SERS substrates based on opal films with gold coating

Substrates for Surface-Enhanced Raman Spectroscopy (SERS) were fabricated by gold sputtering onto surface of synthetic opal films and their characteristics were studied at wavelengths $\lambda$ = 532 and 785 nm. Synthetic opal films were fabricated by self-assembly of spherical SiO$_2$ particles on vertical substrates. It was found that at the concentration of the analyte methylene blue equal to 10$^{-5}$ M the intensity of SERS at the wavelength of 785 nm increased with increasing amount of sputtered gold up to a certain optimal thickness exceeding 35 nm, while at the concentration of 10$^{-6}$ M this dependence was not observed. It is assumed that this is due to the complex amount-dependent morphology of the sputtered gold coating and the presence of “hot spots” of different strengths. For the best samples at a wavelength of $\lambda$ = 785 nm, the SERS enhancement factor was of 7 $\cdot$ 10$^4$ and a detection limit for methylene blue reached 3 $\cdot$ 10$^{-7}$ M that exceeds the results published for similar substrates previously. The SERS parameters obtained for $\lambda$ = 532 nm were less attractive, despite the additional enhancement due to this wavelength was at the edge of the photonic stop-band.