Highly transparent and thermally stable cellulose nanofibril films functionalized with colored metal ions for ultraviolet blocking activities

Although many preparation methods have been reported so far, it is still a great challenge for Ultraviolet (UV) protection films that simultaneously have extremely high transparency and excellent UV shielding properties. Herein, we reported a simple, eco-friendly process for preparing high transparency TOCN-COOM (M: metal ions) films with diverse UV-shielding performances via adsorption of colored metal ions (Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺). The introduction of Fe³⁺ lead to an excellent UV blocking, especially almost 89% absorption of Ultraviolet A (UVA) and full absorption of Ultraviolet B (UVB). Simultaneously an extremely high transmittance in the visible light region (around 88%) was retained. In addition, the UV shielding films performed stable UV-blocking when exposed to UV irradiation or thermal treatment at 120 °C. The formation of metal-carboxylate complexes is also advantageous for improving thermal stability. Hence, the obtained TOCN-COOM films have wide application prospect in the field of UV shielding.