Towards Controlled Synthesis of Water-Soluble Gold Nanoclusters: Synthesis and Analysis

Abstract

Water-soluble gold nanoclusters with well-defined molecular structures and stability possess particular biophysical properties making them excellent candidates for biological applications as well as for fundamental spectroscopic studies. The currently existing synthetic protocols for atomically monodisperse thiolate-protected gold nanoclusters (AuMPCs) have been widely expanded with organothiolates, yet the direct synthesis reports for water-soluble AuMPCs are still deficient. Here, we demonstrate a wet-chemistry pH-controlled synthesis of two large water-soluble nanoclusters utilizing p-mercaptobenzoic acid (pMBA), affording different sizes of plasmonic AuMPCs on the preparative scale (∼7 mg). AuMPCs are essentially homogenous in size and are stable in solution and the solid state. Number of characterization methods were used to gain detailed information about the size, symmetry, molecular composition, and structure of these systems, i.e., high-resolution transmission electron microscopy, powder X-ray diffraction, NMR, UV-vis, thermogravimetry, and polyacrylamide gel electrophoresis. On the basis of the conducted experimental analyses and computationally aided predictions it can be evidenced that both clusters exhibit twinned face-centered cubic symmetry with the molecular composition of Au210-230(pMBA)70-80 and Au426-442(pMBA)112-115, referred from now on as Au250 and Au500, respectively. For future reference, toxicity of both gold clusters in various concentrations on cultures of Gram-positive and Gram-negative bacteria was investigated.