Fast aqueous synthesis of hydrogel-immobilized papain-CuSO₄@SiO₂ for on-site solanine detection

Solanine is a toxic glycoalkaloid naturally present in potatoes and other Solanaceae crops, and its accumulation during storage poses a serious risk to food safety. However, current detection methods such as high-performance liquid chromatography, and mass spectrometry, are costly, labor-intensive, and unsuitable for rapid on-site analysis. In this study, we present a fast, green, and cost-effective colorimetric sensing platform for solanine detection based on a biomimetic papain-CuSO₄@SiO₂ nanozyme. The nanozyme was synthesized in water via a one-step reaction at room temperature, completing within 10 min without the use of precious metals, volatile organic solvents, or high-temperature treatments. The nanozyme mimics natural CuS enzyme active sites, promoting hydroxyl radical (·OH) generation through Fenton-like reactions to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). Incorporation of SiO₂ improves nanozyme dispersion, stability, and active site exposure. The nanozyme was further immobilized within a hydrogel, which can be freeze-dried into an aerogel-like format for long-term storage and rehydrated on demand. A cascade sensing strategy was developed using solanine's inhibition of acetylcholinesterase (ACHE) to modulate H₂O₂ production and control the TMB colorimetric signal. Integrated with smartphone-based RGB analysis, the system achieved sensitive and selective solanine detection with a detection limit of 0.0335 mg L^-1 in 40 min. The platform was successfully validated using potato and lake water samples, demonstrating excellent potential for portable, real-time food safety monitoring.