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cobalt tetrasulfonated phthalocyanine sensor for detection of dopamine
Sensors & Actuators: B. Chemical 285 (2019) 17–23
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A sensitive and selective graphene/cobalt tetrasulfonated phthalocyanine sensor for detection of dopamine
T
Nizam Diaba,b,1, Dulce M. Moralesa,1, Corina Andronescua,c, Muayad Masoudb, ⁎ Wolfgang Schuhmanna, a
Analytical Chemistry - Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany b Chemistry Department, Arab American University, P.O. Box: 240 Jenin, Palestine c Technical Chemistry III, Faculty of Chemistry, University Duisburg-Essen, Carl-Benz-Straße 199, D-47057 Duisburg, Germany
ARTICLE INFO
ABSTRACT
Keywords: Dopamine Graphene Cobalt tetrasulfonate phthalocyanine Differential pulse stripping voltammetry Electrochemical sensor
Electrochemical dopamine-sensing surfaces were fabricated by deposition of graphene sheets modified with cobalt tetrasulfonated phthalocyanine on glassy carbon electrodes (CoTSPc/Gr-GC) using differential pulse amperometry. Differential pulse stripping voltammetry was used to detect dopamine (DA) and the influence of pH value, scan rate, accumulation potential and time as well as dopamine concentration on the performance of CoTSPc/Gr-GC electrodes was investigated. The modified electrodes were successfully used as sensors for the selective and high sensitive determination of DA in presence of high concentrations of ascorbic acid (AA) and uric acid (UA) with a detection limit of 0.87 nM over the dynamic linear range of 20 nM to 220 nM.
1. Introduction Dopamine (DA) is a catecholamine neurotransmitter, which plays a significant role in the central nervous system [1]. A decreased production is correlated with the development of neurodegenerative diseases such as Alzheimer and Parkinson [2]. Therefore, the accurate and high-sensitive detection as well as monitoring of low concentrations of dopamine optimally in-vivo represents an important issue in medical diagnosis. A variety of different analytical methods have been developed for the determination of dopamine, including high performance liquid chromatography (HPLC) with fluorimetric detection [3], gas chromatography-mass spectrometry [4], chemiluminescence [5], colorimetry [6], spectrophotometric methods [7], and capillary electrophoresis [8]. In general, these methods exhibit high operational costs as well as a tedious pretreatment of the samples. On the contrary, electrochemical sensing is preferred as attractive alternative for DA determination due to its simple operation, fast response, low-cost, highsensitivity and excellent selectivity [9]. Moreover, electrochemical methods potentially allow the determination of DA inside the living organisms [10]. Metallophthalocyanine complexes (MPc) are stable compounds which possess electrocatalytic properties that can be tailored by modifying their molecular structure. Changing the central metal [11], the
peripheral substituents [12], as well as the film growing technique [13] have revealed powerful impact on their coordination and subsequent electrochemical behavior. Due to these features, MPcs have found wide applications as sensing materials for the detection of a variety of analytes. Cobalt nanoparticles [14] and complexes [15] have been successfully used for electrochemical sensors for DA. Moreover, among the different MPc complexes, cobalt phthalocyanine and its derivatives (CoPcs) have been among the most widely used materials applied in this field [16]. However, the use of CoPcs as electroactive catalysts at the electrode surface suffers from two major limitations: firstly, the physical adsorption of CoPcs on the electrode surface is weak and related films detach easily from the electrode surface. Secondly, CoPcs possess a low conductivity, which decreases the rate of electron transfer, thus leading to poor electrochemical activity [17]. In order to overcome these limitations, carbon materials, such as graphene, are often employed as conducting additives and supports [18]. Graphene consists of one-atom-thick two-dimensional sheet of sp2hybridized carbon atoms arranged in a honeycomb crystalline lattice [19]. In the past few years, it has received great attention as a conducting support to stabilize electrocatalysts [20] due to its exceptional physicochemical properties, such as large surface area and high conductivity [21]. Moreover, detection of dopamine has been achieved
Corresponding author. E-mail address: [email protected] (W. Schuhmann). 1 Equal contribution. ⁎
https://doi.org/10.1016/j.snb.2019.01.022 Received 8 July 2018; Received in revised form 20 December 2018; Accepted 6 January 2019 Available online 07 January 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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using graphene in combination with cobalt compounds, including cobalt oxide [22], cobalt tetraphenylporphyrin [23], cobalt hexacyanoferrate [24], and cobalt phthalocyanine [25]. However, in all of the aforementioned cases, graphene oxide or reduced graphene oxide were prepared following modified Hummer’s methods [26]. Besides contamination with Mn residues, the obtained graphene oxide and derived reduced graphene oxide show a high number of defects leading to substantially decreased electronic properties [27]. An alternative method for the preparation of a higher-quality pristine graphene is the direct exfoliation of graphite into a suitable liquid phase [27]. Furthermore, it is possible to functionalize graphene by providing planar aromatic molecules during the exfoliation process, such as porphyrins and phthalocyanines, taking advantage of their electronic properties [28] and their π−π stacking capabilities [29]. Herein, we successfully prepared few-layer graphene decorated with cobalt tetrasulfonated phthalocyanine (CoTSPc/Gr) during liquid-phase exfoliation of graphite. The obtained material was deposited onto glassy carbon (GC) electrode surfaces by means of a pulsed amperometry protocol. Cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV) were used to evaluate these modified electrodes with respect to the sensitive and selective determination of dopamine. 2. Experimental
electrode (CoTSPc/Gr-GC). Electrochemical measurements for the detection of DA, AA and UA were performed in a one-compartment cell using either GC or CoTSPc/ Gr-GC electrodes as working electrode, a Pt wire as auxiliary electrode, and a Ag/AgCl/3 M KCl as reference electrode. Before measurements, the modified electrodes were conditioned in 0.1 M PBS by means of cyclic voltammetry from 0 to 0.5 V vs. Ag/AgCl/3 M KCl at a scan rate of 100 mV s−1 until a reproducible response was obtained. Subsequently, cyclic voltammograms were recorded in 0.1 M PBS (pH 7.3) at a scan rate of 100 mV s−1. Investigation of the influence of pH value and scan rate on the oxidation peak currents for 1 mM DA were performed in a pH range from 2.0 to 10.0 with scan rates from 20 to 100 mV s−1, respectively. The electrolyte solutions with different pH values were prepared by addition of phosphoric acid to a 0.1 M PBS solution consisting of KH2PO4 and K2HPO4, with the pH value adjusted using either 0.1 M HCl or 0.1 M NaOH. Differential pulse stripping voltammetry (DPSV) was performed at a scan rate of 5 mV s−1 and a pulse height of 50 mV for the quantitative determination of DA. DA standard solutions were prepared in 4 mL 0.1 M PBS. An accumulation potential (Ea) of −0.8 V vs. Ag/AgCl/3 M KCl and an accumulation time (ta) of 70 s were selected to accumulate DA at the electrode surface prior to the DPSV measurements. All electrochemical measurements were performed with a Gamry Reference 600 potentiostat/galvanostat.
