ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2015, том 70, № 5, с. 531-536
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
STRIPPING VOLTAMMETRIC DETERMINATION OF NICARDIPINE USING P-CYCLODEXTRIN INCORPORATED CARBON NANOTUBE-MODIFIED
GLASSY CARBON ELECTRODE © 2015 K. Zarei1, L. Fatemi, K. Kor
School of Chemistry, Damghan University Damghan, Iran 1E-mail: zarei@du.ac.ir Received 21.09.2013; in final form 24.09.2014
An electrochemical sensor modified with multi-walled carbon nanotubes—P-cyclodextrin (MWCNTs—P-CD) film was constructed and applied to the determination of nicardipine. The electrochemical behavior of nicardipine at the chemically modified electrode was investigated. After stripping of nicardipine on MWCNTs— PCD film at —0.9 V for 1 min, a well defined oxidation peak was produced in 0.04 M NaOH. Determination of nicardipine has been further improved by the formation of inclusion complex of P-CD with nicardipine. The MWCNTs—P-CD film showed preferable analytical characteristics in electrocatalytic oxidation for nicardipine compared with the MWCNTs film and bare glassy carbon electrode (GCE). The surface morphology of MWCNTs—P-CD film was characterized by using electrochemical impedance spectroscopy. The calibration curve was linear from 1.0 x 10-7 to 2.0 x 10 M. The limit of detection was obtained as 1 x 10-8 M. The results demonstrated that this electrochemical sensor has excellent sensitivity and selectivity. The sensor was applied for determination of nicardipine in blood serum with excellent recoveries.
Keywords: P-cyclodextrin, differential pulse voltammetry, multi-walled carbon nanotubes, nicardipine.
DOI: 10.7868/S0044450215050205
Cyclodextrins (CDs, cyclic oligopyranose oligomers) are widely studied host molecular receptors since they have great affinity for hydrophobic molecules in aqueous media [1, 2]. CDs form inclusion complexes with a great variety of analytes [3—13]. P-CD was applied as electrode modifier of carbon paste electrodes for determination of catechin [14], sparfloxacin [15], ascorbic acid [16], dihydroxyben-zene isomers [17] and nifedipine [18]. P-CD was also used as electrode modifier on the pyrolytic graphite electrode for determination of uric acid [19] norepinephrine [20] and simultaneous determination of adenine and guanine [21], and on the glassy carbon electrode for determination of rutin [22], simultaneous determination of quercetin and rutin [23] and adenine, guanine and thymine [24]. P-CD in all above mentioned works was applied as host molecular receptor.
Carbon nanotubes (CNTs) are composed of graphite sheets rolled into closed concentric cylinders with diameter on the order of nanometers and length of micrometers [22]. They have attractive chemical, electronic and mechanical properties [23, 24]. They are found in two types of structures: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes. Application of CNTs for modification of elec-
trodes has been reported in many works such as [17, 20, 22, 25-28].
Synergistic approach of both CDs and CNTs has been adapted for sensitive determination of different species [17, 20, 22, 25,26, 28]. Wang and coworkers [19] have investigated interaction of CDs and CNTs on basis of microscopic FTIR spectrum of CNTs— P-CD. They have reported that P-CD is attached to the CNT wall. Another interaction may originate in hydrophobic property. The surface of carbon nano-tubes is substantially more hydrophobic and may interact strongly with the non-polar regions of P-CD. Therefore, it seems these interactions hinder P-CD leaching from the surface of electrode. However, as far as we know, the determination of nicardipine at the glassy carbon electrode modified with P-cyclodextrin incorporated MWCNTs has not been reported.
Nicardipine (1, 4- dihydro -2,6- dimethyl-4 - [ 3 -ni-trophenyl]-3,5-pyridinecarboxylic acid, methyl-2-[methyl(phenylmethyl)amino]-ethyl ester) belongs to the group of 1,4-dihydropyridine calcium channel antagonists that have become very important in the treatment of heart diseases such as angina pectoris and arterial hypertension [29, 30]. Many of these drugs are highly potent and efficient in relatively low doses; analytical techniques of high sensitivity and selectivity
532 ZAREI
are required for therapeutic monitoring and pharmacokinetic studies. Various methods have been developed for the determination of nicardipine including spectroflourimetry [31], liquid chromatography [29, 32—34], liquid chromatography with electrochemical detection [35] and electroanalytical methods [30, 36]. In comparison to the other options, electroanalytical
h gp.
methods have the advantages of simplicity, fast results and high sensitivity.
Previously, inclusion complex between nicardipine and P-CD was studied and a strong involvement of the phenyl groups in the inclusion mechanism was reported [35]. In addition, it was mentioned that there is no preference for inclusion of a particular aromatic ring, as both forms depicted in Scheme are accepted [35].
h3cooc
h /
cooh2ch2n.
ch3
\
ch
h3c n ch3
3 h 3
h3cooc
no2
cooh2ch2n.
