Effect of organic vapors on Au, Ag, and Au-Ag alloy nanoparticle films with adsorbed 2,6-dimethylphenyl isocyanide.

The physicochemical properties of metallic substrates are affected by the environment in different ways. It is generally difficult to determine these effects because the molecules in the environment interact weakly with metallic substrates. In this work, we demonstrate that even the effect of volatile organic compounds (VOCs) can be identified by utilizing the surface-enhanced Raman scattering of isocyanide molecules. The NC stretching band of 2,6-dimethylphenyl isocyanide (2,6-DMPI) adsorbed on Au, for instance, is blueshifted by 6 cm(-1) under an acetone flow and is redshifted by 20 cm(-1) under an ammonia flow. The same band of 2,6-DMPI adsorbed on Ag and Au0.5Ag0.5 alloy films is, however, redshifted equally by 8 and 13 cm(-1) under acetone and ammonia flows, respectively. This indicates that although the surface plasmons of Au0.5Ag0.5 alloy nanoparticles are clearly distinct from those of Ag (as well as Au) nanoparticles, both Au0.5Ag0.5 and Ag nanoparticles show a similar response to VOCs. These observations led us to conclude that the outermost parts of Au-Ag alloy nanoparticles are enriched with Ag atoms and that only the surfaces of metal nanoparticles, and not the bulk material, are affected by VOCs.