Electron Paramagnetic Resonance for the Detection of Electrochemically Generated Hydroxyl Radicals - Issues Associated with Electrochemical Oxidation of the Spin Trap
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2022-09-26 |
| Journal | ACS Measurement Science Au |
| Authors | Emily Braxton, David J. Fox, Ben G. Breeze, Joshua J. Tully, Katherine J. Levey |
| Institutions | University of Warwick |
| Citations | 41 |
Abstract
Section titled āAbstractāFor the detection of electrochemically produced hydroxyl radicals (HO<sup>Ā·</sup>) from the oxidation of water on a boron-doped diamond (BDD) electrode, electron paramagnetic resonance spectroscopy (EPR) in combination with spin trap labels is a popular technique. Here, we show that quantification of the concentration of HO<sup>Ā·</sup> from water oxidation via spin trap electrochemical (EC)-EPR is problematic. This is primarily due to the spin trap oxidizing at potentials less positive than water, resulting in the same spin trap-OH<sup>Ā·</sup> adduct as formed from the solution reaction of OH<sup>Ā·</sup> with the spin trap. We illustrate this through consideration of 5,5-dimethyl-1-pyrroline <i>N</i>-oxide (DMPO) as a spin trap for OH<sup>Ā·</sup>. DMPO oxidation on a BDD electrode in an acidic aqueous solution occurs at a peak current potential of +1.90 V vs SCE; the current for water oxidation starts to rise rapidly at ca. +2.3 V vs SCE. EC-EPR spectra show signatures due to the spin trap adduct (DMPO-OH<sup>Ā·</sup>) at potentials lower than that predicted thermodynamically (for water/HO<sup>Ā·</sup>) and in the region for DMPO oxidation. Increasing the potential into the water oxidation region, surprisingly, shows a lower DMPO-OH<sup>Ā·</sup> concentration than when the potential is in the DMPO oxidation region. This behavior is attributed to further oxidation of DMPO-OH<sup>Ā·</sup>, production of fouling products on the electrode surface, and bubble formation. Radical scavengers (ethanol) and other spin traps, here <i>N</i>-<i>tert</i>-butyl-<i>Ī±</i>-phenylnitrone, <i>Ī±</i>-(4-pyridyl <i>N</i>-oxide)-<i>N</i>-<i>tert</i>-butylnitrone, and 2-methyl-2-nitrosopropane dimer, also show electrochemical oxidation signals less positive than that of water on a BDD electrode. Such behavior also complicates their use for the intended application.