A fluorescence microscopy platform for time-resolved studies of spin-correlated radical pairs in biological systems

The importance of spin-correlated radical pairs in biology is increasingly recognized, with roles in biological effects of weak magnetic fields and emerging quantum spin-based biomedical applications. Fluorescence microscopy offers sufficient sensitivity to study magnetic field effects on radical pair reactions in living cells, but conventional techniques cannot directly resolve their dynamics because most biologically relevant radical pairs are non-emissive. To overcome this challenge, we introduce two novel microscopy techniques: single color pump-probe (PP) and pump-field-probe (PFP) fluorescence. Here, we describe their working principles, provide their mathematical formulation, and validate both techniques through theoretical analysis and experiments on well-established flavin-based magnetic field sensitive reactions. These approaches offer a sensitive and broadly applicable platform for quantifying and visualizing the quantum spin dynamics of radical pair chemical reactions in biological systems.