Materials Science and Engineering C 39 (2014) 78–85
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Voltammetric behavior of dopamine at a glassy carbon electrode modified with NiFe2O4 magnetic nanoparticles decorated with multiwall carbon nanotubes Ali A. Ensafi a,⁎, B. Arashpour a, B. Rezaei a, Ali R. Allafchian b a b
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 8 February 2014 Accepted 17 February 2014 Available online 22 February 2014 Keywords: NiFe2O4 nanoparticles decorated with multiwall carbon nanotubes Synergic effect Dopamine determination Voltammetry
a b s t r a c t Voltammetric behavior of dopamine was studied on a glassy carbon electrode (GCE) modified-NiFe2O4 magnetic nanoparticles decorated with multiwall carbon nanotubes. Impedance spectroscopy and cyclic voltammetry were used to characterize the behavior of dopamine at the surface of modified-GCE. The modified electrode showed a synergic effect toward the oxidation of dopamine. The oxidation peak current is increased linearly with the dopamine concentration (at pH 7.0) in wide dynamic ranges of 0.05–6.0 and 6.0–100 μmol L−1 with a detection limit of 0.02 μmol L−1, using differential pulse voltammetry. The selectivity of the method was studied and the results showed that the modified electrode is free from interference of organic compounds especially ascorbic acid, uric acid, cysteine and urea. Its applicability in the determination of dopamine in pharmaceutical, urine samples and human blood serum was also evaluated. The proposed electrochemical sensor has appropriate properties such as high selectivity, low detection limit and wide linear dynamic range when compared with that of the previous reported papers for dopamine detection. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dopamine is a catecholamine neurotransmitter that helps control the brain's reward and pleasure centers [1]. This catecholamine is widely distributed in the brain for message transfer in the mammalian central nervous system [2]. Parkinson's disease and schizophrenia are two disorders that appear with dysfunctions of the dopamine production in the brain. People with low dopamine activity may be more susceptible to addiction [3,4]. Dopamine is also used as a medication. It affects the sympathetic nervous system. Dopamine increases heart rate and blood pressure. High doses of dopamine can lead to serious side effects such as heart arrhythmias that can be lifethreatening and kidney damage [5]. So it is necessary to develop a rapid, selective and sensitive method with simple sample preparation and determination steps for dopamine analysis. Recently, several analytical methods have been reported for the determination of dopamine including electrochemistry, high performance liquid chromatography (HPLC) [6,7] and chemiluminescence method [8,9]. Electrochemical techniques based on various chemically modified electrodes have been used to detect dopamine [10–14].
⁎ Corresponding author. Tel.: +98 311 3913269; fax: +98 311 3912350. E-mail address: Ensafi@cc.iut.ac.ir (A.A. Ensafi).
http://dx.doi.org/10.1016/j.msec.2014.02.024 0928-4931/© 2014 Elsevier B.V. All rights reserved.
Common modifiers are organic polymers [15–18], metal complexes [19–21], enzymes [22], nanoparticles and carbon nanotubes [23–26], surfactants [27–29] and organic molecules [10–12,30,31,38–63]. Multiwall carbon nanotubes (MWCNTs) attract high attention as a prospective material due to their electrical and thermal conductivity, strength, stiffness, toughness and chemical stability. One of the most interesting ways of application of MWCNTs is their usage for the modification of the surface of electrodes in electrochemistry [32]. Magnetic nanoparticles provide significant levels of new functionality for electrochemistry due to their high surface area, effective mass transport, catalysis and control over the local microenvironment [33,34]. In the present study, we apply a magnetic nanocomposite of MWCNTs decorated with spinel NiFe2O4 as a modifier to fabricate a modified glassy carbon electrode (GCE) using a citrate sol–gel method. NiFe2O4-nanocomposites were used in electrochemical methods to detect guanine and adenine [35], sotalol [36] and cefixime [37]. The NiFe2O4 nanoparticle incorporated MWCNT modified GCE as a working electrode was utilized as a sensitive and selective electrochemical sensor for the determination of dopamine. The results of our studied showed that the proposed method is simple, rapid, sensitive and selective for the quantitative determination of dopamine at the NiFe 2O 4 –MWCNT modified-GCE surface, using differential pulse voltammetry techniques. Its applicability in the determination of dopamine in pharmaceutical, urine samples and human blood serum was evaluated too.
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2. Experimental 2.1. Apparatus The voltammograms of differential pulse voltammetry and cyclic voltammetry were obtained with a Metrohm instrument, model 797 VA processor (Switzerland). A three-electrode system that consisted of a platinum wire auxiliary electrode, an Ag/AgCl (3.0 mol L− 1 KCl) as a reference electrode and the modified GCE as a working electrode, was used through the experiment. Electrochemical impedance spectroscopy (EIS) was performed in a solution with 0.1 mol L− 1 KCl as a supporting electrolyte, using a current voltage of 5 mV within a frequency range of 100 kHz to 1.0 Hz by Autolab electrochemistry instrument (The Netherlands), and PGSTAT 12 and FRA2 boards, run on a PC using GPES and FRA 4.9 software. Different techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), tunneling electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform IR spectroscopy were used to get information about the surface structure of the modified electrode. AFM was obtained with a Bruker Nanos instrument (Germany). SEM was also performed with a Philips XLC (The Netherlands). TEM was obtained using a Philips CM 200, LaB6-cathode 160 kV (The Netherlands). EDX was obtained with a Philips XLC (The Netherlands). Fourier transform IR spectra were recorded using a JASCO FT-IR (680 plus, Tokyo, Japan). The spectra of solids were obtained using KBr pellets. A coring pH-meter, model 140 (New York, USA), with a glass electrode (conjugated with an Ag/AgCl reference, model 140) was used to measure the solution's pH. 2.2. Chemicals Dopamine was purchased from Merck (Darmstadt, Germany). Ethanol was purchased from Bidestan Co. (Tehran, Iran). Nitric acid, citric acid, Ni(NO3)2∙ 6H2O, Fe(NO3)2∙ 9H2O, dimethyl formamide and ammonium hydroxide were purchased from Sigma-Aldrich (St. Louis, USA) and used without further treatment. Chemicals for interference study including ascorbic acid, salicylic acid, tartaric acid, methionine, sodium nitrate, potassium nitrate, cobalt nitrate, sodium bromide, calcium nitrate, sodium chloride, sodium fluoride, cysteine, valine, uric acid and urea were purchased from Sigma-Aldrich (St. Louis, USA). All the solutions were prepared in mmol L−1 from the chemicals using deionized water. More dilute solutions were prepared daily. A stock solution of 0.01 mol L− 1 dopamine was prepared by dissolving 0.0153 g of pure dopamine hydrochloride in an appropriate volume of water in a 10-mL volumetric flask.
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Phosphate buffer solutions (0.1 mol L − 1 was prepared from H3PO4 and NaOH) with different pH values were used to adjust the solution's pH. Multiwall carbon nanotubes (N 90% MWCNT basis, with a diameter of 20–30 nm and a length of 5–15 μm) were purchased from Sigma-Aldrich.
2.3. Preparation of NiFe2O4–MWCNT modified electrode Citrate sol–gel method was used for the preparation and fabrication of NiFe2 O4 –MWCNT nanohybrid. Before modification of MWCNTs, nitric acid was used to purify MWCNTs. For this propose, 20 mL of 3.0 mol L − 1 nitric acid plus 1.00 g of MWCNTs were mixed in a 50 mL flux. The mixture was refluxed for 15 h and then it was cooled to room temperature. The reaction mixture was passed through a filter paper with 3 μm porosity, and the filtrate was washed with deionized water and dried at room temperature. Such conditions lead to the removal of impurities from the MWCNTs and opened the tube caps [64]. To modify the MWCNTs, 0.750 g of the activated MWCNTs was mixed with 10 mL of 1.0 mol L−1 citric acid. Then, it was placed in an ultrasonic bath for 10 min. Afterward, the suspension was mixed with a 10 mL solution containing 0.5 mol L − 1 Ni(NO3)2 ∙ 6H2O and 1.0 mol L − 1 Fe(NO3 ) 2∙ 9H2 O. In order to adjust the solution pH, 0.10 mol L− 1 ammonium hydroxide (NH 4 OH) was added to the mixture until the pH value reached 9.0. For the completion of the reaction, the mixture was stirred for 48 h at room temperature. The resulting product was dried in an oven at 100 °C for 12 h. Finally the produced mixture was calcined at 630 °C in a furnace under argon atmosphere for 2 h, to remove any impurities [64]. This operation led to the production of NiFe2O4–MWCNTs. Ham et al. [65] illustrated that solvent with high values of the dispersion component is the best for making homogeneous and agglomerate-free dispersions of MWCNTs. Thus, NiFe2O4–MWCNTs were sonicated in dimethyl formamide (1.0 mg per 5 mL) to prepare a suspension. Afterward a GCE (with 3.0 mm in diameter, 0.0707 cm2 area) was polished with aluminum suspensions with a grain size of 0.05 mm on a billiard cloth. Then, the GCE was placed in 1:1 ethanol/water in a sonicator for 5 min. The modified electrode was prepared by dipping 10 μL of the stable suspension onto the top surface of the GCE and then it was dried in air at room temperature. A magnet was used to spread monotonously the nanoparticle modified-MWCNT layer at the electrode surface by moving a magnet above the surface of the GCE before it becomes dry.
