Revisiting the application of molecular probe diagnostics on quantifying aqueous OH radicals in plasma–liquid systems

We revisit one of the most used techniques for quantifying the aqueous OH radicals (OHaq) in plasma–liquid systems, the molecular probe method which obtains the [OHaq] by measuring a stable material formed through a rapid reaction between the molecular probe and the OHaq. In this study, we used disodium terephthalate (NaTA) as the molecular probe; the experimental results with a theoretical analysis suggest that to obtain the correct OHaq concentration, the concentration of the molecular probe should be greater than a certain value, which depends on the types of the plasma–liquid systems. However, this is not the case in most of the existing reports in which the NaTA is often much less than the requisite value. In the molecular probe method, the measured OH concentration in a plasma–liquid system will saturate when the concentration of the molecular probe is greater than a certain value.