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

Abstract

For the detection of electrochemically produced hydroxyl radicals (HO • ) 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 • 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 • adduct as formed from the solution reaction of OH • with the spin trap. We illustrate this through consideration of 5,5-dimethyl-1-pyrroline Noxide (DMPO) as a spin trap for OH • . 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 • ) at potentials lower than that predicted thermodynamically (for water/HO • ) 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-αphenylnitrone, α-(4-pyridyl N-oxide)-N-tert-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.

Fe–Co-Based Metal–Organic Frameworks as Peroxidase Mimics for Sensitive Colorimetric Detection and Efficient Degradation of Aflatoxin B1

“…The electron paramagnetic resonance (EPR) spectra were further used to confirm the production of •OH using 5,5-dimethyl-1-pyrroline N -oxide (DMPO) as a trapping agent. , Apparently, the EPR spectrum of the Fe–Co-MOFs coexisting with H 2 O 2 showed a typical quadruple characteristic peak with a relative signal intensity of 1:2:2:1 that originated from the DMPO/•OH adduct, demonstrating the generation of •OH (Figure H). Compared with Fe-MOFs, Fe–Co-MOFs exhibited a higher characteristic peak intensity, which signifies that more •OH could be formed for Fe–Co-MOFs under the same conditions.…”

Electrodeposited Palladium Nanoparticles Enhancing Atomic Hydrogen-Mediated Electrochemical Recovery of Antimony

“…Atomic H* radicals were trapped in situ using 5,5-dimethyl-1-pyrroline- N -oxide (DMPO, 98%), and subsequently, the solution was extracted to the ESR (Bruker ESR 5000, Germany) test (Texts S3–S4). − Electrochemical measurements, including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry (LSV), were detailed and described in Text S5. The Sb­(III) concentration was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Scientific, iCAP PRO).…”

“…Subsequently, with further increases in trapping time, the signal gradually weakened. This may be due to the limited stable existence time of DMPO-H and the possibility of anodic oxidation of DMPO and DMPO-H. 28 Thus, it was determined that 50 mM DMPO reached its maximum capacity for trapping atomic H* in 300 s. Compared with the standard reagent 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine-1-oxyl (TEMPOL), 27 the atomic H* concentration was ca. 23.16 μM at 300 s (Figures S13 and 4d), suggesting that ca.…”

Photoelectrochemical Conversion of Methane to Ethane and Hydrogen under Visible Light Using Functionalized Tungsten Trioxide Photoanodes with Proton Exchange Membrane

“…The EPR signal is assigned to the • DMPO−OH spin adduct, indicating the • OH formation by the single-electron oxidation of water by a photogenerated hole over the WO 3 photoanode. The signal of • DMPO−OH may be also produced by the nucleophilic addition of hydroxyl ion (OH − ) to DMPO •+ , 35 through the direct oxidation of DMPO (∼1.7 V vs SHE) 34 by photogenerated holes. However, the concentration of OH − over the photoanode surface is scarce due to the formation of excessive protons by PEC water oxidation reactions.…”