Measuring plasma-generated ^·OH and O atoms in liquid using EPR spectroscopy and the non-selectivity of the HTA assay

Abstract

Plasma-generated hydroxyl radicals (·OH) and oxygen atoms (O) produced by the COST reference plasma jet, a micro-scaled atmospheric pressure plasma jet, were investigated using a variety of experimental techniques. Several gas admixtures were studied to distinguish the contributions of the two reactive oxygen species. Large discrepancies between inferred aqueous ·OH densities were noted when using a 2-hydroxyterephthalic acid (HTA) fluorescence assay and electron paramagnetic resonance (EPR) measurements with the spin trap 5,5-dimethyl-1-pyrroline N-oxide—especially when oxygen was present in the feed gas. A series of follow-up experiments including optical emission spectroscopy, H2O2 quantification, and EPR measurements of atomic oxygen using the spin trap 2,2,6,6-tetramethylpiperidine, revealed that the inconsistencies between the measured aqueous ·OH were likely due to the propensity of atomic oxygen to hydroxylate TA in a manner indistinguishable from ·OH. This renders the HTA assay non-selective when both ·OH radicals and atomic oxygen are present, which we report for all three gas admixtures in our experiments. Additionally, considerable degradation of both HTA and the spin adducts measured using EPR spectroscopy was apparent, meaning actual radical densities in the plasma-treated liquid may be considerably higher than implied. Degradation rates compared favorably to previously measured gas phase densities of atomic oxygen in the predecessor of the COST jet and reported degradation of other chemical probes. These results show the prolific role of atomic oxygen in plasma-induced liquid chemistry and caution against diagnostic techniques that are unable to account for it.

Plasma‐driven biocatalysis: In situ hydrogen peroxide production with an atmospheric pressure plasma jet increases the performance of OleTJE when compared to adding the same molar amount of hydrogen peroxide in bolus

“…[34] Here, the He/H 2 O admixture was selected based on the extensive characterization of the gas and liquid phase performed in previous experiments. [30,35,36] To limit the effect of impurities in the discharge, gas delivery was regulated via stainless steel tubing. As a reference jet, the COST-Jet is well studied and previous research has reported its reproducibility, [37] gas-phase densities of reactive species, [34,36,38,39] and liquid phase densities, [30,35,[40][41][42] among others.…”

“…[30,35,36] To limit the effect of impurities in the discharge, gas delivery was regulated via stainless steel tubing. As a reference jet, the COST-Jet is well studied and previous research has reported its reproducibility, [37] gas-phase densities of reactive species, [34,36,38,39] and liquid phase densities, [30,35,[40][41][42] among others. The nozzle of the COST-Jet was placed 4 mm above the liquid surface, see Figure 1.…”

“…Dissipated power was measured during treatment following Golda et al [29,43] and kept constant at 750 ± 10 mW, corresponding to an applied voltage of V RMS ~240 V. This dissipated power was also selected to coincide with previous experiments. [30,35] Before each measurement, the COST-Jet was operated for 30 min to mitigate the influence of impurities.…”

“…That the concentrations in DI water and buffer solution are very similar indicates that the majority of the H 2 O 2 is formed in the gas phase, in agreement with previously reported results for phenol solutions, [45] as well as terephthalic acid and DMPO solutions. [30] It is well documented in the literature that H 2 O 2 delivery can be mediated by changing the water admixture in the feed gas. [31,46] Here experiments were performed to investigate the tunability of H 2 O 2 delivery with a single admixture (He with 2500 ppm H 2 O) by regulating the dissipated power in the active plasma from ignition to arcing (0.25-1.3 W).…”

Radiation Driven Chemistry in Biomolecules—is (V)UV Involved in the Bioactivity of Argon Jet Plasmas?

“…For the quantification of plasma-generated, short-lived ROS (hydroxyl radicals, atomic oxygen) solution of 5 mM terephthalic acid in 25 mM phosphate buffer, pH 7.4 was used despite the limited selectivity [59]. The reaction yield to the fluorescent compound 2-hydroxyterephthalic acid (HTPA) that could be quantified at 318 nm excitation and 426 nm emission using an external calibration curve.…”