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

Abstract

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.

Plasmon-Coupled Directional Emission from Soluplus-Mediated AgAu Nanoparticles for Attomolar Sensing Using a Smartphone

“…In our case, in RK optical configuration, the emission is expected at angles where minimum reflectivity is expected theoretically, in line with our earlier works. 7,19,35 In this context, we have carried out TFClac simulations to theoretically understand the expected angle of emission and to further correlate them with the experimental values. From Figure 2a,b, we note that a sharp dip in the reflectivity is observed for p-polarized or transverse magnetic component of light.…”

“…3−5 Dissimilar metallic or plasmonic nanofilms have been explored to study this phenomenon, along with different thicknesses of the dielectric layer to comprehend the waveguide and plasmon modes. 3,6 SPCE has endowed researchers with opportunities for exploring different optical phenomena, 7 chemical processes, 8 and biophysical sensing applications. 9−11 While plasmonic nanothin films support SPPs propagating over the continuous film, the plasmonic nanoparticles (NPs) support localized surface plasmon resonance (LSPR) sustaining intense electromagnetic (EM)-field localization in the nearfield.…”

“…13 Recently, research efforts have been devoted to synergize PEF and SPCE into a single hybrid platform. [7][8][9][10][11]14 Generally, the enhancement factor of SPCE substrate is 5−10-fold as compared to traditional luminescence. [7][8][9][10][11]15 Lakowicz and coworkers have demonstrated the significance of silver (Ag) NPs spacer layer engineering on SPCE platform to further augment the enhancement to 60-fold.…”

“…[7][8][9][10][11]14 Generally, the enhancement factor of SPCE substrate is 5−10-fold as compared to traditional luminescence. [7][8][9][10][11]15 Lakowicz and coworkers have demonstrated the significance of silver (Ag) NPs spacer layer engineering on SPCE platform to further augment the enhancement to 60-fold. 15 With this as a preliminary step for combining PEF and SPCE, diverse nanoarchitectures have been explored to further augment the SPCE enhancements.…”

“…7−11 The near-field optical confinement provided by the plasmonic NPs generate regions of intense EM "hotspots", when coated on a metal thin film. 16,17 Although a variety of NPs including low-dimensional materials, 18 self-assembled soret colloids, 7 and intermetallics 19,20 have been studied for their photoplasmonic contribution to SPCE, physicochemical evaluation of gold (Au) NPs has been inadequate on account of inevitable quenching of fluorophores located in distances <5 nm. In comparison to other coinage NPs (Ag, Pt, Pd), AuNPs exhibit manifold functionalities due to high stability with precise geometries, extraordinary optoelectronic properties, superior biocompatibility, tunable biophysicochemical properties and ability for multifunctionalization with inorganic/ biological/organic ligands.…”