Research article Received: 17 March 2014,
Revised: 17 June 2014,
Accepted: 18 June 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2742
Sensitive determination of carbidopa through the electrochemiluminescence of luminol at graphene-modified electrodes Morteza Hosseini,a* Nooshin Mirzanasiri,b Morteza Rezapour,c Mohammad Hasan Sheikhha,d Farnoush Faridbod,b Parviz Norouzib and Mohammad Reza Ganjalib† ABSTRACT: Using the concept of electrogenerated chemiluminescence (ECL), a sensitive analytical method for the determination of carbidopa is described. Electro-oxidation of carbidopa on the surface of a graphene oxide (GO)-modified gold electrode (GE) leads to enhancement of the weak emission of oxidized luminol. Under optimum experimental conditions, the ECL signal increases linearly with increasing carbidopa concentrations over a range of 1.0 × 10-9–1.7 × 10-7 M, with a detection limit of 7.4 × 10-10 M. The proposed ECL method was successfully used for the determination of carbidopa in urine samples. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: carbidopa; graphene oxide; electrochemiluminescence; gold electrode; luminol
Introduction Electrogenerated chemiluminescence (also called electrochemiluminescence, ECL) involves the generation of species at electrode surfaces that emit light on undergoing electron-transfer reactions. ECL is a well-known, high-sensitivity detection method in which a chemiluminescence (CL) reaction is initiated and controlled by the application of an electrochemical potential (1,2). The use of ECL in analytical chemistry has grown rapidly in recent years (3–7), and ECL is an interesting method of detection that combines the advantages of luminescence and electrochemical techniques to provide a highly selective response. In ECL, the timing and spatial location of the luminescence reaction can be tightly controlled (3,4,7) and superior sensitivity may be achieved due to low background emission (3,4,7–9). Luminol (3-aminophthalhydrazide) is one of the earliest known synthetic compounds, and shows both CL and ECL phenomena. A promising and rapidly developing area of analytical ECL concerns luminol emission (10–14), which has a high ECL emission quantum yield, lower oxidizing potential and low cost. Also, this ECL reaction can occur in aqueous buffered solutions in the presence of oxygen and other impurities. Graphene oxide (GO) is a layered nanostructure with carbon atoms in pentagonal and hexagonal systems (15,16). As ideal electrode materials of large surface area (16), excellent conductivity (17), unique graphitized basal plane structure and low manufacturing costs (18), graphene and its composites have had an impact on the fields of electrochemical catalysis, sensing and biosensing (19,20). Parkinson’s disease is a neurological degenerative disorder of the extrapyramidal nervous system, the symptoms of which include tremor, rigidity, bradykinesia and loss of control of the skeletal muscular system. The disease is caused by a failure of the blood to produce enough dopamine (21). Dopamine cannot be administered directly because it is unable to cross
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the blood–brain barrier. Carbidopa is used as a source of dopamine because on entering the brain it is decarboxylated to dopamine by aromatic L-amino acid decarboxylase (22,23). The most commonly used methods for the analysis of carbidopa are spectrophotometry (24,25), high-performance liquid chromatography (HPLC) (26) spectrofluorimetry (27), gas chromatography (28), electrochemical methods (29) radioimmunoassay (30), chemiluminescence (31) and potentiometry (32–35). Here, it was found that a weak ECL signal was generated on the electrochemical oxidation of luminol at 0.4 V of electrolytic potential on the surface of an Au electrode in borax buffer (versus a Ag/AgCl reference electrode). The signal was greatly enhanced by the presence of carbidopa (Fig. 1) in the luminol solution. To the best of our knowledge, this effect of carbidopa on an ECL signal in aqueous solution has not been reported previously. Based on this finding, a sensitive, selective and rapid ECL method for the determination of carbidopa has been developed.
* Correspondence to: M. Hosseini, Department of Life Science Engineering, Faculty of New Sciences & Technologies, University of Tehran, Tehran, Iran. E-mail: [email protected] †
Current address: Biosensor Research Center, Endocrinology & Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran
a
Department of Life Science Engineering, University of Tehran, Tehran, Iran
b
Center of Excellence in Electrochemistry, University of Tehran, Tehran, Iran
c
Research Institute of the Petroleum Industry (RIPI), Tehran, Iran
d
Biotechnology Research Center, Shahid Sadoughi University of Medical Science, Yazd, Iran
Copyright © 2014 John Wiley & Sons, Ltd.
M. Hosseini et al.
Figure 1. Chemical structure of carbidopa.
