Mass-producible iron-based nanozyme Lap@Fe for smartphone-based colorimetric detection of ascorbic acid

Rapid and accurate detection of nutritional components in agricultural products is essential for modern food industry. Ascorbic acid (AA) is a key nutritional marker in fruits, making rapid and on-site quantification highly desirable. In this work, we developed a colorimetric sensor based on an iron-based nanozyme, Lap@Fe, which exhibits peroxidase-like activity. The nanozyme is prepared using industrial-grade Laponite (Lap), which is readily available, low-cost, and suitable for mass production. Lap@Fe efficiently catalyzes the H₂O₂-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxTMB. In the presence of AA, oxTMB is reduced, leading to color fading. Based on this principle, the sensor shows a linear detection range of 5-200 μM for AA with a detection limit of 1.4 μM. By further integrating smartphone-based RGB color analysis, the detection limit is significantly improved to 0.045 μM. The proposed method was successfully applied to the quantitative determination of AA in oranges, lemons, and strawberries.