Fe-Doped TiO ₂ Hollow Nanospheres as Light-Responsive Peroxidase-Like Nanozymes for the Colorimetric and Fluorescent Dual-Mode Detection of d -Penicillamine in Foodstuffs and the Environment

Developing light-responsive nanozymes with integrated activity and stability holds significant importance for highly sensitive and accurate sensing in food safety and environmental protection. Herein, we developed a dual-mode (colorimetric and fluorescent) sensor based on an Fe-doped TiO₂ nanosphere for d-penicillamine (D-PA) detection. The unique hollow structure of Fe-TiO₂ provides a large surface area and abundant oxygen vacancies while facilitating rapid Fe(II)/Fe(III) cycling, collectively resulting in enhanced light-responsive peroxidase (POD)-like activity. Under light irradiation, the Fe-TiO₂ promoted the generation of abundant reactive oxygen species, which oxidize TMB to blue oxTMB. Furthermore, the Fukui index and reaction kinetic trials indicated that the detection mechanism was primarily based on the antioxidant effect and redox activity of D-PA. The proposed sensor demonstrated detection limits (LODs) of 0.054 μM (colorimetric) and 0.40 μM (fluorescence), with 92.2-107.4% recovery and relative standard deviation < 4.4% in milk, tap water, and fetal bovine serum, along with outstanding reproducibility and stability. This work provides new insights to synthesize light-responsive nanozymes and integrated multimode sensing platforms.