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Fabrication of SnO2 decorated graphene composite material and its application in electrochemical detection of caffeic acid in red wine
Materials Research Bulletin 126 (2020) 110820
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Fabrication of SnO2 decorated graphene composite material and its application in electrochemical detection of caffeic acid in red wine
T
Ji-Wei Zhanga, Kai-Ping Wanga, Xuan Zhanga,b,* a Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai, 201620, China b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai, 200241, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2decorated graphene Electrochemical sensor Caffeic acid Red wine
Metal oxide/graphene composite has become a promising electrode material to improve the performance of electrochemical sensor. Here SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2, and applied as active electrode material into constructing electrochemical sensor for caffeic acid (CA). The SnO2-RGO composite was characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical sensor (SnO2-RGO/GCE) obtained from SnO2-RGO coating of glass carbon electrode showed excellent electrocatalytic performance for the redox reaction of CA. A linear range of 0.15–25 μM and a detection limit of 80 nM (S/N = 3) were achieved in 0.1 M phosphate buffer (pH 7.0). The present electrochemical sensor also showed high selectivity for CA, excellent reproducibility and stability. Eventually, the sensor SnO2-RGO/GCE was successfully applied for CA detection in commercial red wine samples with satisfactory recovery results.
1. Introduction Polyphenol compounds have been considered as potential agents in neuroprotection [1]. Caffeic acid (CA) (3,4-dihydroxycinnamic acid), one of main phenolic derivatives, usually exists in red wine, green tea, coffee, fruits and vegetables [2–4]. CA plays an extremely important role in human life, such as antibacterial and anti-inflammatory effects [5–8]. However, excessive intake of CA may cause side effect to human health. Therefore, it is important to quantitatively measure the amount of CA in food-related samples. Currently, a variety of analytical techniques have been used to detect CA, such as high performance liquid chromatography (HPLC) [9], capillary electrophoresis [10], sequential injection analysis (SIA) [11], flow injection analysis [12] and gas chromatography-mass spectrometry (GC–MS) [13]. These methods are relatively expensive and the pre-treatment of samples is cumbersome, making them difficult to ready for daily analysis [14]. Electrochemical sensing is one of the most promising techniques for a rapid routine analysis due to its simple operation, low cost, high sensitivity and selectivity [15]. In recent years, a variety of electrochemical sensors have been developed for quantitative analysis of CA [16–19]. However, most of these previous sensors either exhibited low sensitivity, poor stability, or required complicated fabrication processes. Therefore, the
development of electrochemical sensor for CA with simple fabrication process and excellent performance is necessary. Nanocarbon materials such as carbon nanotube, fullerene and graphene have been extensively employed as active electrode material due to their unique physical and chemical properties [15,20,21]. Among these nanocarbon materials, graphene has drawn much attention in electrochemical sensing due to its good electrical conductivity, large specific surface area and rich graphite sources [22]. In addition, metal and metal oxide nanomaterials have also been used as efficient electrode material for electrochemical sensing. Recently, metal oxide/graphene composite materials have revealed promising potential application for electrochemical sensor. For example, Bi2O3/RGO [23], ZnO/ RGO [24], MoS2/RGO [25], Gd2O3/RGO [26], CeO2/RGO [27], Au/ CeO2/RGO [28] have been reported as active electrode materials. In these composites, metal oxide nanoparticles could increase the interlamellar spacing of graphene, preserve the more single-layer graphene structures [29], and therefore improve the performance of electrochemical sensor. In this work, SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2, and applied as an active electrode material for constructing new electrochemical sensor. The developed electrochemical sensor showed high sensitivity and selectivity for CA,
⁎ Corresponding author at: Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai, 201620, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.materresbull.2020.110820 Received 6 August 2019; Received in revised form 13 November 2019; Accepted 18 February 2020 Available online 19 February 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 126 (2020) 110820
J.-W. Zhang, et al.
oxide powder, then ultrasonically rinsed with nitric acid (v: v, 1: 1), ethanol, and water, respectively. The SnO2-RGO suspension in ethanol (5 μL, 2 mg mL–1) was dropped directly onto the surface of the pretreated GCE and dried at 60 °C. For comparison, bare GCE and SnO2 modified electrodes (SnO2/GCE) were also prepared by following the similar procedure.
excellent reproducibility and stability. The concentration of CA in commercial red wine samples was successfully determined with satisfactory recovery results. 2. Experimental 2.1. Chemicals and reagents
2.4. Determination procedure for AP Graphite powder (99.9 %)were purchased from XFNANO(Nanjing, China), Caffeic acid and Tin(II) chloride dihydrate were all purchased from Adamas. Sulfuric acid (H2SO4, 98 %), potassium permanganate (KMnO4) and other common chemicals were obtained from Sinopharm Chemical Reagent Corp. (Shanghai, China). Phosphate buffer (0.1 M) was prepared by mixing 0.1 M K2HPO4 and 0.1 M KH2PO4, and the desired pH was obtained by adjusting the amount of both.
The electrocatalytic activity for caffeic acid oxidation reaction was investigated by cyclic voltammetry (CV) in 0.1 M phosphate buffer (pH 7) containing 10 μM CA between –0.1 and 0.5 V (vs. SCE) with a scan rate of 50 mV s−1. The quantitative analysis of CA was performed on SnO2-RGO/GCE by differential pulse voltammetry (DPV) measurements, where various amounts of CA were added in an electrochemical cell under a stirring condition. The DPV curves were recorded at a potential window between –0.1 and 0.5 V (vs. SCE), with step potential of 4 mV, amplitude of 50 mV, pulse width of 0.2 s and pulse period of 0.5 s, respectively. The electrical conductivity of bare GCE, SnO2, SnO2RGO/GCE was evaluated by electrochemical impedance spectroscopy (EIS) analysis in 0.1 M KCl solution containing 5 mM K3[Fe(CN)6].
