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Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine
Analytica Chimica Acta xxx (xxxx) xxx
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Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine Xia Zhang a, Danqing Li a, Chaoyang Dong a, Jianguo Shi a, Yangang Sun a, Baoxian Ye b, **, Yuandong Xu a, * a b
College of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou, 450001, China College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450001, 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
The kinetic process of the redox reaction of taxifolin on MoS2/ANC is controlled by both adsorption and diffusion. MoS2/ANC sensor displays wide linear range and low detection limit. reveals outstanding MoS2/ANC selectivity in fructus polygoni orientalis detection.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2019 Received in revised form 13 November 2019 Accepted 23 November 2019 Available online xxx
MoS2 and nitrogen doped active carbon composite (MoS2/ANC) is fabricated to detect taxifolin and exhibits superior redox current response and decreased redox potential difference. Further investigation reveals that the kinetic process of the redox reaction of taxifolin on MoS2/ANC electrode is controlled by both adsorption and diffusion process. Under the optimum conditions, the redox peak currents linearly relate with the concentration of taxifolin in the range of 1 109e1 106 mol L1, accompanied by the low detection limit (3 1010 mol L1). Meanwhile, outstanding selectivity, stability and repeatability are also obtained at MoS2/ANC electrode. At last, the proposed method is applied to quantitatively detect taxifolin in fructus polygoni orientalis and satisfactory results have been achieved. © 2019 Elsevier B.V. All rights reserved.
Keywords: Molybdenum sulfide Porous carbon Taxifolin Fructus polygoni Sensor
1. Introduction Flavonoids, as an important part of natural polyphenol, represent one of the most prevalent classes of compounds
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Y. Xu).
containing in medicinal herbs and trees [1,2]. Due to the potential beneficial health effect, flavonoids have aroused increasing focus. Taxifolin (3,30 ,4’,5,7-pentahydroxiflavanon, C15H12O7) is a representative class of flavonoid derived from the rind of Siberian and Dahurian larch, Chinese yew, red onions and milk thistle seeds [3,4]. Taxifolin is the precious raw material for the production of food, medicine and health care products because of its significant biological activity including chemoprevention, antiproliferatives, antioxidant, and anti-inflammatory [5e7]. Therefore, Taxifolin
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reveals outstanding effect in the treatment of cerebral infarction and sequelae, cerebral thrombus, coronary heart disease and angina pectoris [8]. Accordingly, it is urgently necessary to construct accurate and sensitive analytical methods for taxifolin detection. Up to now, various methods have been developed for taxifolin analysis, such as high-performance liquid chromatography (HPLC) [9,10], UVevis spectrophotometry [11], thin layer chromatography (TLC) [12] and capillary zone electrophoresis (CZE) [13,14]. Although these methods have the advantages of sensitivity and accuracy, they are time-consuming and expensive involving a tedious extraction process before detection, which hampers further application. In comparison, electrochemical method has been developed expeditiously due to the merits of sensitivity, simplicity, convenience and inexpensive. Moreover, the redox mechanism of the target molecules can be provided during the electrochemical detection process, and further imply the important information about pharmacological actions [15,16]. Until now, limited reports concern taxifolin based on the electrochemical analysis [17e19]. And the electrode materials used in the existing reports are graphene with narrow linear range and unsatisfactory detection limit. Consequently, exploring a sensitive and feasible electroanalytical method for taxifolin detection is pregnant and necessary. In recent years, as transition metal dichalcogenide, molybdenum disulfide (MoS2) have shown great potential in electrochemical applications like sensors [20], energy storage and conversion [21,22]. It possesses a graphene-like layered structure composed of SeMoeS units chemically bonded to form sheet-like structure. The layer structure is formed by stacking the nanosheet via van der Waals force [22], so that the guest molecules or ions can be embedded in its host structure. MoS2 has been found to be an interesting electrode nanomaterial in electrochemical sensing applications owing to its high electron mobility, highly exposed active sites and layer dependent band gap [20]. However, the poor electroconductivity of MoS2 impedes the quick response of the target molecules at the electrode. Hence, combining MoS2 with an conductive material is an effective approach to enhance the electrical conductivity. Presently, porous carbon plays an important role in electrochemical field owing to the advantages of low cost, environment benignity, widespread resource and large specific surface area. The porous feature can be controlled through an introduction of activated agents during the carbonization process. In addition, nitrogen doping is another effective approach to further improve the electrochemical performance and electrical conductivity of the porous carbon [23]. The introduction of nitrogen atom can not only change the electron cloud distribution of the carbon layer near the nitrogen atom and then increase the ratio of free electrons, but also improve the donor-acceptor characteristics of the carbon skeleton and then increase the electrical conductivity. So the polarity and wettability of the carbon surface are changed, which is conducive to the enhancement of electrocatalytic activity [24,25]. Here in this work, a composite consisted of MoS2 and nitrogendoped porous carbon has been fabricated. This MoS2/ANC composite is used as electrochemical sensor to detect taxifolin using differential pulse voltammetry (DPV) for the first time. Compared with bare glassy carbon electrode, MoS2/ANC exhibits superior current response toward taxifolin and reduce the redox peak separation. Under the optimum conditions, taxifolin at MoS2/ ANC electrode displays wide linear range, low detection limit, excellent selectivity, stability and repeatability. Besides, this proposed method is also applied to detect taxifolin in fructus polygoni orientalis with satisfactory results.
2. Experimental section 2.1. Materials synthesis All used chemicals were analytical reagents. In a typical synthesis process, the active carbon was dispersed in absolute alcohol contained tripolycyanamide. After the alcohol being evaporated, the active carbon dipped with tripolycyanamide was treated by tubular furnace at 700 C for 2 h under nitrogen atmosphere. The calcined sample was further rinsed with deionized water and then dried to acquire N-doped carbon which was designated as NC. The obtained NC was then mixed with KOH and put into tubular furnace at 750 C for 2 h with a heating rate of 4 C min1 under N2 atmosphere. The heat treated sample was further rinsed by 1 M hydrochloric acid and deionized water until to neutral. After being dried at 60 C, the activated NC was obtained and marked as ANC. Subsequently, 0.25 g ANC, 0.1 g sodium molybdate and 0.18 g thiourea were dissolved in 30 ml deionized water under vigorous stirring. Then the solution was transferred into teflon-lined stainless steel autoclave for hydrothermal treatment at 210 C for 2 h. Thereafter, the solid product was vacuum dried at 60 C and the MoS2/ANC composite was achieved. 2.2. Materials characterization The obtained materials were characterized by X-ray powder diffraction (XRD) on Rigaku D/Max-3B diffractometer with Cu Ka radiation. The microstructure was studied by transmission electron microscopy (TEM, JEM-JEOL 2100) and field emission scanning electron microscopy (FESEM, JSM-6701F) equipped with an energy dispersive spectrometer (EDS). The nitrogen adsorption-desorption isotherm was measured using Specific Surface Area and Aperture Aanalyzer (ASAP 2020). The electrochemical experiments was carried out on CHI660E (Chenhua, Shanghai, China) electrochemical work station. 2.3. Electrochemical measurement A standard three-electrode electrochemical cell was used for all electrochemical tests. The bare or modified glassy carbon electrode (GCE, d ¼ 3 mm), a saturated calomel (SCE) and a platinum wire were used as working electrode, reference electrode and auxiliary electrode, respectively. Taxifolin (98%) was purchased from Aladdin. The standard stock solution of taxifolin (1 103 mol L1) was prepared by dissolving taxifolin with methanol and stored at 4 C darkly. The working solution was obtained by diluting the stock solution. 0.1 mol L1 phosphate buffer solution (PBS) was prepared by mixing the stock solution of 0.1 mol L1 NaH2PO4, Na2HPO4 and H3PO4. All aqueous solutions were prepared using newly doubledistilled water and all experiments were performed at room temperature. The cyclic voltammgrams (CV) and differential pulse voltammetry (DPV) were performed to investigate the electrochemical behavior of taxifolin at MoS2/ANC modified GCE (MoS2/ ANC/GCE). Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 102e105 Hz. 2.4. Preparation of modified electrode Before modified, the bare GCE (BGCE) was polished to a mirrorlike surface with 0.3 mm and 0.05 mm alumina slurry, and then sonicated in anhydrous ethanol and double-distilled water, respectively. After that, MoS2/ANC/GCE was fabricated by
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Fig. 1. XRD pattern of ANC, MoS2 and MoS2/ANC.
