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Polyfurfural film modified glassy carbon electrode for highly sensitive nifedipine determination
Electrochimica Acta 186 (2015) 465–470
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Polyfurfural film modified glassy carbon electrode for highly sensitive nifedipine determination Qiang Zeng, Tianyan Wei, Min Wang, Xinjian Huang, Yishan Fang, Lishi Wang* Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 September 2015 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online 29 October 2015
A sensitive and convenient electrochemical strategy is developed for the determination of nifedipine basing on a polyfurfural film modified glassy carbon electrode (GCE) by the one-step electropolymerization of furfural. The prepared polyfurfural film/GCE exhibits excellent electrocatalytic activity towards nifedipine. A series of experimental parameters including the pH of supporting electrolyte, accumulation time and potential for nifedipine is also considered and optimized. Under optimal conditions, the proposed nifedipine sensor has a wide linear detection range from 1 108 to 7 106 mol dm3, with a low detection limit of 5 109 mol dm3. The proposed nifedipine sensor also displays excellent selectivity, stability and reproducibility. In particular, it shows splendid analytical performance for the determination of nifedipine in real pharmaceutical and human urine samples in practice. ã 2015 Published by Elsevier Ltd.
Keywords: Polyfurfural film Electropolymerization Modified electrode Nifedipine sensor Real samples
1. Introduction As an efficient calcium channel blocker, 1,4-dihydro-2,6dimethyl-4-(2-nitrophenyl)-3,5-pyridine-dicarboxylic acid dimethyl ester known as nifedipine (Chart 1) has been extensively applied to the treatment of angina pectoris, arterial hypertension and various cardiovascular diseases [1,2]. However, many undesired side effects, such as nausea, vomiting, dizziness, pounding heartbeats are caused by the overdose of nifedipine in use [3,4]. Thus, a reliable and sensitive method is highly necessary and important for the detection of nifedipine in practice. Various chromatographic and spectroscopic methods have been previously reported for the quantitative determination of nifedipine, such as high performance liquid chromatography (LC) in conjunction with a UV detector [5,6], gas chromatography (GC) coupled to an electron capture detector [7,8], the combination of gas chromatography and mass spectrophotometry (GC-MS) [9], multivariate image analysis-thin layer chromatography [10], LC–MS/MS [11], spectrophotometric detection [12], and fluorometry [13]. However, the main disadvantage of these methods is their timeconsuming and complicated experimental processes.
* Corresponding author at: Room 222, 15th Building, South China University of Technology, 510640. Fax: +86 20 87112906. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.electacta.2015.10.141 0013-4686/ ã 2015 Published by Elsevier Ltd.
Comparing to the conventionally chromatographic and spectroscopic methods, electrochemical monitoring becomes an attractive alternative for the determination of nifedipine because of its rapid response, simplicity of operation as well as the minimal sample pretreatment involved [14–16]. In particular, nifedipine is well known as an electroactive compound, having two redox centers [2,17,18], i.e., the nitroaromatic and dihydropyridine (DHP) groups where the former is electrochemically reducible and the latter is electrooxidizable. Based on these specific electrochemical properties, significant research effort has been made in developing electrochemical nifedipine sensors by various electrode materials, such as b-cyclodextrin incorporated multi-walled carbon nanotubes [17], activated glassy carbon electrode (activated GCE) [2], Ag nanoparticles modified glassy carbon electrode (Ag nanoparticles/ GCE) [19] and modified carbon paste electrode (CPE) [20] or a boron-doped diamond electrode for the quantification of nifedipine-like substances e.g., amlodipine [21]. For the determination of nifedipine, these materials did their jobs, however, the resulted sensors lack satisfied sensitivity. On the other hand, polymermodified electrodes have been also put in the spotlights of sensing applications with superiorities of good stability, containing multiple active sites, homogeneity in electrochemical deposition and strong adherence to electrode surface [22–26]. The strategy of electropolymerization provides advantages to immobilize polymers (e.g., poly-aniline film [27], poly-o-phenylenediamine film [28], poly (naphthol green B) film [29]) onto an electrode surface, giving controllable film thickness, permeation and charge
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NO2 H3COOC
H 3C
COOCH 3
N H
CH3
Chart 1. Chemical structure of nifedipine.
