Electrochemical characterisation of poly(3,4-ethylenedioxythiophene) film modified glassy carbon electrodes prepared in deep eutectic solvents for simultaneous sensing of biomarkers
Graphical abstract
Introduction
Innovative research on conducting polymers since their discovery by MacDiarmid, Heeger and Shirakawa [1], [2] in 1977 has been directed towards development of materials with specific physico-chemical and mechanical characteristics such as high conductivity, higher capacity to accumulate charge, higher elasticity and stability [3], [4]. Electrodeposition of π-conjugated polymers to give conducting polymer modified electrodes has led to application in diverse areas such as in supercapacitors, mechanical actuators, sensors, light emitting diodes, solar cells, electrochromic displays and switching materials [4], [5], [6]. Among these, poly(3,4-ethylenedioxythiophene) (PEDOT) exhibits good electrochemical stability, a wide potential window, low band gap and distinct spectral absorption for oxidized and reduced states [7]. Interesting properties exhibited by these materials is mainly due to their doping/dedoping as a function of applied potential [8].
The electrochemical synthesis of conducting polymers in the presence of different additives such as surfactants, acids and ionic liquids influences the properties of the electrodeposited material in terms of conductivity, redox behaviour and doping levels [9], [10], [11], [12]. Room temperature ionic liquids (RTIL) are alternative solvents utilized for the electrosynthesis of conducting polymers due to their high conductivity, low volatility and wide potential windows compared to conventional aqueous and organic solvents [12]. Although conventional RTIL possess such advantageous properties, they are costlier, involve tedious synthetic protocols and are non-biodegradable [13].
In the present study, we have carried out the electropolymerisation of 3,4-ethylenedioxythiophene (EDOT) in deep eutectic solvents (DES), which are formed by the interaction between suitable hydrogen bond donors and acceptors [13], [14]. These solvents can be easily prepared by mixing a quaternary ammonium salt, which acts as H-bond acceptor, with an H-bond donor (HBD), in specific mole ratios to form a eutectic mixture, with melting point much lower than the two components and which are liquid at room temperature. Commonly used H-bond acceptors are choline chloride, proline, alanine, betaine etc., while a large variety of hydrogen bond donors comes from the family of alcohols, carboxylic acids or amides such as ethylene glycol, glycerol, succinic acid or urea [13], [14], [15]. Advantages of deep eutectic solvents include ease of preparation, low cost, easily available components and biodegradability, in addition to the properties exhibited by conventional ionic liquids. Hence, these solvents possess great potential as electrolytic media for the electrodeposition of metals and polymers [16]. The electrochemical synthesis and characterization of conducting polymers such as polyaniline and polypyrrole in deep eutectic solvents has been reported elsewhere [11], [17]. Hillman et al. conducted studies on the ion transfer mechanism of p-doped PEDOT films prepared in acetonitrile [18]. Recently, we reported the preparation of PEDOT films for highly sensitive electrochemical sensors by electropolymerisation in DES, for the first time [19]. Electrochemical and surface microscopy characterisation of the PEDOT films formed in ethaline, reline and glyceline were carried out.
We now report detailed studies on the electrodeposition of PEDOT in various DES and characterization of the films using quartz crystal microbalance (QCM) gravimetry as well as electrochemical techniques. A gravimetric study using an electrochemical QCM is a powerful tool to detect and quantify the mass change, ion dynamics and polymer film characteristics. Detailed study of the polymerisation parameters, its sequential optimization and comparative evaluation of the sensing characteristics of PEDOT modified electrodes prepared in different DES is carried out using ascorbic acid (AA) as model analyte. Furthermore, the developed modified electrodes were used for the simultaneous detection and quantification of the biomarkers ascorbic acid (AA), dopamine (DA) and uric acid (UA) [20], [21]. Techniques often used to detect and quantify these biomolecules are chemiluminescence [22], UV–vis spectroscopy [23] and spectrofluorimetry [24]. However, these techniques are complicated, expensive, time consuming and usually require prior separation of components by chromatography [25] or electrophoresis [26]. For this reason, voltammetric electrochemical methods have been employed exploiting modified electrodes, which are able to separate the potentials for oxidation of these analytes sufficiently well. The voltammetric oxidation peaks of these compounds are in the range 0.2 to 0.7 V at bare glassy carbon electrodes (GCE) within one broad peak [27]. Modifiers which give selective electrocatalysis such as polypyrrole/poly(4-vinyl pyridine) films [28], [29], carbon based nanomaterials [30], [31], enzymes [23], or semiconductor quantum dots [22] have been employed to obtain well-separated oxidation peaks and high peak currents. Here, we have successfully used PEDOT modified GCE prepared in deep eutectic solvents for the simultaneous detection and quantification of AA, DA and UA.
Section snippets
Reagents and buffer electrolyte solutions
Ethylene glycol, urea, glycerol, choline chloride, 3,4-ethylenedioxythiophene (EDOT), ascorbic acid, dopamine, uric acid, monobasic and dibasic potassium phosphate, sodium chloride and sodium poly(styrene sulfonate) (NaPSS) were from Sigma-Aldrich, Germany. Perchloric acid (70%), potassium chloride and monobasic sodium phosphate were obtained from Fluka, Switzerland. For electrochemical sensing studies, the supporting electrolyte was sodium phosphate buffer saline (NaPBS) (0.1 M phosphate buffer
Results and discussion
Fig. 1 shows typical cyclic voltammograms of the electrochemical synthesis of PEDOT in the eutectic mixtures ethaline, reline and glyceline in the presence of 10 mM EDOT and 4 M HClO4. In order to compare the voltammetric features of PEDOT prepared in DES and other media, the electrochemical synthesis of PEDOT was also carried out in aqueous surfactant medium, 0.1 M PSS, and in aqueous 4 M HClO4.
Characterisation of these PEDOT-modified GCE by cyclic voltammetry, electrochemical impedance and
Conclusions
Procedures for the electrodeposition of thin PEDOT films on glassy carbon electrodes in deep eutectic solvents were developed, and a comparative evaluation of their sensing characteristics was carried out. PEDOT electrodeposited in urea (reline) based DES was found to be better in terms of sensitivity, selectivity, stability and reproducibility compared to that deposited in ethaline or glyceline based DES, using ascorbic acid as model analyte. The plausible reasons and mechanistic pathways for
Acknowledgements
Financial support from Fundação para a Ciência e a Tecnologia (FCT), Portugal PTDC/QUI-QUI/116091/2009, POCH, POFC-QREN (co-financed by FSE and European Community FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade under the projects PEst-C/EME/UI0285/2013) and CENTRO -07-0224 -FEDER -002001 (MT4MOBI)) is gratefully acknowledged. K.P. thanks FCT for a postdoctoral fellowship SFRH/BPD/78939/2011 and DST (India) for an Inspire faculty fellowship –
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