Detection of fluoride ions using carbon quantum dots derived from coal washery rejects via an on-off-on fluorescence mechanism: a smartphone-based image-processing and machine learning approach

Given the high toxicity and easy availability of fluoride ions in surface as well as ground water, an on-off-on fluorescence process-based fluoride ion sensor is developed by using highly fluorescent carbon quantum dots (CQDs) derived from coal washery rejects/waste coal (CWRs). The chemical properties of the fabricated CQDs are evaluated by using high resolution-transmission electron microscopy (HR-TEM), Fourier transform-infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) studies. The as-synthesized CQDs/Fe(3+ )was validated to be an excellent nanoprobe in aqueous conditions based on fluorescence quenching with high sensitivity towards F- ions. The "turn off" fluorescence mechanism of the developed sensor (i.e., CQDs/Fe3+) was investigated by using fluorescence quenching, TRPL (time resolved photoluminescence spectroscopy), XPS, and HR-TEM analysis. The developed CQDs/Fe3+ sensor was found to have promising applications in estimating trace F- ions in real water samples with the working range of 0-20 ppm with a detection limit of as low as 0.3 ppm. The nanoprobe also shows high sensitivity of F- ion detection in the presence of various anions, and was found to be non-toxic, environmentally benign, cheap, rapid, efficient, and most significantly selective. Furthermore, the nanoprobe was satisfactorily employed as a simple and alternative typical laboratory-based fluoride monitoring technique by using a smartphone based on image processing and machine learning approaches. In addition to that, CQDs used in the nanoprobe are considered to be bio-compatible and eco-friendly provided its safety against normal cells with an average cell viability of more than 80% (500 mu g ml-1) and as such no apparent DNA fragmentation in the treated cells was observed at a concentration of 500 mu g ml-1. CQDs will not only serve a significant role in fluoride diagnostic tools, but they are also safer and less hazardous than other conventional nanomaterials.