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Sodium do-decyl benzene sulfate modified carbon paste electrode as an electrochemical sensor for the simultaneous analysis of dopamine, ascorbic acid and uric acid: A voltammetric study
Journal of Molecular Liquids 177 (2013) 32–39
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Sodium do-decyl benzene sulfate modified carbon paste electrode as an electrochemical sensor for the simultaneous analysis of dopamine, ascorbic acid and uric acid: A voltammetric study S. Sharath Shankar, B.E. Kumara Swamy ⁎, B.N. Chandrashekar, K.J. Gururaj Department of P.G. Studies and Research in Industrial Chemistry, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, Shimoga (D), Karnataka (S), India
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 27 September 2012 Accepted 1 October 2012 Available online 24 October 2012 Keywords: Dopamine Simultaneous Cyclic voltammetry Surfactant modified electrode
a b s t r a c t Sodium do-decyl benzene sulfate modified carbon paste electrode (DDBSMCPE) was first employed for the simultaneous determination of dopamine (DA), uric acid (UA), and ascorbic acid (AA). The modified CPE displayed excellent electrochemical catalytic activities. The oxidation over potentials of DA, UA and AA decreases significantly and their oxidation peak currents increases dramatically at DDBSMCPE. Differential pulse voltammetry (DPV) was used for the simultaneous determination of UA, DA and AA in their ternary mixture. The peak separation between UA-DA and DA-AA is 152 mV and 221 mV, respectively, and the detection limit was 0.01 μM for DA. The proposed method improved sensitivity for the determination of DA by more than one order of magnitude. The present method was applied to the determination of DA in real sample by using standard addition method and the obtained results were satisfactory with a good recovery of 98%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A large number of molecular processes in chemically, physically and biologically related systems occur at solid–liquid, liquid–liquid and liquid–gas interfaces, which modify the dynamic behaviour of molecules relative to their bulk properties [1]. The structural and dynamic properties of adsorbed surfactant molecular films are therefore of both fundamental and applied interest [2]. Surfactant molecules exhibit an amphiphilic or amphipathic behaviour and they bear an ionic (zwitterionic, anionic, or cationic) or non-ionic polar head group and a hydrophobic portion. The self-assembled aggregates of amphiphilic surfactant molecules formed on solid surfaces are important as models for biological membranes [3]. In fact, the adsorption of surfactants on solid surfaces allows the simulation of membrane-like structures which can be used in diverse biological/industrial processes such as protein immobilization, charge and mass transfer, membrane solubilization and disruption, etc [4–6]. Parkinson's disease is one of the most dreadful neurodegenerative disorders of the central nervous system, as its complete cure is not possible even today. Tremor, rigidity, bradykinesia and postural instability are some of its common diagnostic features. This disease occurs when dopaminergic neurons malfunction, or are destructed, which is accompanied by a sharp decline in dopamine level [7–9]. DA is a catecholamine neurotransmitter which is generated in various parts of central and peripheral nervous system and has an agonist action
⁎ Corresponding author. Tel.: +91 8282 256225 (office); fax: +91 8282 256255. E-mail address: [email protected] (B.E.K. Swamy). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.10.002
on β adrenoceptors. DA has positive chronotropic and ionotropic effects on myocardium which stimulates cardiac contractility and enhances heart beat rate. Electrochemical detection of DA is a preferred method because DA is electrochemically active. In addition, electrochemical methods offer advantages such as simplicity, speed, and sensitivity. A major problem in the electrochemical detection of DA is the coexistence of other biologically important compounds including ascorbic acid (AA) and uric acid (UA) at a higher concentration [10]. Ascorbic acid is a vital component in human diet. It is known to take part in some biological reactions and is present in mammalian brain [11]. Recent clinical studies have demonstrated that the content of ascorbic acid in biological fluids can be used to assess the amount of oxidation stress in human metabolism [12,13] and excessive oxidative stress lead to cancer, diabetes mellitus and hepatic diseases. Uric acid is the principal final product of purine metabolism in the human body [14]. It has been shown that extreme abnormalities of UA levels are symptoms of several diseases (e.g. gout, hyperuricaemia and Lesch–Nyhan syndrome) [15–17]. Other diseases, such as leukaemia and pneumonia are also associated with enhanced urate levels [16]. In general, electroactive UA can be irreversibly oxidized in aqueous solution and the major product is allantoin [18]. Among many methods for determination of DA, AA and UA in biological samples, electrochemical techniques with modified electrodes have been shown to be powerful tools due to their advantages of being simple, inexpensive and possibility of fast analysis in combination with high sensitivity and selectivity [19]. Thus, different kinds of modified electrodes have been fabricated for detection of DA, AA and UA [19–24]. Even though all these electrodes are having some advantages they have many limitations too. So the real challenge is in developing simple, reliable and efficient sensors with
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enhanced characteristics for effective sensing of DA, AA and UA simultaneously. In recent years, surfactant modified electrodes have provoked much more attention in electro analysis because of their novel physical and chemical properties [23,25–27]. In particular, the catalytic properties of some surfactants cause a decrease in the over potential needed for a redox reaction to become kinetically viable, producing voltammetry which appears more reversible than that displayed by the same material in an unmodified electrode form [28]. Moreover, the use of surfactant-modified electrodes presents some other advantages like high effective mass transport catalyzes and controls the local environment [29,30]. Carbon paste electrode (CPE) is one of the convenient conductive matrices to prepare the chemically modified electrodes (CMEs) by the simple mixing of graphite/binder paste and modifier [31,32]. These kinds of electrodes are inexpensive and possess many advantages such as low background current, wide range of used potential, ease of fabrication, and rapid renewal. In this work, a novel biosensor has been fabricated by using a carbon paste electrode (CPE) immobilized with sodium do-decyl benzene sulfate (DDBS) for simultaneous electrochemical determination of AA, DA and UA. As already known, DDBS is an anionic surfactant which contains benzene in its structure and the SDS has a hydroxy group, which could be covalently bound to the edge plane sites of the carbon surface through the oxygen atom. 2. Materials and methods 2.1. Reagents and Chemicals Dopamine hydrochloride, ascorbic acid, uric acid was purchased from Himedia Company. All chemicals were of analytical grade quality and were used without further purification. 1 × 10−6 M stock solution of DA was prepared by dissolving it in 0.1 M perchloric acid solution. 1 × 10−4 M stock solution of AA was prepared by dissolving it in double distilled water and 1 × 10−4 M stock solution of UA was prepared by dissolving it in 0.1 M NaOH. The supporting electrolyte used was the phosphate buffer solution (PBS). It was prepared by mixing standard stock solutions of 0.1 M disodium-hydrogen phosphate and sodium dihydrogen phosphate by adjusting the pH. 2.2. Apparatus and procedure Cyclic voltammetry (CV) was performed on Model EA-201 Electroanalyser (EA-201, Chemilink System). All the experiments were carried out in a conventional three electrode electrochemical cell. The electrode system contained a carbon paste working electrode (3.0 mm in diameter), a platinum wire counter electrode and a saturated calomel reference electrode (SCE). The bare carbon paste electrode (BCPE) was prepared by grinding 70% graphite powder (particle size 50 mm and bulk density 20–30 g/100 mL from Loba chemical company) and silicon oil (viscosity 300 cps at 20° from Himedia chemical company) to produce a homogeneous carbon paste electrode. The carbon paste was then packed into the cavity of a homemade carbon paste electrode body and smoothened on a weighing paper. DDBS modified carbon paste electrode (DDBSMCPE) was prepared by immobilizing 25 μL of 0.1 mM DDBS on the surface of the carbon paste electrode for 20 minutes.
