Hydrothermal synthesis of nitrogen-doped carbon quantum dots from lignin for formaldehyde determination

Abstract

This work assessed the fabrication of nitrogen-doped CQDs (NCQDs) from alkali lignin (AL) obtained from spruce, representing a green, low-cost biomass generated by the pulp and biorefinery industries. The AL was found to retain its original lignin skeleton and could be used to produce NCQDs with excellent photoluminescence properties by one-pot hydrothermal treatment of AL and m-phenylenediamine. These NCQDs exhibited blue-green fluorescence (FL) with excitation/emission of 390/490 nm under optimal conditions. The NCQDs showed pH and excitation wavelength-dependent FL emission behaviors. On the basis of the exceptional selective response of these NCQDs to specific solvents, we developed a FL probe for the detection of formaldehyde (FA). The FL intensity of NCQDs was found to be directly proportional to the concentration of FA in the range of 0.05 to 2 mM (R ² = 0.993), with a detection limit of 4.64 µM (based on 3σ/K). A composite film comprising NCQDs with poly(vinyl alcohol) was found to act as a sensor with a good FL response to FA gas. When exposed to gaseous FA, this film exhibited increased FL intensity and transitioned from blue-green to blue. A mechanism is proposed in which the NCQDs react rapidly with FA to generate Schiff bases that result in enhanced FL emission and the observed blue shift in color.

Scheme 1. Preparation process of NCQDs from alkaline lignin.

Fig. 1. (a) TEM (HRTEM) images and size distribution histograms of the NCQDs (inset); (b) XRD pattern and (c) FT-IR spectra of the NCQDs. (d) High-resolution C 1s peaks; (e) high-resolution O 1s peaks; (f) high-resolution N 1s peaks.

Fig. 2. (a) UV-vis absorption, Em (emission wavelength = 490 nm) and Ex spectra (excitation wavelength = 390 nm), inset (a) shows a digital photograph of the NCQDs under 365 nm UV light. (b) FL emission spectra of the NCQDs at different excitation wavelengths. (c) FL emission spectra of the NCQDs at different solution pH values (excited at 390 nm). (d) FL emission spectrum of NCQDs aqueous solution mixed with different solvents (excited at 390 nm).

Fig. 3. (a) FL emission spectra of NCQDs solutions at different concentrations of FA (from bottom to top 0, 0.05, 0.1, 0.3, 0.6, 1, 1.5, 2, 3, 4, 5, and 6 mM under excitation at 390 nm). (b) FL intensity as a function of FA concentration. (c) Fluorescence lifetime decay of NCQDs before and after reaction with FA. (d) Schematic of FA gas sensing by NCQDs/PVA composite film. FL microscopy images of NCQDs/PVA composite film under excitation by UV light treated with (e) normal air, (f) water gas, and (g) FA gas. (h) FA gas to air environment.

Fig. 4. (a) FT-IR contrast spectra of NCQDs and NCQDs-FA. (b) Zeta potential of NCQDs and NCQDs-FA. (c) Possible reaction mechanism of NCQDs towards FA.