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Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb Electrochemically selective determination of dopamine in the presence of ascorbic and uric acids on the surface of the modified Nafion/single wall carbon nanotube/poly(3-methylthiophene) glassy carbon electrodes Do Phuc Quan a,∗∗ , Do Phuc Tuyen a , Tran Dai Lam b,∗ , Phan Thi Ngoc Tram a , Nguyen Hai Binh b , Pham Hung Viet a a b Research Centre for Environmental Technology and Sustainable Development, Hanoi University of Science, 334 Nguyen Trai Road, Ha Noi, Viet Nam Institute of Materials Science, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Ha Noi, Viet Nam a r t i c l e i n f o Article history: Received 13 May 2011 Received in revised form 11 August 2011 Accepted 12 August 2011 Available online 22 August 2011 Keywords: Nafion (NF) Single-walled carbon nanotubes (SWCNT) Poly(3-methylthiophene) (PMT) Dopamine (DA) Electrochemical methods a b s t r a c t A voltammetric method based on a combination of incorporated Nafion, single-walled carbon nanotubes and poly(3-methylthiophene) film-modified glassy carbon electrode (NF/SWCNT/PMT/GCE) has been successfully developed for selective determination of dopamine (DA) in the ternary mixture of dopamine, ascorbic acid (AA) and uric acid (UA) in 0.1 M phosphate buffer solution (PBS) pH It was shown that to detect DA from binary DA–AA mixture, the use of NF/PMT/GCE was sufficient, but to detect DA from ternary DA–AA–UA mixture NF/SWCNT/PMT/GCE was required The later modified electrode exhibits superior electrocatalytic activity towards AA, DA and UA thanks to synergic effect of NF/SWCNT (combining unique properties of SWCNT such as high specific surface area, electrocatalytic and adsorptive properties, with the cation selectivity of NF) On the surface of NF/SWCNT/PMT/GCE AA, DA, UA were oxidized respectively at distinguishable potentials of 0.15, 0.37 and 0.53 V (vs Ag/AgCl), to form welldefined and sharp peaks, making possible simultaneous determination of each compound Also, it has several advantages, such as simple preparation method, high sensitivity, low detection limit and excellent reproducibility Thus, the proposed NF/SWCNT/PMT/GCE could be advantageously employed for the determination of DA in real pharmaceutical formulations © 2011 Elsevier B.V All rights reserved Introduction 3,4-Dihydroxyphenyl ethylamine, commonly known as dopamine (DA), has been of interest to neuroscientists and chemists since its discovery in the 1950s [1] As the most significant neurotransmitter, the amount of DA distributed in organs has great influences on human emotions and is directly related to a variety of diseases deriving from the abnormal low concentration level of DA which has been linked to several neurological disorders, e.g., schizophrenia, Huntington’s disease, and Parkinson’s disease (the third most popular one in the world), and even the HIV infection [2–4] Electrochemical detection of DA is a feasible approach because of its good electroactivity and easy oxidation It has been shown that the oxidation of DA is a two-electron irreversible process with transfer of two protons Uric acid (UA) is the major final product of purine catabolism in human body In ∗ Corresponding author Tel.: +84 37564129; fax: +84 438360705 ∗∗ Corresponding author Tel.: +84 38588152; fax: +84 438587964 E-mail addresses: doquan@vnu.edu.vn (D.P Quan), lamtd@ims.vast.ac.vn (T.D Lam) 0927-7765/$ – see front matter © 2011 Elsevier B.V All rights reserved doi:10.1016/j.colsurfb.2011.08.012 a healthy human, the normal level of UA in urine is in mM range where as in serum it is in M range Abnormal levels of UA are symptoms of several diseases such as gout, hyperuricemina, and Lesch–Nyan disease [5] Ascorbic acid (AA) is present in many vegetables, citrus fruits and biological fluids where it acts as an anti-oxidant and free-radical scavenger AA concentration in the body fluids can be used to assess the level of oxidative stress is related to diseases like cancers, diabetes mellitus and hepatic disorders UA and AA is co-present in