Fig. 1. A) X-ray diffraction patterns of MWCNTs (a), and NiFe2O4–MWCNTs; B) FT-IR spectra of MWCNTs (a), and NiFe2O4–MWCNTs (b).
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Fig. 2. A) SEM images of MWCNTs and B) NiFe2O4–MWCNTs; C) TEM image of NiFe2O4–MWCNT modified electrode; D) 2D and E) 3D AFM topology of the surface of NiFe2O4–MWCNT modified electrode.
2.4. Real sample preparation 0.25 mL of dopamine hydrochloride injection solution (40 mg mL−1) was diluted to 100 mL with water. Then, 50 μL of the result solution was injected into a 5-mL volumetric flask and made up to volume with the buffer solution (pH 7.0). The test solution was transferred into the electrochemical cell and the dopamine contents were measured using the proposed method.
Urine and blood plasma samples were obtained from the Isfahan University of Technology Health Center. The samples were stored in a refrigerator after collection. 4.0 mL of each sample was centrifuged for 5 min at 1500 rpm. The solution was diluted with 1.0 mL of 0.10 mol L− 1 buffer (pH 7.0) and was transferred into the electrochemical cell to be analyzed without any further treatment. Standard addition method was used for the determination of dopamine in the samples.
Fig. 3. A) (a): Cyclic voltammograms of (a): unmodified GCE in the blank solution (pH 7.0); (b): MWCNT modified-GCE in the blank solution (pH 7.0); (c): NiFe2O4–MWCNT modified GCE in the blank solution (pH 7.0); (d): unmodified GCE in 10.0 μmol L−1 dopamine at pH 7.0; (e): MWCNT modified GCE in 10.0 μmol L−1 dopamine at pH 7.0; (f): NiFe2O4–MWCNT modified GCE in 10.0 μmol L−1 dopamine at pH 7.0. B) Cyclic voltammograms of (a): NiFe2O4 modified-GCE in the blank solution, pH 7.0; (b): NiFe2O4 modified-GCE in 10.0 μmol L−1 dopamine. Conditions: pulse amplitude of 100 mV, pulse time of 20 ms, and sweep rate of 50 mV s−1.
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3.2. Investigation of electrochemical behavior of dopamine at NiFe2O4–MWCNT modified-GCE
Fig. 4. Nyquist plot of 1.00 mmol L−1 Fe(CN)3−/4− in 0.10 mol L−1 KNO3 a): at unmodified 6 GCE; and b) at NiFe2O4–MWCNT modified electrode. Inset: fitted circuit for electrochemical impedance spectroscopic data.
3. Results and discussion 3.1. Morphology and structure of the modified electrode An XRD spectrum of MWCNTs and the NiFe2O4–MWCNT magnetic nano-composite is shown in Fig. 1A (a and b, respectively). Fig. 1A (b) shows eleven characteristic peaks that occur at 2θ of 30.42°, 35.77°, 37.37°, 44.22°, 51.47°, 57.32°, 63.07°, 71.52°, 74.42°, 75.57° and 79.72°, which are marked by their corresponding indexes (220), (311), (222), (400), (422), (511), (440), (620), (533), (622) and (444), respectively. The diffraction peaks at 2θ of 26.27°, 43.42° and 53.92° are the typical Bragg peaks of pristine CNTs and can be indexed to (002), (101) and (004) reflection of MWCNTs (Fig. 1A, a and b). This reveals that the particles are pure NiFe2O4–MWCNTs with a spinel structure. No diffraction peaks of other impurities such as α-Fe2O3 or NiO were observed. The sizes of the nanoparticles from 30 to 40 nm, which are consistent with the result, are calculated using the Scherer equation. EDX was also used to characterize MWCNT/NiFe2O4 nanohybrid. The EDX spectrum reveals the presence of 52.98% (w/w) Fe, 21.78% Ni, 13.64% O, and 11.60% C. These results demonstrated that MWCNT/NiFe2O4 nanohybrid was successfully synthesized. FT-IR spectra of MWCNTs and MWCNTs decorated with NiFe2O4 are shown in Fig. 1B (a and b, respectively). These spectra clearly show absorption bands at around 1622 and 1380 cm− 1, which are characteristic stretching vibrations of the carboxylate group (C_O) and stretching vibration of C_C, respectively [66]. The absorption band around 1100 cm− 1 is assigned to the stretching vibration of C\C\C group. The band around 500 cm− 1 is due to Fe\O, which is not observed in Fig. 1B (a). This information confirms the presence of NiFe2O4–MWCNTs at the surface of GCE. Fig. 2A and B shows SEM images of MWCNTs and MWCNT/NiFe2O4 nanohybrid. These pictures showed that NiFe2O4 is well distributed on the surface of MWCNTs and also show that the surface of GCE was completely covered with NiFe2O4–MWCNT nanoparticles. Fig. 2C shows TEM image (sample morphology) of NiFe2O4–MWCNTs. This figure confirms that MWCNTs and NiFe2O4 were distributed on the surface of the GCE. The spaghetti-like NiFe2O4–MWCNTs and MWCNTs formed a porous structure. The entangled cross-linked fibrils offered a good accessible surface area. Further information about the surface structure such as roughness and thickness of the NiFe2O4–MWCNT modified electrode was also checked by AFM. The AFM topology of the surface of NiFe2O4–MWCNT modified electrode corresponding to 2D (Fig. 2D) and 3D (Fig. 2E) images recorded over an area of 5.8 × 4.3 μm is shown in Fig. 2C and D. The existence of particles with less than 25 nm at NiFe2O4–MWCNT modified electrode surface is clearly reflected in 2D and 3D AFM images.
The effect of the electrode composition in the voltammetric response of the oxidation of dopamine at the modified and unmodified electrodes was evaluated using cyclic voltammetry. Voltammograms of phosphate buffer (pH 7.0) solution, as an electrolyte, were recorded at the unmodified GCE, at MWCNT modified-GCE and at NiFe 2O 4 –MWCNT modified-GCE as shown in Fig. 3A (curves a, b and c, respectively). Fig. 3A (curves d–f) shows the cyclic voltammograms of 10.0 μmol L − 1 dopamine at pH 7.0 at the surface of GCE (curve d), at MWCNT-GCE (curve e) and at NiFe2 O4 –MWCNT-GCE (curve f). Fig. 3B (curves a and b) shows the cyclic voltammograms of NiFe2O4 modified-GCE in the phosphate buffer at pH 7.0 (curve a) and in 10.0 μmol L− 1 dopamine (at pH 7.0). The experimental results showed that the oxidation peak current of dopamine at the surface of the unmodified-GCE is weak but the use of NiFe2O4 modified-GCE enhanced the peak current. However, the use of NiFe2O4–MWCNTs resulted in a large enhancement in the current response. In other words, the obtained results confirm that the combination of MWCNTs and NiFe2O4 improves the characteristics of the electrode for the oxidation of dopamine. The obtained results represent that the presence of the MWCNTs–NiFe2O4 at GCE surface has a good synergic effect, which may be ascribed to the specific and preferable properties of the magnetic nanoparticles and MWCNTs including their high conductivity, fast electron transfer rate, and good antifouling as well as the inherent catalytic ability of the nanoparticles. This phenomenon increased the oxidation current of dopamine at the surface of MWCNT–NiFe2O4 modified-GCE. In order to estimate the active surface area of the modified electrode, voltammograms of NiFe2O4–MWCNT modified-GCE in 1.0 mmol L− 1
Fig. 5. A) Effect of pH on the peak current; and on the peak potentials (B) of dopamine (30.0 μmol L− 1) oxidation in phosphate buffer (0.1 mol L− 1 ) at NiFe 2O 4–MWCNTmodified electrode. Conditions: pulse amplitude, 100 mV; pulse time, 20 ms; and sweep rate, 50 mV s−1.
