Fabrication of stable bimetallic nanostructures on Nafion membranes for optical applications

Novel stable crystalline bimetallic silver-gold nanostructures homogeneously dispersed on Nafion were prepared by galvanic substitution of a vacuum evaporated silver island film with gold. The method allows control of the composition of the bimetallic nanostructures and tuning of optical properties. Formation of the nanoalloy was monitored by UV-Vis absorption, SEM-EDX, XRD, AFM, ATR-FTIR, and Raman scattering. Bimetallic nanostructure growth was monitored by UV-Vis absorption and surface-enhanced Raman scattering (SERS), by casting an aliquot from a dilute solution of 2-naphthalenethiol (2-NAT) on the composite surface. SERS intensity increases with galvanic substitution, reaching a maximum, and providing a material that delivers SERS enhancement several times higher than those obtained with regular silver and gold island films. Optical enhancement is also fairly homogeneous throughout the treated Nafion surface; this is demonstrated by mapping the average SERS intensity of a mixed Langmuir-Blodgett monolayer bis(benzimidazo) perylene and stearic acid excited with 514 and 633 nm laser lines.