Biosensors and Bioelectronics 71 (2015) 137–142
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Electrochemical sensor for paracetamol recognition and detection based on catalytic and imprinted composite film Ying Teng a,b, Limei Fan a,b, Yunlong Dai a,b, Min Zhong a,b, Xiaojing Lu a,b, Xianwen Kan a,b,n a
College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Anhui Key Laboratory of ChemoBiosensing, Anhui Key Laboratory of Functional Molecular Solids, PR China
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Article history: Received 18 January 2015 Received in revised form 10 April 2015 Accepted 13 April 2015 Available online 14 April 2015
A new strategy for a composite film based electrochemical sensor was developed in this work. A layer of conductive film of poly(p-aminobenzene sulfonic acid) (pABSA) was electropolymerized onto glassy carbon electrode surface and exhibited a high electrocatalytic active for paracetamol (PR) redox. The subsequent formation of a layer of molecular imprinted polymer (MIP) film on pABSA modified electrode endowed the sensor with plentiful imprinted cavities for PR specific adsorption. The advantages of the composite film made the prepared sensor display high sensitivity and good selectivity for PR detection and recognition. Under the optimal conditions, the sensor could recognize PR from its interferents. A linear ranging from 5.0 10 8 to 1.0 10 4 mol/L for PR detection was obtained with a detection limit of 4.3 10 8 mol/L. The sensor has been applied to analyze PR in tablets and human urine samples with satisfactory results. The simple, low cost, and efficient strategy reported here can be further used to prepare electrochemical sensors for other compounds recognition and detection. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electrochemical sensor Conductive polymer Molecular imprinted polymer Paracetamol Electrocatalysis
1. Introduction As a kind of synthetic material, molecular imprinting polymer (MIP) simulates the behavior of natural antibodies and exhibits greater stability than its natural counterparts, resulting in its wide applications in the field of chromatography separation, solid phase extraction, drug controlled release, and electrochemical sensor (Jin et al., 2013; Rossetti et al., 2014; Zeng et al., 2013). Traditional synthetic methods, such as UV and chemically initiated polymerizations, have been investigated and applied for lots of small molecules and biomacromolecules recognition (Wulff et al., 1973; Vlatkis et al., 1993). In electrochemical sensors, electropolymerization methods for MIP preparation exhibit many advantages, such as simple preparation procedures, easy control of the thickness of the film, and uniform polymer distribution on electrode surface. The modified MIP film on electrode surface acts as a molecular recognition element to improve the selectivity of the sensor due to its specific adsorption capacity. Generally, the electropolymerized MIP films are non electroactive, resulting in the lack of direct path for the conduction of electrons from the active sites to electrode (Whitcombe et al., 2011; Wang et al., n Corresponding author at: College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China.. Fax: þ 86 553 3869303. E-mail address: [email protected] (X. Kan).
http://dx.doi.org/10.1016/j.bios.2015.04.037 0956-5663/& 2015 Elsevier B.V. All rights reserved.
2014a; Li et al., 2012a,b). Even though the conductive polymers, such as polypyrrole and polyaniline, were involved, the overoxidation procedure was implemented before the determination also to restrain the non specific adsorption (Turco et al., 2015; Kan et al., 2012). Thus, the sensitivity of the MIP based electrochemical sensors would be seriously limited. Due to the high surface-to-volume ratio, enhancement of the conductivity and acceleration of the electron transfer, various nanomaterials have been introduced into the MIP based electrochemical sensors preparation. Graphene, carbon nanotubes, gold nanoparticles et al. have been used to fabricate effective sensing platforms for electrochemical sensors. These nanomaterials based MIP electrochemical sensors exhibited improved sensitivity for template molecules detection (Riskin et al., 2008; Li et al., 2012a,b; Cai et al., 2010; Xie et al., 2010; Flavin and Resmini, 2009). Besides these nanomaterials, conductive polymers have been widely used in electrochemical sensors since they also can accelerate the electron transfer at the electrode–solution interface and significantly enhance the electrode reaction rate (Kong et al., 2014). Moreover, the advantages of stable coating on electrode and low cost make the conductive polymers perfect for electrochemical sensor preparation to improve the sensitivity in a sense. Yang et al. reported that a poly(malachite green) film-coated electrode displayed high electrocatalytic activity to dopamine (Wang et al., 2007). Cheng et al. investigated the electrocatalytic oxidation of cysteine at screen-printed electrode, which was modified with
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electrogenerated poly(3,4-ethylenediocythiophene) film. The prepared modified polymer film lowered the overpotentials and improved electrochemical behavior of cycteine oxidation (Su and Cheng, 2008). Thionine was used as a monomer for a poly(thionine) modified electrode preparation. Excellent catalytic activity and reversibility for the electrochemical redox reaction of both hydroquinone and catechol were achieved with a really low detection limit (Ahammad et al., 2011). Among the conductive polymers synthesis, sulfonated polyaniline, as the first reported self-doped water-soluble conducting polyaniline derivative, is of interest because of its high stability and good conductivity in a broad pH range (Wei et al., 1996). pAminobenzene sulfonic acid (ABSA) is a kind of electroactive compound and has been successfully electropolymerized to form poly(p-aminobenzene sulfonic acid) (pABSA) film by many researchers under different conditions. Chen et al. electropolymerized a composite of pABSA and flavins on electrode surface, which showed excellent electrocatalytic activity for NADH redox (Kumar and Chen, 2007). A pABSA film modified electrode fabricated by electropolymerization showed an electrocatalytic activity for the oxidation of dopamine and ascorbic acid, resulting in the simultaneously determination of both compounds (Jin et al., 2005). It is of no doubt that the excellent conductivity and electrocatalysis of pABSA could provide a platform to facilitate the conduction of electrons. Hence, the combination of MIP and conductive polymers can be proposed for achieving good recognition capacity as well as high sensitivity of the electrochemical sensor. Paracetamol (PR), as an antipyretic/analgesic, is rapidly and completely metabolized to form inactive metabolites, which would be eliminated in the urine in body (Markas, 1994). However, overdosing and the chronic use of PR produces toxic metabolite accumulation that will cause skin rashes and inflammation of the pancreas (Shiroma et al., 2012). An electrochemical sensor was prepared by modifying graphene on glassy carbon electrode surface for PR detection with a linear range of 1.0 10 7–2.0 10 5 mol/L. A quasi-reversible redox process of PR at the modified electrode and the significant decrease of overpotential of PR were attributed to the electrocatalytic activity of graphene (Kang et al., 2010). A layer of MIP film was reported to be electropolymerized onto multiwalled carbon nanotubes modified electrode surface for PR detection and recognition. The sensor not only recognized PR from its possible interfering substances, but also sensitively detected PR with a linear range of 2.0 10 7–4.0 10 5 mol/L (Peng et al., 2014). Electrochemical sensors based on multiwalled carbon nanotubes and dopamine nanospheres functionalized with gold nanoparticles (Liu et al., 2014), Pd/graphene oxide nanocomposite (J. Li et al., 2014a, Y. Li et al., 2014b), nanogolds (Goyal et al., 2005), have been reported for PR determined with some satisfactory results. These nanomaterials used for sensor preparation can improve the sensitivity of the sensor. Herein, two layers of polymer films were electrpolymerized on glassy carbon electrode (GCE) surface successively to fabricate a novel and facile electrochemical sensor. ABSA was chosen as a monomer to form the first layer of polymer, providing a conductive and catalytic platform for PR sensitive detection. And o-phenylenediamine (OPD) was chosen as a monomer for the second film preparation in the presence of PR to form MIP, endowing the sensor with good selective recognition capacity toward PR.
