The present work describes a novel, simple, and fast electroanalytical methodology for naproxen (NAP) determination in pharmaceutical formulations and biological fluids in the presence of its degradation products. Carbon paste electrodes (CPEs) modified with different carbon nanomaterials, namely, glassy carbon powder (GCE), multiwall carbon nanotubes (MWCNTs), single-walled carbon nanotubes (SWCNTs), graphene nanosheets (Gr), and graphene oxides (GO) were tested. Comprehensive studies were performed on the electrode matrix composition including the nature of the pasting liquids, pH, carbon nanomaterials, and mode of electrode modification. Two anodic oxidation peaks were recorded at 0.890 and 1.18 V in 1 × 10−1 mol·L−1 phosphate buffer solution at pH 6. Oxidation of naproxen (NAP) is an irreversible diffusion-controlled process. Calibration plots were rectilinear in the concentration ranging from 0.067 to 1.0
Naproxen, 2-(6-methoxynaphthalen-2-yl) propanoic acid, is a nonsteroidal anti-inflammatory drug (NSAID) commonly used for the treatment of moderate and severe pain, fever, inflammation, and stiffness [
Determination of NAP in pharmaceutical formulations and biological fluids has been proposed by applying spectrophotometric [
The aforementioned techniques are demandingly laborious and time-consuming and require ancillary instrumentation. Samples prior to derivatization and the use of organic solvents are some of the inherent disadvantages. Electrochemical methods are effective tools for the determination of pharmaceutical compounds as they are faster, cheaper, easier, and more sensitive than spectrometric and HPLC methods. Several reports and comprehensive reviews about voltammetric methods for quantification of drugs have been found in the literature [
Naproxen has been determined voltammetrically by applying mercury electrodes for the cathodic reduction of NAP [
Herein, the differential pulse voltammetric protocol for determination of naproxen in pharmaceutical formulations and biological fluids in presence of its degradation product using carbon paste electrodes modified with different nanomaterials has been suggested.
Ultrapure water with electric resistivity ∼18.3 MΩ·cm (Milli-Q system, Millipore) was used for preparing supporting electrolytes and stock solutions. Britton–Robinson buffer was prepared, and the desired pH value was adjusted with the appropriate amount of 2 × 10−1 mol·L−1 NaOH solution.
Graphite powder (synthetic 1-2 mm, Aldrich) or glassy carbon powder (GC, “Sigradur-G” type, HTW Meitingen, Germany) was used for electrode fabrication. Different carbon nanaomaterials including multiwall carbon nanotubes (MWCNTs, Aldrich), single-walled carbon nanotubes (SWCNTs, Aldrich), graphene nanosheets (Gr, Sigma), and graphene oxides (GO, Sigma) were tested. Paraffin oil (PO; Merk, Germany), silicone oil (SO; Sigma Aldrich), or tricresyl phosphate (TCP, Fluka) were applied as pasting liquids.
An authentic sample of naproxen (C14H14O3, 230.259 g·mol−1) was obtained from the National Organization for Drug Control and Research, Giza, Egypt. The stock drug solution (1 × 10−4 mol·L−1) was freshly prepared by dissolving the appropriate amounts of NAP in 10−2 mol·L−1 NaOH solution.
Naprosyn tablets (250 mg NAP/tablet; Egyptian Group for Pharmaceutical Industries, Cairo, Egypt) were purchased from local drug stores. One tablet was grinded and dissolved in 50 mL NaOH solution. Naproxen content was assayed according to the proposed and HPLC methods [
Aliquots of the biological fluid (plasma, obtained from a healthy male) were spiked with different NAP concentrations, treated with 0.1 mL of 70% perchloric acid diluted to 10 mL, vortexed for 1.0 min, and centrifuged for 10 min at 13000 rpm. The supernatants were neutralized with NaOH to the appropriate pH value, and the volume was completed to 25 mL with water.
All voltammetric experiments were carried out using a Metrohm computrace voltammetric analyzer model 797 VA with software version 1.0 (Metrohm, Switzerland) equipped with Ag/AgCl (3 mol·L−1 KCl) and platinum electrodes as reference and auxiliary electrodes, respectively. The pH measurements were carried out using a 692 pH meter (Metrohm, Herisau, Switzerland) with a combined pH glass electrode (6.0202.100).
