Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements

Single-step environmentally friendly synthesis of biochar dots (BCDs) was developed using hydrothermal treatment of waste biomass. Using soy residue as the carbon precursor, the resultant BCDs had strong and stable photoluminescence. Characterization by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy indicates that the BCDs prepared were water soluble, spherical, oxygenous and nitrogen-doped carbon nanoparticles with 10–20 nm in diameter. The fluorescence quantum yield of the BCDs was 3.7%. The use of the BCDs as a very effective fluorescent probe for label-free, rapid, and selective detection of Hg2+ and Fe3+ ions was further demonstrated with good linear relationships at 0–50 μM and 10–50 μM, respectively. The minimum detection limits of Hg2+ and Fe3+ were 100 nM and 30 nM. Furthermore, the feasibility of using the BCDs for monitoring of Hg2+ and Fe3+ in open waters was also established.