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

Abstract

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.

Fig. 1. Schematic illustration of the preparation process of EDTA–C-dots and Eu–EDTA–C-dots. In the inset, there includes one HRTEM image of precursor C-dots with about 2.5 nm of particle diameter.

Fig. 2. Characterizations of precursor C-dots and EDTA-free x-C-dots: (A) FTIR spectra; (B) XRD patterns; (C) PL of the C-dots; (D) PL of the x-C-dots.

Fig. 3. (A) XRD pattern of Eu–EDTA–C-dots; (B) FTIR spectra of EDTA–C-dots and Eu–EDTA–C-dots; (C) TEM image of EDTA–C-dots; (D) TEM image of Eu–EDTA–C-dots. Inset in (C) is the TEM image of precursor x-C-dots.

Fig. 4. UV-Vis absorption spectra of EDTA–C-dots and Eu–EDTA–C-dots. Inset is the PL spectra of the EDTA–C-dots and Eu–EDTA–C-dots under 340 nm of excitation wavelength.

Fig. 5. Photoluminescence spectra of EDTA–C-dots and Eu–EDTA–C-dots under different excitation wavelength.

Fig. 6. Luminescence decay spectra of EDTA–C-dots (a) and Eu–EDTA–C-dots (b) with 340 nm of excitation wavelength.