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Original Paper. Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid. Saeed Shahrokhian1,2,Г and Mandana Amiri1.
Microchim Acta 157, 149–158 (2007) DOI 10.1007/s00604-006-0665-z Printed in The Netherlands

Original Paper Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid Saeed Shahrokhian1;2; and Mandana Amiri1 1 2

Department of Chemistry, Sharif University of Technology, Tehran 11365-9516, Iran Institute for Nanoscience and Technology, Sharif University of Technology, Tehran 11365-9516, Iran

Received March 4, 2006; accepted July 2, 2006; published online September 29, 2006 # Springer-Verlag 2006

Abstract. A multi-walled carbon nanotube paste electrode (MWCPE) is prepared as an electrochemical sensor with high sensitivity and selectivity in responding to isoniazid. The electrochemical oxidation of isoniazid is investigated in buffered solution by cyclic and differential pulse voltammetry. The electrode is shown to be very effective for the detection of isoniazid in the presence of other biological reductant compounds. The electrochemical oxidation of cysteine, due to the high overvoltage, is completely stopped at the surface of MWCPE. The electrode exhibits a very good resolution between the voltammetric peak of isoniazid and the peaks of ascorbic acid (AA) and dopamine (DA). A resolution of more than 450 mV between the anodic peak potentials makes the MWCPE suitable for simultaneous detection of isoniazid in the presence of AA or DA in clinical and pharmaceutical preparations. Differential pulse voltammetry (DPV) is applied as a sensitive method for the determination of isoniazid. The linear range in these determinations is 1 106 –1 103 M for isoniazid and the detection limit is 5107 M. The electrode was applied to the simultaneous determinations in isoniazid and AA mixtures and also, isoniazid and DA mixture over a wide concentration range. The slope variation for the calibration curves of isoniazid (RSD) was less than  Author for correspondence. E-mail: [email protected]

4.5% (based on ten measurements over a period of three months). Key words: Multi-walled carbon nanotube; modified electrode; carbon-paste electrode; isoniazid; cyclic voltammetry; differential pulse voltammetry.

Isoniazid (pyridine-4-carboxylic acid hydrazide) is one of the most effective antituberculosis agents, which is usually used to prevent the development of clinical tuberculosis. Isoniazid may have bacteriostatic or bacteriocidal action, depending to the concentration of the drug attained at the site of infection and susceptibility of the infecting organism [1]. The poisoning accidents, even death, have sometimes happened due to overdosage in isoniazid [2]. Therefore the control of isoniazid level in human body fluids is very important in clinical chemistry. On the other hand, small concentration difference between effectively therapeutic and toxic dosages makes it necessary to develop a rapid and specific method for determining isoniazid level in body fluids to aid the diagnosis of isoniazid intoxication. Various analytical methods have been presented for determination of isoniazid in clinical and pharmaceutical preparations. Due to very large overpotential at ordinary solid electrodes, the voltammetric detection of this compound is shown to be very difficult [3]. The electrochemical oxidation of isoniazid at the surface of

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palladium-modified microdisk array electrode showed a broad and weak anodic wave with a peak potential more than 0.9 V (vs. Ag=AgCl) [3]. The prepared modified electrode was used as an electrochemical detector in electrophoresis measurements and a detection limit of 5.0 mM is resulted for isoniazid. Cyclic and square-wave voltammetry were used for investigating isoniazid at a carbon paste electrode [4]. An irreversible and relatively broad anodic wave is obtained for isoniazid in a potential about 0.9 V (vs. Ag=AgCl) after 180 s of preconcentration, which contains a considerable amounts of capacitive current. This electrode was used in a square-wave anodic stripping voltammetric procedure and a detection limit of 6.1108 M was obtained for isoniazid. Application of chemically modified electrodes can be considered for lowering the overpotential and consequently, enhancement the voltammetric response of the hydrazines (e.g. isoniazid). Capillary electrophoresis with electrochemical detection using a 4-pyridyl hydroquinone self-assembled platinum micro-disk electrode (as working electrode) has been applied for the detection of isoniazid with a detection limit of 0.2 mM [5]. Some other analytical methods that have been reported for the determination of isoniazid are: liquid chromatography with photometric detection [6, 7], flow injection=chemiluminescence detection [8, 9] and UV-Vis spectrophotometry [10]. However, most of these methods require sophisticated instruments or are time-consuming procedures which are tedious and also, do not have adequate sensitivity. Carbon electrodes, especially glassy and paste electrodes, are widely used in electrochemical investigations because of their low background current, wide potential windows (anodic and cathodic), chemical inertness, low cost and suitability for detection of various organic and biological compounds. Among these, carbon paste electrodes (CPEs), due to unique characteristics, e.g., versatility of chemical modification, renewability of the electrode surface and compatibility with various electron mediators has extensively been used in these studies [11, 12]. It is shown in our previous contributions that various transition metal complexes such as, phthalocyanines [13–15] and Schiff bases [16–19] can be applied for the modification of CPEs. In the recent years by discovering the carbon nanotubes (CNT) [20, 21], due to their specific dimensions and structure-sensitive electrical properties of CNTs [22], their applications have been increased in various

