UV sensors based on nanocomposite hydrogels as a sun exposure alarm

Nowadays there is increasing attention for the dangerous effects of skin exposure to ultraviolet radiation, but, on the other hand, also for the healthy sun exposure. In this work, we present a colorimetric UV sensor based on nanocomposite hydrogel material that should provide, for a wide range of skin sensitivity and phototype, an alert when maximum sun exposure is received. This sensor is based on a biocompatible hydrogel matrix, prepared by combining sodium alginate and porcine gelatin, containing photoactive TiO2 nanoparticles and methylene blue (MB) dye. The TiO2 nanoparticles were synthesized by laser pyrolysis with a mean diameter d=12 nm as resulted by BET analyses. These nanoparticles hosted in the hydrogel matrix are photoactive and catalyse a reaction by which methylene blue is converted into leuko-methylene, causing a well-visible discoloration effect that can be used as an indicator of UV radiation exposure. Nanocomposite hydrogel samples were prepared using different concentrations of TiO2 nanoparticles and characterized by physico-chemical techniques. 2D Raman spectroscopy was used to study the chemical structure of the hydrogel matrix and the nanocomposite. The decreasing of the characteristic Raman band of the MB dye upon exposure to UV and sun radiation was investigated, following the discoloration process as a function of the exposure times, also through colorimetric analysis and luminescence measurements. The results indicate different behaviors in the discoloration process for samples with different concentrations of TiO2 nanoparticles, suggesting further investigations for the development of sensors able to be adapted to different phototypes and skin sensitivities.