Experimental Reagents and chemicals All chemicals were of analytical reagent grade and were used without further purification. Carbidopa was a gift from local pharmaceutical firm (Iran hormon). Luminol, sodium hydroxide and dimethylformamide were obtained from Merck (Darmstadt, Germany). A Nafion perfluorinated ion exchange (5% solution in 90% light alcohol) was obtained from Fluka (Buchs, Switzerland). A 1.0 × 10 2 M stock solution was prepared by dissolving luminol in a small amount of 0.1 M NaOH. Working solutions of luminol were prepared by diluting the stock solution with borax buffer solution (0.1 M, pH 9). A stock solution of carbidopa (1.0 × 10 2 M) was prepared in freshly distilled water. Synthesis of graphene oxide We used an improved method to oxidize graphite for the synthesis of GO (36). First, 3 g of graphite powder, 18 mL of nitric acid (67–70%) and 46 mL of sulfuric acid (98%) were mixed and stirred vigorously at 0–5°C for 15 min in a 500 mL reaction flask immersed in an ice-water bath. Then, 6 g of analytically pure potassium permanganate was added slowly to the above solution over a period of 15 min. After this, the suspended solution was stirred continuously for 2 h in an ice-water bath at 10–15°C, followed by continuous stirring at 35°C for 30 min. Subsequently, 138 mL of distilled water was added slowly to the suspension over a period 10 min and the temperature was held at 95–98°C for 30 min. The suspension was diluted by addition of 210 mL of warm distilled water (40°C) and treated with 18 mL of H2O2 (30%) to reduce any residual permanganate to soluble manganese ions. Finally, to obtain GO, the resulting suspension was filtered, washed with distilled water and dried in a vacuum oven at 60°C for 24 h. Reduction of exfoliated GO with hydrazine hydrate In a typical procedure, 100 mg of GO was placed into a 250 mL round-bottomed flask and 100 mL of water wasadded, yielding an inhomogeneous yellow–brown dispersion. This dispersion was sonicated until it became clear without any visible particulate matter. Hydrazine hydrate was then added and the solution was stirred for 24 h during which time the reduced GO gradually precipitated out as a black solid. This product was isolated by filtration over a medium fritted-glass funnel, washed with water and methanol, and dried on the funnel under a continuous air flow through the solid product cake (36). The scanning electron microscopy (SEM) image, Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) pattern of reduced GO are shown in Fig. 2.
Figure 2. (a) FTIR, (b) XRD and (c) SEM.
of the prepared GO suspension were cast on the surface of the GE and dried under an infrared (IR) lamp to prepare GO-modified GE (GE/GO). Then, 5 μL of Nafion solution (0.5%) was cast on the surface of GE to fabricate a carbidopa sensor (GE/GO/Nafion).
Preparation of carbidopa sensor
Apparatus
The surface of a gold electrode (GE) (8 × 8 mm) was polished successively with 0.3 and 0.1 μm alumina paste to obtain a mirror, and then cleaned ultrasonically in water. One milligram of GO was dispersed in 1 mL of dimethylformamide with ultrasonic agitation for 1 h to achieve a well-dispersed suspension. Five microliters
Cyclic voltammetry (CV) was performed using a PalmSens potentiostat–galvanostat (Ivium Tech., The Netherlands) with a conventional three-electrode set-up in which a GE/GO/Nafion, an Ag/AgCl/KCl(sat) electrode and a platinum wire served as the working, reference and auxiliary electrodes, respectively.
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Copyright © 2014 John Wiley & Sons, Ltd.
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Sensitive determination of carbidopa through electroluminescence Figure 4 shows the Nyquist plots (Zim vs. Zre) of the EIS experiments for bare GE and GE/GO/Nafion. Electrochemical impedance measurements were carried out in a background solution of 1 mM [Fe(CN)6]4 /3 in 0.1 M KCl. The frequency range was 10–105 Hz with a signal amplitude of 10 mV. In order to view GO immobilization on the gold electrode, Faradic impedance spectroscopy on a bare electrode and GE/GO/Nafion was used (Fig. 4). Important differences were observed in the electron transfer resistance (Rct) between the modified and bare gold electrodes. As shown in Fig. 4, the bare electrode exhibits no semicircle, but rather an almost straight line that is characteristic of a diffusion-limiting step in the electrochemical process. After modification of the electrode by GO, a semicircle can be observed, indicating an increase in Rct, from which we can conclude that GO is successfully absorbed on the surface of the GE.
Figure 3. Schematic diagram of the ECL system.
