ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 10, с. 1087-1090
ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
VOLTAMMETRIC DETERMINATION OF NITRENDIPINE
ON COMPOSITE FILM MODIFIED ELECTRODE © 2011 г. Yingliang Wei", Lan Zhang", Luping Zhang4, Chen Shao4, Chao Li4
a Department of Environment Engineering and Chemistry, Luoyang Institute of Science and Technology
Luoyang, 471023, P. R. China b Department of Material and Chemical Engineering, Zhengzhou University of Light Industry
Zhengzhou, 450002, P. R. China Received 22.04.2010; in final form 17.01.2011
This paper was focused on the use of electrochemical methods based on a novel composite film modified electrode for the pharmaceutical analysis. This modified electrode was fabricated by integration of room-temperature ionic liquids (1-butyl-3-methylimidazolium hexafluophosphate) and multi-walled carbon nanotubes with polymeric matrix (chitosan). This sensor showed good stability and high accumulation efficiency. Electrochemical behavior of nitrendipine at this electrode was investigated in detail. A sensitive cathodic peak was observed at —0.76 V on the modified electrode. Under the optimized conditions, the peak current was linear to nitrendipine concentration in the range of 4.0 x 10-7~5.0 x 10-5 M, and the detection limit was estimated to be 1.0 x 10-7 M after an accumulation for 150 s on open circuit. In addition, the proposed method was applied to the determination of nitrendipine in real samples, and the recovery was from 97.6 to 102.5%.
Keywords: voltammetric determination, nitrendipine.
Room temperature ionic liquids (RTILs) are compounds consisting entirely of ions that exist in liquid state around room temperature [1, 2]. They show many characteristics such as good chemical and thermal stability, almost negligible volatility, good ionic conductivity, and wide electrochemical window [3]. Hence they are considered promising solvents for organic chemical reaction, separation and electrochemical application [1]. In direct electrochemistry and electroanalysis field, RTILs have been widely used as a material for electrode preparation [4-8].
Carbon nanotubes have attracted many studies during the past decade because of their unique mechanical, chemical and electrical properties [9]. The subtle electronic behaviors of carbon nanotubes reveal that they have the ability to promote electron-transfer reactions when used as an electrode material. For instance, carbon nanotubes electrodes exhibit catalytic effects to the electrochemical behaviors of norepinephrine [10], re-serpine[11], procaine [12], dopamine and ascorbic acid [13, 14]. Chitosan (CHIT) is a biocompatible, biodegradable and nontoxic biopolymer that exhibits excellent film forming ability [15]. Because of its desirable properties, CHIT has been widely used as an immobilization matrix for biocatalysis. Furthermore, CHIT solution has been used as a solvent to disperse multi-walled carbon nanotubes (MWNTs) and resulted in a biocompatible nanotube aqueous solution [16]. The combination of the benefits of RTILs and MWNTs to electrochemical sensing [17-19] with good properties of CHIT for electrode fabrication may provide a remarkable
synergistic augmentation of electrochemical performance.
In this paper, a novel composite film coated glassy carbon electrode based on room-temperature ionic liquids (RTILs), multi-walled carbon nanotubes(MWNTs) and chitosan(CHIT) was fabricated. This modified electrode demonstrated good performance in sensing nitrendipine and showed the advantages of easy preparation, rapid response, long-term stability, and high sensitivity.
EXPERIMENTAL
Reagents and apparatus. Multi-walled carbon nanotubes (MWNTs) were purchased from the Institute ofNa-nometer Materials of Huazhong Normal University in China (purity >98%, average diameter is 30 nm). 1-Butyl-3-methylimidazolium hexafluophosphate (BMIMPF6, purity 97%) was purchased from Shanghai Chengjie Chemical Co.,Ltd. (China). Chitosan was obtained from Sinopharm Chemical Reagent Company and used as received. A standard solution of 1.0 x 10-3 M nitrendipine (National Institute for the Control ofPharmaceutical and Biological Products, China) was freshly prepared in 50% ethanol and stored under refrigeration. Chitosan was dissolved in 0.05 M acetic acid solutions to prepare 5 mg/mL chitosan aqueous solution. All other reagents were of analytical grade and used directly without purification, and redistilled water was used throughout. The purity of nitrogen gases was 99.99%.
-20 -
-40 -
A * -60 a
-80 -
-100 -
-120 _
-1.2 -1.0
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-0.8 -0.6 E, V(vs SCE)
-0.4
0.2
Linear sweep voltammograms of 1.0 x 10-4 M nitrendipine at different electrodes in pH 7.0 B.R. solution. (a) at bare GCE, (b) at MWNTs/CHIT/GC electrode, (c) at BMIMPF6/MWNTs/CHIT/GC electrode. Scan rate: 0.10 V/s, 150 s accumulation at open circuit.
