2,2’-Bithiophene-5-carboxylic acid (BTA) thin-films on platinum (Pt) electrodes were electrochemically prepared in acetonitrile solution containing 0.1 M tetrabutylammonium perchlorate (TBAP) and 0.05 M BTA. These films were complexes with several metal ions such as Cu2+, Ag+ and Co2+. Their structural characteristics were compared with those of powder complexes chemically prepared from BTA and the corresponding metal ion. IR and XPS techniques reveal that the film complexes with metal ions have the same structures as the corresponding powder complexes. The electrocatalytic activity of BTA film-metal ions has been investigated toward ascorbic acid (AA) oxidation and compared to that obtained on a free BTA film. BTA film-metal ions exhibit good catalytic proprieties and better detection of AA than a free BTA film. This new propriety allows these films to be used as electrochemical sensors. This electronic document is a “live” template. The various components of your paper [title, text, heads, etc.] are already defined on the style sheet, as illustrated by the portions given in this document.
Thiophene and its derivatives can be easily polymerized electrochemically or chemically to give conducting polymers with a range of useful properties that make these materials interesting candidates for a variety of potential applications [1,2], such as electro-chromic and electronic devices, electrical energy storage in rechargeable batteries [
In recent years, there has been a growing interest in conducting polymer-metal systems [8,9], where the metal is directly attached to the π-conjugated backbone, and has the ability to access a continuum of redox states afforded by metal-polymer cooperativity [
The incorporation of metal ions into organic conducting polymers offers potential in the area of organic-inorganic hybrid materials. Polyalkynyl platinum(II) complexes were the first materials prepared in this field [
The oxidation reaction of ascorbic acid (AA) was widely tested on various modified electrodes [13-17]. The role of the AA is known for its properties and its use as an antioxidant agent in foods and drinks, and it is also important for therapeutic purposes and biological metabolism. The determination of AA is a subject of considerable interest and with the use of simple, accurate and advantageous methods. Among of them, the electrochemical determination method is very accurate with high selectivity and best sensitivity. It is recognized as one of the most approaches for the separation of biological compounds such as ascorbic acid and dopamine [18-20]. Therefore, this advantage has a significant attraction in biological and electrochemical research.
With the aim of developing more efficient and novel organic-inorganic hybrid materials to improve their electrocatalytic properties, we were interested here in preparing materials based on 2,2’-bithiophene-5-carboxylic acid (BTA) (Schema 1(a)) and metal ions such as Cu2+, Ag+ and Co2+ that displayed electrocatalytic properties toward ascorbic acid oxidation (AA). BTA is selected for this study because: 1) it has an electroactive functional group (carboxylate group) able to coordinate to metal ions leading to complex materials; 2) the electro-oxidation of BTA leads to the formation of thin films on the electrodes. The BTA films chemically treated with metal ion solutions can be used as electrodes for the electrocatalytic oxidation of AA. The complex materials (film or powder form) were characterized by XPS and IR spectroscopy.
All chemical products and solvents (2,2’-bithiophene-5- carboxylic acid (BTA) (Acros organics); copper chloride CuCl2∙2H2O (Aldrich), cobalt chloride (CoCl2) and silver nitrate (AgNO3) (Fluka) and acetonitrile (Prolabo), were analytical grade and were used without further purification.
The complex was prepared following the procedure described in the literature [
was dissolved in methanol (0.8 mL) and NaOH (15.6 mg, 0.39 mmol) was added to this solution. After 30 min stirring, CuCl2∙2H2O (26.1 mg, 0.19 mmol) in methanol (0.5 mL) was added dropwise. The reaction mixture was stirred for an additional 30 min. The green solution was filtered, washed with methanol several times and dried at 50˚C in vacuum.
Thin films of BTA were first prepared electrochemicaly (cyclic voltammetry or chronoampremetry) in solution of CH3CN/TBAP 0.1 M, containing 5 × 10−2 M of BTA, then the film was rinsed with acetone to remove all traces of monomer and transferred in aqueous metallic solutions (M = Cu, Co, Ag) for concentration of 10−2 M each one for 10 min separately.
Electrochemical measurements were carried out using potentiostat/galvanostat type EG & G Princeton Applied Research model 273 under Chem software, in three electrode and a one-compartment cell. In most of our experiments the working electrode was a platinum (Pt) disc 0.03 cm2 in area. The counter-electrode was a steel grid and the reference was a conventional saturated aqueous potassium chloride-calomel electrode (SCE). The working electrode was polished before each experiment by cloth polishing with 0.1 μm alumina slurry, and rinsed with acetone and ethanol. The supported electrolyte was 0.1 M tetrabutylammonium perchlorate (TBAP) dissolved in argon-purged acetonitrile.
Infrared spectra of the complex and its precursor in the form of pressed KBr pellets were obtained on a Nicolet Magna-860 Fourier transform infrared spectrometer.
