Effect of surface pretreatment of TiO 2 films on interfacial processes leading to bacterial inactivation in the dark and under light irradiation

Abstract

Evidence is presented for radio-frequency plasma pretreatment enhancing the amount and adhesion of TiO 2 sputtered on polyester (PES) and on polyethylene (PE) films. Pretreatment is necessary to attain a suitable TiO 2 loading leading to an acceptable Escherichia coli reduction kinetics in the dark or under light irradiation for PES–TiO 2 and PE–TiO 2 samples. The amount of TiO 2 on the films was monitored by diffuse reflectance spectroscopy and X-ray fluorescence. X-ray electron spectroscopy shows the lack of accumulation of bacterial residues such as C, N and S during bacterial inactivation since they seem to be rapidly destroyed by TiO 2 photocatalysis. Evidence was found for Ti 4+ /Ti 3+ redox catalysis occurring on PES–TiO 2 and PE–TiO 2 during the bacterial inactivation process. On PE–TiO 2 surfaces, Fourier transform infrared spectroscopy (ATR-FTIR) provides evidence for a systematic shift of the n a (CH 2 ) stretching vibrations preceding bacterial inactivation within 60 min. The discontinuous IR-peak shifts reflect the increase in the C–H inter-bond distance leading to bond scission. The mechanism leading to E. coli loss of viability on PES–TiO 2 was investigated in the dark up to complete bacterial inactivation by monitoring the damage in the bacterial outer cell by transmission electron microscopy. After 30 min, the critical step during the E. coli inactivation commences for dark disinfection on 0.1–5% wt PES–TiO 2 samples. The interactions between the TiO 2 aggregates and the outer lipopolysaccharide cell wall involve electrostatic effects competing with the van der Waals forces.