CC BY-NC-ND
0JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
ELSEVIER
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.jfda-online.com
Research Article
Electrochemical oxidation behavior of hydrochlorothiazide on a glassy carbon electrode and its voltammetric determination in pharmaceutical formulation and biological fluids
Ali F. Alghamdi*
Department of Chemistry, Faculty of Science, Taibah University, Madinah, Saudi Arabia
ARTICLE INFO ABSTRACT
Article history:
Received 14 September 2013 Received in revised form 17 November 2013 Accepted 17 December 2013 Available online xxx
Keywords:
Anodic stripping voltammetry Cyclic voltammetry Electro-oxidation Glassy carbon Hydrochlorothiazide
The electrochemical oxidation behavior of hydrochlorothiazide (HCT) on a glassy carbon as a working electrode was investigated in Britton-Robinson (B-R) buffer pH 3, by using anodic stripping voltammetry (ASV) and cyclic voltammetry (CV). This drug gave a well-defined voltammetric oxidation peak at + 1200 mV versus an Ag/AgCl reference electrode. The electrochemical oxidation process was shown to be irreversible and diffusion controlled, with adsorption characterized over the entire pH range. The optimized conditions, such as accumulation time and potential, scan rate, frequency, pulse amplitude, varying of working electrodes, and instrumental parameters were studied. The calibration graph for HCT was obtained from 4 x 10—6 to 4 x 10—5 M (correlation coefficient = 0.997) using the developed electroanalytical method (ASV). The detection limit of this drug was 4.3 x 10—9 M. ASV and CV techniques with adequate precision and accuracy have been developed and applied for direct determination of HCT in commercial tablets without separation or extraction procedures and biological fluids such as urine and plasma.
Copyright © 2014, Food and Drug Administration, Taiwan. Published by Elsevier Taiwan
LLC. All rights reserved.
1. Introduction
Hydrochlorothiazide (HCT) is a diuretic drug of the thiazide class, which acts by inhibiting the ability of the kidneys to retain water. This reduces the volume of the blood, decreasing blood return to the heart and thus cardiac output and, by other mechanisms, is believed to lower peripheral
vascular resistance [1,2]. HCT is a calcium-sparing diuretic; it can help rid the body of excess water, but retain calcium. HCT is frequently used for the treatment of hypertension, congestive heart failure, symptomatic edema, diabetes insipidus, renal tubular acidosis, and the prevention of kidney stones [3]. It is also sometimes used for hypercalciuria, Dent's disease, and Meniere's disease.
* Department of Chemistry, Faculty of Science, Taibah University, P.O. Box 344, Madinah, Saudi Arabia. E-mail address: alifh2006@hotmail.com. http://dx.doi.org/10.1016/jjfda.2013.12.003
1021-9498/Copyright © 2014, Food and Drug Administration, Taiwan. Published by Elsevier Taiwan LLC. All rights reserved.
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
Hydrochlorothiazide is named 6-chloro-1,1-dioxo-3,4-dihydro-2H-1,2,4-benzothiadiazine-7-sulfonamide, as recorded in the IUPAC system. The chemical formula of HCT is C7H8ClN3O4S2.
Anodic stripping voltammetry (ASV) is the most widely used form of stripping analysis. It is considered a voltammetric method for quantitative determination of trace concentrations for heavy metals, dyes, drugs, and so on. The analyte of interest is electroplated on the working electrode surface during a deposition step, and oxidized from the electrode during the stripping step, so the current is measured during the stripping step. The oxidation of species is recorded as a peak in the current signal at the potential at which the species begins to be oxidized. Thereafter, the stripping step can be either linear, staircase, square-wave, or pulse. Many reviews have been published which illustrate the wide spectrum and scope of ASV applications in the analysis of toxic metals [4], pharmaceutical compounds [5-12], and in forensic science [13].
Cyclic voltammetry (CV) is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of CV results from its ability to rapidly provide considerable information on the thermodynamics of redox processes and the kinetics of heterogeneous electron transfer reactions and on coupled chemical reactions or absorption processes. CV is often the first experiment performed in an electrochemical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media on the redox process [14].
ASV is used as an analytical quantitative method capable of monitoring the signal of HCT at low concentration levels. Hence, a wide variety of analytical methods were found to determine trace concentrations of many drugs such as HCT in pharmaceutical formulations and biological fluids [5-12]. Instrumental techniques of chemical analysis were successively used to determine HCT that included spectrophotometry [15-17], chromatography [17,18], liquid chromatography-UV [19], HPLC [20-23], and other electrochemical methods [17,24,25].
