polydopamine hybrid film modified electrode

Journal of Electroanalytical Chemistry 704 (2013) 130–136

Contents lists available at SciVerse ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Selective electrochemical determination of dopamine, using a poly(3,4-ethylenedioxythiophene)/polydopamine hybrid film modified electrode R. Salgado, R. del Rio, M.A. del Valle, F. Armijo ⇑ Departamento de Química Inorgánica, Facultad de Química, Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, Casilla 306, Correo 22, Santiago, Chile

a r t i c l e

i n f o

Article history: Received 31 January 2013 Received in revised form 1 July 2013 Accepted 6 July 2013 Available online 17 July 2013 Keywords: Dopamine Poly(3.4-ethylenedioxythiophene) Polydopamine Modified electrode Ascorbic acid Uric acid

a b s t r a c t The selective determination of dopamine (DA) was performed in the presence of ascorbic (AA) and uric acid (UA), using a platinum electrode (Pt) modified with a hybrid film of poly(3.4-ethylenedioxythiophene)/polydopamine (PEDOT/PDA). PEDOT was obtained using a potential step technique on a bare Pt electrode. 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 interferents, e.g. uric acid (UA) and ascorbic acid (AA). The formed PEDOT/PDA hybrid film was compared with PEDOT in a K4[Fe(CN)6] solution in PBS of pH 7.4. Inhibition of the [Fe (CN)6]4 /[Fe(CN)6]3 redox couple on the PEDOT/PDA surface was observed, indicating the film bears a negative surface charge. The different electrodes were characterized by SEM and FTIR. DA amperometric determination was performed in the presence of AA and UA. The sensor proved to be selective toward DA. Calibration curves, with a linear range 1.5  10 6 to 50  10 6 mol L 1, enabled the limit of detection and quantification to be determined, namely 0.65  10 6 mol L 1 and 1.77  10 6 mol L 1, respectively. The described methodology permits, therefore, to propose this modified electrode as a new electrode for DA determination in the presence of interferents. Ó 2013 Elsevier B.V. All rights reserved.

1. 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 [1–4]. 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]. ⇑ Corresponding author. Tel.: +56 2 6864389; fax: +56 2 6864744. E-mail address: [email protected] (F. Armijo). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.07.005

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)-4,5Dihydroxynaphthalene-2,7-disulfonic acid) film, poly(sulfonazo III) and poly(3-methylthiophene)/Pd, Pt nanoparticle, among others [2,11–18]. The use of electrodes modified with conducting polymers (CPs) in the determination of different kinds of molecules of biological interest [19–28] 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 [29–32]. 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 [26,28,34–36]. 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

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136

dopaminechrome (DAC). DAC can further undergo polymerization reactions on the electrode surface, yielding a deposit of melaninlike 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.

131

range. From the CV profile, the optimum potentiostatic disturbing potential for i–t transient obtention was found to be 1.33 V applied during 240 s of electropolymerization. This corresponds to the potential region where PEDOT is formed. Once PEDOT is obtained, PDA electrodeposition was conducted on the PEDOT coating using 20 consecutive voltammetric cycles between 0.5 V and 0.6 V at 5 mV s 1. All chronoamperometric experimental data sets were smoothed and baseline-corrected. 3. Results and discussion 3.1. PEDOT/PDA modified electrode characterization

Electropolymerization was accomplished at room temperature (20 °C) using an Autolab PGSTAT20 potentiostat coupled to a personal computer, using the GPES V4.7 software. A conventional three-electrode, three-compartment electrochemical cell with a polycrystalline platinum disc (2 mm diameter) as working electrode was used in all the experiments. A Pt coil of large area was the counter electrode while an Ag/AgCl in tetramethylammonium chloride that matches the potential of a saturated calomel electrode (SCE) at room temperature was employed as reference electrode. At least otherwise stated, all potentials quoted in the current work are referred to this electrode [38]. The platinum working electrode was polished to a mirror-like finish with 0.3 lm alumina slurry on a felt pad. PEDOT and PEDOT/PDA FTIR spectra were recorded on a FT-IR Bruker Vector 2255 spectrophotometer; morphological studies were conducted by scanning electron microscopy (SEM) on a Carl Zeiss EVO MA 10, operated at 10.0 kV. pH measurements were done on a pH 330i, WTW, Germany, pH-meter.

