- Email: [email protected]
Novel [email protected] nanohybrid modified glassy carbon electrode for the sensitive determination of melamine
Electrochimica Acta 211 (2016) 689–696
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Novel MOFs@XC-72-Nafion nanohybrid modified glassy carbon electrode for the sensitive determination of melamine Wanqing Zhanga,b , Guangri Xub , Runqiang Liub , Jun Chenb , Xiaobo Lib , Yadong Zhanga,* , Yuping Zhangb,* a b
School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
A R T I C L E I N F O
Article history: Received 12 April 2016 Received in revised form 14 June 2016 Accepted 20 June 2016 Available online 20 June 2016 Keywords: MOFs@XC-72 hybrid electrochemical sensor sensitive melamine liquid milk
A B S T R A C T
A sensitive and selective sensor was fabricated to determine melamine (Mel), based on MIL-53@XC-72 nanohybrid modified glassy carbon electrode (GCE). The MIL-53@XC-72 nanohybrid was synthesized from MIL-53 via hydrothermal method, followed by adulterating with XC-72. The prepared sensor possessed high sensitivity and selectivity due to the synergistic effect of the large surface of MIL-53, the bonding interaction between Mel and MIL-53 as well as the high conductivity of XC-72. Under the optimal conditions, the sensor exhibited rapid electrochemical response to Mel at the potential of 0.7 V in 0.1 M HCl solution. A linear concentration range of Mel was obtained from 0.04 to 10 mM with a linear correlation coefficient of 0.998 and detection limit of 0.005 mM (S/N = 3). In addition, the sensor presented excellent reproducibility, high stability and selectivity, and was successfully applied for the determination of Mel in liquid milk with a good recoveries. Hence, the MOFs@XC-72-Nafion-GCE sensor could be considered as promising applicability for the determination of other electrochemically inactive analytes. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Melamine (Mel) with three amino groups is widely used in the production of melamine formaldehyde for manufacturing resins, coatings, adhesives glues, fertilizer and so on [1,2]. Since Mel contains abundant nitrogen (66% by mass) element, some illegally merchants artificially adulterate it into food products, milk products and other protein sources in order to increase the apparent protein content [1–3]. Though Mel is low-toxic, it can be hydrolyzed to become cyanuric acid in the kidneys under the effection of urine pH. Mel interacts with cyanuric acid to form an insoluble compound, causing urinary bladder tumors in rats and the lethal renal failure in human [1–4]. For instance, thousands of kids have suffered from renal stone even death when they eat milk powder or other milk products adulterated with Mel in September 2008, China. Therefore, it appears that the effective and reliable methods should be further developed to monitor Mel [1–4].
* Corresponding authors. Fax: +86 0371 67781330. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.electacta.2016.06.100 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Many analytical methods such as gas chromatography (GC)/ spectrometry (MS) [5,6], high performance liquid chromatography (HPLC) [7,8], HPLC/MS [9,10], capillary electrophoresis (CE)/MS [11], surface enhanced raman spectroscopy [12,13], nuclear magnetic resonance (NMR) [14], colorimetry [15,16], electrochemilumine- scence [17,18], enzyme-linked immunosorbent assay (ELISA) [19,20], have been applied for the detection of Mel and its analogous contaminants. However, these complex, timeconsuming and expensive methods limit their wide application. In recent years, due to rapidity, economy, convenience and sensitivity, sensors have attracted much more attention on the novel methods for the detection of Mel, such as molecularly imprinted polymer (MIP)-based sensors [21–24], nanoparticle-based sensors [25–30] and electrochemical sensors [31–34]. Metal-organic frameworks (MOFs) are composed of metal clusters and organic ligands. Nano-MIL-53, one of the most typical MOFs, exhibits large surface, appropriate pore volume and accessible cages, enabling it to be the potential modifier to determine Mel. Meanwhile, the poor conductivity of MIL-53 and the low electroactivity of Mel hinder the sensing application of MIL-53 [2,35]. In order to fabricate an ideal sensor, the modified electrode should possess good conductivity and adsorb the Mel
690
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
from aqueous solution on the electrode surface, thus the XC-72 with high conductivity is adulterated with MIL-53 to detect Mel. In this paper, we fabricated a sensitive and selective Mel sensor based on MIL-53@XC-72-Nafion hybrid as the modifier for the first time. The sensor provided a method on how to transform nonelectroactive Mel into electroactive Mel dimer. In addition, compared with other methods for the determination of Mel, the MIL-53@XC-72-Nafion-GCE sensor possessed the advantages of unexpensive reagents, simple operation, moderate linear concentration range and low detection limit. More importantly, the sensor was employed to scan Mel in liquid milk with a satisfactory recovery.
2. Experimental 2.1. Chemicals and Materials Mel was purchased from Tianjin Bodi Chemical Industry Co., Ltd., Tianjin, China. XC-72 carbon was obtained from Cabot Carbon Corporation (USA) and used directly with no further activation. 1,4-benzene dicarboxylic acid (purity 99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Aluminum nitrate nonahydrate (purity 99%) and N,N-dimethylformadine (purity 98%) were obtained from Aladdin Chemistry Co., Ltd. Nafion solution (0.5 wt%) was prepared by diluting 5% Nafion D-520 solution with water and 1-propanol. All other chemicals were at least analytical grade reagents.
2.2. Sythesis of nano-MIL-53 The synthesis of MIL-53 was based on the recipe by J. Zhang et al. [37]. Aluminum nitrate nonahydrate (3.3 mmol) and 1,4-Benzene dicarboxylic acid (5 mmol) were dissolved in N,N-dimethylformadine (33 mL) and stirred for 1 h. The substrate mixture was then transferred to a 50 mL Teflon-lined stainless steel autoclave, and then heated at 493 K for 3 days. The as-synthesized product was filtered and washed with DMF to remove the unreacted BDC ligands. The final product was activated at 333 K under a vacuum drying 18 h.
2.4. Electrode preparation process Prior to each experiment, the GCE was polished with 0.1 and 0.05 mm alumina power, sonicated, and rinsed with deionized water. The GCE was sonicated in 1: 1 nitric acid solution (HNO3: H2O = 1: 1), acetone, and double distilled water, then it was dried in air. 5 mg of the MIL-53@XC-72-Nafion was dispersed in 5 mL H2O under ultrasonication for 30 min, then 10 mL of the suspension was dropped onto the surface of the pretreated GCE. Subsequently, it was dried under infrared lamp. The illustration of the electrode preparation and application process was shown in Scheme 1. For comparison, the bare GCE, MIL-53-Nafion-GCE and XC-72-NafionGCE sensors were also fabricated with similar procedures. Electrochemical impedance spectroscope (EIS), cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were recorded after preconcentrating the analytes for 150 s. All the experiments were performed at room temperature. 3. Results and discussion 3.1. Physical characterization The FTIR spectrum of MIL-53 is shown in Fig. 1a. The absorption peak at 3432 cm1 belongs to OH stretching vibraion peak, which is bridging the alminum atoms in MIL-53. The absorption band at 1682 cm1 is pertained to C¼O vibraion peak, which is resulting from DMF molecule [37,38]. The absorption bands at 1612 and 1512 cm1 are corresponding to COOH asymmetric stretchings, whereas absorption bands at 1412 and 1442 cm1 are assigned to COOH symmetric stretchings [39,40], which can form hydrogen bond with H element of Mel. The results prove that the MIL-53 material contains OH and COOH groups, which are beneficial to form hydrogen bond between MIL-53 and Mel. Power X-ray diffraction is used to verify sample structures, as shown in Fig. 1b. The simulated pattern of the MIL-53 was generated using software from the literature [39]. It can be seen
2.3. Apparatus and instruments FTIR spectrum was recorded on a Lambda 7600. All samples were prepared in 1: 100 sample: KBr pellet. Power X-ray diffraction (XRD) patterns were examined on a DX2700B diffractometer using Cu Ka radiation (l = 1.506 Å). N2 adsorption-desorption isotherms, BET and Langmuir surface area were measured by using a BeiShiDe Instrument-S&T at the temperature of liquid N2 (China). Micropore volumes were calculated using the DFT method. The morphologies were examined using a Quanta 200 scanning electron microscope (FEI, Hillsboro, Oregon, USA). Transmission electron microscope was carried out in an H-7500 electron microscope (Hitachi, Japan). Electrical conductivity was measured by using a four-point-probe conductivity tester (RTS-9, Guangzhou, China) at ambient temperature. All electrochemical experiments were carried out by an electrochemical workstation (Zhengzhou Shiruisi Technology Co., Ltd., China). A three electrode system was used in the measurement composed of glassy carbon electrode or modified GCE as working electrode, the saturated calomel electrode (SCE) as the reference electrode and the platinum wire as the counter electrode.
Scheme 1. The illustration of the electrode preparation and application process.
