Single-nanozyme based hydrogel patch for total antioxidant capacity and pH synchronous visual monitoring

Simultaneously visual monitoring of diversified indicators in sweat and food is crucial for healthcare and food safety assessment. While nanozymes have been explored for colorimetric detection of various biomarkers in sweat, monitoring different biomarkers with a single nanozyme remains challenging. Herein, a novel strategy for real-time synchronous monitoring of total antioxidant capacity (TAC) and pH value using a single nanozyme decorated hydrogel patch has been demonstrated. Fe²⁺ doped calcium peroxide nanoparticles (Fe-CaO₂) were encapsulated in a zeolitic imidazolate framework (ZIF) with the modification of Cu²⁺ to synthesize a novel nanozyme (FCZC). The characteristic of FCZC for self-generating H₂O₂ in an acidic environment eliminated the risks associated with the addition of H₂O₂ externally. FCZC exhibited high peroxidase-like catalytic activity toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) via the Fenton and Fenton-like reactions, with a low Michaelis constant of 0.043 mM. Furthermore, the activity of FCZC exhibited a susceptibility to acidity within a wide pH range. Therefore, based on this, TAC and pH were sensitively detected in the linear ranges of 0.5 μM - 70 μM and 4.0 - 6.8, respectively. A hydrogel patch was constructed by encapsulating FCZC within a sodium alginate hydrogel, enabling synchronous visual detection of TAC and pH in sweat and food samples, which was further integrated with smartphone signaling for in-situ monitoring. The work presented a simple hydrogel sensor enabled the real-time synchronous monitoring of various indicators in sweat and food based on single nanozyme catalysis, facilitating daily health and food safety assessments for the public.