A highly sensitive supported manganese-based voltammetric sensor for the electrocatalytic determination of captopril

Sensors and Actuators B 182 (2013) 80–86

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A highly sensitive supported manganese-based voltammetric sensor for the electrocatalytic determination of captopril D. Habibi a,∗ , A.R. Faraji a,∗ , A. Gil b a b

Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran Department of Applied Chemistry, Los Acebos Building, Public University of Navarre, Campus of Arrosadia, E-31006 Pamplona, Spain

a r t i c l e

i n f o

Article history: Received 30 November 2012 Received in revised form 20 February 2013 Accepted 21 February 2013 Available online 4 March 2013 Keywords: Captopril Voltammetry Supported manganese sensor Electrocatalytic determination

a b s t r a c t Manganese supported on an organo-modified SiO2 /Al2 O3 framework is used as a novel mediator for the selective voltammetric determination of captopril (CAP) in real samples such as drugs and urine. The redox response and electrocatalytic activity of the sensor were studied by cyclic voltammetry (CV), double-step potential chronoamperometry (CA) and linear sweep voltammetry (LSV) methods. Under optimal conditions (pH 6.0), the LSV anodic peak currents show a linear relationship with respect to CAP concentration in the range 3.0 × 10−7 –300 × 10−4 mol/dm3 , with a detection limit of 9.0 × 10−8 mol/dm3 .

1. Introduction (2S)-1-[(2S)-2-methyl-3-sulfanylpropanoyl]pyrrolidine-2carboxylic acid, or captopril (CAP), is one of the oldest drugs used to treat high blood pressure and can also be used to treat diabetes-related kidney problems, congestive heart failure, and to improve survival after a severe heart attack [1]. In order to ensure the quality of CAP in pharmaceutical formulations, and due to the dangers of overdose and the importance of regulating this drug in biological systems such as urine and human blood, analysis of CAP in biological and pharmaceutical samples is vital. Indeed, the reported limit for preventing CAP overdose is less than 450 mg/day. Many methods for determining this compound have been reported to date, including high performance liquid chromatography (HPLC) [2–7], fluorimetry [8–10], colorimetry [11], chemiluminescence and capillary electrophoresis [12–18], electrochemical methods [19–26] and spectrophotometry [27–31]. In this respect, electrochemical sensors for the determination of biological, environmental and pharmaceutical compounds have attracted increasing attention in recent years due to their sensitivity, accuracy, lower cost, high dynamic range and simplicity. Metal nanoparticles and semiconductor and magnetic particles have all been used as novel electrochemical sensors [32–34].

∗ Corresponding authors. Tel.: +98 811 8282807; fax: +98 811 8380709. E-mail addresses: [email protected] (D. Habibi), alireza [email protected] (A.R. Faraji). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.095

© 2013 Elsevier B.V. All rights reserved.

Indeed, various studies have shown that metal nanoparticles provide key properties for the electroanalysis of biological, environmental and other important electroactive compounds. These properties include roughening of the conductive sensing interface, the catalytic behavior of this class of nanomaterials, which allows them to be enlarged with metals, the amplified electrochemical detection of metal deposits, and the redox response of nanoparticles, which allows the electrocatalytic determination of compounds with high over-potentials [28,35]. Herein we report the synthesis of a novel manganese-based voltammetric sensor supported on a SiO2 /Al2 O3 framework and its use for the electrocatalytic determination of CAP in pharmaceuticals and urine samples. 2. Experimental 2.1. Materials and apparatus Fine graphite powder, paraffin oil, CAP and all other reagents were purchased from Merck or Aldrich. All electrochemical measurements were performed using a BHP 2063 Electrochemical Analysis System from Behpajooh (Iran). A 3 mol/dm3 solution of Ag/AgCl/KCl, a platinum wire, and a modified carbon paste electrode (MCPE) were used as reference, supporting and working electrodes, respectively. A digital pH/mV meter (Metrohm model 710) was used for pH measurements. To prepare the 0.001 mol/dm3 CAP solution, 0.022 g of CAP was dissolved in water and the solution diluted to 100 cm3 with distilled water. The resulting solution was kept in a refrigerator at 4 ◦ C in the