transport characteristics by easily adjusting the electrochemical parameters [30]. In particular, the polyfurfural film on different electrodes such as Pt electrode [31,32], low carbon steel [33] and GCE [34] shows good adherence and electrical conductivity, avoiding a complicated electrode preparation process. Moreover, the polyfurfural film modified GCE also exhibits excellent electrocatalytic activity to the oxidation of hydroxy and the reduction of nitro reported by our research group recently [34,35] due to the promoted electron transfer by the conjugated p-electron backbones of polyfurfural [32,34]. In this contribution, a polyfurfural film modified GCE has been fabricated to study the electrochemical behavior of nifedipine by a convenient one-step electropolymerization method. In particular, the polyfurfural film/GCE shows admirable electrocatalytic activity towards nifedipine. Effects of different parameters on the ability of this modified electrode toward the reduction process of nifedipine have been also investigated. Under optimum conditions, differential pulse adsorptive striping voltammetry (DPAdSV) is applied to the direct determination of nifedipine, achieving a significantly wide linear range from 1 108 to 7 106 mol dm3 with a detection limit of 5 109 mol dm3. The proposed sensor is highly sensitive, stable and reproducible and successfully used in the quantitative determination of nifedipine in real pharmaceutical and human urine samples. To best of our knowledge, it is the first report of the electrochemical determination of nifedipine by using the polyfurfural film/GCE. This developed method would have a tremendous meaning to pharmaceutical and biological samples analysis. 2. Experimental 2.1. Reagents and solutions Nifedipine (analytical grade) and Furfural (GC, 98%) were acquired from Aladdin Chemical Reagent Co. Ltd. (Shanghai, China). Sodium perchlorate was obtained from Fuchen Chemical Reagent Company (Tianjin, China). All other chemicals were of analytical reagents grade and used without further purification. Britton-Robinson (BR) buffers solution of pH 3-11 (mixtures of 0.04 mol dm3 of acetic, 0.04 mol dm3 of orthophosphoric, and 0.04 mol dm3 of boric acids; adjusted to the required pH with 0.2 mol dm3 of sodium hydroxide solution) was prepared. Stock solution of nifedipine was prepared as 0.001 mol dm3 in methanol and diluted to different concentrations before use by mixing with BR (pH 9.0). The stock solution should be stored in the dark under refrigeration to avoid decomposition and it showed excellent stability under these conditions for at least 30 days. All aqueous solutions were prepared using double-distilled water. To prepare real pharmaceutical samples, two tablets of nifedipine (labeled 10.0 mg, Sinopharm Group Co. Ltd, Guangzhou, China) were weighed and grounded to a homogeneous fine powder by using a mortar with pestle and then the obtained powder was dissolved in 10 mL methanol by ultrasonication. The adequate
amount (1 mL) of prepared solution with particular amount of nifedipine (giving finally added concentrations as 1.0, 2.0, 3.0 and 4.0 mmol dm3, respectively) was diluted to 100 mL by the BrittonRobinson (BR, pH 9.0) buffer solution. Finally, the resulted solution was transferred to electrochemical cell for the voltammetric determinations. Human urine samples were collected from four healthy volunteers who did not undergo any treatments by pharmaceuticals containing nifedipine. Each sample (5 mL) was centrifuged for 10 min at 12,000 rpm to separate the aqueous and organic layers and the supernatant was filtered using a 0.45 mm filter. The filtrate was diluted by BR buffer solution (pH 9.0) in a 1:3 (urine:buffer) volume ratio to 20 ml. By standard addition method, extra nifedipine was added to these samples for further analysis, making the added concentration of nifedipine as 0.05, 0.1, 0.2 and 0.3 mmol dm3, respectively. 2.2. Fabrication of polyfurfural film modified glassy carbon electrode A 3 mm diameter bare GCE (Xianren, Shanghai, China) was sequentially polished with 0.3 and 0.05 mm alumina powder to obtain a mirror-like surface prior to use, then washed ultrasonically in anhydrous ethanol and doubly distilled water for 5 min, respectively. The cleaned GCE was dried with nitrogen stream for further modification. The electropolymerization of furfural was performed in acetonitrile containing furfural (0.01 mol dm3) and sodium perchlorate (0.06 mol dm3) by cyclic voltammetry between 0.8 V and +2.8 V at a scan rate of 100 mV s1 for 5 cycles. After electropolymerization, the polyfurfural film modified electrode was washed with doubly distilled water to remove all chemicals which were physically absorbed. 2.3. Apparatus and method Electrochemical measurements were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China) with a conventional three-electrode cell. In the process of electropolymerization, a three-electrode configuration consisting of a bare GCE as the working electrode, a Ag/AgCl (0.01 mol dm3 of NaClO4 in acetonitrile) electrode as the reference electrode and a platinum wire as the counter electrode was used. While in the process of determination, the polyfurfural film/GCE working electrode and the saturated calomel reference electrode (SCE) were utilized instead. Electrochemical impedance spectroscopy (EIS) was obtained with a wide frequency range from 0.1 Hz to 10 kHz in 0.1 mol dm3 of KCl solution containing 5 mmol dm3 of K4[Fe(CN)6]/K3[Fe(CN)6] with a sine wave of 5 mV amplitude. The surface morphology was characterized using a field emission scanning electron microscope (FE-SEM; Zeiss Ultra55, Germany). Before each measurement, the accumulation procedures of working solution at the working electrode were carried out at open-circuit for 75 s while the solution was stirred at 400 rpm with a magnetic stirrer. DPAdSV and CV signals were recorded in BR buffer solution (pH 9.0) containing different concentration of nifedipine. For DPAdSV, the parameters such as amplitude (0.05 V), pulse width (0.05 s) and pulse period (0.5 s) have been optimized and used in all measurements. For the determination of nifedipine, the detection limit, Cm, was obtained using equation Eq. (1): Cm = 3Sb/m
(1)
where m is the slope of the calibration plot (1.489 mA mmol dm3) in the linear range (0.01 to 7 mmol dm3), and Sb is the standard deviation of the blank response which is obtained from 20 replicate measurements of the blank BR buffer solution.