The working carbon paste electrodes were prepared by mixing 0.5 g of carbon materials (either graphite powder or GC) with 0.2 g of paraffin oil in a ceramic mortar for 15 min. Alternatively, 10% of carbon powder was replaced with different nanomaterial, and the paste was prepared by the same manner. Homogenous carbon pastes were pushed into the individual Teflon piston holders with conductive electric wires for electric contact with a potentiostat [
For surface modification, 20
An appropriate volume of the NAP stock solution was added to 15 mL of Britton–Robinson buffer at the desired pH value. The voltammograms were recorded using differential pulse voltammetry (DPV), with the following parameters: pulse height, +50 mV; pause before scan, 2 s; pulse width, 100 ms; pulse time, 40 ms; and scan rate, 40 mV·s−1.
The electrochemical behavior of NAP on blank carbon paste electrodes was evaluated by applying cyclic voltammetry and differential pulse voltammetry at pH 6 (Figure
Voltammetric behavior of naproxen on the carbon paste electrode surface in 2 × 10−1 mol·L−1 phosphate buffer solution at pH 6. The scan rate employed was 50 mV·s−1 and NAP concentration 3.0
Differential pulse voltammetry showed sharp and well-defined peaks at 0.976 and 1.221 V with an improved peak height compared with CV; therefore, further studies related to the quantitative determination of naproxen were carried out using the DPV technique. It is noteworthy to mention that the square wave voltammetric technique showed two peaks at 1.06 and 1.25 V but with lower peak height compared with DPV.
The experimental conditions were optimized for the NAP electrochemical response in order to achieve the highest analytical performance. Thus, the effect of electrode matrix compositions, pH of the supporting electrolyte, and electrochemical parameters were studied and optimized.
NAP is considered as a NSAID of the propionic acid group with a pKa value of 4.15 [
Silicon oil showed a wide working pH range from 3 to 8 with the optimum at pH 4 (
Differential pulse voltammograms for 4.5
Application of the paraffin oil as pasting liquids showed improved peak heights with the shift of the second oxidation peak by about 50 mV compared with silicon oil-based electrodes. Moreover, the electrochemical oxidation of NAP at CPE/PO electrodes was investigated at different pH values ranging between 2.0 and 10.0 (Figure
Using NaOH as a solvent for NAP, carbon paste electrodes showed two irreversible oxidation peaks for NAP at 0.880 and 1.14 V, respectively (Figure
Differential pulse voltammograms of NAP using carbon paste electrodes modified in bulk and surface with different nanomaterials (a, c) dissolved in NaOH and (b, d) dissolved in methanol. The scan rate employed was 50 mV·s−1.
Different performances were achieved by replacing 10% of the graphite powder with different nanomaterials (Figure
On application of methanol as solvent for NAP (Figure
The effect of the nanomaterial within the electrode matrix was investigated by varying the SWCNT content from 2.5 to 20%, and 10% was the most promising.
According to the voltammetric peaks represented in Figure
The remarkable enhancement in current response and shifting of the peak potential provide clear evidence of the catalytic effect of the nanomaterial-modified carbon paste electrode which acts as a promoter to enhance the electrochemical reaction, considerably accelerating the rate of electron transfer. Indicative of a mass transport regime that includes a thin-layer diffusional process (entrapment of naproxen species within the carbon nanotube film) is presented as a possible explanation for the lowered oxidation potential and substantial current increase. Moreover, the methoxynaphthyl ring in naproxen may interact strongly with the carbon nanotube structure through
The voltammetric recording for consecutive measurements of NAP on the native carbon paste electrode resulted in a constant decrease in current and shifting of both oxidation peaks toward more positive potential, which may be attributed to the adsorptive properties of NAP or its oxidation products on the electrode surface (Figure
Differential pulse voltammograms for 8 consecutive measurements of 3
In agreement with these results, it is established the hypothesis that some of the naproxen oxidation products become adsorbed at the electrode surface [
Oxidation of naproxen was carried out at different scan rates ranging between 20 and 300 mV·s−1 (Figure
Effect of scan rate on the voltammetric behavior of 3.0
Marotta et al. [
Naproxen showed two oxidation peaks at 0.85 V that correspond to naproxen oxidation with the formation of an intermediate carboxylic radical, followed by decarboxylation at 1.18 V due the formation of ketone (2-acetyl-6-methoxynaphthalene, AMN). Upon photodegradation and formation of AMN, only the first oxidation peak at 0.890 V with the disappearance of the second peak was achieved, allowing the simultaneous voltammetric determination of naproxen in presence of its degradation product (Figure
Differential pulse voltammograms of naproxen and its degradation product on SWCNTs/CPE.
Regression and statistical parameters obtained from differential pulse voltammetry calibration curves of naproxen and its degradation product using SWCNTs/CPE.
