Attack of hydroxyl radicals to α-methyl-styrene sulfonate polymers and cerium-mediated repair via radical cations

Applications of some EPR methods to the investigation of the radical species produced by the reactions of hydroxyl radicals with PEFC-related fluorinated organic acids

Fundamental information on the reactions of ˙OH radicals with perfluoroalkyl sulfonic acids and carboxylic acids is important for understanding the degradation of polymer electrolyte fuel cells (PEFCs). In the present research, the intermediate radicals produced by these reactions were detected and analyzed by means of three methods of electron paramagnetic resonance (EPR) spectroscopy. The conventional CW-EPR technique was applied to both frozen and flowing aqueous solution systems for detecting the reaction intermediates, while the time-resolved (TR) EPR technique was applied to the flowing solution system for analyzing spin dynamics parameters. The reactants tested were CF₃SO₃H, CHF₂CF₂SO₃H, CH₃SO₃H, CF₃COOH, CHF₂COOH, etc., and the ˙OH radical was generated from H₂O₂ by the irradiation of a UV laser. The radicals detected were ˙SO₃^-, ˙CO₂^-, ˙CF₃, ˙CF₂CF₂SO₃H, ˙CF₂COOH, etc. Based on the measurements of TR-EPR spectra, the dependences of the signal intensity on the time and magnetic field were analyzed, and then the longitudinal relaxation time (T₁) and the lifetime of these radical species were evaluated. The three EPR methods for detecting the intermediate radicals were compared to show the limitations of these techniques. Based on the detected radicals, the degradation mechanism reported for perfluoro acids was discussed.

Investigation into the performance decay of proton-exchange membranes based on sulfonated heterocyclic poly(aryl ether ketone)s in Fenton's reagent

Sulfonated N-heterocyclic poly(aryl ether) proton-exchange membranes have potential applications in the fuel-cell field due to their favorable proton conduction capacity and stability. This paper investigates the changes in mass and performance decay, such as proton conduction and mechanical strength, of sulfonated poly(ether ether ketone)s (SPEEKs) and three sulfonated N-heterocyclic poly(aryl ether ketone) (SPPEK, SPBPEK-P-8, and SPPEKK-P) membranes in Fenton's oxidative experiment. The SPEEK membrane exhibited the worst oxidative stability. The oxidative stability of the SPPEK membrane is enhanced due to the introduction of phthalazinone units in the chains. The SPPEKK-P and SPBPEK-P-8 membranes exhibit better radical tolerance than the SPPEK membrane, with proton conductivity retention rates of 66% and 73% for 1 h oxidative treatment, respectively. In addition, the molecular chains of SPPEKK-P and SPBPEK-P-8 exhibit relatively little disruption. The pendant benzenesulfonic groups enhance the steric effects for reducing radical attacks on the ether bonds and reduce the hydration of molecular chains. The introduction of phthalazinone units decreases the rupture points in the main chain. Therefore, the radical tolerance of the membranes is improved. These results provide a reference for the design of highly stable sulfonated heterocyclic poly(aryl ether) membranes.

Impact of substitution on reactions and stability of one-electron oxidised phenyl sulfonates in aqueous solution

Highly reactive aromatic cation radicals have been invoked lately in synthetic routes and in the degradation pathways of hydrocarbon-based polymers. Changes in the electron density of aromatic compounds are expected to alter the reaction pathway following one electron oxidation through altering the pK_a of the formed intermediate cation radical. Electron-donating groups increase its stability, however, little experimental data are known. While, in theory, the cation radical can be repaired by simple electron transfer, electron transfer to or from its deprotonated form, the hydroxycyclohexadienyl radical, will cause permanent modification or degradation. Time-resolved absorption spectroscopy indicates a pK_a ≈ 2–3 for the 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) radical cation, while its parent compound 4-(tert-butyl) phenylsulfonate (BPS) is much more acidic. The stability of both compounds towards oxidation by HO˙ was evaluated under air at pH 5 and pH 0. At pH 5, both BMPS and BPS are unstable, and superstoichiometric degradation was observed. Degradation was slightly reduced for BPS at pH 0. In contrast, the more electron rich BMPS showed 80% lower degradation. We unambigously showed that in the presence of Ce(iii) and H₂O₂ at pH 0 both BMPS and BPS could be catalytically repaired via one electron reduction, resulting in further damage moderation.

Functional groups can be used to modify the equilibrium position and tune the reactivity of one electron oxidised aromatic compounds.

Supersensitive CeOx-based nanocomposite sensor for the electrochemical detection of hydroxyl free radicals

It is well known that an excess of hydroxyl radicals (˙OH) in the human body is responsible for oxidative stress-related diseases. An understanding of the relationship between the concentration of ˙OH and those diseases could contribute to better diagnosis and prevention. Here we present a supersensitive nanosensor integrated with an electrochemical method to measure the concentration of ˙OH in vitro. The electrochemical sensor consists of a composite comprised of ultrasmall cerium oxide nanoclusters (<2 nm) grafted to a highly conductive carbon deposited on a screen-printed carbon electrode (SPCE). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to analyze the interaction between cerium oxide nanoclusters and ˙OH. The CV results demonstrated that this electrochemical sensor had the capacity of detecting ˙OH with a high degree of accuracy and selectivity, achieving a consistent performance. Additionally, EIS results confirmed that our electrochemical sensor was able to differentiate ˙OH from hydrogen peroxide (H2O2), which is another common reactive oxygen species (ROS) found in the human body. The limit of detection (LOD) observed with this electrochemical sensor was of 0.6 μM. Furthermore, this nanosized cerium oxide-based electrochemical sensor successfully detected in vitro the presence of ˙OH in preosteoblast cells from newborn mouse bone tissue. The supersensitive electrochemical sensor is expected to be beneficially used in multiple applications, including medical diagnosis, fuel-cell technology, and food and cosmetic industries.