Sulfur and Nitrogen Co-Doped Graphene Quantum Dots as a Fluorescent Quenching Probe for Highly Sensitive Detection toward Mercury Ions

Sulfur and nitrogen codoped graphene quantum dots (SN-GQDs) were synthesized through an efficient infrared (IR)-assisted pyrolysis of glucose, urea, and ammonia sulfate at 260 °C. These served as a highly selective probe for the sensing of Hg²⁺ ions in an aqueous solution. The IR technique can also prepare N-doped graphene quantum dots (N-GQDs), which have been compared with SN-GQDs for their fluorescence (FL) quenching sensitivities by Hg²⁺ ions. The FL intensities of both GQDs show decreasing functions of concentration of Hg²⁺ ions within the entire concentration ranges of 10 ppb-10 ppm. The sensitivity of SN-GQD is 4.23 times higher than that of N-GQD, based on the calculation of the Stern-Volmer equation. One interband gap structure of SN-GQDs for the detection of mercury ions is proposed. The S doping can coordinate with phenolic groups on the edge of SN-GQDs (i.e., the formation of (C_xO)₂Hg²⁺) and induce the cutting off or alleviation of photon injection paths, thereby leading to significant FL quenching. This work proves that SN-GQD offers sufficient sensitivity for probing the quality of drinking water to ensure that it contains less than 10 ppb of Hg²⁺ ions, as per the World Health Organization standard.