EDTA-bonded multi-connected carbon-dots and their Eu³⁺ complex: preparation and optical properties

EDTA-bonded multi-connected carbon-dots (EDTA-C-dots) were prepared from carbon dot precursors and complexed with Eu³⁺ to give Eu³⁺-coordinated EDTA-bonded multi-connected carbon dots (Eu-EDTA-C-dots). Whereas EDTA-C-dots were readily soluble in DMSO, Eu-EDTA-C-dots could not be easily dissolved in DMSO, water, or other common organic solvents. The newly prepared materials were thoroughly characterized. The X-ray diffraction results showed that no crystalline phase of Eu oxides (europium oxide or europium hydroxide) could be observed in Eu-EDTA-C-dots. The infrared and UV-Vis spectra showed that coordination with Eu³⁺ ions did not damage the structure of the EDTA-C-dots. It was found that EDTA could be easily grafted on the surface of carbon dots and EDTA had minimal influence on the photoluminescence of the carbon dot matrix. In contrast, the existence of Eu³⁺ ions strongly quenched the photoluminescence of Eu-EDTA-C-dots. The measured and fitted decay lifetime indicated that Eu-EDTA-C-dots possessed two photoluminescence decay processes, i.e., radiative recombination and non-radiative recombination.