Various gold nanoparticle shapes such as spheres, rods, wires, and cubes have been prepared and characterized using standard processing techniques by researchers. By varying the shape of the materials, their vibrant optical properties can be suitably tuned from the visible to the IR region of the spectrum. Also enhanced plasmonic properties make Au nanorods of interest for a variety of sensing and biological applications.
Synthesis

Seed mediate process

As per a method outlined in the literature, Au nanorods are synthesized in a two-step seed mediate Process. In this method the surfactant cetyltrimethylammonium bromide (CTAB) is used as a surface passivant. In this mechanism, Au nanoparticle seeds are introduced to a growth solution containing excess CTAB and HAuCl from which the nanorods are grown off the surface of the seeded nucleates. The final rod-like structure possesses a surfactant bilayer on the surface, thus imparting a significant positive charge to the materials, which is the driving force for this solution stability.
Layer-by-layer assembly

The layer-by-layer (LBL) assembly method, combined with the seeded growth technique, has been used to deposit gold shells on the surface of hematite (α-Fe2O3) spindles. The LBL method yields dense coatings of preformed Au nanoparticles, while AuCl−4 ions are further reduced by a mild reducing agent, thicker, rough nanostructured shells can be grown.
Green synthesis

Gold nanorods can be prepared by electrochemically reducing gold salts in a concentrated surfactant solution.

Georgia Tech University researchers describe a wet or 'green' synthesis procedure for the synthesis of gold nanorod (Au NR). The procedure uses Silver nitrate, Ascorbic acid, Sodium borohydride, seed solution, gold solution, & CTAB. Electro deposition methodUsing another procedure, stable Au nanorods with a unidirectional pin-like morphology have been prepared on the surface of glassy carbon electrodes via a potential-step electro deposition method from H2SO4 solution containing Na[AuCl4]. In the presence of cysteine as an additive, there is significant enrichment of Au(100) and Au(110) facets in contrast to the spherical Au nanoparticles deposited in the absence of cysteine.
Uses of gold nanorods

By varying the shape of the materials, their vibrant optical properties can be suitably tuned from the visible to the IR region of the spectrum. Also enhanced plasmonic properties make Au nanorods of interest for a variety of sensing and biological applications. Gold nanorods are useful for the formation of many functional composite materials, for example, with hydrogel, polymers, silica, and bacteria. Gold nanorods also have an axial surface plasmon resonance (SSPR), though one-third that of the LSPR, is still many orders of magnitude greater than quantum dots and nanoshells. Gold nanorods also offer advantages of good biocompatibility, facile preparation, and conjugation with a variety of biomolecular ligands, antibodies, and other targeting moieties. They have therefore found wide applications in biochemical sensing, biological imaging, medical diagnostics, and therapeutics. Further, gold nanorods have found application in materials and optics, including polarizers, filters, and to improve the storage density in compact disks.