2.1. Chemicals
3. Results and discussion
Dopamine (DA), ascorbic acid (AA), uric acid (UA), graphite flakes and phthalocyanine tetrasulfonate hydrate (TSPc) were from SigmaAldrich. TSPc was metallated with cobalt following the method reported by Adler et. al. [30] by adding stoichiometric amounts of TSPc and CoCl2 (Sigma-Aldrich) to DMF (VWR) and keeping the reaction mixture at 100 °C for 2 h. Addition of an equal volume of cold water yielded CoTSPc as precipitate. All other chemicals were of analytical grade and used as received. A 0.1 M phosphate buffer solution (PBS) pH 7.3 was used as electrolyte in all electrochemical experiments for the determination of DA, whereas a 0.1 M NaOH solution was used as electrolyte for electrodeposition of the graphene/CoTSPc catalyst at the glassy carbon (GC) electrode surfaces.
3.1. Electrode surface modification and evaluation of DA oxidation at CoTSPc/Gr-GC electrodes Few-layer graphene modified with CoTSPc (CoTSPc/G) was obtained according to a procedure described previously [31], in which 10 mg mL−1 graphite flakes were dispersed in DMF in the presence of 0.025 mg mL−1 CoTSPc. After 8 h of ultrasonication in an ice bath, followed by centrifugation for 30 min at 4000 rpm, CoTSPc modified few-layer graphene sheets were obtained by careful collection of the supernatants by pipetting, filtering using 0.2 μm pore size nylon membranes and drying at 60 °C overnight. We demonstrated previously [31], that graphene prepared in the presence of CoTSPc exhibits different properties to non-modified graphene, including the degree of defectiveness observed by Raman spectroscopy, the stability of graphene dispersions derived by means of UV–vis spectroscopy, and the electrochemical behavior of the materials prepared using CoTSPc/G as precursor, clearly indicating that the modification of the graphene layers with CoTSPc was successful. For a detailed characterization including microscopic (SEM, AFM) and spectroscopic (UV–vis, EDX, XPS and Raman spectroscopy) investigations, we refer to [31]. Dip- and drop-dry techniques are well established electrode-surface modification methods which have been used with graphene and/or MPcs for the determination of DA [25]. Unfortunately, these methods suffer from high irreproducibility. Moreover, the layer thickness after modification cannot be precisely controlled. In contrast, an electrochemical approach for film deposition on the electrode surface is known to be highly reproducible leading to a controllable film thickness. We applied a previously introduced pulse amperometry protocol [32] to grow a CoTSPc/Gr film on GC electrodes. The protocol consists of alternately applying two short potential pulses, a sufficiently high potential to invoke film deposition ( + 600 mV vs Ag/AgCl/3 M KCl for 1 s) followed by a resting potential to reestablish the bulk concentration of the modified graphene dispersion in front of the electrode surface (−1400 mV vs Ag/AgCl/3 M KCl for 2 s). The pulse sequence is then repeated until a film with the desired characteristics is formed [32]. The current was recorded at the end of each pulse to allow the decay of the capacitive current for the visualization of the film growth as shown in Fig. 1. A gradual current increase was observed with successive pulses, which can be attributed to an increased surface area upon deposition of the CoTSPc/Gr flakes. The conductivity of the deposited film is
2.2. Catalyst synthesis CoTSPc/Gr was prepared as described previously [31]. In short, graphene dispersions were prepared by means of liquid-phase exfoliation mixing graphite flakes and DMF at a concentration of 10 mg mL−1 in the presence of 0.025 mg mL−1 CoTSPc, followed by 8 h ultrasonication in an ice bath and centrifugation for 30 min at 4000 rpm. The supernatants were collected by pipetting and vacuum-filtered using nylon membranes of 0.2 μm pore size (Whatman). The membranes were dried overnight at 60 °C, and the modified graphene powder was collected and ground. XPS analysis revealed a Co content of a pyrolyzed aliquot of CoTSPc/Gr of 0.10 wt% (data not shown, see [31]). 2.3. Electrochemical measurements GC electrodes (0.1134 cm2 geometric area) were polished with 0.05 μm alumina suspension to obtain a mirror-like, shiny electrode surface, then washed with water and sonicated in an ultrasonic bath for 3 min to remove residual alumina particles. The CoTSPc/Gr catalyst (3 mg mL−1) was dispersed in 0.1 M NaOH by sonication for 15 min. A three-electrode electrochemical cell consisting of a GC electrode as working electrode, a Pt wire as auxiliary electrode, and a Ag/AgCl/3 M KCl as reference electrode was used for the electrochemically induced deposition of CoTSPc/Gr on the electrode surface. The electrode potential was pulsed from a starting potential of + 600 mV vs Ag/AgCl/ 3 M KCl for 1 s followed by a final potential of −1400 mV for 2 s for a certain number of cycles to produce a CoTSPc/Gr-modified GC 18
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anodic and cathodic peaks are 100 mV for CoTSPc/Gr-GC, 175 mV for Gr-GC and 357 mV for the unmodified GC, which is also an indication that the oxidation of DA at the CoTSPc/Gr-GC electrode occurred at a considerably faster electron transfer rate [33], which may be due to both the activity provided by CoTSPc and the high electrical conductivity provided by graphene, with a further benefit originating from the π−π interaction between CoTSPc and graphene. 3.2. Optimization of the number of the CoTSPc/Gr deposition pulses To achieve optimal DA oxidation at CoTSPc/Gr film electrodes the number of pulses in the potential pulse sequence (as described in Section 3.1) was varied while monitoring the current response on the modified surfaces for the oxidation of 1 mM DA in PBS (pH 7.3) (Fig. 3A). The highest DA-oxidation current was observed when 60 potential pulse cycles were used for the CoTSPc/Gr deposition, as shown in Fig. 3B. Applying more than 60 deposition cycles resulted in a decreased DA oxidation current. 60 deposition cycles were therefore used for all subsequent experiments.