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3. Results and Discussion 3.1. Characterization of polyfurfural film modified glassy carbon electrode The as-prepared polyfurfural film film/GCE was characterized by both SEM and EIS. As reported previously [34], the SEM image displayed a clear view of a turquoise and uniform polyfurfural film formed on GCE electrode. EIS was also used to probe the interfacial property changes at GCE surface before and after modification by the charge transfer resistance (Rct). Rct of the polymer modified GCE increased dramatically in comparison with that of the bare GCE, suggesting that the polyfurfural film hinders the interfacial charge transfer between the electroactive species in the electrolyte solution and the electrode surface. Therefore, the evidences from both SEM and EIS showed good agreement that a polyfurfural film was successfully coated on GCE. 3.2. Electrochemical behavior of nifedipine at polyfurfural film/GCE The electrochemical behaviours of nifedipine at a bare GCE and a polyfurfural film/GCE were studied by DPAdSV in BR buffer solution (pH 9.0). In Fig. 1, nifedipine showed an irreversible reduction peak at 0.840 V at the base GCE due to the reduction of the nitro-aromatic group to the hydroxylamine derivative [36–38]. And the reaction mechanism was presented in Eq. (2). R-NO2 + 4H+ + 4e ! R-NHOH + H2O
(2)
At the polyfurfural film/GCE, the reduction potential of nifedipine slightly shifted from 0.840 V to 0.812 V and the peak current response was approximately 17-fold higher than that found at the bare GCE. These observations suggest that the reduction of nifedipine can be significantly electro-catalysed by the polyfurfural film. 3.3. The optimum detection conditions In order to enhance the sensitivity of the polyfurfural film/GCE to nifedipine, the method of accumulation was used. As shown in Fig 2, the reduction peak current of 30 mmol dm3 of nifedipine at open-circuit increased gradually with the accumulation time and reached the maximum value when the accumulation time is 75 s. Thus, the optimal accumulation time of 75 s was employed in further experiments. On the other hand, the effect of accumulation potentials on reduction peak current of nifedipine was also investigated with the optimal accumulation time determined above. The experimental data (not shown) revealed that the reduction peak currents were independent on accumulation
Fig. 2. The effect of accumulation time on the reduction peak current of 30 mmol dm3 of nifedipine in BR (pH 9.0) buffer solution at the polyfurfural film/GCE. Accumulation potential was at open-circuit.
potentials. Therefore, an accumulation potential at open-circuit was chosen for nifedipine in subsequent experiments. On the other hand, pH values of electrolyte solutions also showed significant effect on the electrochemical behavior of nifedipine because proton transfer was involved in its overall electrode reaction. In Fig. 3A, the polyfurfural film showed the electrocatalytic activity towards the reduction of nifedipine over the wide range of pH values (3.0–11.0). In particular, the reduction peak current of nifedipine at the polyfurfural film/GCE increased when the pH value of the buffer solution varied from 3.0 to 9.0. After reaching to a maximum at pH 9.0, the reduction peak current dropped rapidly. This observation is attributed to existed mutual repulsion between the reduced nitro-aromatic and negatively charged modified film on the electrode surface at higher pH. In addition, the cathodic peak potential of nifedipine shifted to a more negative value along with the increasing pH (Fig. 3B). A plot of the peak potential versus pH was found to be linear over the pH range of 3.0–11.0 with a linear regression equation Ep(V) = 0.0504 pH – 0.353 (R = 0.9989). According to Eq. (3) [39]: dEp/dpH = 2.303mRT/nF
(3)
where m and n refer to the transferring number of proton and electron, respectively. The value of m/n in this reaction was calculated to be 0.85 and equaled to 1.0 approximately, indicating that the transferring number of proton and electron was same. This result was in line with the reaction mechanism that the nitryl group was reduced to the hydroxylamine in the nifedipine molecule with four electrons and four protons transferring. Therefore, a pH value of 9.0 was chosen as the optimum pH value for latter quantification in this study. Fig. 4 showed the effect of scan rate on the electrochemical response of 30 mmol dm3 nifedipine at the polyfurfural film/GCE in pH 9.0 BR buffer solution with scan rate ranging from 10 to 100 mV s1. The reduction peak current increased linearly with the increase of scan rate, with a linear regression equation of Ip (mA) = 0.302y (mV s1) – 5.864 (R = 0.9993), which indicated that the reduction of nifedipine at the polyfurfural film/GCE was a typical adsorption-controlled process. Moreover, the reduction peak potential exhibited a negative shift of 50 mV when scan rate increased from 10 to 100 mV s1. This observed behaviour is probably due to slow heterogeneous electron transfer or ohmic drop of the polyfurfural film. 3.4. Calibration curve of nifedipine
Fig. 1. DPAdSVs of 30 mmol dm3 nifedipine at (a) bare GCE and (b) the polyfurfural film/GCE in BR (pH 9.0) buffer solution.
For the quantitative analysis of nifedipine using the polyfurfural film/GCE, an analytical curve was obtained by DPAdSV under the
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Fig. 3. The relationship of (A) pH vs. cathodic peak current (Ip) and (B) pH vs. cathodic peak potentials for nifedipine at obtained from DPAdSVs which measured in 30 mmol dm3 nifedipine at different pH values.
Fig. 4. CVs of 30 mmol dm3 nifedipine at the polyfurfural film/GCE at different scan rates (10, 20, 40, 60, 80, 100 mV s1). Inset: The relationship between the Ip values and scan rates.
optimized experimental conditions. In Fig. 5, the cathodic peak current was proportional to nifedipine in the range from 1 108 to 7 106 mol dm3 and the linear regression equation (Fig. 5, inset) can be expressed as Ip (mA) = 1.489C (mmol dm3) –0.0796, with a correlation coefficient (R) of 0.9994. The detection limit (LOD) was determine as 5 109 mol dm3 based on a signal-to-noise ratio of 3 (S/N = 3) by Eq. (1) and the sensitivity was 21.08 mA/mmol dm3 cm2. The therapeutic range of the serum concentration of nifedipine is from 25 to 100 mg dm3 (7.2 108 to 2.9 107 mol dm3) [40], which is significantly covered by the detection range of this proposed sensor even with an upper detection limit up to 24 times overdose of nifedipine. A detailed comparison of the performances of different electrochemical sensors for the determination of nifedipine was summarized in Table 1. In contrast with the previously reported electrochemical sensors using different electrodes, the polyfurfural film/GCE showed a wider linear range with a low detection limit. But most importantly, the one-step preparation of polyfurfural film/GCE was much easier compared with the reported nanomaterials modified electrodes, such as Ag nanoparticles/GCE [19], MWCNTs/b-CD/CPE [20] and hollow PdAg alloy nanoparticles modified ionic liquid functionalized graphene nanoribbons/GCE (GRs-IL-hPdAg/GCE) [41].