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Fig. 6. A) Effect of accumulation potential and accumulation time on the peak current (B) of dopamine (30.0 μmol L− 1) oxidation in phosphate buffer (0.1 mol L− 1) at NiFe2O4–MWCNT-modified electrode. Conditions: pulse amplitude, 100 mV; pulse time, 20 ms; and sweep rate, 50 mV s−1.
K3Fe(CN)6, as a probe, at different scan rates were recorded. For a reversible process the Randles–Sevcik formula (at 25 °C) could be used: 3=2
Ipa ¼ 2:69n
AC 0 D
1=2 1=2
ν
:
ð1Þ
where Ipa (A) refers to the anodic peak current, n is the electron transfer number, A (cm 2) is the microscopic surface area of the electrode, D (cm2 s− 1) is the diffusion coefficient, C0 (mol cm− 3) is the concentration of K 3Fe(CN)6 and ν (V s − 1 ) is the scan rate. In this equation all parameters for potassium hexacyanoferrate are clear as n = 1 and D = (7.6 ± 0.2) × 10− 6 cm2 s− 1, except the surface area of the electrode. The slope of Ipa vs. ν1/2 is equal to the microscopic area of the electrode surface. The comparison of the two resulting slopes showed that the area of NiFe2O4–MWCNT modified-GCE was 6.0 times greater than that of the unmodified-GCE, whereas the surface area for MWCNT modified-GCE was 4 times greater than that for the unmodified GCE. 3.3. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy was used to study the behavior of NiFe2O4–MWCNT modified-GCE surface. This method provides useful information about the changes in resistance of the electrode surface during the oxidation reaction. Nyquist curves of imaginary impedance (Zim) vs. the real impedance (Zre) of the EIS at NiFe2O4–MWCNT modified GCE and at unmodified
Fig. 7. Calibration plot of 0.05–100 μmol L−1 of dopamine; inset: voltammograms of various concentrations of dopamine as 1) 0.05; 2) 0.1; 3) 0.5; 4) 1.0; 5) 6.0; 6) 7.0; 7) 10.0; 8) 30.0; 9) 40.0; 10) 50.0; 11) 60.0; 12) 70.0; 13) 80.0; 14) 90.0; and 15) 100.0 μmol L−1. Conditions: pH, 7.0; and sweep rate, 20 mV s−1.
GCE in a solution containing 1.0 mmol L−1 of Fe(CN)4−/3− in 0.10 mol 6 L−1 KNO3 are shown in Fig. 4. According to the impedance spectrogram, the electrochemical impedance spectroscopic data of the two electrodes are compatible with equivalent circuit, which is shown in Fig. 4B. The circuit consists of Rs (solution resistance), CPE (a constant phase element corresponding to the double layer capacitance), Rct (charge transfer resistance) and Zw (Warburg impedance) that coupled to Rct, which related to Nernstian diffusion. The Nyquist diagram of unmodified GCE (Fig. 4, a) has a semicircle at high frequencies that the diameter of this related to electron transfer resistance (Rct) and straight line with a slope of nearly 45, which is due to a mass transport process via diffusion. The diameter of the semicircle Nyquist diagram of the modified electrode (Fig. 4, b), due to the excessive decrease in the electron transfer resistance, is significantly reduced which is the cause of the presence of high conductive NiFe 2O 4 –MWCNTs. These results show that the performance of NiFe2O4–MWCNTs improves the oxidation process. All these information demonstrated that dopamine could be successfully oxidized at the surface of NiFe2O4–MWCNT modified-GCE. Also, it could be concluded that MWCNTs decorated with NiFe2O4 make nanoparticles that have a synergic effect on the oxidation of dopamine. Fig. 4 (inset) shows fitted circuit for the electrochemical impedance spectroscopic data. 3.4. Optimization of the measurement conditions In order to evaluate the oxidation of dopamine at NiFe2O4–MWCNT modified-GCE the electrochemical behavior of dopamine at the NiFe2O 4–MWCNT modified-GCE was characterized in 0.1 mol L − 1 of phosphate buffer solution with different pH values (3.0–9.0) using differential pulse voltammetry (Fig. 5). Fig. 5A shows that the peak current of dopamine reached to a maximum value at pH 7.0 and then leveled off. The probable reason is that the number of negative carboxylate groups (COO− ) in the activated MWCNTs (on the electrode surface) increased with increasing the solution pH. Therefore, more positive dopamine cations were attracted to the electrode surface. Thus, the peak current increased with increasing the solution pH over the range of 5.0–7.0 (the pH values below acid dissociation constant, pK a = 8.9). Therefore, pH 7.0 was selected for further study. The effect of solution pH on the peak potential of dopamine was investigated too. It was found that the peak potentials (E) for the oxidation process of dopamine shifted negatively with increasing the solution pH as − 0.0545 V pH− 1 (Fig. 5). These results confirm that the ratio of participated protons to the transferred electrons through the NiFe2O4–MWCNTs is 1:1. The slope close to the theoretical value of 59 mV/pH was obtained in the investigated pH range indicating the 2e−/2H+ process [51].
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Fig. 8. The signal of dopamine in the presence of interfering compounds.
The effect of potential scan rate (between 10 and 100 mV s−1) on the peak current was investigated in the presence of 50.0 μmol L−1 dopamine at pH 7.0, using cyclic voltammetry. A linear relationship was observed between the peak current and the scan rate (ν) with a regression equation of Ip(μA) = (0.3571 ± 0.0417) ν (mV s−1) + (6.2103 ± 0.4657), (r2 = 0.9938). This relationship indicates that the oxidation of dopamine was an adsorption-controlled process. To study more about the adsorption of dopamine at NiFe2O4–MWCNT modified-GCE, the influences of accumulation potential and accumulation time on the peak current were evaluated (Fig. 6). The oxidation peak current of 30.0 μmol L− 1 dopamine at different accumulation potentials from − 0.10 V to + 0.20 V was assessed. The results showed (Fig. 6A) that by increasing the accumulation potential from − 0.05 V to 0.10 V the adsorption of dopamine was increased and thus increasing the oxidation peak current. When the potential reached near the oxidation potential of dopamine, the current intensity was decreased. This suggests that in this potential, dopamine was oxidized and it could not remain at the electrode surface. Therefore, + 0.10 V was selected as a suitable accumulation potential. Accumulation time has also affected the peak current. The results of this study showed that the oxidation peak current of 30.0 μmol L− 1 dopamine increased with increasing the accumulation time from 0 to 60 s and then leveled off (Fig. 6B).