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), polytetrafluoroethylene, poly(ethylene terephthalate) and polyimide [27], as well as PDA plates coating the PEDOT structure, different from those found on the above polymers. The differences can be more clearly seen in Fig. 1E where it is observed, to the left, the PEDOT surface and to the right, PDA coated PEDOT surface. Fig. 1F shows a deposit thickness of 2.2 and 1.5 lm for PEDOT/PDA and PEDOT, respectively (Fig. 1 Supplementary material). The surface changes from a dark color for PEDOT to a lighter color when the latter is coated by PDA, demonstrating the PEDOT/PDA formation. Fig. 2 shows the FTIR spectra obtained for DA, PEDOT and PEDOT/PDA, mechanically removed from the Pt sheet. DA exhibited broad and intense bands between 3400 and 3000 cm 1 due to OAH bonds of the DA molecule. Bands at 3144, 3071, 3041 and 2957 cm 1 were assigned to asymmetric stretching vibration of OH groups in the aromatic ring [27,28,40]. NH bond stretching vibrations are observed in a broad and intense band between 3200 cm 1 and 2250 cm 1 and medium intensity bands at 2433, 2539, 2638, 2748 cm 1. Other characteristic bands are: CAOAH bond asymmetric bending vibration at 1320 cm 1, asymmetric CAO vibration at 1190 cm 1 and CAC stretching at 1176 cm 1 [27,28,40]. For PEDOT, quinoid type CAC vibrations are observed at 1460 cm 1 attributed to the thiophene ring and C@C at 1380 cm 1. CAOAC stretching vibrations are observed at 1170 and 1000 cm 1, and thiophene ring CAS vibrations are observed at 974 and 850 cm 1 [41–44]. The formation of PDA on PEDOT can be corroborated by the absence of the broad band between 3000 and 3400 cm 1 and 3200– 2250 cm 1, indicating that the NH2 group disappeared during electro-polymerization. NH2 group bending vibrational modes at 1519 cm 1 also disappeared; the wide spectrum at 1630 cm 1 changes as compared with DA spectrum; this can be ascribed to the stretching vibration of the C@C indole group formed during the electro-polymerization [42]. In the 3500 cm 1 region a broad band belonging to the OAH group showed up. Comparison of these results with previous works revealed that a less overoxidazed PDA film was obtained [28]. Disappearance of PEDOT signals was also observed, consequently it can be inferred that PDA was formed and immobilized on the conductive polymer surface.

2.3. PEDOT/PDA modified electrodes preparation

3.2. PEDOT/PDA electrochemical characterization

The optimum EDOT electro-polymerization potential using cyclic voltammetry (CV) was obtained within the 1.5 V and 1.5 V

The electrochemical characterization was performed by cyclic voltammetry. Fig. 3 shows CV profiles for PDA obtention on PEDOT

2. Experimental 2.1. 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 prepared using 0.1 mol L 1 NaCl (Merck), 2.6  10 3 mol L 1 KCl (JT Baker), 0.04 mol L 1 Na2HPO4 (Merck), and 0.01 mol L 1 KH2PO4 (Merck). Aqueous solutions were prepared with doubly distilled, deionized water. Prior to each experiment, the working solution was deaerated by flushing with high purity, 99.99%, argon for 30 min. All experiments were conducted at room temperature (20 °C) under argon atmosphere. 2.2. Apparatus

132

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136

Fig. 1. Typical SEM images of PEDOT (A and B), PEDOT/PDA (C–F) with low (A, C and E) and high (B, D and F) magnification.

Fig. 2. DA, PEDOT and PEDOT/PDA FTIR spectra.

in a potential range 0.5 to 0.6 V for 20 cycles at a scan rate 5 mV s 1. An anodic peak at 0.17 V is responsible for dopamine oxidation to o-dopaminequinone and the cathodic peak at 0.13 V is related to dopaminequinone reduction back to dopamine. Anodic and cathodic current peaks at 0.25 V and 0.32 V, respectively are associated to the reversible oxidation of leucodopaminochrome to dopaminochrome [28,35]. The latter product is subsequently transformed into melanin-like polymers. Increasing the number of the successive anodic and cathodic sweeps both peaks increase indicating the gradual formation of the polymer film on PEDOT. A IR drop as the number of oxidation cycles increase, Fig. 3 (inset figure), occurs when DA electro-oxidation is attempted on a bare Pt electrode, similar to what occurs on GC and Au electrodes used for the selective detection of DA [34–36]. This suggests that PEDOT allows the formation of a PDA coating and that the obtained hybrid film does not lose its conducting characteristics. The reason for the more effective dopamine polymerization on PEDOT-modified Pt

