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

Although there are numerous techniques for the synthesis of plasmonic (metallic Ag and Au) nanomaterials, eco-friendly and biocompatible methodologies that present diverse applications in nanophotonics and biomedical domains are seldom reported. Soluplus is a graft copolymer of polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol and is extensively used for improving the solubility and bioavailability of poorly water-soluble drugs. However, the utility of their amphiphilic chemical structure, strong UV-light absorbing, and bifunctional properties of the polymer matrix as well as its role as a solubilizer have not been explored for rapid synthesis of plasmonic bimetallic nanohybrids. Although a variety of nanoparticles (NPs) have been studied for surface plasmon-coupled emission (SPCE) spectroscopic applications, AgAu nanohybrids have not been examined in the subject platform hitherto. In this article, we demonstrate a synthesis route for obtaining AgNPs, AuNPs, and AgAu nanohybrids using soluplus as both the reducing and capping agent in a simple UV-light induced one-pot rapid technique. These hybrid nanomaterials were interfaced with propagating surface plasmon polaritons from a 50 nm Ag thin film to understand the plasmonic coupling phenomenon in spacer, cavity, and extended cavity nanoconfigurations. The obtained AgAu nanohybrids aided in addressing the three major long-standing challenges in SPCE technology development, namely, (i) unavoidable quenching in the presence of AuNPs, (ii) chemical instability in AgNPs, and (iii) intrinsic ohmic losses in plasmonic NPs. In addition to overcoming the above-mentioned caveats, unaccustomed >1200-fold sharply directional and polarized SPCE enhancements were achieved using AgAu nanohybrids. The multifold nanogaps generated in AgAu nanohybrid sustained multiple hotspots catering to attomolar sensitivity of the SPCE reporter molecule, rhodamine B (RhB). The proposed methodology to obtain dequenched as well as augmented SPCE enhancements is of utmost utility in biophysicochemical sensor development.