D. Habibi et al. / Sensors and Actuators B 182 (2013) 80–86

dark. Less concentrated solutions were prepared by serial dilution with water. Phosphate buffer solution (PBS) includes NaH2 PO4 , Na2 HPO4 and NaOH at several pH values. CAP tablets were purchased from Darou Pakhsh Company, Iran (25 and 50 mg of CAP per tablet). A CHNO-Rapid Heraeus elemental analyzer (Wellesley, MA) and Shimadzu UV–vis spectrophotometer were used for elemental analysis and diffuse reflectance spectroscopy, respectively. FT-IR spectra were recorded using an FT-IR spectrophotometer (Shimadzu 435-U-04FT, Japan). Transmission electron microscopy (TEM) images were recorded using a Tecnai F30TEM instrument operating at an accelerating voltage of 300 kV. Energy dispersive X-ray analysis (EDX) was also conducted for the elemental analysis of each sample. 2.2. Preparation of organometallic-silica gel Nanosized SiO2 /Al2 O3 was prepared by the sol–gel method [23]. Thus, 3.5 g of nanosized SiO2 /Al2 O3 was activated at 500 ◦ C for 5 h in air and then by refluxing with 4.3 cm3 of trimethoxysilylpropylamine in dry toluene (50 cm3 ) for 24 h. The solid obtained after this process was filtered and washed with methanol, then dried at 100 ◦ C under vacuum for 5 h. Bipyridylketone (BPK) was then added to a suspended solution of SiO2 /Al2 O3 -supported aminopropyl (Si/Al-APTMS) in dry methanol. To synthesize the manganese-based sensor Si/Al-APTMS-BPK-Mn (Scheme 1), 2.0 g of Si/Al-APTMS-BPK was dissolved in 50 cm3 of ethanol in a roundbottomed flask followed and 3.0 mmol of Mn(OAc)2 ·4H2 O added. The resulting mixture was refluxed for 24 h with magnetic stirring. 2.3. Preparation of the electrode In order to prepare the modified carbon paste electrode (MCPE), 0.1 g of precursor was dissolved in diethyl ether and then mixed with graphite powder in a ratio of 1:90. The solvent was evaporated by stirring. It was then blended with a mixture of mediator spiked carbon powder and paraffin oil (weight ratio: 70:30) for 20 min to obtain a uniformly wetted paste. This paste was then packed into the end of a glass tube and a clean copper wire inserted into the glass tube at the back of the mixture to make electrical contact. When necessary, a new surface was created by pushing an excess of paste

OAc

AcO Mn N

N

N

Si HO O

O

O OH

SiO2 /Al2O3 Scheme 1. Proposed structure for the heterogeneous Mn nanomediator.

81

Fig. 1. FT-IR spectra of: (A) Si/Al-APTMS; (B) Si/Al-APTMS-BPK; (C) Si/Al-APTMSBPK-Mn.