S. Shahrokhian and M. Amiri

area of analytical chemistry [23, 24]. Remarkable properties of CNTs including mechanical, structural and electrical properties together with their catalytic effects in electrochemical processes and large microscopic surfaces, which can be obtained using CNTmodified electrodes, has been made CNTs as a new and interesting subject in the construction of very sensitive sensors and biosensors with fast responses [25]. Application of CNT as a past composite with an organic binder has been considerably developed for improvement of the analytical performances of the electrodes, e.g. lowering the background charging current, needed overpotential for the electrode process and increasing the sensitivity (faraday currents) of the electrochemical measurements [26, 27]. The production of CNT films on the surface of the various electrodes is of interests in the preparation of modified surfaces. Simple films of CNTs have been formed on the surface of solid substrates such as GC electrodes via the solvent evaporization from a suspension of CNT in an organic solvent (e.g. DMF). MWNT films were used for the modification of the surfaces of GC [28] and gold electrode [29] and applied in the voltammetric detection of folic acid. GC electrode modified with CNT film clearly showed electrocatalytic activity toward the oxidation of homocysteine at a low potential (0.0 V vs. Ag=AgCl) [30]. The acid treated MWNT were used for the preparation of the modified gold electrode, which was successfully applied for the simultaneous voltammetric detection of dopamine (DA) and ascorbic acid (AA) [31]. In the present work, the design and preparation of the carbon-paste electrodes modified with multi-walled carbon nanotubes (MWNTs) is described. The electrochemical behavior of 4-pyridine carboxylic acid hydrazide (Isoniazid) is investigated at the surface of the prepared electrode and compared with conventional carbon-paste electrode (from graphite powder) using cyclic voltammetry. The MWNT-modified electrode is applied for sensitive determination of isoniazid by the DPV with a sub-micromolar detection limit. This electrode is successfully applied for the detection of isoniazid in the presence of AA and DA. Complete resolution of the differential pulse voltammograms with more than 450 mV differences in Ep,a for these compounds together with considerable improvement of peak currents and lowering the background current makes the prepared electrode very suitable for the simultaneous voltammetric detection of minor amounts of isoniazid in pharmaceutical and clinical samples.

Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid

The application of the MWNT-modified paste electrode for the determination of isoniazid in drug preparations is also described. Experimental Reagents Multi-walled carbon nanotubes (purity more than 95%) with O.D. between 10 and 20 nm, I.D. between 5 and 10 nm and tube length from 0.5 to 200 nm was prepared from Nanostructured & Amorphous Materials (USA, http:==www.nanoamor.com=). Graphite powder20 mM and spectroscopic mineral oil (nujol) for preparation of conventional carbon paste electrode (CPE) were purchased from Aldrich (http:==www.sigma-aldrich.com). 4-pyridinecarboxylic acid hydrazide (Isoniazid), ascorbic acid, dopamine and cysteine for voltammetric investigations were prepared from Merck (http:==www. merck.de). All other chemicals were of analytical reagent grade from Merck. Tablets of isoniazid (from Darou Pakhsh Company, Tehran, Iran) were purchased from local pharmacies.