Fig. 3. Consecutive cyclic voltammograms (20 cycles) of PDA film on PEDOT electrode surface from PBS solution (pH = 7.4) containing 2  10 3 mol L 1 DA in the potential range 0.5 to 0.6 V. m = 5 mV s 1. Inset figure. Consecutive cyclic voltammograms (10 cycles) of PDA film on a Pt bare electrode in the potential range 0.6 to 0.3 V, under similar electropolymerization conditions.

than on bare Pt could be explained by the existence of a hydrophobic interaction between PEDOT film and DA monomer. DA has a less basic or neutral character, and exists as a positively charged or neutral species at pH 7.4. Hence, the possibility of electrostatic interaction between DA and PEDOT film, having positively charged backbones, remained highly improbable. If this hydrophobic interaction is considered, the monomer concentration may have increased at the polymer-solution interface, thereby increasing DA polymerization peak current [37]. Fig. 4 exhibits CV stable voltammetric profiles obtained after 50 cycles in a PBS solution at pH = 7.4 for unmodified Pt (full line with triangles), PEDOT (full line with circles) and PEDOT/PDA (full line with square), over the potential range 0.5 to 0.6 V at 10 mV s 1. It was verified that bare Pt presented a response

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136

Fig. 4. Steady response obtained by cyclic voltammetry in the potential range from 0.5 to 0.6 V in PBS solution (pH = 7.4); Pt bare electrode (line with full triangles), PEDOT (line with full circles) and PEDOT/PDA (line with full square). Inset figure. Cyclic voltammograms of PEDOT (line with full square) and PEDOT/PDA (continuous line) in PBS (pH = 7.4) with 1  10 3 mol L 1 K4[Fe(CN)6]. m = 10 mV s 1.

133

without faradaic redox processes; when the electrode was PEDOT modified, an increase in the capacitive current occurs, but when a PEDOT/PDA layer exists, besides showing higher capacitive response, a faradaic process redox couple at ca. 0.17 V was observed that corresponds to the o-dopaminoquinone/dopamine couple, demonstrating that a hybrid film formation exists. A study of the effect of the potential sweep rate on PEDOT/PDA obtention was then conducted (data not shown). To this end, 10 cycles were performed at 5, 10 and 20 mV s 1. Stable potentiodynamic responses obtained after 50 cycles at 10 mV s 1 revealed that an increase of current takes place as the electro-polymerization scan rate decreased. This enabled inferring that a greater amount of PDA was formed. From these experiments, the selected sweep rate for obtaining modified electrodes was set to 5 mV s 1. Fig. 4 inset shows cyclic voltammetry response of PEDOT (line with full square) and PEDOT/PDA (solid line) modified electrodes in the presence of an anionic redox probe. The continuous line with squares shows the PEDOT response; a redox couple corresponding to the [Fe(CN)6]4 /[Fe(CN)6]3 pair is observed. On the other hand, the solid line is the PEDOT/PDA response and the redox couple is no longer observed. Hence, repulsive forces exist between the PDA modified surface and the redox probe [28]. It may be inferred that the obtained modified electrode must bear a negatively charged surface that would repel anionic species. On the other hand, Fig. 5 (figure inset) depicts the pH vs. Ip plot. These results revealed that the maximum peak current occurred at pH 7.4; this pH was therefore chosen to accomplish electroanalytical experiments. The Ep1/2 vs. pH line exhibits a linear regression slope 0.054 V/pH, R2 = 0.95, very close to the predicted Nernstian value for a two-electron two-proton process [26,28,34–36]. 3.3. Electro-oxidation of AA, DA and UA on PEDOT and PEDOT/PDA

Fig. 5. Cyclic voltammograms at pH between 4.0 and 9.0 for PEDOT/PDA. Inset figure. Plots of peak potential and peak current variation as a function of pH.

