods with LSPR.[32] Similarly, an increase in the proportion of seed to total
an aspect ratio as high as 10 and LSPR up to 1300 nm through gold ions concentration leads to shorter rods while a decrease
either aging of the growth solution or slow addition of gold ions in the total amount of CTAB has a similar effect, but the shape-
after the initial growth (Figure 1c). However, the level of sphe- yield of rods is reduced drastically.[32] Another method to obtain
roidal impurity appears to be significantly higher, as evidenced nanorods with a particular LSPR was proposed by Wei and cow-
by absorbance peak ratio of less than 4:1 compared to 10:1 for orkers who were able to arrest nanorods growth with the addi-
the CTAB-only method. tion of sodium sulfide.[28] Since the nanorods synthesis only
A partial control of the plasmon peak location is possible by consumes about 20–30% of gold ions present in solution, slow
altering the concentration of silver nitrate and gold chloride. reduction of gold onto the rod surface after their initial forma-
Increasing these concentrations led to higher wavelength LSPR tion leads to a gradual increase in diameter and a concomitant
peaks, but only up to a certain point, after which the opposite blue shift of LSPR on the time scale of several days. Addition of
trend was observed, as seen in Figure 1d.[22] In practice, we and sodium sulfide was shown to effectively suppress the post-syn-
others have found that batch to batch variability in LSPRs can thesis drifting of the LSPR peak.[28] Recently, Murray and cow-
be significant and is dependent on several factors.[27,28] In par- orkers demonstrated tuning of LSPR through addition of aro-
ticular, proper synthesis of the seed is particularly important matic salicylate additives as well as hydrochloric acid, which was
as slightly increased amounts of added borohydride can lead explained through an effect on the micellar structure of CTAB
to increased seed particle sizes which can lead to much larger during rod growth.[33] This method can produce higher aspect-
amounts of spheroidal impurities.[29–31] This can be observed ratio gold nanorods with fewer spherical impurities, as can be
through the presence of a reddish hue in the seed solution. Fur- seen in Figure 2a–d. Tuning of the nanorod plasmon can also
thermore, slight variations in the ratio of ascorbic acid to gold be achieved by post-synthetic tip-selective oxidation.[34,35] In this
chloride concentration in the growth solution can lead to large case a solution of gold (III) ions complexed with CTAB serves
Figure 2. Improved synthesis and fine-tuning of gold nanorods. TEM images (a)-(c) show rods synthesized in the presence of 5-bromosalicylic acid
additive and HCl with added amounts of seed and silver nitrate solution, respectively, of (a) 0.4 mL and 48 mL, (b) 0.2 mL and 36 mL, and (c) 0.8 mL
and 30 mL. Corresponding UV-Vis absorbance spectra (d) of rods in (a), (b), and (c) are shown in red, blue, and magenta, respectively. Reproduced
with permission.[33] Copyright 2012 American Chemical Society. Pictures (e) and UV-Vis absorbance spectra (f) of gold nanorods during their gradual
dissolution with HAuCl4. The extent of dissolution increases from right to left in both (e) and (f).
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as an oxidizing agent resulting in a controllable reduction of 19 days, as shown in Figure 3b. This was possible through the
the LSPR wavelength and intensity, as shown in Figure 2d,e. use of a single reducing agent system developed by Tollan and
Many variations on this general scheme have been pub- coworkers[40] which was able to reduce gold ions to the metallic
lished, especially recently. One strategy is to perform a one-pot form only when mixed with a separate feedstock at a higher
synthesis of nanorods by adding sodium borohydride directly to pH.[39]
the nanorod growth solution instead of forming a separate seed
solution.[30,36,37] Although these methods are generally termed
2.1.3. Photochemical, Ultrasonic, and Radiolytic Synthesis
“seedless”, the addition of sodium borohydride still forms seed
gold nanoparticles in situ. While the main benefit of these pro- Besides utilizing ascorbic acid and sodium borohydride as
cedure is the simplicity of preparation, one problem that we reducing agents, it is possible to apply a variety of other pro-
have encountered is that the quality of even “fresh” borohydride cedures, such as the electrochemical synthesis described ear-
can be variable and, given the technique’s sensitivity to boro- lier. The photochemical growth strategy is one such method
hydride concentration and lack of a visible seed in the proce- which utilizes photoreduction to convert Au(III) to Au(0).[41] A
dure, reproducibility of this method can be fairly low. The wet- gold chloride solution within a mixed CTAB-TDTAB surfactant
chemical synthesis of nanorods is generally carried out in small system with silver nitrate, acetone, and hexane additives is irra-
batches in standard laboratory equipment. However, there has diated with a 254 nm UV light for more than 24 hours. Acetone
been an interest in applying the standard silver-nitrate medi- appears to drive the reduction through photochemical genera-