The glassy carbon electrode has been used to determine some chemical compounds in different literatures, as modified electrode to enhance electrochemical study [26-30]. An electrochemical detection used to analysis most compounds in the previous studies [31-33].
ASV is considered a powerful electrochemical procedure that enhances the sensitivity of HCT pharmaceutical content in some pharmaceutical formulations. No published article for analysis of HCT using ASV has been reported in the literature. Therefore, this research was carried out to study the ASV behavior of HCT in order to develop an effective and sensitive electrochemical method for determination it in pharmaceutical formulations and biological fluids.
2. Experimental
2.1. Apparatus
ASV and CV measurements were carried out using 797 VA Computrace (Metrohm, Herisau, Switzerland) controlled by
(VA Computrace 2.0) control software. Anodic stripping and cyclic voltammograms were printed via a HP color laserjet CP1215 printer (China). A three electrode system was used, including a glassy carbon electrode as the working electrode. pH values were studied using a Hanna instrument pH211 (Romania). An Oxford adjustable micropipette (Huawei, Ireland) was used to measure microliter volumes of the drug standard solutions. The Labofuge 200 instrument, Heraeus Sepatech (Germany) was used to centrifuge the urine and plasma samples, which were then suitable for voltammetric analysis.
2.2. Reagents
The chemicals used were of analytical reagent grade and used without further purification. HCT stock solution of 1 x 10-2 M was prepared by dissolving the appropriate amount of HCT in distilled water in a 25 mL volumetric flask. The drug stock solution was stored in a dark place. The standard solutions of HCT with lower concentrations were prepared by diluting the stock solution with distilled water daily. Britton-Robinson (B-R), carbonate, phosphate, and acetate supporting buffers were prepared for the voltammetric analysis of HCT [34].
Procedure
The general procedure which applied for getting all voltam-mograms was as follows: 10 mL of B-R buffer pH 3 was injected in to a dry and clean voltammetric cell and then a required standard solution of HCT was added. All buffer solutions were purged with a nitrogen gas for nearly 3 minutes initially, while these solutions were stirred. The deposition potential of 0.0 V versus an Ag/AgCl reference electrode was applied to a glassy carbon electrode while the solution was stirred for 20 seconds. Anodic scans were carried out over the range 800-1700 mV. The voltammetric measurements were performed at room temperature 25 °C.
Fig. 1 - Anodic stripping voltammogram of hydrochlorothiazide (HCT) in Britton-Robinson (B-R) buffer at pH 3, experimental conditions: tacc = 20 seconds, Eacc = 0.0 V, scan rate = 150 mV s-1, drug concentration = 6 x 10-6 M.
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
NH -2e-
Scheme 1 - Structure and suggested oxidation mechanism of hydrochlorothiazide drug.
Results and discussion
3.1. Anodic voltammetric behavior of HCT
Initially, investigation of HCT indicated that it was deposited effectively onto the solid electrode (glassy carbon) and it could be monitored by ASV after scanning the applied potential in the positive direction. For instance, the electrochemical accumulating of 6 x 10-6 M HCT solution for 20 seconds in B-R supporting buffer pH 3, yielded a single well-defined anodic voltammetric peak at 1200 mV versus the Ag/AgCl reference electrode, as shown in Fig. 1, which shows the anodic stripping voltammogram for HCT. The anodic voltammetric signal is thought to have resulted from the oxidation process through the common active site of the primary amine group (NH2) outside the aromatic ring, where the primary amine function (NH2) is oxidized to the nitro group (NO2). The proposed electrochemical oxidation mechanism is shown in Scheme 1.
By contrast, the resulting oxidized compound was formed as an irreversible reaction, so no cathodic peak was recorded. This irreversible characteristic nature of HCT electrochemical process was confirmed by the CV technique for measuring HCT solution (8 x 10-5 M) at a 50 mV/second scan rate in B-R buffer pH 3, as shown in Fig. 2. The multicyclic voltammo-grams of 8 x 10-5 M of HCT in B-R buffer at pH 3 are shown in Fig. 3, which illustrates that the surface of the glassy carbon electrode was saturated by the analyzed drug after the first sweep, so the values of the current were gradually decreased.
The electrochemical oxidation process was shown to be diffusion controlled with adsorption characterized at different scan rates over the range 10-80 mV/second, as shown in
Fig. 4. Through this analysis, a linear relationship between the oxidation peak current and scan rate was observed.