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Fig. 1. Cyclic voltammogram 1 mM potassium ferrocyanide in 1 M KCl at BCPE (a) and DDBSMCPE (b) with a scan rate of 50 mV/s.
respectively. The peak potential difference (ΔEp) was found to be 32 mV. However, in the same condition DDBSMCPE exhibited good sensitivity for K4Fe(CN)6 and both anodic and cathodic peak current signals got enhanced when compared to the BCPE. The Epa and Epc were located at 257 mV and 196 mV. The ΔEp was found to be 61 mV. The enhancement of peak current shows electro catalytic activity of DDBSMCPE. 3.2. The effect of concentration and immobilization time of DDBS on voltammetric response of K4Fe(CN)6 To improve the performance of the biosensor, various factors influencing the response of the sensor such as the concentration of surfactant and the immobilization time were investigated. It can be seen that the biosensor prepared with 25 μL of 0.1 mM DDBS surfactant solution has a maximum current response. The reason may be that DDBS surfactant molecule diffuses into the carbon paste electrode along with the K4Fe(CN)6 , which results in the increase in current signal. If the concentration of DDBS is low, the rate of diffusion is low and the response of the biosensor is also low. However, if the concentration of DDBS is 0.1 mM it can show maximum current response at 25 μL DDBS. Therefore 25 μL of 0.1 mM DDBS was selected as the suitable amount for further studies. However, a higher concentration of mediator produced a larger background current. Thus, 25 μL DDBS was chosen in the subsequent experiments. 25 μL DDBS was immobilized on the surface of carbon paste electrode and the immobilization time was varied from 5 to 30 minutes. Fig. 2 shows the current response of the modified electrode increased gradually with increase in immobilization time, reaching a maximal value at 20 minutes and then decreases with further increase of immobilization time. Therefore a time gap of 20 minutes was chosen for the diffusion of the DDBS molecule into the porous carbon paste electrode. Such a behavior is typical of a mediator-based sensor [33,34]. Therefore, 20 minutes was selected as the immobilization time for the further analysis.
3. Results and discussion 3.3. Surface morphology of DDBSMCPE 3.1. Electrochemical response of potassium Ferro cyanide on DDBSMCPE Fig. 1 demonstrates the cyclic voltammograms of K4Fe(CN)6 (1 mM) at BCPE and DDBSMCPE at sweep rate of 50 mV/s. The K4Fe(CN)6 showed poor sensitivity and reproducibility at BCPE (dashed line) compared to DDBSMCPE. The cyclic voltammogram of K4Fe(CN)6, in 1 M KCl as supporting electrolyte at sweep rate of 50 mV/s showed its anodic (Epa) and cathodic peak potentials (Epc) at 240 mV and 208 mV
Fig. 3 explains the surface morphology of bare carbon paste electrode (A) and DDBSMCPE (B) using scanning electron microscopy. The surface of bare carbon paste electrode was irregularly shaped micrometer sized flakes of graphite. However, the DDBS film coated carbon paste electrode has typical uniform arrangement of DDBS molecules on the surface of carbon paste electrode. This confirms that the carbon paste electrode was coated by the modifier film.
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Further, blank experiment was carried out without taking any DA with buffer solution (curve b) and no peaks were obtained for the modified electrode. It is clear from the above results that the DDBSMCPE shows high electro catalytic effect on DA. The DDBS molecule forms a monolayer on the CPE surface with “stand” mode. A negative electric field exists around the modified electrode surface. The monolayer adsorption of DDBS on the CPE surface is similar to the adsorption character of SDS on the graphite surface [34]. At 7.4 pH, the amine group in DA molecule is electropositive [35] and the negatively charged DDBS monolayer interacts with the positively charged DA [36]. Therefore electropositive DA is attracted electro-statically to the monolayer and can be oxidized at relatively low potentials and the schematic representation is shown in Fig. 4b. 3.5. Effect of scan rate at DDBSMCPE
Fig. 2. Plot of current vs. immobilization time (from 5 to 30 minutes) in 0.2 M phosphate buffer solution of pH 7.4 with a scan rate of 50 mV/s.
3.4. Electrochemical behavior of dopamine at DDBSMCPE Fig. 4a shows cyclic voltammetric recordings of 0.1 μM DA at the BCPE (curve a) and at DDBSMCPE (curve c) at pH 7.4 phosphate buffer solution. At the BCPE, DA exhibited a poor electrochemical response when compared to the modified electrode. The oxidation peak (Epa) was located at 172 mV and cathodic peak (Epc) was at 134 mV in 0.2 M phosphate buffer solution of pH 7.4. The modified electrode showed increase in peak currents considerably. The cathodic peak potential (Epc) was at 172 mV and anodic peak potential (Epa) at 113 mV.
Fig. 3. SEM images of (A) bare CPE (B) DDBSMCPE.