Electroanalytical studies in the presence of the analytes were performed using cyclic voltammetry and chronoamperometry. Fig. 6A and B shows the electro-oxidation of 5  10 3 mol L 1 AA and 2  10 3 mol L 1 UA, respectively, in PBS at pH 7.4. For PEDOT and different PEDOT/PDA electrodes, obtained with different scan rates and number of cycles in order to study the effect of PDA film thickness, irreversible AA and UA electro-oxidation processes were observed on PEDOT at 0.09 and 0.27 V, respectively. The signal in the presence of AA and UA is suppressed when PEDOT/PDA was obtained using 20 cycles at 5 mV s 1, as more PDA exists, increasing

Fig. 6. Cyclic voltammograms of PEDOT (continuous line) and PEDOT/PDA obtained under different electropolymerization conditions in PBS (pH = 7.4): 10 cycles at 20 mV s 1 (line with full circles), 10 cycles at 10 mV s 1 (line with full triangles), 10 cycles at 5 mV s 1 (line with asterisks) and 20 cycles at 5 mV s 1 (thick solid line) in the presence of (A) 5  10 3 mol L 1 AA and (B) 2  10 3 mol L 1 UA. m = 10 mV s 1.

134

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136

thus the negative surface charge that repels, at the studied pH, the anionic species of these compounds; hence, a decrease in the intensity of the oxidation peak current of AA and UA occurs. This phenomenon can also be described in terms of AA and UA interaction with PEDOT. This polymer is obtained in its oxidized state, i.e. positively charged; at pH = 7.4 AA and UA molecules are in anionic form and therefore electrostatic interactions may take place between PEDOT and these acids. To corroborate the PDA selective effect, an experiment was conducted using Nafion modified PEDOT in presence of AA, UA and DA (Fig. 3 Supplementary material). It was verified that no electrochemical response exists for none of the surveyed analytes. Fig. 7 shows DA electro-oxidation on PEDOT/PDA at 0.17 V. The interaction between DA and PEDOT is weak because at this pH both are in the cationic form. Hydrophobic models for the interaction between these molecules [37] have been proposed, consequently DA exhibits low electrochemical activity. The decrease of AA and UA oxidation current allows saying that PEDOT–AA and PEDOT– UA interaction sites decrease as PDA coating on the PEDOT surface increases. Consequently, a modified electrode for DA selective determination that can be used in the presence of interferents such as AA and UA was obtained. Literature reports indicate higher electrocatalytic activity of a PDA-modified ITO electrode in the presence of hydrazine (1.0  10 3 mol L 1) affording a reduction of the quinone free groups in the PDA skeleton, without interfering in DA determination, and higher anodic currents are obtained. Accordingly, DA amperometric determination was accomplished adding 1.0  10 3 mol L 1 hydrazine to the PBS solution, pH = 7.4 [1]. When amperometric measurements were accomplished in the absence of hydrazine (Fig. 2 Supplementary material), slopes 0.013 and 0.029 lA/lmol L 1 for PEDOT/PDA and PEDOT films, respectively, were obtained. This revealed a sensitivity decrease of the method. 3.4. DA chronoamperometric determination on PEDOT/PDA in the presence of AA and UA The amperometric response of the PEDOT/PDA hybrid film modified electrode towards DA was investigated in a stirred PBS solution (pH 7.4) at a working potential of 0.17 V. Fig. 8 shows typical current–time curves for successive additions of DA of equal concentrations, using the PEDOT/PDA hybrid film modified electrode. The current changed after DA addition and reached another steady-state value within 5 s. Fig. 8 inset shows Ip  DA concentra-

Fig. 8. Typical amperometric curve obtained for PEDOT/PDA in PBS (pH = 7.4) at 0.17 V. Stirring rate 500 rpm. Successive DA addition from 5  10 6 to 50  10 6 mol L 1. Insets show the corresponding calibrations plots with the concentration of DA varying between 5  10 6 to 50  10 6 mol L 1. This experiment conducted in the presence of 1  10 3 molL 1 hydrazine.

