Hydrochromic paper-based dosimeter for monitoring UV light exposure based on the photochemical formation of gold nanoparticles

A paper-based UV dosimeter/indicator based on the solid-state photochemical reduction of gold-cationic surfactant complexes is described. Upon exposure to UV light, the photoreductive elimination of chloride ions from the CTA-AuCl₄^- complex leads to the reduction of Au ions and the formation of small Au nuclei. After hydration, the Au nuclei are washed away by capillary flow and produce gold nanoparticles by coalescence and aggregation, resulting in a blue-purple coloration on the paper surface. The reaction is initiated by UV light of variable wavelength (from UVA at 254 nm tο UVC at 365 nm) and over a wide range of UV doses (up to 3000 mJ/cm² for UVB at 312 nm and UVC at 365 nm). This versatility allows for its application in monitoring light exposure in germicidal sterilization, UV phototherapy, and as a personal solar UV indicator. The user-friendly paper facilitates both the fabrication and use of the devices, which can be activated on demand by simply wetting the paper surface.