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Platinum electrode modified with polyterthiophene doped with metallic nanoparticles, as sensitive sensor for the electroanalysis of ascorbic acid (AA)
⁎Corresponding authors. nmaouche@univ-setif.dz (Naima Maouche), idrissbakas@gmail.com (Idriss Bakas)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A novel electrochemical sensor for ascorbic acid (AA) detection based on platinum electrode modified with polyterthiophene (P3T) and doped with metallic particles (Cu, Co, Ag, Au, Pd) was constructed. The electrocatalytic performances of the modified electrode with polyterthiophene-metallic particles related to the detection of AA, showed a better catalytic activity compared to the modified electrode with polyterthiophene film. The obtained results demonstrate also that the use of P3T–Ag nanocomposite allows a good sensitivity; which gives a high response in oxidation peak of AA. In order to have a good performance using this sensor, several parameters such as polymerization time of the film and immersion time of the film in AgNO3 solution were optimized.
Keywords
Polyterthiophene
Nanoparticles
Ascorbic acid
Electroanalysis
Cyclic voltammetry
1 Introduction
Interest in ascorbic acid (AA) or vitamin C has been persisted since its discovery, due to its important roles in the body, such as collagen synthesis, hormone synthesis, anticoagulant and antioxidant as an antioxidant in foods, animal feed, pharmaceutical formulations, and cosmetic applications (Andre et al., 2010). Owing to these properties, it is used for the prevention and treatment of common cold, mental illness, infertility, cancer, Alzheimer’s disease, atherosclerosis, and AIDS (Shahrokhian and Asadin, 2010; Ensafi et al., 2009). However, at higher concentration levels, AA contributes to the formation of kidney stones.
As a result of its biological importance, it is essential to develop simple, fast, accurate and specific testing methods for this molecule in routine analysis. The most widely used techniques for determining AA are spectrophotometry (Washko et al., 1989; Yamauchi et al., 2008a; Kishida et al., 1992), chromatography (Yamauchi et al., 2008b; Srivastava et al., 1995), and solid phase analysis (Silvestre et al., 2009; Sena et al., 2000; Karatepe, 2004; Moreira et al., 2005). Although all these techniques demonstrate a good performance, they are expensive, time consuming (Yamauchi et al., 2008a; Srivastava et al., 1995; Silvestre et al., 2009; Sena et al., 2000; Agater and Jewsbury, 1997), and require large infrastructure backup and expert knowledge. Thus they are not very appropriate for the analysis of large number of samples. Recently, electrochemical techniques provide a simple and rapid tool for the detection of AA, but even though electroanalytical methods offer simplicity and high selectivity, it is difficult to determine AA concentration electrochemically by direct oxidation on a conventional electrode because of difficulties such as large over potential requirements, fouling of the electrode surface by oxidized products, or lethargic oxidation rates (Azar and Nerbin, 2000). As a result, many different strategies have been used to overcome these problems, including the modification of electrode surfaces by polymers or the use of redox electrocatalysts which provide the selective detection of AA and prevent fouling of the electrode surface (Maouche et al., 2009; Kumar and Chen, 2008; Mohadesi and Taher, 2007).
In this work, we have demonstrated a highly sensitive electrochemical sensor for the determination of ascorbic acid (AA), based on a modified platinum electrode surface with polyterthiophene doped with metallic nanoparticles such as Cu, Co, Ag, Au, and Pd. Cyclic voltammetry (CV) and square wave voltammetry (SWV) were used to study the electrochemical behavior of these new composite materials versus AA. Various parameters affecting the detection process were taken into consideration and optimized.
2 Experimental
2.1 Chemical products
Terthiophene (3T 99%), lithium perchlorate (LiClO4 ⩾ 95%) and acetonitrile anhydrous 99.8% were purchased from Aldrich, and ascorbic acid (AA, 98%) was from Alfa Aesar. Minerals salts including copper (II) chloride dihydrate (CuCl2, 2H2O 99%), cobalt chloride hexahydrated (CoCl2, 6H2O), palladium chloride (PdCl2 99.9%), and gold chloride (AuCl3 99%) were obtained also from Aldrich, and phosphate buffered tablet purchased from Sigma. All reagents are of analytical grade and used as received. The aqueous solutions were prepared with doubled distilled water and high pure nitrogen gas was used for the removal of air molecules from solutions.