out of the tube and polishing it on a weighing paper. An unmodified carbon paste electrode was prepared in the same manner, but without blending mediator into the mixture, for comparison purposes. 2.4. Recommended procedure The MCPE was polished with a clean white paper. Then, to prepare a blank solution, 10.0 cm3 of PBS (pH 6.0) was transferred into an electrochemical cell and the initial and final potentials adjusted to +0.1 and +1.0 V vs. Ag/AgCl, respectively. A linear sweep voltammogram (LSV) was recorded to give the blank signal and labeled as Ipb . Varying amounts of CAP solution were added to the cell, using a micropipette, and the LSV recorded again to establish the analytical signal (Ips ). Calibration curves were constructed by plotting the catalytic peak current vs. CAP concentration. 3. Results and discussion 3.1. Characterization of the materials The FT-IR spectra of Si/Al-APTMS, Si/Al-APTMS-BPK, and Si/AlAPTMS-BPK-Mn are shown in Fig. 1. The strong absorption bands observed in the spectrum of silica at 798 and 1010–1290 cm−1 are due to Si O Si stretching vibrations. The FT-IR spectrum of Si/Al-APTMS shows several signals due to aminopropyl groups in the regions 1450–1560 cm−1 and 2860–2935 cm−1 (C H stretching modes of the propyl group). These bands indicate that the SiO2 /Al2 O3 was successfully modified by amine spacer groups. The N H deformation peak at 1540–1560 cm−1 confirms the successful functionalization of the Si/Al mixed oxide with 3-APTMS. The C N imine vibration signal resulting from the condensation reaction between BPK and the organo-functionalized SiO2 /Al2 O3 was observed at 1630 cm−1 , and the peaks in the range 3020–3066 cm−1 are attributed to the C H stretching vibrations of pyridine groups. Similarly, the peaks at 1434–1437 cm−1 can be assigned to the C C stretching vibration of pyridine groups. Complexation of manganese with the BPK groups immobilized on the modified SiO2 /Al2 O3 led to the appearance of a weak absorption peak at 413 cm−1 due to the Mn N bonds. The loading of Mn on the heterogeneous sensor was characterized by elemental analysis. The final Mn content, as determined by inductively coupled plasma optical emission spectroscopy (ICP–OES), was about 0.35 mmol/g. The UV–vis spectrum of SiO2 /Al2 O3 shows only a side-band adsorption close to 249 nm, whereas the spectra of the Si/AlAPTMS-BPK catalysts were dominated by strong absorptions in the range 255–320 nm due to the  → * and n → * transitions of the ligands. After treatment of Si/Al-APTMS-BPK with Mn(OAc)2 ·4H2 O, the heterogeneous manganese catalyst exhibits broad bands at

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Fig. 4. Plot of Ipa versus 1/2 for the oxidation of MCPE. Insert: cyclic voltammograms of MCPE at various scan rates in the potential range 0.2–1.1 V: (a) 5.0; (b) 10.0; (c) 30.0; (d) 60.0; (e) 100.0; (f) 150.0; (g) 250 and (h) 350 mV/s in 0.1 mol/dm3 PBS (pH 6.0).

Fig. 2. SEM image of the Si/Al-APTMS-BPK-Mn nanomediator.

around 447 nm, probably as a result of ligand-to-metal chargetransfer transitions. The UV–vis spectra also exhibited some new, low intensity metal d–d migration bands at around 490 nm upon complexation. Figs. 2 and 3 present selected scanning electron microscope (SEM) images of Si/Al-APTMS-BPK-Mn and the size distribution of the Mn-based nanosensor (TEM), respectively. It can be seen from Fig. 3 that the nanoparticle diameters range between a minimum of 5–10 nm and a maximum of about 40 nm. As such, in light of the reduced nanoparticle size and ligand capping, which prevents agglomeration, the resulting Mn nanoparticles could be used as a nanosensor for detecting biological materials.

3.2. Electrochemistry of the mediator The electrochemical properties of the modified electrode were studied by cyclic voltammetry in buffer solution (pH 6.0). Fig. 4 shows the cyclic voltammograms obtained for this electrode at various scan rates, ( = 5.0–350 mV/s). The experimental results indicate well-defined and reproducible anodic and cathodic peaks related to the Mn3+ /Mn4+ redox couple, with a quasi-reversible behavior and a peak separation potential (Ep = Epa − Epc ) of 450 mV. The cyclic voltammograms were used to investigate the change in peak current with potential scan rate. The peak current was found to be linearly dependent on 1/2 , with a correlation coefficient of 0.9925 for all scan rates (Fig. 4). This behavior shows that the redox process is diffusion-controlled. 3.3. pH effect The pH of the aqueous solution plays an important role in the electrochemical behavior of MCPE and CAP. Thus, it can be seen from Fig. 5, which shows a plot of Ipa vs. pH, that a pH of 6.0 is optimal for electrocatalytic oxidation of CAP by the Mn3+ incorporated into the carbon paste electrode (CPE) as the potential shift and electrocatalysis peak current values for electrooxidation of CAP are

Fig. 3. TEM image of an Si/Al-APTMS-BPK-Mn sample.

Fig. 5. Net current vs. pH curve for the electro-oxidation of 400 ␮mol/dm3 CAP in 0.1 mol/dm3 phosphate buffer solution at the surface of an MCPE in the potential range 0.2–1.1 V.

