The analysis of drugs is of key importance in drug quality control. In this line, the development of sensitive, simple and rapid methods for the determination of active ingredients in drugs is a daily task for researchers working in analytical sciences. Food and pharmaceutical industries use caffeine (Scheme 1) in a wide range of products such as tea, coffee, soft drinks and medications  . Most neuroprotective and metabolic benefits commonly associated with coffee and tea consumption have often been linked to caffeine content of these beverages . Caffeine promotes gastric acid secretion and diuresis; and stimulates the central nervous    and the cardiovascular systems  . It is however known that a high concentration of caffeine is toxic for the human body: it can cause nausea, trembling, nervousness, depression, hyperactivity and mutation effects  . It constitutes a risk factor for cardiovascular diseases and asthma disease  . The consumption of caffeine above 200 mg/day may lead to death . Therefore, several effective methods have been developed for the detection and quantification of caffeine in various samples. Amongst these methods, the most used are simple chromatographic methods   , coupled in some cases with mass spectrometry  . These techniques are usually expensive and time-consuming when compared to electrochemical methods . However, the high positive oxidation potential of caffeine limits the use of common electrodes such as glassy carbon electrode  and carbon paste electrode . Nafion (Naf) is widely used as chemical modifier due to its capacity to decrease the oxidation potential of some organic compounds. It is a perfluorinated sulfonated cation exchanger polymer widely used in electrochemistry to modify electrodes because it possesses many excellent properties such as chemical inertness, thermal stability, capacity, mechanical strength, huge cation exchange and antifouling properties . Several nafion-based composite electrodes were used for the quantification of caffeine, that include nafion/modified boron-doped diamond , nafion/carbon nanotube , nafion/graphene modified electrode , nafion modified carbon electrode , nafion/graphite nanoplatelets , nafion/attapulgite  and nafion-ruthenium oxide pyrochlore modified glassy carbon electrode .
Besides, fullerene/multiwalled carbon nanotubes (MWCNT) are important electrode modifiers due to their favorable role in the promotion of electron
Scheme 1. Chemical structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H purine-2,6-dione or 1,3,7-trimethylxanthine).
transfer and to their strong electrocatalytic ability in electrochemical reactions . Thus, their application as new effective electrocatalysts for various chemical and biochemical reactions and in the design of electrochemical sensors is undergoing increasing investigation . Therefore, it is expected that, by combining fullerenes carbon nanotubes and nafion as unique electrode modifier, a synergistic effect of these components could lead to a sensitive sensor.
The present work describes the determination of caffeine via differential pulse voltammetry (DPV) using a glassy carbon electrode (GCE) modified by fullerene/MWCNT and nafion. Prior to its use for electroanalytical studies, the thin film electrode (fullerene/MWCNT/Naf) was characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Some parameters were investigated to obtain the best conditions for caffeine determination.
2.1. Apparatus and Reagents
Electrochemical measurements were carried out using the Origalys Potentiostat running with the OrigaMaster 5 software. A standard single compartment three-electrode cell was used, with an Ag/AgCl (3 M KCl) reference electrode and a stainless-steel rod counter electrode. The working electrode was a fullerene/MWCNT/Naf modified GCE prepared as described below. All chemicals were commercially supplied: caffeine (C8H10N4O2), dopamine hydrochloride and D-(+)-glucose from Sigma-Aldrich; citric acid monohydrate (J.T. Baber); K4[Fe(CN)6], K3[Fe(CN)6] (Prolabo), NaOH and NaCl (Fisher scientific international), H2SO4, CH3COOH, HClO4, HNO3 and HCl (Reidel-de-Haen) were used without further purification. Fullerene/MWCNT powder mixture (95%, 3 - 20 nm OD, 2 - 5 nm ID, 0.1 - 10 µm length) and Nafion (5% w/w in EtOH) were obtained from Alfa Aesar. All solutions were prepared using purified water.
2.2. GCE Surface Modification
Prior to its modification by drop-coating, each GCE was polished with alumina pastes on a billiard cloth. They were then placed in a 1:1 ethanol-water solution and properly cleaned in a sonicator for 10 min to eliminate any remaining alumina particles. 3 mg of fullerene/MWCNT were dispersed in 1 mL Nafion (1%) and then in 1 mL ethanol by ultrasonication to give homogeneous fullerene/MWCNT/Naf and fullerene/MWCNT dispersions. The GCE surface was coated with 2 µL of each resulting dispersion and dried at room temperature for 15 min to obtain the working electrode (hereafter referred as fullerene/MWCNT/Naf/GCE). For comparison purposes, a bare GCE covered by a thin film of nafion (Naf/GCE) was also prepared.
