Analytica Chimica Acta 1059 (2019) 36e41
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GUITAR-enhanced facile discrimination of aged Chinese Baijiu using electrochemical impedance spectroscopy Xinyue Jiang b, Yuqun Xie a, b, *, Duanji Wan b, Maobin Chen b, Fuping Zheng a, ** a b
Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University, Beijing, 100048, China School of Bioengineering and Food Science, Hubei University of Technology, Wuhan, Hubei, 430068, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
GUITAR electrode were employed to collect EIS data of verities' aged Baijiu. The obtained Nyquist plots were simulated with the corresponding equivalent circuit. The curve ﬁtting data were dimensionally reduced with principle components analysis (PCA). PCA score image showed a welldeﬁned clustered proﬁle for aged Baijiu discrimination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2018 Received in revised form 7 January 2019 Accepted 24 January 2019 Available online 4 February 2019
The presented research demonstrated a method to evaluate aged Chinese Baijiu by measuring their electrochemical impedance spectrums (EIS) according to the distinctive colloidal impedance phenomena of verities' aged Baijiu. Graphite from the University of Idaho Thermolyzed Asphalt Reaction (GUITAR) electrode was employed for EIS data collection to achieve enhanced resolution. The obtained EIS Nyquist plots were simulated with the corresponding equivalent circuit to extract the numerical values of parameters which characterize the physicochemical properties of aged Baijiu. The curve ﬁtting data were dimensionally reduced with principle components analysis (PCA). PCA score image originated from GUITAR electrode EIS measurement showed a more deﬁned clustered proﬁle in comparison to glassy carbon electrode. These results implicate GUITAR electrode based EIS is potentially available to develop a household device for aged Baijiu classiﬁcation. © 2019 Elsevier B.V. All rights reserved.
Keywords: GUITAR Electrochemical impedance spectroscopy Aged Chinese Baijiu Classiﬁcation
1. Introduction Storing distilled base Baijiu in pottery jars for a certain period of time is an essential step for Chinese Baijiu production process. After
* Corresponding author. Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University, Beijing, 100048, China. ** Corresponding author. E-mail address: [email protected]
(Y. Xie). https://doi.org/10.1016/j.aca.2019.01.050 0003-2670/© 2019 Elsevier B.V. All rights reserved.
storage, the unpleasant and harsh taste of Baijiu is signiﬁcantly reduced and obtained much more delightful and harmonious texture compared to freshly brewed Baijiu. Thus, to produce matured spirits through storage is known as aged Baijiu [1,2]. Several researchers' work on aged Baijiu demonstrated that metal ions gradually dissolve out from pottery jars during the cellar time, these metal cations binding with oxethyl anions to form colloidal particles. The resultant colloidal particles featured with the large speciﬁc surface area and strong absorptivity dominate the trace aroma compounds including alcohols, acids, esters, aldehydes and
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ketones, that uniformly distributed in Baijiu body, resulting in the delightful and harmonious texture of aged Baijiu. It has been veriﬁed that the quantity and particle size of colloid increase with the increase in aging duration via scanning probe micrograph characterization. The delightful and harmonious texture of aged Baijiu is dominated by the physicochemical property of Baijiu colloid, this soliquid aged Baijiu is distinctive from any other distilled alcoholic drink and unripe Chinese Baijiu . Currently, the characteristic aroma components of Baijiu are determined using Infrared spectroscopy, Raman spectroscopy and gas chromatography coupled with mass spectroscopy, and thus Baijiu grade can be well identiﬁed through the multivariate statistical analysis combined with above instrumental analysis technique . However, the aged Baijiu evaluation is difﬁcult to carry out for a long time, and sensory assessment is also extremely unreliable. Recently, Jiang et al. used aroma components' ﬁngerprints as the database to establish the identiﬁcation model, in which Principal Component Analysis (PCA) and Factor Analysis (FA) were integrated to distinguish the age and quality grades of all brands of Baijiu . Zhuang et al. proposed an identiﬁcation approach for aged Baijiu based on the variation of metal ions and colloidal characters within the storage period . Yang et al. adopted the quantity of metal ions, the viscosity of soliquid and the content of the conjugate compounds as indicators to evaluate the aged Baijiu . Xu et al. suggested a method named "volatility identiﬁcation of vintage Baijiu " which is based on the volatility of acetaldehyde, ethyl acetate and acetone in Baijiu to assess the vintage . The above analytical approaches have prominent advantages in some certain aspects. However, serious limitations remain a challenge and none of the methods mentioned previously could systematically characterize the properties of aged Baijiu with solid reliability. There is an urgent demand to develop an effective methodology to