ORIGINAL PAPER Direct monitoring of gold nanorod growth Seong S. Seo & Xiaohong Wang & Davoy Murray Received: 17 July 2007 / Revised: 19 January 2008 / Accepted: 18 March 2008 / Published online: 21 May 2008 # Springer-Verlag 2008 Abstract The seed-mediated growth of gold nanorods is shown to be strongly dependent on the reaction time and chemical environment of the reaction solution. The versatile seed-mediated approach in aqueous surfactant solutions has been used in this study for the synthesis of gold nanorods. Changes in the aspect ratio of gold nanorods were reflected in shifts of the plasmon resonance peaks and were monitored using UV-Visible absorption spectroscopy (UV- Vis) to follow the different stages of gold nanorod formation as a function of time and varying amounts of silver ion. Unlike the use of strong reducing agents to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. Therefore, cyclic voltammetry was used to electrochemically interrogate the entire reaction from gold seed to gold nanorod as a function of time. Data obtained revealed that time-dependent gold species are involved in gold nanorod formation. Keywords Gold nanorods . Surface plasmon resonance . Cyclic voltammetry . Seed-mediated growth Introduction In recent times, gold nanorods have generated great interest because of their shape-dependent optical properties and potentially wide range of applications [1–8]. Many efforts have been devoted to synthetic methods to produce gold nanorods with controlled size and shapes. Several different approaches such as electrochemical [9], photochemical [10, 11], and seed-mediated methods have been extensively used to grow gold nanorods [12–13]. The ability to control the shape of nanocrystals is an important and fundamental challenge to material scientists. Though a number of empirical procedures have been reported to prepare all different shapes of nanoparticles, relatively little is under- stood about the mechanisms that determine crystal struc- tures and morphology [14–17]. The versatile seed-mediated approach in aqueous surfac- tant solutions has been adopted in this study for the synthesis of gold nanorods. The seed solution of gold nanoparticles was prepared by the reduction of Au 3+ particles (HAuCl 4 ) to Au 0 in the presence of NaBH 4 .A growth solution was also prepared, comprising Au 3+ particles, a weak reducing agent (L-ascorbic acid), a surfactant (cetyltrimethylammonium bromide), and the seed solution. UV-Visible absorption spectroscopy (UV-Vis) is a useful qualitative tool for monitoring the gold nanorods, as at each stage, characteristic surface plasmon-resonance modes are exhibited. The major change in the absorption spectrum of gold nanorods is the position of the well- known longitudinal mode. The position of the optical resonance wavelength (λ max ) is influenced by the geometric factors of width, length, and aspect ratio. Red shifts of the longitudinal plasmon peak correspond to an increase of the aspect ratio, while a blue shift corresponds to a decrease in aspect ratio. UV-Vis method was used to monitor the changes in the shape of gold nanorods as functions of time and various concentrations of silver ions [18–19]. Cyclic voltammetric (CV) measurements are sensitive to the chemical environment of solution species so that changes in composition of intermediates also can be monitored. CV was introduced into mechanistic studies to Ionics (2009) 15:67–71 DOI 10.1007/s11581-008-0223-2 S. S. Seo (*) : X. Wang : D. Murray Department of Natural Sciences, Albany State University, 504 College Dr., Albany, GA 31705, USA e-mail: seong.seo@asurams.edu