Guava leaf-derived carbon quantum dots as fluorescent probes for turn-on/off detection of Ba2+, Pb2+, Sn2+, and Hg2+ ions

. Carbon quantum dots (CQDs) are extensively studied zero-dimensional materials due to their high biocompatibility, non-toxicity, trivial size, high solubility, high quantum yield, photoluminescence, etc. In the present research project, bio-based CQDs were synthesized from bio-waste precursor Psidium Guajava (common name- guava) leaves via an inexpensive and environmentally friendly method i.e. pyrolysis at different temperatures. Various characterization procedures such as Fourier transform infrared spectroscopy (FTIR), Zeta potential, UV-visible absorption spectroscopy, Fluorescence spectroscopy, Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM), Energy dispersive analysis of X-rays (EDAX), X-ray photoelectron spectroscopy (XPS) and elemental mapping were employed to understand the shape, size, composition, morphology, surface functionality, optical properties, luminescence etc. of the fabricated CQDs. Globular CQDs of high quantum yield and size 4.6 nm were obtained at annealing temperatures of 420°C. The absorption, excitation and emission wavelengths measured from the UV-visible spectrophotometer were 230 nm, 250 nm and 425 nm respectively. Bio-based CQDs were labored as fluorescent chemo-sensors for the diagnosis of heavy metal ions Sn 2+ , Pb 2+ , Hg 2+ and Ba 2+ from aqueous media. The photoluminescence study revealed that Sn 2+ , Hg 2+ , and Pb 2+ ions showed quenching in the fluorescence whereas Ba 2+ ions displayed enhancement. Highly sensitive CQDs showed selective sensing of metal ions in order as, Sn 2+ >Pb 2+ >Hg 2+ by turn-off mechanism whereas sensing of Ba 2+ ions took place by turn-on mechanism. The sensing features were assessed by using the Stern–Volmer quenching model. Therefore, the present investigation successfully demonstrated the approach of adapting biowaste into high-value optical sensing receptors for sensing applications.