Identification of the key steps in the self-assembly of homogeneous gold metal nanoparticles produced using inverse micelles

Abstract

The self-assembly of gold nanoparticles (Au NPs) using polymer-encapsulated inverse micelles was studied using a set of advanced X-ray techniques (i.e. XAFS, SAXS) in addition to DLS, UV-vis spectroscopy and TEM. Importantly the combination of these techniques with the inverse micelle approach affords us detailed insight and to rationalize the evolving molecular chemistry and how this drives the formation of the Au NPs. We observe that the mechanism comprises three key steps: an initial fast reduction of molecular Au(III) species to molecular Au(I)Cl; the latter species are often very unstable during the self-assembly process. This is followed by a gradual reduction of these molecular Au(I) species and the formation of sub-nanometric Au clusters which coalesce into nanoparticles. It was also found that addition of small amounts of HCl can accelerate the formation of the Au clusters (the second phase) without affecting the final particle size or its particle size distribution. These findings would help us to understand the reaction mechanism of Au NP formation as well as providing insights into how NP properties could be further tailored for a wide range of practical applications.