purification. p-aminobenzene sulfonic acid (ABSA) and paracetamol (PR, 499%) were ordered from Aladdin (Aladdin, China). o-phenylenediamine (OPD), 4-nitrophenol (4-NP), and ascorbic acid (AA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Uric acid (UA), dopamine (DA), and hydroquinone (HQ) were provided by Sigma (Sigma, USA). All other reagents were of at least analytical-reagent grade, and double-distilled deionized water was used for all solutions. 2.2. Apparatus Electrochemical experiments, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) were performed on CHI 660C workstation (ChenHua Instruments Co., Shanghai, China) with a conventional three-electrode system. A bare or modified glassy carbon electrode (GCE) was served as a working electrode. A saturated calomel electrode and a platinum wire electrode were used as a reference electrode and a counter electrode, respectively. Field emission scanning electron microscope (FE-SEM) images were obtained on an S-4800 field emission scanning electron microanalyser (Hitachi, Japan). High-performance liquid chromatography (HPLC, Hitachi L-7100, UV-detector, λ ¼214 nm, VP-ODS C18 150 mm, t¼ 25 °C, mobile phase 0.5% acetic acid in H2O 10%: methanol 90%). 2.3. Preparation of pABSA modified GCE (pABSA/GCE) Prior to the modification, the bare GCE were polished by 0.3 μm alumina slurry on micro-cloth pads and sonicated subsequently in water. Then the clean GCE was immersed into 5 mL phosphate buffer solution (PBS, 0.1 mol/L, pH 7.0) containing 2 10 4 mol/L ABSA. Then CV method was performed from 1.5 V to þ2.5 V for 15 cycles at a scan rate of 100 mV/s, obtaining polymer film modified electrode (pABSA/GCE). 2.4. Fabrication of MIP modified pABSA/GCE (MIP/pABSA/GCE) The prepared pABSA/GCE was immersed into PBS (0.1 mol/L, pH 5.0) containing 5.0 10 3 mol/L PR and 5.0 10 3 mol/L OPD. Then CV was performed from 0.0 V to þ0.8 V for 35 cycles at a scan rate of 75 mV/s, obtaining polymer modified pABSA/GCE. Subsequently, the embedded PR molecules were extracted by incubating the modified electrode into ethanol for 20 min until no obvious oxidation peak of PR could be observed by DPV method, getting MIP modified pABSA/GCE (MIP/pABSA/GCE). The procedure of the fabrication of MIP/pABSA/GCE was depicted in Fig. 1A. As a control, non-molecular imprinting polymer modified electrode (NIP/pABSA/GCE) was prepared and treated in exactly the same way except for the omitting of PR in the electropolymerization process. In order to investigate the electrocatalytic effect of pABSA, other two modified electrodes were prepared by direct electropolymerization of OPD on GCE surface in the presence and absence of PR, which were assigned as MIP/GCE and NIP/GCE, respectively. 2.5. Electrochemical properties measurements
2. Experimental 2.1. Chemicals Commercially available reagents were used without further
Electrochemical measurements to characterize the prepared sensor were carried out in PBS by using CV, EIS, and DPV methods. AA, UA, DA, 4-NP, and HQ were selected as coexisted or structural similar compounds to evaluate the recognition capacity of the prepared sensor.
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Fig. 1. Schematic illustrations of the fabrication procedure of MIP/pABSA/GCE (A) (DPV curves: with the electrocatalysis of pABSA, a higher current response showed on MIP/ pABSA/GCE than that on MIP/GCE), SEM images of pABSA/GCE (B) and MIP/pABSA/GCE (C).
3. Results and discussion 3.1. Electrocatalytic property of pABSA/GCE Fig. 2 showed the CV curves, which were recorded on bare GCE and pABSA/GCE in PBS (0.1 mol/L, pH 7.0) with or without 1.0 10 4 mol/L PR. Compared with the CV curve recorded in blank electrolyte (curve a), an irreversible oxidation peak of PR could be found on bare GCE at þ0.484 V in accordance with the reported results (Su and Cheng, 2010) (curve b). Several pairs of redox peaks appeared in CV curve when it was recorded on pABSA/GCE in PR solution (curve d). To ascribe these redox peaks, another CV curve was recorded by immersing pABSA/GCE in blank electrolyte (curve c). As could be seen, except for a pair of redox peaks appeared at þ0.458 V and þ0.440 V, other redox peaks appeared at the same potentials in curve b and curve d, indicating that these redox peaks were caused by the redox of pABSA itself and the peaks at þ 0.458 V and þ0.440 V were certainly caused by the redox of PR, which corresponded to a two-proton and twoelectron process (Eq. (1)) (Fanjul-Bolado et al., 2009).
