Voltammetric Determination of Diclofenac Sodium Using Tyrosine

Article pubs.acs.org/IECR

Voltammetric Determination of Diclofenac Sodium Using TyrosineModified Carbon Paste Electrode B. K. Chethana,† S. Basavanna,‡ and Y. Arthoba Naik*,† †

Department of Chemistry, School of Chemical Science, Kuvempu University, Shankaraghatta - 577451, India Department of Chemistry, BTL Institute of Technology & Management, Bommasandra Industrial Area, Bangalore - 560 099, India

‡

ABSTRACT: The oxidative behavior of diclofenac sodium (DCF) has been investigated by cyclic voltammetric and differential pulse voltammetric techniques, using a Tyrosine-modified carbon paste electrode (TCPE). Cyclic voltammetry was used to study the influence of pH on the anodic peak current of DCF. The solution conditions and operating parameters were optimized. The phosphate buffer (PBS) of pH 7.0 was selected as a suitable analytical medium in which DCF exhibited a sensitive diffusioncontrolled oxidative peak at 0.67 V (vs Ag/AgCl). The peak current varied linearly with DCF concentration in the range between 10 μM and 140 μM with a detection limit of 3.28 μM. The applicability of the proposed method was illustrated by the determination of DCF present in pharmaceutical samples and human urine samples. A mean recovery of DCF in the tablet was 99.5%, with a relative standard deviation of 1.15%.

1. INTRODUCTION Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most commonly employed drugs for the treatment of several diseases such as ankylosing, spondylitis, acute muscle pain conditions, etc.1 These drug molecules remedying the pain by reducing inflammation rather than opioids, which affect the central nervous system (CNS). Diclofenac is one of the most common nonsteroidal anti-inflammatory drugs used in the treatment of acute, chronic, painful, and inflammatory conditions and it is a potent inhibitor of cyclo-oxygenase enzymes for avoiding the production of prostaglandins.2−4 It is widely prescribed in clinical medicine to treat tuberculosis, urinary tract infections, post-operative or post-traumatic pain, etc.5,6 In addition to its medicinal importance, it is associated with adverse reactions such as hepatitis, hematological toxicity, aplastic anemia, neutropenia, hemolytic anemia, and thrombocytopenia.7 The extensive clinical use of diclofenac triggered the interest for the determination of this drug by simple and rapid techniques.8−10 Drug analysis, which is an important branch of analytical chemistry, plays an important role in drug quality control. Many analytical methods for the determination of diclofenac in biological fluids and in pharmaceutical preparations are reported in the literature. These methods include potentiometry,8−11 spectrophotometry,12−14 capillary zone electrophoresis (CZE),15 thin-layer chromatography,16 high-performance liquid chromatography−mass spectrometry (HPLC-MS),17−19 gas chromatography,20 polarographic analysis,21 chemometric techniques,22,23 etc. However, most of them are expensive and time-consuming; hence, the use of a voltammetric technique for the determination of diclofenac in pharmaceutical samples is of great importance, because of its experimental simplicity, high sensitivity, and selectivity of the procedure.24−30 Carbon paste electrode (CPE) is a mixture of an electrically conducting graphite powder and a pasting liquid. It has been widely used in electroanalytical chemistry as a working electrode. This is because it possesses various advantages, e.g., a wide potential range, simple and fast preparation, convenient © 2012 American Chemical Society

surface renewal, porous surface, and low residual current. In addition, it is inexpensive.31 The CPEs modified by using a catalyst will improve the electro-analytical performance of the electrode.32 In recent years, applications of modified carbon paste electrodes have provided considerable improvements in the electrochemical behavior of biologically important compounds.33−35 Several modified CPEs were reported for the determination of diclofenac in pharmaceutical formulations and in physiological samples.36,37 Chart 1 shows the structure of diclofenac. Chart 1. Structure of Diclofenac

In the present study, cyclic and differential pulse voltammetric techniques were used for the electrochemical determination of diclofenac in pharmaceuticals and human urine samples on Tyrosine-modified carbon paste electrodes (TCPEs).