biological fluids such as blood and urine From the above reasons, it is essential to develop rapid and simple methods to detect/determine the DA concentration One of the biggest challenges of electrochemical detection of DA in real biological matrixes is the coexistence of many interfering compounds In biological systems, AA usually coexists with DA in extracellular fluid at a high concentration level, nearly 1000 times higher than DA Moreover, DA, AA and UA can be oxidized at practically the same potential at bare electrodes, resulting in the peak overlapping as well as poor response resolution in DA determination Homogeneous catalytic oxidation of AA by oxidized DA, interaction of AA and the products of DA oxidation are other difficulties in the DA determination, severely limiting the accuracy of detection D.P Quan et al / Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 To solve these problems, the use of chemically modified electrodes instead of bare ones is preferred Various materials, such as metal or metal oxide nanoparticles (NPs) and metal complexes (Au NP hybrid film and nanogold modified carbon fiber electrodes, palladium nanoparticle loaded carbon nanofibers, zinc oxide composite film, ruthenium oxide, magnetic Fe3 O4 NPs deposited on gold electrode, titanate nanotubes, LaFeO3 nanoparticles [6–15]); organic polymers and composites (polymeric films of aniline, pyrrole, 3-methylthiophene and p-nitrobenzene resorcinol are reported to be useful in the selective detection of DA in excess of AA [16–19], poly(cresol-red) modified electrodespoly(oracet blue) modified electrode, poly(eriochrom black T) modified electrode, poly(naphthalene sulfunic acid) modified electrode, poly(Evans blue) modified electrode, poly(vinyl alcohol) and poly(chromotrope 2B)-modified electrodes are also used to detect DA and/or DA, AA and UA, simultaneously; poly(4amino-1,1′ -azobenzene-3,4′ -disulfonic acid)-coated electrode has been reported for the selective detection of DA in the presence of AA, UA and NADH, poly(3-(3-pyridyl) acrylic acid), and 3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene2,7-disulfonic acid have been reported for simultaneous determination of AA, DA and UA [20–26]) can be used to fabricate modified electrode due to their excellent properties to decrease the over-potential, accelerate the electron and mass transfer rate or greatly enrich the substrates on the electrode surface This article reports the successful combination of poly(3methylthiophene) (PMT) with Nafion (NF) and single wall carbon nanotubes (SWCNTs) in order to provide NF/SWCNT/PMT modified electrodes with enhanced sensitivity and selectivity towards DA in the presence of high excess of AA and UA Glassy carbon electrodes (GCE) are very versatile as electrode material for trace level determination of organic molecules as they provide high sensitivity, negligible porosity, and good mechanical rigidity GCEs have been modified by means of various nanosized additives [27,28] PMT is a widely investigated electronically conducting polymer, which can be easily electrodeposited onto electrode surface by electro-oxidation of its monomer The applications of PMT have been extensively reported and showed excellent electrocatalytic effect, neurotransmitter species [29,30] The choice of SWCNT to develop electrochemical transducers is based on its subtle electronic properties, strong electrocatalytic effect, rapid electron transfer rate, high tensile strength and chemical stability, and ultra-small size effect [27] As for Nafion, a perfluorinated sulfonated cation exchanger, having hydrophilic and hydrophobic domains (in which the later consists primarily of Teflon while the former consists of sulfonic groups, presented at the end of the side chains of the Teflon backbone), excellent antifouling capacity, chemical inertness and high permeability to cations, has been extensively employed as an electrode modifier for organic molecules [31,32] The sensitivity enhancement of the NF membrane is believed to associate with accumulations of DA in the hydrophilic regions or the ion channels