D. Habibi et al. / Sensors and Actuators B 182 (2013) 80–86

Fig. 6. Cyclic voltammograms for 0.1 mol L−1 PBS (pH 6.0) at a scan rate of 10 mV s−1 in the absence (a) and presence (b) of 400 ␮mol/dm3 CAP at an MCPE. Curve c shows the cyclic voltammogram for 400 ␮mol/dm3 CAP at the surface of the unmodified carbon paste electrode.

highest at this pH. As such, this pH was selected for the determination of CAP using the MCPE. 3.4. Catalytic effect The utility of the Mn nanosensor for oxidation of 400 ␮mol/dm3 of CAP was evaluated by CV. As can be seen from Fig. 6, a significant enhancement in the anodic peak current of CAP at the MCPE was achieved at a potential close to the formal potential of the Mn3+ /Mn4+ redox couple along with a concomitant decrease in the cathodic current (Fig. 6b). CAP oxidation does not take place at the surface of the unmodified electrode up to +1.00 V (Fig. 6c). Fig. 6a shows the cyclic voltammograms of the mediator at the surface of the unmodified carbon paste electrode in PBS (pH 6.0). These figures confirm that Mn nanoparticles (as the Mn3+ /Mn4+ redox couple) are chiefly responsible for the electrocatalytic oxidation of CAP. As such, this novel mediator is a suitable catalyst for the electrocatalytic oxidation of captopril (Scheme 2).

Me Me

2 eq. HS

O

Me

N

O COOH

Nano-Mn-sensor

S N

S

83

Fig. 7. Plot of Ipa versus 1/2 for the oxidation of CAP at the MCPE. Insert: Cyclic voltammograms of 400 ␮mol/dm3 CAP at various scan rates in the potential range 0.2–1.1 V: (a) 1.0; (b) 3.0; (c) 6.0; (d) 8.0 and (e) 10 mV/s in 0.1 mol/dm3 PBS (pH 6.0).

The insert to Fig. 7 shows the voltammetric evolution of the MCPE at scan rates ranging from 1.0 to 10.0 mV/s in a solution containing 400 ␮mol/dm3 CAP at pH 6.0. The linear variation of the peak current with the square root of the scan rate (1/2 ) can clearly be seen, thereby suggesting a diffusion-controlled electrooxidation process. As can be seen from Fig. 7, the peak potential for the electrooxidation of CAP shifts to more positive potentials with increasing scan rate, thereby suggesting a kinetic limitation to the reaction between CAP and the redox sites of MCPE. The Tafel plots drawn to evaluate the kinetic parameters (Fig. 8) were based on the points in the Tafel region of the cyclic voltammograms shown in the insert. The results of polarization studies for the electrooxidation of CAP at the MCPE show that the average Tafel slope is 5.3197 1/V. Insertion of this value into the Tafel equation (n(1 − ˛)F/2.3RT) gave an average charge-transfer coefficient (˛) of 0.69. The value of ˛n˛ was found to be 0.55 at the surface of the MCPE in the absence of CAP. These values show that the over-potential of CAP oxidation is reduced at the surface of MCPE and also that the rate of the electron-transfer process is greatly enhanced. This phenomenon was confirmed by the larger Ipa values recorded during cyclic voltammetry at the MCPE.

O

HOOC

N

HOOC

Nano-Mn-sensor

2+

2e-

Electrode Scheme 2. Proposed mechanism for the sensing of captopril using the Mn nanosensor.

Fig. 8. Tafel plot for MCPE in 0.1 mol/dm3 PBS (pH 6.0) at a scan rate of 10 mV/s in the presence of 400 ␮mol/dm3 CAP. Inset: cyclic voltammogram for the MCPE in the presence of 400 ␮mol/dm3 CAP at a scan rate of 10 mV/s.

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D. Habibi et al. / Sensors and Actuators B 182 (2013) 80–86 Table 1 Interference study for the determination of 5.0 ␮mol/dm3 captopril under optimized conditions. Species

Tolerance limits (W/W)

Glucose, Sacarose, Lactose, Feroctose Al3+ , K+ , Na+ , Cl− , Ca2+ , Mg2+ , SO4 2− , F− ,Li+ , ClO4 − Starch Alanine, Phenylalanine, Methionine, Valine, Histidine, Glycine Urea Cysteine, Ascorbic acid

1000 800 Saturated 500 300 4

3.6. Linear dynamic range and limit of detection

Fig. 9. (A) Chronoamperograms obtained for MCPE in the absence (a) and presence of 600 (b) and 800 ␮mol/dm3 CAP at pH 6.0 (c); (B): a Cottrell plot of the data from the chronoamperograms; (C): dependence of Ic/IL on the t1/2 derived from the chronoamperogram data.

