In Situ Doping-Enabled Metal and Nonmetal Codoping in Graphene Quantum Dots: Synthesis and Application for Contaminant Sensing

Fluorescent graphene quantum dots (GQDs) prepared from low-cost and sustainable precursors are highly desirable for various applications, including luminescence-based sensing, optoelectronics, and bioimaging. Among different natural precursors, the unique structural and compositional variety and the abundance of aromatic carbon in lignin make it a unique and renewable precursor for the green synthesis of advanced carbon-based materials including GQDs. However, the inferior photoluminescence quantum yield of GQDs prepared from natural precursors, including lignin, limits their practical utility. Here, for the first time, we demonstrate that the presence of heteroatoms in the innate structure of lignosulfonate can be leveraged to derive in situ heteroatom-doped GQDs with excellent photophysical properties. The as-synthesized lignosulfonate-derived GQDs showed compelling blue fluorescence with a high quantum yield of 23%, which is attributed to in situ S and N doping as confirmed by using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. Assisted by the in situ doping, we further engineered the lignosulfonate-derived GQDs by incorporating a metal atom dopant to derive an enhanced quantum yield of 31%, the highest for any lignin-derived GQDs. Moreover, fundamental photoluminescence studies reveal the presence of multiple emissive centers, with edge states acting as dominant emission centers. Finally, we also demonstrate the applicability of the luminescent, metal- and nonmetal-codoped lignin-derived GQDs as a highly selective sensor for the sub-nanomolar level detection of mercuric ions in water.