Fig. 1. Current–time profile obtained during the electrochemical induced deposition of CoTSPc/G (3 mg mL−1 CoTSPc/G in 0.1 M NaOH) on a GC electrode using sequential pulse profiles: + 600 mV vs. Ag/AgCl/3 M KCl was applied for 1 s followed by −1400 mV vs. Ag/AgCl/3 M KCl for 2 s.
3.3. Influence of the pH value on the peak potential and peak current of DAoxidation
supposed to exceed that of glassy carbon. The obtained modified CoTSPc/Gr-GC was evaluated with respect to its DA oxidation ability by means of cyclic voltammetry in PBS pH 7.3. The redox potential of CoTSPc/Gr-GC was derived from the anodic peak (284.1 mV) in a CV recorded at 100 mV s−1 in PBS (pH = 7.3). Additionally, the non-modified GC electrode and a GC electrode modified with graphene (Gr-GC) were similarly used for DA oxidation for comparison (Fig. 2). There is a significant enhancement of the voltammetric current response for DA oxidation at CoTSPc/Gr-GC electrode compared to the bare GC and the Gr-GC electrodes. In addition to the well-defined peak shape for DA oxidation at CoTSPc/Gr-GC electrode there is an 8 and 2-fold increase in peak current as compared with the unmodified and the Gr-GC electrodes, respectively. Evidently, the faster electron transfer kinetics for DA oxidation at the CoTSPc/Gr-GC electrode (Fig. 2; trace a) leads to an about 50 mV lower peak potential as compared with the Gr-GC electrode (Fig. 2; trace b) and of about 180 mV as compared with the bare GC electrode (Fig. 2; trace c). Moreover, the potential difference (ΔEp) between the
The influence of the pH value in the range from 2.0 to 10.0 on the oxidation peak currents of 1 mM DA was investigated. The anodic peak potentials for DA oxidation shift toward less positive values with increasing pH value (Fig. 4) in agreement with earlier studies on DA oxidation at graphene cobalt(II) complex modified GC electrodes [18], suggesting that protons are involved in the DA oxidation reaction. Although the maximum anodic peak current was obtained at pH 4.0, a pH value of 7.3 was chosen for further experiments to simulate physiological media. 3.4. Effect of scan rate on the DA oxidation peak To investigate the reaction kinetics, the effect of scan rate on the oxidation peak current of 1.0 mM DA in 0.1 M PBS (pH 7.3) was evaluated. The anodic peak current increases with increasing scan rate (ѵ) (Fig. 5A) and the linear correlation of the peak current versus ѵ1/2 from 20 to 100 mV s−1 indicates as expected diffusion controlled DA oxidation (Fig. 5B). 3.5. Effect of accumulation potential and accumulation time on the peak currents of DA oxidation DA determination at CoTSPc/Gr-GC electrodes was performed by means of differential pulse stripping voltammetry (DPSV) after accumulation of DA on the electrode surface by applying a predefined accumulation potential (Ea) for a defined time (ta). The influence of Ea was investigated in the potential range from +0.1 V to −0.9 V vs. Ag/ AgCl/3 M KCl aiming on improved sensitivity for DA determination. Fig. 6A shows DPSVs performed at a scan rate of 5 mV s−1 and a pulse height of 50 mV in 10 μM DA (pH 7.3) after DA accumulation for 20 s at varying Ea. The DA oxidation peak potential is located at 280 mV vs. Ag/AgCl/3 M KCl and increased peak currents are recorded when DA accumulation was performed at more negative potentials (Fig. 6B). Additionally, the effect of ta was studied in the range from 5 to 80 s at constant Ea of −0.8 V vs. Ag/AgCl/3 M KCl. Fig. 6C shows the correlation between ta and the peak current for the oxidation of 10 μM DA in PBS. The DA oxidation peak currents increased up to ta = 70 s with a current saturation at longer ta. Accordingly, Ea of −0.8 V vs. Ag/AgCl/ 3 M KCl and ta of 70 s were selected as optimized parameters for further measurements.
Fig. 2. Cyclic voltammograms of 1 mM dopamine in 0.1 M PBS (pH 7.3) at: (a) CoTSPc/Gr-modified GC electrode, (b) graphene modified GC electrode, and (c) bare GC electrode (scan rate = 100 mV s−1). 19
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Fig. 3. Dependence of the DA oxidation on the number of potential-pulse deposition cycles used to modify GC electrodes with CoTSPc/Gr: A) cyclic voltammograms registered in 1 mM DA solution in PBS (pH 7.3) at 100 mV s−1. B) Variation of the background-subtracted oxidation-peak current with the number of applied deposition pulses: (a) for the bare GC electrode, and for electrodes modified using (b) 15, (c) 30, (d) 60, and (e) 90 deposition pulses.
Fig. 4. pH effect on the oxidation of 1 mM of DA at modified CoTSPc/Gr-GC electrodes; A) cyclic voltammograms in 0.1 M PBS recorded at various pH values at 100 mV s−1, B) DA oxidation peak potential vs. pH.
Fig. 5. Effect of the scan rate on DA oxidation at the CoTSPs/Gr-GC electrode in 1 mM DA in 0.1 M PBS, pH 7.3. A) Cyclic voltammograms at different scan rates. B) Plot of the background-subtracted DA oxidation current vs. the square root of the scan rate (ѵ1/2).
3.6. Determination of DA at CoTSPc/Gr-GC modified electrodes
In addition to the high sensitivity, no substantial differences in the voltammetric signals were observed when the sensor was used few minutes or few days after modifying the GC electrode with CoTSPc/Gr revealing sufficient storage stability. Moreover, the operational stability was evaluated by using the same sensor for about 50 consecutive measurements including cyclic voltammetry and differential pulse voltammetry. For instance, Fig. 7 shows the quantitative determination of DA using DPSV for seven standard solutions, each of which was measured with the same electrode and replicated three times with excellent linearity.
Quantitative determination of DA using the proposed CoTSPc/GrGC modified electrodes has been performed using DPSV and DA standard solutions in 0.1 M PBS. The electrocatalytic oxidation of DA at CoTSPc/Gr-GC modified electrodes at an anodic peak potential of about 280 mV vs. Ag/AgCl/3 M KCl (Fig. 7A) leads to the corresponding calibration graph in a concentration range between 20 and 220 nM DA (Fig. 7B). A sensitivity of 0.302 μA nM DA−1 was determined from the slope of the linear regression and detection limit of 0.87 nM DA was calculated from 3 times the noise level of the background current, clearly outperforming previously reported graphene-supported cobaltbased sensors for the electrochemical detection of DA, which exhibit sensitivities in the μM range [22–25].