3.5. Chronoamperometry studies The catalytic oxidation of CAP at the MCPE was also studied by chronoamperometry (Fig. 9A). Thus, chronoamperometry measurements were performed with various concentrations of CAP at the MCPE by setting the working electrode potential at 600 and 800 mV. The diffusion coefficient (D) for CAP was then calculated as follows. Experimental plots of I versus t−1/2 , which generated the best fits for various concentrations of CAP, were produced (Fig. 9B). The slopes of the resulting straight lines were then plotted against CAP concentration, and the slope of this new plot was entered into the Cottrell equation [36]: I = nFAD1/2 Cb −1/2 t 1/2

(1)

This process gave a diffusion coefficient of 1.55 × 10−4 cm2 /s for CAP. The rate constant for the chemical reaction between CAP and the redox sites in MCPE (kh ) can also be evaluated by chronoamperometry according to the method of Galus [Eq. (2)] [37]: IC 1/2 = 1/2  1/2 −1/2 (kCb t) IL

(2)

where IC is the catalytic current of CAP at the MCPE, IL the limited current in the absence of CAP and t the time elapsed (s). The slope of the IC /IL versus t1/2 plots (Fig. 9C) gives kh for a given CAP concentration. An average value for kh of 6.21 × 102 dm3 /mol s was obtained from the values of the slopes. This value of kh also explains the sharp nature of the catalytic peak observed for catalytic oxidation of CAP at the MCPE surface.

Linear sweep voltammetry was used to determine the CAP concentration. The results indicated two linear segments with different slopes for the CAP concentration. The regression equation for the concentration range 0.3–13 ␮mol/dm3 CAP was ia p = 0.1108C + 2.7690 (r2 = 0.9945, (n = 7)), and that for the CAP concentration range 13–300 ␮mol/dm3 was ia p = 0.0136C + 4.1311 (r2 = 0.9922, (n = 5)), where C is the concentration (␮mol/dm3 ) and ia p the sensitivity (␮A ␮L/mol). The detection limit (3) for CAP was found to be 0.09 ␮mol/dm3 . 3.7. Interference studies We also investigated the ability of various materials to potentially interfere in the determination of CAP. All these experiments were carried out under optimal conditions (5.0 ␮mol/dm3 CAP at pH 6.0). In light of the proposed use for this system, a series of compounds that are usually associated with CAP in pharmaceuticals and/or biological fluids was selected for this investigation. The tolerance limit (w/w) was defined as the maximum concentration of the interfering substance that caused an error of less than ±5% for the determination of CAP. Some of the results obtained are presented in Table 1. 3.8. Stability and reproducibility The reproducibility and stability of the MCPE was investigated by cyclic voltammetry measurements using 50 ␮mol/dm3 of CAP. The relative standard deviation (RSD) for five successive assays was 1.8%. When using four different electrodes, the RSD for five measurements was 2.4%. When the modified electrode wass stored, it retained 98% and 95% of its initial response after 7 and 45 days, respectively. These results show that the MCPE has good stability and reproducibility and could be useful for CAP determination. 3.9. Real sample analysis In order to study applications of this nanosensor for the electrocatalytic determination of CAP in real samples, we selected

Table 2 Determination of captopril in tablet and urine samples. Added (␮mol/dm3 )

Expected (␮mol/dm3 )

Found (␮mol/dm3 )

Published method [24] (␮mol/dm3 )

a

Tablet

– 5.0 15.0

10.0 15.0 30.0

9.61 ± 0.5 15.21 ± 0.4 29.55 ± 0.5

10.30 ± 0.5 – –

Tabletb

– 20.0

10.0 30.0

9.65 ± 0.5 29.75 ± 0.3

9.85 ± 0.6 –

– 5.0

– 5.0



Sample

Urine a b

50 mg tablet, Darou Pakhsh Company, Iran. 25 mg tablet, Darou Pakhsh Company, Iran.

