Solvent-controlled synthesis of nitrogen-doped carbon dots from mangosteen peel extract and application in the detection of mercury ions

Heavy metal contamination poses serious environmental and health risks, even at trace levels, highlighting the need for efficient and sustainable detection strategies. This study presents a green approach for producing nitrogen-doped carbon dots (CDs) from mangosteen peel extract (MPE)—a widely available agricultural waste—via a one-pot solvothermal method using ethylenediamine as a nitrogen source. By valorizing biomass under mild reaction conditions, this method contributes to the development of sustainable nanomaterials. The effects of solvent polarity (water, ethanol, methanol, and isopropanol) on the structural and optical properties of the resulting CDs were systematically investigated. Solvent-dependent fluorescence behavior was observed, with tunable emissions from blue to greenish-yellow and redshift correlating with decreasing solvent polarity. Comprehensive characterization revealed distinct differences in surface functionalities, aggregation tendencies, and emission profiles, all governed by solvent–precursor interactions. The CDs exhibited selective fluorescence quenching in the presence of Hg2+ ions, showing a linear detection range of 0–20 μM and detection limits between 2.78 and 3.00 μM. Furthermore, the CDs demonstrated reliable Hg2+ detection in real wastewater samples, confirming their potential for practical, equipment-free environmental monitoring. These findings underscore the role of solvent environments in modulating the structure-property relationships of biomass-derived carbon nanomaterials, offering design strategies for resource-based sensing platforms and other functional applications.