Nanocomposite hydrogels with TiO2 nanoparticles as colorimetric sensors for monitoring UV radiation exposure

In this study we report the preparation and characterization of nanocomposite hydrogel materials acting as colorimetric sensors upon exposure to UV light. In the last few decades physicians worldwide have stressed the importance of protection from sun exposure. Given the variety of skin types, general exposure limits are not effective when it comes to sun protection. A hydrogel-based sensor to monitor exposure to UV-A radiation and that could possibly be adapted to different types of phototypes would be highly relevant and is presented in this work. The sensor was prepared using a gel matrix made of poly(acrylic acid) in which photoactive TiO2 nanoparticles and methylene blue (MB) dye were integrated. Once irradiated with UV-A light, activation of the TiO2 nanoparticles occurs leading to reactive oxygen species (ROS) formation and photocatalytic degradation of the MB dye. The dye loses its original colour as the degradation progresses, and therefore the colour intensity change can be used as an indicator of radiation exposure. The nanocomposite hydrogel system was prepared using different concentration of TiO2 nanoparticles and characterized by optical and physico-chemical techniques. In particular, Raman mapping was used to determine the network structure of the hydrogel and its water distribution, as well as to quantify the decrease of the characteristic Raman band of the MB dye upon exposure to UV-A radiation. Results indicate that at higher concentrations of TiO2 nanoparticles the loss of colour intensity of the hydrogel sensor becomes more noticeable and saturation is reached at points later in time.