Coencapsulating TMB Probes and Bimetallic MOF Nanozymes in a Hydrogel Patch for Fabricating Reusable Visual VC Sensors

As a typical chromogenic probe, 3,3',5,5'-tetramethylbenzidine (TMB) has been widely applied in the field of visual detection due to its low toxicity and highly sensitive response. Due to the hydrophobic nature of TMB, encapsulating it into a hydrogel, which serves as an ideal matrix for wearable sensors, presents significant challenges that complicate the fabrication of visual wearable devices. Herein, the TMB probe and bimetallic MOF nanozymes are coencapsulated in a hydrogel patch for the fabrication of reusable visual sensors. Hydrophobic TMB is oxidized to hydrophilic ox-TMB by a bimetallic MOF (CuFe-MOF), allowing its diffusion into a hydrophilic agarose hydrogel patch, where it is reduced back to TMB. This process allows the coimmobilization and coencapsulation of CuFe-MOF and TMB within the hydrogel patch. Leveraging the color change between TMB and ox-TMB, as a proof-of-concept application, a reusable visual "On-Off-On" sensor is simply constructed and successfully applied to detect vitamin C in human sweat. Color changes can be quickly read by the naked eye or by smart devices without the need for external equipment. Meanwhile, based on the reversible conversion relationship between TMB and ox-TMB, a reusable sensor construction strategy is proposed. This approach not only facilitates the use of a TMB probe in hydrogel applications but also offers inspiration for the development of point-of-care testing equipment, demonstrating significant application potential.