D. Habibi et al. / Sensors and Actuators B 182 (2013) 80–86 Table 3 Analysis of captopril in human urine samples. Sample a

Urine Urineb Urinec Urined

Proposed method 9.21 9.62 1.51 1.38

± ± ± ±

0.4 0.5 0.3 0.2

Standard method 9.41 9.41 1.68 1.41

± ± ± ±

0.7 0.6 0.5 0.3

Fex e

Ftab e

ttab(95%) e

19 19 19 19

2.4 2.6 2.1 2.3

3.8 3.8 3.8 3.8

±shows the standard deviation. a Sampling was made after 2.5 h from a man who is safe and used captopril. b Sampling was made after 2.5 h from a woman who is safe and used captopril. c Sampling was made after 2.5 h from a woman who had heart problem and used captopril. d Sampling was made after 2.5 h from a man who had heart problem and used captopril. e Fexp is the calculated F-value; Ftab is the F value obtained from one-tailed table of F-test; texp is calculated value of t-student test; ttab is the t-value obtained from the table of student t-test.

a number of tablets and urine samples and analyzed their CAP content. The standard-addition method for measuring CAP concentrations in the real samples was used, and the proposed method compared with a previously published method [30]. The results can be found in Table 2. To further evaluate our method, we analyzed CAP levels in urine from both patients and healthy subjects who had used CAP. The results of this study, which can be found in Table 3, demonstrate the ability of MCPE to voltammetrically determine the levels of CAP in real samples, with good recoveries and reproducibility of the spiked CAP. 4. Conclusions This work has described the preparation of a physically modified carbon paste electrode involving incorporation of a novel Mn nanosensor into a carbon paste electrode. The resulting modified electrode has been used as an electrocatalyst for the voltammetric determination of CAP. The catalytic peak current obtained by LSV has been shown to depend linearly on the CAP concentration, and the limit of detection for CAP has been calculated to be 0.09 ␮mol/dm3 . Finally, this modified electrode has been used to determine the CAP level in real samples with good results. References [1] D.W. Cushman, H.S. Cheung, E.F. Sabo, M.A. Ondetti, in: Z.P. Horovitz (Ed.), Angiotensin-Converting Enzyme Inhibitors, Urban and Schwarzenberg, Munchen, 1981, pp. 3–25. [2] K. Kusmierek, E. Bald, A simple liquid chromatography method for the determination of captopril in urine, Chromatographia 66 (2007) 71–74. [3] T. Huang, Z. He, B. Yang, L. Shao, X. Zheng, G. Duan, Simultaneous determination of captopril and hydrochlorothiazide in human plasma by reverse-phase HPLC from linear gradient elution, Journal of Pharmaceutical and Biomedical Analysis 41 (2006) 644–648. [4] Y. Sun, Z. Zhang, X. Zhang, Determination of captopril by high-performance liquid chromatography with direct electrogenerated chemiluminescence, Spectrochimica Acta, Part A 105 (2013) 171–175. [5] T.D. Karakosta, P.D. Tzanavaras, D.G. Themelis, Automated determination of total captopril in urine by liquid chromatography with post-column derivatization coupled to on-line solid phase extraction in a sequential injection manifold, Talanta 88 (2012) 561–566. [6] M. Bahmaei, A. Khosravi, C. Zamiri, A. Massoumi, M. Mahmoudian, Determination of captopril in human serum by high performance liquid chromatography using solid-phase extraction, Journal of Pharmaceutical and Biomedical Analysis 15 (1997) 1181–1186. [7] T. Huang, Zho. He, B. Yang, L. Shao, X. Zheng, G. Duan, Simultaneous determination of captopril and hydrochlorothiazide in human plasma by reverse-phase HPLC from linear gradient elution, Journal of Pharmaceutical and Biomedical Analysis 4 (2006) 644–648. [8] N. Cheviron, A. Rousseau-Plasse, M.F. Lenfant, M.T. Adeline, P. Potier, J. Thierry, Coumarin-Ser-Asp-Lys-Pro-OH, a fluorescent substrate for determination of angiotensin-converting enzyme activity via high-performance liquid chromatography, Analytical Biochemistry 280 (2000) 58–64.