Fig. 5. Background-subtracted DPAdSVs of 30 mmol dm3 nifedipine at the polyfurfural film/GCE in BR (pH 9.0) buffer solution. Concentrations: 0.01, 0.05, 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 6 and 7 mmol dm3 (from a to m). Inset: The relationship between the Ip values and concentration.
3.5. Selectivity, stability and reproducibility of the polyfurfural film/ GCE In order to evaluate the selectivity of the polyfurfural film/GCE for the determination of nifedipine (30 mmol dm3), the effects of different potentially interfering substances were investigated under optimum conditions. The tolerance limit was defined as the maximum concentration of the potentially interfering substance that caused an error less than 5.0 %. The results showed that 100-fold concentration excess of Na+, Br, K+, NO3, Cu2+, SO42, Mg2+, Cl, Mn2+, Al3+, Ca2+, Ac, CO32 did not affect the reduction peak current of nifedipine. Similarly, no interferences were observed in 10-fold excess of glucose, maltose, ascorbic acid, dopamine, tartaric acid, oxalic acid, L-phenylalanine, Lhistidine under the given experimental conditions. From these results, it may be concluded that the proposed electrode was free from interference by most potentially interfering substances in practical conditions. Stability is a key factor for sensors in their practical applications. The polyfurfural film/GCE possessed good stability for the determination of nifedipine even after stored in refrigerator
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Table 1 Comparison of performances of the polyfurfural film/GCE with other modified electrodes. Modified electrode
Method
Liner range (mol dm3)
LOD (mol dm3)
Ref
Activated GCE Ag NPs/GCE MWCNTs/b-CD/CPE GRs-IL-hPdAg/GCE Polyfurfural film/GCE
LSV DPV DPV DPV DPV
2 105–6 104 8 107–6 105 4.77 108–2.00 105 1 108–4 106 1 108–7 106
– 7.2 107 1.48 108 4 109 5 109
[2] [19] [20] [41] This work
Table 2 Determination of nifedipine in pharmaceutical and human urine samples. Sample (mmol dm3)
Pharmaceuticals
Sample number Blank Standard Additions Sample Responsea Recovery (%) RSD (% n=6)
1 0.8 1.0 1.83 0.04 101.7 2.5
a
Human Urine 2 0.8 2.0 2.75 0.02 98.2 2.0
3 0.8 3.0 3.76 0.03 98.9 2.4
4 0.8 4.0 4.94 0.01 102.0 2.2
10 – 0.05 0.049 0.004 98.0 2.4
20 – 0.10 0.102 0.002 102.0 2.6
30 – 0.20 0.202 0.002 101.4 2.8
40 – 0.30 0.296 0.004 98.7 2.3
Sample responses are expressed as a confidence interval of 95% probability.
at 4 C for 15 and 30 days where the reduction peak current of nifedipine (30 mmol dm3) retained 94.87% and 89.75% of its initial response, respectively. The reproducibility of the polyfurfural film/GCE was investigated by eight replicative measurements of nifedipine (30 mmol dm3) using DPAdSV under optimum conditions. The relative standard deviation (RSD) of the cathodic peak currents of nifedipine by eight replicate measurements was 2.18%. The result revealed that the polyfurfural film/GCE possessed a satisfactory repeatability for the determination of nifedipine.
This research project was funded by the National Nature Science foundation of China (21427809 and 21475046). We also acknowledge the Fundamental Research Funds for the Central Universities (No. 2015ZM050 and 2014ZM0064).
3.6. Real sample determination
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To evaluate the applicability of the polyfurfural film/GCE, the recovery of nifedipine was determined in the pharmaceutical and human urine samples by using the standard addition technique. The reduction peak current of nifedipine was measured by DPAdSV. Six parallel experiments are carried out for all measurements. The results obtained from the pharmaceutical and human urine samples are shown in Table 2. The average recoveries were ranged between 98.2% and 102.0% for the pharmaceutical samples and 98.0% and 102.0% for the human urine samples, indicating that this method can be efficiently used for the determination of nifedipine in real pharmaceutical and human urine samples.
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4. Conclusion In this work, a polyfurfural film modified glassy carbon electrode was prepared by the electropolymerization of furfural in acetonitrile solution. The modified electrode exhibited excellent electrocatalytic activity to the reduction of nifedipine. Under optimal conditions, the polyfurfural film/GCE had a low detection limit (5 109 mol dm3) and a wide linear range (1 108– 7 106 mol dm3). Additionally, the proposed nifedipine sensor was successfully applied to the determination of nifedipine in real samples with satisfactory results. Compared with other methods, this method is much more convenient, sensitive, reliable and reproductive. This study provided an efficient and novel analytical platform for determination of nifedipine and the platform shows wide potential in application of pharmaceutical and biological monitoring.