tion calibration curves using a PEDOT/PDA hybrid film modified electrode; DA concentration ranged from 5  10 6 to 50  10 6 mol L 1, (correlation coefficient 0.995). The linear range and sensitivity observed with PEDOT/PDA were generally comparable with those of most of the modified electrodes reported in literature [1,19–22,24]. Table 1 shows comparison between different PEDOT and PEDOT/PDA modified electrodes in terms of analytical performance. PDA electrode displayed greater sensitivity for dopamine determination and a linear range and limit of detection similar to other reported values. A linear correlation between concentration and peak current was found. To determine amperometric analytical parameters, a calibration curve was constructed between 1.5  10 6 and 50  10 6 mol L 1 that fits the equation Ip (lA) = 0.7436 [DA] (lmol L 1) 0154 (r2 = 0.997) (n = 3). Limit of detection (LOD) and quantification (LOQ), 0.65  10 6 mol L 1 and 1.77  10 6 mol L 1, respectively, were calculated using the following equations: LOD = 3 s/m, LOQ = 10 s/m, where s is the standard deviation of current baseline of the blank (three measurements) and m is the slope of the calibration curve [45,46]. Amperometric responses current in Fig. 9 was obtained by successive addition of 5  10 6, 3  10 6 and 1.5  10 6 mol L 1 DA. Data were recorded in a PBS stirred solution (pH 7.4) at the same potential as used for DA, i.e. 0.17 V. Results clearly pointed out that AA and UA exhibited no obvious interference on DA steady state current at 0.17 V. DA determination in urine, a real sample, under similar conditions (n = 3) (Fig. 4 Supplementary material) exhibited a good linear correlation. Therefore, PEDOT/PDA sensors are sensitive to low DA concentrations and are good candidates to selectively determine DA in the presence of interferents.

3.5. Stability and reproducibility

Fig. 7. PEDOT/PDA cyclic voltammograms in the absence of DA (thick solid line) and PEDOT/PDA obtained after 20 cycles at 5 mV s 1. DA concentration in PBS (pH = 7.4): 1  10 6 mol L 1 (line with full square), 5  10 6 mol L 1 (line with full triangles), 1  10 4 mol L 1 (line with asterisks) and 1  10 3 mol L 1 (thin solid line). m = 10 mV s 1.

Reproducibility of the sensor was assessed by measuring DA concentration of a 1  10 4 mol L 1 in PBS, and the determined relative standard deviation was 2.8% (n = 6), same day and electrode, and 3.8% (n = 3) during three consecutive days using three different electrodes, proving that the device possessed good stability and reproducibility. In addition, to determine the catalytic current during DA oxidation, the PEDOT/PDA electrode was tested in a solution 1  10 4 mol L 1 of DA before and after continuous stirring of the buffered solution for 30 min; the electrode response exhibited no significant difference either in the stirred or unstirred

135

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136 Table 1 Comparision of present work and previus study that use PEDOT and PDA materials for determination of dopamine. Electrode

Analyte Eox (V)

LDR (lM)

LOD

Sensitivity (lA/lM)

Ref

Poly(dopamine) films on indium–tin oxide Copper-modified/PEDOT

DA = +0.28 DA = 0.24

1 nM NR

NR 0.013

1 19

PEDOT/GCE

DA = +0.15

NR

NR

20

PEDOT/Pt in presence SDS.

DA = +0.22

61 nM

NR

21

PEDOT/palladium composite

DA = +0.18

0.5 lM

1.9

22

PEDOT/GCE

DA = +0.21

NR

0.022

24

Pt/PEDOT/PDA

DA = +0.17

0.001–1 6–200 DPV 20–80 DPV 0.5–25 DPV 0.5–1.0 DPV 100–500 SWV 1.5–50 Ch

0.65 lM

0.74

This work

Note: Eox, oxidation potential; GCE, Glassy carbon electrode; SDS, sodium dodecyl sulfate; DA, dopamine; LDR, linear dynamic range; LOD, limit of detection; NR, not reported; CV, cyclic voltammetry; DPV, differential pulse voltammetry; SWV, square wave voltammetry; and Ch, chronoamperometry.