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
691
Fig. 1. (a) The FTIR spectrum of MIL-53, (b) XRD patterns and (c) N2 adsorption-desorption isotherms of the obtained samples.
that the diffraction pattern of MIL-53 as synthesized is in agreement with the published data of the MIL-53 as simulated, demonstrating the successful preparation of MIL-53 [38–40]. For XC-72 sample, the wide diffraction peak at 2u = 24.6 implies that it is an amorphous carbon with fine crystallinity [41]. For MIL53@XC-72 nanohybrid, the material displays the characterisitic diffraction peaks of both MIL-53 and XC-72, indicating uniform distribution of both the materials. N2 adsorption-desorption of the samples is further investigated. Fig. 1c indicates that all the samples show a reversible, Type-I isotherm with no obvious hysteresis, which are typical characteristic of the microporous materials [42]. The Langmuir surface area, BET surface area, micropore volume and average pore diameter of the materials are summarized in Table 2. The relative large surface and pore volume of MIL-53 can efficiently provide contact region between the MIL-53@XC-72-GCE sensor and Mel. Combined with
the electrochemical properties of Mel in Fig. 3b, this demonstrates MIL-53@XC-72 nanohybrid can be used as an excellent modifier. SEM and TEM images of the samples are shown in Fig. 2, clearly showing that all the samples are consisted of nanoscale particles. Fig. 2(a, d) reveal that the MIL-53 is assembled by the apparent crystalline structure with a size of about 50 nm. The XC-72 appears to be spherical particles in shape with sizes of several tens of nanometers in Fig. 2(b, e) [43]. Fig. 2(c, f) indicate that MIL-53 particles are well-dispersed over the XC-72 by means of preserving their major morphology. Four different sites of each MIL-53, XC-72 and MIL-53@XC-72 samples were chosen as conductivity measurements. As shown in Table 1, MIL-53 was barely conductive due to its poor electrical performance (Resistance 1.38 103 V&1, Conductivity 7.5 103 Scm1). After adulterated with XC-72, the conductivity of the MIL53@XC-72 nanohybrid increased to 1.32 Scm1 (about 3 orders of
Fig. 2. SEM and TEM images of MIL-53 (a, d), XC-72 (b, e) and MIL-53@XC-72 (c, f).
692
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
Fig. 3. (a) Electrochemical impedance spectroscope (EIS) of the four electrode sensors in the presence of 5 mM [Fe(CN)6]3/[Fe(CN)6]4 containing 0.1 M KCl and (b) CVs of the four electrode sensors in the presence of 5.0 106 M Mel + 0.1 M HCl. The scan rate: 0.1 V s1. (i) bare GCE, (ii) MIL-53-Nafion-GCE, (iii) XC-72-Nafion-GCE and (iv) MIL53@XC-72-Nafion-GCE.
Table 1 The conductivity data of the MIL-53, XC-72 and MIL-53@XC-72. Samples
Average resistance (V&1)
Average resistivity (Vcm)
Average conductivity (Scm1)
MIL-53 XC-72 MIL-53/XC-72
1.38 103 6.02 8.12
132.7 0.58 0.76
7.5 103 1.72 1.32
Table 2 The textural parameters of the MIL-53, XC-72 and MIL-53@XC-72. Samples
SLangmuir/m2g1
SBET/m2g1
Micropore volume cm3g1
Average pore diameter nm
MIL-53 XC-72 MIL-53@XC-72
1201 324 658
781 213 654
0.48 0.03 0.32
0.7 14.3 7.6
magnitude higher than that of MIL-53). Consequently, the addition of XC-72 could efficiently facilitate electron transfer rate between electrode surface and Mel. EIS is employed to investigate the conductivity of the electrode sensors and the results are shown in Fig. 3a. The semicircle diameter equals the charge-transfer resistance (Rct), which controls the charge-transfer kinetics of the redox probe at the electrode interface [35]. The Rct can be estimated to be 84 V, 1727 V, 4 V and 15 V at bare electrode, MIL-53-Nafion-GCE, XC-
72-Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors, respectively, revealing the low electron-transfer resistance on the XC-72Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors, which is resulted from the good conductivity of XC-72. 3.2. Voltammetric behavior of Mel The CV method is used to monitor the electrochemical behaviors of Mel at the four sensors (Fig. 3b). It is shown that
Fig. 4. Redox mechanism of Mel at XC-72-Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors.
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
the bare GCE and MIL-53-Nafion-GCE sensors have no obvious redox peaks and the MIL-53-Nafion-GCE sensor possesses the lager background current. However, The anodic peak potential (Epa) and the cathodic peak potential (Epc) of the XC-72-Nafion-GCE sensor located at 0.60 V and 0.53 V, respectively. While the Epa and Epc of the MIL-53@XC-72-Nafion-GCE sensor located at 0.67 V and 0.53 V, respectively, showing that the potential separation between anodic and cathodic (DEp = Epa Epc) peaks had been remarkably increased. In addition, by subtracting the background, the peak current of Mel at MIL-53@XC-72-Nafion-GCE sensor was found almost 3.5 times larger than that achieved at the XC-72-NafionGCE sensor. The results indicate that XC-72 and MIL-53@XC-72 nanohybrid have been successfully modified onto the GCE surface and the MIL-53@XC-72-Nafion-GCE sensor is more applicable for the determination of Mel. Based on the above experimental phenomena, Fig. 4 shows that the reaction mechanism of Mel at XC-72-Nafion-GCE and MIL53@XC-72-Nafion-GCE sensors is probably dependent on the redox of amine-containing compounds [20,31,34]. In the first step, Mel (compound 1) generates its corresponding cation radical via single-electron oxidation of the amine group, which is finally accumulated on the glassy carbon electrode by carbon-nitrogen bond. In the second step, Mel is dimerized from single molecule by forming an azo group, which generates compound 2. Finally the compound 2 is reversibly reduced to compound 3, accompanying with a double-electron transfer [34]. Moreover, the large redox peak of MIL-53@XC-72-Nafion-GCE sensor is attributed to the easy adsorption of Mel (compound 1) resulting from the formation of the coordination compound (compound 4) between Al (from MIL53) and N (from Mel) and the hydrogen bond (compound 5) between O (from MIL-53) and H (from Mel). Combined with the BET results, the higher concentration of Mel on the electrode surface is attributed to the large surface and micropore volume of the MIL-53@XC-72 nanohybrid.
693
3.3. Optimization of the experimental parameters The electrode preparation and the experimental measurements processes in solution can remarkablely affect the electrochemical behavior of Mel. In this regard, the ratio of MIL-53: XC-72, the scan rate and the concentration of HCl were detailedly investigated. The first two factors were performed using CV method, and the last factor was used DPV method. As can be seen from Fig. 5a, the peak currents increased due to the increased conductivity of XC-72 until the ratio of MIL-53 and XC-72 was up to 1: 7. Continuing to increase the ratio, the peak currents decreased due to the surface decrease of MIL-53@XC-72Nafion–GCE sensor, resulting in the sharp decline of the absorbed Mel. Besides, the decrease of MIL-53 content led to the weakness of the coordination bond and hydrogen bond between MIL-53 and Mel. To summarize, the large surface of MIL-53, the bonding interaction between Mel and MIL-53 and the conductivity of XC-72 have a synergistic effect, promoting the adsorption and accumulation of Mel on the sensor surface. Based on the results, the ratio of 1: 7 (MIL-53 and XC-72) was selected for the experiments. The CVs of the MIL-53@XC-72-Nafion–GCE sensor at various scan rates from 10 to 330 mV s1 are shown in Fig. 5b. With the increase of scan rates, the oxidation peak shifted to the positive value and the reduction peak to the negative value. The plot of redox peak current versus the scan rate is shown in Fig. 5c. The redox peak current is proportional to the scan rate with the linear equation of Ipa = 48.991 + 0.556 v (R2 = 0.997) and Ipc = 26.4510.294 v (R2 = 0.997), respectively. This result indicated that the electrode reaction was a surface-confined electrochemical process, which was in good agreement with the previous reports [34]. Fig. 5d shows the electrochemical behavior of Mel in HCl solution with different concentrations ranging from 0.02-0.7 M. The current peak of Mel increased with the increase of HCl concentration from 0.02 to 0.1 M, which is probably because it becomes easy to generate compound 2 and 3 in the
Fig. 5. (a) Effect of the ratio of MIL-53 and XC-72. (b) Effect of the scan rate, and (c) The plot of peak current versus scan rate. (d) Effect of the concentration of HCl.
694
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
electrochemical reaction process. However, When the HCl concentration was higher than 0.1 M, The redox peak dropped sharply probably because the protonization of N element in Mel compound caused rapid reduction of absorbed Mel on the electrode surface. To achieve high sensitivity and selectivity of the sensors, the 0.1 M was chosen for the experiments.
relationship can be observed between the peak current and the Mel concentration ranging from 0.04 to 10 mM (in the inset image). The linear regression equation was ip (mA) = 1.551 + 3.197C (mM) (R2 = 0.998). The detection limit was obtained to be 0.005 mM at the signal to noise ratio of 3 (S/N = 3). This illustrates that the MIL53@XC-72-Nafion-GCE sensor possesses wide linear concentration range and low detection limit.
