Germanium – polyelectrolyte – gold nanoparticle composites were prepared and characterized using FTIR-ATR spectroscopy and scanning electron microscopy. The germanium (Ge) element served as internal reflection element and the buildup of the layered system was followed in situ. Positively charged polyelectrolyte poly (allylamine hydrochloride) (PAH) adsorbs spontaneously on negatively charged Ge. Citrate-stabilized gold nanoparticles can then be adsorbed onto the PAH layer. Upon illumination of the device with visible light a prominent absorption over the entire mid infrared region is observed which is due to intervalence band transitions in Ge. The strong infrared signals are evidence for holes in the valence band of the Ge semiconductor, which arise due to electron transfer to the gold nanoparticles (GNP). The electron transfer, as evidenced by the holes in Ge, is affected by the nature of the gap between the Ge and the GNP. Increasing the gap by adsorbing polyelectrolyte multilayers hinders the electron transfer. Also heating and vacuum have a pronounced effect. The device is proposed as a sensor, where the sensing event is transduced into an optical signal in the infrared, as demonstrated for a thiol molecule. The thiol has a large affinity for the gold and therefore affects the germanium – gold nanoparticle gap. This reduces the electron transfer and therefore the absorption in the infrared upon illumination with visible light. Removal of the thiol from the solution leads to a recovering of the signal.

By using a polyelectrolyte layer gold nanoparticles have been assembled onto a Ge internal reflection element. Upon illumination with visible and near infrared light a strong infrared absorption has been observed, which can be traced to intervalence band transitions in Ge. This reveals the existence of holes in the Ge near its valence band edge. The switching between bright and dark states is faster than 160 μs and the device acts as an infrared modulator. The effect develops with a peculiar kinetics, which may indicate the development of an interfacial layer between germanium and gold that allows efficient electron transfer upon illumination.

In this work in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy in a flow-through cell was used to study the effect of visible light irradiation on bovine serum albumin (BSA) adsorbed on porous TiO₂ films. The experiments were performed in water at concentrations of 10^−6 mol/l at room temperature. The curve fitting method of the second derivative spectra allowed us to explore details of the secondary structure of pure BSA in water and conformation changes upon adsorption as well as during and after illumination by visible light. The results clearly show that visible light influences the conformation of adsorbed BSA. The appearance of a shift of the amide I band, in the original spectra, from 1653 cm^−1 to 1648 cm^−1, is interpreted by the creation of random coil in the secondary structure of adsorbed BSA. The second derivative analysis of infrared spectra permits direct quantitative analysis of the secondary structural components of BSA, which show that the percentage of α-helix decreases during visible light illumination whereas the percentage of random coil increases.