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Fabrication of poly (sudan III) modified carbon paste electrode sensor for dopamine: A voltammetric study
Journal of Electroanalytical Chemistry 834 (2019) 71–78
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Fabrication of poly (sudan III) modified carbon paste electrode sensor for dopamine: A voltammetric study T.S. Sunil Kumar Naik, Muthui Martin Mwaurah, B.E. Kumara Swamy
T
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Department of P. G. Studies and Research in Industrial Chemistry, Kuvempu University, Jnanasahyadri, Shankaraghatta - 577451, Shivamoga (D), Karnataka (S), India
ARTICLE INFO
ABSTRACT
Keywords: Sudan (III) Electropolymerization Dopamine Uric acid Ascorbic acid Simultaneous determination Carbon paste electrode
Owing to the challenges faced in simultaneous determination of dopamine(DA), uric acid(UA) and ascorbic acid (AA) in mixtures by electrochemical method, the present work is aimed at developing a simple, sensitive and selective sensor using sudan III as the modifier. The poly (sudan III) modified carbon paste electrode (PS/MCPE) was prepared by electropolymerization of sudan III monomer on the surface of bare carbon paste electrode (BCPE). Electrochemical sensing properties of poly (sudan III) MCPE were evaluated by cyclic voltammetry for individual analytes and findings revealed that it has a good electrocatalytic activity towards the determination of dopamine (DA), uric acid (UA) and ascorbic acid (AA) characterized by a low detection limit of 9.3 × 10−6 M, 1 × 10−5 M and 4.5 × 10−5 M for DA, UA and AA, respectively. The scan rate study reveals that the electrode process is adsorption-controlled. The simultaneous determination of the analytes in a mixture showed that the modified electrode exhibits an outstanding selectivity towards DA, UA and AA with their distinct oxidation potentials occurring at 0.14 V, 0.27 V, and −0.07 V, respectively. The fabricated modified electrode sensor exhibited excellent electrocatalytic properties towards the determination of DA, UA and AA with satisfactory results.
1. Introduction Dopamine (DA), is a monoamine neurotransmitter belongs to catecholamine family [1,2]. It is found in both mammalian central nervous system as well as peripheral nervous system, moreover it's majorly dominant in the corpus stratum of the brain [3,4]. Dopaminergic neurotransmission helps to regulate motion, emotions, motivation, learning and cognition. Abnormal levels of DA in the body results to neurological disorders such as Parkinson's, Alzheimer's and Huntington's diseases, Tourett'e syndrome, schizophrenia and psychosis [5–8]. There has also been documentation of its involvement in manifestation of HIV [9] and drug addiction, thus DA determination is a field of great interest in understanding drug and substance abuse [10]. Determination of DA in biological systems with desired sensitivity and accuracy is very vital in the diagnosis of aforesaid diseases as well as clinical applications in treatment of hypotension and bradycardia [11–18]. Currently, methods employed for the determination of DA include fluorimetry, high-performance liquid chromatography, capillary electrophoresis, flow injection, chemiluminescence and electrochemistry [19]. In comparison to other analytical techniques, electrochemical technique for the determination of dopamine has gained prominence due to its simplicity, sensitivity, fast response, low cost, adaptability and its eco-friendly ⁎
nature [20]. However, dopamine detection is challenging since its oxidation potential is very close to that of interfering molecules such as ascorbic acid which cause overlapping of their oxidation signals at bare carbon paste electrode. Therefore, the oxidation peak potentials of these three electroactive molecules need to be separated from each other for selective and accurate analysis of dopamine [21,22]. This is achieved through chemical modification of the electrode. Furthermore, chemical modification of carbon paste electrode solves the problem of electrode fouling which originate from adsorption of oxidation products of dopamine and other electroactive species on the electrode surface [23,24]. Numerous materials, such as carbon-based nanomaterials, organic redox mediators, ionic liquids, biomaterials, metal nanoparticles, metal oxides, and conductive polymers have been used as modifiers to construct highly sensitive and selective biosensors for dopamine based on carbon paste electrodes [25–30]. However, novel materials for electrode modification are still demanded in attempt to develop superior electrochemical sensors for dopamine. Uric acid (UA) is the end product of purine catabolism in mammals. Hyperuricaemia, a medical condition characterized by excess uric acid in serum is evident in patients with reduced glomerular filtration rate [31]. Prolonged hyperuricaemia causes nucleation and growth of monosodium urate (MSU) crystals in tissues and joints leading to gout [32]. Aside from
Corresponding author. E-mail address: [email protected] (B.E. Kumara Swamy).
https://doi.org/10.1016/j.jelechem.2018.12.054 Received 29 September 2018; Received in revised form 24 December 2018; Accepted 26 December 2018 Available online 28 December 2018 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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gout, abnormal uric acid levels causes other disorders including Fanconi syndrome, Lesch-Nyhan syndrome and renal-disease [33]. Ascorbic acid (AA) also known as vitamin C is distributed widely in both the plant and animal kingdoms. In animals, large amounts of ascorbic acid are evident in leukocytes, liver and anterior pituitary gland and it plays a vital role in wound healing and enhancing the immune system [34,35]. In animals, AA exists as collagen protein which is responsible to maintain bones, blood vessels, and skin. In vegetable cells, ascorbic acid occurs in unbounded state as well as protein-bound form known as ascorbigen. AA is also incorporated in industrial manufacture of animal feeds, pharmaceutical formulations and cosmetic products as an antioxidant [36]. Excess AA concentration in the body may lead to diarrhea, nausea, skin irritation, burning upon urination, and depletion of copper, hence its determination in biological fluids is vital. In biological samples, DA, UA, and AA co-exist in varied concentrations. AA is usually in higher concentrations (about 0.1 mM) while DA has a lower concentration (0.01–1 μM) [37]. As the oxidation potentials of these species are very close, electrochemical interference from each other is often witnessed, thus the ability to selectively and accurately determine these species in a mixed solution is a subject of great significance for both diagnostic research and analytical application. The electrooxidation reactions of DA, UA and AA are shown in the Scheme 1, Scheme 2 and Scheme 3, respectively. Over the years, polymer film modified electrodes have gained much attention in constructing electrochemical sensors and biosensors due of their handy advantages such as selectivity, electrochemical reversibility, low cost, good stability, reproducibility, more active sites, homogeneity in electrochemical deposition and strong adherence to the electrode surface [38,39]. Several polymer films have been applied for the determination of dopamine; Zhang et al. [40] described polymerization of L-lysine on electrode surface which could separate the redox peaks of dopamine, ascorbic acid, and uric acid hence it was utilized in simultaneous electrochemical determination of the aforementioned biomolecules. Additionally, there are other reports of using naphthol green B, pyrrole-3-carboxylic acid, glycine, calmagite, Eriochrome Black-T, Evans Blue [41–46] as monomers for modification of carbon paste electrode by electropolymerization. Sudan III dye also known as (1-(4-phenylazophenylazo)-2-naphthol) is a non-ionic lipophilic azo-aromatic compound that is widely used as a colorant in numerous chemical industries which include oils, plastics, waxes, petrol, printing floor polishing, and spirit varnishing due to its intense red color, low cost and stability [47–50]. In addition sudan III is an electroactive molecule and can undergo polymerization to produce an electrically active polymer. The structure of (sudan III) is shown in Scheme 4. The present work describes the electropolymerization of sudan (III) on the surface of carbon paste electrode by cyclic voltammetry to yield a conducting polymer possessing distinctive electrocatalytic properties towards the determination of dopamine, uric acid and ascorbic acid by enhancing their redox peaks currents at low concentrations and lowering their oxidation potentials.