of NF Thanks to its amphiphilic structure, SWCNT can be homogeneously dispersed in NF solution Thus, a synergistic effect of both NF-SWCNT film modified can be expected to further enhance sensitivity in DA determination The primary objective of this work is not to introduce novel materials for modified electrodes but to emphasize the efforts on designing/optimizing a sensitive and selective interface for simultaneously electrochemical determination of DA in the presence of AA and UA by different voltammetric techniques like cyclic voltammetry (CV) and differential pulse voltammetry (DPV) 765 Experimental 2.1 Chemicals 3-methylthiophene (MT), 5% Nafion® 117 and tetrabuthylammonium perchlorate (TBAP) were from Fluka (Switzerland) Ascorbic acid, dopamine hydrochloride (DA·HCl), uric acid and other chemicals were from Sigma (Germany) and were used without further purification SWCNT was purchased from Chengdu Organic Chemicals Co Ltd (Chengdu, China) Aqueous solutions were prepared with de-ionized (DI) water The other reagents were of analytical reagent grade Highly purity nitrogen was used for deaeration 2.2 Electrode preparation Prior to the electropolymerization, the surface of the Glassy carbon electrode was polished with 15 m and 0.3 m alumina slurry and cleaned by ultrasonication in DI water PMT was electrodeposited on a GCE (to get PMT/GCE) from a solution containing 0.1 M MT and 0.1 M TBAP (dissolved in acetonitrile (CH3 CN)) for 20 s at a constant potential of 1.75 V (vs Ag/AgCl) Afterwards, it was treated at 0.7 V for 10 s Two main types of modified electrodes (denoted as NF/PMT/GCE and NF/SWCNT/PMT/GCE) are prepared First, L of 2.5% NF solution was scrupulously dropped onto the PMT/GCE surfaces, the solvent was evaporated in air to obtain NF/PMT/GCE Second, L of SWCNT dispersion (pre-carboxylated by the mixture of concentrated H2 SO4 and HNO3 (1/3, v/v), then mixed ultrasonically with 0.25 wt% NF solution for 30 min) was scrupulously dropped onto the surfaces of PMT/GCE, then, the solvent was evaporated in air to obtain NF/SWCNT/PMT/GCE All electrodes were carefully rinsed with DI before further characterization 2.3 Electrochemical measurements CV and DPV were performed with Autolab PGSTAT-30 Potentiostat/Galvanostat with GPES software (EcoChemie, The Netherlands) The three-electrode system was employed with Ag/AgCl/saturated KCl reference electrode and Pt wire as auxiliary electrode, the working electrode was either a bared GCE or a modified GCE The electrochemical detection was performed in 0.1 M pH 4.0 PBS containing DA (in absence/presence of AA, UA), purged by high-purity nitrogen All experiments were carried out at room temperature All DPVs were measured/recorded in triplicate (DPV peak height (Ipa ) remains its initial value with a relative standard deviation (R.S.D) less than 2–3% for successive scans) Mean values were used for further calculations of linear regression equation Results and discussion 3.1 Electrochemical behavior of DA at the modified electrode The electrochemical behavior of the modified NF/SWCNT/PMT/GCE was studied by CV Fig shows the voltammograms of (a) bare GCE; (b) PMT/GCE; (c) NF/SWCNT/GCE and (d) NF/SWCNT/PMT/GCE, respectively in 0.1 M PBS (pH 4.0), at scan rate of 100 mV s−1 Curves (a) and (b) confirmed that the electrochemical response to DA at bare GCE and PMT/GCE was very poor, while that at NF/SWCNT/GCE (curve c) and NF/SWCNT/PMT/GCE (curve d) was much better: a well-defined redox couple was recorded, the anodic current value was much higher than that of the two previous cases (curves a and b) The reason for the peak current enhancement may originate in the faster electron transfer rate and/or larger surface area of SWCNT as well as easier mass transfer thanks to an inclusion complex of NF and DA, which 766 D.P Quan et al / Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 A 30 22 108 90.1 73.2 57.9 44.5 32.9 20 20 16 14 I /µA I /µA 18 10 -10 -20 12 23.4 16.2 10 11.4 -30 -40 -0.2 0.0 0.2 0.4 0.6 7.44 µM (a) GCE (b) PMT/GCE (c) NF/SWCNT/GCE (d) NF/SWCNT/PMT/GCE 0.8 PMT/GCE -0.2 E /V vs Ag/AgCl 0.0 0.2 0.4 