Synthesis and functionalization of red-emissive carbon dots towards sensing of copper(II) and ATP in aqueous media

Red-emissive carbon dots, RCDs, were prepared in a bottom-up approach, from citric acid and urea, through a single solvothermal step, which were posteriorly surface-functionalized with di-(2-picolyl)amine (DPA) groups. The functionalized RCDs, henceforth named CD1, were analyzed through high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), dynamic light-scattering (DLS) and infrared spectroscopy (FTIR), and were found to be spherical, with diameter of circa 4 nm. UV-Vis and luminescence spectroscopic studies showed that CD1 display an absorption maximum at 550 nm, with emission maxima at around 640 nm. Fluorescence measurements indicated that Cu(II) induces the strongest changes in the emission of CD1 among eleven investigated metal ions, with a full quenching effect being observed. Further addition of an anionic species, adenosine 5-triphosphate (ATP), resulted in a recovery of the luminescence, which suggests that ATP is capable of sequestering metal ions from the surface of CD1. These results highlight the potential of simple and easy-to-make carbon nanomaterials as sensors for ions in aqueous media.