Quantitative Quantum Relaxometry Framework for Probing Transient Radical-Associated Magnetic Fluctuations under Operando Conditions.

Transient paramagnetic intermediates play central roles in interfacial chemical and biological processes. However, their short lifetimes make dynamic probing difficult. Existing methods typically rely on reporter chemistry or spin adducts, limiting continuous, operando monitoring. Alternatively, the spin fluctuations generated by such intermediates serve as a direct physical observable. Here, we developed a magnetic noise relaxometry framework based on nitrogen-vacancy centers in nanodiamonds to access transient radical-associated dynamics. We established and theoretically validated a quantitative relationship between transient radical concentrations and longitudinal relaxation rates from magnetic fluctuations during reactions by using hydroxyl radicals as an example. Then, we demonstrated the operando monitoring during photocatalysis water splitting by TiO2 as well as the extraction of corresponding effective reaction constants. Our framework provides a promising pathway for rapidly and quantitatively tracking transient paramagnetic intermediates, offering new opportunities for probing in complex chemical and biological environments.