The oxidation peak of PR shifted negatively about 26 mV and
Fig. 2. CV curves recorded on bare GCE in the absence of (a) and in the presence of (b) 1.0 10 4 mol/L PR; CV curves recorded on pABSA/GCE in the absence of (c) and in the presence of (d) 1.0 10 4 mol/L PR.
the peak current at 0.458 V on pABSA/GCE was about seven times larger than that on bare GCE. The results of the increase of peak current and negative shift of oxidized peak potential demonstrated the obvious electrocatalytic characteristics of pABSA for the redox
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of PR, which should improve the sensitivity of the sensor for PR detection. 3.2. Characterization of MIP/pABSA/GCE SEM images of pABSA/GCE and MIP/pABSA/GCE shown in Fig. 1B and C were used to characterize their morphological structures, respectively. A layer of film with scraggy and uneven surface was found in Fig. 1B, indicating the pABSA has been electropolymerized onto the surface of GCE. As MIP film was formed onto pABSA/GCE, a much rougher surface was observed (Fig. 1C), which could be attributed to the electropolymerization of MIP film. The obvious difference on the surface morphologies confirmed that two layer of pABSA and MIP films have been successively prepared onto GCE surface. Electrochemical impedance spectroscopy (EIS) has been carried out to investigate the stepwise constructed process of the sensor. The results were shown in Fig. S1, which also demonstrated the successfully preparation of the sensor. In order to investigate the specific adsorption of MIP and the electrocatalytic effect of pABSA toward the prepared sensor, the electrochemical responses of PR on MIP/GCE, MIP/pABSA/GCE, and NIP/pABSA/GCE were recorded using DPV method, as shown in Fig. 3. Compared with the DPV curve recorded on MIP/GCE (curve b), peak current increased remarkably when DPV was carried out on MIP/pABSA/GCE (curve f), demonstrating the high catalytic activity of pABSA film to the adsorbed PR molecules. It is also apparent that the MIP/pABSA/GCE displayed a much higher current response than that on NIP/pABSA/GCE (curve d) under the same concentration of PR. The high elecrochemical response of the imprinted sensor might result from the imprinted cavities in the MIP and the functional groups in the cavities produced by the template molecules. Therefore, MIP/pABSA/GCE possessed an evident specific adsorption capacity and high electrocatalytic activity to PR. The accumulation time was investigated by recording the current responses of PR on MIP/pABSA/GCE and MIP/GCE, which were immersed in PR solution (5.0 10 5 mol/L) for different time. As shown in Fig. S2, the adsorption of PR on MIP/GCE could not reach equilibrium within 10 min. Compared with MIP/GCE, MIP/pABSA/ GCE showed a faster accumulation equilibrium time within almost 2 min, which could be attributed to catalysis capacity of the modified pABSA film in MIP/pABSA/GCE.
3.3. Optimization of conditions for MIP/pABSA/GCE preparation Different influencing factors including scan cycles for pABSA electropolymerization, scan cycles and scan rate for MIP electropolymerization, and the molar ratio between template molecule and monomer were investigated to fabricate an efficient sensor. DPV was employed to detect the oxidized peak current of PR under different conditions. The pABSA film modified on GCE surface was carried out by CV method with varied scan cycles. As shown in Fig. S3, the peak current of 1.0 10 4 mol/L PR increased on pABPA/GCE with the increase of polymerized cycles from 5 to 15, while decreased with continued increase of the cycles. The reason could be attributed to two competition effects, i.e., enhancing cation-exchange activity of coating and suppressing effect due to the thicker polymer that could reduce the conductivity (Wang et al., 2014a,b). In the imprinted polymers preparation experiments, an imprinted factor (IF, the ratio of oxidized peak currents of 1.0 10 4 mol/L PR recorded on MIP/pABSA/GCE and NIP/pABSA/ GCE) was calculated and compared under each condition. The thickness of the polymer membrane influences the sensitivity of the imprinted electrochemical sensor, which could be controlled by the scanning cycles. As shown in Fig. S4, the low IF was obtained when scan cycles was less than 35, which could be attributed to the formation of fewer imprinted cavities on the electrode surface. However, the IF decreased while the MIP prepared by scanning over 35 cycles, which probably because PR molecules could not be removed completely from the polymer matrix if the imprinted film was too thick. Moreover, accession to the deeply imprinted sites is difficult for the template molecules because of the high mass-transfer resistance, which decreases the detected sensitivity. Therefore, the MIP obtained by scanning 35 cycles achieved the highest sensitivity to PR. Fig. S5 showed the effect of scan rate for MIP film electropolymerization. A tight film would be produced when MIP film was prepared at a slower scan rate, which decreased the accessibility of template molecule to imprinted sites. The IF on the MIP/ pAPBA/GCE was found to increase with an increase of scan rate up to 75 mV/s and decrease as the scan rate increased above that value. A low recognition capacity could be found when a faster scan rate was performed to get a loose and rough film, which would decrease the recognition capacity of MIP film. Thus, the optimum polymerization scan rate was found to be 75 mV/s. The molar ratio between template molecule and monomer used in the polymerization process would affect the amount of imprinted sites in the polymer matrix, further influence the electrochemical behavior of the sensor. A series of MIP/pABSA/GCE sensors were prepared under different molar ratio between template molecules and monomers from 2:1 to 1:3. As shown in Fig. S6, the IF was found to increase with the increase of molar ratio up to 1:1. A considerable decrease of IF below and above this molar ratio was observed. It can be concluded that the optimum molar ratio between template molecules and monomers was about 1:1. 3.4. Selectivity of MIP/pABSA/GCE
Fig. 3. DPV curves recorded on MIP/GCE in the absence of (a) and in the presence of (b) 1.0 10 4 mol/L PR; DPV curves recorded on NIP/pABSA/GCE in the absence of (c) and in the presence of (d) 1.0 10 4 mol/L PR; DPV curves recorded on MIP/ pABSA/GCE in the absence of (e) and in the presence of (f) 1.0 10 4 mol/L.