Scheme 2. Electrochemical oxidation reactions of UA.
Scheme 3. Electrochemical oxidation reactions of AA.
Scheme 4. Structure of sudan III dye.
grade were purchased from Himedia chemical company and used without further purification. Sodium hydroxide, sudan (III), perchloric acid, sodium dihydrogen orthophosphate dehydrate and anhydrous di‑sodium hydrogen phosphate were obtained from Merck. Graphite powder and silicon oil were bought from Lobo Chemie. Stock solutions of 25 × 10−4 M DA, 25 × 10−4 M AA and 25 × 10−4 M UA were prepared by dissolving in 0.1 M perchloric acid solution, double distilled water and 0.1 M NaOH respectively, and all the other reagents solutions were prepared by double distilled water. 2.2. Voltammetric procedure CHI-660c model (CH Instrument-660 electrochemical workstation) comprising of a platinum wire as counter electrode, saturated calomel as reference electrode and carbon paste (3.0 mm in diameter) or poly (sudan III)/MCPE as working electrodes were used to perform cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques at room temperature. 2.3. Preparation of bare carbon paste electrode The bare CPE was prepared by hand mixing graphite powder and silicone oil in the ratio 70:30 (w/w) for about 30 min in an agate mortar to produce a homogeneous mixture. The paste was then packed into the homemade Teflon cavity and smoothened on a weighing paper. Electrical contact was provided at the end of the PVC tube by a copper wire.
2. Experimental 2.1. Reagents and chemicals
2.4. Preparation of poly (sudan III) modified carbon paste electrode (PS/MCPE)
Dopamine (DA), ascorbic Acid (AA) and uric Acid (UA) of analytical
The poly sudan (III) modified CPE was prepared by the electropolymerization of 0.5 × 10−4 M sudan (III) on the surface of the BCPE by performing 15 consecutive cycles of potential scan ranges from −0.8 and 1.0 V at the scan rate of 50 mV s−1 using 0.1 M NaOH as supporting electrolyte. After the electropolymerization, the modified electrode was rinsed with deionized water to remove any physically adsorbed monomer.
Scheme 1. Electrochemical oxidation reactions of DA. 72
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in the presence of 0.2 M PBS. The electrochemical response of DA at modified electrode was found to be pH dependent, an increase in the pH increased the peak currents of the redox system and maximum peak currents was observed at pH 7.4. Hence PBS solution with a pH of 7.4 was selected for all subsequent measurements. It is evident from Fig. 3b that there was a negative shift in the anodic peak potential of DA with increasing the pH (R2 = 0.971) suggesting that the electrode follows Nernst equation. 3.4. Electrochemical behavior of DA at poly (sudan III)/MCPE Fig. 4 depicts the cyclic voltammograms recorded for 0.1 × 10−4 M DA in 0.2 M PBS pH 7.4 at both BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at the scan rate of 50 mV s−1. At BCPE, a pair of redox peaks was observed with an anodic peak potential (Epa) at 0.13 V, cathodic peak potential (Epc) at 0.07 V and the difference in redox peak potential (ΔEp) 0.05 V. Subsequently, poor redox peak current response was observed due to slow electron transfer. However, under identical conditions the poly(sudan III)/MCPE exhibited an increase in redox peak current with an oxidation peak at 0.12 V and reduction peak at 0.1 V and difference in redox peak potential (ΔEp) was found to be 0.02 V. These results illustrates that poly (sudan III)/MCPE has an improved electrocatalytic activity towards DA determination.
Fig. 1. a) Electropolymerization of 1 × 10−4 M sudan (III) monomer in 0.1 M NaOH supporting electrolyte for 15 cycles with Sweep rate of 50 mV s−1, b)Graph of anodic peak current (Ipa) of 0.1 × 10−4 M DA in 0.1 M PBS of pH 7.4 with scan rate 50 mV s−1 versus number of polymerization cycles.
3. Results and discussion 3.1. Electropolymerization of sudan (III) on the surface of BCPE
3.5. Scan rate study of DA at poly (sudan III)/MCPE
Cyclic voltammetry has been employed for the formation of thin polymeric film of poly (sudan III) on the surface of BCPE. Fig. 1a shows the CVs obtained during the electropolymerization of 0.5 × 10−4 M sudan (III) on the surface of BCPE in the presence of 0.1 M NaOH as supporting electrolyte at the scan rate of 50 mV s−1 for 15 cycles. The polymer layer formation was achieved by repetitive cycling the potential between −0.8 V and 1 V. During successive cycles, the voltammograms gradually descended, indicating poly sudan (III) film was being formed and deposited on the surface of the BCPE. Since, number of cycle is proportional to the thickness of the polymer, different polymerization cycles were applied (from 5 to 25) on the surface of BCPE to determine the optimum number of cycles and the corresponding electrocatalytic behavior towards 0.1 × 10−4 M DA in 0.2 M PBS (phosphate buffer solution) of pH 7.4 with sweep rate 50 mV s−1 was examined. A plot of Ipa versus number of cycles as shown in Fig. 1b illustrates a progressive increase in the Ipa from 5 to 25 cycles indicating a good electrocatalytic behavior of the polymer (R2 = 0.998). However, in spite of the continuous increase in the peak current with increasing number of cycles, above 15 cycles the nature of the voltammogram was distorted, hence 15 cycles were chosen since it gave the best cyclic voltammogram for DA.