Electrochemical and ECL measurements were carried out in a 4 mL quartz cell (Fig. 3). The working electrode was mounted in an equatorial position in a quartz cell, with its surface directly in front of the window of a FL-win lab photomultiplier (Perkin-Elmer). The photodetector and ECL cell were enclosed in a light-tight black box. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a solution containing 1 mM [Fe(CN)6]3-/4- and 0.1 M KCl. The frequency ranged from 1 to 10,000 Hz, with signal amplitude of 10 mV. Low-angle XRD patterns were recorded with a Philips X Pert MPD diffractometer (Eindhoven, The Netherlands) using CuKα radiation (40 kV, 40 mA) at a step width of 0.02°. N2 adsorption– desorption isotherms were measured using a BELSORP mini-II (Japan). FTIR spectra were recorded within a 4000–400 cm 1 region on a Bruker Vector 22 infrared spectrophotometer (Australia). SEM analysis was performed on a Philips XL-30 field-emission SEM operated at 16 kV. A pH meter (Metrohm, Switzerland) was used for pH adjustments. All measurements were performed at room temperature.
Results and discussion To investigate the performance of the ECL method as a sensor, EIS was carried out at different stages, together with modification of the impedance of the electrode surface (9,37).
4 /3
Figure 4. Nyquist plots of 1 mM Fe(CN)6
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Electrochemical and ECL behaviors of luminol on GE/GO/Nafion Figure 5(A) shows the CVs of luminol at the GE and GE/GO/ Nafion over the potential range of 0.0 and 0.8 V versus Ag/AgCl/ KCl(sat) at a scan rate of 100 mV/s. As shown in Fig. 5(A), no redox peaks are observed for luminol at the GE. Modification of GE by GO caused a significant increase in the intensity of the anodic peak, indicating an anodic peak current for luminol and also a peak in the charging current (Fig. 5B). The observed high charging currents at the GE and the GE/GO/Nafion are attributed to an increase in the electrode surface area due to the presence of GO and also to the presence of a resistive layer of Nafion on the electrode surface (9). Similar enhancement was also observed in their corresponding ECL signals (Fig. 5B). The intensity of the ECL signal for luminol increased 3.0-fold on modification of the GE with GO. The electrochemical and ECL behaviors of luminol at the GE/GO/Nafion were examined in the presence of carbidopa. The oxidation of luminol at the electrode surface produces a luminol radical anion. In the presence of levodopa, the reaction between the luminol radical anion and H2O2, the product of the oxidation reaction of levodopa, at the electrode surface also produces the excited state 3-aminophthalate anion, and enhancement of the ECL emission at 425 nm is observed. This mechanism has been previously reported (38,39).
couple in 0.1 M KCl solution at (a) GE/Nafion and (b) GE/GO/ Nafion.
Copyright © 2014 John Wiley & Sons, Ltd.
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M. Hosseini et al. 25 b
20 15 a
I/µA
10 5 0 -5 -10 -15
0
0.2
0.4
0.6
0.8
1
E/V
Figure 5. (A) Cyclic voltammograms and (B) corresponding ECL responses of luminol at a GE/Nafion (a) and GE/GO/Nafion (b). Conditions: luminol, 100 μM; supporting electrolyte, 0.1 M borate buffer pH 9; potential scan rate, 100 mV/s.
As shown in Fig. 6, both the oxidation current (Fig. 6A) and the ECL intensity (Fig. 6Bb) increase on addition of carbidopa to the reaction medium. It would appear that the electrocatalytic and ECL activities of GO on the surface of the GE create more active sites for the catalytic redox reaction.
Figure 6. (A) Cyclic voltammograms and (B) corresponding ECL responses of luminol in the absence (a) and presence (b) of carbidopa at GE/GO/Nafion. Conditions: luminol, 100 μM; carbidopa 0.1 μM; supporting electrolyte, 0.1 M borate buffer pH 9; potential scan rate, 100 mV/s.
70
Optimizing of the performance of the ECL biosensor
Effect of pH. The relationship between ECL intensity and pH was investigated over a pH range of 8.0–11.0. Figure 7 shows that the ECL intensity increased considerably with the increase in pH from 8.0 to 9.0. By contrast, when the pH of the buffer solution was > 9.0, the ECL intensity of carbidopa decreased. Thus, borax buffer solution at pH 9.0 was chosen for the ECL reaction, because the most satisfactory response was found at pH 9.0. Effect of GO. The effect of GO loading on the intensity of the ECL signal was evaluated by molding different amounts of GO suspension onto the surface of the GE. It is obvious that the amount of GO on the surface of the electrode is a dominant factor in the oxidation of luminol and therefore on the ECL response. The ECL signal increased as the amount of GO increased, reaching a plateau at 7 mg of GO (1 mg/mL), which was therefore used in fabrication of the sensor. Meanwhile, the effect of the loading Nafion on the intensity of the ECL signal was investigated by applying different amounts of
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60
ECL Intenisty
In order to optimize the performance of the proposed ECL sensor, GE/GO/Nafion, towards carbidopa detection, the effect of pH, GO loading and Nafion on the electrode surface, luminol concentration and scan rate on the intensity of the ECL signal was investigated.
