J.J. Saarinen, D. Valtakari, R. Bollström, M. Stepien, J. Haapanen, J.M. Mäkelä, M. Toivakka
Abo Akademi University,
Keywords: R2R, TiO2, nanoparticles, photocatalysis, controlled wetting, O2 sensor
Summary:We present here ultraviolet A (UVA) light controlled photocatalytic activity of roll-to-roll (R2R) deposited TiO2 nanoparticles on paper, board, and plastic films both for controlled wetting and oxygen sensors for modified atmosphere packages (MAPs). It is well-known that a semiconductor TiO2 is photocatalytically active with anatase crystalline form having bandgap energy of 3.2 eV corresponding to a photon wavelength of 388 nm. Such photocatalytically active TiO2 nanoparticles and thin films have received much attention due to numerous industrial applications ranging from self-cleaning and antibacterial to anti-reflecting and anti-fogging properties. We use a liquid flame spray (LFS) process for large-area, R2R TiO2 nanoparticle deposition on natural fibre based substrates. LFS is a versatile process suitable for formation of various metal and metal oxide nanoparticles that can be deposited on various substrates such as glass and paper. In LFS process a liquid metal salt precursor dissolved in water or alcohol is fed into a high temperature and velocity flame in which the precursor evaporates and nucleates forming nanoparticles. The formed nanoparticle diameter can be controlled from 2 to 200 nm via the process parameters. In LFS the generated nanoparticles can be collected on a moving web, which allows large areas to be coated via roll-to-roll process flow. It was observed that superhydrophobic TiO2 surfaces with water CA of 161° are changed to hydrophilic with water CA of 6° by the UVA light. Full hydrophobicity recovery is achieved by placing the sample into oven for 3 min. The changes in the nanoscale surface chemistry correlate well with wetting as verified by XPS and ToF-SIMS measurements. We also demonstrate here an oxygen indicator based on a methylene blue (MB) indicator ink used with TiO2 nanoparticles. Colorimetric oxygen sensors are based on reversible redox reaction of a dye that results in a change in the light absorption, and thus color. Such colorimetry-based inks can be used to improve food safety and to reduce the amount of wasted food by introducing cost-effective oxygen sensors into MAPs. A reverse gravure coated thin film of MB/TiO2 indicator ink was deposited on a plastic film. We have used UVA light activation with a photomask to generate patterned oxygen indicator films such as 1D and 2D barcodes, and such patterned sensors prevent misinterpretation by unwanted photobleaching during the package storage. First, no oxygen leak has taken place if the pattern is clearly visible. Secondly, there has been an oxygen leak if the whole indicator pattern is faded away to blue i.e. to oxidized, blue state MB. An additional advantage with the patterning approach is that a high image resolution of oxygen indicator films can be achieved with ease. As a conclusion we have studied the photocatalytic activity of TiO2 nanoparticles coated on a paperboard or on a plastic film. We believe that photocatalytically active surfaces with tailorable properties will find many applications in the near future e.g. with printed functional devices.