85

[9] E. Ivashkiv, Spectrofluorometric determination of captopril plus captopril disulfide metabolites in plasma, Journal of Pharmaceutical Sciences 73 (1984) 1427–1430. [10] S.M. Al-Ghannam, A.M. El-Brashy, B.S. Al-Farhan, Fluorimetric determination of some thiol compounds in their dosage forms, Il Farmaco 57 (2002) 625–629. [11] H. Kadin, R.B. Poet, Sequential electrochemical reduction, solvent partition, and automated thiol colorimetry for urinary captopril and its disulfides, Journal of Pharmaceutical Sciences 71 (1982) 1134–1138. [12] S.S.M. Rodrigues, J.L.M. Santos, Chemiluminometric determination of captopril in a multi-pumping flow system, Talanta 96 (2012) 210–215. [13] Z.D. Zhang, W.R. Baeyens, X.R. Zhang, G. Van der Weken, Chemiluminescence flow-injection analysis of captopril applying a sensitized rhodamine 6G method, Journal of Pharmaceutical and Biomedical Analysis 14 (1996) 939–945. [14] J.A.M. Pulgarín, L.F.G. Bermejo, P.F. López, Sensitive determination of captopril by time-resolved chemiluminescence using the stopped-flow analysis based on potassium permanganate oxidation, Analytica Chimica Acta 546 (2005) 60–67. [15] S. Hillaert, W. Van den Bossche, Determination of captopril and its degradation products by capillary electrophoresis, Journal of Pharmaceutical and Biomedical Analysis 21 (1999) 65–73. [16] S. Hillaert, W. Van den Bossche, Determination of captopril on a coated capillary by capillary electrophoresis, Journal de Pharmacie de Belgique 54 (1999) 83–84. [17] J. Russell, D.L. Rabenstein, Speciation and quantitation of underivatized and Ellman’s derivatized biological thiols and disulfides by capillary electrophoresis, Analytical Biochemistry 242 (1996) 136–144. [18] M. Wronski, Separation of urinary thiols as tributyltinmercaptides and determination using capillary isotachophoresis, Journal of Chromatography. B, Biomedical Sciences and Applications 676 (1996) 29–34. [19] N. Sattarahmady, H. Heli, S.E. Moradi, Cobalt hexacyanoferrate/graphene nanocomposite application for the electrocatalytic oxidation and amperometric determination of captopril, Sensors and Actuators B: Chemical 177 (2013) 1098–1106. [20] S. Shahrokhian, M. Karimi, H. Khajehsharifi, Carbon-paste electrode modified with cobalt-5-nitrolsalophen as asensitive voltammetric sensor for detection of captopril, Sensors and Actuators B: Chemical 109 (2005) 278–284. [21] A.A. Ensafi, R. Hajian, Simultaneous determination of captopril and thioguanine in pharmaceutical compounds and blood using cathodic adsorptive stripping voltammetry, Journal of the Brazilian Chemical Society 19 (2008) 405–412. [22] M.B. Gholivand, M. Khodadadian, M. Omidi, Amperometric sensor based on a graphene/copper hexacyanoferrate nano-composite for highly sensitive electrocatalytic determination of captopril, Materials Science and Engineering C 33 (2013) 774–781. [23] A.A. Ensafi, H. Karimi-Maleh, S. Mallakpour, B. Rezaei, Highly sensitive voltammetric sensor based on catechol-derivative-multiwall carbon nanotubes for the catalytic determination of captopril in patient human urine samples, Colloids and Surfaces B 87 (2011) 480–488. [24] H. Karimi-Maleh, A.A. Ensafi, A.R. Allafchian, Fast and sensitive determination of captopril by voltammetric method using ferrocenedicarboxylic acid modified carbon paste electrode, Journal of Solid State Electrochemistry 14 (2010) 9–15. [25] A.A. Ensafi, A. Arabzadeh, A new sensor for electrochemical determination of captopril using chlorpromazine as a mediator at a glassy carbon electrode, Journal of Analytical Chemistry 67 (2012) 486–496. [26] M.A. Khalilzadeh, H. Karimi-Maleh, A. Amiri, F. Gholami, R. Motaghed Mazhabi, Determination of captopril in patient human urine using ferrocenemonocarboxylic acid modified carbon nanotubes paste electrode, Chinese Chemical Letters 21 (2010) 1467–1470. [27] M.S. Arayne, N. Sultana, A. Tabassum, S. Nadir Ali, S. Naveed, Simultaneous LC determination of rosuvastatin, lisinopril, captopril, and enalapril in API, pharmaceutical dosage formulations, and human serum, Medicinal Chemistry Research 21 (2012) 4542–4548. [28] F. Elsebaei, Y. Zhu, Fast gradient high performance liquid chromatography method with UV detection for simultaneous determination of seven angiotensin converting enzyme inhibitors together with hydrochlorothiazide in pharmaceutical dosage forms and spiked human plasma and urine, Talanta 85 (2011) 123–129. [29] Y. Sun, Z. Zhang, X. Zhang, Determination of captopril by high-performance liquid chromatography with direct electrogenerated chemiluminescence, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 171–175. [30] T. Jovanovic, B. Stanovic, Z. Koricanac, Spectrophotometric investigation on complex formation of captopril with palladium (II) and its analytical application, Journal of Pharmaceutical and Biomedical Analysis 13 (1995) 213–217. [31] C.S. Sastry, A. Sailaja, T.T. Rao, Determination of captopril by two simple spectrophotometric methods using oxidative coupling reaction, Pharmazie 46 (1991) 465. [32] A.A. Ensafi, E. Khoddami, B. Rezaei, H. Karimi-Maleh, p-Aminophenol-multiwall carbon nanotubes-TiO2 electrode as a sensor for imultaneous determination of penicillamine and uric acid, Colloids and Surfaces B: Biointerfaces 81 (2010) 42–49. [33] M. Mazloum Ardakani, A. Talebi, H. Naeimi, M. Nejati Barzoky, N. Taghavinia, Fabrication of modified TiO2 nanoparticle carbon pasteelectrode for simultaneous determination of dopamine, uric acid, and l-cysteine, Journal of Solid State Electrochemistry 13 (2009) 1433–1440. [34] H.J. Qiu, G.P. Zhou, G.L. Ji, Y. Zhang, X.R. Huang, Y. Ding, A novel nanoporous gold modified electrode for the selective determination of dopamine in the presence of ascorbic acid, Colloids and Surfaces B: Biointerfaces 69 (2009) 105–108.