ated synthesis to other environments. The synthesis of rods tion of ketyl radicals which act as the active reducing agent.[42]
in a microfluidic system under continuous flow was dem- A good yield of rods of different aspect ratio can be obtained by
onstrated by Boleininger et al.[38] In such a system, it may be changing the silver concentration, leading to LSPRs located at
possible to continuously control the reaction conditions, such 600–800 nm.[41] It is possible to shorten the necessary reaction
as altering the seed to growth ratio or the solution tempera- time to less than 30 minutes by combining the chemical and
ture.[38] More recently, a seedless approach to continuous flow photochemical techniques. In this procedure gold is reduced
synthesis of rods was developed by using a rotating tube proc- to the Au(I) state by ascorbic acid, and the nucleation is subse-
essor (RTP) followed by a narrow channel processor, as shown quently induced by UV irradiation of acetone additive while fur-
in Figure 3a.[39] Two separate and stable stock solutions were ther growth occurs in the dark.[42,43] Interestingly, the ascorbic
mixed in this experiment, leading to the reproducible synthesis acid concentration can be adjusted anywhere from 0.75 to
of nanorods with identical optical absorbance spectra for up to 6.2 times the gold chloride concentration,[44] which is not pos-
sible in the standard seed-mediated synthesis.
More recently, several other techniques have been devel-
oped. Ultrasonication of a growth solution containing CTAB,
gold chloride, silver nitrate, and ascorbic acid was shown to
induce rod formation.[45] It was theorized that ultrasonic irra-
diation led to the formation of hydroxyl and other radicals
which could reduce gold ions to form the initial nuclei nec-
essary for rod growth.[45] Alternatively, radicals can be gener-
ated by γ-irradiation of a growth solution similar to that used
in photochemical synthesis.[46,47] This radical-initiated growth
mechanism may also be operant in the previously described
electrochemical synthesis considering the chemical composi-
tion of the growth solution and the use of similar sonication
procedure.
2.1.4. Crystal Structure
Gold nanorods synthesized by wet chemistry methods in the
presence of silver nitrate display a common crystal struc-
ture based on a single-crystal motif with no twinning faults.
It has been shown that nanorods grow longitudinally along
the direction and have an octagonal cross-section.[25]
The sides of the rods are bound by alternate {100} and {110}
facets that come together at the tips in the form of {110} and
{111} facets, respectively. This view has been predominant
in the literature, but recent analysis by Liz-Marzán and cow-
Figure 3. Seedless, continuous flow synthesis of gold nanords. (a) Design orkers suggests that the actual structure is likely different
of fluidic system in which two stable feedstocks are fed into a rotating
and includes higher-index facets on the sides of the rods,
tube processor (RTP) followed by a narrow channel processor (NCP).
(b) UV-Vis Absorbance spectra of nanorods produced after different times as supported by high-resolution TEM of vertically standing
showing the stability and reproducibility of the procedure. Reproduced nanorods (Figure 4a).[48] Although both models agree with
with permission.[39] Copyright 2011 The Royal Society of Chemistry. the lateral appearance of rods (Figure 4b), the standing view
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Figure 4. Crystal structure of gold nanorods. (a) HRTEM image of a standing rod showing higher-index {250} facets. (b) Old and new models for
rod crystal structure, both of which are consistent with TEM image of side view of rods. (c) Angles between crystal facets measured from HRTEM.
Reproduced with permission.[48]
demonstrates that they are bound by eight higher-index {250} demonstrated that salicylate ions are able to alter the structure
facets with equal surface area, but different angles between of CTAB micelles.
them (Figure 4c). In addition, the and direc- More research has been conducted into the role of the sur-
tions point to the corners of the rod rather than the faces as factant counter-ion in the growth process. Specifically, the pres-
assumed previously. No difference in the tip structure was ence of bromide is known to be key in this synthesis. Garg and
reported compared to earlier work. The implications of these coworkers showed that gold nanorods cannot be synthesized if
structural details for the growth mechanism of rods are par- hexadecyltrimethylammonium chloride (CTAC) or even a 1:2
ticularly important as discussed below. ratio of CTAC:CTAB is used instead of pure CTAB.[51] However,
they demonstrated that by adding an external source of bro-
mide to keep its concentration at 0.1M, it is possible to drop the
2.1.5. Growth Mechanism
CTAB concentration even below its critical micelle concentra-
The growth mechanism of single-crystalline gold nanorods syn- tion and still form rods, although the LSPR:TSPR ratio of 1:1
thesized in CTAB-micellar solutions has received a large amount suggests the procedure is not very efficient. Similarly, Si et al.