respectively Utilizing this procedure a novel DA selective sensor is proposed, that will be previously optimized, particularly with regard to its effective surface area, by preparing PEDOT as nanowire instead of solid, as it has been done herein. Acknowledgements We acknowledge the financial support through Project FONDECYT No. 1110041. R. Salgado thanks CONICYT Scholarship 2010, Folio 63100053. Appendix A. Supplementary material Fig. 9. Typical amperometric curve obtained for a PEDOT/PDA sensor electrode in PBS (pH = 7.4) at 0.17 V. Stirring rate 500 rpm. Successive additions of 5  10 6 M, 3  10 6 and 1.5  10 6 mol L 1 DA and additions of 250  10 6 mol L 1 AA and UA. This experiment conducted in the presence of 1  10 3 mol L 1 hydrazine.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jelechem.2013. 07.005. References

solution. This test demonstrated that reproducible results could be obtained using the proposed PEDOT/PDA electrode. Stability of the PEDOT/PDA hybrid film modified electrode was investigated by storing the device at room temperature in the presence and absence of PBS (pH 7.4). The sensor was stable for 15 days, but thereafter a gradual signal drop was observed. PEDOT/PDA hybrid film modified electrode stored for a week in PBS (pH 7. 4) underwent a 12% DA current drop from its initial value. These results suggest that the PEDOT/PDA sensor has acceptable stability and good reproducibility. This allows the next stage to be designed namely, optimization of the electrode to be applied to biological tissues. 4. 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 interferents, e.g. UA and AA. This will enable the obtainment of a surface capable of detecting just DA in its cationic form and repel acids in their anionic form, the latter showed to decrease the intensity of the electro-oxidation peak current of AA and UA. A linear range between 1.5  10 6 and 50  10 6 mol L 1 DA was found using chronoamperometric technique. The sensor limit of detection and quantification was 0.65  10 6 and 1.77  10 6 mol L 1,

[1] B. Kim, S. Son, K. Lee, H. Yang, J. Kwak, Dopamine detection using the selective and spontaneous formation of electrocatalytic poly(dopamine) films on indium–tin oxide electrodes, Electroanalysis 24 (2012) 993–996. [2] K. Lin, C. Yin, S. Chen, Simultaneous determination of AA, DA, And UA based on bipolymers by electropolymerization of luminol and 3,4ethylenedioxythiophene monomers, Int. J. Electrochem. Sci. 6 (2011) 3951– 3965. [3] P. Rattanarat, W.R. Dungchai, W. Siangproh, O. Chailapakul, C.S. Henry, Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples, Anal. Chim. Acta 744 (2012) 1–7. [4] Y Dong, M.L. Heien, M.M. Maxson, A.G. Ewing, Amperometric measurements of catecholamine release from single vesicles in MN9D cells, J. Neurochem. 107 (2008) 1589–1595. [5] Y.M. Liu, C.Q. Wang, H.B. Mu, J.T. Cao, Y.L. Zheng, Determination of catecholamines by CE with direct chemiluminescence detection, Electrophoresis 28 (2007) 1937–1941. [6] P. Song, O.S. Mabrouk, N.D. Hershey, R.T. Kennedy, In vivo neurochemical monitoring using benzoyl chloride derivatization and liquid chromatography mass spectrometry, Anal. Chem. 84 (2012) 412–419. [7] H. Huang, Y. Gao, F.P. Shi, G.N. Wang, S.M. Shah, X.G. Su, Determination of catecholamine in human serum by a fluorescent quenching method based on a water-soluble fluorescent conjugated polymer–enzyme hybrid system, Analyst 137 (2012) 1481–1486. [8] C.M. Kang, S. Joo, J.H. Bae, Y.R. Kirn, Y. Kim, T.D. Chung, In-channel electrochemical detection in the middle of microchannel under high electric field, Anal. Chem. 84 (2012) 901–907. [9] Y. Hu, W. Xu, J.P. Li, L.J. Li, Determination of 5-hydroxytryptamine in serum by electrochemiluminescence detection with the aid of capillary electrophoresis, Luminescence 27 (2012) 63–68. [10] M.A. del Valle, M. Gacitua, F.R. Diaz, F. Armijo, J.P. Soto, Electro-synthesis and characterization of polythiophene nano-wires/platinum nano-particles composite electrodes. Study of formic acid electro-catalytic oxidation, Electrochim. Acta 71 (2012) 277–282. [11] A. Safavi, N. Maleki, O. Moradlou, F. Tajabadi, Simultaneous determination of dopamine, ascorbic acid, and uric acid using carbon ionic liquid electrode, Anal. Biochem. 359 (2006) 224–229.