3.4. Determination of Mel at MIL-53@XC-72-Nafion–GCE 3.5. Reproducibility, stability and selectivity of the sensor Under the optimization of experimental conditions, Fig. 6(a) shows the DPV peaks measured in a series of different concentrations of Mel at MIL-53@XC-72- Nafion-GCE sensor. A good linear
The reproducibility test of the MIL-53@XC-72-Nafion-GCE sensor was examined in 0.1 M HCl solution containing 5.0 106 M Mel. The relative standard deviation (RSD) for eight successive determinations was 3.1% and five independent measurements was 5.6%. The long-time stability of the optimal sensor was tested in two week period, showing that the MIL-53@XC-72Nafion-GCE sensor can retain its properties with no obvious decline (RSD<4%). The selectivity of the MIL-53@XC-72-NafionGCE sensor was studied in the solution including 5.0 106 M Mel and some metal ions, food additives and the analogues of Mel in the presence of liquid milk. It was confirmed in Fig. 6(b) and (c) that the current response of Mel is significantly higher than that of 500-fold concentration of Cu2+, Fe3+, Ca2+, Mg2+, PO43, SO42, NO3 and 100-fold concentration of benzoguanamine, dicyandiamide, glucose, fructose, ascorbic acid, and uric acid. The satisfacory selectivity of the developed sensor is attributed to the synergistic effect of the large surface of MIL-53, the bonding interaction between Mel and MIL-53 and the high conductivity of XC-72. The results indicate the MIL-53@XC-72-Nafion-GCE sensor has a good reproducibility, stability and selectivity. 3.6. Determination of Mel in milk samples In order to assess the potential practical application of the sensor, it was applied for the analysis of Mel in liquid milk, bought from local supermarket. The liquid milk was mixed with trichloroacetric acid, followed by sharking, centrifuging and filtering, and the final samples were used to be detected. Because there appeared no obvious peak current of Mel in real sample, the standard addition method was selected to calculate the content of Mel in liquid milk. A certain amount of Mel standard solution was added to the liquid milk samples directly. The recoveries of 98.0%-103.5% with a relative standard deviation (RSD) of 3.1%-4.8% and the 100.8% average recovery were obtained according to five measurements, listing in Table 3. The results suggest the optimized method has potential application for the determination of Mel in liquid milk. The results were in comparison with other several techniques, as shown in Table 4. The MIL-53@XC-72-Nafion-GCE sensor presented better electrochemical performance than the HPLC [8], HPLC/MS [10], GC/MS [5], MIP/CL [21] and UV [36]. In addition, the method has cheap regents and simple operation. In contrast to other electrochemical sensor method, such as DOPAC [32], OMCNafion/GCE [34], Oligonucleotides/Au/GCE [25], MIP/potentiometric/GCE [22] and MIP/GCE [24], the MIL-53@XC-72-Nafion-GCE sensor exhibited moderate linear concentration range and low detection limit. 4. Conclusions
Fig. 6. (a) DPV of the MIL-53@XC-72-Nafion-GCE sensor under 0.04, 1.2, 1.9, 3.0, 5.0, 7.9, 10 mM Mel (from a to g) in 0.1 M HCl solution. Inset: the linear relationship between peak current and the Mel concentration. The scan rate: 0.1 V s1. (b) and (c) Selectivity of the MIL-53@XC-72-Nafion-GCE sensor.
In this paper, a new approach for fabrication of the MIL-53@XC72-Nafion-GCE sensor was presented for the first time to determine Mel. The procedure of fabricating sensor was simple, convenient and rapid. The developed sensor exhibited a wide linear concentration range and low detection limit with good reproducibility, stability and high selectivity. Furthermore, the
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
695
Table 3 Recoveries of the determination of Mel in the liquid milk samples. Samples
Added (mM)
Found (mM)
Recovery (%)
RSD (%) (n = 5)
Average recovery (%)
1 2 3 4 5
0.49 1.30 2.0 5.6 9.2
0.48 1.32 2.07 5.58 9.33
98.0 101.5 103.5 99.6 101.4
3.1 4.0 4.3 4.6 4.8
100.8
Table 4 Comparison of the optimized sensors for the detection of Mel with other methods. Methods HPLC HPLC/MS GC/MS MIP/CL UV MIP/potentiometric MIP/GCE * Elec. (DOPAC) OMC-Nafion/GCE Oligonucleotides/Au MIL-53@XC-72 Nafion/GCE *
Linear range 8
6
7.9 10 -7.9 10 M 1.6 108-4.0 106 M 4.0 107-1.6 105 M 7.9 107-4.0 104 M 1.0 105-5.0 105 M 5.0 106-1.0 102 M 4.5 107-4.0 106 M 1.0 108-5.0 106 M 5.0 108-7.0 106 M 3.9 108-3.3 106 M 4.0 108-1.0 105 M
*
Detec.Lim. 9
1.6 10 M 4.4 108 M 7.9 108 M 1.6 107 M 1.6 107 M 1.6 106M 0.36 106M 3.0 109 M 2.4 109 M 9.6 109 M 5.0 109 M
Recovery/%
RSD/%
Ref.
83.6-91.3 66.2-95.5 93.9-102 102.3-104.0 97 95.0-110.0 95.6-102.5 96 94.0-100.6 95.0 98.0-103.5
– 1.6-16.3 3.1-8.7 – 3.0 – 4.2 5.2-6.5 2.5 1.9 4.2
8 10 5 21 36 22 24 32 34 25 This work
Elec represents Electrochemistry and *Detec.Lim represents Detection limit.
proposed method was applied successfully to the determination of Mel in liquid milk with an excellent recovery. The satisfactory results indicated that the MOFs@XC-72 would be potentially applied in modifiers for the determination of other analytes without electrochemical activities. Acknowledgements This work is financially supported by the Iconic Innovation Project Plan of Henan Institute of Science and Technology (No. 2015BZ02) and the Henan Innovation Foundation for Higher Education (No. 2012HASTIT037). We also thank Dr. Yuanchao Li and Shanqin Liu for their valuable discussions in aspects of electrochemical analysis. References [1] F.X. Sun, W. Ma, L.G. Xu, Y.Y. Zhu, L.Q. Liu, C.F. Peng, L.B. Wang, H. Kuang, C.L. Xu, Analytical methods and recent developments in the detection of melamine, Trend. Anal. Chem. 29 (2010) 1239–1249. [2] K. Rovina, S. Siddiquee, A review of recent advances in melamine detection techniques, J. Food. Compos. Anal. 43 (2015) 25–38. [3] Y. Li, J.Y. Xu, C.Y. Sun, Chemical sensors and biosensors for the detection of melamine, RSC. Adv. 5 (2015) 1125–1147. [4] Y. Liu, E.E.D. Todd, Q. Zhang, J.R. Shi, X.J. Liu, Recent developments in the detection of melamine, J. Zhejiang. Univ. Sci. B. (Biomed & Biotechnol) 13 (2012) 525–532. [5] X.M. Xu, Y.P. Ren, Y. Zhu, Z.X. Cai, J.L. Han, B.F. Huang, Y. Zhu, Direct determination of melamine in dairy products by gas chromatography/mass spectrometry with coupled column separation, Anal. Chim. Acta. 650 (2009) 39–43. [6] R.A. Yokley, L.C. Mayer, R. Rezaaiyan, M.E. Manuli, M.W. Cheung, Analytical method for the determination of cyromazine and melamine residues in soil using LC-UV and GC-MSD, J. Agr. Food. Chem. 48 (2000) 3352–3358. [7] S. Ehling, S. Tefera, I.P. Ho, High-performance liquid chromatographic method for the simultaneous detection of the adulteration of cereal flours with melamine and related triazine by-products ammeline ammelide, and cyanuric acid, Food. Addit. Contam. 24 (2007) 1319–1325. [8] R.C. Wei, R. Wang, Q.F. Zeng, M. Chen, T.Z. Liu, High-performance liquid chromatographic method for the determination of cyromazine and melamine residues in milk and pork, J. Chromatogr. Sci. 47 (2009) 581–584. [9] M. Ibáñez, J.V. Sancho, F. Hernández, Determination of melamine in milkbased products and other food and beverage products by ion-pair liquid chromatography-tandem mass spectrometry, Analytica. Chimica. Acta 649 (2009) 91–97.
