Nanoporous gold films prepared by sputtering-dealloying combination for total internal reflection SERS measurement

Abstract

Nanoporous gold (NPG) is an advanced functional material with both propagating and localized surface plasmon resonance (PSPR and LSPR) effects. In this work, uniform NPG films with controlled thickness and small pore size were easily prepared by sputtering deposition followed by low-temperature dealloying. Using slab waveguide spectroscopy, the LSPR absorption peak of the NPG film was measured to shift from 566 nm to 586 nm by changing the surrounding refractive index from 1.333 to 1.368. Total internal reflection (TIR) SERS spectra for Rhodamine 6G (R6G) and Nile blue (NB) molecules adsorbed in the NPG film were investigated with a prism coupler. Upon increasing the incident angle (θ) from the critical value, the intensity of the SERS signal excited with the s-polarized laser beam of 532 nm wavelength gradually decreases but with the p-polarized laser beam exhibits a peak at θ ≈53°. The peak intensity is 2 times stronger than that excited with normal incidence of the same laser beam. The PSPR mode of the NPG film can be excited at 785 nm wavelength, which leads to a strong SERS signal of NB. In contrast, the LSPR effect at 785 nm wavelength is too weak to make the SERS signal undetectable with normal incidence. The findings indicate that the TIR geometry can bring about at least a twofold enhancement of SERS sensitivity of NPG film relative to the conventional normal incidence and that the contributions of LSPR and PSPR effects to the SERS sensitivity of NPG film are different at different incident wavelengths.