136

R. Salgado et al. / Journal of Electroanalytical Chemistry 704 (2013) 130–136

[12] Y. Yue, G. Hu, M. Zheng, Y. Guo, J. Cao, S. Shao, A mesoporous carbon nanofibermodified pyrolytic graphite electrode used for the simultaneous determination of dopamine, uric acid, and ascorbic acid, Carbon 50 (2012) 107–114. [13] A. Salimi, H. MamKhezri, R. Hallaj, Simultaneous determination of ascorbic acid, uric acid and neurotransmitters with a carbon ceramic electrode prepared by sol–gel technique, Talanta 70 (2006) 823–832. [14] J. Huang, Y. Liu, H. Hou, T. You, Simultaneous electrochemical determination of dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbón nanofibers modified electrode, Biosens. Bioelectron. 24 (2008) 632– 637. [15] D. Zheng, J. Ye, L. Zhou, Y. Zhang, C. Yu, Simultaneous determination of dopamine, ascorbic acid and uric acid on ordered mesoporous carbon/Nafion composite film, J. Electroanal. Chem. 625 (2009) 82–87. [16] A.A. Ensafi, M. Taei, T. Khayamian, A differential pulse voltammetric method for simultaneous determination of ascorbic acid, dopamine, and uric acid using poly (3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7disulfonic acid) film modified glassy carbon electrode, J. Electroanal. Chem. 633 (2009) 212–220. [17] A.A. Ensafi, M. Taei, T. Khayamian, A. Arabzadeh, Highly selective determination of ascorbic acid, dopamine, and uric acid by differential pulse voltammetry using poly(sulfonazo III) modified glassy carbón electrode, Sens. Actuators: B 147 (2010) 213–221. [18] N.F. Atta, M.F. El-Kady, Novel poly(3-methylthiophene)/Pd, Pt nanoparticle sensor: Synthesis, characterization and its application to the simultaneous analysis of dopamine and ascorbic acid in biological fluids, Sens. Actuators: B 145 (2010) 299–310. [19] A. Stoyanova, V. Tsakova, Copper-modified poly(3,4-ethylenedioxythiophene) layers for selective determination of dopamine in the presence of ascorbic acid: I. Role of the polymer layer thickness, J. Solid State Electrochem. 14 (2010) 1947–1955. [20] J. Mathiyarasu, S. Senthilkumar, K.L.N. Phani, V. Yegnaraman, PEDOT–Au nanocomposite film for electrochemical sensing, Mater. Lett. 62 (2008) 571– 573. [21] N.F. Atta, A. Galal, R.A. Ahmed, Poly(3,4-ethylene-dioxythiophene) electrode for the selective determination of dopamine in presence of sodium dodecyl sulfate, Bioelectrochemistry 80 (2011) 132–141. [22] S. Harish, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, PEDOT/palladium composite material: synthesis, characterization and application to simultaneous determination of dopamine and uric acid, J. Appl. Electrochem. 38 (2008) 1583–1588. [23] S. Kumar, J. Mathiyarasu, K. Phani, Exploration of synergism between a polymer matrix and gold nanoparticles for selective determination of dopamine, J. Electroanal. Chem. 578 (2005) 95–103. [24] V.S. Vasantha, S. Chen, Electrocatalysis and simultaneous detection of dopamine and ascorbic acid using poly(3,4-ethylenedioxy)thiophene film modified electrodes, J. Electroanal. Chem. 592 (2006) 77–87. [25] A. Stoyanova, V. Tsakova, Copper-modified poly(3,4-ethylenedioxythiophene) layers for selective determination of dopamine in the presence of ascorbic acid: II Role of the characteristics of the metal deposit, J. Solid State Electrochem. 14 (2010) 1957–1965. [26] T. Luczak, Electrocatalytic application of an overoxidized dopamine film prepared on a gold electrode surface to selective epinephrine sensing, Electroanalysis 20 (2008) 1317–1322. [27] J. Jiang, L. Zhu, L. Zhu, B. Zhu, Y. Xu, Surface characteristics of a selfpolymerized dopamine coating deposited on hydrophobic polymer films, Langmuir 27 (2011) 14180–14187. [28] T. Luczak, Preparation and characterization of the dopamine film electrochemically deposited on gold template and its applications for