Conflict of interest There is no conflict of interest. Acknowledgements
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Polyfurfural film modified glassy carbon electrode for highly sensitive nifedipine determination Qiang Zeng, Tianyan Wei, Min Wang, Xinjian Huang, Yishan Fang, Lishi Wang* Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 September 2015 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online 29 October 2015
A sensitive and convenient electrochemical strategy is developed for the determination of nifedipine basing on a polyfurfural film modified glassy carbon electrode (GCE) by the one-step electropolymerization of furfural. The prepared polyfurfural film/GCE exhibits excellent electrocatalytic activity towards nifedipine. A series of experimental parameters including the pH of supporting electrolyte, accumulation time and potential for nifedipine is also considered and optimized. Under optimal conditions, the proposed nifedipine sensor has a wide linear detection range from 1 108 to 7 106 mol dm3, with a low detection limit of 5 109 mol dm3. The proposed nifedipine sensor also displays excellent selectivity, stability and reproducibility. In particular, it shows splendid analytical performance for the determination of nifedipine in real pharmaceutical and human urine samples in practice. ã 2015 Published by Elsevier Ltd.
Keywords: Polyfurfural film Electropolymerization Modified electrode Nifedipine sensor Real samples
1. Introduction As an efficient calcium channel blocker, 1,4-dihydro-2,6dimethyl-4-(2-nitrophenyl)-3,5-pyridine-dicarboxylic acid dimethyl ester known as nifedipine (Chart 1) has been extensively applied to the treatment of angina pectoris, arterial hypertension and various cardiovascular diseases [1,2]. However, many undesired side effects, such as nausea, vomiting, dizziness, pounding heartbeats are caused by the overdose of nifedipine in use [3,4]. Thus, a reliable and sensitive method is highly necessary and important for the detection of nifedipine in practice. Various chromatographic and spectroscopic methods have been previously reported for the quantitative determination of nifedipine, such as high performance liquid chromatography (LC) in conjunction with a UV detector [5,6], gas chromatography (GC) coupled to an electron capture detector [7,8], the combination of gas chromatography and mass spectrophotometry (GC-MS) [9], multivariate image analysis-thin layer chromatography [10], LC–MS/MS [11], spectrophotometric detection [12], and fluorometry [13]. However, the main disadvantage of these methods is their timeconsuming and complicated experimental processes.
* Corresponding author at: Room 222, 15th Building, South China University of Technology, 510640. Fax: +86 20 87112906. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.electacta.2015.10.141 0013-4686/ ã 2015 Published by Elsevier Ltd.
Comparing to the conventionally chromatographic and spectroscopic methods, electrochemical monitoring becomes an attractive alternative for the determination of nifedipine because of its rapid response, simplicity of operation as well as the minimal sample pretreatment involved [14–16]. In particular, nifedipine is well known as an electroactive compound, having two redox centers [2,17,18], i.e., the nitroaromatic and dihydropyridine (DHP) groups where the former is electrochemically reducible and the latter is electrooxidizable. Based on these specific electrochemical properties, significant research effort has been made in developing electrochemical nifedipine sensors by various electrode materials, such as b-cyclodextrin incorporated multi-walled carbon nanotubes [17], activated glassy carbon electrode (activated GCE) [2], Ag nanoparticles modified glassy carbon electrode (Ag nanoparticles/ GCE) [19] and modified carbon paste electrode (CPE) [20] or a boron-doped diamond electrode for the quantification of nifedipine-like substances e.g., amlodipine [21]. For the determination of nifedipine, these materials did their jobs, however, the resulted sensors lack satisfied sensitivity. On the other hand, polymermodified electrodes have been also put in the spotlights of sensing applications with superiorities of good stability, containing multiple active sites, homogeneity in electrochemical deposition and strong adherence to electrode surface [22–26]. The strategy of electropolymerization provides advantages to immobilize polymers (e.g., poly-aniline film [27], poly-o-phenylenediamine film [28], poly (naphthol green B) film [29]) onto an electrode surface, giving controllable film thickness, permeation and charge
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NO2 H3COOC
H 3C
COOCH 3
N H
CH3
Chart 1. Chemical structure of nifedipine.
