Electrochemical determination of acetaminophen (APAP) was successfully performed using a carbon paste electrode (CPE) modified with coffee husks (CH-CPE). Scanning electron microscopy (SEM) and SEM-energy dispersive X-ray spectroscopy (SEM-EDX) were, respectively, used for the morphological and elemental characterization of coffee husks prior to their utilization. The electrochemical oxidation of APAP was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV). SWV technique appeared to be more sensitive since the oxidation current of APAP was twofold higher with the CH-CPE sensor than with the bare CPE, in relation to the increase in the organophilic character of the electrode surface. Furthermore, on CH-CPE, the current response of APAP varied linearly with its concentration in the range of 6.6
Acetaminophen (4′-hydroxyacetanilide or N-acetyl
On the other hand, the exploitation of lignocellulosic materials (LCMs) as effective sorbents for organic compounds such as dyes and pesticides has been largely investigated during the last decade [
In the present work, a simple, reliable, and fast procedure was developed for the determination of acetaminophen by square wave voltammetry, based on a CPE. It is worth mentioning that the use of coffee husks rather than carbon nanotubes [
Prior to the assessment of coffee husks as electrode modifier, their macromolecular composition was determined using the Acid-Detergent Fiber- (ADF-) Neutral Detergent Fiber (NDF) method. The surface of the obtained electrode was characterized using scanning electron microscopy and cyclic voltammetry. Finally, the analytical performance of the proposed CH-CPE for the quantification of APAP was evaluated by square wave voltammetry (SWV) in commercial pharmaceutical tablets.
Acetaminophen (APAP) was purchased from Aldrich as powder and used as received. 0.1
Coffee husks used in this study were collected from a coffee-processing mill in Santchou (Menoua Division, West Cameroon) and dried under sunlight for 3 days. They were ground and crushed, and a series of sieves allowed obtaining their fine fraction (0–100
The electrochemical measurements were performed using an electrochemical analyzer PG580 (Uniscan Instruments, UK) connected to a personal computer. The electrochemical software used was UiEchem version 3.27, from Uniscan Instruments. A classical three-electrode cell configuration was employed, consisting of bare or modified CPEs serving as working electrodes, a saturated calomel reference electrode (SCE), and a platinum wire counter electrode.
The unmodified CPE was prepared by thoroughly hand mixing 30 mg of silicone oil with 70 mg of graphite powder (analytical grade, ultra F, <325 mesh, from Alfa) in a mortar. A portion of the composite mixture was packed into the cylindrical hole of a Teflon® tube equipped with a copper wire serving as electrical contact with the rest of the circuit. The surface exposed to the solution was polished on a weighing paper to give a smooth aspect before use. Coffee husks modified carbon paste electrodes (CH-CPEs) were prepared as described for the bare CPE by using 65 mg of graphite powder, 30 mg of silicone oil, and 5 mg of coffee husks powder. For comparison purposes, a third electrode was prepared with the following composition: 65 mg of graphite powder, 30 mg of silicone oil, and 5 mg of pure cellulose; it is referred to as Ce-CPE throughout the text. When not in use, the CPEs were removed from supporting electrolyte and kept at room temperature.
For the determination of APAP in pharmaceutical formulations, each commercial tablet was carefully weighed, then finely powdered, and dissolved in 1 L PBS. A 1.25 mL aliquot of this solution was then diluted to the mark with PBS in a 50 mL volumetric flask. The samples were finally spiked with known amounts of APAP and the concentration of APAP in solution was determined using the standard addition method.
Morphological analysis of CPE surface was achieved by Field Emission Gun Scanning Electron Microscopy (FEGSEM) on a JSM-6301F apparatus from JEOL (SCIAM, University of Angers, France). The coffee husk powder was immobilized on a SEM sample holder using adhesive carbon tape. Images obtained were from secondary electrons of 3 keV, with magnifications between ×25 and ×20,000. For energy dispersive X-ray spectroscopy (EDX) experiments conducted on the same equipment, the beam energy was 20 keV.
