Green-Synthesized Carbon Quantum Dots for Environmental Monitoring of Heavy Metals: A Critical Review of Mechanisms and Performance

The heavy-metal contamination of water bodies is still a crucial environmental issue and a hazard to the ecosystem as well as human health, and incomplete knowledge of how the synthesis of carbon quantum dots (CQD) can regulate the analytical performance of sensing technologies. Despite the benefits of CQDs reported in many studies, systemic linkages between preparation strategy, surface chemistry, photophysical behaviour, and sensing efficiency are yet to be explained well. This review presents a critical argument of the green bottom-up strategies to fabricate CQDs and compares them to the top-down methods in order to understand the way precursor selection, the introduction of heteroatoms, and the control of defects affect quantum yield, emission mechanisms, and environmental friendliness. The special focus is put on the fluorescence modulation mechanisms such as photoinduced electron transfer, Förster/energy transfer, inner filter effects and static versus dynamic quenching and their application in the detection of priority pollutants, including Pb(II), Hg(II), Cu(II), Cr(V) and Fe(III). In addition to publishing literature research, we compare reported detection limits, linear working ranges and selectivity factors to derive general performance patterns. The analysis has shown that controlled surface passivation and functional groups are the final determinants towards sub-micromolar sensitivity and better anti-interference properties. Other issues raised regarding reproducibility, complexity of real-world samples, long-term stability and scalable green production are also addressed. Combining synthesis-structure-property-performance correlations, this review develops empirical design guidelines of the next-generation CQD sensors and explains feasible ways of developing sustainable monitoring and mitigation of heavy-metal pollution in the environment.