3.7. Interference and reproducibility It is well known that DA, ascorbic acid (AA) and uric acid (UA) 20
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Fig. 6. Optimization of accumulation potential (Ea) and accumulation time (ta) for DA oxidation of 10 μM DA in 0.1 M PBS (pH 7.3) at a CoTSPc/Gr-GC modified electrode. A) DPSVs at a scan rate of 5 mV s−1, pulse height of 50 mV, pulse width: 0.1 s. B) Peak currents as function of Ea for ta = 20 s. C) DPSV peak current as a function of ta at Ea = −0.8 V vs. Ag/AgCl/3 M KCl. Fig. 7. A) DPSVs at a CoTSPc/Gr-GC modified electrode upon successive addition of DA in 0.1 M PBS (pH 7.3): (a) 0 nM, (b) 20 nM, (c) 40 nM, (d) 60 nM, (e) 100 nM, (f) 140 nM, (g) 180 nM, and (h) 220 nM of DA. B) Calibration curve for DA oxidation at a CoTSPc/Gr-GC modified electrode (scan rate = 5 mV s−1; pulse height = 50 mV; pulse width = 0.1 s).
Fig. 8. Cyclic voltammograms of: (a) 10 mM AA, (b) 10 mM UA, and (c) a mixture of 0.3 mM DA, 10 mM AA and 10 mM UA at CoTSPc/Gr-GC modified electrodes in 0.1 M PBS (pH 7.3, scan rate = 100 mV s−1).
Fig. 9. DPSVs of 5 μM DA at CoTSPc/Gr-GC electrodes: (a) in the absence of AA and UA, (b) in presence of 1 mM AA and 1 mM UA, and (c) in presence 2 mM AA and 2 mM UA, in 0.1 M PBS (pH 7.3, Ea = −0.8 V, ta = 70 s, scan rate = 5 mV s−1, pulse height = 50 mV, pulse width = 0.1 s).
coexist in the extra cellular fluid of the central nervous system [34]. They are oxidized at very close potentials at bare electrodes resulting in overlapped voltammetric responses [35]. Therefore, AA and UA were chosen as potentially interfering compounds to demonstrate the selectivity of the CoTSPc/Gr-GC modified electrodes toward DA (Fig. 8). The cyclic voltammograms for AA (Fig. 8; trace a) and UA (Fig. 8; trace b) showed oxidation signals at 570 mV and 477 mV vs. Ag/AgCl/ 3 M KCl, respectively, whereas, the oxidation peak of DA is located at 295 mV vs. Ag/AgCl/3 M KCl (Fig. 2). The CV recorded in the presence of 0.3 mM DA, 10 mM AA and 10 mM UA at the CoTSPc/Gr-GC modified electrode (Fig. 8; trace c) shows two pronounced oxidation peaks with peak potentials of 338 mV and 567 mV, respectively. Although the
first peak can be attributed mainly to the oxidation of DA, it appears at higher potentials in the presence of AA and UA (Fig. 8) compared to DA alone (Fig. 2), clearly indicating that the DA oxidation peak overlaps with those of AA and UA oxidation. The second peak (567 mV) is attributed mainly to an overlap of AA and UA oxidation peaks. Hence, cyclic voltammetry is not a suitable technique to selectively determine DA in the presence of AA and UA. DPSV was therefore used to demonstrate the selectivity of the modified CoTSPc/Gr-GC electrode for oxidation of DA alongside AA and UA. The oxidation current of DA was examined in the presence of various concentrations of AA and UA. Fig. 9 shows the obtained DPSVs 21
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Fig. 10. A) DPSVs of various concentrations of DA in the presence of 10 mM AA and 10 mM UA at CoTSPc/Gr-GC modified electrodes: (a) 0.5 μM, (b) 0.6 μM, (c) 0.7 μM, (d) 0.8 μM, (e) 0.9 μM, and (f) 1.0 μM DA. B) Corresponding plot of the peak current at 270 mV vs. Ag/ AgCl/3 M KCl as a function of the DA concentration in 0.1 M PBS (pH 7.3, Ea = -0.8 V, ta = 70 s, scan rate = 5 mV s−1, pulse height = 50 mV, pulse width = 0.1 s).
for a constant concentration of 5 μM DA at different concentrations of AA and UA. The results clearly demonstrate that DA can be quantitatively determined in the presence of substantially higher concentrations of AA and UA with a relative standard deviation (RSD) of 4.8%. Determination of DA in the 0.5–1.0 μM concentration range was performed in the presence of high concentrations of both AA (10 mM) and UA (10 mM) by DPSV (Fig. 10A). Two clear peaks are observed at 270 mV vs. Ag/AgCl/3 M KCl assigned to DA oxidation and at 508 mV vs. Ag/AgCl/3 M KCl assigned to UA and AA oxidation. Increasing DA concentration leads to an increase in the intensity of the first peak, similar to the response shown in Fig. 7A, whereas just a minor random change was observed for the second peak (RSD = 4.6%). The oxidation peak current was directly proportional to the DA concentration (Fig. 10B). The separation between the DA oxidation peaks (276 mV) and AA and UA oxidation (512 mV) was sufficiently large to allow quantification of DA in presence of more than 10 times higher concentrations of AA and UA. We attribute this high sensitivity in the detection of DA even in the presence of high concentrations of potentially interfering compounds as revealed by the CoTSPc/Gr-GC modified electrodes to the positive charged amino group of DA at the measuring pH value of 7.3 which facilitates the adsorption of DA on the negatively charged CoTSPc surface. Simultaneously, the negatively charged CoTSPc prevents the adsorption of negatively charged AA and UA on the electrode surface. Moreover, the electrochemical behavior of DA may be facilitated by π−π interaction between DA and graphene in contrast to AA [9], whereas only weak π–π interactions occur between UA and graphene [36]. Cobalt as central metal atom may allow axial coordination between CoPc and DA [37] which may be responsible for the observed high sensitivity. The reproducibility of DA determination at CoTSPc/Gr-GC modified electrodes were examined using DPSV at Ea = −0.8 V vs. Ag/AgCl/3 M KCl and ta = 70 s in 0.1 M PBS. A satisfying reproducibility was obtained with a RSD of 4.2% (n = 5) and of 5.2% (n = 5) at DA concentrations of 2 μM and 0.1 μM, respectively. Differences in current intensity among sensitivity and selectivity investigations as shown in Figs. 7A and 10 A, respectively, can be attributed to the difference in DA concentration. Since accumulation parameters were optimized for nM concentrations, it is likely that at larger concentrations a more pronounced accumulation of analyte molecules at the modified electrode surface occurs, leading to a decrease in sensitivity. Hence, the proposed CoTSPc/Gr-GC sensor is more suitable for the quantification of low DA concentrations.