The effect of scan rate on the redox peak current of DA at poly (sudan III) MCPE in presence of 0.2 M PBS was examined by cyclic voltammetry. Fig. 5a depicts the CVs recorded for 0.1 × 10−4 M DA at different sweep rates (10–120 mV s−1). From this data it was determined that the resultant redox peak currents increases lineally with increasing scan rate (υ) and a tiny shifting in the redox potential was observed. The correlation of the oxidation peak current and scan rate was constructed as depicted in Fig. 5b, which exhibited a linear relation between oxidation peak current and scan rate with a correlation coefficient of R2 = 0.999. Moreover, the plot of anodic peak current (Ipa) versus square root of scan rate (ν1/2) (Fig. 5c) was also examined and its correlation coefficient value was found to be R2 = 0.987. Therefore, the obtained result indicating that the electron transfer reaction at the poly (sudan III)/MCPE was found to be adsorption-controlled. 3.6. Effect of concentration The effect of DA concentration was investigated at modified electrode using cyclic voltammetry to determine the limit of detection (LOD) and limit of quantification (LOQ). The CVs obtained for the electrocatalytic oxidation study of different concentrations of DA (0.1 × 10−4 M to 0.9 × 10−4 M) in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE was depicted in Fig. 6a at the scan rate of 50 mV s−1. It is evident that an increase in concentration of DA increases the redox peak currents. A plot anodic peak current (Ipa) versus concentration of DA showed a good linear relationship with a correlation coefficient R2 = 0.994 as illustrated by Fig. 6b. The LOD and LOQ were calculated by the formulas, LOD = 3S/M and LOQ = 10S/M where ‘S’ is the standard deviation and ‘M' is the slope obtained from the calibration plots [52,53] and found to be 9.3 × 10−6 M and 31.2 × 10−6 M, respectively. Comparison of the analytical performance of poly (sudan III)/MCPE towards the detection of DA with the previously reported literature is given in Table 1.
3.2. Surface morphology studies To study the surface morphology of the electrochemically prepared poly (sudan III)/MCPE, scanning electron microscopy (SEM) analysis was performed and depicted in Fig. 2. SEM analysis of BCPE was discussed in our previous article [51] which showed that the surface was made of irregularly shaped graphite flakes thus less surface area was available for the reaction to take place. After the electropolymerization on BCPE, the surface of the modified electrode was characterized by a uniform arrangement of poly (sudan III) molecules, which lead to increase in the active surface area of the modified electrode and intern increasing its electrocatalytic activity.
3.7. Electrochemical behavior UA at poly (sudan III)/MCPE
3.3. Effect of pH
Fig. 7 shows the cyclic voltammograms of 0.2 × 10−4 M UA at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at the scan rate of 50 mV s−1 in 0.2 M PBS of pH 7.4. BCPE exhibited a poor
Fig. 3a represents the CVs recorded for 0.1 × 10−4 M DA at poly (sudan III)/MCPE with scan rate of 50 mV s−1 at different pH (6.4–7.8) 73
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Fig. 2. SEM image of bare(a) and poly (sudan III) MCPE (b).
Fig. 3. a) Cyclic voltammograms recorded at poly sudan (III)/MCPE for 0.1 × 10−4 M DA in 0.2 M PBS solution of different pH values (6.4 to 7.8) at scan rate of 50 mV s−1, b) Plot of Epa versus pH.
Fig. 5. a) Cyclic voltammograms of 0.1 × 10−4 M DA at poly (sudan III)/MCPE with different scan rates (10–120 mV s−1) in 0.2 M PBS of pH 7.4, b) Plot of anodic peak current (Ipa) versus scan rate, c) Plot of anodic peak current (Ipa) versus square root of scan rate.
electrochemical response for UA characterized by low current response and a broad voltammetric oxidation peak which appeared at about 0.29 V. The broad peak suggests slow electron transfer kinetics, presumably due to the fouling of the electrode surface by the oxidation product [25]. However, under the identical conditions, the proposed modified electrode exhibited a remarkable increase in response current which was characterized by a well-defined oxidation peak current with the peak potential at 0.22 V. These phenomena of an increase in current response and shifting of oxidation potential to the negative direction by 0.06 V, gives a clear evidence of the catalytic effect of proposed sensor towards UA analysis. 3.8. Scan rate study of UA at poly (sudan III)/MCPE Fig. 8a shown the cyclic voltammograms of 0.2 × 10−4 M UA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE with different scan rates ranging from 10 to 120 mV s−1. An increase in the scan rate gradually increased the oxidation peak current with a slight shift in oxidation potential to the negative value indicating the occurrence of direct electron transfer between UA and the modified electrode surface. To investigate the kinetics of the electrode reaction, graphs of anodic peak
Fig. 4. Cyclic voltammograms recorded for 0.1 × 10−4 M DA in 0.2 M PSB (pH 7.4) at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at scan rate of 50 mV s−1.
74
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Fig. 8. a) cyclic voltammograms of 0.2 × 10−4 M UA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE at different scan rate ranging from 10 to 120 mV s−1, b) Graph of the anodic peak current (Ipa) versus the scan rate (υ), c) Graph of the anodic peak current (Ipa) versus the square root scan rate (υ1/2).
Fig. 6. a) Cyclic voltammograms of DA at different concentrations (0.1–0.9 × 10–4 M) in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE with scan rate of 50 mV s−1, b) Plot of anodic peak current versus concentration.
3.9. Effect of concentration
Table 1 Comparison of the analytical performance of poly (sudan III)/MCPE with the previously reported sensors. Working electrode
Graphene/GCE Metallothioneins self-assembled gold electrode Mp-GR poly (sudan III)/MCPE
Linear range (μM)
Limit of detection(μM)
DA
DA
Cyclic voltammetry was performed to study the effect of UA concentration at poly (sudan III)/MCPE in presence of 0.2 M PBS of pH 7.4. Fig. 9a shows the CVs recorded for different concentrations of UA ranging from (0.2 × 10−4 M to 1.0 × 10−4 M) at the scan rate of 50 mV s−1. It can be seen that, an increase in the concentration of UA results in an increase in resultant anodic peak current. A plot of Ipa versus concentration of UA shown in Fig. 9b, illustrates that the anodic peak currents (Ipa) changes linearly with concentrations with a correlation coefficient R2 = 0.992. Applying the above mentioned equations the LOD and LOQ were calculated and found to be 1 × 10−5 M and 3.55 × 10−5 M, respectively.
Reference
4–100 20–80
2.6 6
[54] [55]
4–40 10–90
1.5 9.3
[56] This work
3.10. Electrochemical behavior AA at poly (sudan III)/MCPE Fig. 10 shows the cyclic voltammograms recorded for 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) using pH 7.4 (0.2 M PBS). At BCPE, a broad oxidation peak was observed at
Fig. 7. Cyclic voltamogramms recorded for 0.2 × 10−4 M UA at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) in presence of 0.2 M PBS of pH 7.4 with scan rate of 50 mV s−1.
current (Ipa) versus scan rate (υ) (Fig. 8b) and anodic peak current (Ipa) versus square root scan rate (υ1/2) (Fig. 8c) were plotted, and were found to show a linear relation with a correlation coefficient of R2 = 0.999 and R2 = 0.987 respectively. The above results suggest that the electrochemical reaction of UA at MCPE was found to be adsorption-controlled.
Fig. 9. a) Cyclic voltammograms of DA at different concentrations (0.2–1 × 10–4 M) in 0.2 M PBS (pH 7.4) at poly (sudan III)/MCPE with scan rate of 50 mV s−1, b) Graph of Ipa versus concentration of DA. 75
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Fig. 12. a) Cyclic voltammograms of AA at different concentrations (2.5 to 5 × 10−4 M) in 0.2 M PBS (pH 7.4) at poly (sudan III)/MCPE with the scan rate of 50 mV s−1, b) Graph of Ipa versus concentration of AA.
