Silver Soret Nanoparticles for Femtomolar Sensing of Glutathione in a Surface Plasmon-Coupled Emission Platform

Surface plasmon-coupled emission (SPCE) has emerged as an interdisciplinary, versatile sensing platform because of its highly directional, solid-state, and polarized emission. Here, we report the distinct properties rendered by silver Soret colloids (Ag-SCs) such as nanovoids and nanocavities to observe 104-fold enhancement in the emission intensity of omnidirectionally radiating emitter dipoles. Unlike earlier reports utilizing templated silver nanoparticles (AgNPs) in spacer or cavity architectures, here we employ template-free, linker-less Ag-SCs. The Purcell factor (maximum of 120.6) obtained using the finite-difference time-domain simulations for Soret nanocavities is in excellent agreement with the trend in emission enhancements obtained experimentally. The thermal gradient created by adiabatic cooling of AgNPs drives their thermodiffusion, resulting in monodisperse nanoparticle assemblies (Ag-SCs). In addition, we report an extended-cavity architecture with Ag-SCs, as a novel pseudo-metal–dielectric–metal (MDM) interface, for achieving 80-fold SPCE. This study also features the unique properties of Ag-SCs as interfacial nanomaterials on the SPCE platform to achieve femtomolar detection of glutathione (GSH). The quenching of fluorescence from the Alizarin Red S–boric acid (ARS–BA) complex upon the addition of Cu2+ ions and the dequenching upon the GSH addition studied with Ag-SCs as the spacer layer remarkably increased the sensitivity of the analyte. The uniform and intense electromagnetic-field confinement provided by these intricate architectures and hybrid interfaces, along with their ease of fabrication and versatility for a variety of analytes, is critical to achieving augmented SPCE. This is accomplished without compromising the reliability of detection, as demonstrated with the use of a cellphone camera, Commission Internationale de l’Eclairage color space, and luminosity plots for turn-on fluorescence. The emission images were acquired using an android-phone camera by aligning it with its angular emission, making it amenable for point-of-care diagnostics.