[10] Y.T. Wua, C.M. Huang, C.C. Lin, W.A. Ho, L.C. Lin, T.F. Chiu, D.C. Tarng, C.H. Lin, T. Hu Tsai, Determination of melamine in rat plasma, liver, kidney, spleen, bladder and brain by liquid chromatography–tandem mass spectrometry, J. Chromatogr. A 1216 (2009) 7595–7601. [11] H.Y. Huang, C.L. Lin, S.H. Jiang, B. Singco, Y.J. Cheng, Capillary electrochromatography-mass spectrometry determination of melamine and related triazine by-products using poly (divinylbenzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic stationary phases, Analytica. Chimica. Acta 719 (2012) 96–103. [12] A. Kim, S.J. Barcelo, R.S. Williams, Z.Y. Li, Melamine sensing in milk products by using surface enhanced raman scattering, Anal. Chem. 84 (2012) 9303–9309. [13] P.Y. Ma, F.H. Liang, Y. Sun, Y. Jin, Y. Chen, X.H. Wang, H.Q. Zhang, D.J. Gao, D.Q. Song, Rapid determination of melamine in milk and milk powder by surfaceenhanced raman spectroscopy and using cyclodextrin-decorated silver nanoparticles, Microchim. Acta 180 (2013) 1173–1180. [14] D.W. Lachenmeier, E. Humpfer, F. Fang, B. Schutz, P. Dvortsak, C. Sproll, M. Spraul, NMR-spectroscopy for nontargeted screening and simultaneous quantification of health-relevant compounds in foods: the example of melamine, J. Agric. Food Chem. 57 (2009) 7194–7199. [15] Q. Cao, H. Zhao, Y.J. He, X.J. Li, L.X. Zeng, N. Ding, J. Wang, J. Yang, G.W. Wang, Hydrogen-bonding-induced colorimetric detection of melamine by nonaggregation-based Au-NPs as a probe, Biosens. Bioelectron. 25 (2010) 2680–2685. [16] Z.J. Wu, H. Zhao, Y. Xue, Q. Cao, J. Yang, Y.J. He, X.J. Li, Z.B. Yuan, Colorimetric detection of melamine during the formation of gold nanoparticles, Biosens. Bioelectron. 26 (2011) 2574–2578. [17] X.M. Chen, S. Lian, Y. Ma, A.H. Peng, X.T. Tian, Z.Y. Huang, X. Chen, Electrochemiluminescence sensor for melamine based on a Ru (bpy)32+-doped silica nanoparticles/carboxylic acid functionalized multi-walled carbon nanotubes/Nafion hybrid film modified electrode, Talanta 146 (2016) 844–850. [18] Y. Fan, X.R. Tan, X.F. Liu, X. Ou, S.H. Chen, S.P. Wei, H.M. Cao, X.Q. Hu, C.Y. Hu, Y. Zhang, N.Q. Jia, A novel solid-state electrochemiluminescence sensor for melamine with Ru (bpy)32+/mesoporous silica nanospheres/Nafion hybrid modified electrode, Biosens. Bioelectron. 41 (2013) 911–915. [19] W.W. Yin, J.T. Liu, T.C. Zhang, W.H. Li, W. Liu, M. Meng, F.Y. He, Y.P. Wan, C.W. Feng, S.L. Wang, X. Lu, R. Xi, Preparation of monoclonal antibody for melamine and development of an indirect competitive ELISA for melamine detection in raw milk milk powder, and animal feeds, J. Agric. Food Chem. 58 (2010) 8152– 8157. [20] Q. Xu, H.P. Wei, S. Du, H.B. Li, Z.P. Ji, X.Y. Hu, Detection of subnanomolar melamine based on electrochemical accumulation coupled with enzyme colorimetric assay, J. Agric. Food Chem. 61 (2013) 1810–1817. [21] J.H. Yu, C.C. Zhang, P. Dai, S.G. Ge, Highly selective molecular recognition and high throughput detection of melamine based on molecularly imprinted solgel film, Analytica. Chimica. Acta 651 (2009) 209–214. [22] R.N. Liang, R.M. Zhang, W. Qin, Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk, Sensor. Actuat. B 141 (2009) 544–550. [23] A. Pietrzyk, W. Kutner, R. Chitta, M.E. Zandler, F. D’souza, F. Sannicolò, P.R. Mussini, Melamine acoustic chemosensor based on molecularly imprinted polymer Film, Anal. Chem. 81 (2009) 10061–10070.
696
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
[24] Y.T. Liu, J. Deng, X.L. Xiao, L. Ding, Y.L. Yuan, H. Li, X.T. Li, X.N. Yan, L.L. Wang, Electrochemical sensor based on a poly (para-aminobenzoic acid) film modified glassy carbon electrode for the determination of melamine in milk, Electrochim. Acta 56 (2011) 4595–4602. [25] Q. Cao, H. Zhao, L.X. Zeng, J. Wang, R. Wang, X.H. Qiu, Y.J. He, Electrochemical determination of melamine using oligonucleotides modified gold electrodes, Talanta 80 (2009) 484–488. [26] H. Kuang, W. Chen, W.J. Yan, L.G. Xu, Y.Y. Zhu, L.Q. Liu, H.Q. Chu, C.F. Peng, L.B. Wang, N.A. Kotov, C.L. Xu, Crown ether assembly of gold nanoparticles: Melamine sensor, Biosens. Bioelectron. 26 (2011) 2032–2037. [27] N. Vasimalai, S.A. John, Picomolar melamine enhanced the fluorescence of gold nanoparticles: Spectrofluorimetric determination of melamine in milk and infant formulas using functionalized triazole capped gold nanoparticles, Biosens. Bioelectron. 42 (2013) 267–272. [28] M.K. Dey, A.K. Satpati, A.V.R. Reddy, Electrochemical determination of melamine on static mercury drop electrode and on gold nano particle modified carbon paste electrode, Am. J. Anal. Chem. 5 (2014) 598–603. [29] Rosy, R.N. Goyal, Gold nanoparticles decorated poly-melamine modified glassy carbon sensor for the voltammetricestimation of domperidone in pharmaceuticals and biological fluids, Talanta 141 (2015) 53–59. [30] J.J. Du, Z.K. Wang, X.J. Peng, J.L. Fan, In situ colorimetric recognition of melamine based on thymine derivative-functionalized gold nanoparticle, Ind. Eng. Chem. Res. 54 (2015) 12011–12016. [31] R.S. Deinhammer, M. Ho, J.W. Anderegg, M.D. Porter, Electrochemical oxidation of amine-containing compounds: a route to the surface modification of glassy carbon electrodes, Langmuir 10 (1994) 1306–1313. [32] Q. Cao, H. Zhao, Y.J. He, N. Ding, J. Wang, Electrochemical sensing of melamine with 3,4-dihydroxyphenylacetic acid as recognition element, Analytica. Chimica. Acta 675 (2010) 24–28. [33] T.K. Zhao, L.H. Liu, G.M. Li, A.L. Dang, T.H. Li, Electrochemical determination of melamine with a glassy carbon electrode coated with a multi-wall carbon nanotube/chitosan hybrid, J. Electrochem. Soc. 159 (2012) 141–145.
[34] Z. Guo, X.F. Xu, J. Li, Y.W. Liu, J. Zhang, C. Yang, Ordered mesoporous carbon as electrode modification material for selective and sensitive electrochemical sensing of melamine, Sensor. Actuat. B 200 (2014) 101–108. [35] J. Bai, B.P. Lu, X.J. Bo, L.P. Guo, Electrochemical property and electroanalytical application of large mesoporous carbons, Electrochem. Commun. 12 (2010) 1563–1567. [36] J. Rima, M. Abourida, T. Xu, K. Cho, S. Kyriacos, New spectrophotometric method for the quantitative determination of melamine using Mannich reaction, J. Food. Compos. Anal. 22 (2009) 689–693. [37] J. Zhang, S.S. Liu, L.F. Song, C.H. Jiang, C.L. Jiao, S. Wang, Y. Zhang, Hydrogen storage in nano metal-organic framework materials, Materials China 28 (2009) 28–34. [38] W.P. Mounfield III, K.S. Walton, Effect of synthesis solvent on the breathing behavior of MIL-53 (Al), J. Colloid. Interf. Sci. 447 (2015) 33–39. [39] T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille, G. Férey, A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration, Chem. Eur. J. 10 (2004) 1373–1382. [40] G.L. Zi, Z.Y. Yan, Y.X. Wang, Y.J. Chen, Y.L. Guo, J.Q. Wang, Catalytic hydrothermal conversion of carboxymethyl cellulose to value-added chemicals over metalorganic framework MIL-53(Al), Carbohyd. Polym. 115 (2015) 146–151. [41] W.Z. Li, C.H. Liang, W.J. Zhou, J.S. Qiu, Z.H. Zhou, G.Q. Sun, Q. Xin, Preparation and characterization of multiwalled carbon nanotube-supported platinum for cathode catalysts of direct methanol fuel cells, J. Phys. Chem. B 107 (2003) 6292–6299. [42] X.K. Qian, B.L. Yadian, R.B. Wu, Y. Long, K. Zhou, B. Zhu, Y.Z. Huang, Structure stability of metal-organic framework MIL-53(Al) in aqueous solutions, Int. J. Hydrogen. Energ. 38 (2013) 16710–16715. [43] X.M. Ren, D.D. Shao, S.T. Yang, J. Hu, G.D. Sheng, X.L. Tan, X.K. Wang, Comparative study of Pb(II) sorption on XC-72 carbon and multi-walled carbon nanotubes from aqueous solutions, Chem. Eng. J. 170 (2011) 170–177.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Novel MOFs@XC-72-Nafion nanohybrid modified glassy carbon electrode for the sensitive determination of melamine Wanqing Zhanga,b , Guangri Xub , Runqiang Liub , Jun Chenb , Xiaobo Lib , Yadong Zhanga,* , Yuping Zhangb,* a b
School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
A R T I C L E I N F O
Article history: Received 12 April 2016 Received in revised form 14 June 2016 Accepted 20 June 2016 Available online 20 June 2016 Keywords: MOFs@XC-72 hybrid electrochemical sensor sensitive melamine liquid milk
A B S T R A C T
A sensitive and selective sensor was fabricated to determine melamine (Mel), based on MIL-53@XC-72 nanohybrid modified glassy carbon electrode (GCE). The MIL-53@XC-72 nanohybrid was synthesized from MIL-53 via hydrothermal method, followed by adulterating with XC-72. The prepared sensor possessed high sensitivity and selectivity due to the synergistic effect of the large surface of MIL-53, the bonding interaction between Mel and MIL-53 as well as the high conductivity of XC-72. Under the optimal conditions, the sensor exhibited rapid electrochemical response to Mel at the potential of 0.7 V in 0.1 M HCl solution. A linear concentration range of Mel was obtained from 0.04 to 10 mM with a linear correlation coefficient of 0.998 and detection limit of 0.005 mM (S/N = 3). In addition, the sensor presented excellent reproducibility, high stability and selectivity, and was successfully applied for the determination of Mel in liquid milk with a good recoveries. Hence, the MOFs@XC-72-Nafion-GCE sensor could be considered as promising applicability for the determination of other electrochemically inactive analytes. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Melamine (Mel) with three amino groups is widely used in the production of melamine formaldehyde for manufacturing resins, coatings, adhesives glues, fertilizer and so on [1,2]. Since Mel contains abundant nitrogen (66% by mass) element, some illegally merchants artificially adulterate it into food products, milk products and other protein sources in order to increase the apparent protein content [1–3]. Though Mel is low-toxic, it can be hydrolyzed to become cyanuric acid in the kidneys under the effection of urine pH. Mel interacts with cyanuric acid to form an insoluble compound, causing urinary bladder tumors in rats and the lethal renal failure in human [1–4]. For instance, thousands of kids have suffered from renal stone even death when they eat milk powder or other milk products adulterated with Mel in September 2008, China. Therefore, it appears that the effective and reliable methods should be further developed to monitor Mel [1–4].
