Food Chemistry 298 (2019) 125046 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Highly discriminant rate of Dianhong black tea grades based on fluorescent T probes combined with chemometric methods ⁎ Jing Zhu1, Fengyuan Zhu1, Luqing Li, Linlin Cheng, Liang Zhang, Yue Sun, Xiaochun Wan , ⁎ Zhengzhu Zhang State Key Laboratory of Tea Plant Biology and Utilization, and Department of Applied Chemistry, Anhui Agricultural University, Hefei 230036, Anhui, China ARTICLE INFO ABSTRACT Chemical compounds studied in this article: We established a novel Dianhong black tea grades discriminant analytic technique based on a fluorescence image Phenol (PubChem CID: 996) along with carbon quantum dots (CDs) as fluorescent probes. Different grades of Dianhong black tea contain Catechins (PubChem CID: 1203) different various amounts of tea polyphenols. Tea polyphenols can quench the fluorescent intensity ofCDs, Theanine (PubChem CID: 439378) resulting in different fluorescent peaks; Dianhong black tea grades can then be discriminated through theuseof Epigallocatechin Gallate (PubChem CID: principal component analysis and Bayesian analysis. Compared with the additional data processing required in 65064) other methods, the advantage of our method is that the fluorescence curve can be used directly, and it achieves Theaflavin (PubChem CID: 11980943) Ethanol (PubChem CID: 702) satisfactory results. We firstly used CDs combined with chemometrics to identify eight grades of Dianhong black Cobalt Nitrate Hexahydrate (PubChem CID: tea, and we also provide a new method that improves the identification rate using nanotechnology to avoid 24821) performing complex data processing. Sodium Citrate (PubChem CID: 6224) Carbamide (PubChem CID: 1176) Thiourea (PubChem CID: 2723790) Keywords: Fluorescence spectroscopic analysis Grades discriminant Carbon quantum dots Principal component analysis Bayesian discriminant analysis 1. Introduction discriminate the grades on the basis of their own experience. It is ne- cessary to establish an objective analytic procedure for evaluating and Dianhong black tea is a representative and influential black tea discriminating the grades of Dianhong black tea (Pang et al., 2012). brand distributed throughout the world and it is prepared from Camellia Electronic tongue is a new detection method for analysing and re- sinensis grown in Yunnan, where the plant’s bud leaves are soft and cognising the taste of liquid (Huo et al., 2014; Qin et al., 2013). Spectral polyphenol content is high. Because of the abundance of polyphenols in technology combined with chemometrics methods to assess and analyse Dianhong black tea, it possesses antibacterial property (Beresniak, tea grades is a promising area for investigation (He, Li, & Deng, 2007; Duru, Berger, & Bremond-Gignac, 2012) and can lower blood sugar and Hu et al., 2018; Li, Xu, Zhang, Sun, & He, 2017; Luypaert, Zhang, & blood pressure (Ramadan, El-Beih, & El-Ghffar, 2009). Because of Massart, 2003; Ojha & Roy, 2018; Wang, Zheng, Liu, & Fang, 2016). various processing techniques are employed for different grades of tea, Carbon quantum dots (CDs) as a preeminent fluorescent nanoma- obvious differences are perceptible between grades of tea on the basis terial with low cytotoxicity and excellent biocompatibility, are well of their qualities (Wang et al., 2017; Xiao et al., 2017). The price of suited for use in chemical analyses of appointed targets (Xu, Liu, Gao, & Dianhong black tea varies on the basis of its quality grade, and can Wang, 2014; Zong et al., 2014). Their unique characteristics improve range from a few hundred to several thousand renminbi (RMB) per the safety of fluorescence detection. CDs can be synthesized in alab, kilogram. Customers typically cannot evaluate the quality and which makes the method more convenient and cheaper (Zheng, Than, ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Wan), [email protected] (Z. Zhang). 1 These authors contributed equally. https://doi.org/10.1016/j.foodchem.2019.125046 Received 7 November 2018; Received in revised form 18 February 2019; Accepted 18 February 2019 Available online 20 June 2019 0308-8146/ Published by Elsevier Ltd. J. Zhu, et al. Food Chemistry 298 (2019) 125046 Fig. 1. Co-CDs system for sensing Dianhong black tea grades. Ananthanaraya, Kim, & Chen, 2013). Many studies have expended 2.2. Extraction of tea polyphenols considerable effort to use CDs in applications such as fluorescence sensing, bioimaging, and drug delivery. CDs have abundant and low- We used solvent extraction, which exhibits the characteristics of cost carbon precursors (e.g. ground coffee (Hsu & Chang, 2012), used stability, reliability, and simple operation, to extract tea. We used tea (Hsu, Chen, Ou, Chang, & Chang, 2013), candle soot (Liu, Ye, & ethanol 70% or water as solvents. We followed the typical rank order of Mao, 2007), grass (Liu et al., 2012), thus diversifying carbon prepara- Dianhong black tea (Super, First, Second, Third, Fourth, Fifth, Sixth, tion. and