AA, UA, DA, 4-NP, and HQ were selected as interferents to evaluate the recognition capacity of the prepared sensor. The selective experiments were carried out by detecting the current response of 1.0 10 4 mol/L PR or each interferent with the same concentration of PR on MIPs/pABSA/GCE and NIPs/pABSA/GCE. The comparison result was illustrated in Fig. 4. Compared with the current change of AA, UA, DA, and 4-NP on MIPs/pABSA/GCE, the current resulted from PR with the same concentration showed much higher current response, and almost no oxidation peak could be found whether on MIP/pABSA/GCE or NIP/pABSA/GCE
Y. Teng et al. / Biosensors and Bioelectronics 71 (2015) 137–142
Fig. 4. Selectivity of the sensor. DPV current response recorded on MIP/pABSA/GCE and NIP/pABSA/GCE in PR or each interferent solution with the same concentration of 1.0 10 4 mol/L. Each was measured for three repetitive determinations. (RSD o7.65%). ΔI denoted the difference of currents of DPV curves recorded at þ 0.45 V with and without PR in the electrolyte.
when the DPV was carried out in the presence of HQ. The reason may be that the binding sites in imprinted sensor were complementary to PR in terms of the size, shape, and position of their functional groups. These results demonstrated an acceptable selectivity of the sensor for PR. 3.5. Analytical application of the sensor The dependence of the oxidation peak current of PR on MIP/ pABSA/GCE was recorded by DPV under the optimized conditions. As shown in Fig. 5A, the current response increased with the successive addition of PR. Calibration curve a in Fig. 5B revealed that the peak currents were proportional to the concentrations of PR in the range of 5.0 10 8 1.0 10 4 mol/L with a linear regression equation of I (μA) ¼ 0.5310 þ0.171c (μmol/L) (where c is the concentration of PR). And a detected limit calculated was 4.3 10 8 mol/L. Compare with the linear range and detected limit reported in previous studies summarized in Table S1, the present electrochemical sensor showed an excellent detection capacity to PR. The same detection procedure was carried out on NIP/pABSA/GCE and MIP/GCE for comparison. The linear range of 5.0 10 6 1.0 10 4 mol/L obtained on NIP/pABSA/GCE
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(Fig. 5Bb) with a linear regression equation of I (μA)¼ 0.2327 þ0.058c (μmol/L) was much narrower than that on MIP/ pABSA/GCE, which could be attributed to the imprinted cavities in MIP/pABSA/GCE for much more PR molecules adsorption. The same linear range as NIP/pABSA/GCE was obtained on MIP/GCE (Fig. 5Bc) with a linear regression equation of I (μA)¼ 0.7331þ 0.023c (μmol/L). Compared with MIP/GCE and NIP/pAPBA/GCE, the highest sensitivity of MIP/pABSA/GCE confirmed the good electrocatalytic capacity of pABSA and the specific adsorption capacity of MIP. To investigate the regeneration of the constructed sensor, MIP/ pABSA/GCE was used to detect 1.0 10 4 mol/L PR for five times with subsequent cycles of extraction and measuring operations. According to the current responses, a relative standard deviation (RSD) was calculated to be 2.58% (Fig. S7a). The reproducibility of the sensor was investigated by detecting PR on five different MIP/ pABSA/GCE with a RSD of 2.66%, which were prepared under the same conditions (Fig. S7b).The current response of the imprinted sensor decreased to 97.6% after storing for 18 days at 4 °C. These results demonstrated that the prepared sensor had acceptable reproducibility, regeneration, and stability. To evaluate the practical performance of the prepared sensor, two kinds of PR commercial tablets (500 mg/tablet) were determined on MIP/pABSA/GCE. The tablets were ground to powders and then added with ethanol to dissolve PR. After the centrifugation, the supernate was collected and diluted to a working concentration range for electrochemical detection. By using the prepared sensor, the concentration of PR in two kinds of tablets was determined. As shown in Table 1, the results were in good agreement with the manufacturers’ stated contents of PR. Recovery tests were carried out for assay in human urine samples. The samples were obtained from human after 4 h of administration of a tablet containing 500 mg of PR. Prior to analysis, the urine samples were diluted 100 times with PBS. Then the urine sample was added with a known concentration of PR. Using the prepared sensor to detect the concentration of PR in different urine samples before and after spiking, the results obtained were concluded in Table 1. The results showed that the recoveries from the urine samples were excellent, and varied from 96.33% to 102.4%. In order to validate the electrochemical detection, we have detected PR in tablets and human urine samples by HPLC. The results obtained by HPLC also have been included in Table 1, which indicated that the prepared sensor can be used as an effective and reliable sensing platform for detecting PR in real samples.
Fig. 5. DPV curves recorded on MIP/pABSA/GCE with the successive addition of PR (a to l were 5.0 10 8, 1.0 10 7, 5.0 10 7, 1.0 10 6, 3.0 10 6, 5.0 10 6, 7.0 10 6, 1.0 10 5, 3.0 10 5, 5.0 10 5, 7.0 10 5 and 1.0 10 4 mol/L) (A); the calibration plot of the concentration of PR vs. peak current (B) recorded on MIP/ pABSA/GCE (a), NIP/pABSA/GCE (b), and MIP/GCE (c) Each was measured for three repetitive determinations and RSD were calculated to be between 0.48% and 3.57%. ΔI denoted the difference of currents of DPV curves recorded at þ 0.45 V with and without PR in the electrolyte.
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Table 1 Determination of PR in tablets and human urine samples. Samples
Labeled (mmol/ L)
Added (mmol/L)
Detected contentby the sensor (mmol/L)
Recovery (%) RSD (%) Detected content by HPLC (mmol/L)
Recovery (%) RSD (%)
Tablet 1
0.30 0.50
– –
0.31 0.52
102.7 103.0
1.51 3.72
0.29 0.49
95.03 97.90
1.35 5.79
Tablet 2
0.30 0.50
– –
0.32 0.51
105.7 102.6
2.74 2.31
0.28 0.49
91.25 99.74
2.51 4.07
Urine sample 1 – –
0.30 0.50
0.31 0.49
102.4 98.34
5.62 4.68
0.31 0.51
104.0 101.6
4.31 1.66
Urine sample 2 – –
0.30 0.50
0.29 0.49
96.33 97.40
4.55 3.08
0.30 0.51
99.36 101.9
2.07 4.99
4. Conclusion
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Acknowledgements We greatly appreciate the support of the National Natural Science Foundation of China for young program (21005002), Anhui Provincial Natural Science Foundation, China for Young Program (11040606Q35), Anhui University Provincial Natural Science Foundation Key program (KJ2010A138). We gratefully acknowledge Dr. Yunchun Liu for her kind help in HPLC experiments.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.bios.2015.04.037.