Pretreatment of multi-walled carbon nanotubes In order to remove the probable amorphous carbons and catalytic impurities, 500 mg of the prepared MWNT was heated in an oven with 400  C in nitrogen atmosphere for 2 h. The heat processed MWNT was dispersed in 50 mL of 6.0 M HCl for 2 h under ultrasonic agitation under the nitrogen atmosphere; filtered on a Watman 42 filter paper and washed with doubled distilled water until the pH of the solution was neutral. This MWNT was dried under the IR lamp.

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was used for the preparation of the buffer solutions with pHs 4 and 5 and 0.1 phosphate for pHs 3, 6 and 7. All aqueous solutions were prepared with doubly distilled de-ionized water. All voltammetric investigations were performed in deoxygenated solutions by purging the pure argon (99.999% from Roham Gas Company). During the experiments, argon gas was passed over the surface of the test solutions to avoid entrance of oxygen into the solution. To determine isoniazid in pharmaceutical preparations, tablet samples was crushed with a mortar and pestle, and a carefully weighted amount was dissolved in 0.1 M acetate buffer (with pH 4.0).

Results and discussions Cyclic voltammetric studies of isoniazid Previous studies on the electrochemical behavior of CNT-modified electrodes have shown that these electrodes are capable to improve the kinetics of the electrode processes and sensitivity of the measurements [25–27, 32–37]. These improvements accompanied with a considerable decrease in capacitive current and also, enhance the detection limit and resolution of the adjacent waves in electroanalytical measurements. Figure 1 shows the cyclic voltammograms (CVs) for 1 mM isoniazid in 0.1 M acetate buffer (pH 4.0) at the surface of unmodified and modified carbon paste electrode (CPE) with 25 wt% of MWNT. Initial and

Preparation of the carbon-paste electrodes The unmodified carbon-paste electrode was prepared by mixing graphite powder with an appropriate amount of mineral oil (Nujol) and thorough hand mixing in a mortar and pestle (75:25 w=w), and a portion of the composite mixture was packed into the end of a polyamide tube (ca. 2.5 mm i.d.). Electrical contact was made by forcing a copper pin down into the tube and into the back of the composite. The modified electrode was prepared by mixing MWNT and graphite powder with a 25:75 mass ratio in dichloromethane. After the evaporation of the solvent, an appropriate amount of the carbonic mixture with mineral oil (in 75:25 w=w) was transferred to the mortar and pestle for hand mixing. The prepared modified composite was dissolved in 3 mL dichloromethane for homogeneity. The mixture was stirred by a magnetic stirrer till the solvent evaporated, completely. Then, the modified composite was then air dried for 24 h and used in the same way as unmodified electrode.

Apparatus and voltammetric measurements Voltammetric experiments were performed using a Metrohm Computrace Voltammetric Analyzer model 757 VA (http:==www. metrohm.com). A conventional three-electrode system was used with a carbon-paste working electrode (unmodified or modified), a saturated Ag=AgCl reference electrode and a Pt wire as the counter electrode. A digital pH=mV=Ion meter (CyberScan model 2500) was applied for the preparing of the buffer solutions, which were used as the supporting electrolyte in voltammetric experiments. Voltammetric experiments were carried out in the buffered solutions of isoniazid, AA, DA and cysteine. A 0.1 M acetate solution

Fig. 1. Cyclic voltammograms of 1 mM isoniazid at the surface of conventional CPE (dashed lines) and MWNT-modified CPE (solid line), in 0.1 M acetate buffer solution with pH 4.0. Sweep rate was 100 mV s1