depositing MoS2/ANC suspension (5 mL) on freshly cleaned GCE, and then dried under infrared lamp. 2.5. Real sample solution preparation According to the literature, the fructus polygoni orientalis was grinded in a mortar to obtain sample powder [18]. Methanol was used to extract taxifolin under ultrasonic bath, and this step was repeated for several times. Finally, all the extracted solutions were collected together and stored in the dark. The sample solution was diluted quantitatively using PBS supporting electrolyte. 3. Results and discussion 3.1. Sample characterizations The XRD patterns of ANC, MoS2 and MoS2/ANC are shown in Fig. 1. For ANC sample, two obvious characteristic peaks corresponding to the (002) and (100) lattice planes of graphite can be observed at 26 and 43 , respectively [26], indicating the existence of graphite structure in the ANC. It is well-known that the graphite structure exhibits excellent electroconductivity which is beneficial for the fast charge transfer. However, it is needed to point out that the peak width of (002) plane is wide, demonstrating the low order degree of graphite structure. Otherwise, the peaks around 28 , 32 and 40 imply the presence of some potassium compound residual causing during the activation process [27]. As for MoS2, the peaks located at 17, 32 and 57 can be attributed to (002), (100) and (110) crystal planes, respectively, manifesting the hexagonal structure of MoS2 (JCPDS No. 65e0160) [28,29]. The presence of (002) plane demonstrates that the SeMoeS layers are stacking orderly in the MoS2 structure. In addition, the diffraction peaks of MoS2 and ANC can also be found in the XRD pattern of MoS2/ANC composite except that the peak intensity becomes weak. The morphologies of as-prepared MoS2 and MoS2/ANC are revealed by SEM and TEM. In Fig. 2a, MoS2 exhibits regular microspheres with the diameter of approximate 2 mm. The surface is rough with some subunit. Interestingly, after the combination of MoS2 with ANC, MoS2/ANC presents hierarchical microflower structure composed of nanosheet. The microflower is fluffier with the diameter of 1 mm (shown in Fig. 2b). As depicted in TEM images in Fig. 2c and d, the pure MoS2 is not hollow. While the nanosheet of MoS2 microflower in MoS2/ANC is very thin, which is consistent
3
Fig. 2. (a) and (b) are SEM image of MoS2 and MoS2/ANC, (c) and (d) are TEM images of MoS2 and MoS2/ANC.
with SEM results. In addition, EDS mapping scanning tests are performed to observe the element distribution of C, N, Mo and S in MoS2/ANC composite. As shown in Fig. 3, it can be obviously seen that the content of C element is rich in the composite. Mo and S element distribute intensively in the MoS2 microflower. However, N element cannot be clearly observed maybe because of its small content compared with C, Mo and S. So it is not circled in Fig. 3. Nitrogen adsorptionedesorption isotherms of the samples are shown in Fig. 4. The pore parameters calculated from the adsorption isotherms are listed in Table 1. It is obvious that the nitrogen adsorption isotherm of ANC belongs to the type I according to the IUPAC, indication of a prevalence of micropores in the resultant Ndoped carbon (Fig. 4a). As shown in Table 1, the SMicro/SBET and VMicro/VTotal are 83.5% and 68.6%, respectively, manifesting the abundant micropores. However, for the MoS2/ANC sample, the nitrogen adsorption isotherm exhibits obvious hysteresis loop associated with capillary condensation and a high adsorption uptake at p/p0 > 0.9 (Fig. 4b). The SMicro/SBET and VMicro/VTotal is 69.6% and 42%, respectively, which are both lower than that of ANC. This difference suggests that the combination of ANC and MoS2 produces a large amount of mesopores at the expense of micropores, resulting in a decrease of SBET from 450.6 m2 g1 to 255.8 m2 g1 and an increase of average pore diameter from 2.1 nm to 2.8 nm (as shown in Table 1). Furthermore, the pore size characteristics of ANC and MoS2/ANC have also been performed. As shown in Fig. 4c, the curve of dV/dD for ANC sample drops dramatically at 2 nm and is close to zero at 4 nm, suggesting a typical micropore characteristic. Meanwhile the cumulative pore volume curve rises sharply and generates a platform in the range of 3e10 nm, and then increases gradually. These phenomena further demonstrate that ANC mainly contains micropores. In regard to MoS2/ANC sample, the pore size distribution curves (Fig. 4d) are different from ANC, which is due to the occurrence of more mesopores. This result is consistent with the above analysis. 3.2. Voltammetric behavior and EIS measurement Fig. 5 shows the CV curves of taxifolin (3.0 106 mol L1) in 0.1 mol L1 PBS solution (pH 3.0) at BGCE, ANC modified GCE (ANC/ GCE), MoS2 modified GCE (MoS2/GCE) and MoS2/ANC/GCE, respectively. It is clear that redox peaks are absent at BGCE, indicating no response to taxifolin. For MoS2/GCE, as displayed in Fig. 5b, it exhibits large background response, but no redox peaks
Please cite this article as: X. Zhang et al., Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.057
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Fig. 3. The corresponding EDS mapping of C, N, Mo and S elements on the chosen region of MoS2/ANC composite.
appears. However, it should be pointed out that although the pristine MoS2/GCE has no response to taxifolin, it can notably promote the signal response of ANC. Obviously, a pair of oxidationreduction peaks appears at ANC/GCE and MoS2/ANC/GCE. The peak current of MoS2/ANC/GCE is greatly higher than that of ANC/GCE.