3.2. Optimization of experimental parameters
3.2.1. Effect of buffer solution and pH
The study of a suitable medium is a very important parameter fortheASV determination of HCT. Herein,5 x 10—6MofHCTwas analyzed by ASV in B-R, acetate, phosphate, and carbonate buffers at different pH values (3, 7, and 10) after 30 seconds pre-concentration time at 0.0 V deposit potential. The best anodic stripping response in terms of current value, the peak shape, and the smoothness of baseline was observed when the B-R buffer was used. Therefore, B-R buffer was selected as the optimum condition for future experiments. It was generally noticed that the current height of the obtained ASV signal reached its maximum value in acidic media (B-R pH 3). By contrast, the influence of pH variation over the range 2—7 on the anodic current of 5 x 10—6 M HCT was also investigated, and the results are shown in Fig. 5. The highest current was recorded at pH 3, which was selected as the optimum value for further studies.
3.2.2. Effect of accumulation operators
An accumulation process of the studied drug on the glassy carbon electrode surface is another important operator for the sensitive determination of HCT. On the working electrode surface, the accumulated drug depends on the length of the time over which the deposit method is allowed to proceed, in addition to the intensity of anodic stirring and applied deposition (accumulation) potential. The anodic oxidation current of 5 10—6 M for HCT solution was measured as a function of
Fig. 2 — Cyclic voltammogram of 8 x 10—5 M hydrochlorothiazide (HCT) at Britton-Robinson (B-R) buffer pH 3.0, scan rate 100 mV/second, [I(nA) = 780].
Fig. 3 — Multicyclic voltammograms of 8 x 10—5 M hydrochlorothiazide (HCT) at Britton-Robinson (B-R) buffer pH 3.0, scan rate 100 mV/second [I (nA)scans = 750, 400, 300, 180 and 100].
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
Fig. 4 - Relationship between oxidation peak current and scan rate for 8 x 10-5 M of hydrochlorothiazide (HCT) at Britton-Robinson (B-R) buffer pH 3.0.
Fig. 6 - Effect of accumulation time on 5 x 10 M hydrochlorothiazide (HCT) drug in Britton-Robinson (B-R) buffer, pH3, and Eacc = 0.0 V.
accumulation time, as seen in Fig. 6. Through this study, an almost linear relationship between the ASV peak current and accumulation time was recorded from 0.0 seconds to 20 seconds; thereafter, the anodic current decreased. Therefore, 20 seconds was adopted as the optimum for the following ASV studies.
In a different study, the accumulation potential effect on the peak current of this drug was recorded by variation of accumulation potential from +0.8 V to -0.6 V. The maximum anodic peak current value was obtained with an accumulation potential of 0.0 V, which was selected as the optimum value in future experimental studies.
3.2.3. Effect of scan rate, amplitude, and frequency In general, an ASV response depends on various parameters related to the applied potential which was scanned. The anodic current of HCT was found to be proportional to the scan rate, particularly at high scan rate values. The studied scan rate over the range 50-250 mV/second caused the ASV current to increase linearly, but 150 mV/second produced the best shape of peak and it was selected for further works. Furthermore, the influence of the pulse amplitude on the voltammetric signal was also evaluated over the range 10-100 mV. The anodic current of HCT increased almost linearly with the pulse amplitude over this range, but a 30 mV value produced the best shape of peak, so it was selected for further works. By contrast, the influence of frequency on the ASV peak current was studied over the range 10-80 Hz, as can
be seen in Fig. 7. A linear relationship was observed over 10-50 Hz; thereafter, the anodic current decreased. A frequency of 50 Hz was adopted as the optimum value.
3.2.4. Effect of instrumental factor and working electrodes The influence of convection rate on the monitored anodic current was evaluated over the range 0-3000 rpm, as shown in Fig. 8. There was a high anodic current at 1000 rpm, so this value was chosen as the optimum value in further experiments.
By contrast, the effect of the working electrode for analyzed HCT was investigated. Gold, platinum, ultra trace graphite, and glassy carbon were used as working electrodes; the resulting platinum and ultra trace graphite did not record any anodic signal, however gold recorded a high coupling current with an unsuitable peak shape. A high and good anodic peak current was observed by use of a glassy carbon electrode, which was selected over the voltammetric analysis for this drug.
