Plasmon-enhanced fluorescence for sensitive and selective mercury ion (Hg sup(2+)) detection

Abstract

Plasmon-enhanced fluorescence (PEF) has emerged as an interesting platform for biosensing and quantum applications. The strong enhancement of emission from fluorophores arises due to an increase in the excitation and radiative decay rates of fluorophores in the vicinity of the metal surface. Furthermore, the excitation of surface plasmons on the metal accentuates the enhancement mechanism. A PEF-based system combines high sensitivity, photostability, and selectivity, making it a promising candidate for environmental monitoring and for the detection of trace levels of mercury ions (Hg sup(2+)) in aqueous media. Here, we report a study on PEF of CdSe-ZnS core-shell quantum dots and Rhodamine B (RhB) using silica-coated gold nanoparticles. Our observations indicate a strong enhancement in the emission intensity of these fluorophores in the vicinity of the metal nanoparticles. Along with this increase in intensity, there is a decrease in the emitter's lifetimes when mixed with the metal nanoparticles. Our study also shows that the enhancement is maximum at the plasmonic resonance wavelength, implying that this is indeed a plasmon-enhanced process. We performed finite-difference time-domain (FDTD) simulations to numerically investigate the electric-field enhancement in the immediate vicinity of silica-coated gold nanostructures and to determine the spontaneous emission characteristics of emitters. Furthermore, we use this mechanism to detect Hg sup(2+) at ultra-low levels in aqueous media. The system leverages PEF and dynamic quenching mechanisms to achieve a limit of detection (LOD) in the femtomolar (fM) range, surpassing previously reported nanocomposite-based sensors.