UV–visible transmission–absorption spectral study of Au nanoparticles on a modified ITO electrode at constant potentials and under potential modulation

Gold nanoparticles of a diameter of 11 nm were immobilized on a monolayer of amine-terminated siloxane coated on an ITO electrode. The potential dependent behavior of the particles was characterized by combined use of electrochemical and UV–Vis transmission–absorption spectroelectrochemical methods from not only the static but also the dynamic viewpoint. The gold nanoparticles showed a potential dependent shift of the plasmon absorption band due to charging–discharging of the particles; at more negative potentials, a sharper and greater absorption peak was observed at a shorter wavelength. This spectral change was also tracked by potential-modulated UV–Vis transmission–absorption (PMTA) spectroscopy. Analysis by the concerted use of PMTA signals and ac impedance revealed that the PMTA signal synchronizes with the change of the amount of charge on the particles. The potential step coulometry measurement enabled us to estimate the transferred amount of charge as being approximately 1500 electrons V−1 per particle, which is in good agreement with the spectral shift.