Cite this: RSC Advances, 2013, 3, 16345 Received 5th June 2013, Accepted 16th July 2013 Innovative semi-transparent nanocomposite films presenting photo-switchable behavior and leading to a reduction of the risk of infection under sunlight DOI: 10.1039/c3ra42762e www.rsc.org/advances S. Rtimi, a C. Pulgarin,* a R. Sanjines b and J. Kiwi* c Novel sputtered polyethylene–TiO 2 (PE–TiO 2 ) thin films induce fast bacterial inactivation with concomitant photo-switchable hydrophobic to hydrophilic transition under light. RF-plasma pretreatments allowed an increased TiO 2 loading on PE, favorably affecting the photocatalyst performance. ATR-FTIR spectroscopy shows that the increase in the cell lipid-layer fluidity leads to cell wall scission/bacterial inactivation. The stable, strong adhesion of TiO 2 films to glass, iron plates, textile fabrics and thin polymer films active under solar light irradiation is a subject of timely research involving TiO 2 self- cleaning and inactivation properties. 1–4 Several methods are used to prepare TiO 2 thin films, based on sol–gel processing depending on the heat resistant of the selected substrate. Sol–gel processing is the most commercially commonly used method to prepare TiO 2 films 5 but the thickness of the TiO 2 films is not reproducible, they are not mechanically stable, and they exhibit low adhesion since they can be wiped off by a cloth or thumb. 6–8 Polyethylene (PE) is a low cost, inert and mechanically strong thin flexible polymer. For this reason PE has been selected as the support for the semi- transparent TiO 2 films we report in this study. The low surface energy of PE leads to poor particle adhesion. For this reason the PE was pretreated by RF and UVC to produce highly functional self-cleaning and anti-bacterial coatings increas- ing the surface sites able to bind TiO 2 nanoparticles. 7 The plasma pretreatment has been reported to increase the polymer polarity, roughness and hydrophilicity, improving its binding capacity and interfacial adhesion. 2,6 RF-plasma pretreatment of surfaces enhancing the antibacterial activity has been reported. 3–5 The RF-plasma pretreatment of PE induces negatively charged func- tional groups, e.g., carboxylic, percarboxylic, epoxide and peroxide groups by the atomic O, excited O, anionic and cationic O in the gas in the RF chamber. These O-species interact with the functionalized –CH 2 –CH 2 – groups of PE binding TiO 2 by electrostatic attraction. Reports by: Sarakinos, et al., 9 Kelly et al., 10 Parkin et al., 11 Foster et al., 12 Yates, et al., 13 report the sputtering and CVD deposition of TiO 2 on different surfaces. The objectives of this study are: a) to present RF-plasma pretreated TiO 2 –PE sputtered uniform adherent films showing fast bacterial inactivation kinetics, b) to show light induced hydro- phobic to hydrophilic reversible switchable behavior in PE–TiO 2 films and c) to show by ATR-FTIR that the increase in the cell lipid layer fluidity during the loss of cell viability leads to bacterial cell- wall scission. The low density polyethylene (LDPE) consists of highly branched low crystalline semi-transparent film with the formula H(CH 2 –CH 2 ) n H. The PE was pretreated in the cavity of the RF- plasma unit (Harrick Corp. 13.56 MHz, 100 W). 3,8 The details of both pretreatment methods on C-rich surfaces have been recently described. 3–5 The TiO 2 was sputtered by direct current magnetron sputtering (DC) on polyethylene as reported out of our labora- tory. 4,8 Before sputtering the films, the residual pressure P r in the sputtering chamber was set to P r ¡ 10 24 Pa. The substrate target distance was set at 10 cm. The TiO 2 thin films were deposited by reactive DC-magnetron sputtering (DC) in an O 2 gas flow from a 5 cm diameter Ti-target 99.99% pure (Kurt J. Lesker, East Sussex, UK). The current on the Ti target was set at 280 mA, at a power of 128 Watt (2518 V). The samples of Escherichia coli (E. coli K12) were obtained from the Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ) ATCC23716, Braunschweig, Germany, to test the antibacterial activity of the PE–TiO 2 sputtered fabrics, and details of the evaluation of E. coli during the inactivation process have been recently reported. 5,8 The photo-induced super-hydrophilic hydrophobic transformation was evaluated on the PE–TiO 2 surface by the sessile drop method on a DataPhysics OCA 35 unit. The ATR-FTIR spectra were measured in a Portmann Instruments AG spectrophotometer equipped with a Specac attachment (45u one pass diamond crystal). The position of the IR peaks was found by the second derivative of the spectra after Fourier deconvolution. The effective mass of the TiO 2 used in the different samples was determined by X-ray fluorescence (XRF) a Ecole Polytechnique Fe ´de ´rale de Lausanne, EPFL-SB-ISIC-GPAO, Station 6, CH- 1015, Lausanne, Switzerland. E-mail: cesar.pulgarin@epfl.ch; Fax: +41 (0)21 693 56 90; Tel: +41 (0)21 693 47 20 b Ecole Polytechnique Fe ´de ´rale de Lausanne, EPFL-SB-IPMC-LNNME, Bat PH, Station 3, CH-1015, Lausanne, Switzerland c Ecole Polytechnique Fe ´de ´rale de Lausanne, EPFL-SB-ISIC-LPI, Bat Chimie, Station 6, CH1015, Lausanne, Switzerland. E-mail: john.kiwi@epfl.ch RSC Advances COMMUNICATION This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 16345–16348 | 16345 Published on 15 August 2013. Downloaded by ECOLE POLYTECHNIC FED DE LAUSANNE on 16/02/2015 09:55:42. View Article Online View Journal | View Issue