2.2. Instruments The morphology characterization was conducted on transmission electron microscopy (TEM, JEOL JEM-2100 F, Japan). The chemical composition analysis was performed on X-ray photoelectron spectroscopy (XPS, PHI 5400, USA). The crystal structure was determined with X-ray powder diffractometry (XRD, Rigaku D/max 2550, Japan). Electrochemical measurements were carried out on CHI-660D electrochemical workstation (Chenhua, Shanghai) by using a conventional three-electrode system, including glassy carbon electrode (GCE, 3 mm diameter) as working electrode, saturated calomel electrode (SCE) as reference electrode, Pt wire as counter electrode.
3. Results and discussion 3.1. Characterization of SnO2-RGO The microstructure of SnO2-RGO sample was examined by TEM (Figs. 1a and b), where the SnO2 nanocrystals are dispersed on the surface of graphene sheet and SnO2 particles are about 8 nm in diameter. As shown in the Fig. 1c, the XRD patterns of SnO2-RGO composite displayed series sharp diffraction peaks at 26.6°, 33.6°, 51.9°, and 65.2°, matched well with the (110), (101), (211), and (301) planes of tetragonal structure of SnO2 (JCPDS No. 41–1445), respectively [34]. Notably, the typical (002) plane of graphene, usually appeared around 2θ = 24°, was largely overlapped by the broad (110) peak of SnO2 [34]. This confirmed that the SnO2 nanoparticles have been successfully grown on RGO sheets. To investigate the elemental state and chemical composition, XPS measurement was performed toward SnO2-RGO. The survey XPS spectrum (Fig. 2a) showed several characteristic peaks of Sn 3p, Sn 3d, Sn 4d, C 1s and O 1s, indicating the presence of C, O and Sn elements. The quantitative analysis of XPS showed that the percentage of C, O and Sn were 86.7 %, 9.65 % and 3.66 %, respectively. Fig. 2b showed the C 1s high resolution XPS spectrum that could be
2.3. Preparation of SnO2-RGO/GCE Graphene oxide (GO) was firstly synthesized by following the previous procedure [30]. The fabrication procedures for SnO2-RGO/GCE electrochemical sensor were depicted in Scheme 1. In brief, 10 mL of Tin(II) chloride dihydrate solution was added slowly into the an aqueous GO suspension (2 mg mL–1, 20 mL) and sonicated for 0.5 h. Then, the solution was transferred into a Teflon-lined stainless steel autoclave. After being heated at 180 °C for 6 h in an oven, the solution was cooled down to room temperature. Thus the SnO2-RGO was obtained by centrifugation, washing three times using water and ethanol, and drying overnight in vacuum at 40 °C. For a comparison, pure SnO2 were also prepared under the same conditions without adding the GO precursor in the above process. Then the GCE was polished with 0.05 mm alumina
Scheme 1. The fabrication procedures for SnO2-RGO/GCE electrochemical sensor. 2
Materials Research Bulletin 126 (2020) 110820
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Fig. 1. TEM images (a, b) and XRD pattern (c) of SnO2-RGO. The individual component was indicated as dotted cycles in the TEM image (b).
SnO2/GCE and SnO2-RGO/GCE were studied by CV with an external redox probe [Fe(CN)6] 3−/4− in 0.1 M KCl solution. Well-defined pair of CV redox peaks were observed on all three electrodes. While the CV pattern of SnO2/GCE is not much different from that of the bare electrode, SnO2-RGO/GCE showed the highest peak current under the same conditions (Fig. 3a). The electrochemical surface area (ECSA) was then estimated according to the following Randles–Sevcik equation [36]:
deconvolved into three peaks at 284.6 eV, 286.6 eV and 288.8 eV, assigned to the CeC/C=C, CeO and C]O chemical bonds, respectively [30–32]. It can be clearly seen that the intensity of the CeC/C=C band is higher than that of the oxygen-containing functional group (C–O, C] O), revealing the chemical reduction of GO [33]. In the high-resolution XPS spectrum of Sn 3d (Fig. 2c), there are clearly two strong peaks at 487.2 eV and 495.7 eV, which correspond to the Sn 3d3/2 and Sn 3d5/2 states, respectively [34]. Furthermore, as shown in Fig. 2d, the highresolution O 1s spectrum showed three fitted peak, where the peak at 530.6 eV is attributed to the Sn-O bond, and the peaks at 531.4 eV and 533.3 eV are attributed to the C]O and CeO bonds, respectively [35]. These demonstrated that SnO2 decorated graphene material was successfully fabricated.
Ip = (2.69 × 10 5)n3/2AD1/2 Cv1/2 where, Ip is redox peak currents, n is number of electrons, A is electrochemical surface area (cm2), D is diffusion coefficient (cm2 s−1), v is scan rate (V s−1), and C is concentration of [Fe(CN)6]3−/4− (mol cm−3).The ECSA values were calculated to be 0.07, 0.08, and 0.32 cm2 for bare GCE, SnO2/GCE and SnO2-RGO/GCE, respectively. This indicated that the SnO2-RGO/GCE owned the larger ECSA leading to its higher peak current as observed in Fig. 3a. The electrical conductivity of electrode was further evaluated by EIS
3.2. Electrochemical properties of various electrodes As shown in Fig. 3a, the electrochemical properties of bare GCE,
Fig. 2. XPS spectrum of SnO2-RGO composites. Survey scan (a) and high resolution spectra of C 1s (b), Sn 3d (c), and O 1s (d). 3
Materials Research Bulletin 126 (2020) 110820
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Fig. 3. CV curves (a) and EIS plots (b) of the bare GCE, SnO2/GCE and SnO2-RGO/GCE in 0.1 M KCl containing 0.5 mM K3[Fe(CN)6]. CV curves of CA (10 μM) on the bare GCE, SnO2/GCE and SnO2-RGO/GCE in 0.1 M phosphate buffer (pH 7).
