Iron(iii) titanates are composed of earth-abundant elements and are attracting rapidly growing interest as highly promising candidates for solar-energy as well as optoelectronics applications.

Our eyes and skin are routinely exposed to hazardous ultraviolet light (UV) from sunlight as well as to high-energy blue light (HEBL) from modern IT devices. Therefore, it is essential to protect our health by blocking this harmful high-energy radiation. In this study, we demonstrated a biodegradable, transparent, flexible film that blocks this dangerous radiation using cellulose-encapsulated, nitrogen-doped, carbon dots (NCDs). The interaction between cellulose and NCDs by H-bonding enables effective incorporation of NCDs into the cellulose matrix (NCDs@cel). This film blocks UV-A and UV-B as well as HEBL. Absorbed UV and HEBL are converted into lower-energy (longer-wavelength) light in the blue to green region, reflecting its excitation-dependent emission properties. Hence, the film protects us from exposure to harmful photoradiation. The film’s excellent visible transparency at 540 nm, strong UV blocking, and downconverted emission wavelengths can readily be tuned by simply adjusting the concentration of NCDs in the cellulose film. This hybrid film offers promising solutions to efficiently block UV and HEBL light simultaneously, potentially enabling practical applications such as window coatings against sunlight or for display screens.