2.3. Electrochemical Procedure
Cyclic voltammograms were recorded in HClO4 solution containing a given concentration of CAF, in the potential range from +0.6 V to +1.8 V. For trace analysis of caffeine, DPV was performed using the following optimized parameters: pulse amplitude 50 mV, step potential 5 mV, initial potential +1.0 V, final potential +1.7 V, equilibrium time 5 s. The GCE before and after its modification was characterized by EIS set at the frequency range from 1 kHz to 0.01 Hz, and with an amplitude of 15 mV.
For real sample analysis, two pharmaceutical preparations were purchased locally. The analyzed solutions were obtained by dissolving a capsule of Brupanax and Pipadol (Extra) drugs in HClO4. The corresponding molar concentration of the solutions was calculated and a chosen volume of each of them directly added to the measurement cell. For chronocoulometric measurements, the potential was stepped from 0.0 V to +1.4 V for a period of 1 min with a sample time of 0.2 s.
3. Results and Discussion
3.1. Electrochemical Behavior of CAF at the Fullerene/MWCNT/Naf/GCE
Figure 1 shows the cyclic voltammetric responses recorded at 100 mV∙s−1 in 0.1 mol∙L−1 HClO4 of 10−3 mol∙L−1 CAF at the bare GCE (Figure 1(a)), on Naf/GCE (Figure 1(b)), and on fullerene/MWCNT/Naf/GCE (Figure 1(c)). One can notice that, on fullerene/MWCNT/Naf/GCE the peak current is signiﬁcantly higher than those registered on the bare GCE and on Naf/GCE. The oxidation process of CAF on all electrodes is irreversible, which is consistent with other reports . The oxidation mechanism of CAF has been revealed by Hansen et al. . Compared with the GCE andie, Naf/GCE, the oxidation potential at the fullerene/MWCNT/Naf/GCE shifted negatively from 1.522 to 1.393 V. This phenomenon may be an evidence of catalytic effect of fullerene/MWCNT, associated to its large specific surface area  .
Figure 1. Cyclic voltammograms of 10−3 M CAF recorded in 0.1 M HClO4 at 100 mV∙s−1 on (a) bare GCE, (b) Naf/GCE and (c) fullerene/MWCNT/Naf/GCE. (d): blank electrolyte on fullerene/MWCNT/Naf/GCE.
3.2. Electrochemical Impedance Spectroscopy (EIS)
EIS is a valuable method to characterize the electron transfer properties of the electrode surface during the modiﬁcation process . Figure 2 presents the EIS curves recorded in 1.0 mM [Fe(CN)6]3−/4− using the bare GCE, Naf/GCE and fullerene/MWCNT/Naf/GCE. The resistance of charge transfer (Rct) deriving from the recorded Nyquist plots with the lowest value (14 kΩ∙cm2) was obtained on the unmodified GCE (inset in Figure 2). Upon the modification of this later using only a nafion film, the Rct increased to 18.77 kΩ∙cm2 and further to 64.31 kΩ∙cm2 on fullerene/MWCNT/Naf/GCE. This is indicative of a better electrode surface coverage with a non-conductive film from the bare GCE to fullerene/MWCNT/Naf/GCE.
3.3. Chronocoulometric Study
The electrochemical effective surface area of the GCE and fullerene/MWCNT/Naf/GCE was calculated from the slope of the plot of Q versus t1/2 obtained by chronocoulometry (Figure 3) using 1.0 mM K3[Fe(CN)6] as model complex based on Equation (1)  where A is the effective surface area of the working electrode, C is the concentration of the analyte, n is the number of electrons transferred (n = 1 for [Fe(CN)6]4−/3− redox system) and D is the diffusion coefﬁcient of the analyte (7.6 × 10−6 cm2/s for [Fe(CN)6]4−/3−) ; Qdl is the capacitive or double layer charge and Qads is the faradaic component due to the oxidation of adsorbed species.
Based on the slope of the linear relationship between Q and t1/2 (Figure 3(A)),
Figure 2. Nyquist diagrams recorded in 0.1 M KCl containing 10−3 M [Fe(CN)6]−3/−4 on (a) bare GCE, (b) Naf/GCE and (c) Fullerene/MWCNT/Naf/GCE. Frequency range: 1 kHz-0.01 Hz, amplitude: 15 mV. Inset: Rct as a function of electrodes’ type.