3.3. Optimization of determination conditions
analysis and the result was shown in Fig. 3b. The impedance data can be described concisely by the equivalent circuit model (Fig. 3(b), inset). The charge transfer resistance (Rct) can be estimated from the semicircle diameter in the high frequency region [15]. Therefore, it can be clearly seen that the Rct of SnO2-RGO/GCE is smaller than those of the bare GCE and SnO2/GCE. Combining both the higher electrical conductivity and larger ECSA, SnO2-RGO/GCE could serve as a promising electrochemical sensor for CA detection. Fig. 3c showed the electrochemical behavior of CA with three different electrodes. A well-defined pair of CV redox peak was respectively observed on these electrodes. This could be attributed to the oxidation of CA to an ortho-quinone derivative at the oxidation peak potential (+0.215 V) and the reversible reduction of the ortho-quinone derivative to CA by two electrons and two proton processes at the reduction peak potential (+0.185 V) [18,37]. The current response of CA on the bare GCE, SnO2/GCE is relatively weak, whereas SnO2-RGO/GCE showed a significant enhanced intensity. This demonstrated that SnO2RGO/GCE can be used as a potential electrochemical sensor for detecting CA in real sample. The kinetics of the electrochemical reaction was investigated by examining the effect of the scan rate on the redox peak current and potential. As shown in Fig. 4a, with increasing the scan rate (10−100 mV s−1), the redox current intensity increased. Notably, both the oxidation and reduction peak currents were linearly proportional to the scan rates (Fig. 4b), and the linear regression equationis Ipa = 0.7687 v + 4.08 (R2 = 0.9934), Ipc = 0.7481 v -3.1867 (R2 = 0.9944), respectively. This linear dependent relationship suggests that the electrochemical reaction of CA on the SnO2-RGO/GCE is an adsorption controlled process [15].
The experimental conditions, including accumulation time, amount of catalyst and pH of the buffer solution, were firstly optimized by DPV measurements to achieve the best performance of electrochemical sensor. Fig. 5a shows the effect of the amount of catalyst deposited on the GC electrode (2 mg mL−1) on the current of CA. Clearly, the 5 μL catalyst suspension is appropriate amount that provided the strongest current (Fig. 5b). Fig. 5c showed the DPV curve of CA response on SnO2-RGO/GCE recorded at different agitation times. It can be seen that the peak current increased and remained almost stable after 300 s (Fig. 5d). Thus the solution was firstly stirred 300 s before DPV measurement in this work. Fig. 6 showed the effect of pH on the DPV curves of CA as well as on peak current intensity and peak potential. Obviously, both the peak current intensity and peak potential are strongly dependent on the pH value from pH 4–9, indicating that protons participated in the electrochemical oxidation process. The peak current intensity increased with pH value and reached the maximum at pH 7.0, while the peak potential was almost underwent a linearly negative shift. Therefore, a phosphate buffer of pH 7.0 was selected for further study. Notably, the slope of the linear equation between pH value and peak potential is 0.076, which is close to the Nernstian constant of 0.059, indicating that equal number of electrons and protons participate in the transfer process. According to the previous work [19], two protons and electrons were involved into the redox process of CA, and a possible mechanism of CA detection over SnO2-RGO/GCE was similarly proposed (Scheme 2).
3.4. Analytical performance of SnO2-RGO/GCE to CA detection Under the optimal experimental conditions, the determination of CA 4
Materials Research Bulletin 126 (2020) 110820
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Fig. 4. CV curves of CA (20 μM) in 0.1 M phosphate buffer (pH 7.4) measured on SnO2-RGO/GCE at various scan rates (10‒100 mV s−1) (a), and the corresponding plots of the peak currents versus scan rates (b).
Further advantages of the present sensor are its facile fabrication and fast response.
on SnO2-RGO/GCE was performed in 0.1 M phosphate buffer by DPV. Fig. 7a showed the DPV curve of different concentrations of CA on the SnO2-RGO/GCE electrode, where a group of well-defined oxidation peaks of CA are observed at 0.16 V. Fig. 7b showed the calibration curve that revealed the DPV peak current is linearly dependent with the CA concentration (0.15–25 μM). The linear regression equation is Ip = 2.7708 C + 0.0828 (R2 = 0.9989) and the detection limit was estimated to be 80 nM (S/N = 3) for CA. As shown in Table 1, the comparison of analytical performance with those of the previous sensors revealed that the present SnO2-RGO/GCE exhibited good performance. The present sensor showed comparable detection limit with most of previous sensors [39–41] and even lower [38] for CA detection.
3.5. Selectivity, reproducibility and stability SnO2-RGO/GCE The selectivity of electrochemical sensors is particularly important in the presence of various possible interfering compounds. Thus several possible interfering substances were investigated. It was found that 50 μM uric acid (UA), epinephrine (EP), hydroquinone (HQ), catechol (CC), 100 μM Glucose (Glu), ascorbic acid (AA), tartaric acid (TA), and 1 mM CaCl2, KCl, NaCl, MgSO4 showed negligible interference (signal change < 7%). This revealed that the present SnO2-RGO/GCE sensor
Fig. 5. DPV curves of CA (20 μM) on SnO2-RGO/GCE at various usage amounts of catalyst (a), accumulation time (c), and the corresponding peak current versus the volume (b) and time (d), respectively. 5
Materials Research Bulletin 126 (2020) 110820
J.-W. Zhang, et al.