* Corresponding authors. Fax: +86 0371 67781330. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.electacta.2016.06.100 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Many analytical methods such as gas chromatography (GC)/ spectrometry (MS) [5,6], high performance liquid chromatography (HPLC) [7,8], HPLC/MS [9,10], capillary electrophoresis (CE)/MS [11], surface enhanced raman spectroscopy [12,13], nuclear magnetic resonance (NMR) [14], colorimetry [15,16], electrochemilumine- scence [17,18], enzyme-linked immunosorbent assay (ELISA) [19,20], have been applied for the detection of Mel and its analogous contaminants. However, these complex, timeconsuming and expensive methods limit their wide application. In recent years, due to rapidity, economy, convenience and sensitivity, sensors have attracted much more attention on the novel methods for the detection of Mel, such as molecularly imprinted polymer (MIP)-based sensors [21–24], nanoparticle-based sensors [25–30] and electrochemical sensors [31–34]. Metal-organic frameworks (MOFs) are composed of metal clusters and organic ligands. Nano-MIL-53, one of the most typical MOFs, exhibits large surface, appropriate pore volume and accessible cages, enabling it to be the potential modifier to determine Mel. Meanwhile, the poor conductivity of MIL-53 and the low electroactivity of Mel hinder the sensing application of MIL-53 [2,35]. In order to fabricate an ideal sensor, the modified electrode should possess good conductivity and adsorb the Mel
690
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
from aqueous solution on the electrode surface, thus the XC-72 with high conductivity is adulterated with MIL-53 to detect Mel. In this paper, we fabricated a sensitive and selective Mel sensor based on MIL-53@XC-72-Nafion hybrid as the modifier for the first time. The sensor provided a method on how to transform nonelectroactive Mel into electroactive Mel dimer. In addition, compared with other methods for the determination of Mel, the MIL-53@XC-72-Nafion-GCE sensor possessed the advantages of unexpensive reagents, simple operation, moderate linear concentration range and low detection limit. More importantly, the sensor was employed to scan Mel in liquid milk with a satisfactory recovery.
2. Experimental 2.1. Chemicals and Materials Mel was purchased from Tianjin Bodi Chemical Industry Co., Ltd., Tianjin, China. XC-72 carbon was obtained from Cabot Carbon Corporation (USA) and used directly with no further activation. 1,4-benzene dicarboxylic acid (purity 99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Aluminum nitrate nonahydrate (purity 99%) and N,N-dimethylformadine (purity 98%) were obtained from Aladdin Chemistry Co., Ltd. Nafion solution (0.5 wt%) was prepared by diluting 5% Nafion D-520 solution with water and 1-propanol. All other chemicals were at least analytical grade reagents.
2.2. Sythesis of nano-MIL-53 The synthesis of MIL-53 was based on the recipe by J. Zhang et al. [37]. Aluminum nitrate nonahydrate (3.3 mmol) and 1,4-Benzene dicarboxylic acid (5 mmol) were dissolved in N,N-dimethylformadine (33 mL) and stirred for 1 h. The substrate mixture was then transferred to a 50 mL Teflon-lined stainless steel autoclave, and then heated at 493 K for 3 days. The as-synthesized product was filtered and washed with DMF to remove the unreacted BDC ligands. The final product was activated at 333 K under a vacuum drying 18 h.
2.4. Electrode preparation process Prior to each experiment, the GCE was polished with 0.1 and 0.05 mm alumina power, sonicated, and rinsed with deionized water. The GCE was sonicated in 1: 1 nitric acid solution (HNO3: H2O = 1: 1), acetone, and double distilled water, then it was dried in air. 5 mg of the MIL-53@XC-72-Nafion was dispersed in 5 mL H2O under ultrasonication for 30 min, then 10 mL of the suspension was dropped onto the surface of the pretreated GCE. Subsequently, it was dried under infrared lamp. The illustration of the electrode preparation and application process was shown in Scheme 1. For comparison, the bare GCE, MIL-53-Nafion-GCE and XC-72-NafionGCE sensors were also fabricated with similar procedures. Electrochemical impedance spectroscope (EIS), cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were recorded after preconcentrating the analytes for 150 s. All the experiments were performed at room temperature. 3. Results and discussion 3.1. Physical characterization The FTIR spectrum of MIL-53 is shown in Fig. 1a. The absorption peak at 3432 cm1 belongs to OH stretching vibraion peak, which is bridging the alminum atoms in MIL-53. The absorption band at 1682 cm1 is pertained to C¼O vibraion peak, which is resulting from DMF molecule [37,38]. The absorption bands at 1612 and 1512 cm1 are corresponding to COOH asymmetric stretchings, whereas absorption bands at 1412 and 1442 cm1 are assigned to COOH symmetric stretchings [39,40], which can form hydrogen bond with H element of Mel. The results prove that the MIL-53 material contains OH and COOH groups, which are beneficial to form hydrogen bond between MIL-53 and Mel. Power X-ray diffraction is used to verify sample structures, as shown in Fig. 1b. The simulated pattern of the MIL-53 was generated using software from the literature [39]. It can be seen
2.3. Apparatus and instruments FTIR spectrum was recorded on a Lambda 7600. All samples were prepared in 1: 100 sample: KBr pellet. Power X-ray diffraction (XRD) patterns were examined on a DX2700B diffractometer using Cu Ka radiation (l = 1.506 Å). N2 adsorption-desorption isotherms, BET and Langmuir surface area were measured by using a BeiShiDe Instrument-S&T at the temperature of liquid N2 (China). Micropore volumes were calculated using the DFT method. The morphologies were examined using a Quanta 200 scanning electron microscope (FEI, Hillsboro, Oregon, USA). Transmission electron microscope was carried out in an H-7500 electron microscope (Hitachi, Japan). Electrical conductivity was measured by using a four-point-probe conductivity tester (RTS-9, Guangzhou, China) at ambient temperature. All electrochemical experiments were carried out by an electrochemical workstation (Zhengzhou Shiruisi Technology Co., Ltd., China). A three electrode system was used in the measurement composed of glassy carbon electrode or modified GCE as working electrode, the saturated calomel electrode (SCE) as the reference electrode and the platinum wire as the counter electrode.
Scheme 1. The illustration of the electrode preparation and application process.
