Template Synthesis of Subnanometer Gold Clusters in Interfacially Cross-Linked Reverse Micelles Mediated by Confined Counterions

A cationic surfactant with a triallylammonium headgroup was cross-linked photochemically in the presence of a hydrophilic dithiol in the reverse micelle (RM) configuration. The interfacially cross-linked reverse micelles (ICRMs) are unusual templates for nanomaterials synthesis. Our previous work indicated that the ICRMs could extract anionic metal salts such as tetracholoroaurate into the hydrophilic interior, and the entrapped aurate was reduced without externally added reducing agent to form subnanometer luminescent gold clusters [Zhang, S.; Zhao, Y. ACS Nano 2011, 5, 2637-2646]. In this work, the bromide counterions were established as the reducing agent in the template synthesis. The reduction of tetrachloroaurate was proposed to happen through ligand exchange on the aurate by the bromide ions, reductive elimination of halogen, and disproportionation of the Au(I) intermediate. The size of the gold clusters could be tuned rationally by the water-to-surfactant ratio (W(0)) and the reducing agent. Monodisperse Au(4) and Au(9-10) clusters as well as larger Au(18) and Au(23) clusters were obtained from the ICRM templates. The as-prepared, metastable gold clusters were subject to reconstruction triggered by ligand exchange on the surface but could be stabilized through proper surface protection using a chelating dithiol.