Electrochemical characterization of water-soluble Au ₂₅ 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 CH ₂Cl ₂ 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.