VOLTAMMETRIC DETERMINATION OF RUPATADINE AT GRAPHENE MODIFIED GLASSY CARBON ELECTRODE

VOLTAMMETRIC DETERMINATION OF RUPATADINE AT GRAPHENE MODIFIED GLASSY CARBON ELECTRODE

H. Devnani ^*, P. Singh, S. Saxena and S. P. Satsangee

University Science Instrumentation Centre, Dayalbagh Educational Institute, Dayalbagh, Agra - 282005, Uttar Pradesh, India.

ABSTRACT: Electrochemical behavior of rupatadine, an anti-allergic drug was studied at graphene modified glassy carbon electrode in Britton-Robinson buffer (pH 6.5) by square wave voltammetry (frequency, 50 Hz; amplitude, 0.02 V; step potential, 0.005 V) and cyclic voltammetry (scan rates, 20-100 mV/s) techniques. Cyclic voltammetry revealed the oxidation of rupatadine corresponding to N-dealkylation which is the major metabolic pathway of rupatadine in the human body. Atomic force microscopy, scanning electron microscopy and electrochemical impedance spectroscopy was used to study the surface characteristics of the fabricated sensor. The electrode dynamic parameters were investigated and the oxidation mechanism was proposed. The lower limit of detection and quantification were achieved with the modified sensor as compared to the bare sensor due to enhancement in electron transfer process at the surface of modified sensor owing to greater electroactive surface area. The limit of detection was found to be 56.78 ng/ml and limit of quantification, 172.06 ng/ml at graphene modified glassy carbon electrode.

Rupatadine, Graphene, Voltammetry, Electrochemical impedance

INTRODUCTION: The need for clinical and pharmacokinetic studies of pharmaceutical compounds drives the development of the simple, fast, accurate and sensitive analytical method. Drug monitoring plays an important role in drug quality control so it is important to determine their active ingredient ^1-4. Rupatadine (RTD) is 8 chloro 6, 11 dihydro 11-[1-(5-methyl-3-pyridinyl) methyl -4-piperidinyllidene] -5 H- benzo [5, 6] cyclohepta [1, 2-b] pyridine Scheme 1.

SCHEME 1: STRUCTURE OF RTD

It acts as a long-acting, non sedative antagonist at histaminergic H1-receptors and also antagonizes the platelet-activating factor (PAF) ⁵. RTD is a novel compound that shows both antihistamine and anti-PAF effects through its interaction with specific receptors and not due to physiological antagonism ⁶. RTD also has potentially beneficial effects such as inhibition of mast cell degranulation, neutrophil and eosinophil migration, and cytokine release ⁷.

In a literature survey HPLC methods, ^8-12 HPTLC method, ¹³ spectrophotometric methods ¹⁴ and titrimetric method ¹⁵ were reported.

The electrochemical techniques have been used for investigation of the redox mechanism of drugs as their active ingredient is most of the times active in contrast to the excipients. The analyte can be identified by the voltammetric peak potential and thus the technique is selective. The analysis is completed in relatively short time as compared to other techniques ^{16, 17}. In the field of clinical and pharmaceutical analysis, use of various electrodes viz. mercury, solid and modified electrodes for electroanalytical measurements has increased in recent years because of their applicability to the determination of active compounds that undergo redox reactions ¹⁸.

The electrochemical behavior of the analytes can be greatly enhanced by modifying the working electrode surface. Thus different kinds of modified electrodes have been fabricated to enhance the electrode reaction rate and thereby analytical performance ^19-21.

Graphene (GRP), a 2D, honeycomb sp² bonded carbon atom has proved to be very promising and potential nanomaterial. It has an excellent electrical conductivity (200,000 cm²/Vs) ²², large surface area (theoretical value is 2630m²/g) ²³, high thermal conductivity (5000W/mK) ²⁴ and strong mechanical strength ²⁵ which makes it an outstanding material to exhibit plethora of applications such as electronics ²⁶, supercapacitors ²⁷, Lithium-ion batteries ^{23, 28}, fuel cells ²⁹, solar cells ^{30, 31}, electrochemical sensors ^{32, 33} and biosciences/biotechnologies ³⁴ etc.

