Nernstian performance of the optical response of monolayer-protected gold cluster films

Multilayer films of Au147 hexanethiolate monolayer-protected clusters (C6-Au MPCs) of 1.6-nm diameter cores were formed onto indium tin oxide electrodes and the film optical changes arising from sequential single electron removal from the MPC cores in aqueous solutions were followed by UV–vis absorption spectroelectrochemistry. The film absorbance was found to increase proportionally to the MPC core anodic charge up to MPC+5 during the voltammetric scans. When the potential was swept at sufficiently slow scan rates, it was possible to resolve a staircase-shaped curve increment of absorbance with potential. This incremental increase of absorbance, concurrent with each anodic charging event, evidenced that the spectral changes were induced by the discrete charging of the MPCs. The values of the formal potential (E0′) and number of electrons (n) transferred in each charging step were determined from the voltabsorptometric measurements working in conditions close to the equilibrium, where Nernst equation can be applied. This spectroscopic approach yielded n = 1 and E0′values comparable to the voltammetric ones for charging steps up to MPC+5, further proving that the observed spectral features were intimately linked to the quantized capacitance charging of these tiny capacitors.