And the redox peak potentials shift to the negative direction. This behavior indicates good electrocatalytic activity of MoS2/ANC towards taxifolin. The excellent electrochemical response of taxifolin on MoS2/ANC/GCE might be due to the existence of mesopores which is beneficial to the electron transportation.
Fig. 4. (a) N2 adsorption/desorption isotherms and (b) pore size distribution plots of ANC and MoS2/ANC.
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Table 1 Physical properties characterized by N2 adsorption/desorption at 77 K. Sample
SBET /m2 g1
SMicro /m2 g1
SMicro/SBET /%
VTotal /cm3 g1
VMicro /cm3 g1
VMicro/VTotal /%
Dp /nm
ANC MoS2 MoS2/ANC
450.6 290.6 255.8
376.2 0 178.0
83.5 0 69.6
0.2416 0.2442 0.1802
0.1657 0 0.0757
68.6 0 42
2.1442 3.3608 2.8183
To further investigate the charge transfer behavior of the obtained electrode, the electrochemical impedance spectroscopy (EIS) analysis are performed in 5 mM [Fe(CN)6]3-/4- with 0.1 M KCl as supporting electrolyte. Fig. 5c and d displays the Nyquist plots of BGCE, ANC/GCE, MoS2/GCE and MoS2/ANC/GCE electrodes. The semicircle in the high frequency range corresponds to the charge transfer resistance (Rct) of electrolyte ions diffused into the electrode through electrode/electrolyte interface [30,31]. It can be obviously observed that the Rct values for the electrodes are in the order of MoS2/ANC/GCE < ANC/GCE < MoS2/GCE < BGCE, indicating the rapid electron transfer rate and excellent conductivity of MoS2/ ANC/GCE. The straight line in the low frequency range is related to the diffusive resistance of the electrolyte ions into the interior of the electrode. For the purpose of further explain the EIS of the different electrodes, the equivalent circuit is employed for the simulation of impedance spectra, as shown in Fig. S1 in Supporting Information (SI). For all the electrodes, Rs represents the resistance of the solution between the work electrode and reference electrode. At high frequency, the interfacial impedance element is replaced by a constant-phase element (CPE) [32]. The semicircle present in Nyquist plot is due to the parallel combination of resistance and CPE [33,34]. In addition, the Warburg Element (W) corresponds to the straight line in the low frequency in Nyquist plot.
Fig. S2 shows the Nyquist plots of the experimental data and the fitted value, suggesting the good agreement in a wide range of frequencies.
3.3. The influence of the scan rate Useful information involving electrochemical mechanism usually can be acquired from the relationship between peak current and scan rate (v). With the purpose of elucidating the oxidation mechanism of taxifolin at MoS2/ANC/GCE, the scan rate effect on the peak current response is investigated. As shown in Fig. 6a, the peak current of the CV curves increases with the rise of scan rate. The logarithm values of the oxidation current linearly relate with the logarithm of scan rate (Fig. 6b) according to the following equation: logi ¼ 0.7728 logv þ 1.9816 (R ¼ 0.999)
(3)
The slope of 0.7728 manifests that the redox reaction of taxifolin at MoS2/ANC/GCE is an adsorption-diffusion together controlled process [35,36]. During this process, the electron transfer number n can be calculated according to the following equation [37]:
Fig. 5. (a) and (b) are CV curves of taxifolin (3.0 106 mol L1) at different electrodes at a scan rate of 0.1 V s1, (c) and (d) are Nyquist plots of the electrodes (d is the enlarged area from c).
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Fig. 6. (a) CV curves of taxifolin (3.0 106 mol L1) at MoS2/ANC/GCE in PBS (0.1 M, pH ¼ 3.0) at different scan rates, (b) and (c) are logv ~ logi and Ep ~ lnv curves, respectively.
Scheme 1. The redox reaction mechanism of taxifolin at MoS2/ANC/GCE.
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Fig. 7. (a) CV curves of taxifolin (3.0 106 mol L1) at MoS2/ANC/GCE in different pH solutions, (b) The curve of Ep ~ pH, (c) Chronocoulometry curves obtained in the absence and presence of taxifolin at MoS2/ANC/GCE, (d) The corresponding Q ~ t1/2 curves, (e) and (f) are DPV curves and the corresponding current response at different taxifolin concentrations, respectively (scan rate: 0.1 V s1).
ip ¼
nFQv 4RT
(4)
Here, Qv is the peak area of the CV curve and the value of n is 2. Additionally, it should be pointed out that the anodic (Epa) and cathodic (Epc) peak potential shift to the opposite direction as the rise of scan rate, revealing the irreversibility of the taxifolin redox reaction at higher scan rate. And the Epa and Epc linearly relate with the natural logarithm of scan rates (lnv) (shown in Fig. 6c). The regression equations are expressed as follows:
Epa ¼ 0.0152 lnv þ 0.4578 (R ¼ 0.996)
(5)
Epc ¼ 0.0120 lnv þ 0.3375 (R ¼ 0.999)
(6)
According to Laviron’s equation [27], the slope value can be RT expressed as ð1RT aÞnF and anF , respectively. As a result, the value of the electron transfer coefficient a is 0.61. The possible redox reaction mechanism of taxifolin on MoS2/ANC/GCE is speculated in Scheme 1 [18,19]. The semi-quinone structure forms firstly followed by the quinone structure. Otherwise, the formation of dimers and trimers has been testified during the electrooxidation of taxifolin [38].
Table 2 Determination results in fructus polygoni orientalis. Original founda ( 108 mol L1)
Standard added ( 108 mol L1)
Total found ( 108 mol L1)
R.S.D (%)
Recovery (%)
13.285 16.040 21.318
0.6 2.8 2.0 2.3
100.5 98.9 100.4
11.225 2 5 10 a
Average value of the three replicate measurements.