Contents lists available at ScienceDirect
Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios
Electrochemical sensor for paracetamol recognition and detection based on catalytic and imprinted composite film Ying Teng a,b, Limei Fan a,b, Yunlong Dai a,b, Min Zhong a,b, Xiaojing Lu a,b, Xianwen Kan a,b,n a
College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Anhui Key Laboratory of ChemoBiosensing, Anhui Key Laboratory of Functional Molecular Solids, PR China
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a b s t r a c t
Article history: Received 18 January 2015 Received in revised form 10 April 2015 Accepted 13 April 2015 Available online 14 April 2015
A new strategy for a composite film based electrochemical sensor was developed in this work. A layer of conductive film of poly(p-aminobenzene sulfonic acid) (pABSA) was electropolymerized onto glassy carbon electrode surface and exhibited a high electrocatalytic active for paracetamol (PR) redox. The subsequent formation of a layer of molecular imprinted polymer (MIP) film on pABSA modified electrode endowed the sensor with plentiful imprinted cavities for PR specific adsorption. The advantages of the composite film made the prepared sensor display high sensitivity and good selectivity for PR detection and recognition. Under the optimal conditions, the sensor could recognize PR from its interferents. A linear ranging from 5.0 10 8 to 1.0 10 4 mol/L for PR detection was obtained with a detection limit of 4.3 10 8 mol/L. The sensor has been applied to analyze PR in tablets and human urine samples with satisfactory results. The simple, low cost, and efficient strategy reported here can be further used to prepare electrochemical sensors for other compounds recognition and detection. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electrochemical sensor Conductive polymer Molecular imprinted polymer Paracetamol Electrocatalysis
1. Introduction As a kind of synthetic material, molecular imprinting polymer (MIP) simulates the behavior of natural antibodies and exhibits greater stability than its natural counterparts, resulting in its wide applications in the field of chromatography separation, solid phase extraction, drug controlled release, and electrochemical sensor (Jin et al., 2013; Rossetti et al., 2014; Zeng et al., 2013). Traditional synthetic methods, such as UV and chemically initiated polymerizations, have been investigated and applied for lots of small molecules and biomacromolecules recognition (Wulff et al., 1973; Vlatkis et al., 1993). In electrochemical sensors, electropolymerization methods for MIP preparation exhibit many advantages, such as simple preparation procedures, easy control of the thickness of the film, and uniform polymer distribution on electrode surface. The modified MIP film on electrode surface acts as a molecular recognition element to improve the selectivity of the sensor due to its specific adsorption capacity. Generally, the electropolymerized MIP films are non electroactive, resulting in the lack of direct path for the conduction of electrons from the active sites to electrode (Whitcombe et al., 2011; Wang et al., n Corresponding author at: College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China.. Fax: þ 86 553 3869303. E-mail address: [email protected] (X. Kan).
http://dx.doi.org/10.1016/j.bios.2015.04.037 0956-5663/& 2015 Elsevier B.V. All rights reserved.
2014a; Li et al., 2012a,b). Even though the conductive polymers, such as polypyrrole and polyaniline, were involved, the overoxidation procedure was implemented before the determination also to restrain the non specific adsorption (Turco et al., 2015; Kan et al., 2012). Thus, the sensitivity of the MIP based electrochemical sensors would be seriously limited. Due to the high surface-to-volume ratio, enhancement of the conductivity and acceleration of the electron transfer, various nanomaterials have been introduced into the MIP based electrochemical sensors preparation. Graphene, carbon nanotubes, gold nanoparticles et al. have been used to fabricate effective sensing platforms for electrochemical sensors. These nanomaterials based MIP electrochemical sensors exhibited improved sensitivity for template molecules detection (Riskin et al., 2008; Li et al., 2012a,b; Cai et al., 2010; Xie et al., 2010; Flavin and Resmini, 2009). Besides these nanomaterials, conductive polymers have been widely used in electrochemical sensors since they also can accelerate the electron transfer at the electrode–solution interface and significantly enhance the electrode reaction rate (Kong et al., 2014). Moreover, the advantages of stable coating on electrode and low cost make the conductive polymers perfect for electrochemical sensor preparation to improve the sensitivity in a sense. Yang et al. reported that a poly(malachite green) film-coated electrode displayed high electrocatalytic activity to dopamine (Wang et al., 2007). Cheng et al. investigated the electrocatalytic oxidation of cysteine at screen-printed electrode, which was modified with
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electrogenerated poly(3,4-ethylenediocythiophene) film. The prepared modified polymer film lowered the overpotentials and improved electrochemical behavior of cycteine oxidation (Su and Cheng, 2008). Thionine was used as a monomer for a poly(thionine) modified electrode preparation. Excellent catalytic activity and reversibility for the electrochemical redox reaction of both hydroquinone and catechol were achieved with a really low detection limit (Ahammad et al., 2011). Among the conductive polymers synthesis, sulfonated polyaniline, as the first reported self-doped water-soluble conducting polyaniline derivative, is of interest because of its high stability and good conductivity in a broad pH range (Wei et al., 1996). pAminobenzene sulfonic acid (ABSA) is a kind of electroactive compound and has been successfully electropolymerized to form poly(p-aminobenzene sulfonic acid) (pABSA) film by many researchers under different conditions. Chen et al. electropolymerized a composite of pABSA and flavins on electrode surface, which showed excellent electrocatalytic activity for NADH redox (Kumar and Chen, 2007). A pABSA film modified electrode fabricated by electropolymerization showed an electrocatalytic activity for the oxidation of dopamine and ascorbic acid, resulting in the simultaneously determination of both compounds (Jin et al., 2005). It is of no doubt that the excellent conductivity and electrocatalysis of pABSA could provide a platform to facilitate the conduction of electrons. Hence, the combination of MIP and conductive polymers can be proposed for achieving good recognition capacity as well as high sensitivity of the electrochemical sensor. Paracetamol (PR), as an antipyretic/analgesic, is rapidly and completely metabolized to form inactive metabolites, which would be eliminated in the urine in body (Markas, 1994). However, overdosing and the chronic use of PR produces toxic metabolite accumulation that will cause skin rashes and inflammation of the pancreas (Shiroma et al., 2012). An electrochemical sensor was prepared by modifying graphene on glassy carbon electrode surface for PR detection with a linear range of 1.0 10 7–2.0 10 5 mol/L. A quasi-reversible redox process of PR at the modified electrode and the significant decrease of overpotential of PR were attributed to the electrocatalytic activity of graphene (Kang et al., 2010). A layer of MIP film was reported to be electropolymerized onto multiwalled carbon nanotubes modified electrode surface for PR detection and recognition. The sensor not only recognized PR from its possible interfering substances, but also sensitively detected PR with a linear range of 2.0 10 7–4.0 10 5 mol/L (Peng et al., 2014). Electrochemical sensors based on multiwalled carbon nanotubes and dopamine nanospheres functionalized with gold nanoparticles (Liu et al., 2014), Pd/graphene oxide nanocomposite (J. Li et al., 2014a, Y. Li et al., 2014b), nanogolds (Goyal et al., 2005), have been reported for PR determined with some satisfactory results. These nanomaterials used for sensor preparation can improve the sensitivity of the sensor. Herein, two layers of polymer films were electrpolymerized on glassy carbon electrode (GCE) surface successively to fabricate a novel and facile electrochemical sensor. ABSA was chosen as a monomer to form the first layer of polymer, providing a conductive and catalytic platform for PR sensitive detection. And o-phenylenediamine (OPD) was chosen as a monomer for the second film preparation in the presence of PR to form MIP, endowing the sensor with good selective recognition capacity toward PR.
