Probing Plasmonic Effects on the Raman Activity of Ag Nanoparticle-Based Nanostructures through Terphenyl Diisocyanide Adsorption

Nanostructures consisting on Ag nanoparticle (NP) arrays produced by pulsed laser deposition in vacuum and having different morphology are tested as substrates for surface enhanced Raman spectroscopy (SERS) of terphenyl diisocyanide (TPDI). The SERS response strongly depends on the NP size and distribution; its intensity being much stronger in the case of nanostructures containing small NPs with typical size in the range of a few tens of nm and a characteristic interparticle spacing of a few nm. This behavior is related to the existence of plasmonic hot spots between these NPs upon excitation at wavelengths close to that of their characteristic surface plasmon resonance, which favors the Raman signal of TPDI molecules having both isocyanide groups attached to Ag NPs. No changes in the spectral response are observed when the concentration of the molecule varies in the range from 10<sup>–6</sup> to 10<sup>–3</sup> M, which suggests the saturation of the available hot-spot sites at low concentrations. Finally, we investigate the SERS response of covered nanostructures to prevent Ag tarnishing. The SERS response decreases exponentially as the covering thickness increases from 1 to 3 nm, which is related to the progressive reduction of the uncovered Ag surface available to TPDI molecules.