Quenching attenuation and fluorescence augmentation using plasmonic gold nanourchin and dielectric photonic crystal hybrid interface for mercury sensing

Photonic crystal enhanced fluorescence has emerged as a versatile technology for environmental and human health monitoring owing to high sensitivity provided by fluorescence emission amplification, lifetime reduction, and collection efficiency improvement. When plasmonic nanoparticles are used in combination with photonic crystal surfaces to provide fluorescence-enhancing photonic-plasmonic resonator hybrids, quenching phenomena observed in the 'zone of inactivity' represent a performance bottleneck. In particular, although plasmonic Au is an excellent plasmonic material for fluorescence enhancement, its use requires the incorporation of a spacer layer to circumvent quenching effects. In this work, we exploit the properties of the radiating guided mode resonance (GMR) model and sharp-edged plasmonic Au nano-urchins (AuNUs) to realize suppressed quenching and augmented fluorescence output without the use of an optical prism or microscope objective. Rigorous coupled-wave analysis (RCWA) and finite element method (FEM) analysis are used to provide insights corroborating experimentally observed 100-fold dequenched signal intensity. Multiphysics simulations of a fluorescent radiating dipole with different orientations and placements within the AuNU-PC system validate the experimentally observed polarization selectivity. The enhanced local density of states rendered by the synergistic coupling of core-tip plasmons of AuNUs and the GMR of the underlying photonic crystal were applied to demonstrate a 2 part-per-billion limit of detection for mercury (Hg²⁺) ions, thereby presenting a representative example for a quench-free chem-biosensing interface under controlled conditions.