The present work reports an electrochemical method for the study of RTD by square wave voltammetry (SWV) and cyclic voltammetry (CV) at graphene modified glassy carbon electrode (GRP/GCE), using Britton-Robinson (B-R) buffer (pH 6.5) as a supporting electrolyte. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) helped in exploring the surface topology and morphology of the modified sensor. Electrochemical impedance spectroscopy (EIS) detailed lowering in charge transfer resistance and catalytic activity of the fabricated sensor (GRP/GCE) as compared to the bare electrode (GCE).

EXPERIMENTAL:

Instrumentation: All electrochemical studies were carried out using AUTO LAB PGSTAT 302N (Eco-Chemie B.V., Utrecht, The Netherlands) potentiostat-galvanostat with IME663 and software NOVA 1.8. EIS was carried out using FRA 2 module. A standard three-electrode electrochemical assembly was used which consisted of GCE and GRP/GCE as working electrode, platinum wire as counter and Ag│AgCl (3M KCl) as reference electrode. These were fitted in one compartment cell connected with electrochemical workstation through Metrohm 663VA stand. All pH measurements were made on a Mettler Toledo pH meter fitted with a gel electrode and Ag│AgCl electrode as reference. All measurements were carried out at room temperature. AFM study was carried out at Nanosurf Easyscan (Switzerland) with software Nanosurf 1.8. SEM was performed at Japanese Electro-optics Ltd. (JSM-5800LV) involving the INCA software from Oxford (U.K.).

Materials and Reagents: Graphene (12 nm) was procured from Graphene Laboratories, USA. RTD standard (≥99%) was obtained from the Sigma Aldrich. Ultrapure water (Milli-Q water with resistivity 18 MΩ.cm) was obtained from ELGA purification system (U.K). Standard solution of RTD (2mg/mL) was prepared by dissolving pure compound in methanol and was further diluted with B-R buffer to get the concentration in the working range. For real sample analysis, RTD was determined in Rupanex tablet. Solutions at all the stages of the study were prepared by using analytical grade reagents and were used without further purification.

This work was carried out at University Science Instrumentation Centre, Dayalbagh Educational Institute, Dayalbagh, Agra in the year 2013.

Fabrication of GRP/GCE: Before the modification, GCE was polished against the alumina slurry (particle size 0.01-0.3 µm) spread over the Buehler cloth, and thoroughly rinsed with deionized water followed by gently blowing under the nitrogen gas stream to remove the residual alumina particles.

For modification, a suspension of grapheme (GRP) was prepared by dispersing 1 mg of GRP in 1 mL of dimethylformamide (DMF). The obtained suspension was vortexed for 15 min followed by ultrasonication for 2 h. 1 µL of the modifier was withdrawn and drop casted onto the working surface of GCE. The modified electrode surface was dried at room temperature. An increase in the baseline current was observed at higher concentration of GRP, so 1 µL of GRP suspension in DMF (1 mg/mL) was optimized for the modification of GCE.

Analytical Procedure: The stock solution of RTD (2mg/mL) was prepared in methanol. Working solutions were prepared by further dilution with the supporting electrolyte to get the desired concentration. A range of B-R buffer (2-12 pH) was prepared in ultrapure water and used as supporting electrolyte. About 10 mL of electrolyte solution containing an appropriate amount of standard RTD or sample was added to the electrolytic cell. Prior to all the electrochemical measurements, all the solutions were purged with nitrogen gas to remove interference due to oxygen. SWV and CV were recorded in potential range 0 to 1.4 V. The surface of GCE was renewed prior to the modification by abrading it smoothly against the alumina slurry spread over the Buehler cloth. Electrochemical impedance measurements were performed in 5 mM K₃Fe (CN) ₆ prepared with 0.1 M KCl by applying the AC voltage with 5 mV amplitude in a frequency range from 1 Hz to 100 kHz. Nyquist plots were studied for the analysis.

