We take advantage of polyelectrolyte feature exhibited by natural cashew gum (
The physicochemical properties of nanoscale composite are a result of molecular interaction between materials of interest, such as a conducting polymer, promoting greater structural control of the formed films [
The technological interest of several research groups in composites, and more recently in nanocomposites, comes mainly from the mechanical properties and biodegradability, which are both characteristic of natural polymers, allied to the conductive properties of some synthetic polymers, which provides a great versatility of applications in areas such as engineering, biotechnology, and medicine [
Natural gums are macromolecules formed from units of sugars, monosaccharides, linked by glycosidic bonds resulting in natural polymers with long chains and high molecular weight [
The cashew gum (
(a) Chemical structures of material employed in the films and (b) schematic process of multilayer film formation.
The polyaniline (PANI) belongs to the class of conducting polymers with high technological interest due to their potential applications as electroluminescent devices, corrosion protection, sensors, and biosensors [
In this study, LbL films were produced with PANI or PANI-PA and the natural cashew gum (CG) in a bilayer fashion (PANI/CG)
Cashew gum, collected in the state of Ceará (northeast region of Brazil), was isolated and purified using sodium salt, as described by Costa et al. [
PANI was synthesized by the oxidative polymerization of aniline doubly distilled in 1.0 mol L−1 HCl solution containing a proper amount of ammonium persulfate ((NH4)2S2O8, Vetec). The solution temperature was kept between 0°C and 5°C, with continuous mechanical stirring. The product was maintained in ammonia hydroxide (Vetec) for 12 hours to obtain PANI in the form of emeraldine base (EB) [
For the processing of polyaniline solutions, 0.47 g PANI-EB powder (with or without PA dopant) was dissolved in 25 mL dimethylacetamide (DMAc, Vetec) under stirring for 12 h. The solutions were filtered and slowly added to 26 mL of HCl solution, and the pH was adjusted at 2.8. Poly(vinyl sulfonic acid) (PVS) was purchased from Aldrich Co. and used without previous purification in aqueous solutions at a concentration of 0.5 mg mL−1 and pH 2.8. Ultrapure water with a resistivity of 18.3 MΩ cm (Milli-Q, Millipore) was used for preparation of all solutions. The chemical structures of the materials employed are depicted in Scheme
Nanostructured layered films were assembled in a bilayer fashion using PANI or PANI-PA as polycationic solutions in conjunction with CG or PVS as polyanionic solutions. The deposition of each layer consisted in the immersion of the substrate in the dipping solution for 5 min, followed by rinsing in the washing solution (HCl, pH 2.8) and drying in N2 flow. LbL films with four distinct architectures were investigated: (PANI/PVS)
Electrochemical measurements were carried out using a potentiostat Autolab PGSTAT30 and a three-electrode electrochemical cell with 10 mL. A 1.0 cm2 platinum foil and saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. The LbL films onto ITO (0.4 cm−2) were used as the working electrode. All the experiments were performed in inert N2 atmosphere at 22°C in an electrolytic solution of 0.1 mol L−1 HCl. PANI-AP/GC LbL film containing 6 bilayers (
Cyclic voltammograms of ITO unmodified and modified electrodes with LbL film produced with poly(allylaminehydrochloride), PAH, and the natural cashew gum, (PAH/CG)6, were obtained in HCl 0.1 mol L−1 and are shown in Figure
Cyclic voltammograms for bare ITO (—
All films studied were prepared with six bilayers. A previous study about the influence of the size and nature of this anion in the supporting electrolyte for the systems studied here was carried out in HCl or H2SO4, both at 0.1 mol L−1, with scan rate of 50 mV s−1. The processes of oxidation and reduction, characteristic of conducting polymer, were shifted to more positive potentials in H2SO4 media when compared to the profile obtained in HCl solution (data not shown). The potential difference observed for this redox process was 0.05 V. According to Matveeva et al. [
Figure
Cyclic voltammograms for LbL films: (a) (PANI-PA/PVS)6, (b) (PANI/PVS)6, (c) (PANI-PA/CG)6, and (d) (PANI/CG)6 in 0.1 mol L−1 HCl solution at 50 mV s−1.