purification. p-aminobenzene sulfonic acid (ABSA) and paracetamol (PR, 499%) were ordered from Aladdin (Aladdin, China). o-phenylenediamine (OPD), 4-nitrophenol (4-NP), and ascorbic acid (AA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Uric acid (UA), dopamine (DA), and hydroquinone (HQ) were provided by Sigma (Sigma, USA). All other reagents were of at least analytical-reagent grade, and double-distilled deionized water was used for all solutions. 2.2. Apparatus Electrochemical experiments, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) were performed on CHI 660C workstation (ChenHua Instruments Co., Shanghai, China) with a conventional three-electrode system. A bare or modified glassy carbon electrode (GCE) was served as a working electrode. A saturated calomel electrode and a platinum wire electrode were used as a reference electrode and a counter electrode, respectively. Field emission scanning electron microscope (FE-SEM) images were obtained on an S-4800 field emission scanning electron microanalyser (Hitachi, Japan). High-performance liquid chromatography (HPLC, Hitachi L-7100, UV-detector, λ ¼214 nm, VP-ODS C18 150 mm, t¼ 25 °C, mobile phase 0.5% acetic acid in H2O 10%: methanol 90%). 2.3. Preparation of pABSA modified GCE (pABSA/GCE) Prior to the modification, the bare GCE were polished by 0.3 μm alumina slurry on micro-cloth pads and sonicated subsequently in water. Then the clean GCE was immersed into 5 mL phosphate buffer solution (PBS, 0.1 mol/L, pH 7.0) containing 2 10 4 mol/L ABSA. Then CV method was performed from 1.5 V to þ2.5 V for 15 cycles at a scan rate of 100 mV/s, obtaining polymer film modified electrode (pABSA/GCE). 2.4. Fabrication of MIP modified pABSA/GCE (MIP/pABSA/GCE) The prepared pABSA/GCE was immersed into PBS (0.1 mol/L, pH 5.0) containing 5.0 10 3 mol/L PR and 5.0 10 3 mol/L OPD. Then CV was performed from 0.0 V to þ0.8 V for 35 cycles at a scan rate of 75 mV/s, obtaining polymer modified pABSA/GCE. Subsequently, the embedded PR molecules were extracted by incubating the modified electrode into ethanol for 20 min until no obvious oxidation peak of PR could be observed by DPV method, getting MIP modified pABSA/GCE (MIP/pABSA/GCE). The procedure of the fabrication of MIP/pABSA/GCE was depicted in Fig. 1A. As a control, non-molecular imprinting polymer modified electrode (NIP/pABSA/GCE) was prepared and treated in exactly the same way except for the omitting of PR in the electropolymerization process. In order to investigate the electrocatalytic effect of pABSA, other two modified electrodes were prepared by direct electropolymerization of OPD on GCE surface in the presence and absence of PR, which were assigned as MIP/GCE and NIP/GCE, respectively. 2.5. Electrochemical properties measurements
2. Experimental 2.1. Chemicals Commercially available reagents were used without further
Electrochemical measurements to characterize the prepared sensor were carried out in PBS by using CV, EIS, and DPV methods. AA, UA, DA, 4-NP, and HQ were selected as coexisted or structural similar compounds to evaluate the recognition capacity of the prepared sensor.
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Fig. 1. Schematic illustrations of the fabrication procedure of MIP/pABSA/GCE (A) (DPV curves: with the electrocatalysis of pABSA, a higher current response showed on MIP/ pABSA/GCE than that on MIP/GCE), SEM images of pABSA/GCE (B) and MIP/pABSA/GCE (C).
3. Results and discussion 3.1. Electrocatalytic property of pABSA/GCE Fig. 2 showed the CV curves, which were recorded on bare GCE and pABSA/GCE in PBS (0.1 mol/L, pH 7.0) with or without 1.0 10 4 mol/L PR. Compared with the CV curve recorded in blank electrolyte (curve a), an irreversible oxidation peak of PR could be found on bare GCE at þ0.484 V in accordance with the reported results (Su and Cheng, 2010) (curve b). Several pairs of redox peaks appeared in CV curve when it was recorded on pABSA/GCE in PR solution (curve d). To ascribe these redox peaks, another CV curve was recorded by immersing pABSA/GCE in blank electrolyte (curve c). As could be seen, except for a pair of redox peaks appeared at þ0.458 V and þ0.440 V, other redox peaks appeared at the same potentials in curve b and curve d, indicating that these redox peaks were caused by the redox of pABSA itself and the peaks at þ 0.458 V and þ0.440 V were certainly caused by the redox of PR, which corresponded to a two-proton and twoelectron process (Eq. (1)) (Fanjul-Bolado et al., 2009).