transport characteristics by easily adjusting the electrochemical parameters [30]. In particular, the polyfurfural film on different electrodes such as Pt electrode [31,32], low carbon steel [33] and GCE [34] shows good adherence and electrical conductivity, avoiding a complicated electrode preparation process. Moreover, the polyfurfural film modified GCE also exhibits excellent electrocatalytic activity to the oxidation of hydroxy and the reduction of nitro reported by our research group recently [34,35] due to the promoted electron transfer by the conjugated p-electron backbones of polyfurfural [32,34]. In this contribution, a polyfurfural film modified GCE has been fabricated to study the electrochemical behavior of nifedipine by a convenient one-step electropolymerization method. In particular, the polyfurfural film/GCE shows admirable electrocatalytic activity towards nifedipine. Effects of different parameters on the ability of this modified electrode toward the reduction process of nifedipine have been also investigated. Under optimum conditions, differential pulse adsorptive striping voltammetry (DPAdSV) is applied to the direct determination of nifedipine, achieving a significantly wide linear range from 1 108 to 7 106 mol dm3 with a detection limit of 5 109 mol dm3. The proposed sensor is highly sensitive, stable and reproducible and successfully used in the quantitative determination of nifedipine in real pharmaceutical and human urine samples. To best of our knowledge, it is the first report of the electrochemical determination of nifedipine by using the polyfurfural film/GCE. This developed method would have a tremendous meaning to pharmaceutical and biological samples analysis. 2. Experimental 2.1. Reagents and solutions Nifedipine (analytical grade) and Furfural (GC, 98%) were acquired from Aladdin Chemical Reagent Co. Ltd. (Shanghai, China). Sodium perchlorate was obtained from Fuchen Chemical Reagent Company (Tianjin, China). All other chemicals were of analytical reagents grade and used without further purification. Britton-Robinson (BR) buffers solution of pH 3-11 (mixtures of 0.04 mol dm3 of acetic, 0.04 mol dm3 of orthophosphoric, and 0.04 mol dm3 of boric acids; adjusted to the required pH with 0.2 mol dm3 of sodium hydroxide solution) was prepared. Stock solution of nifedipine was prepared as 0.001 mol dm3 in methanol and diluted to different concentrations before use by mixing with BR (pH 9.0). The stock solution should be stored in the dark under refrigeration to avoid decomposition and it showed excellent stability under these conditions for at least 30 days. All aqueous solutions were prepared using double-distilled water. To prepare real pharmaceutical samples, two tablets of nifedipine (labeled 10.0 mg, Sinopharm Group Co. Ltd, Guangzhou, China) were weighed and grounded to a homogeneous fine powder by using a mortar with pestle and then the obtained powder was dissolved in 10 mL methanol by ultrasonication. The adequate
amount (1 mL) of prepared solution with particular amount of nifedipine (giving finally added concentrations as 1.0, 2.0, 3.0 and 4.0 mmol dm3, respectively) was diluted to 100 mL by the BrittonRobinson (BR, pH 9.0) buffer solution. Finally, the resulted solution was transferred to electrochemical cell for the voltammetric determinations. Human urine samples were collected from four healthy volunteers who did not undergo any treatments by pharmaceuticals containing nifedipine. Each sample (5 mL) was centrifuged for 10 min at 12,000 rpm to separate the aqueous and organic layers and the supernatant was filtered using a 0.45 mm filter. The filtrate was diluted by BR buffer solution (pH 9.0) in a 1:3 (urine:buffer) volume ratio to 20 ml. By standard addition method, extra nifedipine was added to these samples for further analysis, making the added concentration of nifedipine as 0.05, 0.1, 0.2 and 0.3 mmol dm3, respectively. 2.2. Fabrication of polyfurfural film modified glassy carbon electrode A 3 mm diameter bare GCE (Xianren, Shanghai, China) was sequentially polished with 0.3 and 0.05 mm alumina powder to obtain a mirror-like surface prior to use, then washed ultrasonically in anhydrous ethanol and doubly distilled water for 5 min, respectively. The cleaned GCE was dried with nitrogen stream for further modification. The electropolymerization of furfural was performed in acetonitrile containing furfural (0.01 mol dm3) and sodium perchlorate (0.06 mol dm3) by cyclic voltammetry between 0.8 V and +2.8 V at a scan rate of 100 mV s1 for 5 cycles. After electropolymerization, the polyfurfural film modified electrode was washed with doubly distilled water to remove all chemicals which were physically absorbed. 2.3. Apparatus and method Electrochemical measurements were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China) with a conventional three-electrode cell. In the process of electropolymerization, a three-electrode configuration consisting of a bare GCE as the working electrode, a Ag/AgCl (0.01 mol dm3 of NaClO4 in acetonitrile) electrode as the reference electrode and a platinum wire as the counter electrode was used. While in the process of determination, the polyfurfural film/GCE working electrode and the saturated calomel reference electrode (SCE) were utilized instead. Electrochemical impedance spectroscopy (EIS) was obtained with a wide frequency range from 0.1 Hz to 10 kHz in 0.1 mol dm3 of KCl solution containing 5 mmol dm3 of K4[Fe(CN)6]/K3[Fe(CN)6] with a sine wave of 5 mV amplitude. The surface morphology was characterized using a field emission scanning electron microscope (FE-SEM; Zeiss Ultra55, Germany). Before each measurement, the accumulation procedures of working solution at the working electrode were carried out at open-circuit for 75 s while the solution was stirred at 400 rpm with a magnetic stirrer. DPAdSV and CV signals were recorded in BR buffer solution (pH 9.0) containing different concentration of nifedipine. For DPAdSV, the parameters such as amplitude (0.05 V), pulse width (0.05 s) and pulse period (0.5 s) have been optimized and used in all measurements. For the determination of nifedipine, the detection limit, Cm, was obtained using equation Eq. (1): Cm = 3Sb/m
(1)
where m is the slope of the calibration plot (1.489 mA mmol dm3) in the linear range (0.01 to 7 mmol dm3), and Sb is the standard deviation of the blank response which is obtained from 20 replicate measurements of the blank BR buffer solution.
