According to the concept of waste utilization and environmental protection, carbon quantum dots (CQDs) with high quantum yield and good stability were synthesized from waste tea leaves and peanut shells by a one-step hydrothermal method. In this study, we explored the synthetic conditions, structures, and optical properties of CQDs. Their unique characteristics of emitting strong and steady blue fluorescence under excitation of ultraviolet light and possessing of plenty of hydrophilic groups on a surface conferred CQD potentials in the field of biomarkers and analytical detection.
Carbon quantum dots (CQDs) are quasizero-dimensional carbon nanoparticles with an average particle size of less than 10 nm, and the surfaces of CQDs contain many organic functional groups. The synthesis of the CQD process is simple, and the raw materials are low-cost. Due to their low toxicity, good biocompatibility, high chemical stability, and good light stability, CQDs have potential applications in many fields such as biomarkers [
Process routing of CQDs.
Ethanol and acetic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents used were of analytical grade without further treatment. The water used throughout was ultrapure water. Dianhong black tea samples were collected from Yunnan Dianhong Black Tea Group Co. Ltd. Abandoned Tieguanyin Tea (purchased on Taobao) was soaked in hot water at 100°C for 5 times and dried in an oven to get the discarded tea. Peanut shell was washed with pure water and dried in an oven at 100°C, then grinded into powder by a crusher, packed in a sealed bag, and kept at 4°C.
A Cary Eclipse fluorescence spectrophotometer (Agilent Technologies, America) and a UV-2100 ultraviolet spectrophotometer (Shimadzu Corporation, Japan) were used to record fluorescence spectra and absorption spectra of CQDs, respectively. A HT-7700 transmission electron microscope (TEM) was used to observe the size and morphology.
The preparation process of CQDs is shown in Figure
Firstly, peanut shell powder was dissolved in 30 mL 3% acetic acid solution and sonicated for 15 min. Then, the mixture was poured into the 50 mL PPL reactor to react at 200°C for 4 h and cooled down to room temperature after the reaction finished. The obtained yellow liquid was filtered twice by a 0.22
The luminescence properties of samples were determined by a fluorescence spectrophotometer and an ultraviolet spectrophotometer. The morphology and size of the samples were observed and determined using a TEM.
The purchased black tea had been divided into 8 experimental grades (Super, First, Second, Third, Fourth, Fifth, Sixth, and Outside) based on its quality. Three samples (0.1 g per sample) were taken from each tea grade, yielding a total of 24 samples. 100 mg of each tea samples was soaked in water at 90°C for 3 min. After adding 3 mL of water to a 5 mL centrifuge tube, 0.2 mL of tea soup and 0.1 mL of quantum dot solution were added and the mixture was shaken. The sample was subjected to fluorescence detection, and the data was saved for analysis.
When using tea as a carbon source, we weighed 0.25 g, 0.50 g, 0.75 g, 1.00 g, 1.25 g, and 1.50 g of tea to get reactions separately. Through the fluorescence spectrum (Figure
Influence of carbon source dosage of (a) tea and (b) peanut shell.
For tea CQDs, we recorded the fluorescence spectrum (Figure
Influence of reaction time of (a) tea and (b) peanut shell.
The tea reaction solution was reacted at 100°C, 150°C, 180°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, and 270°C, for 300 min respectively. As shown in Figure
Influence of reaction temperature of (a) tea and (b) peanut shell.
According to the TEM test results (Figure
Transmission electron microscopy micrograph of CQDs of (a) tea and (b) peanut shell.
The tea CQDs had the strongest fluorescence intensity at 325 nm under different excitation light conditions and emitted blue light fluorescence.
Therefore, the wavelength of the best excitation light was 325 nm; as the wavelength of the excitation light increases, the fluorescence appeared red-shifted. It is shown in Figure
Fluorescence spectrum of CQDs of (a) tea and (b) peanut shell.
Through the infrared spectrum (Figure
Infrared spectrum of CQDs of (a) tea and (b) peanut shell as carbon source.
As we knew, many substances could cause the quenching of quantum dots. The content of tea was very rich, and tea soup could cause the quenching of CQDs. There were differences in the ingredients contained in different grades of tea. However, such a difference was relatively subtle and had high requirements for the accuracy of the test instrument. This high sensitivity of carbon quantum could be used to identify such subtle differences. We took black tea as an experimental sample and divided it into eight grades (Super, First, Second, Third, Fourth, Fifth, Sixth, and Outside). For each grade, three tea samples were taken for experiment. We used SPSS software to perform principal component analysis on the quantum dot fluorescence curve (Figure
Tea grades of (a) fluorescence spectra and (b) distribution map based on principal component.
We used waste tea leaves and peanut shells as carbon sources to synthesize CQDs. This method has advantages of extremely low cost and easy manipulation. The synthesized light yellow CQD solution emitted strong blue fluorescence under the irradiation of ultraviolet light with a wavelength of 365 nm. After the optimization of preparing conditions and characterization of CQDs, we found the best preparation condition is 200°C. It is known that the CQDs are 7~9 nm in size, spherical in shape, and homogenous in water. The surface contains a large number of hydrophilic groups. Furthermore, we used CQDs to discriminate tea grades with high accuracy. The results obtained in this work provided a way to reduce costs for the practical application of carbon quantum dots in the future.
The data used to support the findings of this study are available from the corresponding authors upon request.
The authors declare no conflict of interest.
Jing Zhu and Fengyuan Zhu contributed equally to this work.
This study was financially supported by the National Natural Science Foundation of China (NSFC, 51702004), the Scientific Research Foundation of Anhui Agricultural University (Grant No. yj2017-06), the Youth Research Foundation of Anhui Agricultural University (2016ZR003), the Natural Science Foundation of Anhui Province (1808085QE158), and the Open Fund of State Key Laboratory of Tea Plant Biology and Utilization (SKLTOF20180107).