Position-Dependent Fluorescence Enhancement in Core–Shell Nanoparticles: Synergistic Effects of Dipole Alignment and Multiemitter Coupling

Abstract

The localized surface plasmon resonance (LSPR) on the metal surface enhances the local electric field, which, under optimal conditions (e.g., controlled molecule–metal separation), significantly boosts the fluorescence intensity of adjacent fluorescent molecules. However, the fluorescence enhancement─a key mechanism for sensor signal amplification─is not universal for all fluorescent molecules and metal surfaces, as its occurrence and intensity depend on various factors, particularly the relative position and orientation of the emitter to the metal surface. The fluorescence enhancement of single or multiple quantum emitters near silver core–shell nanoparticles was systematically investigated using COMSOL. The model demonstrated that the maximum enhancement occurred when the quantum emitter’s positional alignment coincided with the plasmonic near-field distribution and its intrinsic dipole orientation. A significant suppression was observed when the emitter’s spatial configuration deviated from parallelism with the electric field vector and dipole axis. Comparative analysis further revealed that single-axis alignment with the localized electric field gradient yielded superior enhancement compared to multidirectional emitter arrangements. Furthermore, the study established a critical dependence of enhancement magnitude on emitter selection criteria, with molecules exhibiting high quantum yield and photostability showing preferential suitability for plasmon-enhanced applications, paving the way for advanced optical sensing technologies.