RESULTS AND DISCUSSIONS:

Characterization of the GRP/GCE and bare GCE: Surface characterization of the GRP/GCE was carried out by SEM and AFM. For this, the suspension of modifier was drop casted over the surface of ITO. SEM image Fig. 1A explains the formation and morphology of the GRP film. It illustrates the flake-like shape of GRP, which is intended to improve the properties of the sensor leading to increase in the interaction of the analyte with the modified surface. AFM helped to interpret the surface profile (topology) of the electrode surface. The roughness parameters are tabulated in Table 1, which imply that the GRP layer is well-coated onto the surface of GCE. Further, it indicates that the graphene nanoparticles were not removed from the surface of GCE by several flushings with deionized water Fig. 1B.

The electroactive surface area of the GRP/GCE and the GCE were obtained by the CV using 1 mM K₃ Fe(CN)₆ as a redox indicator at different scan rate. The peak current Ip can be calculated by the Randles-Sevcik equation at 25 ºC as follows: ³⁵

Ip = (2.69 × 10⁵) n^3/2 A C₀ D^1/2 Ʋ ^½  .....1

For K₃Fe (CN) ₆, n=1 and D= 7.6 × 10^-6 cm²/s. From the slope of Ip (µA) vs. Ʋ ^1/2, the electroactive surface area was determined to be 0.028 cm² for GCE and 0.043 cm² for GRP/GCE. Thus there is enhancement in surface area on modification of the electrode.

TABLE 1: AFM PARAMETERS FOR GRP/ITO

FIG. 1: SCANNING ELECTRON MICROSCOPIC IMAGE OF GRP/ITO (A) AND 3D ATOMIC FORCE MICROSCOPIC IMAGE OF GRP/ITO (B)

Electrochemical Behavior of RTD at GRP/GCE and GCE: Electrochemical behavior of RTD at GCE and GRP/GCE was investigated by CV and SWV techniques. Fig. 2 represents the SWV of RTD (4000 ng/mL) at the bare GCE (b) and GRP/GCE (c) whereas the Fig. 3 represents the CV of RTD (4000 ng/mL) at bare GCE (b) and GRP/GCE (c). At each of the electrodes, a well-defined oxidation peak for RTD was obtained.

FIG. 2: SQUARE WAVE VOLTAMMOGRAMS OF RTD (4000 ng/mL, AMPLITUDE: 20 mV, FREQUENCY: 25 Hz, STEP POTENTIAL: 5 mV, SCAN RATE: 125 mV/s)) AT GCE (curve b), at GRP/GCE (curve c), AND BLANK (curve a)

FIG. 3: CYCLIC VOLTAMMOGRAMS OF RTD (4000ng/mL at 50mV/s) AT GCE (curve b), AT GRP/GCE (curve c), AND BLANK (curve a)

The response characteristics of SWV exhibits that the anodic peak current intensity was enhanced by 64.6% at GRP/GCE compared to the bare GCE whereas the response characteristics of CV represented an enhancement in the peak current intensity by 52.4% at GRP/GCE as compared to GCE. Therefore the electroanalytical performance of GRP/GCE was advantageous over that observed at GCE. No peaks were observed in the reverse scan of CV corresponding to the anodic voltammetric peaks. This indicates that the oxidation reaction is irreversible.

Electrochemical Impedance Spectroscopy: The increase in the electron transfer at the modified electrode was investigated using the EIS technique. For this AC amplitude of 5 mV at a frequency of 1 Hz-100 kHz was applied. The higher electro-catalytic behavior of GRP/GCE was confirmed by the reduction in the charge transfer resistance Fig. 4A.

FIG. 4: NYQUIST PLOTS OF 5 mM K₃Fe(CN)₆ AT GCE (A, a), GRP/GCE (A, b), AND CORRESPONDING EQUIVALENT CIRCUIT (B)

The values observed for the charge transfer resistance, R_CT, fitting an appropriate equivalent circuit, were determined and found to be 17.7 kΩ at GCE and 9.2 kΩ at GRP/GCE Fig. 4B. The decrease in charge transfer resistance can be related to the electrode coverage and are given by the following equation:

(1- Ø)=R°_CT/R_CT .....2

Where Ø is the apparent electrode coverage and R°_CT and R_CT is the charge transfer resistance measured at bare and modified carbon electrode.³⁶ The electrical equivalent circuit compatible with the Nyquist plots comprises of R_S the solution resistance, Q_dl the double layer capacitance, R_CT the charge transfer resistance and W the Warburg impedance. The Warburg type impedance represents the linear line that appeared at lower frequencies because of the diffusion-controlled process. A constant phase element was used instead of the pure capacitance because of the electrode surface inhomogeneities ³⁷.