Figure 3. (A) Plot of Q versus t1/2 of 1.0 mM K3[Fe(CN)6] at GCE (curve (a)), fullerene/MWCNT/Naf/GCE (curve (b)). (B) Plot of Q versus t1/2 of 1.0 mM caffeine at GCE (curve (c)) fullerene/MWCNT/Naf/GCE (curve (d)).
the effective surface area A was evaluated to be 0.029 cm2 and 0.206 cm2, for GCE and fullerene/MWCNT/Naf/GCE, respectively. This increase in effective electrode surface upon modification of the GCE with fullerene/MWCNT and Naf accounts for the enhancement of the oxidation peak current of caffeine. This is also a further proof of successful immobilization of the fullerene/MWCNT/Naf film on the GCE surface.
The diffusion coefﬁcient of 1.0 mM CAF at GCE and fullerene/MWCNT/Naf/GCE was calculated from Equation (1), using the values of surface area of GCE and fullerene/MWCNT/Naf/GCE. The value obtained at the developed sensor (1.146 × 10−4 cm2∙s−1) is higher than that obtained at the bare GCE (9.688 × 10−5 cm2∙s−1), thereby conﬁrming the sensitivity of developed sensor toward CAF oxidation (Figure 3(B)).
3.4. Effect of Scan Rate
The eﬀect of potential scan rate (v) on the DPV current density (jpa) and peak potential (Epa) on fullerene/MWCNT/Naf/GCE in 0.1 M HClO4 (pH 1) + 10−3 M CAF was investigated by CV (Figure 4). As observed, Epa shifted to a positive value with an increase of the scan rate, along with a concurrent increase in anodic peak current. A plot of logjpa vs logv (result not shown) was used to determine whether the oxidative behavior of CAF was due to diﬀusion or adsorption. The slope of 0.87 > 0.5 for a theoretical diﬀusion-controlled process allowed to conclude that a mixed behavior showing both adsorption and diﬀusion-controlled processes were involved . When the scan rate increased, the anodic peak potential at the fullerene/MWCNT/Naf/GCE shifted positively. For further veriﬁcation of the value of α, the Laviron equation for an irreversible electrode process was used  :
where E0 is formal potential, k0 is heterogeneous electron transfer rate constant,
Figure 4. Cyclic voltammograms of 10−3 M CAF at fullerene/MWCNT/Naf/GCE in 0.1 H2SO4 at different scan rates (10, 20, 30, 45, 70, 80 and 100 mV∙s1). Inset (A) jpa as a function of v1/2 and (B) Epa as a function of lnv.
α is electron transfer coeﬃcient, n is the number electron transferred in the rate determining step and v is the scan rate. R = 8.314 J∙mol−1∙K−1, T = 298 K and F = 96.480 C∙mol−1 have their usual meanings. In this work, Epa = 1.272 + 0.0301lnv (Epa in V, v in mV∙s−1, R = 0.998), the electron transfer number (n) was 4 , and the electron transfer coefﬁcient (α) was estimated to be 0.426.
3.5. Effect of Supporting Electrolyte and Its pH
Usually, the nature of the electrolyte solution aﬀects the electrochemical behavior of the analyte, as it was observed in previous studies for CAF    . The electrooxidation of CAF involving protons, diﬀerent decimolar acidic solutions (H2SO4, HClO4, CH3COOH, and HCl) were tested for the diﬀerential pulse voltammetric determination of CAF on fullerene/MWCNT/Naf/GCE.
Although a peak was recorded for all investigated solutions, HClO4 generated the best signal in terms of shape and intensity. This allowed to use 0.1 M HClO4 as supporting electrolyte for further experiments. The redox behavior of most biomolecules is aﬀected by the pH of supporting electrolyte . In this study, the eﬀect of pH on peak current (jpa) and peak potential (Epa) of CAF was evaluated within the acidic pH range (0.5 - 2.5). From pH 0.5, the peak current increased slightly at pH 1, then decreased from pH 1.0 to pH 2.5 (Figure 5(A)). one can also observe that the anodic peak potential of CAF shifted towards less positive values as the pH changes from 0.5 to 2.5 (Figure 5(B)). Therefore, pH 1.0 was chosen for the subsequent experiments (Figure 5).