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
691
Fig. 1. (a) The FTIR spectrum of MIL-53, (b) XRD patterns and (c) N2 adsorption-desorption isotherms of the obtained samples.
that the diffraction pattern of MIL-53 as synthesized is in agreement with the published data of the MIL-53 as simulated, demonstrating the successful preparation of MIL-53 [38–40]. For XC-72 sample, the wide diffraction peak at 2u = 24.6 implies that it is an amorphous carbon with fine crystallinity [41]. For MIL53@XC-72 nanohybrid, the material displays the characterisitic diffraction peaks of both MIL-53 and XC-72, indicating uniform distribution of both the materials. N2 adsorption-desorption of the samples is further investigated. Fig. 1c indicates that all the samples show a reversible, Type-I isotherm with no obvious hysteresis, which are typical characteristic of the microporous materials [42]. The Langmuir surface area, BET surface area, micropore volume and average pore diameter of the materials are summarized in Table 2. The relative large surface and pore volume of MIL-53 can efficiently provide contact region between the MIL-53@XC-72-GCE sensor and Mel. Combined with
the electrochemical properties of Mel in Fig. 3b, this demonstrates MIL-53@XC-72 nanohybrid can be used as an excellent modifier. SEM and TEM images of the samples are shown in Fig. 2, clearly showing that all the samples are consisted of nanoscale particles. Fig. 2(a, d) reveal that the MIL-53 is assembled by the apparent crystalline structure with a size of about 50 nm. The XC-72 appears to be spherical particles in shape with sizes of several tens of nanometers in Fig. 2(b, e) [43]. Fig. 2(c, f) indicate that MIL-53 particles are well-dispersed over the XC-72 by means of preserving their major morphology. Four different sites of each MIL-53, XC-72 and MIL-53@XC-72 samples were chosen as conductivity measurements. As shown in Table 1, MIL-53 was barely conductive due to its poor electrical performance (Resistance 1.38 103 V&1, Conductivity 7.5 103 Scm1). After adulterated with XC-72, the conductivity of the MIL53@XC-72 nanohybrid increased to 1.32 Scm1 (about 3 orders of
Fig. 2. SEM and TEM images of MIL-53 (a, d), XC-72 (b, e) and MIL-53@XC-72 (c, f).
692
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
Fig. 3. (a) Electrochemical impedance spectroscope (EIS) of the four electrode sensors in the presence of 5 mM [Fe(CN)6]3/[Fe(CN)6]4 containing 0.1 M KCl and (b) CVs of the four electrode sensors in the presence of 5.0 106 M Mel + 0.1 M HCl. The scan rate: 0.1 V s1. (i) bare GCE, (ii) MIL-53-Nafion-GCE, (iii) XC-72-Nafion-GCE and (iv) MIL53@XC-72-Nafion-GCE.
Table 1 The conductivity data of the MIL-53, XC-72 and MIL-53@XC-72. Samples
Average resistance (V&1)
Average resistivity (Vcm)
Average conductivity (Scm1)
MIL-53 XC-72 MIL-53/XC-72
1.38 103 6.02 8.12
132.7 0.58 0.76
7.5 103 1.72 1.32
Table 2 The textural parameters of the MIL-53, XC-72 and MIL-53@XC-72. Samples
SLangmuir/m2g1
SBET/m2g1
Micropore volume cm3g1
Average pore diameter nm
MIL-53 XC-72 MIL-53@XC-72
1201 324 658
781 213 654
0.48 0.03 0.32
0.7 14.3 7.6
magnitude higher than that of MIL-53). Consequently, the addition of XC-72 could efficiently facilitate electron transfer rate between electrode surface and Mel. EIS is employed to investigate the conductivity of the electrode sensors and the results are shown in Fig. 3a. The semicircle diameter equals the charge-transfer resistance (Rct), which controls the charge-transfer kinetics of the redox probe at the electrode interface [35]. The Rct can be estimated to be 84 V, 1727 V, 4 V and 15 V at bare electrode, MIL-53-Nafion-GCE, XC-
72-Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors, respectively, revealing the low electron-transfer resistance on the XC-72Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors, which is resulted from the good conductivity of XC-72. 3.2. Voltammetric behavior of Mel The CV method is used to monitor the electrochemical behaviors of Mel at the four sensors (Fig. 3b). It is shown that
Fig. 4. Redox mechanism of Mel at XC-72-Nafion-GCE and MIL-53@XC-72-Nafion-GCE sensors.
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
the bare GCE and MIL-53-Nafion-GCE sensors have no obvious redox peaks and the MIL-53-Nafion-GCE sensor possesses the lager background current. However, The anodic peak potential (Epa) and the cathodic peak potential (Epc) of the XC-72-Nafion-GCE sensor located at 0.60 V and 0.53 V, respectively. While the Epa and Epc of the MIL-53@XC-72-Nafion-GCE sensor located at 0.67 V and 0.53 V, respectively, showing that the potential separation between anodic and cathodic (DEp = Epa Epc) peaks had been remarkably increased. In addition, by subtracting the background, the peak current of Mel at MIL-53@XC-72-Nafion-GCE sensor was found almost 3.5 times larger than that achieved at the XC-72-NafionGCE sensor. The results indicate that XC-72 and MIL-53@XC-72 nanohybrid have been successfully modified onto the GCE surface and the MIL-53@XC-72-Nafion-GCE sensor is more applicable for the determination of Mel. Based on the above experimental phenomena, Fig. 4 shows that the reaction mechanism of Mel at XC-72-Nafion-GCE and MIL53@XC-72-Nafion-GCE sensors is probably dependent on the redox of amine-containing compounds [20,31,34]. In the first step, Mel (compound 1) generates its corresponding cation radical via single-electron oxidation of the amine group, which is finally accumulated on the glassy carbon electrode by carbon-nitrogen bond. In the second step, Mel is dimerized from single molecule by forming an azo group, which generates compound 2. Finally the compound 2 is reversibly reduced to compound 3, accompanying with a double-electron transfer [34]. Moreover, the large redox peak of MIL-53@XC-72-Nafion-GCE sensor is attributed to the easy adsorption of Mel (compound 1) resulting from the formation of the coordination compound (compound 4) between Al (from MIL53) and N (from Mel) and the hydrogen bond (compound 5) between O (from MIL-53) and H (from Mel). Combined with the BET results, the higher concentration of Mel on the electrode surface is attributed to the large surface and micropore volume of the MIL-53@XC-72 nanohybrid.
693
3.3. Optimization of the experimental parameters The electrode preparation and the experimental measurements processes in solution can remarkablely affect the electrochemical behavior of Mel. In this regard, the ratio of MIL-53: XC-72, the scan rate and the concentration of HCl were detailedly investigated. The first two factors were performed using CV method, and the last factor was used DPV method. As can be seen from Fig. 5a, the peak currents increased due to the increased conductivity of XC-72 until the ratio of MIL-53 and XC-72 was up to 1: 7. Continuing to increase the ratio, the peak currents decreased due to the surface decrease of MIL-53@XC-72Nafion–GCE sensor, resulting in the sharp decline of the absorbed Mel. Besides, the decrease of MIL-53 content led to the weakness of the coordination bond and hydrogen bond between MIL-53 and Mel. To summarize, the large surface of MIL-53, the bonding interaction between Mel and MIL-53 and the conductivity of XC-72 have a synergistic effect, promoting the adsorption and accumulation of Mel on the sensor surface. Based on the results, the ratio of 1: 7 (MIL-53 and XC-72) was selected for the experiments. The CVs of the MIL-53@XC-72-Nafion–GCE sensor at various scan rates from 10 to 330 mV s1 are shown in Fig. 5b. With the increase of scan rates, the oxidation peak shifted to the positive value and the reduction peak to the negative value. The plot of redox peak current versus the scan rate is shown in Fig. 5c. The redox peak current is proportional to the scan rate with the linear equation of Ipa = 48.991 + 0.556 v (R2 = 0.997) and Ipc = 26.4510.294 v (R2 = 0.997), respectively. This result indicated that the electrode reaction was a surface-confined electrochemical process, which was in good agreement with the previous reports [34]. Fig. 5d shows the electrochemical behavior of Mel in HCl solution with different concentrations ranging from 0.02-0.7 M. The current peak of Mel increased with the increase of HCl concentration from 0.02 to 0.1 M, which is probably because it becomes easy to generate compound 2 and 3 in the
Fig. 5. (a) Effect of the ratio of MIL-53 and XC-72. (b) Effect of the scan rate, and (c) The plot of peak current versus scan rate. (d) Effect of the concentration of HCl.
694
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
electrochemical reaction process. However, When the HCl concentration was higher than 0.1 M, The redox peak dropped sharply probably because the protonization of N element in Mel compound caused rapid reduction of absorbed Mel on the electrode surface. To achieve high sensitivity and selectivity of the sensors, the 0.1 M was chosen for the experiments.
relationship can be observed between the peak current and the Mel concentration ranging from 0.04 to 10 mM (in the inset image). The linear regression equation was ip (mA) = 1.551 + 3.197C (mM) (R2 = 0.998). The detection limit was obtained to be 0.005 mM at the signal to noise ratio of 3 (S/N = 3). This illustrates that the MIL53@XC-72-Nafion-GCE sensor possesses wide linear concentration range and low detection limit.
