Review—Quantification of Hydrogen Peroxide by Electrochemical Methods and Electron Spin Resonance Spectroscopy

Reactive oxygen species (ROS) exhibit different spatial and temporal distributions as well as concentrations in- and outside the cell, therebyfunctioningassignalingorpathogen-destroyingmolecules.EspeciallytheROSH2O2 isimportantforthepatho/physiological status of an organism. Electrochemistry (EM) and electron spin resonance (ESR)-based techniques allow quantification of H2O2 in artificial and living systems, coping a concentration range from low nM up to mM. Working electrodes for EM are optimized by diverse modifications and, additionally, redox mediators are used. Ultramicroelectrodes allow scanning of single cells to spatially resolveandquantifyextracellularH2O2 inreal-time.WithESRspectroscopy, •O2¯,butnotH2O2,canbedirectlydeterminedbyspin probes in- and outside of cells in suspensions. Monitoring H2O2 requires formation of intermediate radicals, detectable with spin probes. Low μM [H 2O2] can thus be assessed specifically. Using suitable spin traps, in-vivo ESR and immuno-spin trapping can visualize different radicalsat their respective productionsites in small animals, organsand tissues. Here,the redoxreaction cascades may interfere with cell metabolism. Optimization of all methods established for H2O2 determination would be favorable to finally combine them for mutual validation. Thus, a deeper insight into cellular ROS metabolism can be obtained.