sensitivity for DA determination in the nM range, with a low detection limit of 0.87 nM. Moreover, the electrodes showed an excellent selectivity that could be used for quantitation of low concentrations of DA in the presence of high concentrations of AA and UA, suggesting the applicability of these materials for determination of DA in complex solutions. We aim towards the development of corresponding CoTSPc/ Gr modified microelectrodes for potential in-vivo determination of DA in future. Declarations of interest None. Acknowledgments N. Diab and M. Masoud acknowledge the financial support from the Scientific Research Deanship at Arab American University, Jenin (Palestine). N. Diab acknowledges the financial support from Welfare Association (UK) through the Zamala Fellowship Program. D. M. Morales is grateful to Deutscher Akademischer Austauschdienst (Germany) and to Consejo Nacional de Ciencia y Tecnología (Mexico) for the financial support. The authors are grateful for financial support by the Deutsche Forschungsgemeinschaft (Germany) in the framework of the project FLAG-ERA JTC 2015 “Graphtivity” (Schu929/14-1). References [1] K.A. Jennings, ACS Chem. Neurosci. 4 (2013) 704. [2] X. Xia, X. Shen, Y. Du, W. Ye, C. Wang, Sens. Actuator B Chem. 237 (2016) 685. [3] G.E. de Benedetto, D. Fico, A. Pennetta, C. Malitesta, G. Nicolardi, D.D. Lofrumento, F. de Nuccio, V. La Pesa, J. Pharm. Biomed. Anal. 98 (2014) 266. [4] A. Naccarato, E. Gionfriddo, G. Sindona, A. Tagarelli, Anal. Chim. Acta 810 (2014) 17. [5] H. Duan, L. Li, X. Wang, Y. Wang, J. Li, C. Luo, Spectrochim. Acta A 139 (2015) 374. [6] L. Liu, S. Li, L. Liu, D. Deng, N. Xia, Analyst 137 (2012) 3794. [7] M. Mamiński, M. Olejniczak, M. Chudy, A. Dybko, Z. Brzózka, Anal. Chim. Acta 540 (2005) 153. [8] Y.H. Park, X. Zhang, S.S. Rubakhin, J.V. Sweedler, Anal. Chem. 71 (1999) 4997. [9] Y. Wang, Y. Li, L. Tang, J. Lu, J. Li, Electrochem. Commun. 11 (2009) 889. [10] S.-M. Chen, W.-Y. Chzo, J. Electroanal. Chem. Lausanne (Lausanne) 587 (2006) 226. [11] V. Parra, Á.A. Arrieta, J.A. Fernández-Escudero, H. García, C. Apetrei, M.L. Rodríguez-Méndez, J.A.d. Saja, Sens. Actuator B Chem. 115 (2006) 54. [12] J.H. Zagal, S. Griveau, J.F. Silva, T. Nyokong, F. Bedioui, Coord. Chem. Rev. 254 (2010) 2755. [13] F. Bedioui, S. Griveau, T. Nyokong, A.J. Appleby, C.A. Caro, M. Gulppi, G. Ochoa, J.H. Zagal, Phys. Chem. Chem. Phys. 9 (2007) 3383. [14] A. Kutluay, M. Aslanoglu, Anal. Chim. Acta 839 (2014) 59. [15] L. Yang, X. Li, Y. Xiong, X. Liu, X. Li, M. Wang, S. Yan, L.A.M. Alshahrani, P. Liu, C. Zhang, J. Electroanal. Chem. Lausanne (Lausanne) 731 (2014) 14. [16] K. Wang, J.-J. Xu, H.-Y. Chen, Biosens. Bioelectron. 20 (2005) 1388. [17] V. Mani, S.-T. Huang, R. Devasenathipathy, T.C.K. Yang, RSC Adv. 6 (2016) 38463. [18] H. Wang, Y. Bu, W. Dai, K. Li, H. Wang, X. Zuo, Sens. Actuator B Chem. 216 (2015) 298. [19] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. Engl. 48 (2009) 7752. [20] H. Kim, A.A. Abdala, C.W. Macosko, Macromolecules 43 (2010) 6515.
4. Conclusions Graphene sheets decorated with cobalt tetrasulfonated phthalocyanine were electrochemically-induced deposited on GC electrodes using a sequence of potential pulses. The modified electrodes were employed for the electrocatalytic oxidation of dopamine by means of CV and DPSV. After optimization of the accumulation potential and the accumulation time the CoTSPc/Gr-GC modified electrodes exhibited high 22
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N. Diab et al. [21] V. Mani, B. Dinesh, S.-M. Chen, R. Saraswathi, Biosens. Bioelectron. 53 (2014) 420. [22] A. Numan, M.M. Shahid, F.S. Omar, K. Ramesh, S. Ramesh, Sens. Actuator B-Chem. 238 (2017) 1043. [23] K. Deng, J. Zhou, X. Li, Electrochim. Acta 114 (2013) 341. [24] Q. Wang, Q. Tang, Microchim. Acta 182 (2015) 671. [25] J. Yang, Mu Di, Y. Gao, J. Tan, A. Lu, D. Ma, J. Nat. Gas Chem. 21 (2012) 265. [26] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [27] Y. Wei, Z. Sun, Curr. Opin. Colloid Interface Sci. 20 (2015) 311. [28] C.G. Claessens, U. Hahn, T. Torres, Chem. Rec. 8 (2008) 75. [29] D. Parviz, S. Das, H.S.T. Ahmed, F. Irin, S. Bhattacharia, M.J. Green, ACS Nano 6 (2012) 8857. [30] A.D. Adler, F.R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970) 2443.