This is due to the saturation of adsorbed dopamine at the surface of NiFe 2O 4 –MWCNT modified-GCE. Therefore, 60 s was chosen as a suitable accumulation time for further study. The optimum parameters for differential pulse voltammetry (DPV) were selected from the study (using 30.0 μmol L− 1 dopamine) for pulse amplitude, pulse width and scan rate in the range of 20–100 mV, 20–70 ms and 10–100 mV s−1, respectively. The obtained results indicated that the optimum parameters were pulse amplitude of 80 mV, pulse width of 40 ms and sweep rate of 20 mV s−1. 4. Figures of merit Differential pulse voltammetry was used to prepare the calibration curve. Under the optimized conditions, the electrode response to dopamine concentrations was checked at the surface of NiFe2O4–MWCNT modified-GCE (Fig. 7). The results showed that for the range of 0.05–6.0 μmol L−1 dopamine, the regression equation was Ip(μA) = (1.209 ± 0.032)CDopamine + (1.812 ± 0.069), (r2 = 0.996, n = 6), whereas for the range of 6.0–100 μmol L−1 dopamine, the regression equation was Ip(μA) = (0.083 ± 0.001)CDopamine + (8.629 ± 0.103), (r2 = 0.996, n = 6). As the results showed, the sensitivity (slope) of the electrode to dopamine at a lower concentration is better. This
Table 1 Determination of dopamine in real samples at pH 7.0 (n = 3). Sample a
Injection solution Urine Urine Urine Urine Plasma Plasma Plasma Plasma
Dopamine added (μmol L−1)
Dopamine found (μmol L−1)
Recovery (%)
Standard method (μmol L−1)
25.0 – 10.0 20.0 40.0 – 20.0 30.0 50.0
23.5 ± 0.3 bLimit of detection 9.7 ± 0.1 19.8 ± 0.4 39.0 ± 0.7 bLimit of detection 19.3 ± 0.2 29.1 ± 0.6 48.2 ± 1.1
94.0 – 97.0 98.9 97.5 – 96.5 97.0 96.4
24.1 ± 1.7 – 9.8 ± 0.8 20.2 ± 1.1 38.7 ± 1.3 – 20.6 ± 1.1 28.9 ± 1.3 51.2 ± 1.8
Urine and blood plasma samples were taken from women who were safe. a Dopamine injection solution: Caspian Tamin, 200 mg/5 mL, Pharmaceutical Co., Rasht, Iran.
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Table 2 Comparison of the proposed electrochemical sensor with the other reported method for the determination of dopamine. Technique
Limit of detection (μmol L−1)
Linear range (μmol L−1)
Interference compound
Reference
DPV SWV CV Chronoamperometry DPV DPV SWV DPV CV DPV DPV DPV DPV CV SWV DPV DPV DPV CV DPV CV CV CV CV LSV DPV LSV Amperometry DPASV
0.22 0.15 0.6 1.4 3.75 0.1 0.36 0.7 3.2 5.0 0.5 5.0 0.043 0.25 0.35 0.02 0.05 – 0.84 0.015 0.016 0.29 0.15 0.087 0.061 0.04 0.5 0.05 0.002
0.4–150 0.5–150 50–300 1.4–300 20–200 0.5–100 3.2–31.8 1–100 – 10–1100 1–2500 24–384 0.2–60 1-300 20–51 0.04–400 0.05–470 40–5000 0.9–10 0.075–20 30–100 5.0–280 0.4–20 0.1–900 0.5–25 0.1–9.8 2.0–8.0 0.08–600 0.006–0.29
– Not mentioned Not mentioned – – – – Not mentioned – – – – Not mentioned Not mentioned Not mentioned Cysteine Citric acid Not mentioned – Not mentioned – – – – – – Not mentioned – –
[10] [11] [12] [34] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63]
DPV: differential pulse voltammetry; CV: cyclic voltammetry; SWV: square wave voltammetry; LSV: linear sweep voltammetry; DPASV: differential pulse anodic stripping voltammetry.
is due to the fact that at low dopamine concentration, there is a monolayer of the analyte at the electrode surface, whereas at a higher concentration of the analyte, the analyte present at the second layer could not contact directly with the electrode surface, hence the sensitivity (slope) decreased. The detection limit was obtained as 0.02 μmol L− 1 dopamine, according to S/N = 3. The reproducibility and stability of the modified electrode were studied by replicate measurements of 0.50 and 30.0 μmol L−1 of dopamine using DPV under the optimum conditions. The relative standard deviation (RSD%) for ten successive assays of 0.50 and 30.0 μmol L−1 dopamine were 2.2% and 1.9%, respectively. When five different electrodes were used, the RSD% for five measurements of 0.50 and 30.0 μmol L−1 dopamine were 2.3% and 2.1%, respectively. These results confirmed that the modified electrode has excellent stability and reproducibility for the determination of dopamine.
5. Interference studies The selectivity of the new electrochemical sensor for dopamine detection was checked with different potential interfering compounds in the presence of dopamine at the NiFe2O4–MWCNT modified-GCE using DPV under the optimum conditions. The potential interfering compounds were added to a solution containing 30.0 μmol L−1 of dopamine in the buffer solution (pH 7.0). Then, the signal of the mixture was measured at the NiFe2O4–MWCNT modified-GCE. Tolerance limit was taken as the maximum concentration of the interfering substances, which caused a relative error of less than ±5% in the determination of dopamine. The results showed that 1000-fold of ascorbic acid, salicylic acid, tartaric acid, methionine, Na+, K+, Co2+, Ca2+, F−, Cl−, Br−, and NO− 3 , 500-fold of citric acid, cysteine, valine, and 200-fold of uric acid and urea did not affect the selectivity. Fig. 8 also showed the signal of dopamine (DA) in the presence of some interfering compounds. The obtained data showed that the proposed method is highly selective for dopamine determination.
6. Determination of dopamine in injection solution and biological fluids To evaluate the ability of the proposed sensor in real sample analysis, dopamine injection solution, human plasma and urine samples were prepared and analyzed. Standard addition method was used for measuring the dopamine contents in the samples. According to the obtained results, a good agreement can be seen between the proposed method and the standard method [67]. The data, given in Table 1, show the satisfactory results and confirm the capability of the modified GCE for voltammetric determination of dopamine in real samples. 7. Conclusion NiFe 2O 4 –MWCNT modified-GCE shows an excellent synergic behavior toward dopamine oxidation in an aqueous phosphate buffer (pH 7.0) solution. Our study showed that in the case of the unmodified GCE, the voltammograms of dopamine exhibit just a small hump peak, but after modification of the electrode with MWCNT/NiFe2O4, the oxidation peak current of dopamine is significantly enhanced. This technique offers a number of advantages compared to the other published electrochemical methods, such as simplicity in the preparation of the modified electrode and its high selectivity. The modified electrode represents appropriate performance in detecting dopamine and exhibits excellent stability and reproducibility. In the differential pulse voltammetric determination, the detection limit of dopamine was estimated as 0.02 μmol L − 1 . Based on the electrochemical oxidation, the quantitative determination of dopamine in pharmaceutical dosage and biological fluids samples was developed by a simple, rapid, selective and sensitive DPV technique. These properties indicate that the NiFe2O4–MWCNT modified electrode is a good electrochemical sensor for the determination of dopamine. Table 2 shows a comparison of the analytical figures of merit of the proposed method with different reported voltammetric techniques for the determination of dopamine
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[10–12,38–63]. According to available data, this method has appropriate properties compared to the previous studies such as high selectivity, sensitivity, low detection limit and wide linear dynamic range. Acknowledgments The authors wish to thank the Isfahan University of Technology (IUT) Research Council and the Center of Excellence in Sensor and Green Chemistry for their support. References [1] G. Zou, Basic Nerve Pharmacology, Science Press, Beijing, 1999. 203. [2] M. Wightman, L.J. May, A.C. Michael, Anal. Chem. 60 (1998) 769A. [3] K. Jellinger, in: C.D. Marsden, S. Fahn (Eds.), The Pathology of Parkinsonism, Movement Disorders, Butterworths, London, 1987, pp. 124–165. [4] A.M. Sardar, C. Czudek, G.P. Reynolds, Neuroreport 7 (1996) 910. [5] T.E. Smith, in: T.M. Devlin (Ed.), Textbook of Biochemistry with Clinical Correlations, Wiley/Liss, New York, 1992, p. 929. [6] N. Li, J.Z. Guo, B. Liu, H. Cui, L.Q. Mao, Y.Q. Lin, Anal. Chim. Acta. 645 (2009) 48. [7] V. Carrera, E. Sabater, E. Vilanova, M.A. Sogorb, J. Chromatogr. B 847 (2007) 88. [8] S.L. Zhao, Y. Huang, M. Shi, R.J. Liu, Y.M. Liu, Anal. Chem. 82 (2010) 2036. [9] Y.C. Chen, W.Y. Lin, Luminescence 25 (2010) 43. [10] B. Habibi, M. Jahanbakhshi, M.H. Pournaghi-Azar, J. Electrochim. Acta 56 (2011) 2888. [11] X. Tian, C. Cheng, H. Yuan, J. Du, S. Xie, M.F. Choi, D. Xiao, Talanta 93 (2012) 79. [12] S.S. Shankar, B.E. Swamy, K.R. Mahanthesha, T.V. Sathisha, C.C. Vishwanath, J. Anal. Bioanal. Electrochem. 