Theoretical analyses and chemical sensing application of surface plasmon resonance effect of nanoporous gold films

Nanoporous gold films (NPGFs) are chemically robust and thermally stable, have large specific area and salient surface plasmon resonance (SPR) effect. Due to these features, NPGFs are quite applicable for high-sensitivity SPR sensors. In this study, the SPR effect of NPGFs was theoretically analyzed and the dispersion relation was obtained for propagating surface plasmons at the NPGF/air interface. The optimal thickness of NPGF required for optimizing its SPR sensing performance was determined to be ~60 nm. Large-area, uniform and ultrathin NPGFs were prepared by a two-step approach involving sputtering deposition and chemical dealloying. The SPR resonance band in the visible-near-infrared region and the sensing properties of NPGFs were measured with the Kretschmann prism-coupling configuration. Porosity of the NPGF was determined to be ~0.38 by fitting the measured resonance wavelengths based on a combination of the Fresnel formula and the Bruggeman dielectric constant approximation theory. Since the non-modified NPGFs are hydrophilic and enable effective enrichment of bisphenol A (BPA) in water, the NPGF-SPR sensor can easily detect BPA at concentrations as low as 5 nmol∙L^-1. After hydrophobilization of NPGFs, the sensor enables detection of trace amounts of benzo[a]pyrene (BaP) in water, with the detection limit being 1 nmol∙L^-1.