50 40 30 20 10 0 7.5
8.5
9.5
10.5
11.5
pH Figure 7. Effect of pH on the ECL intensity.
Nafion during sensor fabrication. ECL intensity increased gradually on increasing the concentration of Nafion, reaching a maximum at ~ 0.5% (v/v) Nafion; the ECL intensity then decreased. Effect of luminol concentration. Studies on the effect of luminol concentration on the intensity of the ECL signal showed that in the presence of 0.1 μM carbidopa, the signal increased linearly at luminol concentrations between 0.1 and 100 μM. Any further increase in the luminol concentration (up to 200 μM) did not lead to further enhancement in the ECL intensity of the sensor.
Copyright © 2014 John Wiley & Sons, Ltd.
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Sensitive determination of carbidopa through electroluminescence However, the background ECL signal of luminol increased continuously at luminol concentrations from 1 to 200 μM. Therefore, 100 μM was selected as the optimum concentration of luminol for determining carbidopa. Effect of scan rate. The effect of scan rate (v) on the ECL and CV signals of luminol was investigated in the presence of 100 μM carbidopa. The intensity of the ECL signal for luminol reached a maximum at ~ 100 mV/s. At the same time, CVs of luminol at scan rates between 10 and 100 mV/s showed that the anodic peak current increased linearly with v1/2, revealing a diffusioncontrolled redox process (Fig. 8). A scan rate of 100 mV/s was selected for further experiments because maximum ECL sensitivity was achieved at this rate. Interference studies. The influence of some common foreign species on the determination of carbidopa was studied under the optimum experimental conditions given above. The tolerable limit for a foreign species was taken as a relative error of not more than ± 5% in the CL signal of carbidopa. No interference was found with up to 1000-fold Mg2+, K+ and Cl-, 500-fold glucose, 300-fold L-methionine, citric acid, glycine and L-alanine or 100-fold ascorbic acid, dopamine and serotonin.
-9
-8
Figure 9. (A) ECL responses of luminol (100) in the presence of 1.0 × 10 , 1.1 × 10 , -8 -8 -8 -8 -7 -7 -7 3.1 × 10 , 5.1 × 10 , 7.1 × 10 , 9.1 × 10 , 1.1 × 10 , 1.4 × 10 and 1.7 × 10 M carbidopa. (Inset) Linear relationship between ECL intensity and the concentration of carbidopa.
90
Analytical performance
80 70 60
ECL int
Under the optimum conditions given above, the response to carbidopa was linear over the range 1.0 × 10-9 to 1.7 × 10-7 M, with a detection limit of 7.4 × 10-10 M (S/N = 3). The regression equation was: ΔECL = 6.0 × 108C + 19.054 (R2 = 0.999). Figure 9 shows typical calibration traces recorded for carbidopa using the proposed ECL sensor; the relative standard deviation (RSD) was < 4% for the determination of 1.0 × 10-8 mol-1 carbidopa (n = 7). The reproducibility of the sensor for six equivalent electrodes used on different days was 8% (Fig. 10). In addition, when the proposed sensor was used for 1 h per day (after each measurement the electrode should be dried and kept at room temperature), it was found that the proposed sensor could be used for at least 6 weeks without any measurable change in ECL intensity.
50 40 30 20 10 0
0
100
200
300
400
500
Time(s) Figure 10. Successive cyclic ECL responses of the sensor towards 0.1 μM carbidopa over 10 cycles. Conditions: supporting electrolyte, 0.1 M borate buffer pH 9; scan rate, 100 mV/s.
Table 1. The application of proposed method for determination of Caridopa in urine samples Sample 1 2 3
Spiked (M)
Founda (ppm)
Relative error (%)
0,03 0.05 0.09
0.029 ± 0.003 0.051 ± 0.005 0.088 ± 0.003
5.0 3.3 3.3
The reults are based on five replicate mesurements
a
Analytical application
Figure 8. Cyclic voltammograms of luminol (100 μM) in the presence of carbidopa (0.1 μM) on GE/GO/Nafion at different scan rates. (Inset) Plot of peak current versus the square root of scan rate. Conditions: supporting electrolyte, 0.1 M borate buffer pH 9; scan rates from inner to outer, 10, 20, 50, 75 and 100 mV/s.
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To evaluate its applicability to real samples, the proposed method was applied to the determination of carbidopa in urine samples. The carbidopa contents were measured after sample preparation using the standard addition method. The results are given in Table 1 and are in satisfactory agreement with the amounts added.
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M. Hosseini et al.
Conclusions It was found that at an electrolytic potential of 0.6 V (versus Ag/AgCl reference electrode), a weak ECL signal was generated by the electrochemical oxidation of luminol on the surface of an Au electrode modified by GO nanosheets. This signal was greatly enhanced by carbidopa when present in a luminol solution. Based on this finding, a sensitive, selective and rapid ECL method for the determination of carbidopa has been developed. Acknowledgements The authors are grateful to the Research Council of University of Tehran for the financial support of this work.
