Surface-Enhanced Raman Scattering and Infrared Absorption with Plasmonic Ag-SiO₂ Nanocomposite Films for High-Sensitivity Analyte Sensing

Because of the localized surface plasmon resonance (LSPR), molecules adsorbed at the surfaces of the metallic nanostructures exhibit a strongly enhanced electric field. The enhanced field around the nanoscale morphology amplifies the Raman scattering signal and excites the local field across the infrared (IR) vibrational modes, enabling high-sensitivity chemical and biological sensing. Herein, we achieve strong enhancement in surface-enhanced Raman and IR absorption (SERS and SEIRA) spectroscopies with scalable nanocomposite substrates, consisting of ultrathin nanoscale plasmonic Ag-SiO₂ films fabricated by cosputtering. The plasmonic hotspots of a 25-nm-thick nanocomposite film successfully amplify the SERS and SEIRA signals of methylene blue with ultralow detection of 10^–10 M in the SERS mode. The LSPRs of topping silver particles and near-field plasmonic coupling among the neighboring granular silver nanoparticles and topping particle make a combinatorial impact and lead to signal enhancement, with a predicted factor of up to 10⁹ orders of magnitude for Raman scattering and 10² under the IR vibrational modes. These simple nanocomposite films with a high density of plasmonic hotspots are highly suitable as sensors-on-a-chip for label-free chemical and biomolecular identification with outstanding performance.