Fig. 10. Cyclic voltammograms recorded for 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS pH 7.4 with scan rate 50 mV s−1.
Fig. 13. Cyclic voltammograms recorded for a mixture of 0.1 × 10−4 M DA, 0.2 × 10−4 M UA and 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS of pH 7.4 at the scan rate of 50 mV s−1.
Fig. 11. a) cyclic voltammograms of 2.5 × 10−4 M AA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE at different scan rates ranging from 60 to 150 mV s−1, b) Graph of the anodic peak current (Ipa) versus the scan rate (υ), c) Graph of the anodic peak current (Ipa) versus the square root scan rate (υ1/2).
rate (υ1/2) (Fig. 11c) was also plotted which exhibited good linearity of the anodic peak currents with respect to the square root scan rate (υ1/2) and correlation coefficient value was found to be R2 = 0.993 indicating that the electrode transfer reaction was diffusion-controlled.
0.27 V which suggests the slow electron transfer kinetics, presumably as a result electrode fouling [57,58]. However, at MCPE the oxidation peak potential of AA was shifted by 0.15 V towards negative potential, and appeared at around 0.12 V. This phenomena confirms that the MCPE lowered the over potential of AA and favored its electro oxidation. An increase in oxidation peak current suggests that electrode fouling effect was eliminated through modification. Shifting of the oxidation peak of AA to negative potential is highly appreciated since it guarantees the lack of interference with the measurement of DA.
3.12. Effect of concentration The cyclic voltammograms were recorded for the oxidation of 2 × 10−4 M AA in 0.2 M PBS at poly (Sudan III)/MCPE with the scan rate 50 mV s−1 by varying the concentration from 2.5 × 10−4 M to 5 × 10−4 M as shown in Fig. 12a. The obtained results show an increase in anodic peak current due to increase in the concentration of AA. Fig. 12b reveals that the graph of anodic peak current versus concentration of AA exhibits linear relation with a correlation coefficient R2 = 0.989. The LOD and LOQ were found to be 4.5 × 10−5 M and 15.1 × 10−5 M respectively.
3.11. Scan rate study of AA at poly (sudan III)/MCPE Fig. 11a shows the cyclic voltammograms obtained for 2.5 × 10−4 M AA in 0.2 M PBS of pH 7.4 and the scan rate was varied from 60 to 150 mV s−1. The obtained results evident that an increase in the scan rate increased the anodic peak current (Ipa), the graph of anodic peak current (Ipa) versus scan rate (υ) was plotted (Fig. 11b) showed a good linear relation with a correlation coefficient R2 = 0.996. In addition a plot of anodic peak current (Ipa) versus square root scan
3.13. Simultaneous determination of DA, UA and AA DA, UA and AA coexist in extra cellular fluid of the mammalian central nervous system; nevertheless concentration of DA is predominant. In order to establish a sensitive and selective method for the determination of these biomolecules, the poly (sudan III)/MCPE was 76
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Fig. 14. a) DPVs recorded for DA (100 μL–900 μL) in 0.2 M PBS at pH 7.4 at the poly (sudan III) MCPE, b) DPVs recorded for UA (200 μL–1000 μL) in 0.2 M PBS at pH 7.4 at the poly (sudan III) MCPE.
of UA. Fig. 14b depicts the DPVs of the binary mixture of DA and UA where the concentration of DA is constant while that of UA is varied. It can be seen that the anodic peak current of UA was proportional to concentration of the species and there was no change in the anodic peak current and peak potential for DA.
Table 2 Determination of DA injection sample at poly (sudan III)/MCPE (n = 3). Injection sample
Added (μM)
Found (μM)
Recovery (%)
DA
10 20 30 40
9.9 19.7 29.2 38
99 98.6 97.5 95.2
3.15. Real sample analysis To investigate the analytical reliability and potential application, the fabricated poly (sudan III)/MCPE was employed for the determination of DA content in the commercial dopamine hydrochloride injection sample using cyclic voltammetry. The standard addition method was employed [59]. The obtained results were summarized in Table 2 and the recoveries of the proposed method were found in the range of 95.2–99%. Therefore, the obtained result demonstrates the satisfactory accuracy of the proposed sensor for DA determination.
employed to resolve the voltammetric response of DA in presence of AA, and UA. Fig. 13 Illustrates the cyclic voltammograms recorded for a mixture of 0.1 × 10−4 M DA, 0.2 × 10−4 M UA and 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS (pH 7.4) at the scan rate of 50 mV s−1. The BCPE failed to separate the voltammetric signal of DA, AA and UA, thus only one broad oxidation peak was observed at approximately 0.22 V. This is attributed to the fact that the oxidation potential of both AA and UA was similar as that of DA, thus resulting to an overlap in voltammetric response. On the other hand, the CV obtained at MCPE exhibited three well-separated oxidation peaks at potentials around 0.14 V, 0.27 V and − 0.07 V for DA, UA, and AA, respectively with a remarkable increase in the oxidation current response. A significant separation of oxidation peak potentials was observed which 0.22 V was for DA and AA, 0.12 V for DA and UA and finally 1.61 V for AA and UA. These differences in oxidation peak potentials were sufficient to achieve the simultaneous detection of these three compounds in a homogeneous solution. Furthermore, at poly (Sudan III) MCPE in spite of high concentrations of AA a low oxidation peak signal was recorded compared to the low concentrations of DA and UA. These imply that the MCPE may be effectively employed for quantitative determination of DA and UA in the presence of excess amount of AA.
