Bachir Saoudi

Plasma Sterilization : Methods and Mechanisms

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.

Bacterial spore inactivation by atmospheric-pressure plasmas in the presence or absence of UV photons as obtained with the same gas mixture

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 respective roles of UV photons and oxygen atoms in plasma sterilization at reduced gas pressure: the case of N/sub 2/-O/sub 2/ mixtures

In the reduced-pressure (/spl les/10 torr) afterglow stemming from discharges in O/sub 2/- containing mixtures such as N/sub 2/-O/sub 2/, the test-reference spores are ultimately inactivated by UV photons through destruction of their genetic material (DNA). To show this, we assume the inactivation to result from a sufficiently large number of successful hits of the DNA strands by UV photons. This implies that the higher the UV intensity, the shorter the time required to reach the lethal dose. Simultaneously, the increased erosion of the spores by the oxygen atoms as time elapses reduces the incident number of photons required to meet the lethal dose. Erosion, as observed by scanning electron microscopy, also increases with the O/sub 2/ percentage in the mixture. Actually, sterilization time is found to be the shortest when the O/sub 2/ percentage in the mixture is set to maximize the UV emission intensity, which occurs at O/sub 2/ percentages typically below 2%, where erosion is low. This proves the predominant role of UV radiation over erosion as far as spore inactivation is concerned. In any case, plasma sterilization always implies some erosion of the test spores, in contrast to what happens with conventional sterilization methods.

Validation of cold plasma treatment for protein inactivation: a surface plasmon resonance-based biosensor study

Gas plasma is being proposed as an interesting and promising tool to achieve sterilization. The efficacy of gas plasma to destroy bacterial spores (the most resistant living microorganisms) has been demonstrated and documented over the last ten years. In addition to causing damage to deoxyribonucleic acid by UV radiation emitted by excited species originating from the plasma, gas plasma has been shown to promote erosion of the microorganism in addition to possible oxidation reactions within the microorganism. In this work, we used lysozyme as a protein model to assess the effect of gas plasma on protein inactivation. Lysozyme samples have been subjected to the flowing afterglow of a gas discharge achieved in a nitrogen–oxygen mixture. The efficiency of this plasma treatment on lysozyme has been tested by two different assays. These are an enzyme-linked immunosorbent assay (ELISA) and a surface plasmon resonance (SPR)-based biosensor assay. The two methods showed that exposure to gas plasma can abrogate lysozyme interactions with lysozyme-specific antibodies, more likely by destroying the epitopes responsible for the interaction. More specifically, two SPR-based assays were developed since our ELISA approach did not allow us to discriminate between background and low, but still intact, quantities of lysozyme epitope after plasma treatment. Our SPR results clearly demonstrated that significant protein destruction or desorption was achieved when amounts of lysozyme less than 12.5 ng had been deposited in polystyrene 96-well ELISA plates. At higher lysozyme amounts, traces of available lysozyme epitopes were detected by SPR through indirect measurements. Finally, we demonstrated that a direct SPR approach in which biosensor-immobilized lysozyme activity is directly measured prior and after plasma treatment is more sensitive, and thus, more appropriate to define plasma treatment efficacy with more certainty.

Accurate in-situ gas temperature measurements in dielectric barrier discharges at atmospheric pressure

Atmospheric pressure (AP) dielectric barrier discharges are frequently of interest for treating delicate substrates such as polymers or biological materials. In spite of its capital importance, thermometry in AP plasmas is subject to much uncertainty. We report temperature measurements in noble gases, nitrogen, and air using sensitive, accurate fibre-optic instrumentation that is a priori immune towards high voltages and high-frequency electromagnetic fields generally encountered in plasma environments.

Thermometry in noble gas dielectric barrier discharges at atmospheric pressure using optical emission spectroscopy

We measure the temperature, T, of dielectric barrier discharges (DBD), in noble gases using optical emission spectroscopy (OES), by analysing rotational bands in the emission spectra of the first negative system (FNS) of N₂⁺. This has the advantage that rotational structure can be fully resolved even with a spectrograph of average performance, and that the rotational temperature, T_rot (~ T_gas) can then be determined from a conventional Boltzmann plot. Ionization of N2 occurs mainly via Penning transfer from metastable excited states of He (ca. 20 eV) or Ne (ca. 16.6 eV). Using two glass-walled DBD chambers of very different volumes (0.1 and 20 liters), we have studied atmospheric-pressure discharges in flowing helium (He) or neon (Ne) containing traces of nitrogen. Discharges were excited by audio-frequency (10 kHz) high voltage (HV) using a needle as the HV electrode and a dielectric (alumina)-covered planar grounded counter-electrode. OE spectra were acquired with a 0.5 m focal length spectrograph, coupled to an intensified charge coupled device (ICCD) detector. Using the (0-0) R-branch of the FNS N₂⁺ (B²Σ_u⁺ - X²Σ_g⁺) bands near a wavelength of 391.4 nm, we have measured axial (inter-electrode) distributions of T_rot for the two different reactor volumes in both He and Ne. T_rot values were found to be highest at the needle electrode, of about 450 K and 740 K for He and Ne, respectively; in He, T_rot dropped to a minimum of about 405 K at the mid-gap position in the small chamber, and ~ 360 K near the planar electrode in the large chamber. We conclude that temperatures in noble gas discharges depend critically on thermal conductivities of the particular gases (K_He = 1.9; K_Ne = 0.6, both in mW.cm^-1.K^-1) and on other experimental factors that influence heat transfer.