[31] D.M. Morales, J. Masa, C. Andronescu, Y.U. Kayran, Z. Sun, W. Schuhmann, Electrochim. Acta 222 (2016) 1191. [32] W. Schuhmann, C. Kranz, H. Wohlschläger, J. Strohmeier, Biosens. Bioelectron. 12 (1997) 1157. [33] G. Alarcón-Angeles, B. Pérez-López, M. Palomar-Pardave, M.T. Ramírez-Silva, S. Alegret, A. Merkoçi, Carbon 46 (2008) 898. [34] A.J. Downard, A.D. Roddick, A.M. Bond, Anal. Chim. Acta 317 (1995) 303. [35] S.S. Shankar, B.E.K. Swamy, U. Chandra, J.G. Manjunatha, B.S. Sherigara, Int. J. Electrochem. Sci. 4 (2009) 592. [36] C. Cuadrado, L. Ibarra, J. Hurtado, O. García-Beltrán, E. Nagles, Anal. Bioanal. Electrochem. 8 (2016) 910. [37] J. Oni, T. Nyokong, Anal. Chim. Acta 434 (2001) 9.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A sensitive and selective graphene/cobalt tetrasulfonated phthalocyanine sensor for detection of dopamine
T
Nizam Diaba,b,1, Dulce M. Moralesa,1, Corina Andronescua,c, Muayad Masoudb, ⁎ Wolfgang Schuhmanna, a
Analytical Chemistry - Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany b Chemistry Department, Arab American University, P.O. Box: 240 Jenin, Palestine c Technical Chemistry III, Faculty of Chemistry, University Duisburg-Essen, Carl-Benz-Straße 199, D-47057 Duisburg, Germany
ARTICLE INFO
ABSTRACT
Keywords: Dopamine Graphene Cobalt tetrasulfonate phthalocyanine Differential pulse stripping voltammetry Electrochemical sensor
Electrochemical dopamine-sensing surfaces were fabricated by deposition of graphene sheets modified with cobalt tetrasulfonated phthalocyanine on glassy carbon electrodes (CoTSPc/Gr-GC) using differential pulse amperometry. Differential pulse stripping voltammetry was used to detect dopamine (DA) and the influence of pH value, scan rate, accumulation potential and time as well as dopamine concentration on the performance of CoTSPc/Gr-GC electrodes was investigated. The modified electrodes were successfully used as sensors for the selective and high sensitive determination of DA in presence of high concentrations of ascorbic acid (AA) and uric acid (UA) with a detection limit of 0.87 nM over the dynamic linear range of 20 nM to 220 nM.
1. Introduction Dopamine (DA) is a catecholamine neurotransmitter, which plays a significant role in the central nervous system [1]. A decreased production is correlated with the development of neurodegenerative diseases such as Alzheimer and Parkinson [2]. Therefore, the accurate and high-sensitive detection as well as monitoring of low concentrations of dopamine optimally in-vivo represents an important issue in medical diagnosis. A variety of different analytical methods have been developed for the determination of dopamine, including high performance liquid chromatography (HPLC) with fluorimetric detection [3], gas chromatography-mass spectrometry [4], chemiluminescence [5], colorimetry [6], spectrophotometric methods [7], and capillary electrophoresis [8]. In general, these methods exhibit high operational costs as well as a tedious pretreatment of the samples. On the contrary, electrochemical sensing is preferred as attractive alternative for DA determination due to its simple operation, fast response, low-cost, highsensitivity and excellent selectivity [9]. Moreover, electrochemical methods potentially allow the determination of DA inside the living organisms [10]. Metallophthalocyanine complexes (MPc) are stable compounds which possess electrocatalytic properties that can be tailored by modifying their molecular structure. Changing the central metal [11], the
peripheral substituents [12], as well as the film growing technique [13] have revealed powerful impact on their coordination and subsequent electrochemical behavior. Due to these features, MPcs have found wide applications as sensing materials for the detection of a variety of analytes. Cobalt nanoparticles [14] and complexes [15] have been successfully used for electrochemical sensors for DA. Moreover, among the different MPc complexes, cobalt phthalocyanine and its derivatives (CoPcs) have been among the most widely used materials applied in this field [16]. However, the use of CoPcs as electroactive catalysts at the electrode surface suffers from two major limitations: firstly, the physical adsorption of CoPcs on the electrode surface is weak and related films detach easily from the electrode surface. Secondly, CoPcs possess a low conductivity, which decreases the rate of electron transfer, thus leading to poor electrochemical activity [17]. In order to overcome these limitations, carbon materials, such as graphene, are often employed as conducting additives and supports [18]. Graphene consists of one-atom-thick two-dimensional sheet of sp2hybridized carbon atoms arranged in a honeycomb crystalline lattice [19]. In the past few years, it has received great attention as a conducting support to stabilize electrocatalysts [20] due to its exceptional physicochemical properties, such as large surface area and high conductivity [21]. Moreover, detection of dopamine has been achieved
Corresponding author. E-mail address: [email protected] (W. Schuhmann). 1 Equal contribution. ⁎
https://doi.org/10.1016/j.snb.2019.01.022 Received 8 July 2018; Received in revised form 20 December 2018; Accepted 6 January 2019 Available online 07 January 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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using graphene in combination with cobalt compounds, including cobalt oxide [22], cobalt tetraphenylporphyrin [23], cobalt hexacyanoferrate [24], and cobalt phthalocyanine [25]. However, in all of the aforementioned cases, graphene oxide or reduced graphene oxide were prepared following modified Hummer’s methods [26]. Besides contamination with Mn residues, the obtained graphene oxide and derived reduced graphene oxide show a high number of defects leading to substantially decreased electronic properties [27]. An alternative method for the preparation of a higher-quality pristine graphene is the direct exfoliation of graphite into a suitable liquid phase [27]. Furthermore, it is possible to functionalize graphene by providing planar aromatic molecules during the exfoliation process, such as porphyrins and phthalocyanines, taking advantage of their electronic properties [28] and their π−π stacking capabilities [29]. Herein, we successfully prepared few-layer graphene decorated with cobalt tetrasulfonated phthalocyanine (CoTSPc/Gr) during liquid-phase exfoliation of graphite. The obtained material was deposited onto glassy carbon (GC) electrode surfaces by means of a pulsed amperometry protocol. Cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV) were used to evaluate these modified electrodes with respect to the sensitive and selective determination of dopamine. 2. Experimental
electrode (CoTSPc/Gr-GC). Electrochemical measurements for the detection of DA, AA and UA were performed in a one-compartment cell using either GC or CoTSPc/ Gr-GC electrodes as working electrode, a Pt wire as auxiliary electrode, and a Ag/AgCl/3 M KCl as reference electrode. Before measurements, the modified electrodes were conditioned in 0.1 M PBS by means of cyclic voltammetry from 0 to 0.5 V vs. Ag/AgCl/3 M KCl at a scan rate of 100 mV s−1 until a reproducible response was obtained. Subsequently, cyclic voltammograms were recorded in 0.1 M PBS (pH 7.3) at a scan rate of 100 mV s−1. Investigation of the influence of pH value and scan rate on the oxidation peak currents for 1 mM DA were performed in a pH range from 2.0 to 10.0 with scan rates from 20 to 100 mV s−1, respectively. The electrolyte solutions with different pH values were prepared by addition of phosphoric acid to a 0.1 M PBS solution consisting of KH2PO4 and K2HPO4, with the pH value adjusted using either 0.1 M HCl or 0.1 M NaOH. Differential pulse stripping voltammetry (DPSV) was performed at a scan rate of 5 mV s−1 and a pulse height of 50 mV for the quantitative determination of DA. DA standard solutions were prepared in 4 mL 0.1 M PBS. An accumulation potential (Ea) of −0.8 V vs. Ag/AgCl/3 M KCl and an accumulation time (ta) of 70 s were selected to accumulate DA at the electrode surface prior to the DPSV measurements. All electrochemical measurements were performed with a Gamry Reference 600 potentiostat/galvanostat.
