Electron Paramagnetic Resonance for the Detection of Electrochemically Generated Hydroxyl Radicals: Issues Associated with Electrochemical Oxidation of the Spin Trap

For the detection of electrochemically produced hydroxyl radicals (OH˙) from the oxidation of water, electron paramagnetic resonance spectroscopy (EPR) in combination with spin trap labels is a popular technique. Here we show that quantification of the true concentration of OH˙ generated from water oxidation via electrochemical (EC)-EPR is unlikely. This is primarily due to the spin trap oxidising at potentials less positive than water and resulting in the same spin trap adduct as is formed from the solution reaction of OH˙ with the spin trap. We illustrate this through consideration of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to spin trap OH˙. DMPO oxidation on a boron doped diamond (BDD) electrode in stationary solution occurs at a peak current potential of +1.90 V vs SCE, water oxidation commences at +2.35 V vs SCE. EC-EPR spectra shows signatures due to the hydroxyl spin adduct (DMPO-OH˙) at potentials lower than the thermodynamic standard water/HO˙ potential and in the region for DMPO oxidation. Increasing the potential into the water oxidation region, surprisingly, shows a lower DMPO-OH˙ concentration than when the potential is in the DMPO oxidation region. This behavior is attributed to further oxidation of DMPO-OH˙, production of fouling products on the electrode surface and bubble formation. Radical scavengers (ethanol) and other spin traps, here N-tert-butyl-α-phenylnitrone (PBN), α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) and 2-methyl-2-nitrosopropane dimer (MNP), also show oxidation signals less positive than that of water. However, by monitoring ethanol-DMPO adduct versus DMPO-OH˙ product distributions as a function of applied potential, it is possible to identify the potential at which HO˙ generation via water oxidation starts to dominate.