Sensitive and Cumulative UVA Dosage Identification Card Based on Photochromic Crystalline/Amorphous Tungsten Oxides

Ultraviolet A (UVA) irradiation poses a health risk, as excessive skin exposure may lead to serious skin diseases. Therefore, monitoring the cumulative dose of UVA radiation received by the human body over the course of a day is of great importance. In this study, we propose a dual-phase synergistic photochromic strategy based on crystallinity-regulated amorphous/crystalline WO 3 for rapid UVA indication and cumulative UVA dose recording. Benefiting from the synergistic enhancement mechanism at the amorphous–crystalline interface, the optimized WO 3 @400 test paper exhibits outstanding photochromic performance. Upon UVA irradiation, the test paper exhibits a rapid and visually distinguishable color transition from yellow-green to blue-green. It shows a high sensitivity (Δ E = 9.6 within 3 s under 10 mW/cm 2 ) and a large color difference (Δ E = 44.65 after 420 s under 120 mW/cm 2 ). Moreover, the visible/NIR absorption increases markedly after UVA irradiation, with an absorbance difference of 74% at 885 nm, further confirming its excellent photochromic coloration capability associated with polaron absorption. On this basis, two types of test papers with different dose response ranges were fabricated: the low-dose test paper has a response range of 0–100 mJ/cm 2 , serving as a cumulative monitoring threshold for low-intensity UVA exposure over a short period (e.g., indoors near a window or on a cloudy day); the high-dose test paper has a response range of 0–1000 mJ/cm 2 , matching the unweighted 8-h occupational exposure limit for UVA (315–400 nm) established by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). Both test papers exhibit progressive and dose-dependent color evolution with increasing cumulative UVA dose. Furthermore, by mixing samples with different crystallinities, the color gradient can be further tuned to meet diverse application requirements. Therefore, the constructed UVA detection test paper enables visual monitoring of the cumulative UVA dose, providing a simple, low-cost, portable, and power-free technical approach for personal UVA protection.