of attention. It has become quite evident that the presence of showed that, in a one-pot synthesis, total bromide concentra-
silver is essential for rod formation, but some disagreement tion controlled the LSPR position irrespective of its origin.[52]
exists as to the exact mechanism of its action. The synthesis Although bromide cannot be substituted for chloride, the low
of rods can be carried out in the absence of silver under very amount of chloride present from the HAuCl4 starting material
similar conditions, but this generally leads to the synthesis of is not problematic and ionic gold is found mainly as AuBr4− or
pentahedrally-twinned rods (see Section 2.2). Initially, it was AuBr2− after reduction.[53]
theorized that rod-shaped CTAB-micelles were present as soft On the other hand, the presence of iodide can have a pro-
templates for gold nanoparticle growth.[5,23,30,41] The photo- found effect on gold nanorod synthesis. Korgel and coworkers
chemical and electrochemical procedures called for the pres- discovered that micromolar amounts of iodide in the growth
ence of a highly hydrophobic co-surfactant TDTAB along with solution is enough to completely inhibit nanorod growth.[54]
cyclohexane to elongate these surfactant micelles in solution. Earlier reports that CTAB from certain suppliers[55] or even dif-
However, further photochemical[43] and seed-mediated[5,22] pro- ferent batches from the same supplier[56] could not be used for
cedures demonstrated that these species could be removed. the nanorods synthesis was explained by the presence of trace
It is clear that the chemical nature of surfactant is extremely amounts of iodide ions as determined by inductively coupled
important, although there appears to be a limited degree of flex- plasma mass spectroscopy (ICP-MS).[54] This was explained by
ibility in the choice of surfactants. Exchanging CTAB for cetyl- the strong binding of iodide to metallic gold in general, and
triethylammonium bromide was still found to produce rods,[49] specifically to the growing {111} facets on the ends of nano-
although the growth was reported to be slower and the shape- rods.[54] Indeed, the specific binding of different species to var-
yield was reduced significantly based on the optical absorbance ious facets of the gold nanorods during their growth has been
spectrum (2:1 ratio of LSPR:TSPR). It is also possible to use a implicated as the major structure-directing element as opposed
twin-hexadecyl tailed surfactant to synthesize rods at lower con- to any particular soft-templating effects by rod-shaped micelles.
centrations with similar results.[50] As described earlier, the use Differential binding of various species to a particular set of
of a bulkier co-surfactant, BDAC, along with CTAB produced growing facets appears to affect the deposition rate of gold onto
higher aspect ratio nanorods in the seed-mediated synthesis, those facets, thereby controlling the final shape of the nano-
but also led to much lower yield and poor quality of particles.[22] crystal. It is clear that bromide, silver, and the CTA+ cation have
Dropping the CTAB concentration even two times leads to a a large effect on the growth, though the exact surface-binding
large decrease in the yield of rods.[32,51] However, recent work species have not been settled. El-Sayed and coworkers attributed
by Murray and coworkers demonstrated that addition of aro- the enhanced growth of {111} tip facets to the stronger binding
matic additives can reduce the necessary amount of CTAB by affinity of CTAB to the {110} side facets, which have a higher
50% while still maintaining excellent quality of rods and their surface energy due to larger interatomic distances.[22,57] It was
yield with respect to other morphologies.[33] NMR experiments also proposed that AgBr could precipitate onto the nanoparticle
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surface,[58] but later reports indicated that
REVIEW
silver was present in the form of a soluble
AgBr2− CTA+ complex and that this complex
has a high propensity to bind to the nanorod
surface.[53,59] However, further studies of pho-
tochemically synthesized nanorods seemed
to contradict these results. Detailed Extended
X-ray Absorption Fine Structure (EXAFS)
measurements of gold nanorods were per-
formed to determine the chemical and coor-
dination state of silver and demonstrated
that silver is present in metallic rather than
ionic form in the final product.[60,61] It was
also demonstrated that up to several layers
of metallic silver could be present on the
surface of nanorods, a result consistent with Figure 5. Growth of single-crystalline gold nanorods. (a) Predicted growth trajectories of rods
based on a particular side to end rod surface energy ratio of 0.5 suggesting that final aspect ratio
previous ICP-MS analysis.[62] Since the X-ray is determined mainly by relative energies of the side and end rod facets, which can be modu-
characterization data was performed only on lated by surfactant binding/silver underpotential deposition. (b) Cryo-TEM images showing
purified nanorods, this does not rule out the different rod morphologies and different reaction time-points. (c) Schematic of rod growth
importance of the AgBr2− CTA+ complex at including the gradual change in shape supported by cryo-TEM and UV-Vis absorbance as well
some point during the synthesis. as modeling. Reproduced with permission.[65] Copyright 2012 American Chemical Society.