3.3. Analytical performance
3.3.1. Calibration graph
According to optimum experimental conditions, a good linear relationship was obtained between HCT electrochemical anodic signal and its concentration through the range 4 x 10-6 to 4 x 10-5 M (Fig. 9). The parameters of the HCT concentration-anodic current straight line were calculated by
Fig. 5 — Effect of pH on the anodic current of 5 x 10—6 M hydrochlorothiazide (HCT) drug in Britton-Robinson (B-R) buffer, tacc = 30 seconds, and Eacc = 0.0 V.
Fig. 7 - Effect of frequency on 5 x 10-6 M hydrochlorothiazide (HCT) drug in Britton-Robinson (B-R) buffer, pH = 3, tacc = 20 seconds, Eacc = 0.0 V, 150 mV/ second, and 30 mV amplitude.
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
Table 1 - Voltammetric determination of hydrochlorothiazide (HCT) in its commercial tablets by anodic stripping voltammetry (ASV) method.
Labeled content Monozide tablets 25 mg HCT
53 — 52 -51 -50 —
0 500 1000 1500 2000 2500 3000 3500 rpm
Fig. 8 - Effect of convection rate on 5 x 10-6 M hydrochlorothiazide (HCT) drug in Britton-Robinson (B-R) buffer, pH = 3, tacc = 20 seconds, Eacc = 0.0 V, 150 mV/ second, 30 mV amplitude, and 50 Hz.
using the least-square method. The regression equation of the calibration curve is:
ip (nA) = 1.4 x 107 C (M) + 27.45, r = 0.997, n = 9
where ip is the ASV (nano ampere), C is the HCT molar concentration and r is the correlation coefficient.
(mg) Recovery %
24.00 96
23.75 95
23.75 95
23.75 95
23.50 94
Mean 95%
SD (standard deviation) ±0.71
3.3.3. Reproducibility
The electroanalytical precision of this developed method was evaluated from reproducibility of eight determinations of 5 x 10—6 M HCT in B-R buffer at pH 3.0. A relative standard deviation of 0.02% was calculated, which indicated the reproducible deposit and monitoring of the studied HCT drug.
3.3.4. Recovery
The recovery of the developed method, which gives the accuracy of the used ASV technique, was studied by electro-analyzing spiked B-R buffer solution containing 5 x 10—6 M HCT via the optimized ASV procedure. The mean recovery of five measurements was found to be 98% ± 1.0.
3.3.2. Limit of detection
The limit of detection defined as three times the signal-to-noise ratio (S/N = 3) reached in the optimum experimental conditions for monitored HCT drug was 4.3 x 10e9 M (1.3 ppb). This remarkable sensitivity shows the preference of the ASV technique over the other analytical techniques, such as the electrochemical technique, which only achieved a 5 ppb detection limit [25] and the conventional HPLC method with a 1 ppm detection limit [22]. The applied ASV approach obviously enhanced the sensitivity compared to the cited analytical methods.
0.80 1.00 1.20 1.40
Fig. 9 — Calibration graph voltammograms of hydrochlorothiazide (HCT) in Britton-Robinson (B-R) buffer, pH = 3.0, tacc = 20 seconds, Eacc = 0.0 V, and scan rate = 150 mV/second. Drug concentration (M): A = 4 x 1 0—5, B = 3 x 10—5, C = 2.5 x 10—5, D = 2 x 10—5, E = 1.5 x 10—5, F = 1 x 10—5, G = 8 x 10—6, H = 6 x 10—6, and I = 4 x 10—6 M.
3.3.5. Stability
The ASV signal of 5 x 10e6 M HCT was investigated by monitoring the ASV current every 10 minutes. The measured electrochemical response seemed to be fixed over the studied time range (0e90 minutes). This study demonstrates the stability of the analyzed drug under the optimum experimental conditions.
3.4. Interferences study
The influence of some possible interfering substances usually present as ingredients in pharmaceutical formulations (such as starch, sucrose, and so forth) on the anodic current for HCT was also evaluated. These interferences were studied by adding appropriate amounts at different concentrations (1-, 5, and 20-fold) higher than a drug concentration in the used buffer solution (10 mL of B-R buffer, pH 3, containing 2 x 10e5 M HCT). However, the presence of very high concentrations (20-fold) of starch decreased the anodic current by 10% of its original signal. No obvious effect on the voltam-metric response of the analyzed HCT was recorded with all additions of sucrose interference.
