Preparation of carbon quantum dot fluorescent probe from waste fruit peel and its use for the detection of dopamine

Abstract

Carbon quantum dots (CQDs), as a new type of fluorescent nanomaterial, are widely used in the detection of small molecules. Abnormal dopamine secretion can lead to diseases such as Parkinson's disease and schizophrenia. Therefore, it is highly significant to detect dopamine levels in the human body. Using discarded fruit peels to prepare carbon quantum dots can achieve the reuse of kitchen waste, reduce pollution, and create value. Nitrogen-doped carbon quantum dots (N-CQDs) were prepared using the hydrothermal method, with orange peel as the raw material. The fluorescence quantum yield of N-CQDs reached a high value of 35.37% after optimizing the temperature, reaction time, and ethylenediamine dosage. N-CQDs were characterized using various techniques, including ultraviolet visible (UV-vis) spectroscopy, fluorescence spectrophotometer (PL), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR). These analyses confirmed the successful doping of nitrogen in the CQDs. The DA concentration ranged from 0 to 300 μmol L^-1, and the linear equation for fluorescence quenching of N-CQDs was F/F₀ = -0.0056c + 0.98647, with an R² value of 0.99071. The detection limit was 0.168 μmol L^-1. The recovery and precision of dopamine in rabbit serum were 98% to 103% and 2% to 6%, respectively. The prepared N-CQDs could be used as a fluorescent probe to effectively detect DA.

Fig. 1. The mechanism of dopamine quenching the fluorescence of carbon quantum dots under alkaline conditions.

Fig. 2. Optimization of the N-CQDs preparation process. (a) Effect of carbon source type on the fluorescence intensity of N-CQDs. N-CQDs were prepared by mixing ethylenediamine with OP, BP and PP. (b) Influence of carbon source type on the degree of carbonization. (The Raman spectrogram of carbonization degree was completed in Jinan Fine Research Testing Co., Ltd.) (c) Effect of the amount of ethylenediamine doping on the fluorescence intensity of N-CQDs. (The values are means ± SD of three replicates.) (d) Effect of time on the fluorescence intensity of N-CQDs. (The values are means ± SD of three replicates.) (e) Effect of temperature on the fluorescence intensity of N-CQDs. Effect of reaction temperature on the degree of carbonization of N-CQDs. (The values are means ± SD of three replicates.) (f) The effect of temperature on the degree of carbonization.

Fig. 3. Characterization diagram of N-CQDs. (a) The UV-vis absorption spectrum analysis of N-CQDs was determined using a UV-1750. (b) The maximum excitation wavelength and maximum emission wavelength of N-CQDs were determined using an F-2700 PL. (c) TEM image of N-CQDs. The inset graph is a high-power transmission electron microscopy (HRTEM) image of N-CQDs. (TEM characterization was completed at Jinan Jingyan Testing Co., Ltd.) (d) Particle size distribution of N-CQDs was measured using ps software, and the particle size distribution map was generated using origin. (e) FT-IR of N-CQDs. (f) EDS diagram of N-CQDs provided by Jinan Jingyan Testing Co., Ltd for testing.

Fig. 4. Comparison of quantum yields of CQDs produced from biomass in the literature. (This paper (THP).)

Fig. 5. Feasibility test of N-CQDs for DA. (a) Quenching effect of DA concentration on N-CQDs fluorescence. (The fluorescence quenching of N-CQDs was performed with DA solutions of different concentrations in the range of 0–300 μmol L⁻¹.) (b) Influence of common interfering substances in serum on DA detection. (The effects of common metal ions and small biological molecules in serum were investigated.) 1-Blank, 2-K⁺, 3-Zn²⁺, 4-Ca²⁺, 5-triglyceride, 6-urea, 7-phenylalanine, 8-DA, 9-tyrosine, 10-Fe²⁺, 11-EDTA + tyrosine, 12-EDTA + Fe²⁺, 13-EDTA. (The values are means ± SD of three replicates.)

Fig. 6. DA testing process optimization. (The values are means ± SD of three replicates.) (a) The effect of temperature on the performance of N-CQDs for DA detection. (b) Effect of pH on the performance of N-CQDs for DA detection. (c) Effect of time on the performance of N-CQDs DA detection. (d) Effect of N-CQDs dosage on the performance of N-CQDs in DA detection.

Fig. 7. (a) Influence curve of DA concentration on the fluorescence intensity of N-CQDs (the DA concentration was selected as 0.5, 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 100, 200, and 300 μmol L⁻¹). (b) For the quenching of fluorescence intensity of the N-CQDs system by DA concentration, the inset picture shows the linear relationship of fluorescence quenching of N-CQDs by DA concentration. (The values are means ± SD of three replicates.)