The presence of metallic silver coating
supports a different growth mechanism driven by under- (Figure 5c), which was supported by optical absorbance spectra
potential deposition (UPD) of silver onto growing nanorods. and modeling. Evidence also suggests that if a growing seed
According to this hypothesis, silver is present in the growth does not experience symmetry-breaking prior to reaching
solution as Ag(I) and cannot be reduced by ascorbic acid to a certain size (∼5 nm), it may become incompatible for rod
its metallic form in the presence of CTAB under acidic con- growth and will instead transform into a spheroidal particle,
ditions.[26] However, the deposition of silver onto the gold a common impurity in these procedures.[29,65] However, the
surface can happen at lower potential and is expected to exact cause of this symmetry-breaking process and the param-
occur more readily on the {110} facets compared to the {100} eters that control it are still unknown. Also, it is not clear why
facets.[63] Growth along the direction would be slowed the growth of nanorods stops at some specific lengths, espe-
as it would require re-oxidation of silver, thus leading to the cially in the case of a non-uniform growth scenario. This may
unexpected growth along the more thermodynamically stable be explained by Murphy and coworkers hypothesis that rapid
direction. It must also be noted that recent assignment growth continues until metallic silver deposited through UPD
of the higher-index facets to the sides of the rods, as described completely encapsulates the rod, after which the growth rate
earlier,[48] fits well with the growth mechanism based on vari- would rapidly decrease.[62]
able binding between side and tip facets of rods as it would
imply an even larger difference in the openness and binding
propensity (or silver UPD propensity) between higher-index 2.2. Synthesis of Pentahedrally-Twinned Nanorods
sides and lower index tips.
2.2.1. General Synthesis
The strength of binding to various gold facets may not be
the only factor driving the growth of nanorods. One theory It is possible to synthesize crystalline, pentahedrally-twinned
posits that an enhanced electric field exists at the tips of gold nanorods under conditions quite similar to the synthesis
gold rods which leads to a higher collision rate with CTAB- of single-crystalline rods described earlier with the major dif-
bound Au(I) ions and therefore enhancement of anisotropic ference being the absence of silver ions in the growth solution.
growth.[64] Recent work by Cortie and coworkers suggests that This crucial difference was capitalized upon by Murphy and
the main effect of silver complex binding could be in altering coworkers who published a three-step procedure to synthesize
the surface energy of the nanorod sides compared to their pentahedrally-twinned gold nanorods.[66] The first step is to
tips which could be the driving force for anisotropic growth, prepare the seed particles through borohydride reduction of a
as shown in Figure 5a.[65] Supported by combined cryo-TEM, citrate-containing solution of HAuCl4, forming small, roughly
modeling, and optical spectroscopy, they proposed a “popcorn” 3.5 nm particles. Similar to previous methods, the growth
mechanism wherein growing seeds will randomly experience solution contained CTAB and chloroauric acid reduced to
symmetry-breaking and then rapidly reach their final dimen- Au(I) with ascorbic acid. The growth solution is different from
sions. Cryo-TEM imaging of the early stages of growth showed standard seed-mediated synthesis in that the concentration of
a low concentration of nearly fully-formed rods rather than a gold is two times lower, and the ascorbic acid is present in sig-
high concentration of very small rods, indicating that uniform nificantly higher concentration (2.2 mol equiv. with respect to
growth of seeds into rods was not occurring (Figure 5b). Fur- gold chloride). Three such growth solutions are created and a
thermore, initial rod morphology was bowtie-like and gradu- series of sequential transfers of seed to the first growth solu-
ally reshaped into the regular quasi-cylindrical morphology tion, from the first to second, and from the second to the final
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Figure 6. Synthesis of pentahedrally-twinned rods. (a) Three-step procedure for the growth of rods without silver nitrate. (b) TEM images of the
purification of rods through dissolution and sedimentation procedure. Solution goes from highly impure to pure nanorods with increased addition of
chloroauric acid/CTAB solution. Scale bars are 200 nm. Reproduced with permission.[72] Copyright 2008 American Chemical Society. (c) Illustration of
the pentahedrally-twinned structure of high aspect ratio rods. (d) Schematic of surfactant “zipping” process directing anisotropic growth. Reproduced
with permission.[81] Copyright 2003 American Chemical Society. (e) Graph showing the dependence on nanorod aspect-ratio on the gelation state of a
solution containing mixtures of CTAB and octadecyltrimethylammonium bromide (OTAB). On the left, the aspect ratio of rods is depicted as normal
(∼25) with open circles or high (∼50) with closed circles. On the right, the solution state is depicted as liquid with open circles, gelled with closed circles,
or crystallized with closed squares, indicating that gelation leads to the highest aspect ratios. Reprinted with permission.[67] Copyright 2009 Elsevier.