/ch3
\
ch
h3c n ch3
3 h 3
Two proposed schematics for inclusion complex of nicardipine with P-CD [35].
Inclusion complex of nicardipine with P-CD was also prepared and investigated in the solid state [36], but up to date nobody has used this property from the electrochemical point of view.
In this work, application of GCE modified with MWCNTs—P-CD film for determination of nicardipine was successfully investigated. The electrochemical behavior of nicardipine was studied at the GCE modified with MWCNTs—P-CD film and compared with that at the GCE modified with MWCNTs or bare GCE. The response mechanism of the sensor was also discussed. The electrochemical sensor showed excellent sensitivity, selectivity and recovery of nicardipine in blood serum samples.
EXPERIMENTAL
Reagents. All solutions were prepared with doubly distilled water. Used chemicals were of analytical grade. Nicardipine hydrochloride (Sigma) 1 x 10-4 M stock solution in 0.02 M acetic acid was stored in the dark under refrigeration; 0.01 M P-cyclodextrin was prepared by dissolving hydrated P-cyclodextrin (Ald-rich) in water.
Apparatus. Experiments were conducted by using a PAR (Princeton Applied Research) model 394 po-larography (EG&G) and an Autolab electrochemical system with PGSTAT 12 (Eco-Chemie, Utrecht, Netherlands). The Autolab system was run on a PC using GPES and FRA 4.9 software. A conventional
three electrode system, comprising a GCE with an electrode surface area of 3.14 mm2 and GCE modified with MWCNTs—P-CD film as working electrode, a platinum wire counter electrode and an Ag/AgCl (in saturated KCl) reference electrode was used in all experiments. For impedance measurements, a frequency range of 0.10 Hz to 100 kHz was employed. The AC voltage amplitude used was 5 mV, and the equilibrium time was 10 min.
Procedure. The sample solution (10.0 mL), containing 2.0 mL of0.2 M NaOH and appropriate volumes of nicardipine was transferred into the voltammetric cell, and the resulting solution was diluted to the mark with water. The accumulation potential (—0.9 V) was applied to GCE for 1 min whilst stirring the solution. Following the accumulation period, the stirrer was stopped and after 5 s the voltammogram was recorded by applying a positive—going differential pulse scan from —0.7 to —0.3 V with scan rate of 20 mV/s. The peak current for nicardipine was measured at about —430 mV. A blank solution without nicardipine was used to obtain the blank peak current. Between the measurements, the electrode was washed with acetonitrile—water mixture (1 : 1).
To analyze blood serum sample, initially it was diluted with distilled water (1 : 20), then 1 mL from the diluted sample was spiked with different nicardipine amounts and analyzed using standard addition method according to the above procedure.
7 fi
1600 1400 1200 1000 800 600 400 200
0
500
1000
1500
2000 2500 Zre, П
I, цА 15
10
5
0
-5
-1.2 E, V
Fig. 1. Electrochemical impedance spectroscopy of bare GCE (1), MWCNTs/GCE (2) and MWCNTs—
P-CD/GCE (3) for 5 mM Fe(CN)g-/Fe(CN)4- in 0.1 M phosphate buffer solution, pH 7. Applied AC voltage 5 mV, frequency 0.1 Hz to 100 kHz (Zim and Zre refer to imaginary and real impedance, respectively).
Fig. 2. Cyclic voltammograms of 10 ^M nicardipine at first (1) and second (2) scan against blank (electrolyte) solution (background current was subtracted). Electrolyte 0.04 M NaOH, scan rate 100 mV/s.
Preparation of modified electrode. Glassy carbon electrode was carefully polished with Al2O3 slurry and ultrasonically cleaned in 1 : 1 acetonitrile—ethanol solution. The clean electrode was dried under an IR lamp; twenty milligrams of CNT were dispersed for 10 min with the aid of ultrasonic agitation in 10 mL of 0.01 M P-CD aqueous solution to give a 2.0 mg/mL black solution. A CNT—P-CD /GCE was prepared by dropping 4 ^L of the black solution on the glassy carbon electrode surface and heating it under an IR lamp to remove the solvent.
RESULTS AND DISCUSSION
Electrochemical impedance characterization of modified electrode. Carbon nanotubes coated on the electrode surface change the double layer capacitance and interfacial electron transfer resistance of the electrode. The electrochemical impedance spectroscopy provides information on the impedance changes of the electrode surface during the modification process (Fig. 1). The Nyquist diagram includes a semicircular and a linear part. The semicircular part at higher frequencies corresponds to the electron-transfer-limited process, and the diameter is equivalent to the electron transfer resistance (Rct). The linear part at lower frequencies corresponds to the diffusion process. Fig. 1 presents the Nyquist diagrams of bare GCE, MWCNTs/GCE and MWCNTs—P-CD/GCE at 5 mM [Fe(CN)6]3-/ [Fe(CN)6]4- in phosphate buffer solution and applied AC voltage 5 mV in the frequency range from 0.1 Hz to 100 kHz.
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