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.

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