growth solution are carried out, as shown in Figure 6a. The 2.2.3. Synthetic Progress
result is the synthesis of pentahedrally-twinned gold nanorods
This is currently the basis of all available methods to grow crys-
with 20–22 nm diameter and much higher aspect ratio (∼20)
talline high aspect rods, but, unfortunately, it leads to a very low
than those prepared by silver-nitrate mediated synthesis. It is
yield of rods, usually five percent or less,[70] with the majority
presently not possible to decrease the rod diameter much below
of the product consisting mostly of spheroidal particles as
20 nm, although the length can be pushed to above 1 μm while
well as a large number of platelets. As synthesized, the solu-
maintaining nearly the same diameter by altering the reaction
tion has a purple color which comes from the smaller spherical
conditions as described later.[67]
particles. The low yield of this synthesis and the difficulty of
separation of unwanted shapes are the main limitations to this
2.2.2. Crystal Structure technique and several reports exist on the optimization of the
Gold nanorods synthesized in a silver-free environment pos- original three-step procedure to improve these factors. Murphy
sess a pentahedrally-twinned crystal structure as opposed to et al. first reported that increasing the pH of the growth solu-
the single-crystalline nature of rods synthesized in the pres- tion using sodium hydroxide produced rods as the major rather
ence of silver ions. As shown in Figure 6c, the rods are bound than minor product of the synthesis.[70] Interestingly, a later
by {100} or {110} facets while the tips are bound by five tri- report by Wu et al. stated that addition of nitric acid to the final
angular {111} faces and five {111} twin boundaries run lon- growth solution also led to an increase in the quality of the rods,
gitudinally along the rod.[68] The rods are shaped in the form although this was attributed to the presence of nitrate ion rather
of a pentagonal prism in which the different facets may have than a change in pH.[71] Other sources of nitrate were not exam-
different tendencies to bind CTAB in solution, thus contrib- ined though. We have had difficulty reproducing either of these
uting to anisotropic growth as described in the previous sec- techniques, but have been able to utilize part of the procedure
tion. No reports have been published which have challenged of Wu et al. to design a procedure to completely purify rods
this crystallographic assignment, unlike the report of higher- from other impurities.[72] The first step, which was used by Wu
index facets for single-crystalline rods.[48] The initial seed et al. and likely explains why they show such a high concentra-
structure is actually single-crystalline,[68] and, in the case of tion of rods, was to notice that high aspect-ratio rods and large
CTAB-capped seeds, is the same as the seed used in single- platelets settle out of solution after a day or two and the top,
crystalline growth.[69] It is assumed that once the seeds grow to deep purple solution consisting almost exclusively of spherical
a particular size, twinned stacking faults develop to lower the particles and low aspect-ratio rods, can be carefully decanted,
surface energy of the system, possibly through the coalescence thereby easily removing these impurities. Commonly reported
of smaller particles.[68] low-speed centrifugation has a similar effect,[66] but is clearly
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90 percent.[67] Conditions which produced gelation, i.e. higher
REVIEW
more difficult to scale up. Although the overall yield of rods is
still very small, the shape-yield of rods in the precipitate is fairly OTAB concentration and lower temperature, were found to be
high. The other major impurity is large 2D platelets, though its the best combination for the synthesis of rods with the highest
content can be reduced by switching from a citrate-capped seed aspect ratios of 30–50 (Figure 6e). Although the exact mecha-
to a CTAB-capped seed.[69] Our group has developed a separa- nism of this effect is unclear, it is consistent with the previous
tion procedure wherein the rods and platelets are first oxidized report by Murphy et al. in that the even longer octadecyl chain
with the addition of Au(III)-CTAB solution (Figure 6b).[73] This would be expected to lead to higher aspect ratio rods. Further-
oxidation occurs along the entire circumference of platelets, but more, the gelation process could enhance the “zipping” effect
only along the tips of rods, causing the platelets to shrink faster proposed before.[81]
and giving them a higher solubility. Because of that the partially
dissolved platelets stay in solution longer and can be removed
from the precipitated rods. The resulting isolated rods have an 2.3. Templated Synthesis of Gold Nanorods
aspect ratio of about 10, down from the initial 20, but maintain
the same diameter. The variable sedimentation rate of different So far, the discussion has been focused on the wet-chemical
sizes of rods, platelets, and spheres was later explained by Park synthesis of crystalline rods which generally gives access to
and coworkers as a depletion-induced effect which occurs in smaller rods in larger amounts. Other than crystalline growth,
micellar CTAB solutions.[74] The synthesis of pure pentahe- one could consider reduction of gold into some sort of rod-
drally-twinned rods can be important in further growth of rods shaped template. Although rod-shaped CTAB micelles have
into pure particles with controlled morphollogy.[75] been generally disproven to act as a template for rod growth, it