0.6 0.8 E /V vs Ag/AgCl Fig CVs of 100 M DA at: (a) bare GCE; (b) PMT/GCE; (c) NF/SWCNT/GCE; (d), NF/SWCNT/PMT/GCE in 0.1 M PBS of pH 4.0 with the scan rate of 100 mV s−1 B 32.9 12 23.4 11.4 I /µA 7.5 4.5 2.5 1.5 0.5 µM NF/PMT/GCE -0.2 C v (mV/s) 200 40 30 0.4 0.6 0.8 57.9 µM 18 16 32.9 µM 14 12 23.4 µM 10 11.4 µM 7.4 µM NF/SWCNT/PMT/GCE -0,1 50 0.2 20 -2 60 0.0 E /V vs Ag/AgCl I /µA could be dissociated and rapidly diffused more rapidly through the porous layer of SWCNT to the modified electrode surface) Next, to elucidate the process kinetic of DA electro-oxidation at electrode surface, the effect of the scan rate on the peak currents at the NF/SWCNT/PMT/GCE was investigated (Fig 2) The inset showed that the cathodic and anodic peak currents increased linearly along with the square root of scan rates (v1/2 ), suggesting that the electrochemical electro-oxidation of DA was non-surface controlled but diffusion-controlled process, owing to a slow electron hoping across the matrix of the composite film in the studied range of potential sweep rates, according to the following equations: Ipa (A) = −13.3 + 3.03 × v1/2 and Ipc (A) = 10.3 − 2.30 × v1/2 (I in A and v1/2 in mV1/2 s−1/2 ), with the correlation coefficients of 0.99815 and 0.99801, respectively This result was well consistent with other previous studies [30] It is well known that in DPV, current is measured at two points for each pulse, the first point just before the application of the pulse and the second at the end of the pulse These sampling points are selected to minimize non faradic current, thus DPV method is considered more sensitive than CV [33]) So DPV was used here to detect DA at different modified electrodes Fig 3A–C presents the relationship between DA concentration and current signal recorded at (a) PMT/GCE; (b) NF/PMT/GCE and (c) NF/SWCNT/PMT/GCE, respectively It is clear that only at two later cases the linear dependence of current I on [DA] concentration was observed The 10 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 E /V vs Ag/AgCl Fig DPVs for different concentration of DA in 0.1 M PBS of pH 4.0 at (A) PMT/GCE; (B) NF/PMT/GCE; (C) NF/SWCNT/PMT/GCE; (D) the linear regression curve of peak current vs DA concentration at NF/SWCNT/PMT/GCE I /µA 20 10 10 -10 -20 -30 -40 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 E /V vs Ag/AgCl Fig Electrochemical response of 100 M DA in 0.1 M pH 4.0 PBS at NF/SWCNT/PMT/GCE with different scan rates, from 10 to 200 mV s−1 Inset: Ipa − v1/2 and Ipc − v1/2 plots linear regression equations were written as Ipa (A) = 0.267 × [DA] (M) (R2 = 0.998) and Ipa (A) = 0.452 × [DA] (M) (R2 = 0.999) for NF/PMT/GCE and NF/SWCNT/PMT/GCE, respectively Moreover, it is worth noting that NF/SWCNT/PMT/GCE is not only more sensitive (with higher slop value) but also more operational (with twice broader linear range of I–[DA]) compared to that of NF/PMT/GCE, Fig 3D) To the best of our knowledge, the value of sensitivity (0.452 A × M−1 ) is comparable to the best results, recently reported in literature using polypyrrole film doped with sulfonated -cyclodextrins (0.886 A × M−1 ) or poly(3, 4-ethylenedioxythiophene-co-(5-amino-2-naphthalenesulfonic acid)) modified electrode (∼1 A × M−1 ) [34,35] D.P Quan et al / Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 70 AA 20 b 50 DA DA 177 µM 30 145 µM 20 111 µM 40 80 µM 0 50 100 150 50 µM 200 CDA /µM 30 µM 30 a (µA) 10 c 10 I = -1.3 + 0.26488 * C 40 R = 0.99921 60 I /µA I /µA 15 50 a: bare GCE b: PMT/GCE c: NF/PMT/GCE I /µ A 25 767 µM [AA] = 3.7 mM 20 -0.2 0.0 0.2 0.4 0.6 10 0.8 -0,2 0,0 0,2 E /V vs Ag/AgCl As mentioned above, AA coexists with DA in the extracellular fluid of the central nervous system and their concentrations are much higher than that of DA The interference of AA to DA detection arises from two aspects: one is the very similar oxidation potential of AA and DA at bare GCE; the other is the electrocatalytic oxidation of DA by AA Thus, one could expect that the oxidation wave of DA will be affected by the presence of AA Effectively, as presented in Fig 4, current peaks of DA and AA were