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switching potentials in these experiments were respectively, 0.00 and 1.30 volt (vs. Ag=AgCl). As can be seen, the electro-oxidation of isoniazid at unmodified electrode is very sluggish so that a wide anodic wave with anodic peak potential more than 1.000 volt is resulted. On the other hand, using the electrode containing MWNT, a distinct and sharp anodic wave with a peak potential of 0.849 volt is obtained. Such a large enhancement of anodic current revealed that by lowering the anodic overpotential of the electrode process, the kinetics of electron transfer for isoniazid improves remarkably at the MWNT-modified electrode. The effective catalytic role of the MWNT-modified electrode toward isoniazid oxidation can be attributed to some oxygen functional groups, the large microscopic area, specific electronic structure and high electrical conductivity of MWNT for the modified electrode. Such a behavior has been previously reported for CNT modified electrode in the electrochemical oxidation of various organic compounds [37–39]. The results of CVs in Fig. 1 show a considerable decreasing in capacitive background current for the MWNT-modified electrode (Fig. 1). High uniformity in the surface topography of the MWNT-containing electrode together with the hydrophobicity of the organic binder in the matrix of CPE can be presented for decreasing the capacitive current for MWNT-modified electrode. Results of the previous works show that the air oxidation of CNTs in conjunction with acidic treatment in concentrated nitric acid, due to creating hydrophilic oxygenated functional groups in CNT, raises the background current markedly [27, 34]. Therefore, the MWNT with high purity in CNT is used in the present work and pretreatment with concentrated nitric acid and air oxidation is avoided. The prepared MWNT-modified CPE shows high ability of compression in the body of the electrode leading to a compact matrix for electrode with low ohmic resistance. Higher weight percents of CNT in modified electrode are avoided because they did not show any improvement in anodic (faradaic) current or in lowering the capacitive background current. Instead, modified electrodes with higher percents of CNT, due to high mechanical resistance of nanotubes against mechanical stresses, create a relatively rough surface and weaken the reproducibility of electroanalytical measurements. Cyclic voltammograms of isoniazid at the MWNTmodified electrode in various potential sweep rates in the range of 20–200 mV s1 are shown in Fig. 2. The anodic peak current shows a linear dependence to

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Fig. 2. Cyclic voltammograms of 1 mM isoniazid in various potential sweep rates at the surface of MWNT-modified CPE. Supporting electrolyte was 0.1 M acetate buffer with pH 4.0

square root of the sweep rate, indicating that the current is controlled by a semi-infinite linear diffusion. For isoniazid no reverse peak (cathodic peak) is observed in all studied sweep rates and in all potential ranges. This reveals a totally irreversible diffusion controlled process. Cyclic voltammetric behavior of 1 mM isoniazid in buffered solutions with various pHs from 3 to 7 was examined by applying various sweep rates (20– 200 mV s1 ). These investigations show a marked dependence of voltammetric response to the pH. Figure 3 shows the variation of the anodic peak current and peak potential of isoniazid with pH, which is obtained with MWNT modified electrode in the sweep rate of 100 mV s1 . The results indicate a totally irreversible behavior for isoniazid in all pHs. The anodic peak potential for this compound showed a linear shift to less positive values with increasing the pH of the buffer solution according to the equation: Ep.a (mV) ¼ 1100.7–56.1 pH, with a value of 0.9912 for the correlation coefficient (R2). The results indicate a totally irreversible behavior for isoniazid with incorporation of two electrons and two protons in the oxidation process. Such a mechanism has been previously reported for isoniazid at the surface of conventional carbon-paste electrode, in which, oxidation of the amide moiety is performed [4]. By increasing the pH of the buffer solu-

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Fig. 3. Variation of (A) anodic peak potential, and (B) anodic peak current with pH of buffer solution at the modified electrode for 1 mM of isoniazid. Sweep rate was 100 mV s1 (0.1 M acetate is used for pHs, 4, 5 and 0.1 M phosphate is used for pHs, 3, 6, 7)

tion, the anodic peak current decreased (Fig. 3B). Moreover, as pH is increased, the stability of the voltammetric response for isoniazid considerably decreased with time. Instability of the hydrazine group in the structure of isoniazid in solutions with higher pHs can be presented for decreasing the anodic peak current, which accompanied with slight broadening of the peak. Differential pulse voltammetric investigations In analyzing the biological samples using the electrochemical methods the selectivity of the electrode response has a predominant role in resolving the adjacent waves and the accuracy of measurements. In the electrochemical sensing with conventional modified electrodes using transition metal complexes (as electron mediator), especially in clinical samples with a complex matrix (e.g. serum samples), presence of various reducing agents such as, AA, catechol amines (e.g. DA) and cysteine is considered as seriously interfering agents. The MWNT-modified electrode in the present work does not show any voltammetric response to sulfhydryl compounds such as, cysteine, penicillamine and glutathione. This reveals that the electrode is capable to discriminate the effects of such compounds in the electrochemical detection of isoniazid.

