On the quantification of the dissolved hydroxyl radicals in the plasma-liquid system using the molecular probe method

TL;DR

This study evaluates the molecular probe method for quantifying dissolved hydroxyl radicals in plasma-liquid systems, revealing it generally underestimates true concentrations unless correction factors are applied.

Contribution

It provides a validated approach to accurately estimate OHdis concentrations by accounting for derivative destruction and scavengers, improving plasma chemistry measurements.

Findings

Molecular probe method underestimates OHdis without correction.

Correct estimation requires considering HCHO destruction and scavengers.

H2O2 generated by OHdis influences the measurement accuracy.

Abstract

Hydroxyl (OH) radical is the most important reactive species produced by the plasma-liquid interactions, and the OH in the liquid phase (dissolved OH radical, OHdis) takes effect in many plasma-based applications due to its high reactivity. Therefore, the quantification of the OHdis in the plasma-liquid system is of great importance, and a molecular probe method usually used for the OHdis detection might be applied. Herein we investigate the validity of using the molecular probe method to estimate the [OHdis] in the plasma-liquid system. Dimethyl sulfoxide is used as the molecular probe to estimate the [OHdis] in an air plasma-liquid system, and the partial OHdis is related to the formed formaldehyde (HCHO) which is the OHdis-induced derivative. The analysis indicates that the true concentration of the OHdis should be estimated from the sum of three terms: the formed HCHO, the existing OH scavengers, and the OHdis generated H2O2. The results show that the measured [HCHO] needs to be corrected since the HCHO destruction is not negligible in the plasma-liquid system. We conclude from the results and the analysis that the molecular probe method generally underestimates the [OHdis] in the plasma-liquid system. If one wants to obtain the true concentration of the OHdis in the plasma-liquid system, one needs to know the destruction behavior of the OHdis-induced derivatives, the information of the OH scavengers (such as hydrated electron, atomic hydrogen besides the molecular probe), and also the knowledge of the OHdis generated H2O2.