NIR-induced pH-reversible self-healing monitoring with smartphone by wireless hydrogel sensor

Near infrared (NIR)-induced reversible self-healing hydrogel electronics with pH-dependent controllable properties was designed by employing the pH-driven blooming-bridged phenomenon between polydopamine (PDA) and the zwitterionic fluorescent carbon dot (ZFCD). This self-healing hydrogel (PDA@ZFCD-PNIPAAm) was synthesized by incorporating an NIR-responsive PDA-loaded zwitterionic fluorescent carbon dot (PDA@ ZFCD) into a thermo-responsive PNIPAAm hydrogel, and further combined with wireless device to monitor self-healing and electronic property of hydrogel. The change in PDA@ZFCD-PNIPAAm hydrogel properties depended on the blooming-bridged effect of PDA@ZFCD, which controlled the photothermal release and intermolecular hydrogen bonding of PDA under different pH conditions. At the acidic and basic conditions (blooming-bridged state), PDA@ZFCD-PNIPAAm hydrogel displayed a self-healing behavior under NIR irradiation, which did not occur at a neutral pH (bloomless state) owing to different activation of photothermal heat under NIR irradiation with different hydrogel pore size. This hydrogel also demonstrated different thermal and electronic properties in different pH owing to the bridged phenomenon of PDA nanoparticles. Moreover, PDA@ZFCD-PNIPAAm hydrogel indicated reversible fluorescence on/off, self-healing, and electronic properties as the pH conditions changed. In addition, the self-healing phenomenon of hydrogel and the change of its electronic property can be easily monitored in a smartphone as a real-time information via wireless sensor. Therefore, this approach offers new insight on the development of self-healing hydrogels with simple, rapid and real-time monitoring on self-healing behavior, which can be used for future applications of hydrogels.