The competitive adsorption of citrate-capped Ag and Au nanoparticles (∼25 nm in diameter) onto a poly(4-vinyl pyridine) (P4VP) surface has been investigated by means of Raman scattering spectroscopy. The P4VP film prepared on a glass slide was too thin for its normal Raman spectrum to be observed, but the Raman peaks of P4VP could be detected upon the adsorption of Ag and/or Au nanoparticles onto the film, due to the surface-enhanced Raman scattering (SERS) effect associated with the localized surface plasmon of Ag and/or Au nanoparticles. Neither quartz crystal microbalance nor atomic force microscopy (AFM) nor scanning electron microscopy (SEM) methodologies can distinguish between Ag and Au nanoparticles during their adsorption onto P4VP, but it is possible through Raman scattering spectroscopy because Ag (though not Au) nanoaggregates are SERS active at 514.5 nm excitation, while both Ag and Au nanoaggregates are SERS active at 632.8 nm excitation. Coupled with the AFM data, we were thus able to infer that about 120 Ag nanoparticles per 1 μm<sup>2</sup> were adsorbed, along with 60 Au nanoparticles per 1 μm<sup>2</sup>, onto the P4VP film over a period of 1.5 h from a 1 : 1 mixture of Ag and Au sols at 1.6 nM each.

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