5 (2013) 19. [13] Y. Li, X. Liu, W. Wei, Electroanalysis 23 (2011) 2832. [14] A. Naseri, M.R. Majidi, Daru 19 (2011) 270. [15] V.S. Vasantha, S.M. Chen, J. Electroanal. Chem. 592 (2006) 77. [16] M. Majidi, K. Asadpour-Zeynali, S. Gholizadeh, J. Polym. Anal. Charact. 14 (2009) 89. [17] T. Qian, C. Yu, S. Wu, J. Shen, Biosens. Bioelectron. 50 (2013) 157. [18] B. Massoumi, S. Fathalipour, A. Massoudi, M. Hassanzadeh, A.A. Entezami, J. Appl. Polym. Sci. 130 (2013) 2780. [19] I.R.W.Z. Oliveira, A. Neves, I.C. Vieira, Sens. Actuators, B 129 (2008) 424. [20] M. Majidi, K. Asadpour-Zeynali, S. Gholizadeh, Electroanalysis 22 (2010) 1772. [21] J. Oni, Ph. Westbroek, T. Nyokong, Electroanalysis 15 (2003) 847. [22] E.S. Forzani, G.A. Rivas, V.M. Solis, J. Electroanal. Chem. 435 (1997) 77. [23] S. Jo, H. Jeong, S. Ra Bae, S. Jeon, Microchem. J. 88 (2008) 1. [24] L. Zhang, X. Jiang, J. Electroanal. Chem. 583 (2005) 292. [25] S. Yuan, W. Chen, S. Hu, Mater. Sci. Eng. C 25 (2005) 479. [26] L. Zhang, Z. Shi, Q. Lang, J. Solid State Electrochem. 15 (2011) 801.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Voltammetric behavior of dopamine at a glassy carbon electrode modified with NiFe2O4 magnetic nanoparticles decorated with multiwall carbon nanotubes Ali A. Ensafi a,⁎, B. Arashpour a, B. Rezaei a, Ali R. Allafchian b a b
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 8 February 2014 Accepted 17 February 2014 Available online 22 February 2014 Keywords: NiFe2O4 nanoparticles decorated with multiwall carbon nanotubes Synergic effect Dopamine determination Voltammetry
a b s t r a c t Voltammetric behavior of dopamine was studied on a glassy carbon electrode (GCE) modified-NiFe2O4 magnetic nanoparticles decorated with multiwall carbon nanotubes. Impedance spectroscopy and cyclic voltammetry were used to characterize the behavior of dopamine at the surface of modified-GCE. The modified electrode showed a synergic effect toward the oxidation of dopamine. The oxidation peak current is increased linearly with the dopamine concentration (at pH 7.0) in wide dynamic ranges of 0.05–6.0 and 6.0–100 μmol L−1 with a detection limit of 0.02 μmol L−1, using differential pulse voltammetry. The selectivity of the method was studied and the results showed that the modified electrode is free from interference of organic compounds especially ascorbic acid, uric acid, cysteine and urea. Its applicability in the determination of dopamine in pharmaceutical, urine samples and human blood serum was also evaluated. The proposed electrochemical sensor has appropriate properties such as high selectivity, low detection limit and wide linear dynamic range when compared with that of the previous reported papers for dopamine detection. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Dopamine is a catecholamine neurotransmitter that helps control the brain's reward and pleasure centers [1]. This catecholamine is widely distributed in the brain for message transfer in the mammalian central nervous system [2]. Parkinson's disease and schizophrenia are two disorders that appear with dysfunctions of the dopamine production in the brain. People with low dopamine activity may be more susceptible to addiction [3,4]. Dopamine is also used as a medication. It affects the sympathetic nervous system. Dopamine increases heart rate and blood pressure. High doses of dopamine can lead to serious side effects such as heart arrhythmias that can be lifethreatening and kidney damage [5]. So it is necessary to develop a rapid, selective and sensitive method with simple sample preparation and determination steps for dopamine analysis. Recently, several analytical methods have been reported for the determination of dopamine including electrochemistry, high performance liquid chromatography (HPLC) [6,7] and chemiluminescence method [8,9]. Electrochemical techniques based on various chemically modified electrodes have been used to detect dopamine [10–14].
⁎ Corresponding author. Tel.: +98 311 3913269; fax: +98 311 3912350. E-mail address: Ensafi@cc.iut.ac.ir (A.A. Ensafi).
http://dx.doi.org/10.1016/j.msec.2014.02.024 0928-4931/© 2014 Elsevier B.V. All rights reserved.
Common modifiers are organic polymers [15–18], metal complexes [19–21], enzymes [22], nanoparticles and carbon nanotubes [23–26], surfactants [27–29] and organic molecules [10–12,30,31,38–63]. Multiwall carbon nanotubes (MWCNTs) attract high attention as a prospective material due to their electrical and thermal conductivity, strength, stiffness, toughness and chemical stability. One of the most interesting ways of application of MWCNTs is their usage for the modification of the surface of electrodes in electrochemistry [32]. Magnetic nanoparticles provide significant levels of new functionality for electrochemistry due to their high surface area, effective mass transport, catalysis and control over the local microenvironment [33,34]. In the present study, we apply a magnetic nanocomposite of MWCNTs decorated with spinel NiFe2O4 as a modifier to fabricate a modified glassy carbon electrode (GCE) using a citrate sol–gel method. NiFe2O4-nanocomposites were used in electrochemical methods to detect guanine and adenine [35], sotalol [36] and cefixime [37]. The NiFe2O4 nanoparticle incorporated MWCNT modified GCE as a working electrode was utilized as a sensitive and selective electrochemical sensor for the determination of dopamine. The results of our studied showed that the proposed method is simple, rapid, sensitive and selective for the quantitative determination of dopamine at the NiFe 2O 4 –MWCNT modified-GCE surface, using differential pulse voltammetry techniques. Its applicability in the determination of dopamine in pharmaceutical, urine samples and human blood serum was evaluated too.
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2. Experimental 2.1. Apparatus The voltammograms of differential pulse voltammetry and cyclic voltammetry were obtained with a Metrohm instrument, model 797 VA processor (Switzerland). A three-electrode system that consisted of a platinum wire auxiliary electrode, an Ag/AgCl (3.0 mol L− 1 KCl) as a reference electrode and the modified GCE as a working electrode, was used through the experiment. Electrochemical impedance spectroscopy (EIS) was performed in a solution with 0.1 mol L− 1 KCl as a supporting electrolyte, using a current voltage of 5 mV within a frequency range of 100 kHz to 1.0 Hz by Autolab electrochemistry instrument (The Netherlands), and PGSTAT 12 and FRA2 boards, run on a PC using GPES and FRA 4.9 software. Different techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), tunneling electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform IR spectroscopy were used to get information about the surface structure of the modified electrode. AFM was obtained with a Bruker Nanos instrument (Germany). SEM was also performed with a Philips XLC (The Netherlands). TEM was obtained using a Philips CM 200, LaB6-cathode 160 kV (The Netherlands). EDX was obtained with a Philips XLC (The Netherlands). Fourier transform IR spectra were recorded using a JASCO FT-IR (680 plus, Tokyo, Japan). The spectra of solids were obtained using KBr pellets. A coring pH-meter, model 140 (New York, USA), with a glass electrode (conjugated with an Ag/AgCl reference, model 140) was used to measure the solution's pH. 2.2. Chemicals Dopamine was purchased from Merck (Darmstadt, Germany). Ethanol was purchased from Bidestan Co. (Tehran, Iran). Nitric acid, citric acid, Ni(NO3)2∙ 6H2O, Fe(NO3)2∙ 9H2O, dimethyl formamide and ammonium hydroxide were purchased from Sigma-Aldrich (St. Louis, USA) and used without further treatment. Chemicals for interference study including ascorbic acid, salicylic acid, tartaric acid, methionine, sodium nitrate, potassium nitrate, cobalt nitrate, sodium bromide, calcium nitrate, sodium chloride, sodium fluoride, cysteine, valine, uric acid and urea were purchased from Sigma-Aldrich (St. Louis, USA). All the solutions were prepared in mmol L−1 from the chemicals using deionized water. More dilute solutions were prepared daily. A stock solution of 0.01 mol L− 1 dopamine was prepared by dissolving 0.0153 g of pure dopamine hydrochloride in an appropriate volume of water in a 10-mL volumetric flask.