2.2 Instruments
Electrochemical studies were carried out with a Potentiostat/Galvanostat (Voltalab 301), coupled with a computer under voltamaster software. A conventional three-electrode in one compartment cell was employed using platinum disk (d = 2 mm) as working electrode, saturated calomel electrode as reference and a Pt-wire as counter electrode. Cyclic voltammetry (CV) measurements were performed in phosphate buffer solution (pH 7.4) at room temperature. Square wave voltammetry (SWV) measurements were recorded at amplitude of 25 mV and frequency of 15 Hz with the same electrochemical workstation.
2.3 Electrochemical preparation of a modified electrode
Prior to the polymerization of terthiophene, platinum electrode was polished with 0.5 μm alumina slurry on a polishing cloth, followed by rinsing with distilled water. The electropolymerization of terthiophene was made on a platinum electrode by chronoamperometry at an imposed potential of 1.03 V in the 20 mL of a 0.1 M LiClO4/CH3CN as a supporting electrolyte containing 2 · 10−2 mol of terthiophene monomer. After electorpolymerization, the surface of the electrode was washed with CH3CN to remove the excess of monomer. We note that the chronoamperometry is the best way to form the homogenous films, with a good sensitivity compared by cyclic voltammetry (Maouche et al., 2012).
The next step after the polymerization process is to form the composite materials by incubating the obtained polyterthiophene (P3T) film in a solution containing the metallic ions (Scheme 1). Each film doped, was then removed from the soaking solution, rinsed with distilled water and transferred into a solution of phosphate buffer (0.1 M) containing (5 mM) of freshly prepared AA, for electrochemical analysis.
2.4 XPS analysis
A Thermo VG Scientific ESCALAB 250 system fitted with a micro-focused, monochromatic Al Kα X-ray beam (1486.6 eV, 500 μm spot size) was used to record the spectra. The samples were stuck on sample holders using conductive double-sided adhesive tapes and outgassed in the fast entry airlock for at least 1 h at ∼1 × 10–7 mbar. The Avantage software, version 4.67, was used for digital acquisition and data processing. The spectra were calibrated against the C1s main peak component C–C/C–H set at 285 eV.
3 Results and discussion
3.1 SEM characterization of composite material
The SEM studies revealed the morphology of the appearance surface of P3T film electrodeposited on a Pt electrode and impregnated with silver nanoparticles (Fig. 1b and c) in comparison with the morphology of a simple P3T film electrodeposited on a Pt electrode without any modification as control sample (Fig. 1a). By comparing the two images of surface films it can be observed a clear presence of silver nanoparticles more or less great dispersed at least on the major surface of the electrode with different sizes in the order of nanometers and microparticles attached to a porous structure of P3T films.
3.2 XPS characterization
In order to prove the presence of the silver nanoparticles in the P3T film, XPS analysis of the modified film by silver nanoparticles (P3T–Ag) and its corresponding nonmodified film (P3T) have been studied. The XPS survey regions of P3T and P3T–Ag are illustrated in Fig. 2, based on the obtained results, and the survey spectra of P3T show atoms peaks of C1s, O1s, and S2p bands at 285.00, 532.7 and 164.1 eV respectively. In addition, Fig. 2(b) shows the appearances of the peak of Ag 3d at 367.60 eV. By comparing these results to those of the P3T film Fig. 2 (a), in which the Ag peak was not observed, we confirm that the Ag nanoparticles have been successfully immobilized on the surface of the P3T film. We also noted that the presence of the Cl2p peak was due to the supporting electrolyte.
