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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 Hg 2+ and Cu 2+ 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 Hg 2+ and Cu 2+ 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 Hg 2+ and Cu 2+ ions in the gastrointestinal tract of brine shrimp, Artemia.

A sunlight induced simple green route has been developed for the synthesis of polyacrylate functionalized gold nanoparticles (PAA-AuNPs), in which poly(acrylic acid) functions as a reducing as well as stabilizing agent. This material has been characterized on the basis of spectroscopic and microscopic studies; it exhibited selective colorimetric detection of Al in aqueous media, and the Al induced aggregated PAA-AuNPs exhibited detection of F with sharp color change and high selectivity and sensitivity out of a large number of metal ions and anions tested. The mechanistic study revealed that, for Al, the color change is due to a shift of the SPR band because of the Al induced aggregation of PAA-AuNPs, whereas for F, the reverse color change (blue to red) with return of the SPR band to its original position is due to dispersion of aggregated PAA-AuNPs, as F removes Al from the aggregated species by complex formation. Only concentration-dependent fluoride ion can prevent Al from aggregating PAA-AuNPs. The method is successfully used for the detection of F in water collected from various sources by the spiking method, in toothpastes of different brands by the direct method. The solid Al-PAA-AuNPs were isolated, adsorbed on ZIF@8 (zeolitic imidazolate framework) and on a cotton strip, and applied as solid sensing material for detection of F in aqueous media.