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Phosphate buffer solutions (0.1 mol L − 1 was prepared from H3PO4 and NaOH) with different pH values were used to adjust the solution's pH. Multiwall carbon nanotubes (N 90% MWCNT basis, with a diameter of 20–30 nm and a length of 5–15 μm) were purchased from Sigma-Aldrich.
2.3. Preparation of NiFe2O4–MWCNT modified electrode Citrate sol–gel method was used for the preparation and fabrication of NiFe2 O4 –MWCNT nanohybrid. Before modification of MWCNTs, nitric acid was used to purify MWCNTs. For this propose, 20 mL of 3.0 mol L − 1 nitric acid plus 1.00 g of MWCNTs were mixed in a 50 mL flux. The mixture was refluxed for 15 h and then it was cooled to room temperature. The reaction mixture was passed through a filter paper with 3 μm porosity, and the filtrate was washed with deionized water and dried at room temperature. Such conditions lead to the removal of impurities from the MWCNTs and opened the tube caps [64]. To modify the MWCNTs, 0.750 g of the activated MWCNTs was mixed with 10 mL of 1.0 mol L−1 citric acid. Then, it was placed in an ultrasonic bath for 10 min. Afterward, the suspension was mixed with a 10 mL solution containing 0.5 mol L − 1 Ni(NO3)2 ∙ 6H2O and 1.0 mol L − 1 Fe(NO3 ) 2∙ 9H2 O. In order to adjust the solution pH, 0.10 mol L− 1 ammonium hydroxide (NH 4 OH) was added to the mixture until the pH value reached 9.0. For the completion of the reaction, the mixture was stirred for 48 h at room temperature. The resulting product was dried in an oven at 100 °C for 12 h. Finally the produced mixture was calcined at 630 °C in a furnace under argon atmosphere for 2 h, to remove any impurities [64]. This operation led to the production of NiFe2O4–MWCNTs. Ham et al. [65] illustrated that solvent with high values of the dispersion component is the best for making homogeneous and agglomerate-free dispersions of MWCNTs. Thus, NiFe2O4–MWCNTs were sonicated in dimethyl formamide (1.0 mg per 5 mL) to prepare a suspension. Afterward a GCE (with 3.0 mm in diameter, 0.0707 cm2 area) was polished with aluminum suspensions with a grain size of 0.05 mm on a billiard cloth. Then, the GCE was placed in 1:1 ethanol/water in a sonicator for 5 min. The modified electrode was prepared by dipping 10 μL of the stable suspension onto the top surface of the GCE and then it was dried in air at room temperature. A magnet was used to spread monotonously the nanoparticle modified-MWCNT layer at the electrode surface by moving a magnet above the surface of the GCE before it becomes dry.
Fig. 1. A) X-ray diffraction patterns of MWCNTs (a), and NiFe2O4–MWCNTs; B) FT-IR spectra of MWCNTs (a), and NiFe2O4–MWCNTs (b).
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Fig. 2. A) SEM images of MWCNTs and B) NiFe2O4–MWCNTs; C) TEM image of NiFe2O4–MWCNT modified electrode; D) 2D and E) 3D AFM topology of the surface of NiFe2O4–MWCNT modified electrode.
2.4. Real sample preparation 0.25 mL of dopamine hydrochloride injection solution (40 mg mL−1) was diluted to 100 mL with water. Then, 50 μL of the result solution was injected into a 5-mL volumetric flask and made up to volume with the buffer solution (pH 7.0). The test solution was transferred into the electrochemical cell and the dopamine contents were measured using the proposed method.
Urine and blood plasma samples were obtained from the Isfahan University of Technology Health Center. The samples were stored in a refrigerator after collection. 4.0 mL of each sample was centrifuged for 5 min at 1500 rpm. The solution was diluted with 1.0 mL of 0.10 mol L− 1 buffer (pH 7.0) and was transferred into the electrochemical cell to be analyzed without any further treatment. Standard addition method was used for the determination of dopamine in the samples.
Fig. 3. A) (a): Cyclic voltammograms of (a): unmodified GCE in the blank solution (pH 7.0); (b): MWCNT modified-GCE in the blank solution (pH 7.0); (c): NiFe2O4–MWCNT modified GCE in the blank solution (pH 7.0); (d): unmodified GCE in 10.0 μmol L−1 dopamine at pH 7.0; (e): MWCNT modified GCE in 10.0 μmol L−1 dopamine at pH 7.0; (f): NiFe2O4–MWCNT modified GCE in 10.0 μmol L−1 dopamine at pH 7.0. B) Cyclic voltammograms of (a): NiFe2O4 modified-GCE in the blank solution, pH 7.0; (b): NiFe2O4 modified-GCE in 10.0 μmol L−1 dopamine. Conditions: pulse amplitude of 100 mV, pulse time of 20 ms, and sweep rate of 50 mV s−1.
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3.2. Investigation of electrochemical behavior of dopamine at NiFe2O4–MWCNT modified-GCE
Fig. 4. Nyquist plot of 1.00 mmol L−1 Fe(CN)3−/4− in 0.10 mol L−1 KNO3 a): at unmodified 6 GCE; and b) at NiFe2O4–MWCNT modified electrode. Inset: fitted circuit for electrochemical impedance spectroscopic data.
3. Results and discussion 3.1. Morphology and structure of the modified electrode An XRD spectrum of MWCNTs and the NiFe2O4–MWCNT magnetic nano-composite is shown in Fig. 1A (a and b, respectively). Fig. 1A (b) shows eleven characteristic peaks that occur at 2θ of 30.42°, 35.77°, 37.37°, 44.22°, 51.47°, 57.32°, 63.07°, 71.52°, 74.42°, 75.57° and 79.72°, which are marked by their corresponding indexes (220), (311), (222), (400), (422), (511), (440), (620), (533), (622) and (444), respectively. The diffraction peaks at 2θ of 26.27°, 43.42° and 53.92° are the typical Bragg peaks of pristine CNTs and can be indexed to (002), (101) and (004) reflection of MWCNTs (Fig. 1A, a and b). This reveals that the particles are pure NiFe2O4–MWCNTs with a spinel structure. No diffraction peaks of other impurities such as α-Fe2O3 or NiO were observed. The sizes of the nanoparticles from 30 to 40 nm, which are consistent with the result, are calculated using the Scherer equation. EDX was also used to characterize MWCNT/NiFe2O4 nanohybrid. The EDX spectrum reveals the presence of 52.98% (w/w) Fe, 21.78% Ni, 13.64% O, and 11.60% C. These results demonstrated that MWCNT/NiFe2O4 nanohybrid was successfully synthesized. FT-IR spectra of MWCNTs and MWCNTs decorated with NiFe2O4 are shown in Fig. 1B (a and b, respectively). These spectra clearly show absorption bands at around 1622 and 1380 cm− 1, which are characteristic stretching vibrations of the carboxylate group (C_O) and stretching vibration of C_C, respectively [66]. The absorption band around 1100 cm− 1 is assigned to the stretching vibration of C\C\C group. The band around 500 cm− 1 is due to Fe\O, which is not observed in Fig. 1B (a). This information confirms the presence of NiFe2O4–MWCNTs at the surface of GCE. Fig. 2A and B shows SEM images of MWCNTs and MWCNT/NiFe2O4 nanohybrid. These pictures showed that NiFe2O4 is well distributed on the surface of MWCNTs and also show that the surface of GCE was completely covered with NiFe2O4–MWCNT nanoparticles. Fig. 2C shows TEM image (sample morphology) of NiFe2O4–MWCNTs. This figure confirms that MWCNTs and NiFe2O4 were distributed on the surface of the GCE. The spaghetti-like NiFe2O4–MWCNTs and MWCNTs formed a porous structure. The entangled cross-linked fibrils offered a good accessible surface area. Further information about the surface structure such as roughness and thickness of the NiFe2O4–MWCNT modified electrode was also checked by AFM. The AFM topology of the surface of NiFe2O4–MWCNT modified electrode corresponding to 2D (Fig. 2D) and 3D (Fig. 2E) images recorded over an area of 5.8 × 4.3 μm is shown in Fig. 2C and D. The existence of particles with less than 25 nm at NiFe2O4–MWCNT modified electrode surface is clearly reflected in 2D and 3D AFM images.
