Redox-Active Spin Trapping for the Electrochemical Detection, Quantification, and Kinetic Analysis of Sulfate Radicals in Aqueous Media

Abstract Sulfate radicals (SO4•–) are important intermediates in synthetic and environmental chemistry, yet robust methods for their selective detection and quantification remain underdeveloped. Spin traps such as 5–5-dimethyl-1-pyrroline-N-oxide (DMPO) are used to extend the lifetime of these radicals, but the resulting adduct (DMPO–SO4•–) is unstable in water, undergoing nucleophilic substitution to DMPO–OH•, causing misidentification and mechanistic confusion in aqueous systems. Here, we report an electrochemical approach for the selective and quantitative detection of sulfate radical spin-trap adducts in aqueous conditions. Using electrochemical electron paramagnetic resonance (EC-EPR) spectroscopy, we measured the half-life of DMPO–SO4•- in water to be ∼98 s. This short half-life makes the quantification of SO4•– by traditional EPR methods challenging. To overcome this, we used an approach to electrochemically detect spin-trap adducts and found that DMPO–SO4•– could be electrochemically oxidized at ∼0.75 V vs Ag/AgCl. Coupling this approach to microband interdigitated electrode arrays (IDAs) in generation-collection mode, automated electrochemistry, and finite-element simulations, we leveraged the short detection time (ms) of IDAs to capture DMPO–SO4•– before it decays. Quantitative understanding allowed us to estimate the DMPO spin trapping rate constant for SO4•– (4 × 105 M–1s–1). Finally, changes in the DMPO–SO4•– collection current were used to extract the kinetics of a competing reaction (oxidation of benzyl alcohol) in a radical clock-type approach. These results demonstrate the broad utility of redox-active spin trapping for quantifying electrogenerated radical intermediates such as SO4•– with sufficient versatility to capture their transient processes.