The oxidation peak of PR shifted negatively about 26 mV and
Fig. 2. CV curves recorded on bare GCE in the absence of (a) and in the presence of (b) 1.0 10 4 mol/L PR; CV curves recorded on pABSA/GCE in the absence of (c) and in the presence of (d) 1.0 10 4 mol/L PR.
the peak current at 0.458 V on pABSA/GCE was about seven times larger than that on bare GCE. The results of the increase of peak current and negative shift of oxidized peak potential demonstrated the obvious electrocatalytic characteristics of pABSA for the redox
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of PR, which should improve the sensitivity of the sensor for PR detection. 3.2. Characterization of MIP/pABSA/GCE SEM images of pABSA/GCE and MIP/pABSA/GCE shown in Fig. 1B and C were used to characterize their morphological structures, respectively. A layer of film with scraggy and uneven surface was found in Fig. 1B, indicating the pABSA has been electropolymerized onto the surface of GCE. As MIP film was formed onto pABSA/GCE, a much rougher surface was observed (Fig. 1C), which could be attributed to the electropolymerization of MIP film. The obvious difference on the surface morphologies confirmed that two layer of pABSA and MIP films have been successively prepared onto GCE surface. Electrochemical impedance spectroscopy (EIS) has been carried out to investigate the stepwise constructed process of the sensor. The results were shown in Fig. S1, which also demonstrated the successfully preparation of the sensor. In order to investigate the specific adsorption of MIP and the electrocatalytic effect of pABSA toward the prepared sensor, the electrochemical responses of PR on MIP/GCE, MIP/pABSA/GCE, and NIP/pABSA/GCE were recorded using DPV method, as shown in Fig. 3. Compared with the DPV curve recorded on MIP/GCE (curve b), peak current increased remarkably when DPV was carried out on MIP/pABSA/GCE (curve f), demonstrating the high catalytic activity of pABSA film to the adsorbed PR molecules. It is also apparent that the MIP/pABSA/GCE displayed a much higher current response than that on NIP/pABSA/GCE (curve d) under the same concentration of PR. The high elecrochemical response of the imprinted sensor might result from the imprinted cavities in the MIP and the functional groups in the cavities produced by the template molecules. Therefore, MIP/pABSA/GCE possessed an evident specific adsorption capacity and high electrocatalytic activity to PR. The accumulation time was investigated by recording the current responses of PR on MIP/pABSA/GCE and MIP/GCE, which were immersed in PR solution (5.0 10 5 mol/L) for different time. As shown in Fig. S2, the adsorption of PR on MIP/GCE could not reach equilibrium within 10 min. Compared with MIP/GCE, MIP/pABSA/ GCE showed a faster accumulation equilibrium time within almost 2 min, which could be attributed to catalysis capacity of the modified pABSA film in MIP/pABSA/GCE.
3.3. Optimization of conditions for MIP/pABSA/GCE preparation Different influencing factors including scan cycles for pABSA electropolymerization, scan cycles and scan rate for MIP electropolymerization, and the molar ratio between template molecule and monomer were investigated to fabricate an efficient sensor. DPV was employed to detect the oxidized peak current of PR under different conditions. The pABSA film modified on GCE surface was carried out by CV method with varied scan cycles. As shown in Fig. S3, the peak current of 1.0 10 4 mol/L PR increased on pABPA/GCE with the increase of polymerized cycles from 5 to 15, while decreased with continued increase of the cycles. The reason could be attributed to two competition effects, i.e., enhancing cation-exchange activity of coating and suppressing effect due to the thicker polymer that could reduce the conductivity (Wang et al., 2014a,b). In the imprinted polymers preparation experiments, an imprinted factor (IF, the ratio of oxidized peak currents of 1.0 10 4 mol/L PR recorded on MIP/pABSA/GCE and NIP/pABSA/ GCE) was calculated and compared under each condition. The thickness of the polymer membrane influences the sensitivity of the imprinted electrochemical sensor, which could be controlled by the scanning cycles. As shown in Fig. S4, the low IF was obtained when scan cycles was less than 35, which could be attributed to the formation of fewer imprinted cavities on the electrode surface. However, the IF decreased while the MIP prepared by scanning over 35 cycles, which probably because PR molecules could not be removed completely from the polymer matrix if the imprinted film was too thick. Moreover, accession to the deeply imprinted sites is difficult for the template molecules because of the high mass-transfer resistance, which decreases the detected sensitivity. Therefore, the MIP obtained by scanning 35 cycles achieved the highest sensitivity to PR. Fig. S5 showed the effect of scan rate for MIP film electropolymerization. A tight film would be produced when MIP film was prepared at a slower scan rate, which decreased the accessibility of template molecule to imprinted sites. The IF on the MIP/ pAPBA/GCE was found to increase with an increase of scan rate up to 75 mV/s and decrease as the scan rate increased above that value. A low recognition capacity could be found when a faster scan rate was performed to get a loose and rough film, which would decrease the recognition capacity of MIP film. Thus, the optimum polymerization scan rate was found to be 75 mV/s. The molar ratio between template molecule and monomer used in the polymerization process would affect the amount of imprinted sites in the polymer matrix, further influence the electrochemical behavior of the sensor. A series of MIP/pABSA/GCE sensors were prepared under different molar ratio between template molecules and monomers from 2:1 to 1:3. As shown in Fig. S6, the IF was found to increase with the increase of molar ratio up to 1:1. A considerable decrease of IF below and above this molar ratio was observed. It can be concluded that the optimum molar ratio between template molecules and monomers was about 1:1. 3.4. Selectivity of MIP/pABSA/GCE
Fig. 3. DPV curves recorded on MIP/GCE in the absence of (a) and in the presence of (b) 1.0 10 4 mol/L PR; DPV curves recorded on NIP/pABSA/GCE in the absence of (c) and in the presence of (d) 1.0 10 4 mol/L PR; DPV curves recorded on MIP/ pABSA/GCE in the absence of (e) and in the presence of (f) 1.0 10 4 mol/L.