Fig. 6. DPV curves of CA (20 μM) on the SnO2-RGO/GCE in 0.1 M phosphate buffer with various pH between 4 and 9, and the plots of peak current (b) and peak potential (c) versus pH, respectively.
SnO2-RGO/GCE by standard addition method. In this experiment, three samples without any pretreatment were measured by DPV, and the results are summarized in Table 2. Firstly, red wine (10 u L) was injected into 5.0 mL of 0.1 M phosphate buffer (pH 7.0) for a DPV measurement, the current intensity at 0.17 V was recorded, and the concentration was calculated by according to the linear regression equation (section 3.4). Then the certain amounts of CA (0.5–1.5 μM) were spiked into the above samples for DPV measurements, and the corresponding concentration of CA were obtained to evaluate the recovery rate. All DPV measurements were carried out three times to give an average value of CA concentration. The concentration of CA in red wine was determined to be 0.52–1.15 μM. Furthermore, upon addition of certain amounts of CA, the recovery rate of CA was found to be in the range of 93.0–103.9 %. These results proved that the SnO2-RGO/GCE is promising electrochemical sensor for sensitive and selective detection of CA in real red wine.
has excellent selectivity for CA detection. The stability of the SnO2-RGO/GCE was firstly examined by numerous CV measurements in phosphate buffer containing 20 μM of CA (Fig. 8a). It was found that only 6.2 % peak current was reduced even after 100 cycles, suggested the good stability of the SnO2-RGO/GCE electrode. The reproducibility and storage stability of SnO2-RGO/GCE were also investigated using a DPV method in phosphate buffer containing 20 μM of CA. In order to test the reproducibility of SnO2-RGO/ GCE, five continuous measurements were repeated under the same conditions. As shown in Fig. 8b, the DPV curves of the five tests exhibited negligible difference on response current with the relative standard deviation (RSD) of 1.3 %, indicating a superior reproducibility of SnO2-RGO/GCE. The long-term storage stability was further examined by exposing the same SnO2-RGO/GCE electrode in air for one month. As shown in Fig. 8c, the peak current measured every five days on the same SnO2-RGO/GCE electrode remained almost no change, and only 4.6 % current was reduced after one month storage. The above results indicate that SnO2-RGO/GCE showed both good reproducibility and stability, allowing a potential determination of CA in real samples.
4. Conclusions In summary, the SnO2-RGO composite was facilely prepared and used as electrode material to construct electrochemical sensor for CA detection. The electrode material was characterized by TEM and XPS. The electrochemical sensor SnO2-RGO/GCE showed a well-defined
3.6. Determination of CA in red wine samples The determination of CA in the red wine sample was carried out on
Scheme 2. The possible redox mechanism of CA over SnO2-RGO/GCE.
6
Materials Research Bulletin 126 (2020) 110820
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Fig. 7. DPV curves of CA with various concentrations (0.15–35 μM) on SnO2-RGO/GCE (a) and the corresponding calibration plots between the peak current intensity and CA concentration (b).
redox peak of CA. A linear range was found between current intensity and CA concentration of 0.15–25 μM. The present electrochemical sensor showed an excellent selectivity and a good sensitivity with a detection limit of 80 nM, as well as good stability and reproducibility. Furthermore, the present sensor was successfully applied to detect CA in red wines samples with satisfactory recovery results (93.0–103.9 %) and relative standard deviations (0.93–6.2 %). These demonstrated that SnO2-RGO/GCE is a promising electrochemical sensor for practical determination of CA concentration in red wines.
Table 1 Comparison of analytical performance of various electrochemical sensors for CA detection. Electrochemical sensors
Linear ranges (μM)
Limit of detection (nM)
References
3DG/MWCNTs/GCE LDHf/GCE ERGO/Nafion AuNPs/GRNS Activated GCE RGO@PDA SnO2-RGO/GCE
0.2–174 7–180 0.1–10 0.5–50.0 0.1–1 0.005–450 0.15–25
17.8 2600 90 50 68 1.2 80
[17] [38] [39] [40] [41] [42] This work
Fig. 8. The CV cycles of CA (20 μM) measured over the same SnO2-RGO/GCE in 0.1 M phosphate buffer (pH 7.4) (a). The DPV curves of CA (20 μM) measured on five different SnO2-RGO/GCE electrodes in 0.1 M phosphate buffer (pH 7.0) (b). Long-term storage stability tests of SnO2-RGO/GCE in 0.1 M PBS solution containing 20 μM CA (c). 7
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Table 2 Determination of CA in various red wine samples. Samples
Added (μM)
Found (μM)
Recovery (%)
Red wine 1
0 0.50 1.00 1.50 0 0.50 1.00 1.50 0 0.50 1.00 1.50
0.65 1.07 1.59 2.04 0.52 1.06 1.56 2.01 1.15 1.62 2.10 2.61
/ 93.0 96.4 94.9 / 103.9 102.6 99.5 / 98.2 97.7 98.5
Red wine 2
Red wine 3
± 0.01 ± 0.02 ± 0.05 ± 0.07 ± 0.05 ± 0.05 ± 0.04 ± 0.04 ± 0.03
[12]
[13]
[14]
[15]
[16]
[17]
Novelty statement SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2. The electrochemical sensor (SnO2-RGO/GCE) obtained from SnO2-RGO coating of glass carbon electrode showed excellent electrocatalytic performance for the redox reaction of caffeic acid (CA) and a detection limit of 80 nM (S/N = 3) were achieved. The present electrochemical sensor was successfully applied for CA detection in commercial red wine samples with satisfactory recovery results.
