Nanowaveguide-Illuminated Fluorescence Correlation Spectroscopy at Micromolar Concentrations

Single-molecule microscopy and spectroscopy have transformed the real-time observation of molecular dynamics, providing unprecedented insights into molecular interactions. Among these techniques, Fluorescence Correlation Spectroscopy (FCS) has been instrumental in studying biological processes within live cells. Conventional FCS, with diffraction-limited volumes around one femtoliter, is optimal for single-molecule studies at nanomolar concentrations. However, it faces limitations when applied to physiological conditions, where concentrations are typically in the micromolar range thus requiring detection volumes on the order of 100 zeptoliters. In this study, we present the fabrication of dielectric nanowire waveguide devices consisting of a cylindrical titania core with a plasmonic metal cladding. This design overcomes the diffraction limit, achieving sampling volumes of approximately 170 zeptoliters. Furthermore, the near-field confinement of the transmitted light produces an illumination spot around 50 nm laterally and 20 nm longitudinally, making it particularly suited for studying single-molecule dynamics in cell membranes under physiological conditions.