Effect of Scan Rates: To study the effect of scan rate on the oxidation peak of RTD, voltammograms were recorded for different scan rates ranging from 20-100 mV/s at a fix concentration (4000 ng/mL) of RTD. The information involving electrochemical mechanism can be obtained from the investigation of the response characteristics of CV on the electro-oxidation process of RTD Fig. 5.

A linear relationship was obtained between the peak current intensity (Ip) and the square root of scan rate (Ʋ)^1/2, suggesting the diffusion-controlled process at the electrode surface. The linear relationship between Ip and (Ʋ)^1/2 follows Ip(µA)=0.314(Ʋ)^1/2-0.013, R²=0.999. Since the oxidation peak was not accompanied by the reduction peak, therefore, the electro-oxidation process of RTD was proved to be an irreversible diffusion-controlled process.

FIG. 5: CYCLIC VOLTAMMOGRAMS OF RTD (4000 ng/mL IN B-R BUFFER 6.5 pH) AT SCAN RATES 20-100 mV/s (20, 40, 60, 80 AND 100 mV/s, CURVES a – e RESPECTIVELY) AND INSET FIGURE REPRESENTS PLOT OF (Ʋ) ^1/2 vs Ip

Determination of Electrode Dynamics Parameters (α and n) and Proposed Mechanism of RTD: For an irreversible anodic reaction, the relationship between Ep and Ʋ can be described as ³⁸

E_p = E^o’ + RT/αn_α F [0.780 + ln (D^o1/2/k_s) + ln (αn_αF Ʋ/RT) ^1/2] ..... 3

Eº is the formal redox potential, R and F values are 8.314 J/K.mol and 96480 C/mol respectively, k_s is the standard heterogeneous rate constant, and D is the diffusion coefficient.

According to the slope of the straight line of Ep against ln Ʋ, α n = 0.89. The electron participating in the electrode reaction process can be calculated to be 2e-, assuming α is 0.5. Assuming n_α = n, the value of α is 0.44.

Thus the voltammetric technique can be utilized to elucidate the electroactivity and possible surface activity of various compounds, as seen in the case of RTD. CV and SWV of RTD exhibited well defined anodic voltammetric peak which is attributed to the N dealkylation of the piperidine nitrogen Scheme 2.

SCHEME 2: PROPOSED MECHANISM FOR RTD

Effect of Supporting Electrolyte and its pH: The influence of various supporting electrolytes such as KCl, phosphate buffer, acetate buffer and B-R buffer on the electrooxidation of RTD (4000 ng/mL) was investigated. B-R buffer was found to be the best supporting electrolyte with well defined current responses, the stability of the solution system and prominent peak shape. The anodic peak current of RTD was found to be sensitive towards the pH of the solution and was analyzed in the range 2.5 to 12. A sequential shift of the anodic peak potential with the change in solution pH was observed implying that the protons had participated in the electrode reaction process of RTD ^{39, 40}. The best result with respect to sensitivity was obtained at 6.5 pH Fig. 6B. Therefore, pH 6.5 was selected and used throughout the analysis. Fig. 6A explains the shifting of peak potential with increase in pH values following the linear equation Ep (mV) =1192-41.54 pH, R²=0.983. According to Nernst equation: ⁴¹

E_p = E^o’- 2.303RTmpH/ (1-α) nF .....4

Where, m is the proton number of electrode reaction, which was calculated to 1 from the slope of Ep–pH equation

FIG. 6: DEPENDENCE OF PEAK POTENTIAL (Ep, A) AND PEAK CURRENT (Ip, B) OF RTD (4000 ng/mL) ON THE pH (2.5-10.5) OF SUPPORTING ELECTROLYTE (B-R BUFFER)

Calibration Curve and Detection Limit: SWV recorded with the increasing amounts of RTD under the optimized conditions exhibited that the peak currents increased linearly with the increasing concentration Fig. 7. A linear relationship between the peak current and the RTD concentration was observed in the range of 400 ng/mL to 1400 ng/mL The calibration plot of Ip (µA) vs. concentration (ng/mL) followed the linear equation, Ip(µA) = 0.018 (ng/mL) + 0.920, R² = 0.998 for GRP/GCE.