poorly distinguished at bare GCE (curve a) However, the peak overlapping was reduced at PMT/GCE (curve b), which can be assigned to the role of the electroactive PMT The different mechanism of interaction of PMT with DA and with AA made their diffusion to the electrode surface differentiated Further, with NF addition in NF/PMT/GCE, the interference of AA to DA could be significantly eliminated, as shown in curve c At pH DA (pKa = 8.87) exits in cationic form, while AA (pKa = 4.17) can be found almost equally in anionic as well as cationic forms Even the concentration of AA was 10 times larger than that of DA, much smaller detected peak of AA, compared to that of DA (curve c), clearly demonstrated that pH of is a good choice It should be emphasized that in many studies reported in literature the detection occurs in PBS medium of pH 6.5–7 [29,35] Except for maintaining the physiological environment, the important reason for their choice was generally based on purely electrostatic interaction, suggesting that at this pH the negatively charged NF will exclude AA anions (pKa = 4.17 < pH 7) and provides a preferential transport channel for DA cations (pKa = 8.87 > pH 7) Further, the qualitative determination of DA in the presence of AA could be excellently performed in the range of 5.0–177.0 M of DA (Fig 5, inset) The linear regression equation was calculated as: Ipa (A) = −0.13 + 0.26488 × [DA] (M) with R2 = 0.999 Moreover, the peak intensity and position of AA did not change with the variation of the concentration of DA in the above mentioned range, signifying that AA could not interfere to the sensitivity of the DA at NF/PMT/GCE and the selective detection of DA is possible from binary DA–AA mixture at the modified NF/PMT/GCE 3.3 Selective determination of DA in ternary DA–AA–UA mixture on NF/SWCNT/PMT/GCE Like AA, UA also coexists with DA in the extracellular fluid of the central nervous system UA is the second major interference for DA detection as they both are oxidized at the same potential 0,8 [25] Hence, this investigation further extended, on the verification of UA effect on the DA oxidation at modified electrodes From Fig 6A, it can be seen that the oxidation potentials of AA and UA at PMT/GCE were quite close to each other, making the oxidation peaks of AA–DA and DA–UA merged significantly At NF/PMT/GCE, the overlapping was less pronounced but still presented (Fig 6B), implying that purely electrostatic repulsion, based on NF component was not enough to avoid the interference of A AA UA I /µA 3.2 Selective determination of DA in binary DA–AA mixture on NF/PMT/GCE 0,6 Fig DPVs for different concentration of DA in 0.1 M PBS of pH 4.0 at NF/PMT/GCE Inset: the linear regression curve of peak current vs DA concentration DA -0.2 0.0 0.2 0.4 0.6 0.8 E /V vs Ag/AgCl B 18 60 µM 16 I /µA Fig DPVs of mixture of 100 M DA and 1000 M AA in 0.1 M PBS of pH 4.0 at (a) bare GCE; (b) PMT/GCE; (c) NF/PMT/GCE 0,4 E /V vs Ag/AgCl DA + UA 40 µM 14 25 µM 15 µM 12 11 µM 10 µM µM µM AA -0.2 0.0 0.2 0.4 0.6 0.8 E /V vs Ag/AgCl Fig (A) DPVs for mM AA, mM UA and 50 M DA, in 0.1 M PBS of pH 4.0, at PMT/GCE (B) DPVs for different concentrations (2–60 M) of DA, mM of AA and 0.47 mM of UA, in 0.1 M PBS of pH 4.0, at NF/PMT/GCE 768 D.P Quan et al / Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 A 0.45x10-4 DA 0.43x10-4 0.40x10-4 0.38x10-4 i/A 0.35x10-4 UA 0.33x10-4 0.30x10-4 AA 0.28x10-4 -4 0.25x10 0.23x10-4 0.20x10-4 0.100 0.200 0.300 0.400 0.500 0.600 0.700 E/V B 0.15x10-4 pH3 -4 0.13x10 pH7 0.10x10-4 i/A 0.08x10-4 0.05x10-4 0.03x10-4 -0.03x10-4 -0.300 -0.050 0.200 0.450 0.700 E/V Fig (A) DPVs of mixture solution of 0.1 mM DA, 2.5 mM AA and 0.415 mM UA in 0.1 M PBS of pH 4.0 at NF/SWCNT/PMT/GCE (B) DPVs of mixture solution of 0.1 mM DA, 2.5 mM AA and 0.415 mM UA in 0.1 M PBS at NF/SWCNT/PMT/GCE, in function of pH UA to the detection of DA in the mixture solution (effectively, in terms of purely electrostatic interaction, the negatively charged NF cannot exclude UA (pKa = 5.4 > pH 4) Therefore, it is impossible to determine the individual concentration of each compound