Outside), and we collected 15 tea samples (0.2 g per sample) of In this work, our objective was to design a discriminant model to each tea grade, yielding a total of 120 samples. Each sample was placed identify Dianhong black tea grades by using Co-modified CDs combined in a 10-mL centrifuge tube and labelled with a corresponding number in with chemometrics methods (Fig. 1). No study has performed an ex- the range of D1–D120 (Super: D1–D15, First: D16–D30, Second: periment to identify eight grades of a type of tea by using Co-modified D31–D45, Third: D46–D60, Fourth: D61–D75, Fifth: D76–D90, Sixth: CDs as fluorescent sensors because of higher number of grades signifies, D91–D105, and Outside: D106–D120). Subsequently, 8 mL of 70% smaller differences, and thus greater difficulty in tea identification. We ethanol solution was added to each sample, and the sample was then also used PCA and Bayes discriminant analysis to analyse the data ac- placed in an ultrasonic cleaning instrument at 70 °C. Ultrasonic ex- quired from the application of the CDs fluorescence sensor to Dianhong traction was performed for 45 min, which entailed shaking the cen- black tea. PCA can identify the grade information of the sample, and trifuge tube at intervals of 15 min to prevent the accumulation of tea Bayes discriminant analysis can predict each grade of Dianhong black powder at the bottom of the tube, which would affect the extraction tea accurately. effectiveness. The extract was then centrifuged at a speed of2191g- force for 10 min, and the upper liquid was filtered using a 0.22-μm filter membrane. The filtrate was collected into a corresponding E1–E120 5- 2. Methods mL centrifuge tube and stored at 4 °C without light. The procedure for extracting tea polyphenols with water followed 2.1. Materials the same method: 0.2 g of each sample was placed in a 10-mL centrifuge tube labelled W1–W120. Next, 8 mL of ultrapure water was added to All chemicals and reagents used in this work were of analytical each sample, and the sample was then placed in a constant-temperature grade. Sodium citrate (C6H5NaO7·2H2O), cobalt nitrate hexahydrate water bath of 70 °C to extract for 45 min. The extraction process also (Co(NO3)2·6H2O), iron chloride hexahydrate (FeCl3·6H2O), thiourea entailed shaking the centrifuge tube at intervals of 15 min. After ex- (CS(NH2)2), carbamide (CN2H4O), and D-glucose anhydrous (C6H12O6) traction, centrifugation and filtration were performed as in the previous were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, operation. The filtrate was collected into the corresponding W1–W120 China). The resistivity of the test water was greater than 18 MΩ·cm−1. 5-mL centrifuge tubes and stored at 4 °C without light. Chitosan (C6H11NO4)n was procured from Beijing Solarbio Science & Technology Co. Ltd (Beijing, China). Acetic acid glacial (C2H4O2) was 2.3. Synthesis of fluorescent CDs purchased from Tedia Company (Ohio, America). Fluorescence measurements were performed using a Carry Eclipse We used a one-pot hydrothermal synthetic approach to obtain CDs. Fluorescence Spectrophotometer (Aglient Technologies). Dianhong Carbon sources for synthesizing are extensive, and we selected sodium black tea samples were collected from Yunnan Dianhong Black Tea citrate and chitosan as carbon sources in this experiment. Briefly, Group Co. Ltd. To reduce heterogeneity, the black tea samples were 0.28 mmol sodium citrate was added into 30 mL of ultrapure water and grinded ground into powder by a soymilk machine, sieved through a mixed with 1.26 mmol carbamide and 0.42 mmol thiourea. After com- 40–80-mesh sieve, and stored in a refrigerator at 4 °C. plete dissolution, the mixture was transferred into a 50-mL PTFE 2 J. Zhu, et al. Food Chemistry 298 (2019) 125046 Fig. 2. (A). Fluorescence spectra of CDs system reacted with ethanol-extracted samples; (B). Content of tea polyphenols in eight grades of Dianhong black tea samples. reactor and placed in an oven at 185 °C. After 6 h of reaction, the re- variables from raw data (Bilge, Velioglu, Sezer, Eseller, & Boyaci, 2016; actor was cooled to room temperature. The CDs solution, a brownish Bro & Smilde, 2014; Gurdeniz & Ozen, 2009). PCA focuses on the vector yellow liquid, was purified using filtration through a 0.22-μm filter space for a superior description of the raw data, whereas LDA for the membrane and stored at 4 °C for further use. In another synthetic op- vector space that distinguishes the data most effectively, taking full eration, 50 mmol chitosan was fully dissolved in 20 mL of 2.5% acetic advantage of the class information of the training samples (Li, Xie, acid solution. Subsequently, the solution was transferred to a 50-mL Ning, Chen, & Zhang, 2018). We used PCA and LDA to analyse the reaction kettle and heated in an oven at 200 °C. The reactor was then fluorescence spectrum, and determine whether the fluorescence spec- cooled to room temperature after 4 h of reaction.