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Revised: 17 June 2014,
Accepted: 18 June 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2742
Sensitive determination of carbidopa through the electrochemiluminescence of luminol at graphene-modified electrodes Morteza Hosseini,a* Nooshin Mirzanasiri,b Morteza Rezapour,c Mohammad Hasan Sheikhha,d Farnoush Faridbod,b Parviz Norouzib and Mohammad Reza Ganjalib† ABSTRACT: Using the concept of electrogenerated chemiluminescence (ECL), a sensitive analytical method for the determination of carbidopa is described. Electro-oxidation of carbidopa on the surface of a graphene oxide (GO)-modified gold electrode (GE) leads to enhancement of the weak emission of oxidized luminol. Under optimum experimental conditions, the ECL signal increases linearly with increasing carbidopa concentrations over a range of 1.0 × 10-9–1.7 × 10-7 M, with a detection limit of 7.4 × 10-10 M. The proposed ECL method was successfully used for the determination of carbidopa in urine samples. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: carbidopa; graphene oxide; electrochemiluminescence; gold electrode; luminol
Introduction Electrogenerated chemiluminescence (also called electrochemiluminescence, ECL) involves the generation of species at electrode surfaces that emit light on undergoing electron-transfer reactions. ECL is a well-known, high-sensitivity detection method in which a chemiluminescence (CL) reaction is initiated and controlled by the application of an electrochemical potential (1,2). The use of ECL in analytical chemistry has grown rapidly in recent years (3–7), and ECL is an interesting method of detection that combines the advantages of luminescence and electrochemical techniques to provide a highly selective response. In ECL, the timing and spatial location of the luminescence reaction can be tightly controlled (3,4,7) and superior sensitivity may be achieved due to low background emission (3,4,7–9). Luminol (3-aminophthalhydrazide) is one of the earliest known synthetic compounds, and shows both CL and ECL phenomena. A promising and rapidly developing area of analytical ECL concerns luminol emission (10–14), which has a high ECL emission quantum yield, lower oxidizing potential and low cost. Also, this ECL reaction can occur in aqueous buffered solutions in the presence of oxygen and other impurities. Graphene oxide (GO) is a layered nanostructure with carbon atoms in pentagonal and hexagonal systems (15,16). As ideal electrode materials of large surface area (16), excellent conductivity (17), unique graphitized basal plane structure and low manufacturing costs (18), graphene and its composites have had an impact on the fields of electrochemical catalysis, sensing and biosensing (19,20). Parkinson’s disease is a neurological degenerative disorder of the extrapyramidal nervous system, the symptoms of which include tremor, rigidity, bradykinesia and loss of control of the skeletal muscular system. The disease is caused by a failure of the blood to produce enough dopamine (21). Dopamine cannot be administered directly because it is unable to cross
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the blood–brain barrier. Carbidopa is used as a source of dopamine because on entering the brain it is decarboxylated to dopamine by aromatic L-amino acid decarboxylase (22,23). The most commonly used methods for the analysis of carbidopa are spectrophotometry (24,25), high-performance liquid chromatography (HPLC) (26) spectrofluorimetry (27), gas chromatography (28), electrochemical methods (29) radioimmunoassay (30), chemiluminescence (31) and potentiometry (32–35). Here, it was found that a weak ECL signal was generated on the electrochemical oxidation of luminol at 0.4 V of electrolytic potential on the surface of an Au electrode in borax buffer (versus a Ag/AgCl reference electrode). The signal was greatly enhanced by the presence of carbidopa (Fig. 1) in the luminol solution. To the best of our knowledge, this effect of carbidopa on an ECL signal in aqueous solution has not been reported previously. Based on this finding, a sensitive, selective and rapid ECL method for the determination of carbidopa has been developed.
* Correspondence to: M. Hosseini, Department of Life Science Engineering, Faculty of New Sciences & Technologies, University of Tehran, Tehran, Iran. E-mail: [email protected] †
Current address: Biosensor Research Center, Endocrinology & Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran
a
Department of Life Science Engineering, University of Tehran, Tehran, Iran
b
Center of Excellence in Electrochemistry, University of Tehran, Tehran, Iran
c
Research Institute of the Petroleum Industry (RIPI), Tehran, Iran
d
Biotechnology Research Center, Shahid Sadoughi University of Medical Science, Yazd, Iran
Copyright © 2014 John Wiley & Sons, Ltd.
M. Hosseini et al.
Figure 1. Chemical structure of carbidopa.
