Functional Gold Nanorods: Synthesis,

 Self-Assembly,and Sensing Applications

The fascinating size-dependent properties of noble metal nanoparticleshave created a great promise for their use in a varietyof electronic, optical, and biomedical applications. Gold nanorods,specifi cally, have received a great deal of attention dueto their unusual physical properties. The nanoscale confi nementof electrons on the surface of gold nanoparticles grantsthem shape- and size-dependent properties not seen in largerparticles.  Initially, spherical or quasi-spherical gold nanoparticlesreceived the most attention due to the ease of synthesisof such structures. This is perhaps unsurprising giventhat the spherical shape is often the most thermodynamicallyand kinetically favorable morphology. In order to access morecomplicated structures, it is necessary to fi nd reaction conditionswhich can break the propensity towards isotropic growthand instead direct the nanoparticle growth into an anisotropicdimension. The fi rst class of anisotropic nanoparticles to gainthe most popularity has been gold nanorods, which were fi rstsynthesized in the mid-1990s through anapproach based on electrochemical reductioninto rod-shaped templates.  Due tothe limitations of this technique such asthe low total yield of the procedure, morewidespread adoption of gold nanorods intoresearch did not occur until the advent ofwet-chemistry synthetic techniques, whichdid not appear until Murphy and coworkers’seminal work published in 2001. Continued improvements in syntheticmethodology have led to better reliabilityand have increased the shape-yield of rodsto greater than 90 percent.As synthetic capabilities improved, sodid the understanding of the physicalproperties of nanorods including their anisotropicoptical and electronic properties.Excellent reviews have been publishedthat describe the origins and modelingof the physical properties of gold nanorodsand so will not be included in thisreview in detail. Briefl y, gold nanorods, like spherical goldnanoparticles and other noble metal nanoparticles, have theability to absorb light of varying wavelength due to the creationof plasmon resonances on their surface. These plasmons representcollective oscillations of the electrons surrounding thenanoparticles and the intensity and wavelength of these surfaceplasmon resonances (SPR) can be highly shape- and sizedependent. Due to the anisotropic shape of gold nanorods,they display two separate SPR bands corresponding to theirwidth and length known as the transverse (TSPR) and longitudinalplasmon bands (LSPR). The TSPR is located at just above500 nm while the LSPR varies widely according to the nanorodaspect ratio and the overall size. Through careful synthesis, itis possible to create single crystalline gold nanorods with anLSPR anywhere from the visible (600 nm) all the way into thenear IR (1100 + nm) portion of the electromagnetic spectrum.The ability of nanorods to absorb near IR light makes them particularlywell-suited to biomedical applications since the absorbanceof the surrounding tissue in this region is low. This reviewwill not, however, focus on these applications as several excellentreviews have already been published on the topic. For nearly all applications, the ability to properly functionalizethe nanorod surface can determine the success or failureof the project. In general, the functionalization of gold nanorodscan be signifi cantly more challenging than the functionalizationof spherical particles, even through the well-knowngold-thiol chemistry, due to the unique surfactant capping of

as-synthesized nanorods. While spherical particles may be

directly thiol-coated during the synthesis or coated only with a

weakly-bound anion, gold nanorods are usually synthesized in

the presence cetyltrimethylammonium bromide (CTAB), which

binds more strongly to the nanorod surface. Complete or partial

aggregation can easily occur during functionalization if the

CTAB structure around the rods is disturbed, leading to loss of

desired optical properties. Thus, general functionalization strategies

as well as specifi c examples pertaining to nanorod applications

will be discussed.

Although the goal of a particular application may be to create

individual nanorods, their assemblies can also be highly desirable

due to the modulation of their physical properties as they

are brought close together. This review will cover the signifi cant

progress that has been made on controlling nanorod assembly

in the past several years, which has allowed for the production of

interesting structures such as nanorod chains, rings, and threedimensional

supercrystals. Importantly, these assembly techniques

have found signifi cant application in sensing and detection

of a variety of analytes including environmental toxins 

and biomarkers.  Thus, detection modalities based on the

anisotropic properties of rods such as their surface-enhanced

Raman scattering (SERS) ability will be examined in detail.

 Synthesis

There are various methods to produce gold nanorods with different

structures. The fi rst class of synthetic techniques that will

be discussed are the various aqueous wet-chemical CTAB-mediated

synthetic procedures which have become the most popular

as originated by Murphy et al and El-Sayed et al. While all

of these techniques produce crystalline nanorods, they can be

subdivided into those that lead to rods with single-crystalline

or pentahedrally-twinned structure. This is an important distinction

as the purity, length-scale, and further manipulations

can depend highly on this difference. The second class of techniques

are those based on reduction of gold inside a template

of some sort, most often an anodized aluminum oxide (AAO)

membrane, which produces polycrystalline structures in limited

quantities. Finally, several methods exist to synthesize

nanorods in organic solvents which generally lead to much different

morphologies including ultrathin rods and wires.

Silver-Mediated Synthesis of Single-Crystalline Nanorods

 Electrochemical Synthesis

The first report of reasonably high quality gold nanorods used

an electrochemical approach which was the precursor of the

most common seed-mediated procedure.  Reported by

Wang and coworkers, this approach utilized a two-electrode

electrochemical cell in which the gold anode provided the gold

source for the reaction while the template for rod-growth was

a mixed surfactant system of CTAB and tetradodecylammonium

bromide (TDTAB). Small amounts of acetone and hexane

additives were also present and the entire setup was sonicated

throughout the reaction. The presence of a silver plate,which was theorized to produce silver ions in solution, led to

increased rod yield and length.  Nanorods were synthesized

with aspect ratios anywhere from 1 to 7 with a corresponding

longitudinal plasmon as high as 1050 nm with rod diameters of

about 10 nm. Although the exact mechanism was not known,

it was theorized that TDTAB was the rod-directing agent andthat growth may have occurred on the surface of the electrode,

with sonication responsible for freeing the rods into solution.

El-Sayed and coworkers carried out a crystallographic examination

of these rods and determined that the majority of the

rods were single-crystalline in nature and grew along the [001]

direction, the same structure as that created by silver-assisted

seed-mediated .