Fluorescent N-functionalized carbon nanodots from carboxymethylcellulose for sensing of high-valence metal ions and cell imaging

Abstract

A convenient and sensitive reversible-fluorescence sensing platform for accurate monitoring of high-valence metal ions is still very challenging. As a green kind of fluorescent carbon nanomaterials, carbon dots (CDs) have captured considerable attention because of the stable fluorescence property and low cost. Herein, we fabricated a type of nitrogen-functionalized carbon dots (N-CDs) from CMC as a fluorescent reversible sensing platform for detecting various high-valence metal ions. N-CDs with a mean size of 2.3 nm were obtained and possessed 22.9% quantum yields (QY). A label-free fluorescent probe for detection of high-valence metal ions (Fe³⁺, Cr⁶⁺, Mn⁷⁺) was established via the fluorescence quenching response. Among them, the detection limit (LOD) toward Fe³⁺ ions reached 0.8 µM. We have explored the quenching mechanism of N-CDs to explain the valence state-related electron-transfer fluorescence quenching between high-valence metal ions and N-CDs. Moreover, the valence state-related fluorescence quenching phenomenon of N-CDs in aqueous solution could be effectively recovered by introducing a reducing agent (Ti³⁺). This "turn off-on" fluorescence recovery system of N-CDs could be applied in different applications covering the selective detection of environmental high-valence metal ions and cellular imaging.

Scheme 1. The synthetic diagram of fluorescent N-CDs.

Fig. 1. TEM image (a) and particle size distribution (b) of N-CDs, XPS survey of CMC, CDs and N-CDs (c), N 1s of N-CDs (d), C 1s (e) and O 1s (f) of CMC, CDs and N-CDs.

Fig. 2. FTIR spectra of CMC, CDs and N-CDs (a), Raman spectra (b) and XRD pattern (c) of N-CDs.

Fig. 3. UV-vis and the optimum excitation and emission fluorescence spectra of N-CDs (a), fluorescent properties of N-CDs with various excitation wavelengths (b), different pH (c) and concentrations of sodium chloride (d).

Fig. 4. Fluorescent properties of N-CDs containing 1 mM Fe³⁺ change with time (a) and the F₀/F value of N-CDs various metal ions (b), fluorescence quenching spectra of N-CDs and plots of quenching by Fe³⁺ (c and d), MnO₄⁻ (e and f), CrO₄²⁻ (g and h) and Cr₂O₇²⁻ (i and j).

Scheme 2. Schematic of the N-CDs “turn off-on” fluorescence sensing system.

Fig. 5. XPS spectra of N-CDs interacted with (a) Fe³⁺, (b) Fe²⁺, (c and e) Fe³⁺ and Ti³⁺, (d) Ti³⁺.

Fig. 6. Fluorescent recovery time of N-CDs/Fe³⁺ (1 mM) with Ti³⁺ (1 mM) (a), fluorescent recovery spectra and the plots of quenching of N-CDs/Fe³⁺ (b and c), MnO₄⁻ (d and e), CrO₄²⁻ (f and g) and Cr₂O₇²⁻ (h and i) systems with various concentrations Ti³⁺.

Fig. 7. The cell viability in various concentrations of N-CDs (a), microscopy photographs of N-CDs without/with Fe³⁺ (1 mM) under the bright field (b) and (e), λ_ex = 488 nm (c) and (f), overlay (d) and (g), respectively.