4. Conclusion The poly (sudan III)/MCPE was fabricated using sudan III as monomer in presence of 0.1 M NaOH as the supporting electrolyte. The developed sensor exhibited excellent electrocatalytic activity towards the oxidation of DA, UA and AA by significantly lowering their oxidation potentials and enhancing the anodic peak currents. The calibration curves for individual analysis of DA, UA and AA gave a low detection limit at 9.37 μM, 10.6 μM and 45.5 μM, respectively. Cyclic voltammetric technique using poly (sudan III)/MCPE was successfully employed for the individual and simultaneous determination of these biomolecules in their homogeneous mixture with excellent selectivity, sensitivity and reproducibility. References
3.14. Interference study
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DA, UA and AA coexist in the biological fluids and often determination of these biomolecules in a homogeneous mixture is challenging due to interference caused by overlapping of their oxidation potentials. The anti-interference ability of the poly (sudan III)/MCPE sensor was examined by differential pulse voltammetry (DPV) technique on an homogeneous mixture of DA and UA in 0.2 M PBS of pH 7.4 with the scan rate of 50 mV s−1. Fig. 14a shows the DPVs recorded for the mixture containing DA and UA, in which the concentration of UA was kept constant while that of DA was varied. From the above mentioned results it's evident that the oxidation peak current of DA increased linearly with increase in concentration of DA and no significant alteration was observed in the oxidation peak current and peak potential 77
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Fabrication of poly (sudan III) modified carbon paste electrode sensor for dopamine: A voltammetric study T.S. Sunil Kumar Naik, Muthui Martin Mwaurah, B.E. Kumara Swamy
T
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Department of P. G. Studies and Research in Industrial Chemistry, Kuvempu University, Jnanasahyadri, Shankaraghatta - 577451, Shivamoga (D), Karnataka (S), India
ARTICLE INFO
ABSTRACT
Keywords: Sudan (III) Electropolymerization Dopamine Uric acid Ascorbic acid Simultaneous determination Carbon paste electrode
Owing to the challenges faced in simultaneous determination of dopamine(DA), uric acid(UA) and ascorbic acid (AA) in mixtures by electrochemical method, the present work is aimed at developing a simple, sensitive and selective sensor using sudan III as the modifier. The poly (sudan III) modified carbon paste electrode (PS/MCPE) was prepared by electropolymerization of sudan III monomer on the surface of bare carbon paste electrode (BCPE). Electrochemical sensing properties of poly (sudan III) MCPE were evaluated by cyclic voltammetry for individual analytes and findings revealed that it has a good electrocatalytic activity towards the determination of dopamine (DA), uric acid (UA) and ascorbic acid (AA) characterized by a low detection limit of 9.3 × 10−6 M, 1 × 10−5 M and 4.5 × 10−5 M for DA, UA and AA, respectively. The scan rate study reveals that the electrode process is adsorption-controlled. The simultaneous determination of the analytes in a mixture showed that the modified electrode exhibits an outstanding selectivity towards DA, UA and AA with their distinct oxidation potentials occurring at 0.14 V, 0.27 V, and −0.07 V, respectively. The fabricated modified electrode sensor exhibited excellent electrocatalytic properties towards the determination of DA, UA and AA with satisfactory results.
1. Introduction Dopamine (DA), is a monoamine neurotransmitter belongs to catecholamine family [1,2]. It is found in both mammalian central nervous system as well as peripheral nervous system, moreover it's majorly dominant in the corpus stratum of the brain [3,4]. Dopaminergic neurotransmission helps to regulate motion, emotions, motivation, learning and cognition. Abnormal levels of DA in the body results to neurological disorders such as Parkinson's, Alzheimer's and Huntington's diseases, Tourett'e syndrome, schizophrenia and psychosis [5–8]. There has also been documentation of its involvement in manifestation of HIV [9] and drug addiction, thus DA determination is a field of great interest in understanding drug and substance abuse [10]. Determination of DA in biological systems with desired sensitivity and accuracy is very vital in the diagnosis of aforesaid diseases as well as clinical applications in treatment of hypotension and bradycardia [11–18]. Currently, methods employed for the determination of DA include fluorimetry, high-performance liquid chromatography, capillary electrophoresis, flow injection, chemiluminescence and electrochemistry [19]. In comparison to other analytical techniques, electrochemical technique for the determination of dopamine has gained prominence due to its simplicity, sensitivity, fast response, low cost, adaptability and its eco-friendly ⁎
nature [20]. However, dopamine detection is challenging since its oxidation potential is very close to that of interfering molecules such as ascorbic acid which cause overlapping of their oxidation signals at bare carbon paste electrode. Therefore, the oxidation peak potentials of these three electroactive molecules need to be separated from each other for selective and accurate analysis of dopamine [21,22]. This is achieved through chemical modification of the electrode. Furthermore, chemical modification of carbon paste electrode solves the problem of electrode fouling which originate from adsorption of oxidation products of dopamine and other electroactive species on the electrode surface [23,24]. Numerous materials, such as carbon-based nanomaterials, organic redox mediators, ionic liquids, biomaterials, metal nanoparticles, metal oxides, and conductive polymers have been used as modifiers to construct highly sensitive and selective biosensors for dopamine based on carbon paste electrodes [25–30]. However, novel materials for electrode modification are still demanded in attempt to develop superior electrochemical sensors for dopamine. Uric acid (UA) is the end product of purine catabolism in mammals. Hyperuricaemia, a medical condition characterized by excess uric acid in serum is evident in patients with reduced glomerular filtration rate [31]. Prolonged hyperuricaemia causes nucleation and growth of monosodium urate (MSU) crystals in tissues and joints leading to gout [32]. Aside from
Corresponding author. E-mail address: [email protected] (B.E. Kumara Swamy).
https://doi.org/10.1016/j.jelechem.2018.12.054 Received 29 September 2018; Received in revised form 24 December 2018; Accepted 26 December 2018 Available online 28 December 2018 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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gout, abnormal uric acid levels causes other disorders including Fanconi syndrome, Lesch-Nyhan syndrome and renal-disease [33]. Ascorbic acid (AA) also known as vitamin C is distributed widely in both the plant and animal kingdoms. In animals, large amounts of ascorbic acid are evident in leukocytes, liver and anterior pituitary gland and it plays a vital role in wound healing and enhancing the immune system [34,35]. In animals, AA exists as collagen protein which is responsible to maintain bones, blood vessels, and skin. In vegetable cells, ascorbic acid occurs in unbounded state as well as protein-bound form known as ascorbigen. AA is also incorporated in industrial manufacture of animal feeds, pharmaceutical formulations and cosmetic products as an antioxidant [36]. Excess AA concentration in the body may lead to diarrhea, nausea, skin irritation, burning upon urination, and depletion of copper, hence its determination in biological fluids is vital. In biological samples, DA, UA, and AA co-exist in varied concentrations. AA is usually in higher concentrations (about 0.1 mM) while DA has a lower concentration (0.01–1 μM) [37]. As the oxidation potentials of these species are very close, electrochemical interference from each other is often witnessed, thus the ability to selectively and accurately determine these species in a mixed solution is a subject of great significance for both diagnostic research and analytical application. The electrooxidation reactions of DA, UA and AA are shown in the Scheme 1, Scheme 2 and Scheme 3, respectively. Over the years, polymer film modified electrodes have gained much attention in constructing electrochemical sensors and biosensors due of their handy advantages such as selectivity, electrochemical reversibility, low cost, good stability, reproducibility, more active sites, homogeneity in electrochemical deposition and strong adherence to the electrode surface [38,39]. Several polymer films have been applied for the determination of dopamine; Zhang et al. [40] described polymerization of L-lysine on electrode surface which could separate the redox peaks of dopamine, ascorbic acid, and uric acid hence it was utilized in simultaneous electrochemical determination of the aforementioned biomolecules. Additionally, there are other reports of using naphthol green B, pyrrole-3-carboxylic acid, glycine, calmagite, Eriochrome Black-T, Evans Blue [41–46] as monomers for modification of carbon paste electrode by electropolymerization. Sudan III dye also known as (1-(4-phenylazophenylazo)-2-naphthol) is a non-ionic lipophilic azo-aromatic compound that is widely used as a colorant in numerous chemical industries which include oils, plastics, waxes, petrol, printing floor polishing, and spirit varnishing due to its intense red color, low cost and stability [47–50]. In addition sudan III is an electroactive molecule and can undergo polymerization to produce an electrically active polymer. The structure of (sudan III) is shown in Scheme 4. The present work describes the electropolymerization of sudan (III) on the surface of carbon paste electrode by cyclic voltammetry to yield a conducting polymer possessing distinctive electrocatalytic properties towards the determination of dopamine, uric acid and ascorbic acid by enhancing their redox peaks currents at low concentrations and lowering their oxidation potentials.