An intermediate process between the transitions related above is defined as acidic degradation of PANI with the formation of benzoquinone (oxidation) and hydroquinone (reduction) pair. The electrochemical behavior for the systems in Figure
The phosphonic acid (PA) used in this study acts as both a modifying agent PANI, increasing its solubility, and a dopant acid promoting a further increase in current values observed in the redox processes of PANI-PA (Figures
For the films PANI-PA/PVS (Figure
Cyclic voltammograms for LbL films from (a) (PANI-PA/PVS)
When PVS was replaced by cashew gum (Figure
Representation of the mechanisms of the adsorption process proposed for the PANI and PVS: (a) adsorption in the presence of electric charge and (b) adsorption in the absence of electric charge [
Probably the adsorption processes proposed for the PANI and PVS must be somehow related to the oxidation process observed in the region of 0.86 V in Figure
In Figure
Cyclic voltammograms for LbL films from (a) (PANI-PA/PVS)6 and (b) (PANI-PA/CG)6 after five and twenty (5th cycle —
Therefore, the polyaniline suffers acid degradation, promoted by the high polarization potential and by electrolyte of HCl, and in the case in the PANI-PA/PVS system this process is enhanced by the presence of PA modifier. On the other hand, when PVS was replaced by CG the degradation process of PANI was reduced indicating that the cashew gum protects the film from the degradations processes mentioned above, presenting a greater stability during scanning in an acid medium and polarizations at 0.90 V compared to PANI-PA/PVS or PANI/PVS films studied. Therefore, the cashew gum acts as a kind of antioxidant for polyaniline. Previous works from our group [
The process of charge transfer from the ITO-modified electrode with PANI-PA/CG film containing 6 bilayers was studied by varying the scan rate (
The dependence on pH solution of the electrochemical behavior of PANI-PA/CG film was studied and shown in Figure
Cyclic voltammograms for LbL films from (PANI-PA/CG)6 in HCl electrolyte at different pHs (—■— pH 1 initial; —|— pH 2; - - - pH 3; —+— pH 4; — pH 5; —
LbL films of cashew gum containing modified polyaniline with PA dopant are very interesting for applications in electrochemical sensors due to their high reproducibility and stability. Thus, the self-assembled film of PANI-PA/CG containing 6 bilayers was selected to be applied to detect dopamine (DA), an important neurotransmitter in the central nervous system of mammals [
Figure
(a) Cyclic voltammograms for LbL films from (PANI-PA/CG)6 in 0.1 mol L−1 HCl solution in the presence of DA at concentrations ranging from 0.01 mmol L−1 to 0.23 mmol L−1 (from bottom to top). (b) The dependence of peak current (
The anodic current peak (
Table
Comparative performance of different electrodes for dopamine determination.
Electrode | Method | Dynamic range (mmol L−1) | LD (mol L−1) | Reference |
---|---|---|---|---|
ITO modified with LbL film from (PANI-PA/CG)6 | CVa | 0.01–0.23 | 1.5 × 10−8 | This work |
ITO modified with LbL film from (PAH/Chichá gum/PAH/NiTsPcb)5 | CV | 0.3–250 | 1.05 × 10−5 | [ |
ITO modified with LbL film from ([PAMAM-MWCNTs]c/NiTsPc)3 | CV | 2.5 × 10−3–0.24 | 5.4 × 10−7 | [ |
GCEd/MWCNTs modified with LbL film from (Nafion/[PVI-dmeOs]e)3 | DPVf | 1.0 × 10−4–0.01 | 5.0 × 10−8 | [ |
GCE modified with poly(flavin adenine dinucleotide) | CV | 2.5 × 10−6–4.0 × 10−5 | 5.0 × 10−9 | [ |
ITO modified with LB film of (PANI/Rupyg)21 | CV | 0.04–1.2 | 4.0 × 10−5 | [ |
GCE modified with (LiTCNE/PLL)h membrane | DPV | 1.0 × 10−5–0.01 | 5.0 × 10−10 | [ |
aCyclic voltammetry.
bTetrasulfonated metallophthalocyanine of nickel.
cPolyamidoamine-multiwalled carbon nanotubes.
dGlassy carbon electrode.
ePoly(vinylimidazole)-Os(4,4′-dimethylbpy)2Cl.
fDifferential pulse voltammetry.
gRuthenium complex
hLithium tetracyanoethylenide/poly-L-lysine.
The relationship of peak current (
In order to investigate the selectivity of the PANI-PA/CG modified electrode, we tested the simultaneous detection of 0.01 mmol L−1 DA in different ascorbic acid (AA) concentrations, the natural interfering of DA [
Cyclic voltammograms for LbL films from (PANI-AP/GC)6 film in the absence and presence of 0.01 mmol L−1 DA containing ascorbic acid at a concentration of (1) 0.01 mmol L−1 (2) 0.02 mmol L−1 and (3) 0.03 mmol L−1 in 0.1 mol L−1 HCl solution, at 50 mV s−1.
Additionally it is important to observe that this new nanocomposite using cashew gum, which is a natural and biocompatible polymer, in the multilayer structure, gives rise to new applications as biosensor [
The electrochemical profiles observed for the films studied showed the redox intrinsic characteristic transitions of polyaniline. The presence of CG increases the electrochemical stability of the film, suggesting that it acts by protecting the conductive polymer from acid degradation. LbL films of PANI/PVS and PANI-PA/PVS show an oxidation process around 0.86 V, which can be related to interaction between PANI and PVS. Detailed studies showed that the electrochemical reaction in the PANI-PA/CG film is governed by a charge transfer mechanism at the surface electrode via electron hopping. The ITO-modified electrode with PANI-PA/CG film showed high reproducibility and stability, encouraging its use as a sensor of DA. This modified electrode was able to detect electroactive molecule of DA around 0.63 V in detection limits consistent with the pharmaceuticals formulations.
The S. B. A. Barros is indebted to CAPES for MSc fellowship. The authors thank the financial support from the Brazilian funding agencies FAPEPI, CAPES, and CNPq, as well as the technical support from LAPETRO-UFPI. The authors are grateful for the English revision done by our dear friend Mrs. Claudia Hissette.