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3.4. The pH effect of the supporting electrolyte The pH effect of PBS to the current response of taxifolin (3 106 mol L1) at MoS2/ANC/GCE is investigated by CV. As displayed in Fig. 7a, as the pH value increasing from 2.0 to 5.0, both anodic and cathodic peak potential shift to the negative direction with different current response, indicating that the oxidation process of taxifolin at MoS2/ANC/GCE is a proton-loss reaction. The relationship between the peak potential and pH value is revealed in Fig. 7b. The linear regression equations are shown as follows: Epa(V) ¼ 0.5646e0.0494 pH (R ¼ 0.996)
(1)
Epc(V) ¼ 0.5617e0.0649 pH (R ¼ 0.999)
(2)
The slope values are close to the theoretical value of 0.059, demonstrating that the equal number of protons and electrons participate in the redox reaction process [19]. Meanwhile, the best current response is observed at pH ¼ 3.0 which is denoted as the optimum pH during the subsequent measurements. 3.5. Saturated adsorption capacity Considering the adsorption-diffusion process of taxifolin on MoS2/ANC/GC, the diffusion coefficient (D) and saturated adsorption capacity (G*) should be provided. Chronocoulometry technique is adopted in the blank and taxifolin (3 106 mol L1) solution (Fig. 7c). To Extract data from Fig. 7c, the corresponding Q ~ t1/2 curves are acquired and shown in Fig. 7d. It can be clearly observed that the Q ~ t1/2 curves are in good linear relationship and the regression equations are presented as follows: Q (106 C) ¼ 37.56 t1/2 þ 91.21 (R ¼ 0.999) (taxifolin solution)(7) Q (106 C) ¼ 31.14 t1/2 þ 28.44 (R ¼ 0.999) (blank solution) (8) According to Anson’s theory [20,28], the following equations can be established:
Q¼
2nFAcðDtÞ1=2
p1=2
Qads ¼ nFAG
þ Qdl þ Qads
*
manifesting an excellent sensitivity of MoS2/ANC electrode. Otherwise, the parallel results of the calibration curves shown in Fig. S3 in SI suggests the consistent results with Fig. 7f. 3.7. Repeatability, stability and interference studies The repeatability is performed through two ways: one electrode to test six parallel taxifolin solutions and six modified electrodes to detect one taxifolin solution, the relative standard deviation (RSD) of the peak current response is 2.7% and 3.8%, respectively. To store MoS2/ANC/GCE for one week, the current response only decreases by 5.4%. The above results demonstrate the outstanding repeatability and stability of this sensor for taxifolin detection. The interference measurement is carried out to evaluate the selectivity of MoS2/ANC/GCE (shown in Fig. S4 in SI). Ascorbic acid, glucose, 2 Ca2þ, Mn2þ, Zn2þ, CO2 3 , SO4 , Br and Cl (the concentrations are all 3 104 mol L1) are added into the taxifolin contained PBS (pH 3.0) solution, exhibiting no interference toward taxifolin detection. 3.8. Real sample analysis Real sample analysis is significant to examine the practicability of the proposed method. Herein, standard addition method is adopted to detect taxifolin in fructus polygoni orientalis. As the detection results listed in Table 2, the recovery of taxifolin in fructus polygoni orientalis is calculated in the range of 98.9e100.5%, indicating the availability of the proposed sensor for the determination of taxifolin. 4. Conclusions In a word, an electrochemical sensor based on MoS2/ANC is fabricated and exhibits sensitive electroanalytical activity for taxifolin detection. The proposed modified electrode displays wide linear range of 1 109e1 106 mol L1 and low detection limit of 3 1010 mol L1. Meanwhile, high adsorption capacity, excellent selectivity, stability, and repeatability are also achieved. At last, this sensor is further applied to detect taxifolin in fructus polygoni orientalis with satisfactory result, manifesting a promising platform for detecting taxifolin in traditional Chinese medicine.
(9) Author contributions section
(10)
Hereon, c is the concentration of taxifolin, A is the area of the electrode, Qdl and Qads are the double-layer and Faradaic charge, respectively. Other symbols have their usual meanings. Qads is calculated as 6.28 105 C. As a result, D and G* are 6.76 107 cm2 s1 and 4.58 109 mol cm2, respectively.
Xia Zhang: Conceptualization, Methodology, Formal analysis, Writing-Review & Editing. Danqing Li: Validation, Investigation, Formal analysis. Chaoyang Dong: Investigation, Software. Jianguo shi: Resources, Data Curation. Yangang Sun: Validation, Software. Baoxian Ye: Visualization, Project administration. Yuandong Xu: Supervision, Funding acquisition.
3.6. Calibration curve Declaration of competing interest Fig. 7e displays the differential pulse voltammetry (DPV) response of taxifolin with different concentrations. Obviously, the oxidation peak current increases with taxifolin concentrations. As revealed in Fig. 7f, in the range of 1 109e1 106 mol L1, a linear relationship can be established between ipa and the concentration of taxifolin. The linear regression equation is listed infra: i (mA) ¼ 58.29 C (mmol L1) þ 1.1150 (R2 ¼ 0.981)
(11)
Based on the signal-to-noise ratio (S/N) of 3, the detection limit of taxifolin at MoS2/ANC/GCE is 3 1010 mol L1 which is superior to the relevant references shown in Table S1 in SI [17e19,39],
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (NSFC, No. 51702088), The Training Program for Key Young Teachers in Henan Institutions (2019GGJS092), The Institutions of Higher Learning Key Scientific Research Project (19B150005), High-level Talent Fund Project of Henan University of
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Technology (2016BS011), The Youth Backbone Teacher Project of Henan University of Technology (2015007), and The National College Students Innovation Training Program (201910463014). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.11.057. References [1] C. Li, C. Huo, M. Zhang, Q. Shi, Chemistry of Chinese yew, Taxus chinensis var. mairei, Biochem. Syst. Ecol. 36 (2008) 266e282. [2] R. Slimestad, T. Fossen, I.M. Vågen, Onions: a source of unique dietary flavonoids, J. Agric. Food Chem. 55 (2007) 10067e10080. [3] O. Sytar, K. Bruckova, E. Hunkova, M. Zivcak, K. Konate, M. Brestic, The application of multiplex fluorimetric sensor for the analysis of flavonoids content in the medicinal herbs family Asteraceae, Lamiaceae, Rosaceae, Biol. Res. 48 (2015) 5e14. [4] S.X. Zhou, Y. Shao, J.H. Fu, L. Xiang, Y.N. Zheng, W. Li, Characterization and quantification of taxifolin related flavonoids in larix olgensis henry Var. koreana nakai extract analysis and its antioxidant activity assay, Int. J. Pharmacol. 14 (2018) 534e545. [5] C. Wu, S. Cao, T. Hong, Y. Dong, C. Li, Q. Wang, J. Sun, R.S. Ge, Taxifolin inhibits rat and human 11b-hydroxysteroid dehydrogenase 2, Fitoterapia 121 (2017) 112e117. [6] M.B. Plotnikov, D.M. Plotnikov, O.I. Aliev, M.Y. Maslov, A.S. Vasiliev, V.M. Alifirova, N.A. Tyukavkina, Hemorheological and antioxidant effects of ascovertin in patients with sclerosis of cerebral arteries, Clin. Hemorheol. Microcirc. 30 (2004) 449e452. http://www.medsci.cn/sci/submit.do? id¼c8de1549. [7] Y. Wei, X. Chen, X. Jiang, Z. Ma, J. Xiao, Determination of taxifolin in polygonum orientale and study on its antioxidant activity, J. Food Compos. Anal. 22 (2009) 154e157. [8] M.B. Plotnikov, O.I. Aliev, M.J. Maslov, A.S. Vasiliev, N.A. Tjukavkina, Correction of the high blood viscosity syndrome by a mixture of diquertin and ascorbic acid in vitro and in vivo, Phytother Res. 17 (2003) 276e278. n ~ ez, J.K. Takemoto, [9] K.R. Vega Villa, C.M. Remsberg, Y. Ohgami, J.A. Ya P.K. Andrews, N.M. Davies, Stereospecific high-performance liquid chromatography of taxifolin, applications in pharmacokinetics, and determination in tu fu ling (Rhizoma smilacis glabrae) and apple (Malus domestica), Biomed. Chromatogr. 23 (2009) 638e646. [10] O.N. Pozharitskaya, M.V. Karlina, A.N. Shikov, V.M. Kosman, M.N. Makarova, V.G. Makarov, Determination and pharmacokinetic study of taxifolin in rabbit plasma by high-performance liquid chromatography, Phytomedicine 16 (2009) 244e251. [11] D. Liu, S. Lin, G. Liang, Determination of the content of taxifolin in princefeather fruit by spectrophotometry, Prog. Mod. Biomed. 