Scheme 2. Electrochemical oxidation reactions of UA.
Scheme 3. Electrochemical oxidation reactions of AA.
Scheme 4. Structure of sudan III dye.
grade were purchased from Himedia chemical company and used without further purification. Sodium hydroxide, sudan (III), perchloric acid, sodium dihydrogen orthophosphate dehydrate and anhydrous di‑sodium hydrogen phosphate were obtained from Merck. Graphite powder and silicon oil were bought from Lobo Chemie. Stock solutions of 25 × 10−4 M DA, 25 × 10−4 M AA and 25 × 10−4 M UA were prepared by dissolving in 0.1 M perchloric acid solution, double distilled water and 0.1 M NaOH respectively, and all the other reagents solutions were prepared by double distilled water. 2.2. Voltammetric procedure CHI-660c model (CH Instrument-660 electrochemical workstation) comprising of a platinum wire as counter electrode, saturated calomel as reference electrode and carbon paste (3.0 mm in diameter) or poly (sudan III)/MCPE as working electrodes were used to perform cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques at room temperature. 2.3. Preparation of bare carbon paste electrode The bare CPE was prepared by hand mixing graphite powder and silicone oil in the ratio 70:30 (w/w) for about 30 min in an agate mortar to produce a homogeneous mixture. The paste was then packed into the homemade Teflon cavity and smoothened on a weighing paper. Electrical contact was provided at the end of the PVC tube by a copper wire.
2. Experimental 2.1. Reagents and chemicals
2.4. Preparation of poly (sudan III) modified carbon paste electrode (PS/MCPE)
Dopamine (DA), ascorbic Acid (AA) and uric Acid (UA) of analytical
The poly sudan (III) modified CPE was prepared by the electropolymerization of 0.5 × 10−4 M sudan (III) on the surface of the BCPE by performing 15 consecutive cycles of potential scan ranges from −0.8 and 1.0 V at the scan rate of 50 mV s−1 using 0.1 M NaOH as supporting electrolyte. After the electropolymerization, the modified electrode was rinsed with deionized water to remove any physically adsorbed monomer.
Scheme 1. Electrochemical oxidation reactions of DA. 72
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in the presence of 0.2 M PBS. The electrochemical response of DA at modified electrode was found to be pH dependent, an increase in the pH increased the peak currents of the redox system and maximum peak currents was observed at pH 7.4. Hence PBS solution with a pH of 7.4 was selected for all subsequent measurements. It is evident from Fig. 3b that there was a negative shift in the anodic peak potential of DA with increasing the pH (R2 = 0.971) suggesting that the electrode follows Nernst equation. 3.4. Electrochemical behavior of DA at poly (sudan III)/MCPE Fig. 4 depicts the cyclic voltammograms recorded for 0.1 × 10−4 M DA in 0.2 M PBS pH 7.4 at both BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at the scan rate of 50 mV s−1. At BCPE, a pair of redox peaks was observed with an anodic peak potential (Epa) at 0.13 V, cathodic peak potential (Epc) at 0.07 V and the difference in redox peak potential (ΔEp) 0.05 V. Subsequently, poor redox peak current response was observed due to slow electron transfer. However, under identical conditions the poly(sudan III)/MCPE exhibited an increase in redox peak current with an oxidation peak at 0.12 V and reduction peak at 0.1 V and difference in redox peak potential (ΔEp) was found to be 0.02 V. These results illustrates that poly (sudan III)/MCPE has an improved electrocatalytic activity towards DA determination.
Fig. 1. a) Electropolymerization of 1 × 10−4 M sudan (III) monomer in 0.1 M NaOH supporting electrolyte for 15 cycles with Sweep rate of 50 mV s−1, b)Graph of anodic peak current (Ipa) of 0.1 × 10−4 M DA in 0.1 M PBS of pH 7.4 with scan rate 50 mV s−1 versus number of polymerization cycles.
3. Results and discussion 3.1. Electropolymerization of sudan (III) on the surface of BCPE
3.5. Scan rate study of DA at poly (sudan III)/MCPE
Cyclic voltammetry has been employed for the formation of thin polymeric film of poly (sudan III) on the surface of BCPE. Fig. 1a shows the CVs obtained during the electropolymerization of 0.5 × 10−4 M sudan (III) on the surface of BCPE in the presence of 0.1 M NaOH as supporting electrolyte at the scan rate of 50 mV s−1 for 15 cycles. The polymer layer formation was achieved by repetitive cycling the potential between −0.8 V and 1 V. During successive cycles, the voltammograms gradually descended, indicating poly sudan (III) film was being formed and deposited on the surface of the BCPE. Since, number of cycle is proportional to the thickness of the polymer, different polymerization cycles were applied (from 5 to 25) on the surface of BCPE to determine the optimum number of cycles and the corresponding electrocatalytic behavior towards 0.1 × 10−4 M DA in 0.2 M PBS (phosphate buffer solution) of pH 7.4 with sweep rate 50 mV s−1 was examined. A plot of Ipa versus number of cycles as shown in Fig. 1b illustrates a progressive increase in the Ipa from 5 to 25 cycles indicating a good electrocatalytic behavior of the polymer (R2 = 0.998). However, in spite of the continuous increase in the peak current with increasing number of cycles, above 15 cycles the nature of the voltammogram was distorted, hence 15 cycles were chosen since it gave the best cyclic voltammogram for DA.
The effect of scan rate on the redox peak current of DA at poly (sudan III) MCPE in presence of 0.2 M PBS was examined by cyclic voltammetry. Fig. 5a depicts the CVs recorded for 0.1 × 10−4 M DA at different sweep rates (10–120 mV s−1). From this data it was determined that the resultant redox peak currents increases lineally with increasing scan rate (υ) and a tiny shifting in the redox potential was observed. The correlation of the oxidation peak current and scan rate was constructed as depicted in Fig. 5b, which exhibited a linear relation between oxidation peak current and scan rate with a correlation coefficient of R2 = 0.999. Moreover, the plot of anodic peak current (Ipa) versus square root of scan rate (ν1/2) (Fig. 5c) was also examined and its correlation coefficient value was found to be R2 = 0.987. Therefore, the obtained result indicating that the electron transfer reaction at the poly (sudan III)/MCPE was found to be adsorption-controlled. 3.6. Effect of concentration The effect of DA concentration was investigated at modified electrode using cyclic voltammetry to determine the limit of detection (LOD) and limit of quantification (LOQ). The CVs obtained for the electrocatalytic oxidation study of different concentrations of DA (0.1 × 10−4 M to 0.9 × 10−4 M) in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE was depicted in Fig. 6a at the scan rate of 50 mV s−1. It is evident that an increase in concentration of DA increases the redox peak currents. A plot anodic peak current (Ipa) versus concentration of DA showed a good linear relationship with a correlation coefficient R2 = 0.994 as illustrated by Fig. 6b. The LOD and LOQ were calculated by the formulas, LOD = 3S/M and LOQ = 10S/M where ‘S’ is the standard deviation and ‘M' is the slope obtained from the calibration plots [52,53] and found to be 9.3 × 10−6 M and 31.2 × 10−6 M, respectively. Comparison of the analytical performance of poly (sudan III)/MCPE towards the detection of DA with the previously reported literature is given in Table 1.
