Utilizing a plasma to achieve sterilization is a possible alternative to conventional sterilization means as far as sterilization of heat-sensitive materials and innocuity of sterilizing agents are concerned. A major issue of plasma sterilization is the respective roles of ultraviolet (UV) photons and reactive species such as atomic and molecular radicals. At reduced gas pressure (£10 torr) and in mixtures containing oxygen, the UV photons dominate the inactivation process, with a significant contribution of oxygen atoms as an erosion agent. Actually, as erosion of the spore progresses, the number of UV photons successfully interacting with the genetic material increases. The different physicochemical processes at play during plasma sterilization are identified and analyzed, based on the specific characteristics of the spore survival curves.

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This paper comprises two main parts: a review of the literature on atmospheric-pressure discharges used for micro-organism inactivation, focused on the inactivation mechanisms, and a presentation of our research results showing, in particular, that UV photons can be the dominant species in the inactivation process.The possibility of achieving spore inactivation through UV radiation using an atmospheric-pressure discharge or its flowing afterglow is the object of a continuing controversy. In fact, the review of the literature that we present shows that a majority of researchers have come to the conclusion that, at atmospheric pressure, chemically reactive species such as free radicals, metastable atoms and molecules always control the inactivation process, while UV photons play only a minor role or no role at all. In contrast, only a few articles suggest or claim that UV photons coming from atmospheric-pressure discharges can, in some cases, inactivate micro-organisms, but the experimental data presented and the supporting arguments brought forward in that respect are relatively incomplete.Using a dielectric-barrier discharge operated at atmospheric pressure in an N2–N2O mixture, we present, for the first time, experiments where micro-organisms are subjected to plasma conditions such that, on the one hand, UV radiation is strong or, on the other hand, there is no UV radiation, the two different situations being obtained with the same experimental arrangement, including the same gas mixture, N2–N2O. To achieve maximum UV radiation, the concentration of the oxidant molecule (N2O) added to N2 needs to be tuned carefully, resulting then in the fastest inactivation rate. The concentration range of the oxidant molecule in the mixture for which the UV intensity is significant is extremely narrow, a fact that possibly explains why such a mode of plasma sterilization was not readily observed. The survival curves obtained under dominant UV radiation conditions are, as we show, akin to those recorded at reduced pressure. Relatively fast spore inactivation can also be obtained under no UV radiation as a result of radicals diffusing deeply inside the spores, leading to oxidative lethal damage.

The flowing afterglow of an N2–O2 discharge in the 0.6–10 Torr range is examined in the perspective of achieving sterilization of medical devices (MDs) under conditions ensuring maximum UV intensity with minimum damage to polymer-based MDs. The early afterglow is shown to be responsible for creating strong erosion damage, requiring that the sterilizer be operated in a dominant late-afterglow mode. These two types of afterglow can be characterized by optical emission spectroscopy: the early afterglow is distinguished by an intense emission from the 1st negative system (band head at 391.4 nm) while the late afterglow yields an overpopulation of the v′ = 11 ro–vibrational level of the N2(B) state, indicating a reduced contribution from the early afterglow N2 metastable species. We have studied the influence of operating conditions (pressure, O2 content in the N2–O2 mixture, distance of the discharge from the entrance to the afterglow (sterilizer) chamber) in order to achieve a dominant late afterglow that also ensures maximum and almost uniform UV intensity in the sterilization chamber. As far as operating conditions are concerned, moving the plasma source sufficiently far from the chamber entrance is shown to be a practical means for significantly reducing the density of the characteristic species of the early afterglow.Using the NO titration method, we obtain the (absolute) densities of N and O atoms in the afterglow at the NO injection inlet, a few cm before the chamber entrance: the N atom density goes through a maximum at approximately 0.3–0.5% O2 and then decreases, while the O atom density increases regularly with the O2 percentage. The spatial variation of the N atom (relative) density in the chamber is obtained by recording the emission intensity from the 1st positive system at 580 nm: in the 2–5 Torr range, this density is quite uniform everywhere in the chamber. The (relative) densities of N and O atoms in the discharge are determined by using the actinometry method: the density of N atoms decreases from its maximum value at 0% O2 as the percentage of O2 is increased while the density of O atoms increases, almost linearly, as a function of the percentage of O2, as in the afterglow. The intensity variation of the NOβ UV emission as a function of the percentage of O2 is characterized by a maximum around 0.6% O2 (2 Torr) followed by an approximately exponential decay. We observe that, in the 0–1% O2 range, the UV emission is limited by the availability of O atoms. Beyond this point, the decrease of the UV intensity follows the decrease in the N atom density, while on the average, the O atom density keeps on increasing with O2%. Erosion of polymer microspheres is found to be strongest at the chamber axis when no O2 is present, implying a dominant early afterglow. Adding even only 1% O2 causes a strong quenching of the N2 metastable species, leading to a dominant late afterglow and therefore considerably reducing the etching rate at the axis. In contrast, at 5 cm from the axis under the same operating conditi...

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