Optical emission spectroscopy characterization of oxygen plasma during degradation of Escherichia coli

Optical emission spectroscopy was applied for plasma characterization during sterilization of substrates contaminated with bacteria. The amount of 1010/ml cells of Escherichia coli was carefully applied to glass substrates and exposed to oxygen plasma glow discharge at different pressures between 30 and 200 Pa. Plasma was created in a glass discharge tube by an inductively coupled rf generator at the frequency of 27.12 MHz and output power of about 250 W. The electron temperature and plasma density were estimated with a double Langmuir probe. They were between 3 and 5 eV and 2 and 35x1015 m-3. Density of neutral oxygen atoms was measured with a catalytic probe, and was between 2 and 6x1021 m-3. Optical emission spectroscopy was performed with a low resolution spectrometer. The emission from carbon monoxide and nitrogen molecules was used to monitor the evolution of bacteria degradation. Both signals expressed a well defined maximum corresponding to peak erosion of bacteria by plasma radicals. As the sterilization was accomplished, both CO and N2 lines fell below the detection limit of the spectrometer. The bacteria degradation was also monitored by scanning electron microscope (SEM) and culturing. The SEM images corresponded well with the evolution of CO and N2 lines so the optical emission spectroscopy found a reliable tool for monitoring the sterilization process