Plasmonic Nanocavity-Based Method for Measuring Electric Charges in Solution

Electric charges play a fundamental role in shaping the structure, function, and interactions of biomolecules, yet precisely measuring these charges at the single-molecule level remains a significant technical challenge. Here, we introduce a novel experimental methodology that utilizes plasmonic nanocavities to quantify molecular electric charges in solution with high sensitivity. Our approach exploits an externally applied electric field to induce the spatial redistribution of charged molecules confined within a planar metallic nanocavity, while simultaneously leveraging nanocavity-induced fluorescence lifetime modulation as a highly sensitive readout. We demonstrate the feasibility of this method through proof-of-concept experiments, where we measure the fluorescence lifetimes of positively and negatively charged fluorescent dye molecules as a function of the applied electric field across the cavity. The experimental results are validated through a rigorous theoretical framework, incorporating statistical thermodynamics and electrodynamic modeling to accurately describe the observed data. The proposed method offers a calibration-free, experimentally simple, and rapid alternative to existing charge measurement techniques, opening new avenues for precise quantification of molecular electric charges down to the single-molecule level.