3.6. Interference Study, Repeatability and Stability of the Electrode
The next step in the work focused on the evaluation of the specificity of the sensor when operating in the presence of interfering agents frequently found in commercialized drinks and drugs. A CAF solution (10−4 mol∙L−1) spiked with various species was evaluated under the same experimental conditions in 0.1 M HClO4 by DPV. The selected interfering compounds include citric acid, sulfite ions, glucose and dopamine that were added in the electrochemical cell, with concentrations 100 to 200-fold higher than that of CAF (10−4 mol∙L−1). The tolerance limit, defined as the concentration ratio of the additive over CAF causing ±5.0% relative error was used to measure the interfering effect. The results obtained (Table 1) showed that the oxidation peak of CAF is slightly affected by citric acid as more than 100-fold excess of this compound did not significantly influence the peak current of caffeine. However, dopamine and sulfite ions were the most interfering agents, somewhat limiting the selectivity of the proposed sensor for media containing high amount of such interfering compounds.
Several measurements were performed to determine the repeatability of the electrochemical responses of fullerene/MWCNT/Naf modiﬁed GCE for 1.0 mM CAF in 0.1 M HClO4 solution. The results of ten successive measurements showed a relative standard deviation of 4.56 % for CAF, indicating that the proposed modified GCE has good repeatability. The long-term stability of the electrode was also evaluated by measuring the response for 1.0 mM CAF after keeping the electrode in HClO4 solution for 7 days. The electrode retained about 95% of its original activity for CAF, indicating the electrode was very stable.
Figure 5. (A) Effect of detection medium pH (0.5, 1, 1.5, 2, and 2.5) on the anodic peak position of 10−4 M CAF, at fullerene/MWCNT/Naf/GCE. (B) Variation of the peak current as a function of pH of the detection medium (HClO4) and peak potential as a function of the pH of the detection medium.
Table 1. Interference study on the determination of 104 M CAF.
Figure 6. DPV curves obtained under optimized conditions in 0.1 M HClO4 (pH 1) at fullerene/MWCNT/Naf/GCE for various concentrations of CAF (a - j): 0, 10, 30, 180, 270, 450, 540, 650, 800 and 1000 µM. The inset shows the corresponding calibration graph.
Table 2. Comparison of the performance of some CAF sensors.
LOD: Low limit of detection, 4-CuIITAPc: Copper (II) phthalocyanine. SAM: Self-assembled monolayer, Pt-GR: Platinum graphene, GORG: Graphene oxide-reduced glutathione, SDS: Sodium dodecylsulfate, NCOM: Nano-Cobalt (II, III) oxide modified, GrRAC: Cork-graphite, PDDA: Poly (diallyldimethylammonium chloride).
3.7. Variation of CAF Concentration, Calibration Graph and Analytical Application
The DPV curves of CAF in the concentration range from 10 to 1000 µM at fullerene/MWCNT/Naf/GCE in 0.1 M HClO4 (pH 1.0) are shown in Figure 6.
The recorded peak current increased linearly with an increase in the concentration of CAF. A calibration plot of the oxidation peak current against CAF concentration is linear, with a correlation coeﬃcient of 0.999 and a detection limit of 72.89 nM (S/N = 3).
As compared with other modiﬁed electrodes, it is obvious that the present method is comparable or better in the determination of CAF since it provides a signiﬁcantly low detection limit (Table 2). On the other hand, the preparation of the modiﬁed electrode is simple and reproducible.
The applicability of the developed method to the determination of caffeine in real samples was investigated using the standard addition method. The experimentally detected values and the labeled values are compared in Table 3. It was found that the results obtained using fullerene/MWCNT/Naf/GCE are in a good agreement with the content label.
This work showed the ability of fullerene/MWCNT/Naf/GCE electrode towards the electrochemical quantification of caffeine. The obtained results showed that the electrocatalytic oxidation of caffeine at the surface of the modified electrode occurs at a potential of about 1.33 V. The anodic peak current of caffeine was proportional to the concentration in the range from 10 to 1000 µM, with a detection limit 72.89 nM and correlation coefficient of 0.999. The simple preparation, good stability, excellent reproducibility, sensitivity and rapid analysis make the developed method suitable for routine determination of caffeine in real samples.
The authors acknowledge the support of The World Academy of Sciences (TWAS) for the advancement of science in developing countries (RGA N˚16-515 RG/CHE/AF/AC_G-FR3240293302) allowed to Ignas K. Tonle.
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