Experimental Reagents and chemicals All chemicals were of analytical reagent grade and were used without further purification. Carbidopa was a gift from local pharmaceutical firm (Iran hormon). Luminol, sodium hydroxide and dimethylformamide were obtained from Merck (Darmstadt, Germany). A Nafion perfluorinated ion exchange (5% solution in 90% light alcohol) was obtained from Fluka (Buchs, Switzerland). A 1.0 × 10 2 M stock solution was prepared by dissolving luminol in a small amount of 0.1 M NaOH. Working solutions of luminol were prepared by diluting the stock solution with borax buffer solution (0.1 M, pH 9). A stock solution of carbidopa (1.0 × 10 2 M) was prepared in freshly distilled water. Synthesis of graphene oxide We used an improved method to oxidize graphite for the synthesis of GO (36). First, 3 g of graphite powder, 18 mL of nitric acid (67–70%) and 46 mL of sulfuric acid (98%) were mixed and stirred vigorously at 0–5°C for 15 min in a 500 mL reaction flask immersed in an ice-water bath. Then, 6 g of analytically pure potassium permanganate was added slowly to the above solution over a period of 15 min. After this, the suspended solution was stirred continuously for 2 h in an ice-water bath at 10–15°C, followed by continuous stirring at 35°C for 30 min. Subsequently, 138 mL of distilled water was added slowly to the suspension over a period 10 min and the temperature was held at 95–98°C for 30 min. The suspension was diluted by addition of 210 mL of warm distilled water (40°C) and treated with 18 mL of H2O2 (30%) to reduce any residual permanganate to soluble manganese ions. Finally, to obtain GO, the resulting suspension was filtered, washed with distilled water and dried in a vacuum oven at 60°C for 24 h. Reduction of exfoliated GO with hydrazine hydrate In a typical procedure, 100 mg of GO was placed into a 250 mL round-bottomed flask and 100 mL of water wasadded, yielding an inhomogeneous yellow–brown dispersion. This dispersion was sonicated until it became clear without any visible particulate matter. Hydrazine hydrate was then added and the solution was stirred for 24 h during which time the reduced GO gradually precipitated out as a black solid. This product was isolated by filtration over a medium fritted-glass funnel, washed with water and methanol, and dried on the funnel under a continuous air flow through the solid product cake (36). The scanning electron microscopy (SEM) image, Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) pattern of reduced GO are shown in Fig. 2.
Figure 2. (a) FTIR, (b) XRD and (c) SEM.
of the prepared GO suspension were cast on the surface of the GE and dried under an infrared (IR) lamp to prepare GO-modified GE (GE/GO). Then, 5 μL of Nafion solution (0.5%) was cast on the surface of GE to fabricate a carbidopa sensor (GE/GO/Nafion).
Preparation of carbidopa sensor
Apparatus
The surface of a gold electrode (GE) (8 × 8 mm) was polished successively with 0.3 and 0.1 μm alumina paste to obtain a mirror, and then cleaned ultrasonically in water. One milligram of GO was dispersed in 1 mL of dimethylformamide with ultrasonic agitation for 1 h to achieve a well-dispersed suspension. Five microliters
Cyclic voltammetry (CV) was performed using a PalmSens potentiostat–galvanostat (Ivium Tech., The Netherlands) with a conventional three-electrode set-up in which a GE/GO/Nafion, an Ag/AgCl/KCl(sat) electrode and a platinum wire served as the working, reference and auxiliary electrodes, respectively.
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Luminescence 2014
Sensitive determination of carbidopa through electroluminescence Figure 4 shows the Nyquist plots (Zim vs. Zre) of the EIS experiments for bare GE and GE/GO/Nafion. Electrochemical impedance measurements were carried out in a background solution of 1 mM [Fe(CN)6]4 /3 in 0.1 M KCl. The frequency range was 10–105 Hz with a signal amplitude of 10 mV. In order to view GO immobilization on the gold electrode, Faradic impedance spectroscopy on a bare electrode and GE/GO/Nafion was used (Fig. 4). Important differences were observed in the electron transfer resistance (Rct) between the modified and bare gold electrodes. As shown in Fig. 4, the bare electrode exhibits no semicircle, but rather an almost straight line that is characteristic of a diffusion-limiting step in the electrochemical process. After modification of the electrode by GO, a semicircle can be observed, indicating an increase in Rct, from which we can conclude that GO is successfully absorbed on the surface of the GE.
Figure 3. Schematic diagram of the ECL system.