The effect of the scan rate on the peak current of DA was investigated. Fig. 5a shows that the CVs of the DDBSMCPE are dependent on the various scan rates in 0.2 M PBS (pH 7.4). The anodic or cathodic peak currents are proportional to scan rates from 50 to 350 mV/s and the curves are linear with scan rate (Fig. 5b). The corresponding linear regression equation is expressed as ip (μA) =−0.31 +0.130ν (mV/s). The result indicates that the reaction of DA at the DDBSMCPE is controlled by the adsorption process. 3.6. Effect of pH Cyclic voltammetry was used to investigate the effects of pH value in electrochemical determination of DA in the presence of AA at the DDBSMCPE. Fig. 6a shows the cyclic voltammograms obtained at the DDBSMCPE in 0.2 M phosphate buffer solutions of different pH values containing 0.1 μM DA at a scan rate of 50 mV/s. Fig. 6b shows the effect of pH on the peak potential (Epa) and peak currents in aqueous buffer solution with the increasing pH from 3.4 to 8.4. The Epa shifted negatively and were dependent linearly on pH with the slopes of − 57 mV/pH which indicates that the proportion of the electrons and protons involved in the redox reaction of DA was 1:1 and the oxidation of DA at the DDBSMCPE had been inferred to be a doubleelectron transfer reaction [37]. It could also be observed that both the oxidation and reduction peak current obtained a maximum at pH 7.4 (Fig. 6c), which could be partly explained on the basis of the dissociation ability of –SO3Na(H) group of DDBS film in different pH environments. When the solution pH was equal to 7.4, the –SO3Na group of DDBS film could dissociate into a negative charge group –SO3−. Under this condition, the –NH2 group of DA molecules (pKa: 8.9) [38] could obtain a proton and form the cation of DA. Therefore, the negative charge group –SO3− on the surface of DDBSMCPE has a well affinity to the DA positive ions and could catalyze and promote the oxidation of DA in the neutral buffer solution (pH 7.4). While pH was below 7.4, the –SO3Na group of DDBS film could form –SO3H and exclude the DA positive ions. However, when pH value was beyond 7.4, the decreased DA peak currents seemed to be contradicted to the above-mentioned dissociation resumption. Actually, to interpret the effect of pH on the electrocatalytic oxidation of DA, it should not be overlooked that DDBS film at electrode acted as a mediator of electron transfer for DA's oxidation, whose electron transfer process could be affected by pH. Indeed, the anodic peak current of DDBS film reaction at carbon electrode was observed to decrease gradually with increasing pH from 7.4 to 8.4. It was identified that with increasing pH, the rate of electron transfer of the DDBS film could also decrease gradually, which was disadvantageous to the electrocatalytic reaction of DA at the DDBS coated electrode. Thus, pH was a dual conditioner to the oxidation of DA at the DDBS coated carbon electrode. On other hand, increasing pH of electrolyte solution from 3 .4 to 7.4, could promote the formation of the –SO3− anions on DDBS film showing that the DDBSMCPE had a well affinity to the DA
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Fig. 4. a. Cyclic voltammograms in 0.2 M PBS of pH 7.4 on (a) BCPE (c) DDBSMCPE with 0.1 μM DA and (b) on DDBSMCPE without DA at a scan rate of 50 mV/s. b. Schematic diagram represents oxidation of DA at DDBSMCPE surface.
positive ions and enhanced the oxidation of DA. Hence, the pH 7.4 was an optimum protocol for the electrocatalytic oxidation of DA at the DDBSMCPE.
detection limit of various electroanalytic methods proposed for determination of DA is compared with our analytical data in Table 1. LOD ¼ 3S=M
ð1Þ
LOQ ¼ 10S=M
ð2Þ
3.7. Effect of concentration of dopamine at DDBSMCPE From Fig. 7 it is clear that the relationship between anodic peak current and concentration of DA is linear between 0.1 μM and 2.5 μM with the linear regression equations as ipa (μA) = 0.0183 + 0.3519 (C)DA μM/L. The correlation coefficient was found to be 0.998. The detection limit for DA was found to be 0.01 μM and quantification limit was 0.05 μM by DPV method and was calculated by using the formulae (1) and (2) [39,40], where S is the standard deviation and M is the slope obtained from the three calibration plots. The
3.8. Electrochemical oxidation of ascorbic acid at DDBSMCPE Fig. 8 shows the electro-catalytic behavior of AA at the DDBSMCPE and the BCPE in the PBS of pH 7.4 that contained 10 μM of AA. The cyclic voltammograms of AA in the PBS produced a single irreversible oxidation peak using both DDBSMCPE and BCPE. The reason for this phenomenon was the reaction of the oxidized form of ascorbic acid
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Fig. 5. a. Cyclic voltammograms of 0.1 μM DA in 0.2 M PBS (pH 7.4) on the DDBSMCPE at different scan rates (a–g: 50, 100, 150, 200, 250, 300, 350 mV/s.). b. Effect of variation of scan rate on the anodic peak current of 0.1 μM DA in 0.2 M PBS of pH 7.4.
with water that resulted in an irreversible electrochemical behavior [46–48]. The oxidation of ascorbic acid occurred at around 182 mV at the BCPE, whereas the same occurred at the DDBSMCPE at about − 25 mV at pH 7.4 buffer aqueous solution. This negative shifting in its oxidation potential could be due to the repulsive force between the negatively charged SO3 layer of the modified electrode and anionic form of AA. 3.9. Electrochemical oxidation of UA at DDBSMCPE Fig. 9 shows the cyclic voltammograms of 10 μM uric acid at the BCPE (curve a) and DDBSMCPE (curve b) in 0.2 M phosphate buffer solution of pH 7.4 as a supporting electrolyte at scan rate of 50 mV/ s in the range of − 250 to 650 mV. At the BCPE, UA gives an oxidation peak at about 304 mV. However at DDBSMCPE UA gives one anodic and one cathodic peak at a potential of 298 mV and 245 mV, respectively. The peak to peak separation was found to be 53 mV. Further the concentration of UA was increased for the confirmation of redox peak. While increasing the concentration, both anodic and cathodic peak got increased. The DDAB accelerates the electron
Fig. 6. a. Cyclic voltammogram of 0.1 μM DA in PBS with different pH (a–e; 3.4, 4.4, 5.4, 6.4, 7.4 and 8.4). Scan rate: 50 mV/s. b. Graph of anodic peak potential of 0.1 μM DA vs. pH. c. Graph of anodic peak current of 0.1 μM DA vs. pH.
transfer and hence the irreversible nature of UA changes to reversible at the DDBSMCPE. This result indicates good electro catalytic activity of DDBS film towards the oxidation of UA.
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Fig. 7. Effect of concentration of dopamine on the anodic peak current in 0.2 M PBS of pH 7.4 with a scan rate of 50 mV/s.
Fig. 8. Cyclic voltammograms obtained for the oxidation of 10 μM AA in 0.2 M PBS (pH 7.4) at bare CPE (a) and DDBSMCPE (b). Scan rate: 50 mV/s.