An estimation of the three parietal constituents (cellulose, hemicellulose, and lignin) contained in the coffee husks was made using the ADF-NDF method of Van Soest and Wine [
The contact angle measurements were performed on the unmodified CPE and modified CH-CPE by the sessile drop technique which allows comparison between materials. In this aim, the electrodes were freshly prepared and dried at room temperature in a desiccator tank. After complete drying, a droplet of ultrapure water (20
For the evaluation of the real surface area of the electrodes, CPE, CH-CPE, and Ce-CPE were used to record by cyclic voltammetry the curves of a 5 mM [Fe(CN)6]3− solution in 0.1 M PBS (pH 7.4). The peak intensity (
The SEM 2D images of the bare CPE and CH-CPE are shown in Figure
2D-SEM images of (a) bare CPE and (b) CH-CPE. (c) EDX spectrum obtained by difference between EDX spectra of the bare CPE and CH-CPE.
To ascertain the presence of coffee husks within the modified CPE, EDX experiments were performed and the response obtained by difference between the EDX spectra of the bare CPE and CH-CPE is presented in Figure
The organophilic character of CPEs was evaluated using sessile drop contact measurements. A progressive decrease was observed in contact angle from
To measure the electrochemically active surface areas of CPE and CH-CPE, a third electrode was prepared which contained pure cellulose as modifier (Ce-CPE). The geometric surfaces of these electrodes were calculated using the formula
The three electrodes were then used to record by cyclic voltammetry the signal of a 75 mg L−1 APAP solution in PBS (curves not shown). The ratios of peak intensities and of real surface areas obtained with modified CPEs, in relation to the bare CPE, are reported in Table
Ratios of real surface areas and of peak intensities.
Electrodes | Ratio of |
Ratio of real surface areas of modified electrodes versus bare CPE | Chemical effect (hydrophilic character of cellulose) |
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CPE | 1 | 1 | 0 |
CH-CPE | 2.000 | 1.182 | 0.820 |
Ce-CPE | 2.600 | 1.719 | 0.881 |
EIS is an accurate tool for determination of the electrical properties of an electrode/electrolyte interface. It was used to analyze the charge transfer rate for both bare CPE and coffee husks modified CPE. Figure
Nyquist plots of (a) CPE and (b) CH-CPE obtained in 5 mM [Fe(CN)6]3−/4− (1 : 1) solution containing 0.1 M PBS (pH 7.4). The frequency range was from 0.1 to 104 Hz at the formal potential of 0.18 V.
A marked difference was observed between the bare CPE (Figure
APAP bears hydroxyl and amine functional groups in its structure, and it is expected that acidity of the supporting electrolyte could affect its redox behavior. Cyclic voltammetry was used to investigate the electrochemical behavior of APAP on the bare CPE. Since the proposed sensor is devoted to the analysis of APAP in pharmaceutical formulations, 0.1 M phosphate buffer solution (PBS) was chosen as supporting electrolyte to meet physiological conditions. Thus, the influence of pH on the oxidation peak current of APAP was investigated in acidic, neutral, and basic media, that is, in 0.1 M PSB with pH fixed at 5.5, 7.4, and 8.6. As shown in Figure S1 (Supporting Information, in Supplementary Material available online at
Electrochemical oxidation of APAP.
Cyclic voltammograms of 75 mg L−1 APAP in PBS at the (a) bare CPE and at (b) CH-CPE. Potential scan rate: 100 mV s−1.
The corresponding current density was found to be 600
To yield more information about the behavior of APAP on CH-CPE, the effect of scan rate (
(a) CV curves recorded on CH-CPE with 25 mg L−1 APAP in PBS at different scan rates from 50 to 250 mV s−1. (b) Linear relationship between the electrode responses and the square root of the potential scan rate.
Moreover, the relationship between the peak currents (anodic and catholic) and the square root of
These results indicate that a diffusion controlled process takes place at the electrode, which is the mass transport rate of APAP to the surface of the electrode across a concentration gradient. It clearly appears that the presence of coffee husks increases the oxidation peak intensity of APAP corresponding to a good sensitivity of the modified electrode. These results open the way for further development of a sensor for APAP determination.
Prior to the optimization of the sensor based on CH-CPE, the stripping currents of a solution of 75
SWV and DPV curves obtained at CH-CPE in 75 mg L−1 APAP in PBS (potential scan rate: 100 mV s−1).
The operating parameters of both techniques were optimized in order to reach maximum electroanalytical sensitivity, following the methodology reported by Pontié et al. [
Considering the inherent advantages of SWV such as minimization on the contribution from the capacitive charging current to the current signal as reported elsewhere [
The oxidation peak current of APAP was directly proportional to its concentration (Figure
SWV of APAP at different concentrations in PBS on the CH-CPE. APAP concentration (A–L): 1, 3, 5, 9, 15, 25, 35, 40, 50, 60, 70, and 75 mg L−1. Insert shows peak current versus APAP concentration.