discriminate aged Baijiu comprehensively. Electrochemical impedance spectroscopy (EIS) measures the impedance of a system over a series of frequencies. EIS offers kinetics and electrode interface information in the electrochemical electrode process . Thus, EIS is a powerful non-destructive technique for analyzing the complex electrochemical resistance system and is sensitive to surface phenomena and changes in bulk properties . This technique has grown tremendously in stature over decades and is now being widely employed in a wide variety of scientiﬁc areas, for instance, in the ﬁelds of metal electrodeposition [10e12], corrosion research [13e15], chemical power [16e18], food safety detection [19e21], and biological sensor [22e24]. EIS is sensitive and can be advantageously exploited for sensing on Graphite from the University of Idaho Thermolyzed Asphalt Reaction (GUITAR) platform, in particular . This is due to the unique electrochemical properties of GUITAR when compared to graphene, pyrolitic graphite and glassy carbon. In general, the heterogeneous electron transfer rates of edge and basal planes of graphite and graphene are indistinguishable for certain compounds, the basal plane is signiﬁcantly less active than the edge plane of graphite , which is problematic for EIS measurement with [Fe(CN)6]3-/4- as a redox probe. Herein, we applied a GUITAR electrode which the standard heterogeneous rate constant across the basal plane with [Fe(CN)6]3-/4- is 2e7 orders of magnitude greater than graphite and glassy carbon [27,28]. In addition with the large surface area of GUITAR electrode provides more basal plane and active interaction sites for targets than that of other graphitic carbon. Therefore, the enhanced sensitivity with above GUITAR electrode for EIS measurement is promised. The objective of this work is to demonstrate a novel technique to discriminate aged Chinese Baijiu by measuring its electrochemical impedance spectrum, according to the distinctive colloidal impedance phenomena of verities' aged Baijiu. Fig. 1 presents the
schematic diagram of this work. EIS spectra were simulated with the corresponding equivalent circuit to extract the numerical values of parameters which characterize the physicochemical properties of aged Baijiu. The resultant multi-elements data were dimensionally reduced with Principle components analysis (PAC). The PCA score plots show that aged Baijiu data points are clustered in each region of PCs space corresponding to their storage periods with a deﬁned resolution. The PCA score image indicates that GUITAR electrode based EIS coupled with PCA is a facile, economical and promising technique to discriminate aged Chinese Baijiu. The presented research implicates EIS is potentially available to develop a household device for aged Baijiu discrimination. 2. Experimental 2.1. Reagents and apparatus K3[Fe(CN)6] (Sinopharm Chemical Reagent Co., Ltd, Shanghai), Baijiu (1st year, 3rd year, 5th year, 9th year, 12th year, 15th year, 20th year and 30th year) are obtained from a local Chinese Baijiu industrial Baiyunbian Group. All electrochemical experiments were carried out by CS electrochemical workstation (CorrTest, Wuhan) with a conventional three-electrode system, including the glassy carbon electrode and synthesized GUITAR as the working electrode, Ag/AgCl electrode as the reference electrode, and the carbon rod as the auxiliary electrode. KQ5200DE CNC ultrasonic cleaning machine (Shanghai), and XB220A electronic analytical balance (Shanghai). 2.2. Preparation of GUITAR electrode GUITAR working electrode were produced in a similar fashion as described in a previous publication . Hydrophobic organic groups were functionalized on the silicon wafer substrates in order to support the synthesized GUITAR ﬁlms in aqueous solution . Prior to each EIS measurement, GUITAR electrode was rinsed with deionized water, followed by with 75% acetone cleaning, ﬁnally sonicating with deionized water for 1 min in the ultrasonic cleaner. 2.3. Electrochemical measurements The Faradic impedance measurements were carried out in 9 ml Baijiu solution with the addition of 1 mL 0.01 M potassium ferrocyanide in 0.5 M NaCl as supporting electrolyte. The EIS measurements were taken with a three-electrodes system. The working electrode was GUITAR and glassy carbon electrode, the Ag/AgCl electrode was used as the reference electrode, and the graphite carbon rod was an auxiliary electrode. 10 mV of AC amplitude was employed with the frequency range from 0.1 M Hz to 0.01 Hz. 2.4. Analyzing softwares Analog equivalent circuit software: CorrTest.CSStudio; Principal component analysis software: MATLAB. 3. Results and discussion 3.1. The nyquist plots of aged Baijiu Comparison of GUITAR and glassy carbon electrode for faradic impedance measurements were carried out to obtain a superior resolution for aged Baijiu discrimination. Fig. 2 presents the EIS spectra taken from a glassy carbon electrode(Fig. 2a)and a GUITAR electrode (Fig. 2b), respectively. Hardly any feasible information
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Fig. 1. (a) The schematic diagram illustrates the EIS behavior of colloidal particles in aged Baijiu. (b) A ﬂowchart outlines the strategy applied in the proposed research.
