Generation and delivery of free hydroxyl radicals using a remote plasma

We demonstrate a new gas-based OH∙ generation source using a low power radio frequency driven atmospheric pressure plasma configured to deliver the radical flux into the far effluent region, well away from interference from other plasma factors such as electric fields, currents, and ultraviolet radiation. Using He–H2O gas chemistry isolated from the laboratory air, the plasma generated flux contains OH∙ and other radicals including, O and HO2 as well as H2O2 which, along with, was found to vary with H2O vapour content and absorbed power density. Peak flux values were 2.3 nmol s−1 and 0.23 nmol s1 for H2O2 and OH∙ respectively at a distance of 50 mm from the plasma, with 790 ppmv H2O and a power density of ∼108 W m−3. The maximum OH∙ flux density was 4.5 × 1019 m−2s−1 falling to 1.7 × 1019 m2 s1 at 110 mm, equivalent to generation rates of 74 µM s1 and 28 µM s−1. Despite high OH∙ recombination rates at the plasma exit, the escaping flux is still significant, indicating a viable delivery capability to downstream targets. Its performance with regard to OH∙ generation rates compares well with traditional OH∙ generation techniques such as radiolysis, advanced oxidation processes and enhanced Fenton-chemistry approaches where OH∙ production rates are sub-µM s−1. Delivering precisely quantifiable OH∙ fluxes provides new opportunities for scientific studies and technological opportunities in cell biology, atmospheric chemistry, protein unfolding and systematic dose studies for plasma-based and other OH∙ related potential medical treatments.