Further insight into the three-step procedure was gained by is possible to fabricate harder templates out of various materials
studying the effects of the size and charge of the seed particles that possess rod-shaped openings. Initial applications of this
on the nanorods synthesis.[58] Seeds of different size stabilized technique were focused on gold reduction inside of nanoporous
by citrate, CTAB, glucose, and 4-mercaptobenzoic acid were used membranes fabricated out of polycarbonate, alumina, or other
and a general trend of decreasing aspect ratio was observed with materials. Polycarbonate membranes can be ion-track etched
increasing the seed size, although this did not appear to apply to create uniform cylindrical pores of diameter down to 10 nm
to CTAB-capped seeds. The ability of so many different kind with pore diameters up to 109 pores/cm2, though these are usu-
of seeds to induce rod growth is highly surprising, especially ally purchased rather than made in the lab.[83] Porous alumina
considering the extreme sensitivity to the seed structure in the membranes can be produced in the lab through electrochemical
silver-mediated growth synthesis (vide supra). One possibility anodization of aluminum.[84] Anodization in an acidic solution
is that the large excess of borohydride which is used in most produces uniform pores in the alumina that can reach diameter
of the seed syntheses is carried over into the growth solutions as low as 5 nm with length depending on the anodization time
and is responsible for in-situ seed formation. Although seeds that is usually on the micron scale.[83] Pore density on the order
were aged for three hours at 25 °C, aqueous borohydride solu- of 1011 pores/cm2 can be achieved.[83] Electrochemical deposi-
tions are actually more stable than might be expected due to the tion of gold into the pores leads to the growth of gold rods and
basification of the solution with time.[76] However, the growth wires.
mechanism of penta-twinned rods is clearly different from that Early reports of rod synthesis using this technique came
of single-crystalline ones. Several reports of synthesis confined from Martin and coworkers who used both polycarbonate and
to surfaces exist[77–80] and seem to show that anisotropic growth alumina membranes along with electrochemical deposition of
does not start until seed-particles grow to just under 20 nm in gold inside the pores.[3,4,83,85–87] Rods are grown according to
size and that rod diameter does not increase appreciably after the process depicted in Figure 7a and then can be released by
that point.[79] The effect of varying the surfactant chain length dissolving away the alumina template pictured in Figure 7b.
and structure was also studied.[81] By changing the alkyl chain This can be accomplished in a way that leaves a standing array
length in the alkyl-trimethylammonium bromide from decyl to of rods (Figure 7c).[88] Nanorods are produced with diameter
hexadecyl, Murphy et al. demonstrated that increasing the chain that is equivalent to the diameter of the pores, making it fairly
length led to a higher aspect ratio. More importantly, reducing controllable. Although initially the diameter of the rods was
the chain length to ten carbon atoms led to complete disappear- high as compared to wet-chemically synthesized rods, around
ance of high aspect ratio rods. It was suggested that the growth 50 nm or more,[3,4] this was brought down to 16 nm,[89] which
was driven by the preference of the surfactant to bind to the is similar to solution-synthesized rods. The length of rods is
sides of rods during the growth through “zipping” interactions more difficult to control because the plating current efficiency
along the surfactant bilayer caused by increased van der Waals varies from membrane to membrane.[86] There is as much as
interactions between the hydrocarbon tails (Figure 6d).[81] The 30% variance in length among rods due to uneven deposi-
importance of surfactant head group binding was also noted tion, leading to rod aspect ratios from 1 all the way to 19.[89]
given that cetylpyridinium chloride could not induce rod for- Schönenberger and coworkers were also able to form rods with
mation,[81] although another report showed 1-dodecyl-3-methyl- diameters as low as 11 nm and aspect ratios up to 17 using this
imidazolium bromide actually could.[82] Recently, it was also method.[90,91] Other groups also developed similar techniques
reported that seed-mediated growth in a surfactant mixture for rod synthesis.[84,92–95]
comprised of CTAB and octadecyltrimethylammonium bro- This technique clearly has several benefits as well as limi-
mide (OTAB) under conditions in which gelation occurred led tations. A large limitation exists in the amount of material
to the synthesis of high aspect ratio rods in yields greater than that can be synthesized at once as well as the scalability of the
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REVIEW
template, but used only an HAuCl4 growth
solution, although the focus was on silver
rather than gold rod synthesis.[97] Recently, a
promising synthetic strategy was developed
that used silica nanotubes as the template for
gold (or other metal) rod growth.[98] In this
technique, the silica nanotubes, which can
be synthesized in gram scale by templating
from nickel-hydrazine complex nanorods,[99]
are selectively functionalized on their inner
surface. This allows the selective deposition
of gold inside the tubes and the formation
of rods with a diameter of 17 nm and aspect
ratio ranging from 3.5 to 21 after the disso-
lution of the template, as seen in Figure 8.
