Observing Ultra-Small Gold Cluster to Plasmonic Nanoparticle Evolution in a One-Pot Aqueous Synthesis

Abstract

Studies of metal nanoclusters can provide valuable information on how various properties evolve in the nanoscale materials. Previously often mixed-sized products have been separated in order to study their size-related properties. We present here a one-pot aqueous synthesis protocol that allows direct observations of the evolutions of size-dependent optical and electrochemical properties of the gold nanoclusters while the clusters grow from a few-atom, ultra-small size to the plasmonic nanoparticle stage. Our selection of a mixed ligand system comprising of methionine and CTAB for the aqueous synthesis of gold nanoclusters by the chemical reduction of gold ions led to the slow growth of the nanoclusters making direct observations of the cluster growth and concomitant evolution of properties possible. UV-visible spectrophotometric and dynamic light scattering studies demonstrated the size-dependent transition from the molecule-like, non-plasmonic to plasmonic properties in the evolving nanoclusters. Differential pulse voltammetry studies demonstrated how the electrochemical properties changed from the molecule-like redox behavior to a quantized double-layer charging type with the growth of the nanocluster with time. Thermogravimetric analysis, MALDI-TOF mass spectrometry, etc. were employed to further characterize the nanocluster systems. This report will inspire further studies in understanding the evolution of the size-dependent properties of the nanoscale materials.