3.4. Determination of Mel at MIL-53@XC-72-Nafion–GCE 3.5. Reproducibility, stability and selectivity of the sensor Under the optimization of experimental conditions, Fig. 6(a) shows the DPV peaks measured in a series of different concentrations of Mel at MIL-53@XC-72- Nafion-GCE sensor. A good linear
The reproducibility test of the MIL-53@XC-72-Nafion-GCE sensor was examined in 0.1 M HCl solution containing 5.0 106 M Mel. The relative standard deviation (RSD) for eight successive determinations was 3.1% and five independent measurements was 5.6%. The long-time stability of the optimal sensor was tested in two week period, showing that the MIL-53@XC-72Nafion-GCE sensor can retain its properties with no obvious decline (RSD<4%). The selectivity of the MIL-53@XC-72-NafionGCE sensor was studied in the solution including 5.0 106 M Mel and some metal ions, food additives and the analogues of Mel in the presence of liquid milk. It was confirmed in Fig. 6(b) and (c) that the current response of Mel is significantly higher than that of 500-fold concentration of Cu2+, Fe3+, Ca2+, Mg2+, PO43, SO42, NO3 and 100-fold concentration of benzoguanamine, dicyandiamide, glucose, fructose, ascorbic acid, and uric acid. The satisfacory selectivity of the developed sensor is attributed to the synergistic effect of the large surface of MIL-53, the bonding interaction between Mel and MIL-53 and the high conductivity of XC-72. The results indicate the MIL-53@XC-72-Nafion-GCE sensor has a good reproducibility, stability and selectivity. 3.6. Determination of Mel in milk samples In order to assess the potential practical application of the sensor, it was applied for the analysis of Mel in liquid milk, bought from local supermarket. The liquid milk was mixed with trichloroacetric acid, followed by sharking, centrifuging and filtering, and the final samples were used to be detected. Because there appeared no obvious peak current of Mel in real sample, the standard addition method was selected to calculate the content of Mel in liquid milk. A certain amount of Mel standard solution was added to the liquid milk samples directly. The recoveries of 98.0%-103.5% with a relative standard deviation (RSD) of 3.1%-4.8% and the 100.8% average recovery were obtained according to five measurements, listing in Table 3. The results suggest the optimized method has potential application for the determination of Mel in liquid milk. The results were in comparison with other several techniques, as shown in Table 4. The MIL-53@XC-72-Nafion-GCE sensor presented better electrochemical performance than the HPLC [8], HPLC/MS [10], GC/MS [5], MIP/CL [21] and UV [36]. In addition, the method has cheap regents and simple operation. In contrast to other electrochemical sensor method, such as DOPAC [32], OMCNafion/GCE [34], Oligonucleotides/Au/GCE [25], MIP/potentiometric/GCE [22] and MIP/GCE [24], the MIL-53@XC-72-Nafion-GCE sensor exhibited moderate linear concentration range and low detection limit. 4. Conclusions
Fig. 6. (a) DPV of the MIL-53@XC-72-Nafion-GCE sensor under 0.04, 1.2, 1.9, 3.0, 5.0, 7.9, 10 mM Mel (from a to g) in 0.1 M HCl solution. Inset: the linear relationship between peak current and the Mel concentration. The scan rate: 0.1 V s1. (b) and (c) Selectivity of the MIL-53@XC-72-Nafion-GCE sensor.
In this paper, a new approach for fabrication of the MIL-53@XC72-Nafion-GCE sensor was presented for the first time to determine Mel. The procedure of fabricating sensor was simple, convenient and rapid. The developed sensor exhibited a wide linear concentration range and low detection limit with good reproducibility, stability and high selectivity. Furthermore, the
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
695
Table 3 Recoveries of the determination of Mel in the liquid milk samples. Samples
Added (mM)
Found (mM)
Recovery (%)
RSD (%) (n = 5)
Average recovery (%)
1 2 3 4 5
0.49 1.30 2.0 5.6 9.2
0.48 1.32 2.07 5.58 9.33
98.0 101.5 103.5 99.6 101.4
3.1 4.0 4.3 4.6 4.8
100.8
Table 4 Comparison of the optimized sensors for the detection of Mel with other methods. Methods HPLC HPLC/MS GC/MS MIP/CL UV MIP/potentiometric MIP/GCE * Elec. (DOPAC) OMC-Nafion/GCE Oligonucleotides/Au MIL-53@XC-72 Nafion/GCE *
Linear range 8
6
7.9 10 -7.9 10 M 1.6 108-4.0 106 M 4.0 107-1.6 105 M 7.9 107-4.0 104 M 1.0 105-5.0 105 M 5.0 106-1.0 102 M 4.5 107-4.0 106 M 1.0 108-5.0 106 M 5.0 108-7.0 106 M 3.9 108-3.3 106 M 4.0 108-1.0 105 M
*
Detec.Lim. 9
1.6 10 M 4.4 108 M 7.9 108 M 1.6 107 M 1.6 107 M 1.6 106M 0.36 106M 3.0 109 M 2.4 109 M 9.6 109 M 5.0 109 M
Recovery/%
RSD/%
Ref.
83.6-91.3 66.2-95.5 93.9-102 102.3-104.0 97 95.0-110.0 95.6-102.5 96 94.0-100.6 95.0 98.0-103.5
– 1.6-16.3 3.1-8.7 – 3.0 – 4.2 5.2-6.5 2.5 1.9 4.2
8 10 5 21 36 22 24 32 34 25 This work
Elec represents Electrochemistry and *Detec.Lim represents Detection limit.
proposed method was applied successfully to the determination of Mel in liquid milk with an excellent recovery. The satisfactory results indicated that the MOFs@XC-72 would be potentially applied in modifiers for the determination of other analytes without electrochemical activities. Acknowledgements This work is financially supported by the Iconic Innovation Project Plan of Henan Institute of Science and Technology (No. 2015BZ02) and the Henan Innovation Foundation for Higher Education (No. 2012HASTIT037). We also thank Dr. Yuanchao Li and Shanqin Liu for their valuable discussions in aspects of electrochemical analysis. References [1] F.X. Sun, W. Ma, L.G. Xu, Y.Y. Zhu, L.Q. Liu, C.F. Peng, L.B. Wang, H. Kuang, C.L. Xu, Analytical methods and recent developments in the detection of melamine, Trend. Anal. Chem. 29 (2010) 1239–1249. [2] K. Rovina, S. Siddiquee, A review of recent advances in melamine detection techniques, J. Food. Compos. Anal. 43 (2015) 25–38. [3] Y. Li, J.Y. Xu, C.Y. Sun, Chemical sensors and biosensors for the detection of melamine, RSC. Adv. 5 (2015) 1125–1147. [4] Y. Liu, E.E.D. Todd, Q. Zhang, J.R. Shi, X.J. Liu, Recent developments in the detection of melamine, J. Zhejiang. Univ. Sci. B. (Biomed & Biotechnol) 13 (2012) 525–532. [5] X.M. Xu, Y.P. Ren, Y. Zhu, Z.X. Cai, J.L. Han, B.F. Huang, Y. Zhu, Direct determination of melamine in dairy products by gas chromatography/mass spectrometry with coupled column separation, Anal. Chim. Acta. 650 (2009) 39–43. [6] R.A. Yokley, L.C. Mayer, R. Rezaaiyan, M.E. Manuli, M.W. Cheung, Analytical method for the determination of cyromazine and melamine residues in soil using LC-UV and GC-MSD, J. Agr. Food. Chem. 48 (2000) 3352–3358. [7] S. Ehling, S. Tefera, I.P. Ho, High-performance liquid chromatographic method for the simultaneous detection of the adulteration of cereal flours with melamine and related triazine by-products ammeline ammelide, and cyanuric acid, Food. Addit. Contam. 24 (2007) 1319–1325. [8] R.C. Wei, R. Wang, Q.F. Zeng, M. Chen, T.Z. Liu, High-performance liquid chromatographic method for the determination of cyromazine and melamine residues in milk and pork, J. Chromatogr. Sci. 47 (2009) 581–584. [9] M. Ibáñez, J.V. Sancho, F. Hernández, Determination of melamine in milkbased products and other food and beverage products by ion-pair liquid chromatography-tandem mass spectrometry, Analytica. Chimica. Acta 649 (2009) 91–97.