AA, UA, DA, 4-NP, and HQ were selected as interferents to evaluate the recognition capacity of the prepared sensor. The selective experiments were carried out by detecting the current response of 1.0 10 4 mol/L PR or each interferent with the same concentration of PR on MIPs/pABSA/GCE and NIPs/pABSA/GCE. The comparison result was illustrated in Fig. 4. Compared with the current change of AA, UA, DA, and 4-NP on MIPs/pABSA/GCE, the current resulted from PR with the same concentration showed much higher current response, and almost no oxidation peak could be found whether on MIP/pABSA/GCE or NIP/pABSA/GCE
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Fig. 4. Selectivity of the sensor. DPV current response recorded on MIP/pABSA/GCE and NIP/pABSA/GCE in PR or each interferent solution with the same concentration of 1.0 10 4 mol/L. Each was measured for three repetitive determinations. (RSD o7.65%). ΔI denoted the difference of currents of DPV curves recorded at þ 0.45 V with and without PR in the electrolyte.
when the DPV was carried out in the presence of HQ. The reason may be that the binding sites in imprinted sensor were complementary to PR in terms of the size, shape, and position of their functional groups. These results demonstrated an acceptable selectivity of the sensor for PR. 3.5. Analytical application of the sensor The dependence of the oxidation peak current of PR on MIP/ pABSA/GCE was recorded by DPV under the optimized conditions. As shown in Fig. 5A, the current response increased with the successive addition of PR. Calibration curve a in Fig. 5B revealed that the peak currents were proportional to the concentrations of PR in the range of 5.0 10 8 1.0 10 4 mol/L with a linear regression equation of I (μA) ¼ 0.5310 þ0.171c (μmol/L) (where c is the concentration of PR). And a detected limit calculated was 4.3 10 8 mol/L. Compare with the linear range and detected limit reported in previous studies summarized in Table S1, the present electrochemical sensor showed an excellent detection capacity to PR. The same detection procedure was carried out on NIP/pABSA/GCE and MIP/GCE for comparison. The linear range of 5.0 10 6 1.0 10 4 mol/L obtained on NIP/pABSA/GCE
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(Fig. 5Bb) with a linear regression equation of I (μA)¼ 0.2327 þ0.058c (μmol/L) was much narrower than that on MIP/ pABSA/GCE, which could be attributed to the imprinted cavities in MIP/pABSA/GCE for much more PR molecules adsorption. The same linear range as NIP/pABSA/GCE was obtained on MIP/GCE (Fig. 5Bc) with a linear regression equation of I (μA)¼ 0.7331þ 0.023c (μmol/L). Compared with MIP/GCE and NIP/pAPBA/GCE, the highest sensitivity of MIP/pABSA/GCE confirmed the good electrocatalytic capacity of pABSA and the specific adsorption capacity of MIP. To investigate the regeneration of the constructed sensor, MIP/ pABSA/GCE was used to detect 1.0 10 4 mol/L PR for five times with subsequent cycles of extraction and measuring operations. According to the current responses, a relative standard deviation (RSD) was calculated to be 2.58% (Fig. S7a). The reproducibility of the sensor was investigated by detecting PR on five different MIP/ pABSA/GCE with a RSD of 2.66%, which were prepared under the same conditions (Fig. S7b).The current response of the imprinted sensor decreased to 97.6% after storing for 18 days at 4 °C. These results demonstrated that the prepared sensor had acceptable reproducibility, regeneration, and stability. To evaluate the practical performance of the prepared sensor, two kinds of PR commercial tablets (500 mg/tablet) were determined on MIP/pABSA/GCE. The tablets were ground to powders and then added with ethanol to dissolve PR. After the centrifugation, the supernate was collected and diluted to a working concentration range for electrochemical detection. By using the prepared sensor, the concentration of PR in two kinds of tablets was determined. As shown in Table 1, the results were in good agreement with the manufacturers’ stated contents of PR. Recovery tests were carried out for assay in human urine samples. The samples were obtained from human after 4 h of administration of a tablet containing 500 mg of PR. Prior to analysis, the urine samples were diluted 100 times with PBS. Then the urine sample was added with a known concentration of PR. Using the prepared sensor to detect the concentration of PR in different urine samples before and after spiking, the results obtained were concluded in Table 1. The results showed that the recoveries from the urine samples were excellent, and varied from 96.33% to 102.4%. In order to validate the electrochemical detection, we have detected PR in tablets and human urine samples by HPLC. The results obtained by HPLC also have been included in Table 1, which indicated that the prepared sensor can be used as an effective and reliable sensing platform for detecting PR in real samples.
Fig. 5. DPV curves recorded on MIP/pABSA/GCE with the successive addition of PR (a to l were 5.0 10 8, 1.0 10 7, 5.0 10 7, 1.0 10 6, 3.0 10 6, 5.0 10 6, 7.0 10 6, 1.0 10 5, 3.0 10 5, 5.0 10 5, 7.0 10 5 and 1.0 10 4 mol/L) (A); the calibration plot of the concentration of PR vs. peak current (B) recorded on MIP/ pABSA/GCE (a), NIP/pABSA/GCE (b), and MIP/GCE (c) Each was measured for three repetitive determinations and RSD were calculated to be between 0.48% and 3.57%. ΔI denoted the difference of currents of DPV curves recorded at þ 0.45 V with and without PR in the electrolyte.
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Table 1 Determination of PR in tablets and human urine samples. Samples
Labeled (mmol/ L)
Added (mmol/L)
Detected contentby the sensor (mmol/L)
Recovery (%) RSD (%) Detected content by HPLC (mmol/L)
Recovery (%) RSD (%)
Tablet 1
0.30 0.50
– –
0.31 0.52
102.7 103.0
1.51 3.72
0.29 0.49
95.03 97.90
1.35 5.79
Tablet 2
0.30 0.50
– –
0.32 0.51
105.7 102.6
2.74 2.31
0.28 0.49
91.25 99.74
2.51 4.07
Urine sample 1 – –
0.30 0.50
0.31 0.49
102.4 98.34
5.62 4.68
0.31 0.51
104.0 101.6
4.31 1.66
Urine sample 2 – –
0.30 0.50
0.29 0.49
96.33 97.40
4.55 3.08
0.30 0.51
99.36 101.9
2.07 4.99
4. Conclusion
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Acknowledgements We greatly appreciate the support of the National Natural Science Foundation of China for young program (21005002), Anhui Provincial Natural Science Foundation, China for Young Program (11040606Q35), Anhui University Provincial Natural Science Foundation Key program (KJ2010A138). We gratefully acknowledge Dr. Yunchun Liu for her kind help in HPLC experiments.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.bios.2015.04.037.