Electrochemical and ECL measurements were carried out in a 4 mL quartz cell (Fig. 3). The working electrode was mounted in an equatorial position in a quartz cell, with its surface directly in front of the window of a FL-win lab photomultiplier (Perkin-Elmer). The photodetector and ECL cell were enclosed in a light-tight black box. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a solution containing 1 mM [Fe(CN)6]3-/4- and 0.1 M KCl. The frequency ranged from 1 to 10,000 Hz, with signal amplitude of 10 mV. Low-angle XRD patterns were recorded with a Philips X Pert MPD diffractometer (Eindhoven, The Netherlands) using CuKα radiation (40 kV, 40 mA) at a step width of 0.02°. N2 adsorption– desorption isotherms were measured using a BELSORP mini-II (Japan). FTIR spectra were recorded within a 4000–400 cm 1 region on a Bruker Vector 22 infrared spectrophotometer (Australia). SEM analysis was performed on a Philips XL-30 field-emission SEM operated at 16 kV. A pH meter (Metrohm, Switzerland) was used for pH adjustments. All measurements were performed at room temperature.
Results and discussion To investigate the performance of the ECL method as a sensor, EIS was carried out at different stages, together with modification of the impedance of the electrode surface (9,37).
4 /3
Figure 4. Nyquist plots of 1 mM Fe(CN)6
Luminescence 2014
Electrochemical and ECL behaviors of luminol on GE/GO/Nafion Figure 5(A) shows the CVs of luminol at the GE and GE/GO/ Nafion over the potential range of 0.0 and 0.8 V versus Ag/AgCl/ KCl(sat) at a scan rate of 100 mV/s. As shown in Fig. 5(A), no redox peaks are observed for luminol at the GE. Modification of GE by GO caused a significant increase in the intensity of the anodic peak, indicating an anodic peak current for luminol and also a peak in the charging current (Fig. 5B). The observed high charging currents at the GE and the GE/GO/Nafion are attributed to an increase in the electrode surface area due to the presence of GO and also to the presence of a resistive layer of Nafion on the electrode surface (9). Similar enhancement was also observed in their corresponding ECL signals (Fig. 5B). The intensity of the ECL signal for luminol increased 3.0-fold on modification of the GE with GO. The electrochemical and ECL behaviors of luminol at the GE/GO/Nafion were examined in the presence of carbidopa. The oxidation of luminol at the electrode surface produces a luminol radical anion. In the presence of levodopa, the reaction between the luminol radical anion and H2O2, the product of the oxidation reaction of levodopa, at the electrode surface also produces the excited state 3-aminophthalate anion, and enhancement of the ECL emission at 425 nm is observed. This mechanism has been previously reported (38,39).
couple in 0.1 M KCl solution at (a) GE/Nafion and (b) GE/GO/ Nafion.
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20 15 a
I/µA
10 5 0 -5 -10 -15
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Figure 5. (A) Cyclic voltammograms and (B) corresponding ECL responses of luminol at a GE/Nafion (a) and GE/GO/Nafion (b). Conditions: luminol, 100 μM; supporting electrolyte, 0.1 M borate buffer pH 9; potential scan rate, 100 mV/s.
As shown in Fig. 6, both the oxidation current (Fig. 6A) and the ECL intensity (Fig. 6Bb) increase on addition of carbidopa to the reaction medium. It would appear that the electrocatalytic and ECL activities of GO on the surface of the GE create more active sites for the catalytic redox reaction.
Figure 6. (A) Cyclic voltammograms and (B) corresponding ECL responses of luminol in the absence (a) and presence (b) of carbidopa at GE/GO/Nafion. Conditions: luminol, 100 μM; carbidopa 0.1 μM; supporting electrolyte, 0.1 M borate buffer pH 9; potential scan rate, 100 mV/s.
70
Optimizing of the performance of the ECL biosensor
Effect of pH. The relationship between ECL intensity and pH was investigated over a pH range of 8.0–11.0. Figure 7 shows that the ECL intensity increased considerably with the increase in pH from 8.0 to 9.0. By contrast, when the pH of the buffer solution was > 9.0, the ECL intensity of carbidopa decreased. Thus, borax buffer solution at pH 9.0 was chosen for the ECL reaction, because the most satisfactory response was found at pH 9.0. Effect of GO. The effect of GO loading on the intensity of the ECL signal was evaluated by molding different amounts of GO suspension onto the surface of the GE. It is obvious that the amount of GO on the surface of the electrode is a dominant factor in the oxidation of luminol and therefore on the ECL response. The ECL signal increased as the amount of GO increased, reaching a plateau at 7 mg of GO (1 mg/mL), which was therefore used in fabrication of the sensor. Meanwhile, the effect of the loading Nafion on the intensity of the ECL signal was investigated by applying different amounts of
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ECL Intenisty
In order to optimize the performance of the proposed ECL sensor, GE/GO/Nafion, towards carbidopa detection, the effect of pH, GO loading and Nafion on the electrode surface, luminol concentration and scan rate on the intensity of the ECL signal was investigated.
50 40 30 20 10 0 7.5
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pH Figure 7. Effect of pH on the ECL intensity.