2. EXPERIMENTAL SECTION 2.1. Apparatus. Electrochemical measurements were performed with the help of a CHI660D electrochemical workstation (CH Instruments, USA) coupled with a conventional three-electrode system. The carbon paste electrode modified with Tyrosine (TCPE) was used as working electrode. Received: Revised: Accepted: Published: 10287

December 14, 2011 June 18, 2012 July 14, 2012 July 15, 2012 dx.doi.org/10.1021/ie202921e | Ind. Eng. Chem. Res. 2012, 51, 10287−10295

Industrial & Engineering Chemistry Research

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A platinum wire and Ag/AgCl (3 M KCl) were used as counter and reference electrodes, respectively. The analysis of the samples by high-performance liquid chromatography coupled with mass spectroscopy (HPLC-MS) was carried out in a Thermo Finnigan Surveyor, comprising a BDS HYPERSIL C18 (250 mm × 4.6 mm) column. A photodiode array with a UV detector (280 nm) was used as the detector, and the data were processed by the Xcalibur software. Mass spectra were recorded using Thermo LCQ Deca XP MAX, with source voltage of 4.5 kV. 2.2. Chemicals. All the chemicals were of analytical-reagent grade and were used as received without any further purification. Potassium ferricyanide and diclofenac sodium (DCF) were purchased from Sigma−Aldrich. The tablets and injections containing DCF were purchased from the pharmaceutical shops. The 0.1 M phosphate buffer solution was prepared and the pH of the solutions were adjusted by H3PO4 or NaOH. Doubly distilled water was used for the preparation of all solutions. 2.3. Preparation of Modified Carbon Paste Electrodes. The TCPE was prepared by mixing graphite powder, paraffin oil (mineral oil), and modifier in the ratio of 77:20:3 (w/w) and grinding the resultant mixture in a mortar for a period of 30 min. The homogenized paste was packed carefully into the glass tube (3 mm inner diameter and a height of 6 cm) without any air gap. A copper wire was inserted to establish electrical contact. The external surface of the carbon paste was mechanically renewed and polished with weighing paper to get a smooth and shiny surface. Usually, this simple operation was made before starting a new set of experiments. The carbon paste electrode (CPE) was prepared in the same way without the addition of modifier (Tyrosine).

Figure 2. Effect of Tyrosine content of TCPE on anodic peak current of 1 mM DCF at a scan rate 10 mV s−1.

NaCl at CPE and TCPE. The cyclic voltammograms of [Fe(CN)6]3−/4− couple show a peak-to-peak separation (ΔEp) of 0.157 mV at CPE and 0.130 mV at TCPE. The reduction in ΔEp value at TCPE clearly indicated the more-reversible charge-transfer process of TCPE than that of CPE.38 From Figure 1, it can also be observed that the magnitude of redox peak current for [Fe(CN)6]3−/4− couple has been enhanced (2fold) at TCPE, when compared to CPE. The increase in peak current and the decrease in ΔEp at TCPE is attributed to the catalytic effect of Tyrosine, which enhances the active surface area of the electrode.39,40 These results revealed that TCPE catalyzed the redox reaction of [Fe(CN)6]3−/4−. The active surface area of the electrode was obtained by the cyclic voltammetric method using 1 mM K3[Fe(CN6)] as a probe at different scan rates. For a reversible process, the following Randles−Sevcik equation can be used:

3. RESULTS AND DISCUSSION 3.1. Electrochemical Behavior of [Fe(CN)6]3−/4− Couple. In order to better understand the electrochemical properties of TCPE, [Fe(CN)6]3−/4− was used as the electrochemical redox probe. Figure 1 shows the cyclic voltammograms of 1 mM K3[Fe(CN)6] containing 0.1 M

Ip = (2.69 × 105)n3/2AD01/2C0*ν1/2

where Ip refers to the anodic peak current, n is the number of electrons transferred, A is the surface area of the electrode, D0 represents the diffusion coefficient, υ denotes the scan rate, and C0* is the concentration of K3[Fe(CN)6]. For 1 mM K3[Fe(CN)6] in 0.1 M NaCl electrolyte, n = 1 and D0 = 7.6 × 10−6 cm2 s−1; then, from the slope of the plot of Ipa vs υ1/2, the active surface area of the electrode has been calculated. For CPE, the electrode active surface area was found to be 0.0147 cm2 and, for TCPE, it was 0.01985 cm2. 3.2. Influence of Modifier (Tyrosine). Mixing of the carbon paste with some sort of adsorptive species (modifiers) remarkably increases the sensitivity of the CPEs.41,42 In the present study, Tyrosine was tested as a modifier, because it possesses good adsorptive properties.43 The effect of the modifier concentration (Tyrosine) in the CPE on its voltammetric response was evaluated in phosphate buffer solution (pH 7.0) with 1 mM diclofenac, shown in Figure 2. The best modifier concentration should give the highest current value and resolution and the longest distance from the discharging current of the background.41,44 From Figure 2, it can be observed that a 3-wt %-Tyrosine-modified carbon paste electrode gives the highest peak intensity values; in contrast, other compositions of the electrode give weak signals. This may