was obtained from Fig. 2a and b, due to the random distribution of Nyquist curves for each sample. Data pre-processing is essential at this point. Nyquist plots simulation was carried out based on the equivalent circuit with CorrTest.CSStudio . Fig. 2c and d displayed the experimental and simulated Nyquist plots for glassy carbon and GUITAR EIS measurements according to their corresponding equivalent circuits. The simulated curves are precisely ﬁtted with experimental Nyquist plots at high frequency in which dominates the core features of EIS measurement  and at a low frequency, a trivial deviation appeared. Simulated curves remained the major electrochemical properties of aged Baijiu entirely. Thus, the characteristic of Nyquist plots was translated into a set of circuit elements numerical values after curve ﬁtting based on equivalent circuits (supplementary material data tables). 3.2. Data visualization by PCA The principal component analysis aims at reducing a large set of variables to a small set that still contains most of the information from the original data set [33,34]. PCA was performed on the data of the equivalent circuit from each sample, which reduces 6 components (from glassy carbon) and 5 (from GUITAR) to 3 principle components, respectively. Table 1 are variance decomposition principal components extraction table for glassy carbon and GUITAR electrode impedance measurements, which showed the
accumulative reliabilities of PC1, PC2, PC3, PC4, PC5 and PC6. The cumulative reliability of the ﬁrst three principal components is 93.8765% (glassy carbon electrode) and 95.8804% (GUITAR electrode) respectively. It suggests that these 3 principal components dominate all the index information. They provide the best discrimination of the classes. The rest components are statistically insigniﬁcant for classiﬁcation analysis. PCA was performed on 8 groups out of 40 samples' EIS spectrum data in total. The 8 groups Baijiu samples are classiﬁed as their storage duration, they are stored for 1st year, 3rd years, 5th years, 9th years, 12th years, 15th years, 20th years and 30th years. Scatter plots were built based on the scores of three principal components from glassy carbon and GUITAR EIS data. They were named ‘PCA scores image’, shown in Fig. 3a and b. The advantage of principal components scores image is that it displays the clustering information of all the aged Baijiu samples . The scatter plot of glassy carbon electrode based EIS is presented in Fig. 3a, PC1 (variability; 52.67%) vs. PC2 (variability; 24.01%) vs. PC3 (variability; 17.2%) accounted for 93.88% of the variation in the spectra. The location of each data points appears to be incompact and lack of distinctive boundary for each aged Baijiu group. Fig. 3b is the PCA score image for GUITAR electrode measured EIS. In sum of PC1 (variability; 73.27%) vs. PC2 (variability; 16.9%) vs. PC3 (variability; 5.71%) is 95.88% of the variation in the spectra. The data points represent for storage duration aggregated in each region of
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Fig. 2. Nyquist plots (a) from glassy carbon electrode and (b) GUITAR electrode measured in a frequency range of 0.1 M Hz to 0.01 Hz with 10 mV of AC amplitude. Representative Simulated Nyquist curves superposed with the experimental measurements according to corresponding equivalent circuits (c) glassy carbon electrode and (d) GUITAR electrode for 5th years Baijiu sample.
