Exploring the potential of branched submicron channels-based structures in ultra-thin Ag films for SERS molecular sensing

Abstract

Noble metal-based nanostructures are actively studied due to their promising prospects for fabricating efficient substrates for Surface Enhanced Raman Spectroscopy (SERS)-based molecular sensing. In this work, we report the gross potential of SERS substrates based on branched submicron channels in ultra-thin Ag films. These films were fabricated by magnetron sputtering and then annealed in high vacuum conditions to induce branched channels growth by thermal dewetting. Different sets of nominally equal samples were tested during a determined period of time, using confocal Raman spectroscopy and methylene blue (MB) as Raman-active molecule, to assess their performance for SERS in ambient aging conditions. Micro-Raman intensity mapping studies demonstrated the emergence of “hot spots” enhancing the Raman signal in the branched submicron channels. A good mass sensitivity and fast spectral acquisition were achieved using these nanostructures, observing an exceptional spectral resolution and identifying all main vibrational states of MB in a few seconds for samples with a MB surface mass density of about 2 ng/mm2. A good spectral resolution was achieved even using shorter measurement times from 1.00 to 0.05 s, suggesting the potential for fast SERS determinations. Samples showed good reproducibility in the Raman spectral response during the tested period, demonstrating the marginal impact of the aging effects on the Raman signal enhancement and ensuring a time-stable SERS performance in the short term. Our results show that the proposed nanostructures are promising candidates for the development of substrates with competitive-sensitivity, time-stability, and fast SERS response, representing a simple and attractive alternative for efficient SERS-based molecular sensing.