Figure 1. Cyclic voltammograms of (a) blank solution at TCPE and 1 mM K3[Fe(CN)6] in 0.1 M NaCl on (b) CPE and (c) TCPE. Scan rate = 50 mV s−1. 10288

dx.doi.org/10.1021/ie202921e | Ind. Eng. Chem. Res. 2012, 51, 10287−10295


Article

Figure 3. (A) Cyclic voltammograms of blank solution at TCPE (curve a) and with 1 mM of DCF on CPE (curve b) and TCPE in 0.1 M PBS pH 7.0 (curve c); the scan rate was 10 mV s−1. (B) Successive cyclic voltammograms of 1 mM DCF on TCPE in 0.1 M PBS pH 7.0; scan rate = 10 mV s−1.

be due to the fact that the CPE with a modifier concentration more than 3 wt % gives resistive voltammetric signals.44 The modifier (Tyrosine) also influences the oxidation potential values, although the shifts used in the peaks of the diclofenac are not significantly relevant. Taking previous considerations into account, 3 wt % of Tyrosine was selected as the best concentration to determine the diclofenac in further experiments. 3.3. Cyclic Voltammetric Studies of DCF. The cyclic voltammograms (Figure 3A) were obtained in the absence (curve a) and presence of 1 mM DCF at CPE (curve b) and TCPE (curve c) in phosphate buffer solution (pH 7.0). DCF exhibits an oxidation peak and peak potential was observed at +0.67 V (Vs Ag/AgCl) on TCPE, with an increase in anodic peak current, compared to that of CPE. The remarkable enhancement of anodic peak current provides clear evidence of the catalytic effect of TCPE. On reverse scan, no reduction peak was observed, indicating that the electron transfer process at the electrode surface of the DCF is an irreversible process.28 Nevertheless, it was found that the oxidation peak current of

Figure 4. (A) Cyclic voltammograms of 1 mM DCF on TCPE at different scan rates ((10 mV s−1 (curve a), 30 mV s−1 (curve b), 50 mV s−1 (curve c), 70 mV s−1 (curve d), 90 mV s−1 (curve e), and 120 mV s−1 (curve f) in 0.1 M phosphate buffer solution, pH 7.0. (B) Relationship between anodic peak current and square root of scan rate. (C) Relationship between anodic peak potential and logarithm scan rate. 10289



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Figure 6. Relationship between peak current and solution pH at TCPE in the sweep rate of 10 mV s−1 and the presence of 1 mM DCF.

Table 2. Determination of DCF Present in Real Samples (n = 4) analyte

detected (μM)

spiked (μM)

found (μM)

recovery (%)

60.04 72.74 28.75

20 20 20

79.94 92.61 48.62

99.5 99.3 99.3

20.31

20

40.07

98.8

tablet injection human urine sample 1 human urine sample 2

cyclic voltammograms were recorded on TCPE at different scan rates (Figure 4A). The plot of square root of scan rate versus peak current showed a linear relationship, which is a characteristic of a diffusion-controlled process,48 and the corresponding equation can be expressed as Ipa = 6.921 υ1/2 − 8.59, r = 0.9843 (Figure 4B). A linear relationship was observed between log Ipa and log υ, and the corresponding equation can be expressed as log Ipa = 0.601 log υ + 0.577; r = 0.9940. The value 0.601 is close to the theoretical value of 0.500, which clearly indicates a diffusioncontrolled electrode process.48 As the scan rate increased, the peak potential shifted to a more positive value, and a linear relationship was observed in the range of 10−120 mV s−1 (Figure 4C). The equation can be expressed as Epa = 0.082 log υ + 0.5754; r = 0.9911. As for an irreversible electrode process, according to Laviron,49 Ep is defined by the following equation:

Figure 5. (A) Differential pulse voltammograms of solutions containing various concentrations of DCF (10 μM (curve a), 40 μM (curve b), 80 μM (curve c), 100 μM (curve d), and 140 μM (curve e)). Supporting electrolyte in all measurements was 0.1 M phosphate buffer with pH 7.0. (B) Dependence of the anodic peak current (Ipa) on concentration for the oxidation of DCF at the surface of TCPE obtained from the data of panel (A).