from the merged voltammetric peak on the surface of NF/PMT/GCE In contrast to NF/PMT/GCE, the situation was substantially improved when NF/SWCNT/PMT/GCE was used Fig 7A showed that all three compounds of AA, DA and UA were oxidized at distinguishable potentials of 0.15, 0.37 and 0.53 V vs Ag/AgCl, for AA, DA and UA, respectively to form well-defined and sharp peaks This large peak separation is expected to allow the selective determination of DA even at the concentration of AA and UA of 20 times larger than that of DA As mentioned above, these findings cannot be interpreted on the basis of the above electrostatic interaction mechanism The more probable explanation for this is the synergetic effect of NF and SWNT at NF/SWCNT/PMT/GCE which contains the cation exchanger, NF, having a selective cation exchange enriching property due to the electrostatic interaction as well as SWCNT, displaying attractive characteristics, such as much larger specific surface area, excellent adsorptive ability and catalytic ability Thus, it can be inferred that the electro-oxidation of the analyzed molecules became facile at the surface of NF/SWCNT/PMT/GCE It is clear that ultimate purpose of DA detection is to effectively discriminate it against AA and UA In our study, we emphasize the efforts on maximizing the peak separation thus minimizing peak overlapping Two important experimental parameters such as NF amount and pH will be discussed and their chosen values will be justified below First, it should be reminded that NF has the ability not only to extract DA but also to decrease the mass transfer rate Thus, the addition of a small amount of NF forms a thin film resulting in poor sensitivity to DA, while a larger amount of NF forms a relatively thick film, decreasing the mass transfer rate of DA and the transfer rate of electrons within the NF film Thus in order to enhance the performance of the hybrid film modified electrode the NF amount should be optimized Investigating the relationship between peak currents and the amount of NF, experimentally varied from 0% to 2.5% in can be inferred that at the beginning, the peak current increases with increasing amount of NF, but when the amount exceeds 0.25% the peak current decreases (figure not shown) With increasing amount of NF, the sites of ion exchange increase, and the adsorption on the NF-SWCNT modified electrode is also amplified Hence, the peak current increases But, when the amount of NF is increased beyond a certain value, the NF thickness will induce higher resistance for the electrochemical process, therefore hinder the electron exchange between DA and NF/SWCNT/PMT/GCE, leading to a decrease of the electrode sensitivity Thus, in our study, 4.0 L of 1.0 mg mL−1 NF-SWCNT (0.25 wt.%) was considered as an optimal value and was chosen for NF/SWCNT/PMT/GCE preparation in all further experiments Second, the pH of the electrolyte solution has a strong influence on the oxidation of AA, DA and UA at electrode surface, when varying both the peak current and potential The effect of pH of the electrolyte solution on the peak current and peak potential was examined by recording DPV of AA, DA and UA of concentration 0.1 mM, 2.5 mM and 0.415 mM, respectively in a series in the pH range from to The response of peak current to pH is shown in Fig 7B For DA the anodic peak current was higher at pH and decreases gradually with increasing pH Similarly, UA also gave higher peak current at pH The peak potential of DA and UA were linearly shifted to positive side with decreasing pH as shown in Table with a slope of −55.3 mV per pH unit for DA confirming two protons and two electrons were involved in oxidation process respectively From this table, in view of simultaneous determination of AA, DA and UA, it is obvious that the lower pH value, the larger peak separation and the sharper peak forms thus higher sensitivity and higher selectivity will be For above reasons, pH was preferred to physiological pH Next, the typical DPVs of the ternary mixture at the NF/SWCNT/PMT/GCE for the DA concentration range from to 120 M were shown in Fig It can