[10] Y.T. Wua, C.M. Huang, C.C. Lin, W.A. Ho, L.C. Lin, T.F. Chiu, D.C. Tarng, C.H. Lin, T. Hu Tsai, Determination of melamine in rat plasma, liver, kidney, spleen, bladder and brain by liquid chromatography–tandem mass spectrometry, J. Chromatogr. A 1216 (2009) 7595–7601. [11] H.Y. Huang, C.L. Lin, S.H. Jiang, B. Singco, Y.J. Cheng, Capillary electrochromatography-mass spectrometry determination of melamine and related triazine by-products using poly (divinylbenzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic stationary phases, Analytica. Chimica. Acta 719 (2012) 96–103. [12] A. Kim, S.J. Barcelo, R.S. Williams, Z.Y. Li, Melamine sensing in milk products by using surface enhanced raman scattering, Anal. Chem. 84 (2012) 9303–9309. [13] P.Y. Ma, F.H. Liang, Y. Sun, Y. Jin, Y. Chen, X.H. Wang, H.Q. Zhang, D.J. Gao, D.Q. Song, Rapid determination of melamine in milk and milk powder by surfaceenhanced raman spectroscopy and using cyclodextrin-decorated silver nanoparticles, Microchim. Acta 180 (2013) 1173–1180. [14] D.W. Lachenmeier, E. Humpfer, F. Fang, B. Schutz, P. Dvortsak, C. Sproll, M. Spraul, NMR-spectroscopy for nontargeted screening and simultaneous quantification of health-relevant compounds in foods: the example of melamine, J. Agric. Food Chem. 57 (2009) 7194–7199. [15] Q. Cao, H. Zhao, Y.J. He, X.J. Li, L.X. Zeng, N. Ding, J. Wang, J. Yang, G.W. Wang, Hydrogen-bonding-induced colorimetric detection of melamine by nonaggregation-based Au-NPs as a probe, Biosens. Bioelectron. 25 (2010) 2680–2685. [16] Z.J. Wu, H. Zhao, Y. Xue, Q. Cao, J. Yang, Y.J. He, X.J. Li, Z.B. Yuan, Colorimetric detection of melamine during the formation of gold nanoparticles, Biosens. Bioelectron. 26 (2011) 2574–2578. [17] X.M. Chen, S. Lian, Y. Ma, A.H. Peng, X.T. Tian, Z.Y. Huang, X. Chen, Electrochemiluminescence sensor for melamine based on a Ru (bpy)32+-doped silica nanoparticles/carboxylic acid functionalized multi-walled carbon nanotubes/Nafion hybrid film modified electrode, Talanta 146 (2016) 844–850. [18] Y. Fan, X.R. Tan, X.F. Liu, X. Ou, S.H. Chen, S.P. Wei, H.M. Cao, X.Q. Hu, C.Y. Hu, Y. Zhang, N.Q. Jia, A novel solid-state electrochemiluminescence sensor for melamine with Ru (bpy)32+/mesoporous silica nanospheres/Nafion hybrid modified electrode, Biosens. Bioelectron. 41 (2013) 911–915. [19] W.W. Yin, J.T. Liu, T.C. Zhang, W.H. Li, W. Liu, M. Meng, F.Y. He, Y.P. Wan, C.W. Feng, S.L. Wang, X. Lu, R. Xi, Preparation of monoclonal antibody for melamine and development of an indirect competitive ELISA for melamine detection in raw milk milk powder, and animal feeds, J. Agric. Food Chem. 58 (2010) 8152– 8157. [20] Q. Xu, H.P. Wei, S. Du, H.B. Li, Z.P. Ji, X.Y. Hu, Detection of subnanomolar melamine based on electrochemical accumulation coupled with enzyme colorimetric assay, J. Agric. Food Chem. 61 (2013) 1810–1817. [21] J.H. Yu, C.C. Zhang, P. Dai, S.G. Ge, Highly selective molecular recognition and high throughput detection of melamine based on molecularly imprinted solgel film, Analytica. Chimica. Acta 651 (2009) 209–214. [22] R.N. Liang, R.M. Zhang, W. Qin, Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk, Sensor. Actuat. B 141 (2009) 544–550. [23] A. Pietrzyk, W. Kutner, R. Chitta, M.E. Zandler, F. D’souza, F. Sannicolò, P.R. Mussini, Melamine acoustic chemosensor based on molecularly imprinted polymer Film, Anal. Chem. 81 (2009) 10061–10070.
696
W. Zhang et al. / Electrochimica Acta 211 (2016) 689–696
[24] Y.T. Liu, J. Deng, X.L. Xiao, L. Ding, Y.L. Yuan, H. Li, X.T. Li, X.N. Yan, L.L. Wang, Electrochemical sensor based on a poly (para-aminobenzoic acid) film modified glassy carbon electrode for the determination of melamine in milk, Electrochim. Acta 56 (2011) 4595–4602. [25] Q. Cao, H. Zhao, L.X. Zeng, J. Wang, R. Wang, X.H. Qiu, Y.J. He, Electrochemical determination of melamine using oligonucleotides modified gold electrodes, Talanta 80 (2009) 484–488. [26] H. Kuang, W. Chen, W.J. Yan, L.G. Xu, Y.Y. Zhu, L.Q. Liu, H.Q. Chu, C.F. Peng, L.B. Wang, N.A. Kotov, C.L. Xu, Crown ether assembly of gold nanoparticles: Melamine sensor, Biosens. Bioelectron. 26 (2011) 2032–2037. [27] N. Vasimalai, S.A. John, Picomolar melamine enhanced the fluorescence of gold nanoparticles: Spectrofluorimetric determination of melamine in milk and infant formulas using functionalized triazole capped gold nanoparticles, Biosens. Bioelectron. 42 (2013) 267–272. [28] M.K. Dey, A.K. Satpati, A.V.R. Reddy, Electrochemical determination of melamine on static mercury drop electrode and on gold nano particle modified carbon paste electrode, Am. J. Anal. Chem. 5 (2014) 598–603. [29] Rosy, R.N. Goyal, Gold nanoparticles decorated poly-melamine modified glassy carbon sensor for the voltammetricestimation of domperidone in pharmaceuticals and biological fluids, Talanta 141 (2015) 53–59. [30] J.J. Du, Z.K. Wang, X.J. Peng, J.L. Fan, In situ colorimetric recognition of melamine based on thymine derivative-functionalized gold nanoparticle, Ind. Eng. Chem. Res. 54 (2015) 12011–12016. [31] R.S. Deinhammer, M. Ho, J.W. Anderegg, M.D. Porter, Electrochemical oxidation of amine-containing compounds: a route to the surface modification of glassy carbon electrodes, Langmuir 10 (1994) 1306–1313. [32] Q. Cao, H. Zhao, Y.J. He, N. Ding, J. Wang, Electrochemical sensing of melamine with 3,4-dihydroxyphenylacetic acid as recognition element, Analytica. Chimica. Acta 675 (2010) 24–28. [33] T.K. Zhao, L.H. Liu, G.M. Li, A.L. Dang, T.H. Li, Electrochemical determination of melamine with a glassy carbon electrode coated with a multi-wall carbon nanotube/chitosan hybrid, J. Electrochem. Soc. 159 (2012) 141–145.
[34] Z. Guo, X.F. Xu, J. Li, Y.W. Liu, J. Zhang, C. Yang, Ordered mesoporous carbon as electrode modification material for selective and sensitive electrochemical sensing of melamine, Sensor. Actuat. B 200 (2014) 101–108. [35] J. Bai, B.P. Lu, X.J. Bo, L.P. Guo, Electrochemical property and electroanalytical application of large mesoporous carbons, Electrochem. Commun. 12 (2010) 1563–1567. [36] J. Rima, M. Abourida, T. Xu, K. Cho, S. Kyriacos, New spectrophotometric method for the quantitative determination of melamine using Mannich reaction, J. Food. Compos. Anal. 22 (2009) 689–693. [37] J. Zhang, S.S. Liu, L.F. Song, C.H. Jiang, C.L. Jiao, S. Wang, Y. Zhang, Hydrogen storage in nano metal-organic framework materials, Materials China 28 (2009) 28–34. [38] W.P. Mounfield III, K.S. Walton, Effect of synthesis solvent on the breathing behavior of MIL-53 (Al), J. Colloid. Interf. Sci. 447 (2015) 33–39. [39] T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille, G. Férey, A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration, Chem. Eur. J. 10 (2004) 1373–1382. [40] G.L. Zi, Z.Y. Yan, Y.X. Wang, Y.J. Chen, Y.L. Guo, J.Q. Wang, Catalytic hydrothermal conversion of carboxymethyl cellulose to value-added chemicals over metalorganic framework MIL-53(Al), Carbohyd. Polym. 115 (2015) 146–151. [41] W.Z. Li, C.H. Liang, W.J. Zhou, J.S. Qiu, Z.H. Zhou, G.Q. Sun, Q. Xin, Preparation and characterization of multiwalled carbon nanotube-supported platinum for cathode catalysts of direct methanol fuel cells, J. Phys. Chem. B 107 (2003) 6292–6299. [42] X.K. Qian, B.L. Yadian, R.B. Wu, Y. Long, K. Zhou, B. Zhu, Y.Z. Huang, Structure stability of metal-organic framework MIL-53(Al) in aqueous solutions, Int. J. Hydrogen. Energ. 38 (2013) 16710–16715. [43] X.M. Ren, D.D. Shao, S.T. Yang, J. Hu, G.D. Sheng, X.L. Tan, X.K. Wang, Comparative study of Pb(II) sorption on XC-72 carbon and multi-walled carbon nanotubes from aqueous solutions, Chem. Eng. J. 170 (2011) 170–177.