Biogenic Carbon Dot-Embedded Chitosan Hydrogels as a Two-Stage Fluorescence OFF–ON Probe for Sequential and Ratiometric Detection of Fe(III) and Glutathione

Biogenic carbon dots (BCDs) are useful fluorescence sensors for selective detection of toxic metals. In the present work, we propose to synthesize a hybrid fluorescent chitosan gel using BCDs for detection and enrichment of Fe(III) ions and subsequent ratiometric detection of glutathione (GSH). Carbon dots were synthesized using waste lemon peels (LCDs) as the precursor. The sensing studies showed the selectivity and ultrasensitivity of the LCD sensor toward Fe(III) in the presence of other metal ions with a limit of detection as low as 17 pM. The visual as well as PL studies demonstrated that the LCD sensing probe is free from any significant interference. The LCDs were successfully fabricated to develop "in-house" paper-based kits for in situ and on-site detection of Fe(III). Further, a fluorescent chitosan-based hydrogel (CDG) was synthesized by conjugation of LCDs for the selective uptake and enrichment of Fe(III), which led to quenching of fluorescence with >90% encapsulation. Post enrichment, the Fe(III) enriched LCD hybrid hydrogel (Fe@CDG) was further applied for the selective and ultrasensitive detection of GSH with the limit of detection of 15.22 pM. The sensing studies showed that the detection of GSH is free from any interference. Thus, the fluorescent hydrogel exhibited quenching on entrapment of Fe(III) (Turn OFF) and recurrence of fluorescence on the release of entrapped Fe(III) in the presence of GSH (Turn ON). Additionally, LCDs and the CDG demonstrated potential antibacterial activity, annihilating bacteria in water samples in less than 10 min of photoexcitation, due to the potential ROS generation ability of the LCDs.