Nafion during sensor fabrication. ECL intensity increased gradually on increasing the concentration of Nafion, reaching a maximum at ~ 0.5% (v/v) Nafion; the ECL intensity then decreased. Effect of luminol concentration. Studies on the effect of luminol concentration on the intensity of the ECL signal showed that in the presence of 0.1 μM carbidopa, the signal increased linearly at luminol concentrations between 0.1 and 100 μM. Any further increase in the luminol concentration (up to 200 μM) did not lead to further enhancement in the ECL intensity of the sensor.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Sensitive determination of carbidopa through electroluminescence However, the background ECL signal of luminol increased continuously at luminol concentrations from 1 to 200 μM. Therefore, 100 μM was selected as the optimum concentration of luminol for determining carbidopa. Effect of scan rate. The effect of scan rate (v) on the ECL and CV signals of luminol was investigated in the presence of 100 μM carbidopa. The intensity of the ECL signal for luminol reached a maximum at ~ 100 mV/s. At the same time, CVs of luminol at scan rates between 10 and 100 mV/s showed that the anodic peak current increased linearly with v1/2, revealing a diffusioncontrolled redox process (Fig. 8). A scan rate of 100 mV/s was selected for further experiments because maximum ECL sensitivity was achieved at this rate. Interference studies. The influence of some common foreign species on the determination of carbidopa was studied under the optimum experimental conditions given above. The tolerable limit for a foreign species was taken as a relative error of not more than ± 5% in the CL signal of carbidopa. No interference was found with up to 1000-fold Mg2+, K+ and Cl-, 500-fold glucose, 300-fold L-methionine, citric acid, glycine and L-alanine or 100-fold ascorbic acid, dopamine and serotonin.
-9
-8
Figure 9. (A) ECL responses of luminol (100) in the presence of 1.0 × 10 , 1.1 × 10 , -8 -8 -8 -8 -7 -7 -7 3.1 × 10 , 5.1 × 10 , 7.1 × 10 , 9.1 × 10 , 1.1 × 10 , 1.4 × 10 and 1.7 × 10 M carbidopa. (Inset) Linear relationship between ECL intensity and the concentration of carbidopa.
90
Analytical performance
80 70 60
ECL int
Under the optimum conditions given above, the response to carbidopa was linear over the range 1.0 × 10-9 to 1.7 × 10-7 M, with a detection limit of 7.4 × 10-10 M (S/N = 3). The regression equation was: ΔECL = 6.0 × 108C + 19.054 (R2 = 0.999). Figure 9 shows typical calibration traces recorded for carbidopa using the proposed ECL sensor; the relative standard deviation (RSD) was < 4% for the determination of 1.0 × 10-8 mol-1 carbidopa (n = 7). The reproducibility of the sensor for six equivalent electrodes used on different days was 8% (Fig. 10). In addition, when the proposed sensor was used for 1 h per day (after each measurement the electrode should be dried and kept at room temperature), it was found that the proposed sensor could be used for at least 6 weeks without any measurable change in ECL intensity.
50 40 30 20 10 0
0
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Time(s) Figure 10. Successive cyclic ECL responses of the sensor towards 0.1 μM carbidopa over 10 cycles. Conditions: supporting electrolyte, 0.1 M borate buffer pH 9; scan rate, 100 mV/s.
Table 1. The application of proposed method for determination of Caridopa in urine samples Sample 1 2 3
Spiked (M)
Founda (ppm)
Relative error (%)
0,03 0.05 0.09
0.029 ± 0.003 0.051 ± 0.005 0.088 ± 0.003
5.0 3.3 3.3
The reults are based on five replicate mesurements
a
Analytical application
Figure 8. Cyclic voltammograms of luminol (100 μM) in the presence of carbidopa (0.1 μM) on GE/GO/Nafion at different scan rates. (Inset) Plot of peak current versus the square root of scan rate. Conditions: supporting electrolyte, 0.1 M borate buffer pH 9; scan rates from inner to outer, 10, 20, 50, 75 and 100 mV/s.
Luminescence 2014
To evaluate its applicability to real samples, the proposed method was applied to the determination of carbidopa in urine samples. The carbidopa contents were measured after sample preparation using the standard addition method. The results are given in Table 1 and are in satisfactory agreement with the amounts added.
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M. Hosseini et al.
Conclusions It was found that at an electrolytic potential of 0.6 V (versus Ag/AgCl reference electrode), a weak ECL signal was generated by the electrochemical oxidation of luminol on the surface of an Au electrode modified by GO nanosheets. This signal was greatly enhanced by carbidopa when present in a luminol solution. Based on this finding, a sensitive, selective and rapid ECL method for the determination of carbidopa has been developed. Acknowledgements The authors are grateful to the Research Council of University of Tehran for the financial support of this work.
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