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3. Results and Discussion 3.1. Characterization of polyfurfural film modified glassy carbon electrode The as-prepared polyfurfural film film/GCE was characterized by both SEM and EIS. As reported previously [34], the SEM image displayed a clear view of a turquoise and uniform polyfurfural film formed on GCE electrode. EIS was also used to probe the interfacial property changes at GCE surface before and after modification by the charge transfer resistance (Rct). Rct of the polymer modified GCE increased dramatically in comparison with that of the bare GCE, suggesting that the polyfurfural film hinders the interfacial charge transfer between the electroactive species in the electrolyte solution and the electrode surface. Therefore, the evidences from both SEM and EIS showed good agreement that a polyfurfural film was successfully coated on GCE. 3.2. Electrochemical behavior of nifedipine at polyfurfural film/GCE The electrochemical behaviours of nifedipine at a bare GCE and a polyfurfural film/GCE were studied by DPAdSV in BR buffer solution (pH 9.0). In Fig. 1, nifedipine showed an irreversible reduction peak at 0.840 V at the base GCE due to the reduction of the nitro-aromatic group to the hydroxylamine derivative [36–38]. And the reaction mechanism was presented in Eq. (2). R-NO2 + 4H+ + 4e ! R-NHOH + H2O
(2)
At the polyfurfural film/GCE, the reduction potential of nifedipine slightly shifted from 0.840 V to 0.812 V and the peak current response was approximately 17-fold higher than that found at the bare GCE. These observations suggest that the reduction of nifedipine can be significantly electro-catalysed by the polyfurfural film. 3.3. The optimum detection conditions In order to enhance the sensitivity of the polyfurfural film/GCE to nifedipine, the method of accumulation was used. As shown in Fig 2, the reduction peak current of 30 mmol dm3 of nifedipine at open-circuit increased gradually with the accumulation time and reached the maximum value when the accumulation time is 75 s. Thus, the optimal accumulation time of 75 s was employed in further experiments. On the other hand, the effect of accumulation potentials on reduction peak current of nifedipine was also investigated with the optimal accumulation time determined above. The experimental data (not shown) revealed that the reduction peak currents were independent on accumulation
Fig. 2. The effect of accumulation time on the reduction peak current of 30 mmol dm3 of nifedipine in BR (pH 9.0) buffer solution at the polyfurfural film/GCE. Accumulation potential was at open-circuit.
potentials. Therefore, an accumulation potential at open-circuit was chosen for nifedipine in subsequent experiments. On the other hand, pH values of electrolyte solutions also showed significant effect on the electrochemical behavior of nifedipine because proton transfer was involved in its overall electrode reaction. In Fig. 3A, the polyfurfural film showed the electrocatalytic activity towards the reduction of nifedipine over the wide range of pH values (3.0–11.0). In particular, the reduction peak current of nifedipine at the polyfurfural film/GCE increased when the pH value of the buffer solution varied from 3.0 to 9.0. After reaching to a maximum at pH 9.0, the reduction peak current dropped rapidly. This observation is attributed to existed mutual repulsion between the reduced nitro-aromatic and negatively charged modified film on the electrode surface at higher pH. In addition, the cathodic peak potential of nifedipine shifted to a more negative value along with the increasing pH (Fig. 3B). A plot of the peak potential versus pH was found to be linear over the pH range of 3.0–11.0 with a linear regression equation Ep(V) = 0.0504 pH – 0.353 (R = 0.9989). According to Eq. (3) [39]: dEp/dpH = 2.303mRT/nF
(3)
where m and n refer to the transferring number of proton and electron, respectively. The value of m/n in this reaction was calculated to be 0.85 and equaled to 1.0 approximately, indicating that the transferring number of proton and electron was same. This result was in line with the reaction mechanism that the nitryl group was reduced to the hydroxylamine in the nifedipine molecule with four electrons and four protons transferring. Therefore, a pH value of 9.0 was chosen as the optimum pH value for latter quantification in this study. Fig. 4 showed the effect of scan rate on the electrochemical response of 30 mmol dm3 nifedipine at the polyfurfural film/GCE in pH 9.0 BR buffer solution with scan rate ranging from 10 to 100 mV s1. The reduction peak current increased linearly with the increase of scan rate, with a linear regression equation of Ip (mA) = 0.302y (mV s1) – 5.864 (R = 0.9993), which indicated that the reduction of nifedipine at the polyfurfural film/GCE was a typical adsorption-controlled process. Moreover, the reduction peak potential exhibited a negative shift of 50 mV when scan rate increased from 10 to 100 mV s1. This observed behaviour is probably due to slow heterogeneous electron transfer or ohmic drop of the polyfurfural film. 3.4. Calibration curve of nifedipine
Fig. 1. DPAdSVs of 30 mmol dm3 nifedipine at (a) bare GCE and (b) the polyfurfural film/GCE in BR (pH 9.0) buffer solution.
For the quantitative analysis of nifedipine using the polyfurfural film/GCE, an analytical curve was obtained by DPAdSV under the
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Fig. 3. The relationship of (A) pH vs. cathodic peak current (Ip) and (B) pH vs. cathodic peak potentials for nifedipine at obtained from DPAdSVs which measured in 30 mmol dm3 nifedipine at different pH values.
Fig. 4. CVs of 30 mmol dm3 nifedipine at the polyfurfural film/GCE at different scan rates (10, 20, 40, 60, 80, 100 mV s1). Inset: The relationship between the Ip values and scan rates.
optimized experimental conditions. In Fig. 5, the cathodic peak current was proportional to nifedipine in the range from 1 108 to 7 106 mol dm3 and the linear regression equation (Fig. 5, inset) can be expressed as Ip (mA) = 1.489C (mmol dm3) –0.0796, with a correlation coefficient (R) of 0.9994. The detection limit (LOD) was determine as 5 109 mol dm3 based on a signal-to-noise ratio of 3 (S/N = 3) by Eq. (1) and the sensitivity was 21.08 mA/mmol dm3 cm2. The therapeutic range of the serum concentration of nifedipine is from 25 to 100 mg dm3 (7.2 108 to 2.9 107 mol dm3) [40], which is significantly covered by the detection range of this proposed sensor even with an upper detection limit up to 24 times overdose of nifedipine. A detailed comparison of the performances of different electrochemical sensors for the determination of nifedipine was summarized in Table 1. In contrast with the previously reported electrochemical sensors using different electrodes, the polyfurfural film/GCE showed a wider linear range with a low detection limit. But most importantly, the one-step preparation of polyfurfural film/GCE was much easier compared with the reported nanomaterials modified electrodes, such as Ag nanoparticles/GCE [19], MWCNTs/b-CD/CPE [20] and hollow PdAg alloy nanoparticles modified ionic liquid functionalized graphene nanoribbons/GCE (GRs-IL-hPdAg/GCE) [41].
