Electrosynthesis and Characterisation of Poly(Safranine T) Electroactive Polymer films
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Thin Solid Films 517 (2009) 5435–5441 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Electrosynthesis and characterisation of poly(safranine T) electroactive polymer films Rasa Pauliukaite a,b, Ausra Selskiene a, Albertas Malinauskas a, Christopher M.A. Brett b,⁎ a Institute of Chemistry, A. Gostauto 9, LT-01108 Vilnius, Lithuania b Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal article info abstract Article history: The phenazine safranine T has been electropolymerised by potential cycling at carbon film electrodes in Received 9 May 2008 nitrate-containing solutions at different pH: 2.0, 5.5, and 7.0, as well as from chloride solution, pH 5.5. The Received in revised form 9 January 2009 electroactive polymer obtained, poly(safranine T), has been analysed by cyclic voltammetry in different Accepted 21 January 2009 electrolyte solutions and its morphology examined using scanning electron microscopy. The surface coverage Available online 29 January 2009 was calculated and found to be the highest for the film formed in chloride solution, pH 5.5, with 8.7 nmol cm−2. The other films had lower coverages, from 5.3 to 7.8 nmol cm− 2, and are similar to those obtained for other Keywords: Safranine T phenazine dye polymers. Although the fastest polymerisation occurred in solutions at pH 5.5, the best Phenazine electroactive polymer electrochemical properties resulted from electropolymerisation at pH 2.0. Analysis of the cyclic voltammo- Electropolymerisation grams showed that in most cases the electrochemical processes at the polymer coated electrodes are limited by Cyclic voltammetry surface reactions. Scanning electron microscopy data confirmed the results obtained by cyclic voltammetry and showed that the most compact film was obtained in chloride solution. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The application of the dye monomer as a mediator in a biosensing system with nitrite and nitrate reductase has been reported [23]. Polymer films possessing conductive properties have become A redox polymer containing safranine has been prepared by a attractive for many purposes since the electrosynthesis of polyaniline chemical procedure [24]. The amine groups present in safranine O [1,2]. Many other semiconducting polymer films have been reported, were condensed with poly(methacroylchloride) in dry dimethylfor- in particular polypyrrole, polyphenols, polythiophene [3,4], as well as mamide and allowed to reflux for a few days; the product was then polymer films electrodeposited from substituted monomers [3–6]. precipitated by addition of water to the reaction mixture. A structure Some of these polymers also exhibit electrocatalytic properties and of the product was not given, but cross-linking due to reaction of both they have been widely applied in the preparation of different sensing amino groups in safranine O molecules was seen as being possible. The surfaces [3,7]. important role of counterion diffusion during the polymer redox A special interest is that of polymers with direct charge transport processes was demonstrated. properties and which can therefore serve as redox mediators [3,7]. In other work, a polysafranine dye derivative poly[5-phenyl-7- Such films can be formed from azines, particularly various phenazines, (N,N-diethylamino) phenazinium] chloride, was prepared by decom- by electropolymerisation of their monomers [7,8]. The most-used position of the diazonium salt of 3-amino[5-phenyl-7-(N,N-diethylamino) azines for this purpose, as polymeric mediator, that have been phenazonium] chloride, and was used as a levelling component for reported so far are azur A [9,10], methylene blue [7,11–17], methylene electrodeposited copper coatings [25]. green [7,12,13,17], neutral red [7,12–18], brilliant cresyl blue [12,13], The chemical oxidative polymerisation of safranine and phenosa- phenothiazine [12,13], toluidine blue [7,12,13], and phenosafranine franine using peroxydisulfate initiator has been recently reported [19,20]. together with structural characterisation of the oligomers formed by Safranine T (ST), 3,7-diamino-2,8-dimethyl-5-phenylphenazinium Fourier transform infrared, Raman and ultraviolet spectroscopies [26]. chloride (Fig. 1), is a water-soluble red-coloured phenazine-type dye Safranine, dissolved in dilute hydrochloric acid, was oxidized by used in the textile, pharmaceutical, paper and cosmetic industries [21] addition of ammonium peroxydisulfate at room temperature to give, as and as a marker in biological processes [22]. This dye, like many product after 24 h, a dark red insoluble precipitate suspended in the phenazine dyes, is difficult to degrade due to its complex structure and colourless aqueous medium. The polymerisation mode was determined to one of the ways to achieve this is electrochemical degradation [21]. be coupling of the monomer molecules to give oligomeric-type structures with a molecular mass of several thousand, corresponding to about ⁎ Corresponding author. Tel./fax: +351 239835295. 