3.10. Simultaneous electro chemical detection of dopamine, uric acid and ascorbic acid at DDBSMCPE
3.11. Interference study
The interference due to the presence of other electroactive species such as AA and UA are one of the major problems in determination of dopamine. Both AA and UA get electrochemically oxidized at the potentials close to oxidation of dopamine and normally cause an overlapped oxidation potential on the electrodes used for detection. These interferences result in the oxidized species having an overlap of peaks in unmodified electrode to detect dopamine. Another problem is that ascorbic acid has a concentration ranging from hundred folds than dopamine, which results in poor selectivity and sensitivity for dopamine detection [46–48]. The simultaneous determination of 0.1 μM dopamine in presence of 10 μM ascorbic acid and 10 μM uric acid at DDBSMCPE was investigated by CV technique at the scan rate of 50 mV/s in PBS (pH 7.4). As shown in Fig. 10a, the BCPE shows one broad and relatively low single anodic peak (curve a) and at the same time DDBSMCPE reduces the anodic over potentials (curve b) and well-defined anodic peaks are observed at − 23, 182 and 317 mV for AA, DA and UA. The electrochemical peak separation between DA and AA was found to be 206 mV and for UA and DA to be 134 mV, which indicates a sufficient separation of three-well defined peaks. Further, the electrochemical separation study was carried out by DPV technique as shown in Fig. 10b because of its higher current sensitivity and better resolution than cyclic voltammetric technique. The anodic peak potentials for DA, AA and UA were at − 20, 169 and 302 mV respectively. The peak to peak separation for DA-AA and UA-DA were 189 mV and 132 mV, which are large enough to identify them simultaneously.
The electrochemical behavior of dopamine with different concentration in presence of 10 μM ascorbic acid and 10 μM uric acid at DDBSMCPE was studied by DPV. The DA concentration was varied from 0.1 to 2 μM (Fig. 11a). The anodic peak current of DA increased with the concentration of DA. AA was increased from 10 to 25 μM and UA from 10 to 30 μM (Figs. 11b and c respectively). The experimental results showed that there was no shift in anodic peak potentials of all electro-active species by DPV. The DDBSMCPE was able to determine DA in the presence of higher concentration of AA and UA. The obtained result could explain that the oxidation peaks of DA, AA and UA existed independently. 3.12. Stability and reproducibility of the DDBSMCPE The stability of the electrochemical sensor was investigated by recording a cyclic voltammogram of DDBSMCPE in 0.2 M PBS (pH 7.4). After 50 successive potential scans, the anodic and cathodic peak
Table 1 Comparison of DDBSMCPE with other modified electrodes. Electrode
Detection limit (μmol/L)
Method
Reference
Nano-Au/CA/GCE Hydrogenated cylindrical carbon electrodes MPPA modified gold electrode GNP/Ch/GCE GCE/PLME DDBSMCPE
0.04 0.75 0.15 0.12 0.42 0.01
DPV CV DPV DPV CV DPV
[41] [42] [43] [44] [45] This work
Fig. 9. Cyclic voltammograms obtained for the oxidation of 10 μM UA in 0.2 M PBS (pH 7.4) at bare CPE (dashed line) and DDBSMCPE (solid line). Scan rate: 50 mV/s and pulse width of 25.
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Fig. 10. a. Cyclic voltammograms obtained at bare CPE (a) and DDBSMCPE (b) in 0.2 M PBS (pH 7.4) containing a mixture of 0.1 μM DA and 10 μM AA and 10 μM UA at a scan rate 50 mV/s. b. Differential pulse voltammogram of DDBSMCPE in 0.2 M PBS pH 7.4 containing 0.1 μM DA and 10 μM AA and μM UA at a scan rate 50 mV/s.
currents of DA at the DDBSMCPE decreased only by 3.4%. The storage stability of DDBSMCPE was tested after storing in 0.2 M PBS (pH 7.4) for two weeks and it retained 96% of the initial response. The background current variation of the DDBSMCPE surface at five newly prepared DDBSMCPE was less than 2% which validates the reproducible nature of the DDBSMCPE and the result suggests it possesses good stability and reproducibility.
3.13. Analytical applications The DDBSMCPE was applied to the determination of dopamine injection with a specified content of DA of 40 mg/mL (Sterile Specialties India Private Ltd.,). The procedure for sample assay was as follows. 1 mL portion of dopamine hydrochloride injection was pipetted in to an 8 mL calibrated tube and was diluted up to the mark with PBS. Using the proposed methods described above, the injection of dopamine hydrochloride was analyzed by standard addition method and the results are shown in Table 2. It demonstrated a good performance of the DDMCPE with satisfactory reproducibility and the recoveries were acceptable, showing that the proposed methods could be efficiently used for the determination of DA in injection samples.
Fig. 11. DPVs of (a) Different concentration of DA (0.1 to 2 μM) in presence of constant AA (10 μM) and UA (10 μM) (b) Different concentration of AA (10 to 25 μM) in presence of constant DA (0.1 μM) and UA (10 μM) (c) Different concentration of UA (10 to 30 μM) in presence of constant AA (10 μM) and DA (0.1 μM). Scan rate of 20 mV/s.