2.1. Chemicals
3. Results and discussion
Dopamine (DA), ascorbic acid (AA), uric acid (UA), graphite flakes and phthalocyanine tetrasulfonate hydrate (TSPc) were from SigmaAldrich. TSPc was metallated with cobalt following the method reported by Adler et. al. [30] by adding stoichiometric amounts of TSPc and CoCl2 (Sigma-Aldrich) to DMF (VWR) and keeping the reaction mixture at 100 °C for 2 h. Addition of an equal volume of cold water yielded CoTSPc as precipitate. All other chemicals were of analytical grade and used as received. A 0.1 M phosphate buffer solution (PBS) pH 7.3 was used as electrolyte in all electrochemical experiments for the determination of DA, whereas a 0.1 M NaOH solution was used as electrolyte for electrodeposition of the graphene/CoTSPc catalyst at the glassy carbon (GC) electrode surfaces.
3.1. Electrode surface modification and evaluation of DA oxidation at CoTSPc/Gr-GC electrodes Few-layer graphene modified with CoTSPc (CoTSPc/G) was obtained according to a procedure described previously [31], in which 10 mg mL−1 graphite flakes were dispersed in DMF in the presence of 0.025 mg mL−1 CoTSPc. After 8 h of ultrasonication in an ice bath, followed by centrifugation for 30 min at 4000 rpm, CoTSPc modified few-layer graphene sheets were obtained by careful collection of the supernatants by pipetting, filtering using 0.2 μm pore size nylon membranes and drying at 60 °C overnight. We demonstrated previously [31], that graphene prepared in the presence of CoTSPc exhibits different properties to non-modified graphene, including the degree of defectiveness observed by Raman spectroscopy, the stability of graphene dispersions derived by means of UV–vis spectroscopy, and the electrochemical behavior of the materials prepared using CoTSPc/G as precursor, clearly indicating that the modification of the graphene layers with CoTSPc was successful. For a detailed characterization including microscopic (SEM, AFM) and spectroscopic (UV–vis, EDX, XPS and Raman spectroscopy) investigations, we refer to [31]. Dip- and drop-dry techniques are well established electrode-surface modification methods which have been used with graphene and/or MPcs for the determination of DA [25]. Unfortunately, these methods suffer from high irreproducibility. Moreover, the layer thickness after modification cannot be precisely controlled. In contrast, an electrochemical approach for film deposition on the electrode surface is known to be highly reproducible leading to a controllable film thickness. We applied a previously introduced pulse amperometry protocol [32] to grow a CoTSPc/Gr film on GC electrodes. The protocol consists of alternately applying two short potential pulses, a sufficiently high potential to invoke film deposition ( + 600 mV vs Ag/AgCl/3 M KCl for 1 s) followed by a resting potential to reestablish the bulk concentration of the modified graphene dispersion in front of the electrode surface (−1400 mV vs Ag/AgCl/3 M KCl for 2 s). The pulse sequence is then repeated until a film with the desired characteristics is formed [32]. The current was recorded at the end of each pulse to allow the decay of the capacitive current for the visualization of the film growth as shown in Fig. 1. A gradual current increase was observed with successive pulses, which can be attributed to an increased surface area upon deposition of the CoTSPc/Gr flakes. The conductivity of the deposited film is
2.2. Catalyst synthesis CoTSPc/Gr was prepared as described previously [31]. In short, graphene dispersions were prepared by means of liquid-phase exfoliation mixing graphite flakes and DMF at a concentration of 10 mg mL−1 in the presence of 0.025 mg mL−1 CoTSPc, followed by 8 h ultrasonication in an ice bath and centrifugation for 30 min at 4000 rpm. The supernatants were collected by pipetting and vacuum-filtered using nylon membranes of 0.2 μm pore size (Whatman). The membranes were dried overnight at 60 °C, and the modified graphene powder was collected and ground. XPS analysis revealed a Co content of a pyrolyzed aliquot of CoTSPc/Gr of 0.10 wt% (data not shown, see [31]). 2.3. Electrochemical measurements GC electrodes (0.1134 cm2 geometric area) were polished with 0.05 μm alumina suspension to obtain a mirror-like, shiny electrode surface, then washed with water and sonicated in an ultrasonic bath for 3 min to remove residual alumina particles. The CoTSPc/Gr catalyst (3 mg mL−1) was dispersed in 0.1 M NaOH by sonication for 15 min. A three-electrode electrochemical cell consisting of a GC electrode as working electrode, a Pt wire as auxiliary electrode, and a Ag/AgCl/3 M KCl as reference electrode was used for the electrochemically induced deposition of CoTSPc/Gr on the electrode surface. The electrode potential was pulsed from a starting potential of + 600 mV vs Ag/AgCl/ 3 M KCl for 1 s followed by a final potential of −1400 mV for 2 s for a certain number of cycles to produce a CoTSPc/Gr-modified GC 18
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anodic and cathodic peaks are 100 mV for CoTSPc/Gr-GC, 175 mV for Gr-GC and 357 mV for the unmodified GC, which is also an indication that the oxidation of DA at the CoTSPc/Gr-GC electrode occurred at a considerably faster electron transfer rate [33], which may be due to both the activity provided by CoTSPc and the high electrical conductivity provided by graphene, with a further benefit originating from the π−π interaction between CoTSPc and graphene. 3.2. Optimization of the number of the CoTSPc/Gr deposition pulses To achieve optimal DA oxidation at CoTSPc/Gr film electrodes the number of pulses in the potential pulse sequence (as described in Section 3.1) was varied while monitoring the current response on the modified surfaces for the oxidation of 1 mM DA in PBS (pH 7.3) (Fig. 3A). The highest DA-oxidation current was observed when 60 potential pulse cycles were used for the CoTSPc/Gr deposition, as shown in Fig. 3B. Applying more than 60 deposition cycles resulted in a decreased DA oxidation current. 60 deposition cycles were therefore used for all subsequent experiments.
