Microwave-Assisted Polyol Synthesis of Water Dispersible Red-Emitting Eu3+-Modified Carbon Dots

Abstract

Eu³⁺-modified carbon dots (C-dots), 3-5 nm in diameter, were prepared, functionalized, and stabilized via a one-pot polyol synthesis. The role of Eu²⁺/Eu³⁺, the influence of O₂ (oxidation) and H₂O (hydrolysis), as well as the impact of the heating procedure (conventional resistance heating and microwave (MW) heating) were explored. With the reducing conditions of the polyol at the elevated temperature of synthesis (200-230 °C), first of all, Eu²⁺ was obtained resulting in the blue emission of the C-dots. Subsequent to O₂-driven oxidation, Eu³⁺-modified, red-emitting C-dots were realized. However, the Eu³⁺ emission is rapidly quenched by water for C-dots prepared via conventional resistance heating. In contrast to the hydroxyl functionalization of conventionally-heated C-dots, MW-heating results in a carboxylate functionalization of the C-dots. Carboxylate-coordinated Eu³⁺, however, turned out as highly stable even in water. Based on this fundamental understanding of synthesis and material, in sum, a one-pot polyol approach is established that results in H₂O-dispersable C-dots with intense red Eu³⁺-line-type emission.

Keywords: carbon dot; europium; microwave; polyol; surface conditioning.

Figure 1

One-pot polyol synthesis of Eu-modified C-dots with the role of Eu²⁺, Eu³⁺, O₂, H₂O, and the influence of (a) conventional resistance heating and (b) MW-heating on the surface functionalization and PL (excitation via UV-LED, λ_max = 365 nm, SI: Figure S2).

Figure 2

Dry air treatment of conventionally heated Eu-modified C-dots: (a) photographs with time-depending emission (UV-LED excitation); (b) PL spectra (normalized on C-dot emission at 440 nm); and (c) emission intensity of Eu³⁺ at 614 nm (all spectra with λ_exc = 366 nm).

Figure 3

Photoluminescence decay curves Eu-modified C-dots (in PEG400, λ_exc = 375 nm): (a) as-prepared (Eu²⁺-modified, λ_em = 440 nm); (b) treatment with dry air (2 months, λ_em = 440 nm); and (c) treatment with dry air (two months, λ_em = 615 nm).

Figure 4

H₂O-driven quenching of conventionally-heated Eu-modified C-dots: (a) photographs and PL spectra with time-depending emission upon addition of water (UV-LED excitation); and (b) PL spectra during humid air treatment (λ_exc = 366 nm; normalized on C-dot emission at 440 nm).

Figure 5

Fluorescence of MW-heated Eu³⁺-modified C-dots: (a) photos of powder samples and aqueous suspension (under daylight and under excitation); and (b) excitation (λ_em = 615 nm) and emission (red line: λ_exc = 300 nm; black dash: λ_exc = 366 nm) spectra of aqueous suspensions.

Figure 6

Composition and surface functionalization of MW-heated Eu³⁺-modified C-dots: (a) HAADF-STEM image; (b) Eu elemental mapping; (c) HRTEM image with lattice distance; and (d) FT-IR spectra of MW-heated C-dots (red) in comparison to conventionally heated C-dots (black).