The DPV was used to determinations in the binary mixtures of isoniazid with AA or DA. Figure 4A, shows the DPV for a solution containing 0.1 mM of AA and isoniazid at pH 4. By using conventional and MWNT-modified CPEs. In order to obtaining suitable voltammograms of the electroactive species in these experiments, the potential ramp was applied in the range of 0.00–1.00 volt. As can be seen, at the CPE two relatively weak anodic waves with poor resolution are resulted. On the other hand, by using the MWNTmodified electrode two very sharp waves with a very good resolution is resulted for these two compounds. Observation of a Ep ¼ 475 mV together with lowering the background current and increasing the sharpness of the waves exhibit an appropriate strategy for simultaneous voltammetric detection of the minor amounts of AA and isoniazid. Similar results for mixture solutions of DA and isoniazid is obtained using two electrodes. By applying the MWNT-modified electrode, a Ep ¼ 438 mV is obtained for the solution of 0.1 mM of DA and isoniazid (Fig. 4B). Therefore, the modified electrode can be used successfully for the selective voltammetric detection of isoniazid in the presence of AA and DA. The DPV measurements for isoniazid at the MWNT-modified electrode show a linear relationship between anodic peak current and

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Fig. 4. Differential pulse voltammograms of a mixture of 1  104 M isoniazid and (A) 1.0  104 M AA, (B) 1.0  104 M dopamine at unmodified CPE (dashed line), and MWNT-modified CPE (solid line) in solutions with pH 4.0 (acetate, 0.1 M) and sweep rate 100 mV s1

concentrations in the range of 1106 –1103 M with a detection limit of 5107 M, which is obtained in buffered solutions with pH 4.0 (0.1 M acetate). A relative standard deviation of 3.6% was obtained for the slope of the calibration curve (peak current versus isoniazid concentration) based on five replicates (during a period of 1 month). Figure 5A shows the DPVs for solutions containing a constant background of 5.0105 M of AA and three different concentrations of isoniazid, in buffered solutions with pH 4.0. These results for solutions with a constant background of isoniazid and three different concentrations of AA (in a reverse manner) are presented in Fig. 5B. The results clearly express a high efficiency for the prepared modified electrode for simultaneous determination of AA and isoniazid in pharmaceutical and clinical preparations. Similar results for buffered solutions (pH 4.0) containing various mixtures of DA and isoniazid are presented in Fig. 6. The MWNT-modified electrode was used for the determination of minor amounts of isoniazid in pharmaceutical formulations using DPV technique. Tablet samples of isoniazid (with labeled values of 100 and 300 mg isoniazid=tablet) were powdered and an aliquot equal to 5.0104 M of isoniazid was prepared in 0.1 M acetate buffer with pH 4.0. The standard addition method was applied for drawing the calibra-

tion curves of current versus isoniazid concentration and also, determination of recovery in spiking of isoniazid to the pharmaceutical samples. The accuracy of the method for isoniazid, based on the labeled value, was 94.5 and 93.6% for 100 and 300 mg tablet samples, respectively. The precision of method for isoniazid determination in drug samples, based on the five replicates of analysis, was between 3.4 and 3.8%. Figure 7 compares the typical linear calibration curves for isoniazid in background buffer solution (pH ¼ 4.0) and in a background of the tablet sample buffered to pH 4.0. The slope of the calibration curve, which is obtained by the standard solutions of isoniazid in the range of 1105 –1103 M, was 0.0152 A=M with a correlation coefficient of (R2) 0.9994. A similar linear calibration curve for the standard addition to the drug sample (with a labeled value of 4.5104 M) was obtained with a slope of 0.0161 A=M and a correlation coefficient of 0.9974. By comparing the two slopes of the standard and spiked drug samples, a recovery of 105.9% was obtained for the method, revealing the efficiency of the method for accurate determination of isoniazid in pharmaceutical samples. The MWNTmodified CPE has performance characteristics, ease of preparation, modification and simplicity of the renewability of its surface by simple polishing. The detection system is very stable and the prepared electrode can be

Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid

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Fig. 5. (A) DPVs of solutions containing constant 1.0  104 M AA and (    ) 5.0  105 , (- - - - -) 7.5  105 and (——) 1.0  104 M isoniazid. (B) DPVs for solutions containing (I) constant 1.0  104 M isoniazid and (    ) 5.0  105 , (- - - - -) 7.5  105 and (——) 1.0  104 M AA. Supporting electrolyte in all measurements was 0.1 M acetate buffer with pH 4.0