[18]
[19]
[20] [21]
Declaration of Competing Interest [22]
The authors have declared that no conflicting interests exist. Acknowledgments
[23]
This work was financially supported by Shanghai Municipal Natural Science Foundation (16ZR1401700) and the Open Research Fund of Key Laboratory of Polar Materials and Devices, Ministry of Education.
[24]
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Fabrication of SnO2 decorated graphene composite material and its application in electrochemical detection of caffeic acid in red wine
T
Ji-Wei Zhanga, Kai-Ping Wanga, Xuan Zhanga,b,* a Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai, 201620, China b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai, 200241, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SnO2decorated graphene Electrochemical sensor Caffeic acid Red wine
Metal oxide/graphene composite has become a promising electrode material to improve the performance of electrochemical sensor. Here SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2, and applied as active electrode material into constructing electrochemical sensor for caffeic acid (CA). The SnO2-RGO composite was characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical sensor (SnO2-RGO/GCE) obtained from SnO2-RGO coating of glass carbon electrode showed excellent electrocatalytic performance for the redox reaction of CA. A linear range of 0.15–25 μM and a detection limit of 80 nM (S/N = 3) were achieved in 0.1 M phosphate buffer (pH 7.0). The present electrochemical sensor also showed high selectivity for CA, excellent reproducibility and stability. Eventually, the sensor SnO2-RGO/GCE was successfully applied for CA detection in commercial red wine samples with satisfactory recovery results.
1. Introduction Polyphenol compounds have been considered as potential agents in neuroprotection [1]. Caffeic acid (CA) (3,4-dihydroxycinnamic acid), one of main phenolic derivatives, usually exists in red wine, green tea, coffee, fruits and vegetables [2–4]. CA plays an extremely important role in human life, such as antibacterial and anti-inflammatory effects [5–8]. However, excessive intake of CA may cause side effect to human health. Therefore, it is important to quantitatively measure the amount of CA in food-related samples. Currently, a variety of analytical techniques have been used to detect CA, such as high performance liquid chromatography (HPLC) [9], capillary electrophoresis [10], sequential injection analysis (SIA) [11], flow injection analysis [12] and gas chromatography-mass spectrometry (GC–MS) [13]. These methods are relatively expensive and the pre-treatment of samples is cumbersome, making them difficult to ready for daily analysis [14]. Electrochemical sensing is one of the most promising techniques for a rapid routine analysis due to its simple operation, low cost, high sensitivity and selectivity [15]. In recent years, a variety of electrochemical sensors have been developed for quantitative analysis of CA [16–19]. However, most of these previous sensors either exhibited low sensitivity, poor stability, or required complicated fabrication processes. Therefore, the
development of electrochemical sensor for CA with simple fabrication process and excellent performance is necessary. Nanocarbon materials such as carbon nanotube, fullerene and graphene have been extensively employed as active electrode material due to their unique physical and chemical properties [15,20,21]. Among these nanocarbon materials, graphene has drawn much attention in electrochemical sensing due to its good electrical conductivity, large specific surface area and rich graphite sources [22]. In addition, metal and metal oxide nanomaterials have also been used as efficient electrode material for electrochemical sensing. Recently, metal oxide/graphene composite materials have revealed promising potential application for electrochemical sensor. For example, Bi2O3/RGO [23], ZnO/ RGO [24], MoS2/RGO [25], Gd2O3/RGO [26], CeO2/RGO [27], Au/ CeO2/RGO [28] have been reported as active electrode materials. In these composites, metal oxide nanoparticles could increase the interlamellar spacing of graphene, preserve the more single-layer graphene structures [29], and therefore improve the performance of electrochemical sensor. In this work, SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2, and applied as an active electrode material for constructing new electrochemical sensor. The developed electrochemical sensor showed high sensitivity and selectivity for CA,
⁎ Corresponding author at: Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai, 201620, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.materresbull.2020.110820 Received 6 August 2019; Received in revised form 13 November 2019; Accepted 18 February 2020 Available online 19 February 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
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oxide powder, then ultrasonically rinsed with nitric acid (v: v, 1: 1), ethanol, and water, respectively. The SnO2-RGO suspension in ethanol (5 μL, 2 mg mL–1) was dropped directly onto the surface of the pretreated GCE and dried at 60 °C. For comparison, bare GCE and SnO2 modified electrodes (SnO2/GCE) were also prepared by following the similar procedure.
excellent reproducibility and stability. The concentration of CA in commercial red wine samples was successfully determined with satisfactory recovery results. 2. Experimental 2.1. Chemicals and reagents
2.4. Determination procedure for AP Graphite powder (99.9 %)were purchased from XFNANO(Nanjing, China), Caffeic acid and Tin(II) chloride dihydrate were all purchased from Adamas. Sulfuric acid (H2SO4, 98 %), potassium permanganate (KMnO4) and other common chemicals were obtained from Sinopharm Chemical Reagent Corp. (Shanghai, China). Phosphate buffer (0.1 M) was prepared by mixing 0.1 M K2HPO4 and 0.1 M KH2PO4, and the desired pH was obtained by adjusting the amount of both.
The electrocatalytic activity for caffeic acid oxidation reaction was investigated by cyclic voltammetry (CV) in 0.1 M phosphate buffer (pH 7) containing 10 μM CA between –0.1 and 0.5 V (vs. SCE) with a scan rate of 50 mV s−1. The quantitative analysis of CA was performed on SnO2-RGO/GCE by differential pulse voltammetry (DPV) measurements, where various amounts of CA were added in an electrochemical cell under a stirring condition. The DPV curves were recorded at a potential window between –0.1 and 0.5 V (vs. SCE), with step potential of 4 mV, amplitude of 50 mV, pulse width of 0.2 s and pulse period of 0.5 s, respectively. The electrical conductivity of bare GCE, SnO2, SnO2RGO/GCE was evaluated by electrochemical impedance spectroscopy (EIS) analysis in 0.1 M KCl solution containing 5 mM K3[Fe(CN)6].