[29]

[30]

[31]

[32] [33] [34] [35]

[36]

[37]

[38] [39]

[40] [41]

[42]

[43]

[44]

[45]

[46]

dopamine sensing in aqueous solution, Electrochim. Acta 53 (2008) 5725– 5731. J. Heinze, B.A. Frontana-Uribe, S. Ludwigs, Electrochemistry of conducting polymers-persistent models and new concepts, Chem. Rev. 110 (2010) 4477– 4724. M.A. del Valle, M.A. Gacitúa, L.I. Canales, F.R. Díaz, Oligomer chain length effect on the nucleation and growth mechanisms (NGM) of polythiophene, J. Chil. Chem. Soc. 54 (2009) 260–266. R. Schrebler, P. Grez, P. Cury, C. Veas, M. Merino, H. Gómez, R. Córdova, M.A. del Valle, Nucleation and growth mechanisms of poly(thiophene) – Part 1. Effect of electrolyte and monomer concentration in dichloromethane, J. Electroanal. Chem. 430 (1997) 77–90. M.A. del Valle, P. Cury, R. Schrebler, Solvent effect on the nucleation and growth mechanisms of poly(thiophene), Electrochim. Acta 48 (2002) 397–405. H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. H. Chang, D. Kim, Y.C. Park, Electrochemically degraded dopamine film for the determination of dopamine, Electroanalysis 18 (2006) 1578–1583. Y. Li, M. Liu, C. Xiang, Q. Xie, S. Yao, Electrochemical quartz crystal microbalance study on growth and property of the polymer deposit at gold electrodes during oxidation of dopamine in aqueous solutions, Thin Solid Films 497 (2006) 270–278. M. Li, C. Deng, Q. Xie, Y. Yang, S. Yao, Electrochemical quartz crystal impedance study on immobilization of glucose oxidase in a polymer grown from dopamine oxidation at an Au electrode for glucose sensing, Electrochim. Acta 51 (2006) 5478–5486. V.S. Vasantha, S.-M. Chen, Effect of reaction conditions on electropolimerization of melatonin on bare electrodes and PEDOT-modified electrodes, J. Electrochem. Soc. 152 (2005) D151–D159. G.A. East, M.A. del Valle, Easy-to-make Ag/AgCl reference electrode, J. Chem. Educ. 77 (2000) 97. S. Patra, K. Barai, N. Munichandraiah, Scanning electron microscopy studies of PEDOT prepared by various electrochemical routes, Synth. Met. 158 (2008) 430–435. D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Elucidating the structure of poly(dopamine), Langmuir 28 (2012) 6428–6435. W. Chiu, J. Travas-Sejdic, R. Cooney, G. Bowmaker, Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4ethylenedioxythiophene), Synth. Met. 155 (2005) 80–88. K. Mueller, M. Park, M. Klapper, W. Knoll, K. Muellen, Synthesis and layer-bylayer deposition of spherical poly(3,4-ethylenedioxythiophene) nanoparticles – toward fast switching times between reduced and oxidized states, Macromol. Chem. Phys. 208 (2007) 1394–1401. B. Kowalewska, K. Miecznikowski, O. Makowski, B. Palys, L. Adamczyk, P.J. Kulesza, Preparation and spectroelectrochemical characterization of composite films of poly(3,4-ethylenedioxythiophene) with 4-(pyrrole-1-yl) benzoic acid, J. Solid State Electrochem. 11 (2007) 1023–1030. C. Kvarnstrom, H. Neugebauer, A. Ivaska, N. Sariciftci, Vibrational signatures of electrochemical p- and n-doping of poly(3,4-ethylenedioxythiophene) films: an in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study, J. Mol. Struct. 521 (2000) 271–277. L. Molero, M. Faúndez, M.A. del Valle, R. del Río, F. Armijo, Electrochemistry of methimazole on fluorine-doped tin oxide electrodes and its square-wave voltammetric determination in pharmaceutical formulations, Electrochim. Acta 88 (2013) 871–876. M.P. Miranda, R. del Río, M.A. del Valle, M. Faúndez, F. Armijo, Use of fluorinedoped tin oxide electrodes for lipoic acid determination in dietary supplements, J. Electroanal. Chem. 668 (2012) 1–6.