XPS spectra were recorded using a Thermo VG ESCALAB 250 system equipped with a micro-focused, monochromatic Al Ka X-ray source (1486.6 eV) and a magnetic lens which increases the sensitivity of the apparatus. The specimens were pressed against doublesided adhesive tapes mounted on sample holders, and then pumped overnight in the fast-entry lock at 5 × 10−8 mbar before introduction into the analysis chamber. A 650 mm X-ray beam was used at a power of 10 mA × 15 kV. The spectra were acquired in the constant analyser energy mode, with pass energies of 150 and 40 eV for the survey and the narrow regions, respectively. All samples (powders and plates) were subjected to charge compensation achieved with an electron flood gun combined with an argon ion gun. The argon partial pressure was 2 × 10−8 mbar in the analysis chamber. Avantage software, version 3.51, was used for digital data acquisition and processing. The peak binding energy positions were calibrated by setting the C1s peak maxima to 285.0 eV. The apparent surface compositions were determined by considering the integrated areas of the core-level peaks and their respective sensitivity factors.
By applying an anodic polarization to the platinum electrode in a 0.1 M TBAP acetonitrile solution of 5 × 10−2 M BTA, we obtained cyclic voltammetric curves between 0 and +1600 mV with an oxidation peak at about 1460 mV (
A similar study was undertaken with the BTA-Cu2+ complex (chemically synthesized its proposed structure is shown in Schema 1(b)) under the same conditions. The voltammetric behaviour of this complex is characterized, on the first scan, by a shoulder about 1100 mV and a peak at 1360 mV (
Thin films freshly prepared either by cyclic voltammetry or by the potentiostatic method were cycled in CH3CN containing 0.1 M TBAP or in H2O containing 0.1 M LiClO4 to study their electrochemical behaviour. In both cases, these films present no redox peaks in the range of potential from 0 to 1600 mV (
Despite the lack of electroactivity in both the above solutions, the BTA dimer films having carboxylic functional groups are able to complex electroactive species such as Cu2+, Co2+ and Ag+, which allow them to be used as sensors. These films were immersed in aqueous solutions containing either Cu2+ or Co2+ for 10 min, then rinsed with water and transferred to an electrochemical cell containing either CH3CN + 0.1 M TBAP or water + 0.1 M LiClO4.
In the first case, the cyclic voltammogram (CV) displayed oxidation and reduction peaks at 590 and 340 mV (
In the second case, similar observations to those of the first case were observed, but the two redox peaks shifted to more negative potentials, at 100 and −100 mV, respectively (
A similar study was carried out by immersing BTA thin-films in an aqueous solution of CoCl2 or AgNO3. The redox properties of BTA thin-film-Co2+ or BTA thin-film-Ag+ in aqueous solution exhibit features similar to those observed for BTA thin-film-Cu2+ (data not shown), which confirms the presence of metallic ions in BTA thin-films [23-25].
To verify and confirm the presence of metal ions in the BTA thin-films, XPS measurements were carried out on the free BTA thin-film, BTA thin-film-Cu2+ and BTACu2+ complex powder chemically prepared.
The survey spectra of a BTA thin-film and after its immersion in a 0.1 M CuCl2 solution for 10 min are shown in
case of a BTA-Cu2+ complex powder. These results suggest that Cu2+ is complexed by BTA thin-film. In addition, when comparing the binding energy (BE) values of the O1s in BTA and BTA film-Cu2+, we note that the oxygen BE is shifted about 0.8 eV (from 532.5 to 531.7 eV) due to O-Cu bonding [
Similar XPS analysis were performed on BTA thinfilm-Ag+, BTA thin-film-Co2+ and on the corresponding complexes under form of powders. These analyses reveal the presence of Ag+ and Co2+ in the BTA thin-films and powder complexes (data not shown). In our study, the detection of an O-Cu bond by XPS spectrum confirmed the formation of the BTA-Cu2+ complex [
IR measurements were performed on the ligand (BTA), BTA film-Cu2+ deposited on an ITO electrode and a
BTA-Cu2+ powder. By comparing the IR frequencies of the free ligand with those of BTA film-Cu2+ and BTACu2+ powder can be highlighted the formation of the complex (
Many compounds have been found to be active electron transfer mediators of the electocatalytic oxidation of ascorbic acid (AA) [13,33,34]. In our case also, the electrocatalytic behaviour of BTA thin-film modified by ion metals has been studied toward ascorbic acid oxidation in a 0.1 M phosphate buffer at pH 7.4.
BTA thin-films were immersed for 10 min in various solutions containing CuCl2, CoCl2 and AgNO3; then the electrode was removed, rinsed with distilled water and
transferred to a freshly prepared ascorbic acid solution. Cyclic voltammetry was used to examine the electroactivity of BTA thin-film-metal ion complexes deposited on Pt electrodes. The cyclic voltammograms of 0.1 M phosphate buffer containing 5 mM of AA clearly show (
As shown in
In this work, we have prepared and characterized BTA films deposed electrochemically on Pt electrodes. We have shown that these films after immersing in metal ion
solutions such as Cu2+, Ag+ or Co2+ had an electrocatalytic process for AA oxidation as analyte. This electrocatalytic activity toward such an analyte is particularly interesting owing to the fundamental importance for several applications, especially in the pharmaceutical and food fields. Moreover, we have shown by spectroscopic techniques (IR, XPS) that BTA can form a complex with Cu2+, Ag+ or Co2+, and that these metallic ions remain in the film after its use as an electrode in the reaction of AA oxidation. These electrodes may be used as sensors for the detection of AA.