Molecularly imprinted polymer SiO2 nanostructures combined with anthocyanin from purple sweet potato peel for on-site colorimetric sensing of benzoic acid in milk.

Benzoic acid is an important preservative-related marker in dairy products, and its rapid on-site detection remains challenging because conventional analytical methods generally require laboratory-based instrumentation and time-consuming procedures. In this study, a sustainable and portable colorimetric sensing platform was developed for benzoic acid detection in milk by integrating molecularly imprinted SiO2 nanostructures with anthocyanin extracted from purple sweet potato peel. SiO2 nanoparticles were synthesized using a modified Stöber sol-gel method and subsequently functionalized through a surface molecular imprinting polymer (MIP) strategy using benzoic acid as the template molecule and a tetraethyl orthosilicate-(3-aminopropyl)triethoxysilane (TEOS-APTES) silane system to form MIP@SiO2 recognition sites. Anthocyanin served as a natural pH-responsive chromogenic probe, converting the selective rebinding of benzoic acid into measurable optical signals. The sensing mechanism was attributed to the synergistic effect of benzoic acid recognition within the imprinted cavities and the microenvironment-induced structural transformation of anthocyanin. The developed MIP@SiO2/anthocyanin system exhibited a concentration-dependent colorimetric response toward benzoic acid, with a limit of detection of 27.2 mg L-1. The sensor showed good selectivity against common milk-related interferents, including urea, melamine, sodium chloride, and calcium chloride dihydrate. Practical applicability was demonstrated in spiked milk samples, achieving satisfactory recoveries of 96.0-99.0% using smartphone red green blue (RGB) analysis and 96.32-97.20% using the portable device, with low relative standard deviations. Furthermore, a compact detection device integrating a white LED, AS7341 multispectral sensor, Arduino microcontroller, and bluetooth transmission enabled portable optical readout without requiring a conventional spectrophotometer. These results demonstrate that the proposed MIP@SiO2/anthocyanin sensing platform provides a low-cost, sustainable, and field-deployable strategy for rapid benzoic acid screening in milk.