8 (2008) 331e333. [12] J. He, Y. Zhai, TLC method for the determination of taxifolin content in fructus polygoni orientalis, J. Liaoning Coll. Tradit. Chin. Med. 7 (2005) 622e623. [13] Q.F. Zhang, S.C. Li, W.P. Lai, H.Y. Cheung, b-Cyclodextrin facilitates simultaneous analysis of six bioactive components in Rhizoma Smilacis Glabrae by capillary zone electrophoresis, Food Chem. 113 (2009) 684e691. [14] Q.F. Zhang, H.Y. Cheung, The content of astilbin and taxifolin in concentrated extracts of Rhizoma Smilacis Glabrae and turtle jelly vary significantly, Food Chem. 119 (2010) 907e912. [15] A.S. Farag, N.K. Bakirhan, I. Svancara, S.A. Ozkan, A new sensing platform based on NH2fMWCNTs for the determination of antiarrhythmic drug Propafenone in pharmaceutical dosage forms, J. Pharma. Biomed. 174 (2019) 534e540. [16] P. Puthongkham, S.T. Lee, B.J. Venton, Mechanism of histamine oxidation and electropolymerization at carbon electrodes, Anal. Chem. 91 (2019) 8366e8373. [17] Y.J. Wu, M.X. Lv, B. Li, J.D. Ge, L. Gao, A rapid, green and controllable strategy to fabricate electrodeposition of reduced graphene oxide film as sensing materials for determination of taxifolin, Nano: Brief Rep. Rev. 10 (2015) 110e117. [18] F. Wang, Y.J. Wu, K. Lu, B.X. Ye, A sensitive voltammetric sensor for taxifolin based on graphene nanosheets with certain orientation modified glassy carbon electrode, Sens. Actuators B Chem. 208 (2015) 188e194.
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine Xia Zhang a, Danqing Li a, Chaoyang Dong a, Jianguo Shi a, Yangang Sun a, Baoxian Ye b, **, Yuandong Xu a, * a b
College of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou, 450001, China College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450001, 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
The kinetic process of the redox reaction of taxifolin on MoS2/ANC is controlled by both adsorption and diffusion. MoS2/ANC sensor displays wide linear range and low detection limit. reveals outstanding MoS2/ANC selectivity in fructus polygoni orientalis detection.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 October 2019 Received in revised form 13 November 2019 Accepted 23 November 2019 Available online xxx
MoS2 and nitrogen doped active carbon composite (MoS2/ANC) is fabricated to detect taxifolin and exhibits superior redox current response and decreased redox potential difference. Further investigation reveals that the kinetic process of the redox reaction of taxifolin on MoS2/ANC electrode is controlled by both adsorption and diffusion process. Under the optimum conditions, the redox peak currents linearly relate with the concentration of taxifolin in the range of 1 109e1 106 mol L1, accompanied by the low detection limit (3 1010 mol L1). Meanwhile, outstanding selectivity, stability and repeatability are also obtained at MoS2/ANC electrode. At last, the proposed method is applied to quantitatively detect taxifolin in fructus polygoni orientalis and satisfactory results have been achieved. © 2019 Elsevier B.V. All rights reserved.
Keywords: Molybdenum sulfide Porous carbon Taxifolin Fructus polygoni Sensor
1. Introduction Flavonoids, as an important part of natural polyphenol, represent one of the most prevalent classes of compounds
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Y. Xu).
containing in medicinal herbs and trees [1,2]. Due to the potential beneficial health effect, flavonoids have aroused increasing focus. Taxifolin (3,30 ,4’,5,7-pentahydroxiflavanon, C15H12O7) is a representative class of flavonoid derived from the rind of Siberian and Dahurian larch, Chinese yew, red onions and milk thistle seeds [3,4]. Taxifolin is the precious raw material for the production of food, medicine and health care products because of its significant biological activity including chemoprevention, antiproliferatives, antioxidant, and anti-inflammatory [5e7]. Therefore, Taxifolin
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reveals outstanding effect in the treatment of cerebral infarction and sequelae, cerebral thrombus, coronary heart disease and angina pectoris [8]. Accordingly, it is urgently necessary to construct accurate and sensitive analytical methods for taxifolin detection. Up to now, various methods have been developed for taxifolin analysis, such as high-performance liquid chromatography (HPLC) [9,10], UVevis spectrophotometry [11], thin layer chromatography (TLC) [12] and capillary zone electrophoresis (CZE) [13,14]. Although these methods have the advantages of sensitivity and accuracy, they are time-consuming and expensive involving a tedious extraction process before detection, which hampers further application. In comparison, electrochemical method has been developed expeditiously due to the merits of sensitivity, simplicity, convenience and inexpensive. Moreover, the redox mechanism of the target molecules can be provided during the electrochemical detection process, and further imply the important information about pharmacological actions [15,16]. Until now, limited reports concern taxifolin based on the electrochemical analysis [17e19]. And the electrode materials used in the existing reports are graphene with narrow linear range and unsatisfactory detection limit. Consequently, exploring a sensitive and feasible electroanalytical method for taxifolin detection is pregnant and necessary. In recent years, as transition metal dichalcogenide, molybdenum disulfide (MoS2) have shown great potential in electrochemical applications like sensors [20], energy storage and conversion [21,22]. It possesses a graphene-like layered structure composed of SeMoeS units chemically bonded to form sheet-like structure. The layer structure is formed by stacking the nanosheet via van der Waals force [22], so that the guest molecules or ions can be embedded in its host structure. MoS2 has been found to be an interesting electrode nanomaterial in electrochemical sensing applications owing to its high electron mobility, highly exposed active sites and layer dependent band gap [20]. However, the poor electroconductivity of MoS2 impedes the quick response of the target molecules at the electrode. Hence, combining MoS2 with an conductive material is an effective approach to enhance the electrical conductivity. Presently, porous carbon plays an important role in electrochemical field owing to the advantages of low cost, environment benignity, widespread resource and large specific surface area. The porous feature can be controlled through an introduction of activated agents during the carbonization process. In addition, nitrogen doping is another effective approach to further improve the electrochemical performance and electrical conductivity of the porous carbon [23]. The introduction of nitrogen atom can not only change the electron cloud distribution of the carbon layer near the nitrogen atom and then increase the ratio of free electrons, but also improve the donor-acceptor characteristics of the carbon skeleton and then increase the electrical conductivity. So the polarity and wettability of the carbon surface are changed, which is conducive to the enhancement of electrocatalytic activity [24,25]. Here in this work, a composite consisted of MoS2 and nitrogendoped porous carbon has been fabricated. This MoS2/ANC composite is used as electrochemical sensor to detect taxifolin using differential pulse voltammetry (DPV) for the first time. Compared with bare glassy carbon electrode, MoS2/ANC exhibits superior current response toward taxifolin and reduce the redox peak separation. Under the optimum conditions, taxifolin at MoS2/ ANC electrode displays wide linear range, low detection limit, excellent selectivity, stability and repeatability. Besides, this proposed method is also applied to detect taxifolin in fructus polygoni orientalis with satisfactory results.