All the voltammetric experiments were carried out with a CHI-830B Electrochemical Workstation (CH Instruments, USA). A conventional three-electrode system was employed, including a BMIMPF6/MWNTs/CHIT composite modified GC electrode as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. All the electrode potentials were reported versus SCE. All experiments were performed at 25°C.
Fabrication of modified electrode. Multi-walled carbon nanotubes (MWNTs) were refluxed for 10 h in concentrated HNO3 [20]. It is well known that this treatment causes segmentation and carboxylation for MWNTs at their terminus. To obtain good voltammetric response of nitrendipine at BMIMPF6/MWNTs/CHIT/GC electrode, the concentrations and the mass ratios of MWNTs, CHIT and BMIMPF6 were optimized in control experiments. Finally, a homogeneous suspension containing 0.25 mg/mL CHIT, 3 mg/mL MWNTs and 5% (VRTIL/VTotal) BMIMPF6 was achieved by sonicating dispersion. Before modification, the GCE was polished with a 0.05 ^m aluminum slurry (CH instruments), rinsed thoroughly with redistilled water and sonicated successively in ethanol, 1 : 1 HNO3, and redistilled water, each for 2 min. Then 3 ^L suspension solution was dropped on the GCE and let it dry at room temperature, thus a uniform film coated electrode (BMIMPF6/MWNTs/CHIT/GCE) was obtained. For comparison, a MWNTs/CHIT/GCE was fabricated through similar method.
Analytical procedure. Firstly, the modified electrode was activated in B.R. (Britton Robinson) buffer by successive cyclic sweeps between —0.2 and —1.2 V until the cyclic voltammograms were stable. After that, a measured-volume of nitrendipine standard solution was add-
ed into the cell. Before the measurement, the analytical solution was deaerated by bubbling N2 for at least 20 minutes, and the linear sweep voltammograms were recorded from —0.2 to —1.2 V at scan rate of 0.10 V/s. The peak currents were measured at —0.76 V for nitrendipine. After every measurement, the cyclic sweeps were repeated successively for 10 times in a blank solution for the electrode surface to regenerate.
RESULTS AND DISCUSSION
Electrochemical behavior of nitrendipine on BMIMPF6/MWNTs/CHIT/GCE. The voltammetric responses of nitrendipine at different electrodes in B.R. buffer were compared by linear sweep voltammetry (LSV). Figure shows the LSVs of different modified electrodes in pH 7.0 B.R containing 1.0 x 10-4 M nitrendipine at 0.10 V/s, respectively. As can be seen, the reduction peak of nitrendipine are observed at —0.76 V on BMIMPF6/ MWNTs/CHIT/GC electrode. The reduction peak current increases (curve b), compared with the bare GC electrode. At the BMIMPF6/MWNTs/CHIT/ GCE, the peak current(curve c) increases greatly, which is about 2.7 times and 1.5 times as large as that obtained at the bare GC electrode and the MWNTs/ CHIT/GCE, respectively, indicating the addition of BMIMPF6 into the composite further promoted the electron transfer rate, This should be ascribed to the synergistic effect of BMIMPF6 and MWNTs. MWNTs can improve the surface area of electrode, and BMIMPF6 can enhance the current response of nitrendipine to a great extent by extracting molecules. Furthermore, the ionic liquid can interact with carbon nanotubes through n-n interaction, which makes the entangled MWNTs bundles form much finer bundles. This is expected to improve the electrochemical catalysis of MWNTs [21].
Optimization of the experimental conditions. Optimizing of supporting electrolyte. The electrode reaction might be affected by the buffer solution. The effect of different electrolyte on the current responses was investigated. Some electrolytes including KH2PO4—NaOH, HAc-NaAc, B.R., Na2HPO—NaH2PO4 and NH3—NH4Cl were studied (each 0.1 M). The results showed that relatively positive reduction potential and high current peak were obtained in B.R. buffer. High current peak and good peak shape were obtained when pH 7.0 B.R. buffer was used. So this solution was applied in the subsequent studies.
Influence of scan rate. The influence of scan rate on the Ipc and Epc of nitrendipine at the BMIMPF6/ MWNTs/CHIT/GCE was tested. As the result, the peak current varies linearly with the square root of scan rate, and the regression equation is:
Грс =- 3.78 (±0.02) + 46.42 (± 0.03)v (r = 0.0998, 1рс : цА, v: V/s).
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VOLTAMMETRIC DETERMINATION OF NITRENDIPINE
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This reflects the diffusion-controlled feature of an electrochemical process, which seems to be disagreement with the fact that nitrendipine is accumulated on the electrode. This may relate to the diffusion of nitrendipine accumulated in the BMIMPF6 film to the MWNTs surface to undergo electrochemical reaction. Meanwhile, the log Ipc is proportional to the logv over the full scan rate range. The regression equation is:
logIpc = - 4.26 (±0.03) + 0.73 (± 0.01) logv
(r = 0.995, Ipc : A, : v : V/s). (2)
The slope is between 0.5 and 1.0, indicating that the electrochemical process essentially belongs to a mixture of diffusion controlled
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