Real-Time Detection of Hydroxyl Radical Generated at Operating Electrodes via Redox-Active Adduct Formation Using Scanning Electrochemical Microscopy

Abstract

The hydroxyl radical (•OH) is one of the most attractive reactive oxygen species due to its high oxidation power and its clean (photo)­(electro)­generation from water, leaving no residues and creating new prospects for efficient wastewater treatment and electrosynthesis. Unfortunately, in situ detection of •OH is challenging due to its short lifetime (few ns). Using lifetime-extending spin traps, such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to generate the [DMPO–OH]• adduct in combination with electron spin resonance (ESR), allows unambiguous determination of its presence in solution. However, this method is cumbersome and lacks the necessary sensitivity and versatility to explore and quantify •OH generation dynamics at electrode surfaces in real time. Here, we identify that [DMPO–OH]• is redox-active with E 0 = 0.85 V vs Ag|AgCl and can be conveniently detected on Au and C ultramicroelectrodes. Using scanning electrochemical microscopy (SECM), a four-electrode technique capable of collecting the freshly generated [DMPO–OH]• from near the electrode surface, we detected its generation in real time from operating electrodes. We also generated images of [DMPO–OH]• production and estimated and compared its generation efficiency at various electrodes (boron-doped diamond, tin oxide, titanium foil, glassy carbon, platinum, and lead oxide). Density functional calculations, ESR measurements, and bulk calibration using the Fenton reaction helped us unambiguously identify [DMPO–OH]• as the source of redox activity. We hope these findings will encourage the rapid, inexpensive, and quantitative detection of •OH for conducting informed explorations of its role in mediated oxidation processes at electrode surfaces for energy, environmental, and synthetic applications.

Bioinspired Single‐Atom Sites Enable Efficient Oxygen Activation for Switching Anodic/Cathodic Electrochemiluminescence

“…It can be oxidized at a certain potential and generate a current response at the ultramicroelectrode (UME) probe. [35] The oxidation potential of [DMPO-OH] * is 0.8 V, confirmed by the Fenton reaction (Figure S14). When an O 2 reduction potential (À 0.4 V) is applied to the substrate working electrode, the FeN 5 SACs system exhibits a prominently higher [DMPO-OH] * oxidation collection current signal on the probe than FeN 4 © SACs system (Figure 3b).…”

“…In short, a trapping agent, 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO), is employed to capture ⋅OH to form redox‐active [DMPO‐OH]⋅ composites. It can be oxidized at a certain potential and generate a current response at the ultramicroelectrode (UME) probe [35] . The oxidation potential of [DMPO‐OH]⋅ is 0.8 V, confirmed by the Fenton reaction (Figure S14).…”

Explaining the Decay of Nucleic Acid-Based Sensors under Continuous Voltammetric Interrogation

“…This idea is supported by square wave voltammograms measured at t = 0 and t = 48 h on the same PNA-based NBEs (Figure S3), which show significant increases in capacitive and oxygen reduction currents relative to DNA-based analogues. A recent publication by Barroso-Martínez et al reports a novel redox reporter that can chemically trap reactive oxygen species and allow their electrochemical quantification on electrode surfaces . Such measurements are beyond the scope of this work but could be used to further understand the rates of NBE signal decay as a function of nucleic acid chemistries.…”

Imaging Analysis of Scanning Electrochemical Microscopy in Energy Catalysis

“…31,32 Barroso-Marti ́nez et al developed a technique that enables real-time monitoring of newly generated •OH on the electrode surface, using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to react with •OH to form a long-lived adduct and then collecting the adduct produced on the electrode surface through a needle tip (Figure 3B). 33 Ma et al measured the size of the Pt nanoparticle (NP) at nanometer resolution by in situ SECM. 34 In a high-concentration HClO 4 solution, the tip generated hydrogen instead of bubbles by controlling the tip−substrate distance in TG-SC mode (Figure 3C), so as to obtain the steady-state current of Pt NP deposition at the tip and thus measure the size of Pt NP.…”

“…(iii) When the BDD electrode was turned on and off, the ESR spectra of a solution containing 10 mM DMPO and 0.1 M Na 2 SO 4 (pH 4) were collected near the BDD electrode surface. Adapted from ref . Copyright 2022 American Chemical Society.…”