Sensitive and Selective Detection of Clenbuterol in Meat Samples by a Graphene Quantum Dot Fluorescent Probe Based on Cationic-Etherified Starch

Abstract

The use of clenbuterol (CLB) in large quantities in feedstuffs worldwide is illegal and potentially dangerous for human health. In this study, we directly prepared nitrogen-doped graphene quantum dots (N-GQDs) by a one-step method using cationic-etherified starch as raw material without pollution, which has the advantages of simple, green, and rapid synthesis of N-GQDs and high doping efficiency of nitrogen elements, compared with the traditional nitrogen doping method of reacting nitrogen source raw material with quantum dots. The N-GQDs synthesized by cationic etherification starch with different substitution degrees (DSs) exhibit good blue-green photoluminescence, good fluorescence stability, and water solubility. By comparing the fluorescence emission intensity of the two methods, the N-GQDs prepared by this method have higher fluorescence emission intensity and good fluorescence stability. Based on the static quenching mechanism between CLB and N-GQDs, a fluorescent probe was designed to detect CLB, which exhibited a wide linear range in the concentration range of 5 × 10^-10~5 × 10^-7 M (R² = 0.9879) with a limit of detection (LOD) of 2.083 × 10^-13 M. More excitingly, the N-GQDs fluorescent probe exhibited a satisfactory high selectivity. Meanwhile, it can be used for the detection of CLB in chicken and beef, and good recoveries were obtained. In summary, the strategic approach in this paper has potential applications in the detection of risky substances in the field of food safety.

Keywords: cationic-etherified starch; clenbuterol; fluorescence; fluorescence quenching; nitrogen-doped graphene quantum dots.

Scheme 1

Schematic diagram of the detection of clenbuterol by N-GQD fluorescent sensors and its application to real samples.

Figure 1

(a) TEM image of N-GQDs and the inset of 1a is size distribution; (b) HRTEM image of N-GQDs; (c) potential distribution of N-GQDs, N-GQDs with a degree of substitution of 0.2; (d) UV–vis absorption spectrum of N-GQDs and GQDs. The inset of 1d shows an aqueous solution of N-GQDs in sunlight (right) and 365 nm UV light (left).

Figure 2

(a) Full-scan range of XPS spectra of synthetic N-GQDs (DS: 0.2); (b) full-scan range of XPS spectra of synthetic N-GQDs (DS: 0.1); (c) full-scan range of XPS spectra of synthetic N-GQDs (DS: 0.05); (d) fluorescence stability of N-GQDs. X-ray photoelectron spectroscopy (XPS) spectrum of C 1 s (d), O 1 s (e), and N 1 s (f), respectively.

Figure 3

(a) Fluorescence emission intensity of N-GQDs or GQDs synthesized from different raw materials at an excitation wavelength of 360 nm; (b–d) emission spectra of N-GQDs prepared from cationic starch with different degrees of substitution (0.2, 0.1, 0.05) at different excitation wavelengths between 310 and 390 nm, respectively (in steps of 10 nm); (e) emission fluorescence intensity of N-GQDs prepared from cationic starches with different degrees of substitution at an excitation wavelength of 360 nm; (f) PL excitation and emission spectra of the N-GQDs; (g) N-GQD fluorescence decay profiles.

Figure 4

(a) Effect of different pH conditions on the fluorescence emission intensity of N-GQDs; (b) effect of different reaction temperatures on the fluorescence emission intensity of N-GQDs; (c,d) comparison of the fluorescence stability of Y-N-GQDs prepared from cationic-etherified starch with N-GQDs prepared from tapioca starch and urea.

Figure 5

(a) Fluorescence emission spectra of 0.2 mg mL⁻¹ N-GQDs in solution with and without the addition of 10⁻⁵ M CLB. (b) Fluorescence emission spectra of different concentrations of CLB (0–10⁻⁵ M) when added to N-GQDs. (c) Linear plot of the degree of variation of the system fluorescence intensity, F/F₀, versus log C(CLB) concentration values (CLB concentrations of 5 × 10⁻¹⁰ to 5 × 10⁻⁷ M), with F and F₀ being the fluorescence emission intensity in the presence or absence of CLB in the N-GQDs solution, respectively. (d) Fluorescence emission decay curve of 0.2 mg ml⁻¹ N-GQDs and N-GQDs@CLB (10⁻⁵ M).

Figure 6

Testing the selectivity of N-GQDs for CLB in the coexistence of different interfering substances (10⁻⁵ M for the other analytes and 10 times the concentration of CLB for the metal ions).