3.5. Analytical applications
The developed ASV method was applied to determine HCT in some real samples, such as pharmaceutical tablets and biological fluids. The HCT content of commercially available tablets (Monozide 25, consisting of 25 mg of hydrochlorothi-azide, the Jordanian Pharm., Mfg.co., Jordanian) was analyzed directly by the developed ASV method after the dissolving and filtration steps. Five aliquots of the dissolved real sample were diluted to the required concentration level and measured by
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
Table 2 — Voltammetric determination of hydrochlorothiazide (HCT) in biological fluids by anodic stripping voltammetry (ASV) method.
the standard additions approach. The results of this study show a recovery mean of 95% with a standard deviation of ±0.71%, as can be seen in Table 1.
By contrast, the applicability of the ASV technique for analysis of HCT in biological fluids was also evaluated by obtaining its recovery from spiked human urine and plasma. A fast and simple pretreatment process [35], which is actually considered a slight modification of the sample preparation method for the determination of some drugs, was used. It was carried out by adding a small amount of 5% ZnSO4.7H2O solution, NaOH, and methanol to the biological samples and centrifuging the mixture to remove and eliminate most of interfering substances by precipitation. The determination of HCT in spiked urine and plasma resulted in mean recoveries of 90% ± 1.22 (standard deviation) and 88% ± 0.71, respectively, as can be seen from Table 2.
Conflicts of interest
The author declares no conflicts of interest.
Acknowledgments
The author would like to thank Mr. Awad Alqarni and Ahmad Alribdy at Qassim Pharmaceutical Plant Buraydah, Spimaco Addwaeih, Saudi Arabia for their assistance and for supplying standard drugs.
REFERENCES
[1] Beermann B, Groschinsky-Grind M, Rosen A. Absorption, metabolism, and excretion of hydrochlorothiazide. Clin Pharmacol Ther 1976;19:531-7.
[2] Duarte JD, Cooper-DeHoff RM. Mechanisms for blood pressure lowering and metabolic effects of thiazide and thiazide-like diuretics. Expert Rev Cardiovasc Ther 2010;8:793-802.
[3] "Hydrochlorothiazide". The American Society of Health-System Pharmacists; 2011.
[4] Ostapczuk P, Valenta P, Rutzel H, et al. Application of differential pulse anodic stripping voltammetry to the determination of heavy metals in environmental samples. Sci Total Environ 1987;60:1—16.
[5] Vire JC, Kauffmann JM, Patriarche GJ. Adsorptive stripping voltammetry applied to drug analysis: a powerful tool. J Pharm Biomed Anal 1989;7:1323—35.
[6] Golcu A, Dogan B, Ozkan SA. Anodic voltammetric behavior and determination of cefixime in pharmaceutical dosage forms and biological fluids. Talanta 2005;67:703—12.
[7] Wahdan T, El-Ghany NA. Determination of domperidone in tablet dosage form by anodic differential pulse voltammetry. Il Farmaco 2005;60:830—3.
[8] Radi AM. Anodic voltammetric assay of lansoprazole and omeprazole on a carbon paste electrode. J Pharm Biomed Anal 2003;31:1007—12.
[9] Gao L, Wu Y, Liu J, et al. Anodic voltammetric behaviors of methotrexate at a glassy carbon electrode and its determination in spiked human urine. J Electroanal Chem 2007;610:131—6.
[10] Erk N. Differential pulse anodic voltammetric determination of pantoprazole in pharmaceutical dosage forms and human plasma using glassy carbon electrode. Anal Biochem 2003;323:48—53.
[11] Zhang H, Xu L, Zheng J. Anodic voltammetric behavior of resveratrol and its electroanalytical determination in pharmaceutical dosage form and urine. Talanta 2007;71:19—24.
[12] Farghaly OA, Taher MA, Naggar AH, et al. Square wave anodic stripping voltammetric determination of metoclopramide in tablet and urine at carbon paste electrode. J Pharm Biomed Anal 2005;38:14—20.
[13] Liu JH, Lin WF, Taylor L. The application of anodic stripping voltammetry to forensic science: II. Anodic stripping voltammetric analysis of gunshot residues. Forensic Sci Int 1980;16:43—52.
[14] Wang J. Analytical electrochemistry. 3rd ed. New Jersey: Wiley-VCH; 2006. p. 29.
[15] Erk N. Three new spectrophotometric methods applied to the simultaneous determination of hydrochlorothiazide and irbesartan. Die Pharmazie 2003;58:543—8.
[16] Gangola R, Singh N, Gaurav A, et al. Spectrophotometric simultaneous determination of hydrochlorothiazide and telmisartan in combined dosage form by dual wavelength method. Pharmacie Globale (IJCP) 2011;2:1—3.