Fig. 5. Background-subtracted DPAdSVs of 30 mmol dm3 nifedipine at the polyfurfural film/GCE in BR (pH 9.0) buffer solution. Concentrations: 0.01, 0.05, 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 6 and 7 mmol dm3 (from a to m). Inset: The relationship between the Ip values and concentration.
3.5. Selectivity, stability and reproducibility of the polyfurfural film/ GCE In order to evaluate the selectivity of the polyfurfural film/GCE for the determination of nifedipine (30 mmol dm3), the effects of different potentially interfering substances were investigated under optimum conditions. The tolerance limit was defined as the maximum concentration of the potentially interfering substance that caused an error less than 5.0 %. The results showed that 100-fold concentration excess of Na+, Br, K+, NO3, Cu2+, SO42, Mg2+, Cl, Mn2+, Al3+, Ca2+, Ac, CO32 did not affect the reduction peak current of nifedipine. Similarly, no interferences were observed in 10-fold excess of glucose, maltose, ascorbic acid, dopamine, tartaric acid, oxalic acid, L-phenylalanine, Lhistidine under the given experimental conditions. From these results, it may be concluded that the proposed electrode was free from interference by most potentially interfering substances in practical conditions. Stability is a key factor for sensors in their practical applications. The polyfurfural film/GCE possessed good stability for the determination of nifedipine even after stored in refrigerator
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Table 1 Comparison of performances of the polyfurfural film/GCE with other modified electrodes. Modified electrode
Method
Liner range (mol dm3)
LOD (mol dm3)
Ref
Activated GCE Ag NPs/GCE MWCNTs/b-CD/CPE GRs-IL-hPdAg/GCE Polyfurfural film/GCE
LSV DPV DPV DPV DPV
2 105–6 104 8 107–6 105 4.77 108–2.00 105 1 108–4 106 1 108–7 106
– 7.2 107 1.48 108 4 109 5 109
[2] [19] [20] [41] This work
Table 2 Determination of nifedipine in pharmaceutical and human urine samples. Sample (mmol dm3)
Pharmaceuticals
Sample number Blank Standard Additions Sample Responsea Recovery (%) RSD (% n=6)
1 0.8 1.0 1.83 0.04 101.7 2.5
a
Human Urine 2 0.8 2.0 2.75 0.02 98.2 2.0
3 0.8 3.0 3.76 0.03 98.9 2.4
4 0.8 4.0 4.94 0.01 102.0 2.2
10 – 0.05 0.049 0.004 98.0 2.4
20 – 0.10 0.102 0.002 102.0 2.6
30 – 0.20 0.202 0.002 101.4 2.8
40 – 0.30 0.296 0.004 98.7 2.3
Sample responses are expressed as a confidence interval of 95% probability.
at 4 C for 15 and 30 days where the reduction peak current of nifedipine (30 mmol dm3) retained 94.87% and 89.75% of its initial response, respectively. The reproducibility of the polyfurfural film/GCE was investigated by eight replicative measurements of nifedipine (30 mmol dm3) using DPAdSV under optimum conditions. The relative standard deviation (RSD) of the cathodic peak currents of nifedipine by eight replicate measurements was 2.18%. The result revealed that the polyfurfural film/GCE possessed a satisfactory repeatability for the determination of nifedipine.
This research project was funded by the National Nature Science foundation of China (21427809 and 21475046). We also acknowledge the Fundamental Research Funds for the Central Universities (No. 2015ZM050 and 2014ZM0064).
3.6. Real sample determination
References
To evaluate the applicability of the polyfurfural film/GCE, the recovery of nifedipine was determined in the pharmaceutical and human urine samples by using the standard addition technique. The reduction peak current of nifedipine was measured by DPAdSV. Six parallel experiments are carried out for all measurements. The results obtained from the pharmaceutical and human urine samples are shown in Table 2. The average recoveries were ranged between 98.2% and 102.0% for the pharmaceutical samples and 98.0% and 102.0% for the human urine samples, indicating that this method can be efficiently used for the determination of nifedipine in real pharmaceutical and human urine samples.
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4. Conclusion In this work, a polyfurfural film modified glassy carbon electrode was prepared by the electropolymerization of furfural in acetonitrile solution. The modified electrode exhibited excellent electrocatalytic activity to the reduction of nifedipine. Under optimal conditions, the polyfurfural film/GCE had a low detection limit (5 109 mol dm3) and a wide linear range (1 108– 7 106 mol dm3). Additionally, the proposed nifedipine sensor was successfully applied to the determination of nifedipine in real samples with satisfactory results. Compared with other methods, this method is much more convenient, sensitive, reliable and reproductive. This study provided an efficient and novel analytical platform for determination of nifedipine and the platform shows wide potential in application of pharmaceutical and biological monitoring.
Conflict of interest There is no conflict of interest. Acknowledgements
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