3.2. Surface morphology studies To study the surface morphology of the electrochemically prepared poly (sudan III)/MCPE, scanning electron microscopy (SEM) analysis was performed and depicted in Fig. 2. SEM analysis of BCPE was discussed in our previous article [51] which showed that the surface was made of irregularly shaped graphite flakes thus less surface area was available for the reaction to take place. After the electropolymerization on BCPE, the surface of the modified electrode was characterized by a uniform arrangement of poly (sudan III) molecules, which lead to increase in the active surface area of the modified electrode and intern increasing its electrocatalytic activity.
3.7. Electrochemical behavior UA at poly (sudan III)/MCPE
3.3. Effect of pH
Fig. 7 shows the cyclic voltammograms of 0.2 × 10−4 M UA at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at the scan rate of 50 mV s−1 in 0.2 M PBS of pH 7.4. BCPE exhibited a poor
Fig. 3a represents the CVs recorded for 0.1 × 10−4 M DA at poly (sudan III)/MCPE with scan rate of 50 mV s−1 at different pH (6.4–7.8) 73
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Fig. 2. SEM image of bare(a) and poly (sudan III) MCPE (b).
Fig. 3. a) Cyclic voltammograms recorded at poly sudan (III)/MCPE for 0.1 × 10−4 M DA in 0.2 M PBS solution of different pH values (6.4 to 7.8) at scan rate of 50 mV s−1, b) Plot of Epa versus pH.
Fig. 5. a) Cyclic voltammograms of 0.1 × 10−4 M DA at poly (sudan III)/MCPE with different scan rates (10–120 mV s−1) in 0.2 M PBS of pH 7.4, b) Plot of anodic peak current (Ipa) versus scan rate, c) Plot of anodic peak current (Ipa) versus square root of scan rate.
electrochemical response for UA characterized by low current response and a broad voltammetric oxidation peak which appeared at about 0.29 V. The broad peak suggests slow electron transfer kinetics, presumably due to the fouling of the electrode surface by the oxidation product [25]. However, under the identical conditions, the proposed modified electrode exhibited a remarkable increase in response current which was characterized by a well-defined oxidation peak current with the peak potential at 0.22 V. These phenomena of an increase in current response and shifting of oxidation potential to the negative direction by 0.06 V, gives a clear evidence of the catalytic effect of proposed sensor towards UA analysis. 3.8. Scan rate study of UA at poly (sudan III)/MCPE Fig. 8a shown the cyclic voltammograms of 0.2 × 10−4 M UA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE with different scan rates ranging from 10 to 120 mV s−1. An increase in the scan rate gradually increased the oxidation peak current with a slight shift in oxidation potential to the negative value indicating the occurrence of direct electron transfer between UA and the modified electrode surface. To investigate the kinetics of the electrode reaction, graphs of anodic peak
Fig. 4. Cyclic voltammograms recorded for 0.1 × 10−4 M DA in 0.2 M PSB (pH 7.4) at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) at scan rate of 50 mV s−1.
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Fig. 8. a) cyclic voltammograms of 0.2 × 10−4 M UA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE at different scan rate ranging from 10 to 120 mV s−1, b) Graph of the anodic peak current (Ipa) versus the scan rate (υ), c) Graph of the anodic peak current (Ipa) versus the square root scan rate (υ1/2).
Fig. 6. a) Cyclic voltammograms of DA at different concentrations (0.1–0.9 × 10–4 M) in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE with scan rate of 50 mV s−1, b) Plot of anodic peak current versus concentration.
3.9. Effect of concentration
Table 1 Comparison of the analytical performance of poly (sudan III)/MCPE with the previously reported sensors. Working electrode
Graphene/GCE Metallothioneins self-assembled gold electrode Mp-GR poly (sudan III)/MCPE
Linear range (μM)
Limit of detection(μM)
DA
DA
Cyclic voltammetry was performed to study the effect of UA concentration at poly (sudan III)/MCPE in presence of 0.2 M PBS of pH 7.4. Fig. 9a shows the CVs recorded for different concentrations of UA ranging from (0.2 × 10−4 M to 1.0 × 10−4 M) at the scan rate of 50 mV s−1. It can be seen that, an increase in the concentration of UA results in an increase in resultant anodic peak current. A plot of Ipa versus concentration of UA shown in Fig. 9b, illustrates that the anodic peak currents (Ipa) changes linearly with concentrations with a correlation coefficient R2 = 0.992. Applying the above mentioned equations the LOD and LOQ were calculated and found to be 1 × 10−5 M and 3.55 × 10−5 M, respectively.
Reference
4–100 20–80
2.6 6
[54] [55]
4–40 10–90
1.5 9.3
[56] This work
3.10. Electrochemical behavior AA at poly (sudan III)/MCPE Fig. 10 shows the cyclic voltammograms recorded for 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) using pH 7.4 (0.2 M PBS). At BCPE, a broad oxidation peak was observed at
Fig. 7. Cyclic voltamogramms recorded for 0.2 × 10−4 M UA at BCPE (dashed line) and poly (sudan III)/MCPE (solid line) in presence of 0.2 M PBS of pH 7.4 with scan rate of 50 mV s−1.
current (Ipa) versus scan rate (υ) (Fig. 8b) and anodic peak current (Ipa) versus square root scan rate (υ1/2) (Fig. 8c) were plotted, and were found to show a linear relation with a correlation coefficient of R2 = 0.999 and R2 = 0.987 respectively. The above results suggest that the electrochemical reaction of UA at MCPE was found to be adsorption-controlled.
Fig. 9. a) Cyclic voltammograms of DA at different concentrations (0.2–1 × 10–4 M) in 0.2 M PBS (pH 7.4) at poly (sudan III)/MCPE with scan rate of 50 mV s−1, b) Graph of Ipa versus concentration of DA. 75
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Fig. 12. a) Cyclic voltammograms of AA at different concentrations (2.5 to 5 × 10−4 M) in 0.2 M PBS (pH 7.4) at poly (sudan III)/MCPE with the scan rate of 50 mV s−1, b) Graph of Ipa versus concentration of AA.
Fig. 10. Cyclic voltammograms recorded for 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS pH 7.4 with scan rate 50 mV s−1.
