Diffusion limited green synthesis of ultra-small gold nanoparticles at room temperature

Synthesis of biocompatible and very stable ultra-small gold nanoparticles (Au-USNPs, 1-3 nm) by green methods has posed a challenge for a long time. The well-established Turkevich method involves naturally occurring reducing agents, like citrate, but it yields particles with a mean diameter >= 20 nm. Here, we extend the Turkevich method to the low nanometer range and enables the formation of Au-USNPs by implementing a diffusion limited mechanism. We attained tight control over the particle growth by carring the process (i) at room temperature and (ii) using micro-molar amounts of poly-vinyl-pyrrolidone (PVP) in (iii) conditions of Brownian motion. Particle size distribution along with UV-vis spectroscopy data confirm that particle growth is diffusion limited and can be explained by particle migration and coalescence model. Resulting nanoparticles display (i) very high stability, (ii) biocompatibility and (iii) functionalizable surface. These Au-USNPs may easily find applications in biomedical engineering, medicine, biotechnology and related fields.