The relatively low polydispersity of the rods
combined with the potential for large-scale
synthesis addresses some of the shortcom-
ings of the previous template-based methods
and could make this an attractive alternative
to wet-chemical synthesis of rods.
Figure 7. Porous membrane-templated synthesis of gold nanorods. (a) In the first step, alu-
minium, gold and tantalum oxide films are deposited onto a glass substrates. The porous
membrane is formed by the anodization of the aluminum film with sulfuric acid. In the last 2.4. Other Synthetic strategies
step, gold nanorods are electrochemically deposited inside the template. Reproduced with
permission.[95] Copyright 2008 The Royal Microscopical Society (b) SEM image of porous alu-
mina membrane. Reproduced with permission.[91] Copyright 2000 American Chemical Society Almost all the synthetic techniques for gold
(c) SEM image of free-standing array of gold nanorods after dissolution of alumina template. nanorod synthesis are carried out in an
Reproduced with permission.[88] Copyright 2011 Nature Publishing Group. aqueous environment. However, recently,
there have been several reports which utilize
an organic environment and produce rather
technique since the rod growth is limited to a two-dimensional different morphologies. Xia and coworkers demonstrated
area of a membrane rather than taking place in a three dimen- the formation of ultrathin gold nanorods by mixture of an
sional volume as in the wet-chemical synthesis. This could AuCl(oleylamine) complex with amorphous iron nanoparticles
make this synthetic technique very difficult to apply for real in chloroform and aging for one week.[100] Iron nanoparticles
applications. While the rod diameter can be determined rather act as the reducing agent and lead to the creation of nano-
accurately based on the template, length control is more diffi- particles rich in defects which are slowly converted to single-
cult and polydispersity in the sample can lead to broadening of crystalline rods with 2 nm diameter and average aspect ratio
the longitudinal plasmon.[91] In addition, gold is deposited in a of 30, apparently through an etching and redeposition process.
polycrystalline fashion inside the pores which are not perfectly Growth of the rods occur along the direction and is pos-
smooth, leading to an increase in roughness which can also tulated to be directed by oleylamine. Around the same time,
broaden the LSPR,[91] although Mirkin and coworkers recently several other groups published the formation of ultrathin gold
proposed a method to electrochemically reduce this rough- nanowires in organic solvent.[101–103] It appears that formation
ness by a factor of five.[92] Functionalization and stabilization of of aurophilic polymers can lead to the organization of gold into
template-synthesized rods can be achieved with the use of poly- the nanowire form, after which it is reduced into single crystal-
meric stabilizers such as poly(vinylpyrrolydone)[90] and the com- line growth along the same direction. Interestingly, the
pletely bare surface of the rods could make them particularly choice of solvent is critical as the formation of rods rather than
attractive for some applications compared to wet-chemically wires only occurred in chloroform where the aurophilic poly-
synthesized rods that have relatively strongly bound CTAB. mers do not form. A separate example exists wherein pentahe-
Recent reports show that nanoporous membranes are not the drally-twinned rods are synthesized by anisotropic overgrowth
only possible templates in which gold nanorods can be made. of gold decahedron seeds in a high-temperature synthesis.[104]
Two examples exist of growing rods in the pores of mesopo- Gold decahedrons of size 40 nm and 64 nm prepared through a
rous silica SBA-15. In one report, an amine-modified SBA-15 polyol synthesis are grown in refluxing pentanediol in the pres-
is impregnated with gold seeds through a calcination approach ence of poly(vinylpyrrolidone) and silver. Anisotropic growth
and rods are further grown under conditions modified from is proposed to occur due to differential binding to tip and side
the wet-chemical growth of rods including CTAB, silver nitrate, facets of the decahedra of PVP as well as UPD-deposited silver
etc.[96] This leads to the growth of thin rods with a diameter of and leads to rods with minimum diameter of about 70 nm and
6-7 nm and adjustable length with apparently single-crystalline highest aspect ratio of about 2.4. Finally, the synthesis of single-
structure inside the template, which can be dissolved with HF crystalline rods in an imidazolium-based ionic liquid was car-
to free the rods. A second report also utilized an SBA-15 growth ried out, though the morphology was somewhat irregular.[105]
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REVIEW
organic solvents, making them of limited usefulness for many
applications.[106,107]
It is first necessary to understand the general structure and