Fig. 13. Cyclic voltammograms recorded for a mixture of 0.1 × 10−4 M DA, 0.2 × 10−4 M UA and 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS of pH 7.4 at the scan rate of 50 mV s−1.
Fig. 11. a) cyclic voltammograms of 2.5 × 10−4 M AA in 0.2 M PBS of pH 7.4 at poly (sudan III)/MCPE at different scan rates ranging from 60 to 150 mV s−1, b) Graph of the anodic peak current (Ipa) versus the scan rate (υ), c) Graph of the anodic peak current (Ipa) versus the square root scan rate (υ1/2).
rate (υ1/2) (Fig. 11c) was also plotted which exhibited good linearity of the anodic peak currents with respect to the square root scan rate (υ1/2) and correlation coefficient value was found to be R2 = 0.993 indicating that the electrode transfer reaction was diffusion-controlled.
0.27 V which suggests the slow electron transfer kinetics, presumably as a result electrode fouling [57,58]. However, at MCPE the oxidation peak potential of AA was shifted by 0.15 V towards negative potential, and appeared at around 0.12 V. This phenomena confirms that the MCPE lowered the over potential of AA and favored its electro oxidation. An increase in oxidation peak current suggests that electrode fouling effect was eliminated through modification. Shifting of the oxidation peak of AA to negative potential is highly appreciated since it guarantees the lack of interference with the measurement of DA.
3.12. Effect of concentration The cyclic voltammograms were recorded for the oxidation of 2 × 10−4 M AA in 0.2 M PBS at poly (Sudan III)/MCPE with the scan rate 50 mV s−1 by varying the concentration from 2.5 × 10−4 M to 5 × 10−4 M as shown in Fig. 12a. The obtained results show an increase in anodic peak current due to increase in the concentration of AA. Fig. 12b reveals that the graph of anodic peak current versus concentration of AA exhibits linear relation with a correlation coefficient R2 = 0.989. The LOD and LOQ were found to be 4.5 × 10−5 M and 15.1 × 10−5 M respectively.
3.11. Scan rate study of AA at poly (sudan III)/MCPE Fig. 11a shows the cyclic voltammograms obtained for 2.5 × 10−4 M AA in 0.2 M PBS of pH 7.4 and the scan rate was varied from 60 to 150 mV s−1. The obtained results evident that an increase in the scan rate increased the anodic peak current (Ipa), the graph of anodic peak current (Ipa) versus scan rate (υ) was plotted (Fig. 11b) showed a good linear relation with a correlation coefficient R2 = 0.996. In addition a plot of anodic peak current (Ipa) versus square root scan
3.13. Simultaneous determination of DA, UA and AA DA, UA and AA coexist in extra cellular fluid of the mammalian central nervous system; nevertheless concentration of DA is predominant. In order to establish a sensitive and selective method for the determination of these biomolecules, the poly (sudan III)/MCPE was 76
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Fig. 14. a) DPVs recorded for DA (100 μL–900 μL) in 0.2 M PBS at pH 7.4 at the poly (sudan III) MCPE, b) DPVs recorded for UA (200 μL–1000 μL) in 0.2 M PBS at pH 7.4 at the poly (sudan III) MCPE.
of UA. Fig. 14b depicts the DPVs of the binary mixture of DA and UA where the concentration of DA is constant while that of UA is varied. It can be seen that the anodic peak current of UA was proportional to concentration of the species and there was no change in the anodic peak current and peak potential for DA.
Table 2 Determination of DA injection sample at poly (sudan III)/MCPE (n = 3). Injection sample
Added (μM)
Found (μM)
Recovery (%)
DA
10 20 30 40
9.9 19.7 29.2 38
99 98.6 97.5 95.2
3.15. Real sample analysis To investigate the analytical reliability and potential application, the fabricated poly (sudan III)/MCPE was employed for the determination of DA content in the commercial dopamine hydrochloride injection sample using cyclic voltammetry. The standard addition method was employed [59]. The obtained results were summarized in Table 2 and the recoveries of the proposed method were found in the range of 95.2–99%. Therefore, the obtained result demonstrates the satisfactory accuracy of the proposed sensor for DA determination.
employed to resolve the voltammetric response of DA in presence of AA, and UA. Fig. 13 Illustrates the cyclic voltammograms recorded for a mixture of 0.1 × 10−4 M DA, 0.2 × 10−4 M UA and 2 × 10−4 M AA at BCPE (dashed line) and poly (Sudan III)/MCPE (solid line) in 0.2 M PBS (pH 7.4) at the scan rate of 50 mV s−1. The BCPE failed to separate the voltammetric signal of DA, AA and UA, thus only one broad oxidation peak was observed at approximately 0.22 V. This is attributed to the fact that the oxidation potential of both AA and UA was similar as that of DA, thus resulting to an overlap in voltammetric response. On the other hand, the CV obtained at MCPE exhibited three well-separated oxidation peaks at potentials around 0.14 V, 0.27 V and − 0.07 V for DA, UA, and AA, respectively with a remarkable increase in the oxidation current response. A significant separation of oxidation peak potentials was observed which 0.22 V was for DA and AA, 0.12 V for DA and UA and finally 1.61 V for AA and UA. These differences in oxidation peak potentials were sufficient to achieve the simultaneous detection of these three compounds in a homogeneous solution. Furthermore, at poly (Sudan III) MCPE in spite of high concentrations of AA a low oxidation peak signal was recorded compared to the low concentrations of DA and UA. These imply that the MCPE may be effectively employed for quantitative determination of DA and UA in the presence of excess amount of AA.
4. Conclusion The poly (sudan III)/MCPE was fabricated using sudan III as monomer in presence of 0.1 M NaOH as the supporting electrolyte. The developed sensor exhibited excellent electrocatalytic activity towards the oxidation of DA, UA and AA by significantly lowering their oxidation potentials and enhancing the anodic peak currents. The calibration curves for individual analysis of DA, UA and AA gave a low detection limit at 9.37 μM, 10.6 μM and 45.5 μM, respectively. Cyclic voltammetric technique using poly (sudan III)/MCPE was successfully employed for the individual and simultaneous determination of these biomolecules in their homogeneous mixture with excellent selectivity, sensitivity and reproducibility. References
3.14. Interference study
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DA, UA and AA coexist in the biological fluids and often determination of these biomolecules in a homogeneous mixture is challenging due to interference caused by overlapping of their oxidation potentials. The anti-interference ability of the poly (sudan III)/MCPE sensor was examined by differential pulse voltammetry (DPV) technique on an homogeneous mixture of DA and UA in 0.2 M PBS of pH 7.4 with the scan rate of 50 mV s−1. Fig. 14a shows the DPVs recorded for the mixture containing DA and UA, in which the concentration of UA was kept constant while that of DA was varied. From the above mentioned results it's evident that the oxidation peak current of DA increased linearly with increase in concentration of DA and no significant alteration was observed in the oxidation peak current and peak potential 77
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