Self-assembled Au nanoparticles as substrates for surface-enhanced vibrational spectroscopy: optimization and electrochemical stability.

Three-dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self-assembly. Nanostructures consisting of one to 20 depositions of 13 nm-diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection-absorption spectroscopy. Surface-enhanced polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) is observed from Au NPs modified by the probe molecule 4-hydroxythiophenol. The limitation of this kind of substrate for surface-enhanced PM-IRRAS is discussed. The surface-enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au-NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au-NP depositions, and the absolute SERS intensities from different batches are reproducible within 20%. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between -800 and +200 mV (vs. Ag/AgCl/sat. Cl(-)).