The effect of the electrode composition in the voltammetric response of the oxidation of dopamine at the modified and unmodified electrodes was evaluated using cyclic voltammetry. Voltammograms of phosphate buffer (pH 7.0) solution, as an electrolyte, were recorded at the unmodified GCE, at MWCNT modified-GCE and at NiFe 2O 4 –MWCNT modified-GCE as shown in Fig. 3A (curves a, b and c, respectively). Fig. 3A (curves d–f) shows the cyclic voltammograms of 10.0 μmol L − 1 dopamine at pH 7.0 at the surface of GCE (curve d), at MWCNT-GCE (curve e) and at NiFe2 O4 –MWCNT-GCE (curve f). Fig. 3B (curves a and b) shows the cyclic voltammograms of NiFe2O4 modified-GCE in the phosphate buffer at pH 7.0 (curve a) and in 10.0 μmol L− 1 dopamine (at pH 7.0). The experimental results showed that the oxidation peak current of dopamine at the surface of the unmodified-GCE is weak but the use of NiFe2O4 modified-GCE enhanced the peak current. However, the use of NiFe2O4–MWCNTs resulted in a large enhancement in the current response. In other words, the obtained results confirm that the combination of MWCNTs and NiFe2O4 improves the characteristics of the electrode for the oxidation of dopamine. The obtained results represent that the presence of the MWCNTs–NiFe2O4 at GCE surface has a good synergic effect, which may be ascribed to the specific and preferable properties of the magnetic nanoparticles and MWCNTs including their high conductivity, fast electron transfer rate, and good antifouling as well as the inherent catalytic ability of the nanoparticles. This phenomenon increased the oxidation current of dopamine at the surface of MWCNT–NiFe2O4 modified-GCE. In order to estimate the active surface area of the modified electrode, voltammograms of NiFe2O4–MWCNT modified-GCE in 1.0 mmol L− 1
Fig. 5. A) Effect of pH on the peak current; and on the peak potentials (B) of dopamine (30.0 μmol L− 1) oxidation in phosphate buffer (0.1 mol L− 1 ) at NiFe 2O 4–MWCNTmodified electrode. Conditions: pulse amplitude, 100 mV; pulse time, 20 ms; and sweep rate, 50 mV s−1.
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Fig. 6. A) Effect of accumulation potential and accumulation time on the peak current (B) of dopamine (30.0 μmol L− 1) oxidation in phosphate buffer (0.1 mol L− 1) at NiFe2O4–MWCNT-modified electrode. Conditions: pulse amplitude, 100 mV; pulse time, 20 ms; and sweep rate, 50 mV s−1.
K3Fe(CN)6, as a probe, at different scan rates were recorded. For a reversible process the Randles–Sevcik formula (at 25 °C) could be used: 3=2
Ipa ¼ 2:69n
AC 0 D
1=2 1=2
ν
:
ð1Þ
where Ipa (A) refers to the anodic peak current, n is the electron transfer number, A (cm 2) is the microscopic surface area of the electrode, D (cm2 s− 1) is the diffusion coefficient, C0 (mol cm− 3) is the concentration of K 3Fe(CN)6 and ν (V s − 1 ) is the scan rate. In this equation all parameters for potassium hexacyanoferrate are clear as n = 1 and D = (7.6 ± 0.2) × 10− 6 cm2 s− 1, except the surface area of the electrode. The slope of Ipa vs. ν1/2 is equal to the microscopic area of the electrode surface. The comparison of the two resulting slopes showed that the area of NiFe2O4–MWCNT modified-GCE was 6.0 times greater than that of the unmodified-GCE, whereas the surface area for MWCNT modified-GCE was 4 times greater than that for the unmodified GCE. 3.3. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy was used to study the behavior of NiFe2O4–MWCNT modified-GCE surface. This method provides useful information about the changes in resistance of the electrode surface during the oxidation reaction. Nyquist curves of imaginary impedance (Zim) vs. the real impedance (Zre) of the EIS at NiFe2O4–MWCNT modified GCE and at unmodified
Fig. 7. Calibration plot of 0.05–100 μmol L−1 of dopamine; inset: voltammograms of various concentrations of dopamine as 1) 0.05; 2) 0.1; 3) 0.5; 4) 1.0; 5) 6.0; 6) 7.0; 7) 10.0; 8) 30.0; 9) 40.0; 10) 50.0; 11) 60.0; 12) 70.0; 13) 80.0; 14) 90.0; and 15) 100.0 μmol L−1. Conditions: pH, 7.0; and sweep rate, 20 mV s−1.
GCE in a solution containing 1.0 mmol L−1 of Fe(CN)4−/3− in 0.10 mol 6 L−1 KNO3 are shown in Fig. 4. According to the impedance spectrogram, the electrochemical impedance spectroscopic data of the two electrodes are compatible with equivalent circuit, which is shown in Fig. 4B. The circuit consists of Rs (solution resistance), CPE (a constant phase element corresponding to the double layer capacitance), Rct (charge transfer resistance) and Zw (Warburg impedance) that coupled to Rct, which related to Nernstian diffusion. The Nyquist diagram of unmodified GCE (Fig. 4, a) has a semicircle at high frequencies that the diameter of this related to electron transfer resistance (Rct) and straight line with a slope of nearly 45, which is due to a mass transport process via diffusion. The diameter of the semicircle Nyquist diagram of the modified electrode (Fig. 4, b), due to the excessive decrease in the electron transfer resistance, is significantly reduced which is the cause of the presence of high conductive NiFe 2O 4 –MWCNTs. These results show that the performance of NiFe2O4–MWCNTs improves the oxidation process. All these information demonstrated that dopamine could be successfully oxidized at the surface of NiFe2O4–MWCNT modified-GCE. Also, it could be concluded that MWCNTs decorated with NiFe2O4 make nanoparticles that have a synergic effect on the oxidation of dopamine. Fig. 4 (inset) shows fitted circuit for the electrochemical impedance spectroscopic data. 3.4. Optimization of the measurement conditions In order to evaluate the oxidation of dopamine at NiFe2O4–MWCNT modified-GCE the electrochemical behavior of dopamine at the NiFe2O 4–MWCNT modified-GCE was characterized in 0.1 mol L − 1 of phosphate buffer solution with different pH values (3.0–9.0) using differential pulse voltammetry (Fig. 5). Fig. 5A shows that the peak current of dopamine reached to a maximum value at pH 7.0 and then leveled off. The probable reason is that the number of negative carboxylate groups (COO− ) in the activated MWCNTs (on the electrode surface) increased with increasing the solution pH. Therefore, more positive dopamine cations were attracted to the electrode surface. Thus, the peak current increased with increasing the solution pH over the range of 5.0–7.0 (the pH values below acid dissociation constant, pK a = 8.9). Therefore, pH 7.0 was selected for further study. The effect of solution pH on the peak potential of dopamine was investigated too. It was found that the peak potentials (E) for the oxidation process of dopamine shifted negatively with increasing the solution pH as − 0.0545 V pH− 1 (Fig. 5). These results confirm that the ratio of participated protons to the transferred electrons through the NiFe2O4–MWCNTs is 1:1. The slope close to the theoretical value of 59 mV/pH was obtained in the investigated pH range indicating the 2e−/2H+ process [51].
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Fig. 8. The signal of dopamine in the presence of interfering compounds.