be found that the DPV peak height was linearly related to the DA concentration over two concentration regions, namely, of 1.5–20 M and 20–120 M, for which the linear regression equations were written respectively as follows: Ipa1 (A) = 0.37 + 0.42×[DA] (M) (R2 =0.9974), (1.5−20 M) Ipa2 (A) = 0.9 + 0.14 × [DA] (M) (R2 = 0.9897) (20 − 50 M) It is interesting to note that the slope variation for the two regions may be an evidence of mechanism change of DA transport towards the electrode surface, from adsorptive to diffusional mode, accordingly characterized for the lower and higher regions of DA concentration (Fig 8, inset) 3.4 Interference studies on the NF/SWCNT/PMT/GCE The interference from selected organic compounds and metal ions was evaluated Interference tests were investigated by DPV, at D.P Quan et al / Colloids and Surfaces B: Biointerfaces 88 (2011) 764–770 769 Table Epa,DA–AA and Epa,DA–UA (mV) in function of pH pH Epa,DA (mV) Epa,AA (mV) Epa,UA (mV) Epa,DA–AA (mV) Epa,DA–UA (mV) 401 371 282 246 187 175 157 68 32 -22 562 532 425 377 318 226 214 220 214 209 161 161 143 131 131 4th Add 3rd Add 2nd Add I /µA NF/SWCNT/PMT/GCE DA concentration was fixed at 100 M Interfering species of 50 or 400 times higher concentration than that of DA were added to the solution It was found that no interference could be observed for glucose (concentration ratio of interference/DA, n = 400), citric acid (n = 50), NaCl (n = 400), KCl (n = 400), CaCl2 (n = 400), MgCl2 (n = 400), NaNO3 (n = 400), NH4 NO3 (n = 400), indicating that NF/SWCNT/PMT/GCE has excellent selectivity for DA and its determination was insignificantly affected by the most common interfering species 1st Add Sample 3.5 Reproducibility and stability of the NF/SWCNT/PMT/GCE Another advantage of the NF/SWCNT/PMT/GCE was its working stability, which was tested by measuring the voltammetric current decay during repetitive DPV cycling It was found that the DPV peak height (Ipa ) remains practically its initial value with a relative standard deviation less than 5% for 30 successive measurements, indicating an excellent reproducibility of above proposed modified electrode 3.6 Determination of DA in real samples 0,2 0,3 0,4 0,5 0,6 0,7 E /V vs Ag/AgCl Fig DPVs for dopamine injection drug sample Inset: the linear regression curve of peak current vs DA concentration In the present study, NF/PMT and NF/SWCNT/PMT were synthesized at the surface of GCE By means of the CV and DPV, the selective DA determination in the solution with the excess AA and UA of binary (DA–AA) and ternary (DA–AA–UA) mixtures has been shown It was found that owing to the synergetic effect of NF and SWNT at NF/SWCNT/PMT/GCE the electro-oxidation of each molecule became facile and distinguishable at the surface of NF/SWCNT/PMT/GCE and the biosensor showed the excellent features, such as wide linear response range, high sensitivity and selectivity, good reproducibility and long time stability This sensor proved to be successfully used for DA determination in pharmaceutical and clinical preparations Acknowledgements 14 12 10 I /µA I /µA 0,1 Conclusions In order to verify the reliability of the method for analysis of DA in pharmaceutical product, SWCNTs/PMT/GCE was applied to determine DA·HCL (dopamine hydrochloride, Rotexmedica, Germany) injection (labeled 40 mg mL−1 , used in heart treatment) The result from 10 random samples, analyzed by the standard addition method (Fig 9) and the relationship between the height of the peak current and DA concentration, demonstrated that the content of DA·HCl in the drug was 39.851 ± 0.213 mg mL−1 The average content of DA·HCl, calculated as 39.851 mg mL−1 , was less than 0.4% different from the labeled content, meaning that the proposed NF/SWCNT/PMT/GCE could be applicable for direct DA determination in real samples 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 -0,1 -1 0,0 This work was supported by Vietnam’s National Foundation for Science and Technology Development (NAFOSTED) under Grant 107.04.108.09 DA UA -10 10 20 30 C /µM 40 References 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