Kinetic studies on water-soluble gold nanoparticles coordinated to poly(vinylpyrrolidone): isotropic to anisotropic transformation and morphology Md. Habib Ullah • Tafazzal Hossain • Chang-Sik Ha Received: 1 March 2011 / Accepted: 28 May 2011 / Published online: 14 June 2011 Ó Springer Science+Business Media, LLC 2011 Abstract The growth kinetics, isotropic-to-anisotropic transformation, structural properties and surface mor- phology of polyvinylpyrrolidone (PVP)-coordinated gold nanoparticles are reported in this work. The reduction of gold ions, kinetics, and growth mechanism of gold nano- particles, and the coordination between PVP and gold are explored for the first time in this single report. The layer- by-layer growth mechanism (adsorption of gold ions to the nuclei and their subsequent reduction) was observed in the growth of isotropic nanoparticles during the initial stage of the reaction, whereas the Ostwald ripening mechanism (growth of larger particles at the expense of smaller par- ticles) was observed in the growth of the anisotropic nanoparticles in the later stage of the reaction. The surface plasmon resonance band for the anisotropic nanoparticles (average size for a typical sample was ca. 9 nm) was blue- shifted (20 nm) toward that of the isotropic nanoparticles (whose average size is much smaller than that of the anisotropic nanoparticles). The increased effective electron density on the surface of anisotropic particles was the cause of this blue shift. The resultant gold colloids were very stable because the PVP molecules were coordinated through both the C–N and C=O groups, instead of the C=O group alone. The positions of the surface plasmon band and morphology of the gold products were strongly dependent on the amount of PVP. Introduction Colloids of noble metal nanoparticles exhibit a strong optical resonance phenomenon that is absent in the bulk phase. The surface plasmon resonance is the coherent excitation of all of the ‘free’ electrons within the conduc- tion band, leading to an in-phase oscillation [1]. For very small particles less than 2 nm in size, the plasmon oscil- lation is strongly damped and exhibits a monotonically increasing absorbance toward higher energies [1–3]. This is because the electron density in the conduction band becomes very small. For larger particles whose size is still small compared with the wavelength of light, excitations of the surface resonance can take place in visible light and give rise to a characteristic surface plasmon band [1]. The plasmon band of a specific metal is sensitive to the nano- particle size, shape, local dielectric environment, and interparticle gap [1–16]. In addition to their role in optical resonance, metal (e.g., silver, gold, and platinum) nano- particles play important roles in many different areas, such as electronics, catalysis, information storage, and surface- enhanced Raman scattering (SERS) applications. Gold is an important noble metal. Gold nanoparticles are a promising candidate for biomedical applications, such as drug delivery, biodetection, and biolabeling, which require nanoparticles that are nontoxic and highly soluble in water as well as in physiological solutions [17–21]. The size distri- bution of gold nanoparticles is considered to be important for their biomedical applications. The size-dependent cytotox- icity of gold nanoparticles was recently studied by Pan et al. [22]. They reported that very small gold compounds and Md. Habib Ullah  C.-S. Ha (&) Pioneer Research Center for Nanogrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea e-mail: csha@pnu.edu Md. Habib Ullah  T. Hossain Department of Physics, School of Natural Science, American International University-Bangladesh, Banani, Dhaka 1213, Bangladesh 123 J Mater Sci (2011) 46:6988–6997 DOI 10.1007/s10853-011-5667-5