Enhancement of infrared signals from polyelectrolyte adsorbed on gold nanoparticles (GNPs) was studied in situ by attenuated total reflection infrared spectroscopy. Nanoparticles and polyelectrolytes were deposited using layer-by-layer techniques, and the infrared signal was studied as a function of particle size, particle density, and distance from particle surface. It was observed that enhancement is more pronounced for larger nanoparticles and it decreases with increasing distance from the particle surface. Furthermore, at high GNP coverage, the signal from the first polyelectrolyte layer is particularly enhanced, and the signal increases slowly with time, in contrast to subsequent layers. We assign this to polyelectrolyte adsorption within narrow gaps between nanoparticles, where the electric field is enhanced. Furthermore, enhanced absorption was observed in the gap between the GNPs and the germanium internal reflection element, which was confirmed by polarized measurements. This enhancement is more pronounced for silver particles, and it represents a promising route for analysis of surfaces by infrared spectroscopy.

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.

Attenuated total reflection infrared (ATR-IR) spectroscopy is used to study the adsorption of gold and silver nanoparticles and the layer-by-layer (LBL) growth of polyelectrolyte multilayers on a Ge ATR crystal. The Ge ATR crystal is first functionalized using positively charged polyelectrolyte poly(allylamine hydrochloride) (PAH). Then citrate-stabilized gold or silver nanoparticles are adsorbed onto the modified Ge ATR crystal. When gold or silver nanoparticles are adsorbed, a drastic increase of the water signal is observed which is attributed to an enhanced absorption of IR radiation near the nanoparticles. This enhancement was much larger for the silver nanoparticles (SNP). On top of the nanoparticles multilayers of oppositely charged polyelectrolytes PAH and poly(sodium 4-styrenesulfonate) (PSS) were deposited, which allowed to study the enhancement of the IR signals as a function of the distance from the nanoparticles. Furthermore, adsorption of a thiol, N-acetyl-l-cysteine, on the nanoparticles confirmed the enhancement. In the case of SNP an absorbance signal of about 15% was observed, which is a factor of about 40 times larger compared to typical signals measure without nanoparticles.