Green and structurally tailored synthesis of g-CNQDs for highly selective dopamine detection

A facile and green one-step hydrothermal synthesis of graphitic carbon nitride quantum dots (g-CNQDs) using polyethylenimine (PEI) as a structure-directing agent is reported. The inherent structural features of PEI, including its branched architecture and abundant amine groups, drive a remarkable self-cyclization process that spontaneously generates triazine-based g-CNQDs exhibiting bright blue fluorescence. Unlike traditional synthetic strategies that involve complex procedures and hazardous reagents, our streamlined protocol produces g-CNQDs with a moderate quantum yield, outstanding aqueous dispersibility, and favorable biocompatibility. The resulting g-CNQDs exhibit exceptional sensing capabilities based on electrostatic interactions between their positively charged surfaces and negatively charged dopamine (DA) molecules. Upon binding, a stable non-fluorescent g-CNQDs-DA complex forms, generating a highly sensitive "turn-off" fluorescence response that correlates linearly with DA concentration. The developed probe achieves a low detection limit of 43 nM (S/N = 3) across a wide linear range of 0.1-300 μM. Validation in human urine and serum samples confirms the probe's high selectivity for DA over potentially interfering biomolecules and metal ions, with recoveries ranging from 97 to 106%. This approach effectively addresses the challenges of green synthesis and selective detection, offering a promising platform for sustainable and specific fluorescent probes in diverse bioanalytical applications.