Electrochemical Characterization of Water-Soluble Au25 Nanoclusters Enabled by Phase-Transfer Reaction

We report the synthesis and electrochemical characterization of a new water-soluble Au-25 cluster protected with (3-mercaptopropyl)sulfonate. The presence of sulfonate terminal groups on the surface of the cluster enabled facile phase transfer of the water-soluble cluster to organic phase by ion-pairing with hydrophobic counterions. The phase-transferred form of the water-soluble Au-25 cluster was found to retain its integrity and allowed investigation of its electrochemical properties in organic media. The voltammetric investigation of the phase-transferred Au-25 cluster in mixtures of CH2Cl2 and toluene has revealed that the cluster exhibits the characteristic Au-25 peak pattern, but the electrochemical HOMO-LUMO energy gap of the cluster varies from 1.39 to 1.66 V depending on the solvent polarity. The origin of the solvent dependence is explained by the electrostatic field effect of the sulfonate anion on the redox potentials of the Au-25 cluster.