Cold atmospheric-pressure plasma (CAP) is a relatively new method being investigated for antimicrobial activity. However, the exact mode of action is still being explored. Here we report that CAP efficacy is directly correlated to bacterial cell wall thickness in several species. Biofilms of Gram positiveBacillus subtilis, possessing a 55.4 nm cell wall, showed the highest resistance to CAP, with less than one log₁₀reduction after 10 min treatment. In contrast, biofilms of Gram negativePseudomonas aeruginosa, possessing only a 2.4 nm cell wall, were almost completely eradicated using the same treatment conditions. Planktonic cultures of Gram negativePseudomonas libanensisalso had a higher log₁₀reduction than Gram positiveStaphylococcus epidermidis.Mixed species biofilms ofP. aeruginosaandS. epidermidisshowed a similar trend of Gram positive bacteria being more resistant to CAP treatment. However, when grown in co-culture, Gram negativeP. aeruginosawas more resistant to CAP overall than as a mono-species biofilm. Emission spectra indicated OH and O, capable of structural cell wall bond breakage, were present in the plasma. This study indicates that cell wall thickness correlates with CAP inactivation times of bacteria, but cell membranes and biofilm matrix are also likely to play a role.

Detailed plasma kinetics modelling is presented of a low electron energy N ₂ –H ₂ atmospheric-pressure discharge for ammonia synthesis. The model considers both electron and vibrational kinetics, including excited N ₂ ( X , ν ) and H ₂ ( X , ν ) species, and surface reactions such as those occurring by the Eley–Rideal and Langmuir–Hinshelwood mechanisms and dissociative adsorption of molecules. The predictions of the model are compared to the measured NH ₃ concentration produced in a packed-bed dielectric barrier discharge reactor as a function of process parameters such as input gas composition and applied voltage. Unlike typical low-pressure plasma processes, under the plasma conditions considered here (reduced electric field E / N in the range 30–50 Td, electron density of the order 10 ⁸ cm ⁻³ ), the influence of ions is not significant. Instead, the reactions between radicals and vibrationally-excited molecules are more important. The active species in surface reactions, such as surface-adsorbed atomic nitrogen N(s) or hydrogen H(s), are found to be predominantly generated through the dissociative adsorption of molecules, in contrast to previously proposed mechanisms for plasma catalysis under low-pressure, high- E / N conditions. It is found that NH radicals play an important role at the early stages of the NH ₃ -generation process, NH in turn is produced from N and H ₂ ( ν ). Electron kinetics is shown to play a critical role in the molecular dissociation and vibrational excitation reactions that produce these precursors. It is further found that surface-adsorbed atomic hydrogen H(s) takes a leading role in the formation of NH ₃ , which is another significant difference from the mechanisms in conventional thermo-chemical processes and low-pressure plasmas. The applied voltage, the gas temperature, the N ₂ :H ₂ ratio in the input gas mixture and the reactivity of the surface material are all found to influence the ammonia production. The calculated results reproduce the observed trends in the dependence of NH ₃ production on the N ₂ /H ₂ ratio in the input gas mixture and the applied voltage.