DCF showed a remarkable decrease during the successive cyclic voltammetric sweeps (Figure 3B). A decrease in the oxidation peak current occurs with an increase in the number of successive sweeps. This phenomenon may be due to the fact that the diffusion of its oxidative product occurs at the electrode surface and leads to saturation of the active surface area of the electrode.45−47 Therefore, the voltammograms corresponding to the first cycle was generally recorded. 3.4. Influence of Scan Rate. In order to study the nature of the electrode process occurring at the electrode surface,

Ep = E 0 +

⎛ 2.303RT ⎞ ⎛ RTk0 ⎞ ⎛ 2.303RT ⎞ ⎜ ⎟log⎜ ⎟log ν ⎟+⎜ ⎝ αnF ⎠ ⎝ αnF ⎠ ⎝ αnF ⎠

Table 1. Comparison of Electrochemical Data along with Reported Literature Values for the Determination of Diclofenac electrode

analytical method

linear dynamic range (mol/L)

limit of detection (μ mol/L)

ref

PVC membrane electrode Ni-Curcumin complex modified electrode Ni-hydroxide modified Ni electrode ion-selective-membrane electrode TCPE

poteniometry amperometry amperometry potentiometry differential pulse voltammetry

1 × 10−2−5 × 10−5 1.96 × 10−4−15.5 × 10−4 1.96 × 10−4−26.5 × 10−4 2 × 10−5−1 × 10−4 10 × 10−6−140 × 10−6

23 27.9 31.7 99 3.28

10 29 30 11 present work

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calculated from the slope of Ep vs log υ. In this system, the slope is 0.082, taking T = 298 K, R = 8.314, and F = 96 480, αn was calculated to be 0.7194. Generally, α is assumed to be 0.5 in a total irreversible electrode process.50 Therefore, the number of electrons (n) transferred in the electro-oxidation of DCF was calculated and found to be 2. The value of k0 can be determined from the intercept of Figure 4C if the value of E0 is known. The value of E0 can be obtained from the intercept of the plotted Ep vs υ curve by extrapolating to the vertical axis at υ = 0.51 In the present system, the intercept of Ep vs log υ plot was 0.5754; k0 was calculated to be 2808.98 s−1. 3.5. Differential Pulse Voltammetric (DPV) Studies. To develop a voltammetric method for determining the drug, we selected the differential pulse voltammetry mode, because the peaks are sharper and better defined at lower concentration with a lower background current and improved resolution.52,53 Based on the results, it could be possible to apply this technique for the quantitative analysis of DCF. The phosphate buffer solution of pH 7.0 was selected as the supporting electrolyte for the quantification of DCF. Differential pulse voltammograms showed that the peak current increased linearly as the concentration of DCF increased (see Figure 5A). Linear calibration curves were obtained (Figure 5B) for DCF in the concentration range from 10 × 10−6 to 140 × 10−6 M. The linear equation was Ip (μA) = 0.1905C (μM) + 14.406; r = 0.9905. The detection limit (S/N = 3) was 3.28 μM. The limit of detection (LOD) values of experimentally determined diclofenac and reported LOD values are tabulated in Table 1. The relative standard deviation after 10 successive scans at the same electrode was found to be 3.5% for 30 μM DCF,

Figure 7. Demonstration of determining diclofenac sodium (DCF) in the Voveran tablet using the standard addition method. Curve (a) shows the differential pulse voltammogram of diclofenac in the Voveran tablet, and curve (b) shows the differential pulse voltammogram of diclofenac after adding 20 μM diclofenac standard solutions. Experimental condition are the same as those in Figure 5A.

where α is the charge-transfer coefficient, k0 the standard rate constant of the reaction, n the number of electrons transferred, υ the scan rate, R the gas constant, F the Faraday constant, and E0 the formal redox potential. Ther value of αn can be

Figure 8. HPLC-MS spectra of pure DCF in a methanol−water solvent. 10291



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Figure 9. High-performance liquid chromatography (HPLC) of (a) Voveran tablet, (b) klofenac injection, and (c) human urine sample 1.

peak current was observed at pH 8.0; however, from an analytical point of view, pH 7.0 was selected for further studies of diclofenac, since pH 7.0 is considered to be the physiological pH.40,56 3.7. Analytical Applications. 3.7.1. Pharmaceutical Analysis. To evaluate the applicability of the proposed method in the analysis of pharmaceutical sample, the determination of DCF in Voveron-50 and injection (klonefac) samples has been undertaken. All the results are summarized in Table 2. Four tablets of Voveran-50 were weighed and ground to a fine powder using a mortar. A suitable amount (0.8 g) of this powder was taken in 50 mL of phosphate buffer solution (pH 7.0) and sonicated for 15 min at room temperature to achieve