The effect of potential scan rate (between 10 and 100 mV s−1) on the peak current was investigated in the presence of 50.0 μmol L−1 dopamine at pH 7.0, using cyclic voltammetry. A linear relationship was observed between the peak current and the scan rate (ν) with a regression equation of Ip(μA) = (0.3571 ± 0.0417) ν (mV s−1) + (6.2103 ± 0.4657), (r2 = 0.9938). This relationship indicates that the oxidation of dopamine was an adsorption-controlled process. To study more about the adsorption of dopamine at NiFe2O4–MWCNT modified-GCE, the influences of accumulation potential and accumulation time on the peak current were evaluated (Fig. 6). The oxidation peak current of 30.0 μmol L− 1 dopamine at different accumulation potentials from − 0.10 V to + 0.20 V was assessed. The results showed (Fig. 6A) that by increasing the accumulation potential from − 0.05 V to 0.10 V the adsorption of dopamine was increased and thus increasing the oxidation peak current. When the potential reached near the oxidation potential of dopamine, the current intensity was decreased. This suggests that in this potential, dopamine was oxidized and it could not remain at the electrode surface. Therefore, + 0.10 V was selected as a suitable accumulation potential. Accumulation time has also affected the peak current. The results of this study showed that the oxidation peak current of 30.0 μmol L− 1 dopamine increased with increasing the accumulation time from 0 to 60 s and then leveled off (Fig. 6B).
This is due to the saturation of adsorbed dopamine at the surface of NiFe 2O 4 –MWCNT modified-GCE. Therefore, 60 s was chosen as a suitable accumulation time for further study. The optimum parameters for differential pulse voltammetry (DPV) were selected from the study (using 30.0 μmol L− 1 dopamine) for pulse amplitude, pulse width and scan rate in the range of 20–100 mV, 20–70 ms and 10–100 mV s−1, respectively. The obtained results indicated that the optimum parameters were pulse amplitude of 80 mV, pulse width of 40 ms and sweep rate of 20 mV s−1. 4. Figures of merit Differential pulse voltammetry was used to prepare the calibration curve. Under the optimized conditions, the electrode response to dopamine concentrations was checked at the surface of NiFe2O4–MWCNT modified-GCE (Fig. 7). The results showed that for the range of 0.05–6.0 μmol L−1 dopamine, the regression equation was Ip(μA) = (1.209 ± 0.032)CDopamine + (1.812 ± 0.069), (r2 = 0.996, n = 6), whereas for the range of 6.0–100 μmol L−1 dopamine, the regression equation was Ip(μA) = (0.083 ± 0.001)CDopamine + (8.629 ± 0.103), (r2 = 0.996, n = 6). As the results showed, the sensitivity (slope) of the electrode to dopamine at a lower concentration is better. This
Table 1 Determination of dopamine in real samples at pH 7.0 (n = 3). Sample a
Injection solution Urine Urine Urine Urine Plasma Plasma Plasma Plasma
Dopamine added (μmol L−1)
Dopamine found (μmol L−1)
Recovery (%)
Standard method (μmol L−1)
25.0 – 10.0 20.0 40.0 – 20.0 30.0 50.0
23.5 ± 0.3 bLimit of detection 9.7 ± 0.1 19.8 ± 0.4 39.0 ± 0.7 bLimit of detection 19.3 ± 0.2 29.1 ± 0.6 48.2 ± 1.1
94.0 – 97.0 98.9 97.5 – 96.5 97.0 96.4
24.1 ± 1.7 – 9.8 ± 0.8 20.2 ± 1.1 38.7 ± 1.3 – 20.6 ± 1.1 28.9 ± 1.3 51.2 ± 1.8
Urine and blood plasma samples were taken from women who were safe. a Dopamine injection solution: Caspian Tamin, 200 mg/5 mL, Pharmaceutical Co., Rasht, Iran.
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Table 2 Comparison of the proposed electrochemical sensor with the other reported method for the determination of dopamine. Technique
Limit of detection (μmol L−1)
Linear range (μmol L−1)
Interference compound
Reference
DPV SWV CV Chronoamperometry DPV DPV SWV DPV CV DPV DPV DPV DPV CV SWV DPV DPV DPV CV DPV CV CV CV CV LSV DPV LSV Amperometry DPASV
0.22 0.15 0.6 1.4 3.75 0.1 0.36 0.7 3.2 5.0 0.5 5.0 0.043 0.25 0.35 0.02 0.05 – 0.84 0.015 0.016 0.29 0.15 0.087 0.061 0.04 0.5 0.05 0.002
0.4–150 0.5–150 50–300 1.4–300 20–200 0.5–100 3.2–31.8 1–100 – 10–1100 1–2500 24–384 0.2–60 1-300 20–51 0.04–400 0.05–470 40–5000 0.9–10 0.075–20 30–100 5.0–280 0.4–20 0.1–900 0.5–25 0.1–9.8 2.0–8.0 0.08–600 0.006–0.29
– Not mentioned Not mentioned – – – – Not mentioned – – – – Not mentioned Not mentioned Not mentioned Cysteine Citric acid Not mentioned – Not mentioned – – – – – – Not mentioned – –
[10] [11] [12] [34] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63]
DPV: differential pulse voltammetry; CV: cyclic voltammetry; SWV: square wave voltammetry; LSV: linear sweep voltammetry; DPASV: differential pulse anodic stripping voltammetry.
is due to the fact that at low dopamine concentration, there is a monolayer of the analyte at the electrode surface, whereas at a higher concentration of the analyte, the analyte present at the second layer could not contact directly with the electrode surface, hence the sensitivity (slope) decreased. The detection limit was obtained as 0.02 μmol L− 1 dopamine, according to S/N = 3. The reproducibility and stability of the modified electrode were studied by replicate measurements of 0.50 and 30.0 μmol L−1 of dopamine using DPV under the optimum conditions. The relative standard deviation (RSD%) for ten successive assays of 0.50 and 30.0 μmol L−1 dopamine were 2.2% and 1.9%, respectively. When five different electrodes were used, the RSD% for five measurements of 0.50 and 30.0 μmol L−1 dopamine were 2.3% and 2.1%, respectively. These results confirmed that the modified electrode has excellent stability and reproducibility for the determination of dopamine.
5. Interference studies The selectivity of the new electrochemical sensor for dopamine detection was checked with different potential interfering compounds in the presence of dopamine at the NiFe2O4–MWCNT modified-GCE using DPV under the optimum conditions. The potential interfering compounds were added to a solution containing 30.0 μmol L−1 of dopamine in the buffer solution (pH 7.0). Then, the signal of the mixture was measured at the NiFe2O4–MWCNT modified-GCE. Tolerance limit was taken as the maximum concentration of the interfering substances, which caused a relative error of less than ±5% in the determination of dopamine. The results showed that 1000-fold of ascorbic acid, salicylic acid, tartaric acid, methionine, Na+, K+, Co2+, Ca2+, F−, Cl−, Br−, and NO− 3 , 500-fold of citric acid, cysteine, valine, and 200-fold of uric acid and urea did not affect the selectivity. Fig. 8 also showed the signal of dopamine (DA) in the presence of some interfering compounds. The obtained data showed that the proposed method is highly selective for dopamine determination.
6. Determination of dopamine in injection solution and biological fluids To evaluate the ability of the proposed sensor in real sample analysis, dopamine injection solution, human plasma and urine samples were prepared and analyzed. Standard addition method was used for measuring the dopamine contents in the samples. According to the obtained results, a good agreement can be seen between the proposed method and the standard method [67]. The data, given in Table 1, show the satisfactory results and confirm the capability of the modified GCE for voltammetric determination of dopamine in real samples. 7. Conclusion NiFe 2O 4 –MWCNT modified-GCE shows an excellent synergic behavior toward dopamine oxidation in an aqueous phosphate buffer (pH 7.0) solution. Our study showed that in the case of the unmodified GCE, the voltammograms of dopamine exhibit just a small hump peak, but after modification of the electrode with MWCNT/NiFe2O4, the oxidation peak current of dopamine is significantly enhanced. This technique offers a number of advantages compared to the other published electrochemical methods, such as simplicity in the preparation of the modified electrode and its high selectivity. The modified electrode represents appropriate performance in detecting dopamine and exhibits excellent stability and reproducibility. In the differential pulse voltammetric determination, the detection limit of dopamine was estimated as 0.02 μmol L − 1 . Based on the electrochemical oxidation, the quantitative determination of dopamine in pharmaceutical dosage and biological fluids samples was developed by a simple, rapid, selective and sensitive DPV technique. These properties indicate that the NiFe2O4–MWCNT modified electrode is a good electrochemical sensor for the determination of dopamine. Table 2 shows a comparison of the analytical figures of merit of the proposed method with different reported voltammetric techniques for the determination of dopamine
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