S.S. Shankar et al. / Journal of Molecular Liquids 177 (2013) 32–39 Table 2 Determination of DA in Dopamine hydrochloride injections (n = 6) by standard addition method. Sample
Added (mg mL−1)
Found (mg mL−1)
RSD (%)
Recovery (%)
1 2 3
5 5 5
4.85 ± 0.070 4.63 ± 0.062 4.90 ± 0.078
0.91 1.82 1.98
97 92.6 98
4. Conclusion In this work the electrochemical behavior of DA was investigated on DDBS modified CPE, which showed electrochemical catalytic promotion of DA oxidation. The oxidation peak current was in proportion to the DA concentration with a detection limit of 0.01 μM. The electrode process was controlled by adsorption. The DDBSMCPE showed good ability to separate the electrochemical responses of DA, AA and UA in the mixed solution. This method was further applied to the determination of DA in injection samples with satisfactory results. The application of this electrode creates a new approach to determine the important of neurotransmitter dopamine sensitively and simultaneously in real sample also. References [1] W. Adamson, Physical Chemistry of Surfaces, John Wiley and Sons, New York, 1990. Chapter XIII. [2] P.S.C. Prete, K. Gomes, S.V.P. Malheiros, N.C. Meirelles, E. de Paula, Biophys. Chem. 97 (2002) 45–54. [3] F. Giess, M.G. Friedrich, J. Heberle, R.L. Naumann, W. Knoll, Biophys. J. 87 (2004) 3213–3220. [4] S. Schreier, S.V.P. Malheiros, E.D. Paula, Biochim. Biophys. Acta 1508 (2000) 210–234. [5] M. Tanaka, E. Sackmann, Nature 437 (2005) 656–663. [6] D.M. Soares, W.E. Gomes, M.A. Tenan, Langmuir 23 (2007) 4383–4388. [7] M.C. Shih, M.Q. Hoexter, L. Andrade, R. Bressan, Sao Paulo Med. J. 124 (2006) 168–175. [8] M. Velasco, A. Luchsinger, Am. J. Ther. 5 (1998) 37–43. [9] R.N. Goyal, S.P. Singh, Carbon 46 (2008) 1556–1562. [10] R.N. Goyal, V.K. Gupta, M. Oyama, N. Bachheti, Talanta 72 (2007) 976–983. [11] D.W. Martin Jr., in: D.W. Martin Jr., P.A. Mayes, V.W. Rodwell (Eds.), Harpers Review of Biochemistry, 19th ed., Lange, Los Altos, CA, 1983, p. 112.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Sodium do-decyl benzene sulfate modified carbon paste electrode as an electrochemical sensor for the simultaneous analysis of dopamine, ascorbic acid and uric acid: A voltammetric study S. Sharath Shankar, B.E. Kumara Swamy ⁎, B.N. Chandrashekar, K.J. Gururaj Department of P.G. Studies and Research in Industrial Chemistry, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, Shimoga (D), Karnataka (S), India
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 27 September 2012 Accepted 1 October 2012 Available online 24 October 2012 Keywords: Dopamine Simultaneous Cyclic voltammetry Surfactant modified electrode
a b s t r a c t Sodium do-decyl benzene sulfate modified carbon paste electrode (DDBSMCPE) was first employed for the simultaneous determination of dopamine (DA), uric acid (UA), and ascorbic acid (AA). The modified CPE displayed excellent electrochemical catalytic activities. The oxidation over potentials of DA, UA and AA decreases significantly and their oxidation peak currents increases dramatically at DDBSMCPE. Differential pulse voltammetry (DPV) was used for the simultaneous determination of UA, DA and AA in their ternary mixture. The peak separation between UA-DA and DA-AA is 152 mV and 221 mV, respectively, and the detection limit was 0.01 μM for DA. The proposed method improved sensitivity for the determination of DA by more than one order of magnitude. The present method was applied to the determination of DA in real sample by using standard addition method and the obtained results were satisfactory with a good recovery of 98%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A large number of molecular processes in chemically, physically and biologically related systems occur at solid–liquid, liquid–liquid and liquid–gas interfaces, which modify the dynamic behaviour of molecules relative to their bulk properties [1]. The structural and dynamic properties of adsorbed surfactant molecular films are therefore of both fundamental and applied interest [2]. Surfactant molecules exhibit an amphiphilic or amphipathic behaviour and they bear an ionic (zwitterionic, anionic, or cationic) or non-ionic polar head group and a hydrophobic portion. The self-assembled aggregates of amphiphilic surfactant molecules formed on solid surfaces are important as models for biological membranes [3]. In fact, the adsorption of surfactants on solid surfaces allows the simulation of membrane-like structures which can be used in diverse biological/industrial processes such as protein immobilization, charge and mass transfer, membrane solubilization and disruption, etc [4–6]. Parkinson's disease is one of the most dreadful neurodegenerative disorders of the central nervous system, as its complete cure is not possible even today. Tremor, rigidity, bradykinesia and postural instability are some of its common diagnostic features. This disease occurs when dopaminergic neurons malfunction, or are destructed, which is accompanied by a sharp decline in dopamine level [7–9]. DA is a catecholamine neurotransmitter which is generated in various parts of central and peripheral nervous system and has an agonist action
⁎ Corresponding author. Tel.: +91 8282 256225 (office); fax: +91 8282 256255. E-mail address: [email protected] (B.E.K. Swamy). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.10.002
on β adrenoceptors. DA has positive chronotropic and ionotropic effects on myocardium which stimulates cardiac contractility and enhances heart beat rate. Electrochemical detection of DA is a preferred method because DA is electrochemically active. In addition, electrochemical methods offer advantages such as simplicity, speed, and sensitivity. A major problem in the electrochemical detection of DA is the coexistence of other biologically important compounds including ascorbic acid (AA) and uric acid (UA) at a higher concentration [10]. Ascorbic acid is a vital component in human diet. It is known to take part in some biological reactions and is present in mammalian brain [11]. Recent clinical studies have demonstrated that the content of ascorbic acid in biological fluids can be used to assess the amount of oxidation stress in human metabolism [12,13] and excessive oxidative stress lead to cancer, diabetes mellitus and hepatic diseases. Uric acid is the principal final product of purine metabolism in the human body [14]. It has been shown that extreme abnormalities of UA levels are symptoms of several diseases (e.g. gout, hyperuricaemia and Lesch–Nyhan syndrome) [15–17]. Other diseases, such as leukaemia and pneumonia are also associated with enhanced urate levels [16]. In general, electroactive UA can be irreversibly oxidized in aqueous solution and the major product is allantoin [18]. Among many methods for determination of DA, AA and UA in biological samples, electrochemical techniques with modified electrodes have been shown to be powerful tools due to their advantages of being simple, inexpensive and possibility of fast analysis in combination with high sensitivity and selectivity [19]. Thus, different kinds of modified electrodes have been fabricated for detection of DA, AA and UA [19–24]. Even though all these electrodes are having some advantages they have many limitations too. So the real challenge is in developing simple, reliable and efficient sensors with
