Self-healable and self-adhesive hydrogel based gas sensor using carbon quantum dots embedded ZnMn2O4 platelets for ppb level sensing of toluene at room temperature

Self-healing and self-adhesive materials are some of the most promising smart materials that are being actively explored for sensing applications. In this work, we report a multiresponsive, self-adhesive, and self-healing hydrogel based on carbon quantum dots (CQDs) embedded in zinc manganite (ZnMn2O4) for the detection of toluene at room temperature. CQDs are synthesized from stone apple shell via hydrothermal method and ZnMn2O4 is synthesized using a co-precipitation process. An optimized ratio of composite crosslinked by polyvinyl alcohol, boric acid, and acrylic acid-based hydrogel (CQDs/ZnMn2O4 HG) is prepared using cyclic freezing-thawing method. Detailed morphological analysis reveals the even distribution of CQDs on the ZnMn2O4 platelets which are then encapsulated in the polymer matrix. The hydrogel shows green luminescence under 365 nm wavelength of UV light and photoluminescence (PL) properties in aqueous medium. The performance of ZnMn2O4, CQDs/ZnMn2O4, composite, and CQDs/ZnMn2O4 HG towards toluene sensing is investigated where CQDs/ZnMn2O4 HG shows the best response. The CQDs/ZnMn2O4 HG-based toluene sensor exhibits a wide dynamic range of 550 ppb to 50 ppm with fast response/recovery speed (26 s/36 s), and a low detection limit of ∼ 502 ppb. This hydrogel sensor also displays an excellent selectivity towards toluene when compared to other volatile organic compounds (VOCs). A comprehensive sensing mechanism is also discussed using band energy diagram. The improved toluene detection performance is attributed to the structural merits and synergetic effects including the formation of p-p heterojunction, high specific surface, and abundance of O2 functional groups (Oc% = 72.24 %). Further, this hydrogel shows outstanding mechanical properties in addition to self-adhesion and self-healing efficiency (within 5 min) thus establishing itself as a robust material for wearable applications. © 2025 Elsevier B.V.