The various energy values of the chemical elements detected in the film surface of P3T and P3T–Ag obtained from the XPS are summarized in Table 1.
| Samples | Binding energy (eV) | ||||
|---|---|---|---|---|---|
| C1s | O1s | S2p | Ag3d | Cl2p | |
| P3T | 285.00 | 532.70 | 164.10 | – | 200.60 |
| P3T–Ag | 285.00 | 533.00 | 164.10 | 367.60 | 200.60 |
3.3 Electrocatalytic application of modified films
P3T modified films by different metals (Au, Ag, Pd, Cu, and Co) were tested for the electrocatalytic study of ascorbic acid (AA). The electrocatalysis of AA was performed in a phosphate buffer solution PBS (0.1 M, pH 7.4) at room temperature, using films prepared by chronoamperometry. Of particular interest, we thoroughly investigated some parameters affecting the electrochemical detection of AA as effect of the metal nature, formation time of the film, immersion time, scan rate, and concentration of AA.
3.3.1 Effect of the nature of metal
A series of P3T films were synthesized by electropolymerization using chronoamperometry technique for 60 s, and the films synthesized were then immersed for 20 min in various solutions containing CuCl2, CoCl2, AgNO3, AuCl3 and PdCl2. After incubation, the doped films were removed from the solution, rinsed with distilled water and transferred into a solution of phosphate buffer (0.1 M) containing (5 mM) of freshly prepared AA. To examine the influence of the metal nature impregnated on the film on the catalytic oxidation of AA, Fig. 3 shows cyclic voltammograms obtained for platinum electrode, unmodified P3T film, P3T–Au, Cu–P3T, Co–P3T, P3T–Ag and P3T–Pd electrodes for the 5 mM AA. As it can be observed the peak of irreversible oxidation of AA obtained on a platinum is at 0.443 V whereas no response was observed on the film P3T. However, the irreversible oxidation of AA is observed at 0.132 V, 0.236 V, 0.397 V, 0.089 V and 0.148 V respectively with films P3T–Cu, P3T–Co, P3T–Ag, P3T–Pd, and P3T–Au. We noted also that all the potential values corresponding to the oxidation peak of AA using different electrode (films) are less positive than the potential when we used platinum electrode, and the largest current intensities were acquired for P3T–Ag which shows the higher sensitivity for AA, compared with films modified by other metals. A larger shift of potential toward more negative value is observed when polyterthiophene film is modified by Pd with a very high value of 0.354 V as energy gain. The potential shift obtained using different metallic nanoparticles compared to that of the bare electrode, can be related to the electrocatalytic activity of the material immobilized on the electrode. The good sensitivity of the film P3T–Ag toward the target molecule AA, due to the high electron conductivity and good stability of the silvers nanoparticles in aqueous solution (Lia et al., 2011), will be used in further experiments as working electrode.
3.3.2 Effect of polymerization time of the film
The effect of the formation time of the film on the catalytic activity is studied under the same conditions as described previously. Fig. 4 shows the electrocatalytic activity of the polyterthiophenes films prepared by chronoamperometry with different polymerization times (30, 60, 90, 120, 480 s) and then modified by silver cations (P3T–Ag) as working electrode. The results of these experiments indicate an increase in the current intensity of the AA oxidation peak when the polymerization time of the film comprises between 30 and 60 s, which is probably related to the increase of the specific surface covered by the film and the decrease of the roughness when we increased the deposition time. Further increase in the polymerization time of the film (t > 60 s) induced a decrease in peak intensity of AA, which can be explained by the fact that the film thickness becomes more thicker and therefore blocks the charge transfer between the electrode and the AA. Based on these results, we can deduce that the best detection was obtained with thin films especially when the polymerization time was for 60 s.
3.3.3 Effect of immersion time
To find out the suitable immersion time of P3T-film for effective analysis of AA, the modified electrode by P3T film was immersed in the solutions AgNO3 for different times 2, 5, 10, 20 and 30 min. The films were then used after immersion for the detection of ascorbic acid and the obtained results are shown in Fig. 5. As can be seen from the voltammograms, the electrochemical responses of the AA change upon changing on the immersion time of the film and the peak intensity of the AA oxidation increases when the incubation time of the electrode in the solution increases from 2 to 20 min. “This behavior due to the important amount of Ag deposed on the film when increasing the immersion time leading to the formation of enough active sites to bind the target molecule AA, which increases its detection (Los et al., 1993; Hitz and Lasia, 2001). After 30 min of immersion, the current intensity decreases, this phenomenon can be attributed to the drop of the Ag particles which gathered on the film surfaces as aggregate, and the effect of their weight let them to fall down in the solution.”