20 monomer units. Some parameters of the polysafranine obtained −10 −1 −3 E-mail address: [email protected] (C.M.A. Brett). were determined: conductivity 6.2×10 Scm ;density1.34gcm ; 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.092 5436 R. Pauliukaite et al. / Thin Solid Films 517 (2009) 5435–5441 KCl electrode served as reference. Electrochemical measurements were performed using a computer-controlled BAS Epsilon potentio- stat/galvanostat analyser running with DigiSim software (BAS, West Lafayette, IN, USA). The pH measurements were carried out with a Mettler Toledo pH- meter. The surfaces of the samples were examined using an EVO 50EP (Carl Zeiss SMT AG, Germany) scanning electron microscope. All observations were carried out with a secondary electron (SE) detector in high-vacuum mode at 15 kV accelerating voltage. Fig. 1. Chemical structure of safranine T (3,7-diamino-2,8-dimethyl-5-phenylphenazinium chloride). 3. Results and discussion molecular weight, Mw, 45,000; soluble in N-methylpyrrolidone and 3.1. Electropolymerisation of safranine T dimethylsulfoxide [26]. Carbon film electrodes are attractive supports for mediator-based ST was polymerised electrochemically by cycling the applied sensors because they are cheap, easy to produce, small and are adapt- potential from −0.8 to 1.2 V vs. Ag/AgCl 18 times in solutions of able as disposable or short-term use sensors or biosensors [27–29]. different composition in order to find the best polymerisation − They have properties similar to those of glassy carbon and after elec- conditions. Since it is known that NO3 catalyses the polymerisation trochemical pre-treatment have a large potential window and low of some phenazine derivatives such as neutral red and methylene background current [30]. Recently, electrochemical enzyme biosen- blue [12], potassium nitrate electrolyte solutions were used at dif- sors on carbon film electrode supports have been investigated using ferent pH: 2.0 (S1), 5.5 (S2), and 7.0 (S3). At higher pH, in strongly various poly(azine) dyes – poly(neutral red), poly(methylene blue) alkaline media, ST does not form a stable polymer, the same hap- and poly(methylene green) – for measurements in complex matrices pens with neutral red [12]. Potentiodynamic mode was chosen be- [14–18]. cause it is simple to observe film growth by following the changes The present work aims to study the electrochemical polymerisation in cyclic voltammograms. The potential window has been chosen in of ST in different media and characterise the polymer films obtained order to follow the peaks of the redox processes of monomer/polymer using cyclic voltammetry and scanning electron microscopy. (~−0.55 V vs. Ag/AgCl) and irreversible monomer oxidation (between 0.9 and 1.0 V) and the position of these peaks varied only 2. Experimental details 2.1. Materials and reagents Safranine T was obtained from La Chema (Czech Republic). KCl was from Reachim (Russia), HNO3 and HCl were from Sigma, Na2HPO4 and NaH2PO4 were purchased from Riedel-de Haen (Germany). All solu- tions were analytical grade purity and were used as obtained with- out any further purification. Purified water (conductivity≤1µScm− 1, DEMIWA 10rosa system, Watek, Czech Republic) was used for preparation of all solutions. 2.2. Electrode preparation Cylindrical carbon film electrodes were made from carbon film resistors of diameter 1.5 mm and length 6 mm (resistance ~1.5 Ω). The electrode preparation protocol is described elsewhere [30,31]. The exposed electrode geometric area was ~0.20 cm2. Prior to first use, the electrodes were electrochemically pre-treated by cycling the applied potential between 0.0 and +1.0 V vs. Ag/AgCl in 0.1 M KNO3 solution for not less than 10 cycles, until stable cyclic voltammograms were obtained. Poly(safranine T) (PST) films were obtained by electrochemical polymerisation, cycling the applied potential from −0.8 to 1.2 V vs. Ag/AgCl at scan rate 50 mV s−1 in the polymerisation solution containing 0.5 mM ST, and different electrolytes: 10 mM HNO3 + 100 mM KNO3 (S1), 50 mM phosphate buffer (pH 5.5)+100 mM KNO3 (S2), 50 mM phosphate buffer (pH 7.0)+100 mM KNO3 (S3), or 50 mM phosphate buffer (pH 5.5)+100 mM KCl (S4). The polymer films obtained from the polymerisation solutions S1, S2, S3 and S4 will be referred to as PST1, PST2, PST3 and PST4, respectively. 2.3. Instruments A three-electrode electrochemical cell was used for electrochemi- cal measurements. It contained a PST-coated carbon film working electrode, a platinum foil as counter electrode, and an Ag/AgCl/sat. Fig. 2. Mechanism of dimer formation from safranine T. R. Pauliukaite et al. / Thin Solid Films 517 (2009) 5435–5441 5437 and one irreversible oxidation, similar to what was obtained in Ref. [21] at Pt electrodes. The first redox couple at −0.5 V is the redox process for STox/STred, as found for neutral red [33].