Fig. 6. (A) DPVs of solutions containing constant 5.0  105 M DA and (——) 1.0  105 , (    ) 3.0  105 , (- - - - -) 7.0  105 and (-   -   -) 1.0  104 M isoniazid. (B) DPVs of solutions containing constant 5.0  105 M isoniazid and (——) 1.0  105 , (    ) 3.0  105 , (- - - - -) 7.0  105 and (-   -   -) 1.0  104 M DA. Supporting electrolyte in all measurements was 0.1 M acetate buffer with pH 4.0

used for more than three months without any considerable changes in its voltammetric behavior. Using the MWNT-modified electrode, the capacitive background

current is considerably decreased. By considering the catalytic effect of MWNT in improving the rate of the electrode process and anodic current, an excellent

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Fig. 7. Differential pulse calibration curves for anodic current versus isoniazid concentration in a background of 0.1 M acetate buffer (), standard addition to a tablet sample buffered with 0.1 M acetate buffer with pH 4.0 ()

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sub-micromolar detection limit is provided for the detection of the trace amounts of isoniazid. Figure 8 shows the differential pulse voltammograms of a 1106 M isoniazid at the surface of two conventional and MWNT-modified CPEs. As can be seen, the modified electrode exhibits a distinct anodic wave for this solution, whereas at the surface of the unmodified CPE such a wave cannot be obtained. The MWNT-modified electrode was applied for obtaining the recovery of the isoniazid added to the human serum samples. The concentration of each component in the human synthetic serum was chosen to be near to its normal level in the real human serum, except cysteine which does not exist in this sample [40]. The results indicate that most components of the synthetic serum do not show any interference with the electrochemical response of isoniazid on the surface of the modified electrode. Except tryptophan, this shows an irreversible anodic wave with a peak potential of 870 mV. In the experimental conditions, the anodic wave for isonizid in the spiked range of 1105 – 1104 M is appeared in 720 mV, which is 150 mV less than that of tryptophan. These investigations proved that the tryptophan concentrations less than 1105 M do not show any pronounced effect on the recovery of isoniazid. Therefore, in all recovery examinations, various concentrations of isoniazid have been applied with a 10-fold diluted concentration of human synthetic serum and reached to volume by acetate buffer solution (0.1 M, pH ¼ 4.0). The results for DA spiked to the diluted serum samples (10-fold) in the concentration range between 10 and 100 mM were in the range of 96.2–105.0%. The relative standard deviation (RSD %) for five replicates in the spiked range of isoniazid concentration was less than 4.0%. Conclusions

Fig. 8. (A) DPVs of solutions containing constant 1.0  106 M isoniazid at the surface of conventional CPE (dashed lines) and MWNT-modified CPE (solid line), in 0.1 M acetate buffer solution with pH 4.0. Pulse amplitude was 50 mV

The modified carbon-paste electrode incorporating multi-walled carbon nanotube (MWNT) provides a convenient and very effective method for lowering the capacitive background current and improving the voltammetric responses. The MWNT-modified electrode in this work shows high catalytic activity for the electro-oxidation of isoniazid and produce sharp voltammetric waves in both cyclic and differential pulse voltammetric investigations. High resolutions between the voltammetric responses of isoniazid, AA and DA, together with the lowering of background and enhancement the anodic currents, exhibit a suitable

Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid

procedure for the simultaneous voltammetric determinations in real samples with reasonable accuracy and reproducibility. The prepared MWNT-modified electrode does not show any voltammetric response to sulfhydryl compounds (e.g. cysteine, penicillamine and glutathione). This reveals the high selectivity of the electrode for the determination of isoniazid. The modified electrode can be prepared and renewed easily by the mechanical polishing. This electrode showed good reproducibility, long time stability, low detection limit (sub-micromolar) and high sensitivity for determining the minor amounts of isoniazid in the presence of many other biological reducing agents. Acknowledgements. The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran, Iran. The authors acknowledge gratefully Professor Mehdi JalaliHeravi for his valuable discussion and recommendations.

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Multi-walled carbon nanotube paste electrode for selective voltammetric detection of isoniazid

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