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enhanced characteristics for effective sensing of DA, AA and UA simultaneously. In recent years, surfactant modified electrodes have provoked much more attention in electro analysis because of their novel physical and chemical properties [23,25–27]. In particular, the catalytic properties of some surfactants cause a decrease in the over potential needed for a redox reaction to become kinetically viable, producing voltammetry which appears more reversible than that displayed by the same material in an unmodified electrode form [28]. Moreover, the use of surfactant-modified electrodes presents some other advantages like high effective mass transport catalyzes and controls the local environment [29,30]. Carbon paste electrode (CPE) is one of the convenient conductive matrices to prepare the chemically modified electrodes (CMEs) by the simple mixing of graphite/binder paste and modifier [31,32]. These kinds of electrodes are inexpensive and possess many advantages such as low background current, wide range of used potential, ease of fabrication, and rapid renewal. In this work, a novel biosensor has been fabricated by using a carbon paste electrode (CPE) immobilized with sodium do-decyl benzene sulfate (DDBS) for simultaneous electrochemical determination of AA, DA and UA. As already known, DDBS is an anionic surfactant which contains benzene in its structure and the SDS has a hydroxy group, which could be covalently bound to the edge plane sites of the carbon surface through the oxygen atom. 2. Materials and methods 2.1. Reagents and Chemicals Dopamine hydrochloride, ascorbic acid, uric acid was purchased from Himedia Company. All chemicals were of analytical grade quality and were used without further purification. 1 × 10−6 M stock solution of DA was prepared by dissolving it in 0.1 M perchloric acid solution. 1 × 10−4 M stock solution of AA was prepared by dissolving it in double distilled water and 1 × 10−4 M stock solution of UA was prepared by dissolving it in 0.1 M NaOH. The supporting electrolyte used was the phosphate buffer solution (PBS). It was prepared by mixing standard stock solutions of 0.1 M disodium-hydrogen phosphate and sodium dihydrogen phosphate by adjusting the pH. 2.2. Apparatus and procedure Cyclic voltammetry (CV) was performed on Model EA-201 Electroanalyser (EA-201, Chemilink System). All the experiments were carried out in a conventional three electrode electrochemical cell. The electrode system contained a carbon paste working electrode (3.0 mm in diameter), a platinum wire counter electrode and a saturated calomel reference electrode (SCE). The bare carbon paste electrode (BCPE) was prepared by grinding 70% graphite powder (particle size 50 mm and bulk density 20–30 g/100 mL from Loba chemical company) and silicon oil (viscosity 300 cps at 20° from Himedia chemical company) to produce a homogeneous carbon paste electrode. The carbon paste was then packed into the cavity of a homemade carbon paste electrode body and smoothened on a weighing paper. DDBS modified carbon paste electrode (DDBSMCPE) was prepared by immobilizing 25 μL of 0.1 mM DDBS on the surface of the carbon paste electrode for 20 minutes.
33
Fig. 1. Cyclic voltammogram 1 mM potassium ferrocyanide in 1 M KCl at BCPE (a) and DDBSMCPE (b) with a scan rate of 50 mV/s.
respectively. The peak potential difference (ΔEp) was found to be 32 mV. However, in the same condition DDBSMCPE exhibited good sensitivity for K4Fe(CN)6 and both anodic and cathodic peak current signals got enhanced when compared to the BCPE. The Epa and Epc were located at 257 mV and 196 mV. The ΔEp was found to be 61 mV. The enhancement of peak current shows electro catalytic activity of DDBSMCPE. 3.2. The effect of concentration and immobilization time of DDBS on voltammetric response of K4Fe(CN)6 To improve the performance of the biosensor, various factors influencing the response of the sensor such as the concentration of surfactant and the immobilization time were investigated. It can be seen that the biosensor prepared with 25 μL of 0.1 mM DDBS surfactant solution has a maximum current response. The reason may be that DDBS surfactant molecule diffuses into the carbon paste electrode along with the K4Fe(CN)6 , which results in the increase in current signal. If the concentration of DDBS is low, the rate of diffusion is low and the response of the biosensor is also low. However, if the concentration of DDBS is 0.1 mM it can show maximum current response at 25 μL DDBS. Therefore 25 μL of 0.1 mM DDBS was selected as the suitable amount for further studies. However, a higher concentration of mediator produced a larger background current. Thus, 25 μL DDBS was chosen in the subsequent experiments. 25 μL DDBS was immobilized on the surface of carbon paste electrode and the immobilization time was varied from 5 to 30 minutes. Fig. 2 shows the current response of the modified electrode increased gradually with increase in immobilization time, reaching a maximal value at 20 minutes and then decreases with further increase of immobilization time. Therefore a time gap of 20 minutes was chosen for the diffusion of the DDBS molecule into the porous carbon paste electrode. Such a behavior is typical of a mediator-based sensor [33,34]. Therefore, 20 minutes was selected as the immobilization time for the further analysis.
3. Results and discussion 3.3. Surface morphology of DDBSMCPE 3.1. Electrochemical response of potassium Ferro cyanide on DDBSMCPE Fig. 1 demonstrates the cyclic voltammograms of K4Fe(CN)6 (1 mM) at BCPE and DDBSMCPE at sweep rate of 50 mV/s. The K4Fe(CN)6 showed poor sensitivity and reproducibility at BCPE (dashed line) compared to DDBSMCPE. The cyclic voltammogram of K4Fe(CN)6, in 1 M KCl as supporting electrolyte at sweep rate of 50 mV/s showed its anodic (Epa) and cathodic peak potentials (Epc) at 240 mV and 208 mV
Fig. 3 explains the surface morphology of bare carbon paste electrode (A) and DDBSMCPE (B) using scanning electron microscopy. The surface of bare carbon paste electrode was irregularly shaped micrometer sized flakes of graphite. However, the DDBS film coated carbon paste electrode has typical uniform arrangement of DDBS molecules on the surface of carbon paste electrode. This confirms that the carbon paste electrode was coated by the modifier film.
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Further, blank experiment was carried out without taking any DA with buffer solution (curve b) and no peaks were obtained for the modified electrode. It is clear from the above results that the DDBSMCPE shows high electro catalytic effect on DA. The DDBS molecule forms a monolayer on the CPE surface with “stand” mode. A negative electric field exists around the modified electrode surface. The monolayer adsorption of DDBS on the CPE surface is similar to the adsorption character of SDS on the graphite surface [34]. At 7.4 pH, the amine group in DA molecule is electropositive [35] and the negatively charged DDBS monolayer interacts with the positively charged DA [36]. Therefore electropositive DA is attracted electro-statically to the monolayer and can be oxidized at relatively low potentials and the schematic representation is shown in Fig. 4b. 3.5. Effect of scan rate at DDBSMCPE
Fig. 2. Plot of current vs. immobilization time (from 5 to 30 minutes) in 0.2 M phosphate buffer solution of pH 7.4 with a scan rate of 50 mV/s.
3.4. Electrochemical behavior of dopamine at DDBSMCPE Fig. 4a shows cyclic voltammetric recordings of 0.1 μM DA at the BCPE (curve a) and at DDBSMCPE (curve c) at pH 7.4 phosphate buffer solution. At the BCPE, DA exhibited a poor electrochemical response when compared to the modified electrode. The oxidation peak (Epa) was located at 172 mV and cathodic peak (Epc) was at 134 mV in 0.2 M phosphate buffer solution of pH 7.4. The modified electrode showed increase in peak currents considerably. The cathodic peak potential (Epc) was at 172 mV and anodic peak potential (Epa) at 113 mV.