2.2. Instruments The morphology characterization was conducted on transmission electron microscopy (TEM, JEOL JEM-2100 F, Japan). The chemical composition analysis was performed on X-ray photoelectron spectroscopy (XPS, PHI 5400, USA). The crystal structure was determined with X-ray powder diffractometry (XRD, Rigaku D/max 2550, Japan). Electrochemical measurements were carried out on CHI-660D electrochemical workstation (Chenhua, Shanghai) by using a conventional three-electrode system, including glassy carbon electrode (GCE, 3 mm diameter) as working electrode, saturated calomel electrode (SCE) as reference electrode, Pt wire as counter electrode.
3. Results and discussion 3.1. Characterization of SnO2-RGO The microstructure of SnO2-RGO sample was examined by TEM (Figs. 1a and b), where the SnO2 nanocrystals are dispersed on the surface of graphene sheet and SnO2 particles are about 8 nm in diameter. As shown in the Fig. 1c, the XRD patterns of SnO2-RGO composite displayed series sharp diffraction peaks at 26.6°, 33.6°, 51.9°, and 65.2°, matched well with the (110), (101), (211), and (301) planes of tetragonal structure of SnO2 (JCPDS No. 41–1445), respectively [34]. Notably, the typical (002) plane of graphene, usually appeared around 2θ = 24°, was largely overlapped by the broad (110) peak of SnO2 [34]. This confirmed that the SnO2 nanoparticles have been successfully grown on RGO sheets. To investigate the elemental state and chemical composition, XPS measurement was performed toward SnO2-RGO. The survey XPS spectrum (Fig. 2a) showed several characteristic peaks of Sn 3p, Sn 3d, Sn 4d, C 1s and O 1s, indicating the presence of C, O and Sn elements. The quantitative analysis of XPS showed that the percentage of C, O and Sn were 86.7 %, 9.65 % and 3.66 %, respectively. Fig. 2b showed the C 1s high resolution XPS spectrum that could be
2.3. Preparation of SnO2-RGO/GCE Graphene oxide (GO) was firstly synthesized by following the previous procedure [30]. The fabrication procedures for SnO2-RGO/GCE electrochemical sensor were depicted in Scheme 1. In brief, 10 mL of Tin(II) chloride dihydrate solution was added slowly into the an aqueous GO suspension (2 mg mL–1, 20 mL) and sonicated for 0.5 h. Then, the solution was transferred into a Teflon-lined stainless steel autoclave. After being heated at 180 °C for 6 h in an oven, the solution was cooled down to room temperature. Thus the SnO2-RGO was obtained by centrifugation, washing three times using water and ethanol, and drying overnight in vacuum at 40 °C. For a comparison, pure SnO2 were also prepared under the same conditions without adding the GO precursor in the above process. Then the GCE was polished with 0.05 mm alumina
Scheme 1. The fabrication procedures for SnO2-RGO/GCE electrochemical sensor. 2
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Fig. 1. TEM images (a, b) and XRD pattern (c) of SnO2-RGO. The individual component was indicated as dotted cycles in the TEM image (b).
SnO2/GCE and SnO2-RGO/GCE were studied by CV with an external redox probe [Fe(CN)6] 3−/4− in 0.1 M KCl solution. Well-defined pair of CV redox peaks were observed on all three electrodes. While the CV pattern of SnO2/GCE is not much different from that of the bare electrode, SnO2-RGO/GCE showed the highest peak current under the same conditions (Fig. 3a). The electrochemical surface area (ECSA) was then estimated according to the following Randles–Sevcik equation [36]:
deconvolved into three peaks at 284.6 eV, 286.6 eV and 288.8 eV, assigned to the CeC/C=C, CeO and C]O chemical bonds, respectively [30–32]. It can be clearly seen that the intensity of the CeC/C=C band is higher than that of the oxygen-containing functional group (C–O, C] O), revealing the chemical reduction of GO [33]. In the high-resolution XPS spectrum of Sn 3d (Fig. 2c), there are clearly two strong peaks at 487.2 eV and 495.7 eV, which correspond to the Sn 3d3/2 and Sn 3d5/2 states, respectively [34]. Furthermore, as shown in Fig. 2d, the highresolution O 1s spectrum showed three fitted peak, where the peak at 530.6 eV is attributed to the Sn-O bond, and the peaks at 531.4 eV and 533.3 eV are attributed to the C]O and CeO bonds, respectively [35]. These demonstrated that SnO2 decorated graphene material was successfully fabricated.
Ip = (2.69 × 10 5)n3/2AD1/2 Cv1/2 where, Ip is redox peak currents, n is number of electrons, A is electrochemical surface area (cm2), D is diffusion coefficient (cm2 s−1), v is scan rate (V s−1), and C is concentration of [Fe(CN)6]3−/4− (mol cm−3).The ECSA values were calculated to be 0.07, 0.08, and 0.32 cm2 for bare GCE, SnO2/GCE and SnO2-RGO/GCE, respectively. This indicated that the SnO2-RGO/GCE owned the larger ECSA leading to its higher peak current as observed in Fig. 3a. The electrical conductivity of electrode was further evaluated by EIS
3.2. Electrochemical properties of various electrodes As shown in Fig. 3a, the electrochemical properties of bare GCE,
Fig. 2. XPS spectrum of SnO2-RGO composites. Survey scan (a) and high resolution spectra of C 1s (b), Sn 3d (c), and O 1s (d). 3
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Fig. 3. CV curves (a) and EIS plots (b) of the bare GCE, SnO2/GCE and SnO2-RGO/GCE in 0.1 M KCl containing 0.5 mM K3[Fe(CN)6]. CV curves of CA (10 μM) on the bare GCE, SnO2/GCE and SnO2-RGO/GCE in 0.1 M phosphate buffer (pH 7).
