Surface-Enhanced Fluorescence on SiO2-Coated Silver Island Films

Abstract

Fluorescence is one of the molecular spectroscopic properties that is enhanced by placing the molecule on the rough surface of a coinage metal. The surface-enhanced fluorescence (SEF) can be directly observed in steady-state fluorescence experiments. The observations are the results of a delicate balance between the enhanced emission and the quenching due to energy transfer to nonradiative surface plasmons. In the present report, SiO₂-coated silver films were fabricated at varying dielectric thickness. The surface of the films was analyzed with the use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM confirms the surface roughness and XPS analysis indicates that the SiO₂ coverage was successful. SEF and SERS (surface-enhanced Raman scattering) were observed on active 6, 10, and 14 nm silver films coated with SiO₂. Similar results were obtained with a 6 nm silver film coated with 6 nm SiO. The SEF work was carried out on fluorescent molecules with different quantum yield, and the typical enhancement factor obtained for the fluorescent signal was approximately 10. Both the SiO₂ and SiO overlayers provide stable surfaces with well-defined hydrophilic properties. Such stable constructions have applicability towards the advancement of SERS and SEF as routine analytical techniques in bio- and chemical sensors.