On-line monitoring of a recirculating-flow fluorescent capillary system for exploring the interaction mechanism of carbon dots/metal ions

Abstract

The interaction mechanism between carbon dots (CDs) and metal ions is essential for optimizing their design, synthesis, and application. However, it must be accurately distinguished and quantified because of CDs' complex structure, composition, and coexisting various response mechanisms or products. Herein, a recirculating-flow fluorescence capillary analysis (RF-FCA) system was developed to online monitor the fluorescence kinetics of CDs interacting with metal ions. The fluorescence kinetics of purification and dissociation of CDs/metal ion complexes were easy to monitor online by integrating immobilized CDs and RF-FCA. Here, CDs derived from citric acid and ethylenediamine were used as a model system. We found that the fluorescence of CDs is quenched by Cu(II) and Hg(II) only through the formation of a coordination complex, by Cr(VI) only through the inner filtering effect, and by Fe(III) through the above two mechanisms. Then the kinetics of the competitive interaction between metal ions were used to address the difference of binding sites on CDs with metal ions, wherein Hg(II) was bound to other sites of CDs besides the same sites of CDs with Fe(III) and Cu(II). Finally, from the fluorescence kinetics of fluorescent molecules in the CD structure with metal ions, the difference was due to the presence of two fluorescent centers in the carbon core and molecular state in the CDs. Therefore, the RF-FCA system can distinguish and quantify the interaction mechanism between metal ions and CDs effectively and accurately and be a potential detection or performance characterization method.