3.3. Optimization of determination conditions
analysis and the result was shown in Fig. 3b. The impedance data can be described concisely by the equivalent circuit model (Fig. 3(b), inset). The charge transfer resistance (Rct) can be estimated from the semicircle diameter in the high frequency region [15]. Therefore, it can be clearly seen that the Rct of SnO2-RGO/GCE is smaller than those of the bare GCE and SnO2/GCE. Combining both the higher electrical conductivity and larger ECSA, SnO2-RGO/GCE could serve as a promising electrochemical sensor for CA detection. Fig. 3c showed the electrochemical behavior of CA with three different electrodes. A well-defined pair of CV redox peak was respectively observed on these electrodes. This could be attributed to the oxidation of CA to an ortho-quinone derivative at the oxidation peak potential (+0.215 V) and the reversible reduction of the ortho-quinone derivative to CA by two electrons and two proton processes at the reduction peak potential (+0.185 V) [18,37]. The current response of CA on the bare GCE, SnO2/GCE is relatively weak, whereas SnO2-RGO/GCE showed a significant enhanced intensity. This demonstrated that SnO2RGO/GCE can be used as a potential electrochemical sensor for detecting CA in real sample. The kinetics of the electrochemical reaction was investigated by examining the effect of the scan rate on the redox peak current and potential. As shown in Fig. 4a, with increasing the scan rate (10−100 mV s−1), the redox current intensity increased. Notably, both the oxidation and reduction peak currents were linearly proportional to the scan rates (Fig. 4b), and the linear regression equationis Ipa = 0.7687 v + 4.08 (R2 = 0.9934), Ipc = 0.7481 v -3.1867 (R2 = 0.9944), respectively. This linear dependent relationship suggests that the electrochemical reaction of CA on the SnO2-RGO/GCE is an adsorption controlled process [15].
The experimental conditions, including accumulation time, amount of catalyst and pH of the buffer solution, were firstly optimized by DPV measurements to achieve the best performance of electrochemical sensor. Fig. 5a shows the effect of the amount of catalyst deposited on the GC electrode (2 mg mL−1) on the current of CA. Clearly, the 5 μL catalyst suspension is appropriate amount that provided the strongest current (Fig. 5b). Fig. 5c showed the DPV curve of CA response on SnO2-RGO/GCE recorded at different agitation times. It can be seen that the peak current increased and remained almost stable after 300 s (Fig. 5d). Thus the solution was firstly stirred 300 s before DPV measurement in this work. Fig. 6 showed the effect of pH on the DPV curves of CA as well as on peak current intensity and peak potential. Obviously, both the peak current intensity and peak potential are strongly dependent on the pH value from pH 4–9, indicating that protons participated in the electrochemical oxidation process. The peak current intensity increased with pH value and reached the maximum at pH 7.0, while the peak potential was almost underwent a linearly negative shift. Therefore, a phosphate buffer of pH 7.0 was selected for further study. Notably, the slope of the linear equation between pH value and peak potential is 0.076, which is close to the Nernstian constant of 0.059, indicating that equal number of electrons and protons participate in the transfer process. According to the previous work [19], two protons and electrons were involved into the redox process of CA, and a possible mechanism of CA detection over SnO2-RGO/GCE was similarly proposed (Scheme 2).
3.4. Analytical performance of SnO2-RGO/GCE to CA detection Under the optimal experimental conditions, the determination of CA 4
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Fig. 4. CV curves of CA (20 μM) in 0.1 M phosphate buffer (pH 7.4) measured on SnO2-RGO/GCE at various scan rates (10‒100 mV s−1) (a), and the corresponding plots of the peak currents versus scan rates (b).
Further advantages of the present sensor are its facile fabrication and fast response.
on SnO2-RGO/GCE was performed in 0.1 M phosphate buffer by DPV. Fig. 7a showed the DPV curve of different concentrations of CA on the SnO2-RGO/GCE electrode, where a group of well-defined oxidation peaks of CA are observed at 0.16 V. Fig. 7b showed the calibration curve that revealed the DPV peak current is linearly dependent with the CA concentration (0.15–25 μM). The linear regression equation is Ip = 2.7708 C + 0.0828 (R2 = 0.9989) and the detection limit was estimated to be 80 nM (S/N = 3) for CA. As shown in Table 1, the comparison of analytical performance with those of the previous sensors revealed that the present SnO2-RGO/GCE exhibited good performance. The present sensor showed comparable detection limit with most of previous sensors [39–41] and even lower [38] for CA detection.
3.5. Selectivity, reproducibility and stability SnO2-RGO/GCE The selectivity of electrochemical sensors is particularly important in the presence of various possible interfering compounds. Thus several possible interfering substances were investigated. It was found that 50 μM uric acid (UA), epinephrine (EP), hydroquinone (HQ), catechol (CC), 100 μM Glucose (Glu), ascorbic acid (AA), tartaric acid (TA), and 1 mM CaCl2, KCl, NaCl, MgSO4 showed negligible interference (signal change < 7%). This revealed that the present SnO2-RGO/GCE sensor
Fig. 5. DPV curves of CA (20 μM) on SnO2-RGO/GCE at various usage amounts of catalyst (a), accumulation time (c), and the corresponding peak current versus the volume (b) and time (d), respectively. 5
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Fig. 6. DPV curves of CA (20 μM) on the SnO2-RGO/GCE in 0.1 M phosphate buffer with various pH between 4 and 9, and the plots of peak current (b) and peak potential (c) versus pH, respectively.