2. Experimental section 2.1. Materials synthesis All used chemicals were analytical reagents. In a typical synthesis process, the active carbon was dispersed in absolute alcohol contained tripolycyanamide. After the alcohol being evaporated, the active carbon dipped with tripolycyanamide was treated by tubular furnace at 700 C for 2 h under nitrogen atmosphere. The calcined sample was further rinsed with deionized water and then dried to acquire N-doped carbon which was designated as NC. The obtained NC was then mixed with KOH and put into tubular furnace at 750 C for 2 h with a heating rate of 4 C min1 under N2 atmosphere. The heat treated sample was further rinsed by 1 M hydrochloric acid and deionized water until to neutral. After being dried at 60 C, the activated NC was obtained and marked as ANC. Subsequently, 0.25 g ANC, 0.1 g sodium molybdate and 0.18 g thiourea were dissolved in 30 ml deionized water under vigorous stirring. Then the solution was transferred into teflon-lined stainless steel autoclave for hydrothermal treatment at 210 C for 2 h. Thereafter, the solid product was vacuum dried at 60 C and the MoS2/ANC composite was achieved. 2.2. Materials characterization The obtained materials were characterized by X-ray powder diffraction (XRD) on Rigaku D/Max-3B diffractometer with Cu Ka radiation. The microstructure was studied by transmission electron microscopy (TEM, JEM-JEOL 2100) and field emission scanning electron microscopy (FESEM, JSM-6701F) equipped with an energy dispersive spectrometer (EDS). The nitrogen adsorption-desorption isotherm was measured using Specific Surface Area and Aperture Aanalyzer (ASAP 2020). The electrochemical experiments was carried out on CHI660E (Chenhua, Shanghai, China) electrochemical work station. 2.3. Electrochemical measurement A standard three-electrode electrochemical cell was used for all electrochemical tests. The bare or modified glassy carbon electrode (GCE, d ¼ 3 mm), a saturated calomel (SCE) and a platinum wire were used as working electrode, reference electrode and auxiliary electrode, respectively. Taxifolin (98%) was purchased from Aladdin. The standard stock solution of taxifolin (1 103 mol L1) was prepared by dissolving taxifolin with methanol and stored at 4 C darkly. The working solution was obtained by diluting the stock solution. 0.1 mol L1 phosphate buffer solution (PBS) was prepared by mixing the stock solution of 0.1 mol L1 NaH2PO4, Na2HPO4 and H3PO4. All aqueous solutions were prepared using newly doubledistilled water and all experiments were performed at room temperature. The cyclic voltammgrams (CV) and differential pulse voltammetry (DPV) were performed to investigate the electrochemical behavior of taxifolin at MoS2/ANC modified GCE (MoS2/ ANC/GCE). Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 102e105 Hz. 2.4. Preparation of modified electrode Before modified, the bare GCE (BGCE) was polished to a mirrorlike surface with 0.3 mm and 0.05 mm alumina slurry, and then sonicated in anhydrous ethanol and double-distilled water, respectively. After that, MoS2/ANC/GCE was fabricated by
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Fig. 1. XRD pattern of ANC, MoS2 and MoS2/ANC.
depositing MoS2/ANC suspension (5 mL) on freshly cleaned GCE, and then dried under infrared lamp. 2.5. Real sample solution preparation According to the literature, the fructus polygoni orientalis was grinded in a mortar to obtain sample powder [18]. Methanol was used to extract taxifolin under ultrasonic bath, and this step was repeated for several times. Finally, all the extracted solutions were collected together and stored in the dark. The sample solution was diluted quantitatively using PBS supporting electrolyte. 3. Results and discussion 3.1. Sample characterizations The XRD patterns of ANC, MoS2 and MoS2/ANC are shown in Fig. 1. For ANC sample, two obvious characteristic peaks corresponding to the (002) and (100) lattice planes of graphite can be observed at 26 and 43 , respectively [26], indicating the existence of graphite structure in the ANC. It is well-known that the graphite structure exhibits excellent electroconductivity which is beneficial for the fast charge transfer. However, it is needed to point out that the peak width of (002) plane is wide, demonstrating the low order degree of graphite structure. Otherwise, the peaks around 28 , 32 and 40 imply the presence of some potassium compound residual causing during the activation process [27]. As for MoS2, the peaks located at 17, 32 and 57 can be attributed to (002), (100) and (110) crystal planes, respectively, manifesting the hexagonal structure of MoS2 (JCPDS No. 65e0160) [28,29]. The presence of (002) plane demonstrates that the SeMoeS layers are stacking orderly in the MoS2 structure. In addition, the diffraction peaks of MoS2 and ANC can also be found in the XRD pattern of MoS2/ANC composite except that the peak intensity becomes weak. The morphologies of as-prepared MoS2 and MoS2/ANC are revealed by SEM and TEM. In Fig. 2a, MoS2 exhibits regular microspheres with the diameter of approximate 2 mm. The surface is rough with some subunit. Interestingly, after the combination of MoS2 with ANC, MoS2/ANC presents hierarchical microflower structure composed of nanosheet. The microflower is fluffier with the diameter of 1 mm (shown in Fig. 2b). As depicted in TEM images in Fig. 2c and d, the pure MoS2 is not hollow. While the nanosheet of MoS2 microflower in MoS2/ANC is very thin, which is consistent
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Fig. 2. (a) and (b) are SEM image of MoS2 and MoS2/ANC, (c) and (d) are TEM images of MoS2 and MoS2/ANC.