3.3.4 Effect of scan rate
The scan rate effect was studied to evaluate the kinetics of electrode reaction. Hence, the voltammograms of AA were recorded by varying the scan rate (see Fig. 6A). As it can be seen in the cyclic voltammograms, the scan rate exhibits an obvious effect on the oxidation peak current of AA, and we noted an increase in anodic peak currents of AA with an increase of scan rate from 10 to 120 mV/s. The variation of current density Ip versus v1/2 was plotted, the graph obtained was nearly straight line (Fig. 6B), and the relationship between the peak currents against square root of scan rate was described with linear equations: T = 22.57 v1/2 −5.6208 which indicates that the electrode transfer reaction was controlled by diffusion. This behavior is observed for AA oxidation on some polymer modified electrodes (Chen et al., 2005; Roy et al., 2004).
In addition, the oxidation peak potential shifted to more positive potentials when the scan rate increases, suggesting a kinetic limitation in the reaction between the redox sites P3T-nanoparticles and AA (Raoof et al., 2006).
3.3.5 Effect of AA concentration
The effect of initial concentration on the catalytic activity of the film toward AA was studied in the range of 10−3–10−9 M. Cyclic voltammetry responses (see Fig. 7(a)) show that AA detection is highly concentration dependent and the intensity of AA oxidation peak increases regularly as its concentration is increased. This increases in detection capacity with electroactive species concentration due to a high driving force for mass transfer (Bakas et al., 2014). The minimum concentration which can be detected using this method was about 10−6 M. However, by using square wave voltammetry (SWV) as an electroanalytical method, the oxidation peak current for AA increases regularly as its concentration is increasing Fig. 7 (b), and that at 10−9 mol L−1, a very small peak is still observed for the uptake of AA. These results prove that the sensor sensitivity using square wave voltammetry (SWV) technique is better than using cyclic voltammetry.
3.3.6 Detection limit and sensor stability
In order to determine the LOD at S/N = 3 it was necessary to record the baseline in pure PBS without any added ascorbic acid. The average blank output signals were found to be 5 μA, using SWV in PBS solution. From the linear plots determined in Fig. 8 and setting I = 3 SD (SD: standard deviation), the LOD value was found to be 5.17 · 10−10 mol L−1. Indeed, the recovery percentage after six uses of the sensor remains high 95.0% (±4.3%) showing that this device can be reused at least six times.
3.3.7 Selectivity
To investigate the specificity of the developed electrochemical sensor, and in order to confirm that the current signal was based on the specific interaction between the P3T–Ag and ascorbic acid and was not caused by nonspecific adsorption, the electroanalysis was done by using two other molecules dopamine and quercetin as nonspecific analytes instead of acid ascorbic. A very small oxidation peaks corresponding to dopamine (∼100 mV) and quercetin (∼30 mV) compared to that of AA were observed (see Fig. 9). These results confirmed that the developed sensor is very selective for AA than others molecules.
4 Conclusion
In this paper, a novel sensor for the detection of AA has been developed based on platinum electrode modified with polyterthiophene (P3T) and doped with silver nanoparticles. The P3T film doped with Ag showed a high sensitivity compared to those doped with other metallic particles such as Cu, Co, Au, and Pd, also the detection using square wave voltammetry (SWV) increases the oxidation signals than cyclic voltammetry (CV). According to the obtained results it was observed that a good detection of AA depends critically on several parameters such as polymerization time of the film, immersion time of the film in AgNO3 solution, which found to be 60 s, 20 min respectively. The limit of detection was determined at S/N = 3 and found to be 5.17 · 10−10 mol L−1 by square wave voltammetry (SWV). The sensors provide a good selectivity to AA, and could be reused up to 6 times with an excellent recovery (∼95%). Based on this, a sensitive, rapid and simple electrochemical method was developed for the simultaneous detection of AA.
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