Potential Regulation of the Spectroelectrochemical Response of Monolayer-Protected Gold Cluster Films by Electrolyte Composition

The spectroelectrochemistry of multilayer films of hexanethiolate-protected Au 147 clusters (C6-Au 147 MPCs) supported on gold electrodes has been investigated by UV−vis reflectance spectroelectrochemical quartz crystal microbalance in aqueous solutions containing electrolyte anions with different degrees of hydrophobicity (PF 6 -, ClO 4 -, BF 4 -, and NO 3 - ). The multiresponse technique has enabled us to measure simultaneously potential-dependent reflectance and gravimetric changes in the film during the stepwise charging of the C6-Au 147 MPCs in different electrolytic media. Changes in the film reflectance, due to quantized charging of the C6-Au 147 MPCs, proved to be very sensitive to both the hydrophobicity and concentration of the electrolyte anion used. Thus, the onset potential for the charging current and consequently, the reflectance attenuation, shifted anodically with decreasing concentration/hydrophobicity of the electrolyte anion. Moreover, the magnitude of the film reflectance changes resulting from each single-electron-transfer process varied between electrolytes. Electrochemical quartz crystal microbalance (EQCM) measurements enabled us to rationalize the differences noted in the spectroelectrochemical responses in terms of dissimilar ion-binding strengths of the electrolyte anions with the hydrophobic MPCs. Hence, the results presented here demonstrate the feasibility of regulating the electro-optical performance of C6-Au 147 MPCs by simple chemical manipulation of the sequential electron transfer by solution ions.