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D. Habibi et al. / Sensors and Actuators B 182 (2013) 80–86

[35] W. Zheng, Y.F. Zheng, K.W. Jin, N. Wang, Direct electrochemistry and electrocatalysis of hemoglobin immobilized in TiO2 nanotube films, Talanta 74 (2008) 1414–1419. [36] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamentals and Applications, 2nd Ed., Wiley, New York, 2001. [37] Z. Galus, Fundamentals of Electrochemical Analysis, Ellis Horwood, Michigan, 1994.

(England) in 1987 and he received his PhD in 1991 in Organic Polymer Chemistry in the same University. His current research interests are coupling reactions, green chemistry and application of heterogeneous catalysts in the synthesis of new heterocyclic compounds.

Biographies

A.R. Faraji was born in Tehran, Iran in 1978. He has received his BS (2005) from Arak University (Iran) and MS (2007) from Bu-Ali Sina University. He is currently a PhD student in Organic Chemistry at Bu-Ali Sina University (Hamedan, Iran). His work focuses on the preparation, characterization of heterogeneous catalysts and synthesis of heterocyclic compounds.

D. Habibi Professor, Head of the Department of Organic Chemistry at Bu-Ali Sina University (Hamedan, Iran), is an author and co-author of over 50 publications in the field of electrosynthesis and Heterocyclic Chemistry. He was born in Saveh, Iran in 1954. He was graduated in Chemistry (MSc) from Manchester University

A. Gil is Professor of Chemical Engineering in the Applied Chemistry Department at the Public University of Navarra, Spain. He received his M.S. (1989) and Ph.D. (1994) in chemistry from the University of Basque Country, San Sebastian, Spain. He has coauthored more than 350 book chapters, journal and conference papers related to adsorption, environmental catalysis and environmental technologies.