with SEM results. In addition, EDS mapping scanning tests are performed to observe the element distribution of C, N, Mo and S in MoS2/ANC composite. As shown in Fig. 3, it can be obviously seen that the content of C element is rich in the composite. Mo and S element distribute intensively in the MoS2 microflower. However, N element cannot be clearly observed maybe because of its small content compared with C, Mo and S. So it is not circled in Fig. 3. Nitrogen adsorptionedesorption isotherms of the samples are shown in Fig. 4. The pore parameters calculated from the adsorption isotherms are listed in Table 1. It is obvious that the nitrogen adsorption isotherm of ANC belongs to the type I according to the IUPAC, indication of a prevalence of micropores in the resultant Ndoped carbon (Fig. 4a). As shown in Table 1, the SMicro/SBET and VMicro/VTotal are 83.5% and 68.6%, respectively, manifesting the abundant micropores. However, for the MoS2/ANC sample, the nitrogen adsorption isotherm exhibits obvious hysteresis loop associated with capillary condensation and a high adsorption uptake at p/p0 > 0.9 (Fig. 4b). The SMicro/SBET and VMicro/VTotal is 69.6% and 42%, respectively, which are both lower than that of ANC. This difference suggests that the combination of ANC and MoS2 produces a large amount of mesopores at the expense of micropores, resulting in a decrease of SBET from 450.6 m2 g1 to 255.8 m2 g1 and an increase of average pore diameter from 2.1 nm to 2.8 nm (as shown in Table 1). Furthermore, the pore size characteristics of ANC and MoS2/ANC have also been performed. As shown in Fig. 4c, the curve of dV/dD for ANC sample drops dramatically at 2 nm and is close to zero at 4 nm, suggesting a typical micropore characteristic. Meanwhile the cumulative pore volume curve rises sharply and generates a platform in the range of 3e10 nm, and then increases gradually. These phenomena further demonstrate that ANC mainly contains micropores. In regard to MoS2/ANC sample, the pore size distribution curves (Fig. 4d) are different from ANC, which is due to the occurrence of more mesopores. This result is consistent with the above analysis. 3.2. Voltammetric behavior and EIS measurement Fig. 5 shows the CV curves of taxifolin (3.0 106 mol L1) in 0.1 mol L1 PBS solution (pH 3.0) at BGCE, ANC modified GCE (ANC/ GCE), MoS2 modified GCE (MoS2/GCE) and MoS2/ANC/GCE, respectively. It is clear that redox peaks are absent at BGCE, indicating no response to taxifolin. For MoS2/GCE, as displayed in Fig. 5b, it exhibits large background response, but no redox peaks
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Fig. 3. The corresponding EDS mapping of C, N, Mo and S elements on the chosen region of MoS2/ANC composite.
appears. However, it should be pointed out that although the pristine MoS2/GCE has no response to taxifolin, it can notably promote the signal response of ANC. Obviously, a pair of oxidationreduction peaks appears at ANC/GCE and MoS2/ANC/GCE. The peak current of MoS2/ANC/GCE is greatly higher than that of ANC/GCE.
And the redox peak potentials shift to the negative direction. This behavior indicates good electrocatalytic activity of MoS2/ANC towards taxifolin. The excellent electrochemical response of taxifolin on MoS2/ANC/GCE might be due to the existence of mesopores which is beneficial to the electron transportation.
Fig. 4. (a) N2 adsorption/desorption isotherms and (b) pore size distribution plots of ANC and MoS2/ANC.
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Table 1 Physical properties characterized by N2 adsorption/desorption at 77 K. Sample
SBET /m2 g1
SMicro /m2 g1
SMicro/SBET /%
VTotal /cm3 g1
VMicro /cm3 g1
VMicro/VTotal /%
Dp /nm
ANC MoS2 MoS2/ANC
450.6 290.6 255.8
376.2 0 178.0
83.5 0 69.6
0.2416 0.2442 0.1802
0.1657 0 0.0757
68.6 0 42
2.1442 3.3608 2.8183
To further investigate the charge transfer behavior of the obtained electrode, the electrochemical impedance spectroscopy (EIS) analysis are performed in 5 mM [Fe(CN)6]3-/4- with 0.1 M KCl as supporting electrolyte. Fig. 5c and d displays the Nyquist plots of BGCE, ANC/GCE, MoS2/GCE and MoS2/ANC/GCE electrodes. The semicircle in the high frequency range corresponds to the charge transfer resistance (Rct) of electrolyte ions diffused into the electrode through electrode/electrolyte interface [30,31]. It can be obviously observed that the Rct values for the electrodes are in the order of MoS2/ANC/GCE < ANC/GCE < MoS2/GCE < BGCE, indicating the rapid electron transfer rate and excellent conductivity of MoS2/ ANC/GCE. The straight line in the low frequency range is related to the diffusive resistance of the electrolyte ions into the interior of the electrode. For the purpose of further explain the EIS of the different electrodes, the equivalent circuit is employed for the simulation of impedance spectra, as shown in Fig. S1 in Supporting Information (SI). For all the electrodes, Rs represents the resistance of the solution between the work electrode and reference electrode. At high frequency, the interfacial impedance element is replaced by a constant-phase element (CPE) [32]. The semicircle present in Nyquist plot is due to the parallel combination of resistance and CPE [33,34]. In addition, the Warburg Element (W) corresponds to the straight line in the low frequency in Nyquist plot.
Fig. S2 shows the Nyquist plots of the experimental data and the fitted value, suggesting the good agreement in a wide range of frequencies.
3.3. The influence of the scan rate Useful information involving electrochemical mechanism usually can be acquired from the relationship between peak current and scan rate (v). With the purpose of elucidating the oxidation mechanism of taxifolin at MoS2/ANC/GCE, the scan rate effect on the peak current response is investigated. As shown in Fig. 6a, the peak current of the CV curves increases with the rise of scan rate. The logarithm values of the oxidation current linearly relate with the logarithm of scan rate (Fig. 6b) according to the following equation: logi ¼ 0.7728 logv þ 1.9816 (R ¼ 0.999)
(3)
The slope of 0.7728 manifests that the redox reaction of taxifolin at MoS2/ANC/GCE is an adsorption-diffusion together controlled process [35,36]. During this process, the electron transfer number n can be calculated according to the following equation [37]:
Fig. 5. (a) and (b) are CV curves of taxifolin (3.0 106 mol L1) at different electrodes at a scan rate of 0.1 V s1, (c) and (d) are Nyquist plots of the electrodes (d is the enlarged area from c).
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Fig. 6. (a) CV curves of taxifolin (3.0 106 mol L1) at MoS2/ANC/GCE in PBS (0.1 M, pH ¼ 3.0) at different scan rates, (b) and (c) are logv ~ logi and Ep ~ lnv curves, respectively.
Scheme 1. The redox reaction mechanism of taxifolin at MoS2/ANC/GCE.
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Fig. 7. (a) CV curves of taxifolin (3.0 106 mol L1) at MoS2/ANC/GCE in different pH solutions, (b) The curve of Ep ~ pH, (c) Chronocoulometry curves obtained in the absence and presence of taxifolin at MoS2/ANC/GCE, (d) The corresponding Q ~ t1/2 curves, (e) and (f) are DPV curves and the corresponding current response at different taxifolin concentrations, respectively (scan rate: 0.1 V s1).
ip ¼
nFQv 4RT
(4)
Here, Qv is the peak area of the CV curve and the value of n is 2. Additionally, it should be pointed out that the anodic (Epa) and cathodic (Epc) peak potential shift to the opposite direction as the rise of scan rate, revealing the irreversibility of the taxifolin redox reaction at higher scan rate. And the Epa and Epc linearly relate with the natural logarithm of scan rates (lnv) (shown in Fig. 6c). The regression equations are expressed as follows:
Epa ¼ 0.0152 lnv þ 0.4578 (R ¼ 0.996)
(5)
Epc ¼ 0.0120 lnv þ 0.3375 (R ¼ 0.999)
(6)
According to Laviron’s equation [27], the slope value can be RT expressed as ð1RT aÞnF and anF , respectively. As a result, the value of the electron transfer coefficient a is 0.61. The possible redox reaction mechanism of taxifolin on MoS2/ANC/GCE is speculated in Scheme 1 [18,19]. The semi-quinone structure forms firstly followed by the quinone structure. Otherwise, the formation of dimers and trimers has been testified during the electrooxidation of taxifolin [38].
