Quantized spectroelectrochemical behaviour of monolayer-protected gold cluster films assessed by reflectance spectroelectrochemical quartz crystal microbalance

The spectral changes occurring in multilayer films of hexanethiolate monolayer-protected Au147 clusters (C6–Au147 MPCs) as a consequence of quantized MPC core charging have been investigated in aqueous solutions using a multiresponse technique, UV–vis reflectance spectroelectrochemical quartz crystal microbalance (SEQCM). The joint technique, a combination of UV–vis near-normal incidence reflectance spectroelectrochemistry and electrochemical quartz crystal microbalance, has enabled us to follow both reflectance and gravimetric changes taking place in the MPC film concurrently with each single electron transfer event. Reversible film reflectance drops were observed upon anodic MPC charging, which were linearly dependent on the MPC charge state. The values of the formal potential and number of electrons transferred in each charging step, determined from the potential dependence of the reflectance changes, proved that the spectral features were induced by the discrete charging of the MPCs. Simultaneously, the gravimetric signal monitored with EQCM yielded values of the number of MPC-bound electrolyte ions as a function of the MPC redox state, both during voltammetric and potential step charging of the MPC films. Additionally, the dynamics of electron transfers in these multilayer MPC films has been investigated by electrochemical impedance spectroscopy (EIS). Thus, the film capacitance, the resistance to charge transfer, and the electron-transfer rate constant for MPC oxidation have been estimated.