Water-Dispersible Fluorescent Carbon Dots as Bioimaging Agents and Probes for Hg²⁺ and Cu²⁺ Ions

Fluorescent carbon dots (N-CDs) were synthesized from ascorbic acid and urea following a green route and were characterized on the basis of analytical, spectroscopic, and microscopic techniques. The surface functional groups of the carbon dots were identified as −NH, −OH, C═O, etc. using FT-IR. The N-CDs are highly stable under the wide ranges of pH, ionic strength, and solvents. It also exhibited an excitation wavelength dependent fluorescence property: the emission band with maximum intensity was observed at 415 nm by excitation at 340 nm. The metal ion sensing study revealed that N-CD detects Hg2+ and Cu2+ in aqueous media out of a large number of metal ions tested. A detailed study suggested that the metal ions interact with the surface bound functional groups of the multiple N-CDs, promoting aggregation, as evident from the TEM images, and quenching in emission intensity takes place through a PET process. For field application, paper-based sensing strips were prepared by coating N-CDs on filter paper, which was successfully tested for sensing of Hg2+ and Cu2+ in aqueous media as well as in solid state. N-CD was found to be nontoxic and was used as bioimaging agent for detection of Hg2+ and Cu2+ ions in the gastrointestinal tract of brine shrimp, Artemia.