Fig. 3. SEM images of (A) bare CPE (B) DDBSMCPE.
The effect of the scan rate on the peak current of DA was investigated. Fig. 5a shows that the CVs of the DDBSMCPE are dependent on the various scan rates in 0.2 M PBS (pH 7.4). The anodic or cathodic peak currents are proportional to scan rates from 50 to 350 mV/s and the curves are linear with scan rate (Fig. 5b). The corresponding linear regression equation is expressed as ip (μA) =−0.31 +0.130ν (mV/s). The result indicates that the reaction of DA at the DDBSMCPE is controlled by the adsorption process. 3.6. Effect of pH Cyclic voltammetry was used to investigate the effects of pH value in electrochemical determination of DA in the presence of AA at the DDBSMCPE. Fig. 6a shows the cyclic voltammograms obtained at the DDBSMCPE in 0.2 M phosphate buffer solutions of different pH values containing 0.1 μM DA at a scan rate of 50 mV/s. Fig. 6b shows the effect of pH on the peak potential (Epa) and peak currents in aqueous buffer solution with the increasing pH from 3.4 to 8.4. The Epa shifted negatively and were dependent linearly on pH with the slopes of − 57 mV/pH which indicates that the proportion of the electrons and protons involved in the redox reaction of DA was 1:1 and the oxidation of DA at the DDBSMCPE had been inferred to be a doubleelectron transfer reaction [37]. It could also be observed that both the oxidation and reduction peak current obtained a maximum at pH 7.4 (Fig. 6c), which could be partly explained on the basis of the dissociation ability of –SO3Na(H) group of DDBS film in different pH environments. When the solution pH was equal to 7.4, the –SO3Na group of DDBS film could dissociate into a negative charge group –SO3−. Under this condition, the –NH2 group of DA molecules (pKa: 8.9) [38] could obtain a proton and form the cation of DA. Therefore, the negative charge group –SO3− on the surface of DDBSMCPE has a well affinity to the DA positive ions and could catalyze and promote the oxidation of DA in the neutral buffer solution (pH 7.4). While pH was below 7.4, the –SO3Na group of DDBS film could form –SO3H and exclude the DA positive ions. However, when pH value was beyond 7.4, the decreased DA peak currents seemed to be contradicted to the above-mentioned dissociation resumption. Actually, to interpret the effect of pH on the electrocatalytic oxidation of DA, it should not be overlooked that DDBS film at electrode acted as a mediator of electron transfer for DA's oxidation, whose electron transfer process could be affected by pH. Indeed, the anodic peak current of DDBS film reaction at carbon electrode was observed to decrease gradually with increasing pH from 7.4 to 8.4. It was identified that with increasing pH, the rate of electron transfer of the DDBS film could also decrease gradually, which was disadvantageous to the electrocatalytic reaction of DA at the DDBS coated electrode. Thus, pH was a dual conditioner to the oxidation of DA at the DDBS coated carbon electrode. On other hand, increasing pH of electrolyte solution from 3 .4 to 7.4, could promote the formation of the –SO3− anions on DDBS film showing that the DDBSMCPE had a well affinity to the DA
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Fig. 4. a. Cyclic voltammograms in 0.2 M PBS of pH 7.4 on (a) BCPE (c) DDBSMCPE with 0.1 μM DA and (b) on DDBSMCPE without DA at a scan rate of 50 mV/s. b. Schematic diagram represents oxidation of DA at DDBSMCPE surface.
positive ions and enhanced the oxidation of DA. Hence, the pH 7.4 was an optimum protocol for the electrocatalytic oxidation of DA at the DDBSMCPE.
detection limit of various electroanalytic methods proposed for determination of DA is compared with our analytical data in Table 1. LOD ¼ 3S=M
ð1Þ
LOQ ¼ 10S=M
ð2Þ
3.7. Effect of concentration of dopamine at DDBSMCPE From Fig. 7 it is clear that the relationship between anodic peak current and concentration of DA is linear between 0.1 μM and 2.5 μM with the linear regression equations as ipa (μA) = 0.0183 + 0.3519 (C)DA μM/L. The correlation coefficient was found to be 0.998. The detection limit for DA was found to be 0.01 μM and quantification limit was 0.05 μM by DPV method and was calculated by using the formulae (1) and (2) [39,40], where S is the standard deviation and M is the slope obtained from the three calibration plots. The
3.8. Electrochemical oxidation of ascorbic acid at DDBSMCPE Fig. 8 shows the electro-catalytic behavior of AA at the DDBSMCPE and the BCPE in the PBS of pH 7.4 that contained 10 μM of AA. The cyclic voltammograms of AA in the PBS produced a single irreversible oxidation peak using both DDBSMCPE and BCPE. The reason for this phenomenon was the reaction of the oxidized form of ascorbic acid
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Fig. 5. a. Cyclic voltammograms of 0.1 μM DA in 0.2 M PBS (pH 7.4) on the DDBSMCPE at different scan rates (a–g: 50, 100, 150, 200, 250, 300, 350 mV/s.). b. Effect of variation of scan rate on the anodic peak current of 0.1 μM DA in 0.2 M PBS of pH 7.4.
with water that resulted in an irreversible electrochemical behavior [46–48]. The oxidation of ascorbic acid occurred at around 182 mV at the BCPE, whereas the same occurred at the DDBSMCPE at about − 25 mV at pH 7.4 buffer aqueous solution. This negative shifting in its oxidation potential could be due to the repulsive force between the negatively charged SO3 layer of the modified electrode and anionic form of AA. 3.9. Electrochemical oxidation of UA at DDBSMCPE Fig. 9 shows the cyclic voltammograms of 10 μM uric acid at the BCPE (curve a) and DDBSMCPE (curve b) in 0.2 M phosphate buffer solution of pH 7.4 as a supporting electrolyte at scan rate of 50 mV/ s in the range of − 250 to 650 mV. At the BCPE, UA gives an oxidation peak at about 304 mV. However at DDBSMCPE UA gives one anodic and one cathodic peak at a potential of 298 mV and 245 mV, respectively. The peak to peak separation was found to be 53 mV. Further the concentration of UA was increased for the confirmation of redox peak. While increasing the concentration, both anodic and cathodic peak got increased. The DDAB accelerates the electron
Fig. 6. a. Cyclic voltammogram of 0.1 μM DA in PBS with different pH (a–e; 3.4, 4.4, 5.4, 6.4, 7.4 and 8.4). Scan rate: 50 mV/s. b. Graph of anodic peak potential of 0.1 μM DA vs. pH. c. Graph of anodic peak current of 0.1 μM DA vs. pH.
transfer and hence the irreversible nature of UA changes to reversible at the DDBSMCPE. This result indicates good electro catalytic activity of DDBS film towards the oxidation of UA.
