Smartphone integrated portable oxidase-like hydrogel kit for UO₂²⁺ colorimetric analysis

Uranyl ion (UO₂²⁺) pollution in marine environments poses a serious threat to human health and aquatic ecosystems. To overcome the dependence of traditional detection methods on large-scale instruments, a portable smartphone-integrated hydrogel kit was reported for on-site quantitative detection of UO₂²⁺ with nanozyme technology. Spindle-shaped FeOOH/MnO₂ nanozymes with intrinsic oxidase-like activity were fabricated within an agarose hydrogel matrix in a 96-well plate. FeOOH/MnO₂ catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) without the need of unstable hydrogen peroxide (H₂O₂). UO₂²⁺ causes a distinct blue-to-colorless transition by forming a complex with OX-TMB. Quantification of UO₂²⁺ was achieved using either a microplate reader or a smartphone by analyzing the RGB values extracted from the hydrogel wells. The detection limit was 5.53 ppb, which is below the drinking water safety standard of 30 ppb stipulated by the World Health Organization (WHO). To the best of our knowledge, it is the first example of a portable oxidase-like hydrogel kit for UO₂²⁺ detection. By merging nanozyme catalysis, hydrogel encapsulation, and smartphone-based readout, this platform provides a reliable, high-throughput, and field-deployable strategy for monitoring uranium contamination and may inspire the development of detection strategies for other marine pollutants.