indicating that the TCPE had excellent reproducibility. All these results indicated the good analytical applicability of TCPE for the determination of DCF in real samples. 3.6. Effect of pH. The influence of solution pH on the oxidation of diclofenac at the TCPE was investigated. The effect of the pH in the range of 3.0−8.0 on the anodic peak current has been presented in Figure 6. The results showed that the anodic peak current for DCF has increased semilinearly with increasing solution pH. The peak current increases at pH 4.0, because DCF is a monobasic acid with a dissociation constant (pKa) of 4.0 ± 0.2 at 25 °C in water.54,55 From Figure 6, it was also observed that the oxidation peak current increases gradually with an increase in pH from 5.0 to 8.0. The maximum 10292



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the pharmaceutical samples. The proposed method offers the advantages of accuracy, as well as a simplicity of reagents and apparatus. In addition, the analysis of DCF present in spiked urine samples demonstrated the applicability of the method for real sample analysis. Furthermore, the present method could possibly be adopted for pharmacokinetic as well as clinical and quality-control laboratories.

complete dissolution. Solutions of tablets and injections were diluted separately so that the DCF concentration lies in the range of calibration curve. Differential pulse voltammograms were then recorded at TCPE and, keeping the dilution factor in consideration, the concentration of DCF in the pharmaceutical formulations was determined (Figure 7). The proposed method shows that Voveran-50 and injection (klonefac) samples contain 6.12% and 7.27% DCF, respectively, which were in good agreement with the labeled values of the samples. 3.7.2. Determination of DCF in Human Urine. The utilization of the proposed modified electrode in real sample analysis was also investigated by the direct analysis of DCF present in a human urine samples. Human urine samples were obtained from two healthy volunteers of different age groups. Aliquots were centrifuged at 3000 rpm for ∼5 min at room temperature. Resultant urine samples were diluted up to 5 times with the phosphate buffer solution and subjected to voltammetric analysis. The urine sample then was spiked with a known concentration of DCF (Table 2). The concentrations of DCF in the human urine sample and the spiked analytes were estimated using the calibration plot (Figure 5B). The results obtained for different urine samples, before and after spiking, are given in Table 2. The recoveries of the spiked analytes were found to be 99.3% and 98.8%, indicating that the detection procedures were free from interference of the matrix present in the urine sample matrix. 3.8. Interference Effect. Pharmaceutical and biological samples contain many excipients, which may affect the selectivity of the modified electrodes.25 Differential pulse voltammetric procedure for the analysis of diclofenac at TCPE was examined under optimized experimental condition in the presence of some foreign substances (such as glucose, ascorbic acid, and uric acid) in order to check the selectivity of developed method. The results showed that the presence of foreign substances do not affect the oxidation potential of diclofenac, even if concentrations of interferents are 10-fold higher than that of diclofenac. Obtained results are comparable with the reported literatures,25,29 which clearly indicate the efficiency of method for the determination of diclofenac in the presence of the possible interfering species. Therefore, the developed method can be considered as selective for the determination of diclofenac in real samples. 3.9. HPLC-MS Studies. HPLC-MS studies were carried out for DCF, Voveran-50, Klofenac injection, and human urine samples. Figure 8 represents the HPLC-MS spectrum of pure DCF in methanol−water system. The retention time was observed at 11.53 min and mass peak at 296.24 (m/z) on BDS HYPERSIL C18 column. In Figure 9, the peaks observed at 13.57, 13.78, and 14.16 min correspond to the retention times of DCF present in tablet, injection, and human urine samples, respectively. A small shift in the retention time in case of real samples is attributed to the presence of other ingredients.57,58 These results confirmed the presence of DCF in the tablet, injection, and human urine samples.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-9448855078. Fax: +91-08282 256255. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Department of Science and Technology, New Delhi, India for providing financial assistance. The authors wish to thank Kuvempu University, Shankaragatta, India for providing laboratory facilities to carry out this work. The authors are thankful to Prof. K. J. Rao and Dr. H. N. Vasan (SSCU, Indian Institute of Science, India) for their encouragement and useful discussions.

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REFERENCES

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4. CONCLUSIONS The present work illustrates the oxidation behavior of DCF on the Tyrosine-modified carbon paste electrode (TCPE). When the potential was made to move, DCF produced one anodic peak at ∼0.67 V in 0.1 M phosphate buffer solution (pH 7.0). The oxidation process was irreversible and diffusion-controlled. This method could be used successfully to determine DCF in 10293



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Voltammetric Determination of Diclofenac Sodium Using Tyrosine

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