Parameters | NAP | Degradation product | |
---|---|---|---|
At 0.85 V | At 1.18 V | ||
Concentration range ( |
0.067–0.67 | 0.067–1.00 | 0.067–0.73 |
Slope of regression line ( |
0.9083 | 1.2264 | 0.7696 |
|
0.0154 | 0.0321 | 0.0096 |
Intercept of regression line (b) ( |
0.0021 | −0.0779 | 0.0141 |
|
0.00626 | 0.0194 | 0.0043 |
Correlation coefficient ( |
0.9988 | 0.9956 | 0.9993 |
LOD ( |
0.1042 | 0.0812 | 0.045 |
LOQ ( |
0.31572 | 0.2461 | 0.135 |
SD | 0.023 | 0.025 | 0.015 |
RSD (%) | 3.04 | 2.78 | 3.46 |
At the optimum measuring conditions applying carbon paste electrodes incorporated with 10% SWCNTs, the standard calibration curves using SWCNT-modified carbon paste electrodes were performed. The peak current at 0.85 V increased linearly with increasing the NAP in the concentration range 0.067 to 0.67
The linearity with regression parameters was calculated according to ICH guidelines (Table
According to the obtained results, it was possible to apply this technique to the quantitative analysis of NAP in pure form, dosage form, and plasma. The proposed method was successfully applied for the determination of NAP in its pharmaceutical dosage form (Naprosyn tablets; 250 mg NAP/tablet) using SWCNTs/CPEs.
The results obtained by the proposed method were compared with those obtained from the reported method [
Application of the proposed and reference method for the determination of NAP in pure form, dosage form, and plasma.
Parameters | Pure form | Dosage form | Plasma | |||
---|---|---|---|---|---|---|
Method | Proposed method |
Reference method [ |
Proposed method |
Reported method [ |
Proposed method |
Reported method [ |
% found | 102.14 | 101.2 | 100.17 | 100.8 | 96.54 | 97.9 |
100.45 | 100.65 | 100.22 | 99.7 | 97.32 | 99.0 | |
99.9 | 99.8 | 99.6 | 98.8 | 98.0 | 99.2 | |
Mean ± S.D. | 100.83 ± 1.67 | 100.55 ± 0.71 | 99.99 ± 0.34 | 99.77 ± 1.0 | 97.28 ± 0.73 | 98.7 ± 0.7 |
|
0.922 (2.776) | 0.876 (2.776) | 0.842 (2.776) | |||
|
2.345 (19) | 1.512 (19) | 1.344 (19) |
To validate the suggested procedure, the linearity, range, limit of detection, limit of quantification, accuracy, and robustness were measured according to the ICH guidelines.
In this study, it was shown that carbon paste electrodes modified with carbon nanomaterials can be considered as a sensitive working electrode for simultaneous voltammetric determination of naproxen in presence of its degradation product. From the different method for electrode fabrication, bulk modification with SWCNTs in the PO/CPE showed an effective electrocatalytic activity toward the anodic oxidation of naproxen, which leads to a great increase in the peak current (more than 8-fold). The present study showed comparable sensitivity with previously published NAP sensors (Table
Comparison of analytical parameters of different naproxen electrodes.
Working electrode | Electrochemical technique | Linear range ( |
LOD ( |
Sample | Degradation product | Reference |
---|---|---|---|---|---|---|
Glassy carbon electrode | DPV | 10–125 | 0.3 | Tablets | No | [ |
Boron-doped diamond | DPV | 0.5–50 | 0.03 | Tablets | Yes | [ |
Platinum electrode | DPV | 4.03–108 | 1.04 | Tablets | No | [ |
ZnO/MWCNTs/CPE | SWV | 1.0–200 | 0.23 | Tablets | No | |
MWCNTs-Gr-Il/GCE | DPV | 1–100 | 0.125 | Blood plasma | No | [ |
MWCNTs/GCE | Amperometry | 10–100 | 0.6 | Tablets | No | [ |
Graphite electrode | DPV | 4.9–123 | 4.45 | Tablets | No | [ |
SWCNTs/CPE | DPV | 4.35–65.5 | 6.255 | Tablets and blood plasma | Yes | Present work |
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors would like to express their gratitude to the National Research Center (project no. 110090360) and National Organization for Drug Control and Research for providing instruments and the means necessary to accomplish this work.
S1: differential pulse voltammograms for 10.0 × 10–6 mol·L−1 NAP using silicon oil carbon paste electrodes at different pH values. S2: FT-IR spectra of naproxen and its degradation product.