Table 1 Variance decomposition principal components extraction table from all Baijiu samples. (a) EIS measurements with glassy carbon electrode Principal components
Principal component variance Total
1 2 3 4 5 6
0.2288 0.1043 0.0747 0.0235 0.0019 0.0012
56.6703 24.0101 17.1961 5.4098 0.4374 0.2762
52.6703 76.6805 93.8765
0.2288 0.1043 0.0747
56.6703 24.0101 17.1961
52.6703 76.6805 93.8765
Extraction factor variance Total reliabilities/%
73.2748 90.1715 95.8804
0.3504 0.0808 0.0273
73.2748 90.1715 95.8804
(b) EIS measurements with GUITAR electrode Composition Principal component variance Total reliabilities/% 1 2 3 4 5
0.3504 0.0808 0.0273 0.0144 0.0053
73.2748 16.8967 5.7089 3.0113 1.1083
Extraction factor variance
PCs space with a deﬁned resolution . Fig. 3b shows that the GUITAR electrode-based EIS offers an enhanced resolution for aged Baijiu discrimination compared to glassy carbon. This can be reasoned by the unique electrochemical properties of utilized GUITAR electrode in this study, in which the standard heterogeneous rate constant across the basal plane with [Fe(CN)6]3-/4- is 2e7 orders of magnitude greater than glassy carbon . In addition, large surface area of GUITAR electrode exposures more basal
73.2748 16.8967 5.7089
plane and absorbs more targets than that of glassy carbon, which leading to enhanced performance . 4. Conclusion and discussion The contribution of this work is that a facile and non-destructive approach for aged Baijiu discrimination with EIS and PCA was demonstrated. Faradic impedance spectroscopy taking with
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Fig. 3. PCA score image of (a) glassy carbon electrode and (b) GUITAR electrode.
GUITAR electrode provides higher resolution than glassy carbon electrode EIS measurements. The presented discriminating model was built based on the differences in physiochemical properties of aged Baijiu other than its subtle aroma compounds variation, which ensured the reliability of the above methodology . We characterized aged Baijiu with EIS and established a discrimination model according to the EIS information for the ﬁrst time. Further research works are needed to interpret the speciﬁc colloidal impedance behavior of aged Chinese Baijiu. Discovering the regular impedance phenomena of aged Baijiu and employing EIS to elucidate the mechanism of colloid formation in aged Chinese Baijiu are also interesting and critical in the future. This study holds great promise in developing a household device for aged Chinese Baijiu discrimination. Acknowledgement This work is sponsored by National Science Foundation of China Project No. 31601560. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.01.050. References  Z.C. Xu, Study of scientiﬁc identiﬁcation techniques of the storage ageo of Chinese liquordage liquor identiﬁcation by volatilization coefﬁcient, J. LiquorMaking Science & Technology 44 (2) (2008) 96e98.  T. Yang, G.Y. Li, M.Y. Zhuang, Identiﬁcation of vintage of Chinese distillled spirit, J. Liquor Making 35 (5) (2008) 33e38.  G. X Wang, Recognition of Liquor Age Based on Raman Spectra and Support Vector Regression, D. Nanjing University of Finance and Economics, 2015.  Z.J. Jiang, J.M. Li, Study on the relationship between brandy composition and liquor age, J. Food and Fermentation Industries 34 (3) (2008) 137e140.  M.Y. Zhuang, Talk about year wine and distinguishing method, J. LiquorMaking Science & Technology 44 (7) (2008) 28e31.  M.Y. Zhuang, Properties of sol and its application principle and method in Chinese liquor, J. Liquor Making. 29 (1) (2002) 27e30.  Z.C. Xu, Invention of ‘Volatilization-coefﬁcient-method to idntify the vintage of Chinese liquor’ breach the worldwide problem, J. Sichuan Food and Fermentation 44 (1) (2008) 1e3.  M.H. Chakrabarti, N.P. Brandon, S.A. Hajimolana, F. Tariq, V. Yuﬁt, M.A. Hashim, M.A. Hussain, C.T.J. Low, P.V. Aravind, Application of carbon materials in redox ﬂow batteries, J. Power Sources 3 (5) (2014) 544e549.  L. Yang, R. Bashir, Electrical/electrochemical impedance for rapid detection of foodborne pathogenic bacteria, Biotechnol. Adv. 26 (2) (2008) 135e150. pez De Mishima, J.R. Vilche, An electro L.M. Gassa, H.T. Mishima, B.A. Lo chemical impedance spectroscopy study of electrodeposited manganese oxide ﬁlms in borate buffers, Electrochim. Acta 42 (11) (1997) 1717e1723. vy-Cle ment, I. Mora-Sero , Y. Luo, J. Bisquert,  R. Tena-Zaera, J. Elias, C. Le Electrodeposition and impedance spectroscopy characterization of ZnO nanowire arrays, Phys. Status Solidi Appl. Mater. Sci. 205 (10) (2008) 2345e2350.
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