Covalently Coupled Phenylenediamine and Citric Acid-Based Carbon Dots: A Dual-Emissive Architecture for Rapid Optical Cr(VI) Sensing and Live-Cell Imaging.

Carbon dots (CDs) are fluorescent nanomaterials known for their excellent photostability, tunable fluorescence, and biocompatibility, making them ideal for various applications. Within this class, dual-emissive CDs are particularly noteworthy, as they can exhibit two distinct emission peaks under a single excitation wavelength. This study reports the development of covalently coupled dual-emissive CDs, synthesized from 1,2-phenylenediamine and citric acid, which exhibit distinct dual emission peaks at 455 and 555 nm under a single 390 nm excitation wavelength. These coupled CDs served as a selective fluorescent probe for Cr-(VI) detection, demonstrating a linear response of the emission peak at 455 nm across the range of 5 to 500 μM. The quenching mechanism was investigated through time-resolved photoluminescence (TRPL) and spectral overlap analysis. The results indicated that the fluorescence response was predominantly structured by the inner filter effect, while the nearly unchanged fluorescence lifetime suggests that dynamic collisional quenching is not the dominant pathway. A key advantage of this system is its rapid Cr-(VI) detection capability, with a response time of less than 15 s. Furthermore, the probe was successfully applied for visualizing Cr-(VI) detection within NIH/3T3 cells through fluorescence imaging, and the probe showed low cytotoxicity under the short-term imaging conditions, with cell viability remaining above 90% after 4 h exposure at concentrations up to 5 mg/mL. The covalent coupling strategy yields a stable dual-emissive material with enhanced sensing performance for Cr-(VI) compared to individual or simply mixed CDs, highlighting its potential for environmental monitoring and bioimaging applications.