properties of the CTAB coating on the surface of rods. Gold rods
have been shown to be stabilized by a partially-interdigitated
bilayer of CTAB which is similar in structure to a micelle.[108,109]
El-Sayed and coworkers used FTIR to show binding of the
ammonium bromide head group to the gold surface.[108] Ther-
mogravimetric analysis (TGA) also demonstrated the presence
of CTAB molecules with different desorption properties corre-
sponding to a weak binding outer layer and a stronger binding
inner layer.[108] Characterization of this structure is generally
difficult because of its noncovalent binding to the gold sur-
face and dynamic character in solution. Gómez-Graña and
coworkers carried out small angle x-ray (SAXS) and neutron
(SANS) scattering experiments to measure the structure of the
surfactant bilayer.[109] By using SAXS to characterize the gold
core and SANS to characterize the surfactant bilayer, they were
able to determine a bilayer thickness of 3.2 nm, which is less
than a fully extended bilayer thickness of 4 nm and thus leads
to the likelihood of partial interdigitation between the two CTAB
layers.[109] This bilayer is necessary to stabilize the rods, and, as
the authors note, the rods tend to aggregate if the concentration
of CTAB in solution drops below the critical micelle concen-
tration.[109] The stability of the bilayer can also be disrupted in
other ways, such as the addition of organic solvents, which can
be either highly problematic or can actually be used to one’s
advantage. For instance, a technique was developed in our lab
to functionalize the nanorod surface with mercaptophenol.[31,110]
Addition of a THF solution of mercaptophenol to a 0.1 M CTAB
solution of rods allows for the gradual replacement of CTAB
with mercaptophenol. As the exchange proceeds, rods begin
to gradually aggregate and crash out of solution. If the proper
amount of THF and a large excess of mercaptophenol is used,
Figure 8. Silica nanotube-templated synthesis of gold nanorods. (a) The
schematic of the growth procedure. TEM images show each step of rods will be sufficiently functionalized and can be brought back
the procedure: (b) silica nanotubes with amine functionality only on fully into organic solution through esterification with carboxyl-
the inner surface, (c) nanotubes impregnated with seed gold nanoparti- containing organic polymers (Figure 9a).[111–114] This procedure
cles, (d) nanotubes filled with gold, and (e) nanorods which have been also points out a particular limitation on the functionalization of
released after silica etching. Reproduced with permission.[98] Copyright rods with small molecules: the resulting rod may simply not be
2011 American Chemical Society. soluble enough due to the large size of nanorod core compared
to the small organic functionality. This can also lead to complete
loss of solubility of rods before they can be functionalized.
The use of the classic gold-thiol bond chemistry is one
3. General Functionalization Strategies
common way to functionalize gold rods, though, as the previous
As is the case for most nanoparticles, the synthesis of gold example suggests, simple addition of any thiol may not lead to
nanorods is often not, by itself, enough to make them useful complete functionalization. For instance, direct exchange of
for most applications. The nanorod surface must first be CTAB for the commonly used thiol mercaptoundecanoic acid
functionalized with the proper organic or inorganic material (MUA) generally leads to irreversible aggregation or only par-
to grant stability under the necessary conditions as well as to tial exchange.[115] The addition of an ethanolic solution of MUA
provide added functionality. In particular, nanorods synthe- to rods has been reported.[116] This is generally combined with
sized wet-chemically in the presence of CTAB, by far the most heating to promote the direct exchange and constant sonication
used type of rods, present a set of unique challenges for their to keep rods from aggregating.[117] A separate procedure was
functionalization and further applications. The CTAB capping developed by Wijaya and coworkers based on a round-trip phase
agent presents a surface different to that of the more “bare” transfer ligand exchange procedure wherein CTAB-capped rods
surfaces of gold nanoparticles capped with citrate or similar are first transferred into a highly-concentrated dodecanethiol-
ligands and thus specific methods have been developed for its acetone solution, centrifuged to remove excess thiol, and then
functionalization. In general, the stability of CTAB-capped rods heated in the presence of an alkyl thiol acid such as MUA
is known to be poor under a variety of conditions including until aggregation occurs, after which the rods become soluble
high salt content, low CTAB concentration, and addition of in aqueous environment (Figure 9b).[118] The question of what
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