Biocompatible Mn(II)-Enhanced N–S-Codoped Carbon Dots: A Versatile Fluorescence Sensor for Sensitive Hg²⁺ Detection in Coastal Seawater and Living Cells

Despite the promising potential of carbon dots (CDs) as a photoluminescent nanomaterial in advancing spectral analysis techniques for the detection of various harmful heavy metal ions such as Hg2+, Cu2+, Cd2+, and Pb2+, the fundamental challenge of effectively eliminating the interference of transition metal ions in multi-ion systems persists. In this study, we present straightforward, efficient, and versatile manganese(II)-enhanced nitrogen and sulfur codoped carbon dots (Mn(II)-N,SCDs) specifically designed for the highly selective and sensitive detection of Hg2+ ions. Mn(II)-N,SCDs exhibited uniform particle size (∼2.0 nm) and demonstrated excellent fluorescence performance, characterized by high fluorescence intensity and quantum yield (QY = 48.71%). The incorporation of Mn2+ not only enhances the fluorescence characteristics but also serves to effectively block the surplus transition metal ion binding sites on the surface of carbon dots, thereby leading to a heightened selective response to Hg2+. Furthermore, the synthesized Mn(II)-N,SCDs also exhibited low cytotoxicity and efficient cellular uptake, enabling fluorescence imaging of living cells. Importantly, the developed fluorescence sensor exhibited a highly specific response to Hg2+ ions even in the presence of other metal ions in phosphate-buffered solution (PBS), with a low detection limit of 0.29 nM (S/N = 3). The efficacy of the probe was successfully demonstrated through the determination of Hg2+ in live cells and natural coastal water samples.