Eco-Friendly Self-Passivated Fluorescent Carbon Quantum Dots for Rapid Paper-Based Nanoscale Sensing of Mercury in Water Matrices

Agro-industrial waste and coal are abundant carbon resources endowed with pre-existing functional groups that can advance the development of value-added materials of societal relevance. Developing eco-friendly fluorescent carbon quantum dots as sensors from these natural precursors to detect mercury in aqueous matrices is a significant challenge that has not yet been reported. In this work, we report a green, cost-effective carbon quantum dot-based mercury sensor prepared from a mixture of sulfur-containing Indian coal and biochar derived from potato peel, which provides abundant sulfur- and nitrogen-containing functionalities without the use of hazardous chemicals. The carbon quantum dots exhibit strong blue fluorescence, excellent water dispersibility, and approximate diameters of 3 nm. Fluorescence quenching of carbon quantum dots by divalent mercury enables sensitive and selective detection, attributed to strong interactions between mercury ions and sulfur-nitrogen-containing surface functionalities of the prepared materials. The fluorescent sensor displays a broad linear detection range of 0–10 nM, a low detection limit of 0.98 nM, and a strong resistance to interference from coexisting ions. The developed fluorescence-quenching-based technique was shown to be effective by validating its findings against a Lumex mercury analyzer. The practical applicability was validated through successful mercury detection in real aqueous samples and the development of a paper-based sensing chip. This work emphasizes the potential utilization of naturally abundant materials for developing environmentally benign carbon-based nanosensors for mercury monitoring.