SnO2-RGO/GCE by standard addition method. In this experiment, three samples without any pretreatment were measured by DPV, and the results are summarized in Table 2. Firstly, red wine (10 u L) was injected into 5.0 mL of 0.1 M phosphate buffer (pH 7.0) for a DPV measurement, the current intensity at 0.17 V was recorded, and the concentration was calculated by according to the linear regression equation (section 3.4). Then the certain amounts of CA (0.5–1.5 μM) were spiked into the above samples for DPV measurements, and the corresponding concentration of CA were obtained to evaluate the recovery rate. All DPV measurements were carried out three times to give an average value of CA concentration. The concentration of CA in red wine was determined to be 0.52–1.15 μM. Furthermore, upon addition of certain amounts of CA, the recovery rate of CA was found to be in the range of 93.0–103.9 %. These results proved that the SnO2-RGO/GCE is promising electrochemical sensor for sensitive and selective detection of CA in real red wine.
has excellent selectivity for CA detection. The stability of the SnO2-RGO/GCE was firstly examined by numerous CV measurements in phosphate buffer containing 20 μM of CA (Fig. 8a). It was found that only 6.2 % peak current was reduced even after 100 cycles, suggested the good stability of the SnO2-RGO/GCE electrode. The reproducibility and storage stability of SnO2-RGO/GCE were also investigated using a DPV method in phosphate buffer containing 20 μM of CA. In order to test the reproducibility of SnO2-RGO/ GCE, five continuous measurements were repeated under the same conditions. As shown in Fig. 8b, the DPV curves of the five tests exhibited negligible difference on response current with the relative standard deviation (RSD) of 1.3 %, indicating a superior reproducibility of SnO2-RGO/GCE. The long-term storage stability was further examined by exposing the same SnO2-RGO/GCE electrode in air for one month. As shown in Fig. 8c, the peak current measured every five days on the same SnO2-RGO/GCE electrode remained almost no change, and only 4.6 % current was reduced after one month storage. The above results indicate that SnO2-RGO/GCE showed both good reproducibility and stability, allowing a potential determination of CA in real samples.
4. Conclusions In summary, the SnO2-RGO composite was facilely prepared and used as electrode material to construct electrochemical sensor for CA detection. The electrode material was characterized by TEM and XPS. The electrochemical sensor SnO2-RGO/GCE showed a well-defined
3.6. Determination of CA in red wine samples The determination of CA in the red wine sample was carried out on
Scheme 2. The possible redox mechanism of CA over SnO2-RGO/GCE.
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Fig. 7. DPV curves of CA with various concentrations (0.15–35 μM) on SnO2-RGO/GCE (a) and the corresponding calibration plots between the peak current intensity and CA concentration (b).
redox peak of CA. A linear range was found between current intensity and CA concentration of 0.15–25 μM. The present electrochemical sensor showed an excellent selectivity and a good sensitivity with a detection limit of 80 nM, as well as good stability and reproducibility. Furthermore, the present sensor was successfully applied to detect CA in red wines samples with satisfactory recovery results (93.0–103.9 %) and relative standard deviations (0.93–6.2 %). These demonstrated that SnO2-RGO/GCE is a promising electrochemical sensor for practical determination of CA concentration in red wines.
Table 1 Comparison of analytical performance of various electrochemical sensors for CA detection. Electrochemical sensors
Linear ranges (μM)
Limit of detection (nM)
References
3DG/MWCNTs/GCE LDHf/GCE ERGO/Nafion AuNPs/GRNS Activated GCE RGO@PDA SnO2-RGO/GCE
0.2–174 7–180 0.1–10 0.5–50.0 0.1–1 0.005–450 0.15–25
17.8 2600 90 50 68 1.2 80
[17] [38] [39] [40] [41] [42] This work
Fig. 8. The CV cycles of CA (20 μM) measured over the same SnO2-RGO/GCE in 0.1 M phosphate buffer (pH 7.4) (a). The DPV curves of CA (20 μM) measured on five different SnO2-RGO/GCE electrodes in 0.1 M phosphate buffer (pH 7.0) (b). Long-term storage stability tests of SnO2-RGO/GCE in 0.1 M PBS solution containing 20 μM CA (c). 7
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Table 2 Determination of CA in various red wine samples. Samples
Added (μM)
Found (μM)
Recovery (%)
Red wine 1
0 0.50 1.00 1.50 0 0.50 1.00 1.50 0 0.50 1.00 1.50
0.65 1.07 1.59 2.04 0.52 1.06 1.56 2.01 1.15 1.62 2.10 2.61
/ 93.0 96.4 94.9 / 103.9 102.6 99.5 / 98.2 97.7 98.5
Red wine 2
Red wine 3
± 0.01 ± 0.02 ± 0.05 ± 0.07 ± 0.05 ± 0.05 ± 0.04 ± 0.04 ± 0.03
[12]
[13]
[14]
[15]
[16]
[17]
Novelty statement SnO2 decorated graphene (SnO2-RGO) composite was facilely fabricated by a hydrothermal reduction of graphene oxide with SnCl2. The electrochemical sensor (SnO2-RGO/GCE) obtained from SnO2-RGO coating of glass carbon electrode showed excellent electrocatalytic performance for the redox reaction of caffeic acid (CA) and a detection limit of 80 nM (S/N = 3) were achieved. The present electrochemical sensor was successfully applied for CA detection in commercial red wine samples with satisfactory recovery results.
[18]
[19]
[20] [21]
Declaration of Competing Interest [22]
The authors have declared that no conflicting interests exist. Acknowledgments
[23]
This work was financially supported by Shanghai Municipal Natural Science Foundation (16ZR1401700) and the Open Research Fund of Key Laboratory of Polar Materials and Devices, Ministry of Education.
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