Plasmon-enhanced fluorescence from synergistic engineering of graphene oxide and sharp-edged silver nanorods mediated with castor protein for cellphone-based attomolar sensing

Environmentally benign plasmonic nanomaterials are increasingly used for sensing applications in nano-photonics and biomedical diagnostics. This study reports the utility of castor proteins as a unique biomaterial for plasmonic sensing application. We demonstrate an intriguing strategy to obtain rod-shaped silver nanoparticles (AgNPs) using biocompatible castor protein as both the reducing and capping agent in a frugal UV-light induced one-pot rapid synthesis. These AgNPs thus synthesized were juxtaposed with propagating surface plasmon polaritons of surface plasmon-coupled emission (SPCE) platform to generate the photo-plasmonic coupling phenomenon in the three nano-configurations of spacer, cavity, and extended cavity. Dramatically enhanced, sharply directional and distinctly polarized fluorescence enhancement was achieved using AgNPs on account of the sharp edges and pointed ends. The observed 1000-fold fluorescence enhancement is attributed to the hybrid coupling of the pi-plasmons of graphene oxide (GO) with the AgNPs supporting lightning rod effect on account of the rod-shaped nano-morphology. Such realization of abundant plasmonic hotspots over the SPCE substrate assisted in attaining attomolar sensitivity. The performance of the subject platform was further validated using MATLAB simulations and user-friendly smartphone-based detector system for decentralizing the lab-confined biosensing architectures. We firmly believe that the eco-friendly method developed for nanosynthesis and the user-friendly technique demonstrated using smartphone technology would immensely benefit the biosensing approaches in resource-limited settings.