The detection limit (DL) calculated with a signal-to-noise ratio of 3 was found to be 0.66
Comparison of detection limits (DLs) of different modified CPEs reported for the detection of APAP.
Modified electrodes | pH | Linear range ( |
DL ( |
Reference |
---|---|---|---|---|
Ethynylferrocene-NiO/MWCNT nanocomposite modified CPE | 6 | 0.8–600 | 0.500 | [ |
CPE modified with CNT and PAP | 5 | 10–100 | 1.1 | [ |
ZrO2 nanoparticles-modified CPE | 7 | 1.0–2500 | 0.912 | [ |
Graphene-CoFe2O4 nanocomposite |
7 | 0.03–12 | 0.025 | [ |
PEDOT graphene oxide composites | 4.8 | 10–1000 | 0.57 | [ |
Carbon nanotube modified CPE | 7 | 1–1000 | 0.46 | [ |
CPE modified by coffee husks | 7.4 | 6.6–500 | 0.66 | This work |
PEDOT: poly(3,4-ethylenedioxythiophene); PAP: poly(3-aminophenol).
It can be seen that the detection limit obtained in the present study (0.66
The influence of some electroactive species that commonly coexist in physiological fluids was investigated under the optimal experimental conditions defined above. The selected potential interfering species were hydroquinone, para-aminophenol, dopamine, ascorbic acid, aspartic acid, and glucose. The tolerance limit for each compound was considered the maximum concentration which caused approximately ±5% relative error in the determination of APAP. Thus, known amounts of each of these species were added to a 25 mg L−1 APAP solution and the obtained solutions were analyzed by SWV. Results are summarized in Table
Effect of potential interfering species on the response of CH-CPE to 25 mg L−1 APAP in PBS.
Interfering species | Concentration (mg L−1) of added species | % variation in the SWV peak current (with |
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Hydroquinone | 25 |
|
50 |
|
|
75 |
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|
100 |
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Para-aminophenol | 25 |
|
50 |
|
|
75 |
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|
100 |
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Dopamine | 25 |
|
50 |
|
|
75 |
|
|
100 |
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Ascorbic acid | 25 |
|
50 |
|
|
75 |
|
|
100 |
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Aspartic acid | 25 |
|
50 |
|
|
75 |
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Glucose | 25 |
|
50 |
|
|
75 |
|
The presence of up to 2-fold excess of hydroquinone, para-aminophenol, dopamine, and ascorbic acid in the APAP solution did not influence the response of this latter. Likewise, glucose at the same concentration was found to interfere slightly with APAP. The most dramatic effect was noticed with aspartic acid which greatly reduced the SWV signal of APAP when added at the same concentration. This fact leads us to suggest the elimination of aspartic acid from any matrices before the quantification of APAP.
The sensor based on CH-CPE was applied for direct detection of APAP in commercial tablets, Doliprane 500 and Doliprane 1000, using the common analytical method of internal standards. As illustrated in Figure
Determination of APAP in commercial tablets using CH-CPE (all values are in mg, except for recovery rate in %).
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Tablet mass obtained after powdering |
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Theoretical APAP mass in the weighted tablet |
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APAP mass determined with CH-CPE |
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Recovery (%) |
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Tablet mass obtained after powdering |
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Theoretical APAP mass in the weighted tablet |
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APAP mass determined with CH-CPE |
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Recovery (%) |
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SWV curves recorded for the commercial tablet Doliprane 1000, upon addition of known amounts of APAP in the supporting electrolyte. Insert displays the corresponding calibration curve.
This work demonstrates the ability of a CPE chemically modified by coffee husks for the electrochemical determination of acetaminophen in pharmaceutical formulations. The beneficial organophilic effect of coffee husks in the bulk of the carbon paste electrode was first demonstrated towards the accumulation of acetaminophen, using cyclic voltammetry and SWV. It was shown that the process occurring at the modified electrode for acetaminophen is controlled by diffusion. The calibration curve of biosensor showed a linear range from 6.6
The authors declare that they have no competing interests.
The authors wish to thank R. Mallet (SCIAM, Angers University, France) for recording the FEGSEM images. They also thank the University of Angers (France) for funds allocated to Serge Foukmeniok Mbokou for a scientific stay in France (ARIANES program). Maxime Pontie acknowledges the support of Origalys Electrochem SAS (France) for a travel in Cameroon.