[17] Burcin B, Mehmet G, Dogan TB, et al. Fully validated simultaneous determination of bisoprolol fumarate and hydrochlorothiazide in their dosage forms using different voltammetric, chromatographic, and spectrophotometric analytical methods. J AOAC Inter 2013;96:42—51.
[18] Ivanovic D, Medenica M, Janeic B, et al. Validation of an analytical procedure for simultaneous determination of hydrochlorothiazide, lisinopril, and their impurities. Acta Chromatographica 2007;18:143—56.
[19] Qutab SS, Razzaq SN, Ashfaq M, et al. Simple and sensitive LC—UV method for simultaneous analysis of hydrochlorothiazide and candesartan cilexetil in pharmaceutical formulations. Acta Chromatographica 2007;19:119—29.
[20] Violeta I, Dragica Z, Marina S. HPLC determination of hydrochlorothiazide in urine after solid-phase extraction. Macedonian Pharm Bulletin 2005;51:23—8.
[21] Menon GN, White LB. Simultaneous determination of hydrochlorothiazide and triamterene in capsule formulations by high performance liquid chromatography. J Pharm Sci 1981;70:1083—5.
[22] Farthinq D, Fakhry I, Ripley EB, et al. Simple method for determination of hydrochlorothiazide in human urine by
JOURNAL OF FOOD AND DRUG ANALYSIS XXX (2014) 1 —7
high performance liquid chromatography utilizing narrowbore chromatography. J Pharm Biomed Anal 1998;17:1455—9.
[23] Cooper MJ, Sinaiko AR, Anders MW, et al. High pressure liquid chromatographic determination of hydrochlorothiazide in human serum and urine. Anal Chem 1976;48:1110—1.
[24] Gholirand MB, Khodadadian M. Simultaneous voltammetric determination of captopril and hydrochlorothiazide on a graphene/ferrocene composite carbon paste electrode. Electroanal 2013;25:1263—70.
[25] Abdel Razak O. Electrochemical study of hydrochlorothiazide and its determination in urine and tablets. J Pharm Biomed Anal 2004;34:433—40.
[26] Baiqing Y, Xiaoying Z, Chunying X, et al. Electrochemical modification of graphene oxide bearing different types of oxygen functional species for the electro-catalytic oxidation of reduced glutathione. Sensors and Actuators B 2013;184:15—20.
[27] Baiqing Y, Chunying X, Dehua D, et al. Graphene oxide/nickel oxide modified glassy carbon electrode for supercapacitor and nonenzymatic glucose sensor. Electrochimica Acta 2013;88:708—12.
[28] Baiqing Y, Xiaoying Z, Dehua D, et al. Electrochemical determination of glutathione based on an electrodeposited nickel oxide nanoparticles-modified glassy carbon electrode. Anal Methods 2013;5:1779—83.
[29] Baiqing Y, Chunying X, Lin L, et al. Cu2O/NiOx/graphene oxide modified glassy carbon electrode for the enhanced
electrochemical oxidation of reduced glutathione and nonenzyme glucose sensor. Electrochimica Acta 2013;104:78-83.
[30] Junxia W, Jingnan M, Jing M, et al. Determination of rutin and puerarin in teas and pharmaceutical preparations using poly (evans blue) film-modified electrodes. J Food Drug Anal 2012;20:611-6.
[31] Baiqing Y, Jianshe H, Jinying S, et al. A novel technique for NACE coupled with simultaneous electrochemiluminescence and electrochemical detection for fast analysis of tertiary amines. Electrophoresis 2009;30:479-86.
[32] Huan P, Yunfeng S, Jimin D, et al. Porous nickel oxide microflowers synthesized by calcination of coordination microflowers and their applications as glutathione electrochemical sensor and supercapacitors. Electrochimica Acta 2012;85:256-62.
[33] Ghorbani-bidkorbeh F, Shahrokhian S, Mohammadi A, et al. Preparation of a naltrexone HCl potentiometric sensor and its application to pharmaceutical analysis and drug determination in biological fluids. J Food Drug Anal 2011;19:445-51.
[34] Alghamdi A, Kooli F. A sensitive procedure for the rapid electrochemical determination of Eosin-Y dye using voltammetric techniques at a mercury electrode. J Mater Environ Sci 2013;4:762-9.
[35] Stubauer G, Obendorf D. Determination of trace levels of niguldipine in urine and blood by adsorptive stripping voltammetry at the hanging mercury drop electrode. Analyst 1996;121:351-6.