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Fig. 7. Effect of concentration of dopamine on the anodic peak current in 0.2 M PBS of pH 7.4 with a scan rate of 50 mV/s.
Fig. 8. Cyclic voltammograms obtained for the oxidation of 10 μM AA in 0.2 M PBS (pH 7.4) at bare CPE (a) and DDBSMCPE (b). Scan rate: 50 mV/s.
3.10. Simultaneous electro chemical detection of dopamine, uric acid and ascorbic acid at DDBSMCPE
3.11. Interference study
The interference due to the presence of other electroactive species such as AA and UA are one of the major problems in determination of dopamine. Both AA and UA get electrochemically oxidized at the potentials close to oxidation of dopamine and normally cause an overlapped oxidation potential on the electrodes used for detection. These interferences result in the oxidized species having an overlap of peaks in unmodified electrode to detect dopamine. Another problem is that ascorbic acid has a concentration ranging from hundred folds than dopamine, which results in poor selectivity and sensitivity for dopamine detection [46–48]. The simultaneous determination of 0.1 μM dopamine in presence of 10 μM ascorbic acid and 10 μM uric acid at DDBSMCPE was investigated by CV technique at the scan rate of 50 mV/s in PBS (pH 7.4). As shown in Fig. 10a, the BCPE shows one broad and relatively low single anodic peak (curve a) and at the same time DDBSMCPE reduces the anodic over potentials (curve b) and well-defined anodic peaks are observed at − 23, 182 and 317 mV for AA, DA and UA. The electrochemical peak separation between DA and AA was found to be 206 mV and for UA and DA to be 134 mV, which indicates a sufficient separation of three-well defined peaks. Further, the electrochemical separation study was carried out by DPV technique as shown in Fig. 10b because of its higher current sensitivity and better resolution than cyclic voltammetric technique. The anodic peak potentials for DA, AA and UA were at − 20, 169 and 302 mV respectively. The peak to peak separation for DA-AA and UA-DA were 189 mV and 132 mV, which are large enough to identify them simultaneously.
The electrochemical behavior of dopamine with different concentration in presence of 10 μM ascorbic acid and 10 μM uric acid at DDBSMCPE was studied by DPV. The DA concentration was varied from 0.1 to 2 μM (Fig. 11a). The anodic peak current of DA increased with the concentration of DA. AA was increased from 10 to 25 μM and UA from 10 to 30 μM (Figs. 11b and c respectively). The experimental results showed that there was no shift in anodic peak potentials of all electro-active species by DPV. The DDBSMCPE was able to determine DA in the presence of higher concentration of AA and UA. The obtained result could explain that the oxidation peaks of DA, AA and UA existed independently. 3.12. Stability and reproducibility of the DDBSMCPE The stability of the electrochemical sensor was investigated by recording a cyclic voltammogram of DDBSMCPE in 0.2 M PBS (pH 7.4). After 50 successive potential scans, the anodic and cathodic peak
Table 1 Comparison of DDBSMCPE with other modified electrodes. Electrode
Detection limit (μmol/L)
Method
Reference
Nano-Au/CA/GCE Hydrogenated cylindrical carbon electrodes MPPA modified gold electrode GNP/Ch/GCE GCE/PLME DDBSMCPE
0.04 0.75 0.15 0.12 0.42 0.01
DPV CV DPV DPV CV DPV
[41] [42] [43] [44] [45] This work
Fig. 9. Cyclic voltammograms obtained for the oxidation of 10 μM UA in 0.2 M PBS (pH 7.4) at bare CPE (dashed line) and DDBSMCPE (solid line). Scan rate: 50 mV/s and pulse width of 25.
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Fig. 10. a. Cyclic voltammograms obtained at bare CPE (a) and DDBSMCPE (b) in 0.2 M PBS (pH 7.4) containing a mixture of 0.1 μM DA and 10 μM AA and 10 μM UA at a scan rate 50 mV/s. b. Differential pulse voltammogram of DDBSMCPE in 0.2 M PBS pH 7.4 containing 0.1 μM DA and 10 μM AA and μM UA at a scan rate 50 mV/s.
currents of DA at the DDBSMCPE decreased only by 3.4%. The storage stability of DDBSMCPE was tested after storing in 0.2 M PBS (pH 7.4) for two weeks and it retained 96% of the initial response. The background current variation of the DDBSMCPE surface at five newly prepared DDBSMCPE was less than 2% which validates the reproducible nature of the DDBSMCPE and the result suggests it possesses good stability and reproducibility.
3.13. Analytical applications The DDBSMCPE was applied to the determination of dopamine injection with a specified content of DA of 40 mg/mL (Sterile Specialties India Private Ltd.,). The procedure for sample assay was as follows. 1 mL portion of dopamine hydrochloride injection was pipetted in to an 8 mL calibrated tube and was diluted up to the mark with PBS. Using the proposed methods described above, the injection of dopamine hydrochloride was analyzed by standard addition method and the results are shown in Table 2. It demonstrated a good performance of the DDMCPE with satisfactory reproducibility and the recoveries were acceptable, showing that the proposed methods could be efficiently used for the determination of DA in injection samples.
Fig. 11. DPVs of (a) Different concentration of DA (0.1 to 2 μM) in presence of constant AA (10 μM) and UA (10 μM) (b) Different concentration of AA (10 to 25 μM) in presence of constant DA (0.1 μM) and UA (10 μM) (c) Different concentration of UA (10 to 30 μM) in presence of constant AA (10 μM) and DA (0.1 μM). Scan rate of 20 mV/s.
S.S. Shankar et al. / Journal of Molecular Liquids 177 (2013) 32–39 Table 2 Determination of DA in Dopamine hydrochloride injections (n = 6) by standard addition method. Sample
Added (mg mL−1)
Found (mg mL−1)
RSD (%)
Recovery (%)
1 2 3
5 5 5
4.85 ± 0.070 4.63 ± 0.062 4.90 ± 0.078
0.91 1.82 1.98
97 92.6 98
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