Selective electrochemical determination of dopamine, using a poly(3,4-ethylenedioxythiophene)/polydopamine hybrid film modified electrode
Introduction
Dopamine (DA) is a neurotransmitter of great biological interest, and therefore many studies have been focused on the selective determination of this molecule. In this field the electrochemical methods have provided efficient determination of DA through its electro-oxidation, but the electro-oxidation potential of some molecules are very close to that reported for DA. In a healthy human brain, ∼50 nmol g−1 DA is found and in extracellular fluids 0.01 to 1 × 10−6 mol L−1 [1], [2], [3], [4]. A number of analytical methods have been developed to provide fast but sensitive, selective and reliable quantification in complex biological samples, including capillary electrophoresis [5], liquid chromatography [6], spectrofluorometry [7], microchip electrophoresis [8] and electrogenerated chemiluminescence [9]. The problem with these methods is the time required for sample preparation, the need of expensive equipment and that they can only be carried out at laboratory level. On the other hand, the development of conducting polymer modified electrodes for use in DA determination methods possesses advantages such as low cost, portability, high sensitivity and ability to make direct measurements in different analytical matrices [10].
Simultaneous determination of AA, UA and DA has been reported on various bipolymer modified electrodes, carbon ionic liquid, mesoporous carbon nanofiber, carbon ceramic, palladium nanoparticle-loaded carbon nanofibers, ordered mesoporous carbon/Nafion composite film, poly(3-(5-chloro-2-hydroxyphenylazo)-4,5-Dihydroxynaphthalene-2,7-disulfonic acid) film, poly(sulfonazo III) and poly(3-methylthiophene)/Pd, Pt nanoparticle, among others [2], [11], [12], [13], [14], [15], [16], [17], [18]. The use of electrodes modified with conducting polymers (CPs) in the determination of different kinds of molecules of biological interest [19], [20], [21], [22], [23], [24], [25], [26], [27], [28] has recently attracted considerable attention. One of the most utilized CPs is poly(3,4-ethylenedioxythiophene) (PEDOT), a polymer with exceptional stability in the oxidized state, exhibiting high conductivity [29]. Conductive polymers electropolymerization depends on the experimental conditions, e.g. solvent, supporting electrolyte, monomer concentration, and so on [29], [30], [31], [32]. Consequently the surface modification with new materials and its applications is necessary. Among these, the use of biopolymers, such as polydopamine (PDA), has been investigated. PDA obtention was accomplished using direct immersion of different electrode materials into an alkaline DA solution [1], [33] or by electropolymerization [26], [28], [34], [35], [36]. DA is oxidized to dopaminequinone (DAQ); DAQ intramolecular cyclization via 1,4-Michael addition leads to the more readily oxidizable leucodopaminechrome (LDAC), and subsequently LDAC is oxidized to dopaminechrome (DAC). DAC can further undergo polymerization reactions on the electrode surface, yielding a deposit of melanin-like polymer, responsible for the gradual electrode activity loss [26], [28], [35]. This disadvantage could be overcome by using a PEDOT film modified electrode and subsequent PDA growth upon this film. Chen studied electro-polymerization of melatonin on PEDOT unmodified and modified electrodes, finding that polymelatonine electro-polymerization depends on the experimental conditions, e.g. monomer oxidation potential and PEDOT film thickness [37]. Different catecholamines growth on PEDOT could generate a series of selective electrodes based on the principle “like recognizes like”.
Consequently, the use of electrochemical methods for molecules oxidation and polymers formation on electrodes can warrant the formation of the same amount of polymeric material, provided the electro-polymerization conditions are the same (starting unit, solvent, concentration and type of monomer and supporting electrolyte, electro-polymerization potential). Herein the obtention of a PEDOT/PDA hybrid film is proposes that enables DA selective determination in the presence of AA and UA.
Section snippets
Reagents
3,4-Ethylenedioxythiophene (EDOT), dopamine hydrochloride (DA), hydrazine sulfate, uric acid, L-ascorbic acid, potassium hexacyanoferrate(II) trihydrate, tetrabutylammonium hexafluorophosphate, and acetonitrile were purchased from Sigma–Aldrich (USA). PEDOT was prepared from 0.01 mol L−1 EDOT solution using 0.1 mol L−1 TBAPF6 as supporting electrolyte in anhydrous acetonitrile. 2.0 × 10−3 mol L−1 DA in PBS solution was utilized for PEDOT/PDA preparation.
A phosphate buffer solution (PBS, pH 7.4) was
PEDOT/PDA modified electrode characterization
The structural studies of the PDA and hybrid PEDOT/PDA film were performed using SEM and infrared spectroscopy. Fig. 1A and B shows PEDOT images obtained by SEM on a sheet of Pt. A globular morphology, similar to those already reported, is observed [21], [39]. Fig. 1C and D shows PDA images obtained on PEDOT revealing the formation of a hybrid PEDOT/PDA film. Different forms are observed, such as PDA sphere groups similar to those obtained on other polymers, e.g. poly(vinylidene fluoride),
Conclusions
The selective determination of dopamine (DA) was performed in the presence of ascorbic (AA) and uric (UA) acid, using a platinum electrode (Pt) modified with a hybrid film of poly(3.4-ethylenedioxythiophene)/polydopamine (PEDOT/PDA). PEDOT was electro-deposited on a bare Pt electrode using a potential step technique. PDA was subsequently obtained on the PEDOT coated electrode by cyclic voltammetry. The PDA thickness effect allows observing just the DA oxidation signal in the presence of some
Acknowledgements
We acknowledge the financial support through Project FONDECYT No. 1110041. R. Salgado thanks CONICYT Scholarship 2010, Folio 63100053.
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