Limit of detection (LOD) and limit of quantification (LOQ) (estimated as 3 S/m and 10 S/m, respectively, ‘S’ being the standard deviation and ‘m’ being the slope of the calibration curve) were obtained as 56.78ng/mL and 172.06ng/mL for GRP/GCE. Various statistical parameters for the linear regression equation have been evaluated and reported in Table 2.

TABLE 2: SQUARE-WAVE VOLTAMMETRIC METHOD VALIDATION PARAMETERS FOR STANDARD LINEARITY

FIG. 7: SQUARE WAVE VOLTAMMOGRAMS OF RTD AT DIFFERENT CONCENTRATION LEVELS, (400ng/mL-1400ng/mL, a- e), BLANK (curve a) AND INSET FIGURE REPRESENTS PLOT OF CONCENTRATION vs. CURRENT Ip (400, 600, 800, 1000, 1200 AND 1400 ng/mL)

Determination of RTD in Pharmaceutical Formulation (Rupanex): The modified sensor, GRP/GCE was successfully applied for the determination of RTD in Rupanex (Mfd. by Dr. Reddy’s). No sample pre-treatment and time-consuming extraction process were required prior to analysis. Tablets were crushed and powdered using a mortar and pestle. A weighed amount was then added to a specific volume of the suitable solvent. In this case, 20 mg of drug was added to 10 ml methanol. To achieve maximum solubility of drug, sonication was carried out for 5 min and then centrifuged at 3200 rpm for about 5-10 min. The clear supernatant was used for the analysis. It was diluted with pH 6.5 BR buffer to get final concentration in working range. SWV was run under optimized conditions as described in the text. The concentration of RTD was then determined using the standard addition method by the following equation: ⁴²

C_u =I_p1 C_sV_s / [I_p2 (V_u+V_s)] - I_p1V_u .....5

After getting the voltammogram of the supporting electrolyte, known volume (V_u) of unknown concentration (C_u) of the investigated drug was added and the resulting voltammogram was recorded and its peak current (I_p1) measured. The known volume (V_s) of known concentration (C_s) of the standard pure drug was then added and its peak current (I_p2) measured. The concentration calculated by the proposed method is compared with that claimed in tablet (Rupanex) in Table 3.

TABLE 3: ANALYTICAL RESULTS FOR RTD IN RUPANEX TABLET

^a [standard found/standard added] × 100

*Amount found represents the average of six observations (n = 6)

CONCLUSION: GRP/GCE proved to be effective for the selective and sensitive determination of RTD in B-R buffer at pH 6.5 without any major interference. The modified electrode offered higher electrocatalytic activity towards RTD which can be attributed to the increase in the surface area and interaction of the graphene with the solution system thereby enhancing the analyte response. The oxidation mechanism and the electron transfer process was analyzed and discussed in the present communication. The electrode reaction process dynamics parameters were also investigated and CV with the variation of scan rate study showed that the sensor follows the irreversible diffusion-controlled process. Calibration plot revealed linearity within the range of 400-1400ng/mL with correlation coefficient of 0.998. LOD and LOQ were found to be 56.78ng/mL and 172.06ng/mL respectively.

ACKNOWLEDGEMENT: Financial support from the Ministry of Human Resource and Development, India under NMEICT project is gratefully acknowledged. The authors are also thankful to Prof. M. Shyama Prasad and Mr. Areef A. Sardar for providing SEM lab facility at National Institute of Oceanography, Goa, India.

CONFLICT OF INTEREST: Nil

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    How to cite this article:

    Devnani H, Singh P, Saxena S and Satsangee SP: Voltammetric determination of rupatadine at graphene modified glassy carbon electrode. Int J Pharm Sci & Res 2014; 5(11): 4792-99. doi: 10.13040/IJPSR.0975-8232.5(11).4792-99.

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