Table 2 Determination results in fructus polygoni orientalis. Original founda ( 108 mol L1)
Standard added ( 108 mol L1)
Total found ( 108 mol L1)
R.S.D (%)
Recovery (%)
13.285 16.040 21.318
0.6 2.8 2.0 2.3
100.5 98.9 100.4
11.225 2 5 10 a
Average value of the three replicate measurements.
Please cite this article as: X. Zhang et al., Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.057
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3.4. The pH effect of the supporting electrolyte The pH effect of PBS to the current response of taxifolin (3 106 mol L1) at MoS2/ANC/GCE is investigated by CV. As displayed in Fig. 7a, as the pH value increasing from 2.0 to 5.0, both anodic and cathodic peak potential shift to the negative direction with different current response, indicating that the oxidation process of taxifolin at MoS2/ANC/GCE is a proton-loss reaction. The relationship between the peak potential and pH value is revealed in Fig. 7b. The linear regression equations are shown as follows: Epa(V) ¼ 0.5646e0.0494 pH (R ¼ 0.996)
(1)
Epc(V) ¼ 0.5617e0.0649 pH (R ¼ 0.999)
(2)
The slope values are close to the theoretical value of 0.059, demonstrating that the equal number of protons and electrons participate in the redox reaction process [19]. Meanwhile, the best current response is observed at pH ¼ 3.0 which is denoted as the optimum pH during the subsequent measurements. 3.5. Saturated adsorption capacity Considering the adsorption-diffusion process of taxifolin on MoS2/ANC/GC, the diffusion coefficient (D) and saturated adsorption capacity (G*) should be provided. Chronocoulometry technique is adopted in the blank and taxifolin (3 106 mol L1) solution (Fig. 7c). To Extract data from Fig. 7c, the corresponding Q ~ t1/2 curves are acquired and shown in Fig. 7d. It can be clearly observed that the Q ~ t1/2 curves are in good linear relationship and the regression equations are presented as follows: Q (106 C) ¼ 37.56 t1/2 þ 91.21 (R ¼ 0.999) (taxifolin solution)(7) Q (106 C) ¼ 31.14 t1/2 þ 28.44 (R ¼ 0.999) (blank solution) (8) According to Anson’s theory [20,28], the following equations can be established:
Q¼
2nFAcðDtÞ1=2
p1=2
Qads ¼ nFAG
þ Qdl þ Qads
*
manifesting an excellent sensitivity of MoS2/ANC electrode. Otherwise, the parallel results of the calibration curves shown in Fig. S3 in SI suggests the consistent results with Fig. 7f. 3.7. Repeatability, stability and interference studies The repeatability is performed through two ways: one electrode to test six parallel taxifolin solutions and six modified electrodes to detect one taxifolin solution, the relative standard deviation (RSD) of the peak current response is 2.7% and 3.8%, respectively. To store MoS2/ANC/GCE for one week, the current response only decreases by 5.4%. The above results demonstrate the outstanding repeatability and stability of this sensor for taxifolin detection. The interference measurement is carried out to evaluate the selectivity of MoS2/ANC/GCE (shown in Fig. S4 in SI). Ascorbic acid, glucose, 2 Ca2þ, Mn2þ, Zn2þ, CO2 3 , SO4 , Br and Cl (the concentrations are all 3 104 mol L1) are added into the taxifolin contained PBS (pH 3.0) solution, exhibiting no interference toward taxifolin detection. 3.8. Real sample analysis Real sample analysis is significant to examine the practicability of the proposed method. Herein, standard addition method is adopted to detect taxifolin in fructus polygoni orientalis. As the detection results listed in Table 2, the recovery of taxifolin in fructus polygoni orientalis is calculated in the range of 98.9e100.5%, indicating the availability of the proposed sensor for the determination of taxifolin. 4. Conclusions In a word, an electrochemical sensor based on MoS2/ANC is fabricated and exhibits sensitive electroanalytical activity for taxifolin detection. The proposed modified electrode displays wide linear range of 1 109e1 106 mol L1 and low detection limit of 3 1010 mol L1. Meanwhile, high adsorption capacity, excellent selectivity, stability, and repeatability are also achieved. At last, this sensor is further applied to detect taxifolin in fructus polygoni orientalis with satisfactory result, manifesting a promising platform for detecting taxifolin in traditional Chinese medicine.
(9) Author contributions section
(10)
Hereon, c is the concentration of taxifolin, A is the area of the electrode, Qdl and Qads are the double-layer and Faradaic charge, respectively. Other symbols have their usual meanings. Qads is calculated as 6.28 105 C. As a result, D and G* are 6.76 107 cm2 s1 and 4.58 109 mol cm2, respectively.
Xia Zhang: Conceptualization, Methodology, Formal analysis, Writing-Review & Editing. Danqing Li: Validation, Investigation, Formal analysis. Chaoyang Dong: Investigation, Software. Jianguo shi: Resources, Data Curation. Yangang Sun: Validation, Software. Baoxian Ye: Visualization, Project administration. Yuandong Xu: Supervision, Funding acquisition.
3.6. Calibration curve Declaration of competing interest Fig. 7e displays the differential pulse voltammetry (DPV) response of taxifolin with different concentrations. Obviously, the oxidation peak current increases with taxifolin concentrations. As revealed in Fig. 7f, in the range of 1 109e1 106 mol L1, a linear relationship can be established between ipa and the concentration of taxifolin. The linear regression equation is listed infra: i (mA) ¼ 58.29 C (mmol L1) þ 1.1150 (R2 ¼ 0.981)
(11)
Based on the signal-to-noise ratio (S/N) of 3, the detection limit of taxifolin at MoS2/ANC/GCE is 3 1010 mol L1 which is superior to the relevant references shown in Table S1 in SI [17e19,39],
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (NSFC, No. 51702088), The Training Program for Key Young Teachers in Henan Institutions (2019GGJS092), The Institutions of Higher Learning Key Scientific Research Project (19B150005), High-level Talent Fund Project of Henan University of
Please cite this article as: X. Zhang et al., Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.057
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Please cite this article as: X. Zhang et al., Molybdenum sulfide-based electrochemical platform for high sensitive detection of taxifolin in Chinese medicine, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.057