Enhanced Monitoring of Photocatalytic Reactive Oxygen Species: Using Electrochemistry for Rapid Sensing of Hydroxyl Radicals Formed during the Degradation of Coumarin

Abstract

Many recent research studies have reported indirect methods for the detection and quantification of OH radicals generated during photocatalysis. The short lifespan and high reactivity of these radicals make indirect detection using probes such as coumarin a more viable quantification method. Hydroxyl radical production is commonly monitored using fluorescence spectroscopy to determine the concentration of the compound 7hydroxycoumarin, which is formed from hydroxyl radical attack on coumarin. There are, however, a number of additional hydroxylated coumarins generated during this process, which are less amenable to detection by fluorescence spectroscopy. Consequently, limitations and inaccuracies of this method have previously been reported in the literature. As an alternative approach to those previously reported, this work has developed an electrochemical screening method using coumarin as a OH radical trap, that is capable of in situ monitoring of not only 7-hydroxycoumarin, but all the main mono-hydroxylated products formed. As a result, this technique is a more representative and comprehensive method for the quantification of OH radicals produced by photocatalysts using coumarin as a probe molecule. Moreover, the electroanalytical method provides a portable, rapid, sensitive, and accurate in situ method for the monitoring of OH radical formation without the need for sample preparation.

Quantifying hydroxyl radicals generated by a low-temperature plasma using coumarin: methodology and precautions

“…22,34,35 3- and 4-hydroxycoumarin, if formed, are below the detection limit, which was also reported for photolysis and radiolysis of coumarin aqueous solutions, probably due to unfavorable electronic configuration on the lactone ring. 22,24,33 Based on the results in Fig. 2, it can be concluded that the same hydroxycoumarin isomers are formed and their quantity increases with the irradiation time, attesting for a growing production of HO˙ with irradiation time and/or dose.…”

“…Among them, only 7-hydroxycoumarin (7-OHCou) possesses a high fluorescence yield, 22 allowing an indirect but sensitive detection of HO by a simple fluorescence assay. Considering the numerous advantages of coumarin as a probe of HO , it was applied for a large variety of systems such as exposition to ionizing radiation, 20,23 UV photolysis, 24,25 plasma irradiation, 7,26 sonication, 27 Fenton reaction 28 and also to biological hydroxylating systems. 29 In the case of LTP irradiation, coumarin was already used as a HO probe in only a few studies.…”