Fig. 1. Current–time profile obtained during the electrochemical induced deposition of CoTSPc/G (3 mg mL−1 CoTSPc/G in 0.1 M NaOH) on a GC electrode using sequential pulse profiles: + 600 mV vs. Ag/AgCl/3 M KCl was applied for 1 s followed by −1400 mV vs. Ag/AgCl/3 M KCl for 2 s.
3.3. Influence of the pH value on the peak potential and peak current of DAoxidation
supposed to exceed that of glassy carbon. The obtained modified CoTSPc/Gr-GC was evaluated with respect to its DA oxidation ability by means of cyclic voltammetry in PBS pH 7.3. The redox potential of CoTSPc/Gr-GC was derived from the anodic peak (284.1 mV) in a CV recorded at 100 mV s−1 in PBS (pH = 7.3). Additionally, the non-modified GC electrode and a GC electrode modified with graphene (Gr-GC) were similarly used for DA oxidation for comparison (Fig. 2). There is a significant enhancement of the voltammetric current response for DA oxidation at CoTSPc/Gr-GC electrode compared to the bare GC and the Gr-GC electrodes. In addition to the well-defined peak shape for DA oxidation at CoTSPc/Gr-GC electrode there is an 8 and 2-fold increase in peak current as compared with the unmodified and the Gr-GC electrodes, respectively. Evidently, the faster electron transfer kinetics for DA oxidation at the CoTSPc/Gr-GC electrode (Fig. 2; trace a) leads to an about 50 mV lower peak potential as compared with the Gr-GC electrode (Fig. 2; trace b) and of about 180 mV as compared with the bare GC electrode (Fig. 2; trace c). Moreover, the potential difference (ΔEp) between the
The influence of the pH value in the range from 2.0 to 10.0 on the oxidation peak currents of 1 mM DA was investigated. The anodic peak potentials for DA oxidation shift toward less positive values with increasing pH value (Fig. 4) in agreement with earlier studies on DA oxidation at graphene cobalt(II) complex modified GC electrodes [18], suggesting that protons are involved in the DA oxidation reaction. Although the maximum anodic peak current was obtained at pH 4.0, a pH value of 7.3 was chosen for further experiments to simulate physiological media. 3.4. Effect of scan rate on the DA oxidation peak To investigate the reaction kinetics, the effect of scan rate on the oxidation peak current of 1.0 mM DA in 0.1 M PBS (pH 7.3) was evaluated. The anodic peak current increases with increasing scan rate (ѵ) (Fig. 5A) and the linear correlation of the peak current versus ѵ1/2 from 20 to 100 mV s−1 indicates as expected diffusion controlled DA oxidation (Fig. 5B). 3.5. Effect of accumulation potential and accumulation time on the peak currents of DA oxidation DA determination at CoTSPc/Gr-GC electrodes was performed by means of differential pulse stripping voltammetry (DPSV) after accumulation of DA on the electrode surface by applying a predefined accumulation potential (Ea) for a defined time (ta). The influence of Ea was investigated in the potential range from +0.1 V to −0.9 V vs. Ag/ AgCl/3 M KCl aiming on improved sensitivity for DA determination. Fig. 6A shows DPSVs performed at a scan rate of 5 mV s−1 and a pulse height of 50 mV in 10 μM DA (pH 7.3) after DA accumulation for 20 s at varying Ea. The DA oxidation peak potential is located at 280 mV vs. Ag/AgCl/3 M KCl and increased peak currents are recorded when DA accumulation was performed at more negative potentials (Fig. 6B). Additionally, the effect of ta was studied in the range from 5 to 80 s at constant Ea of −0.8 V vs. Ag/AgCl/3 M KCl. Fig. 6C shows the correlation between ta and the peak current for the oxidation of 10 μM DA in PBS. The DA oxidation peak currents increased up to ta = 70 s with a current saturation at longer ta. Accordingly, Ea of −0.8 V vs. Ag/AgCl/ 3 M KCl and ta of 70 s were selected as optimized parameters for further measurements.
Fig. 2. Cyclic voltammograms of 1 mM dopamine in 0.1 M PBS (pH 7.3) at: (a) CoTSPc/Gr-modified GC electrode, (b) graphene modified GC electrode, and (c) bare GC electrode (scan rate = 100 mV s−1). 19
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Fig. 3. Dependence of the DA oxidation on the number of potential-pulse deposition cycles used to modify GC electrodes with CoTSPc/Gr: A) cyclic voltammograms registered in 1 mM DA solution in PBS (pH 7.3) at 100 mV s−1. B) Variation of the background-subtracted oxidation-peak current with the number of applied deposition pulses: (a) for the bare GC electrode, and for electrodes modified using (b) 15, (c) 30, (d) 60, and (e) 90 deposition pulses.
Fig. 4. pH effect on the oxidation of 1 mM of DA at modified CoTSPc/Gr-GC electrodes; A) cyclic voltammograms in 0.1 M PBS recorded at various pH values at 100 mV s−1, B) DA oxidation peak potential vs. pH.
Fig. 5. Effect of the scan rate on DA oxidation at the CoTSPs/Gr-GC electrode in 1 mM DA in 0.1 M PBS, pH 7.3. A) Cyclic voltammograms at different scan rates. B) Plot of the background-subtracted DA oxidation current vs. the square root of the scan rate (ѵ1/2).
3.6. Determination of DA at CoTSPc/Gr-GC modified electrodes
In addition to the high sensitivity, no substantial differences in the voltammetric signals were observed when the sensor was used few minutes or few days after modifying the GC electrode with CoTSPc/Gr revealing sufficient storage stability. Moreover, the operational stability was evaluated by using the same sensor for about 50 consecutive measurements including cyclic voltammetry and differential pulse voltammetry. For instance, Fig. 7 shows the quantitative determination of DA using DPSV for seven standard solutions, each of which was measured with the same electrode and replicated three times with excellent linearity.
Quantitative determination of DA using the proposed CoTSPc/GrGC modified electrodes has been performed using DPSV and DA standard solutions in 0.1 M PBS. The electrocatalytic oxidation of DA at CoTSPc/Gr-GC modified electrodes at an anodic peak potential of about 280 mV vs. Ag/AgCl/3 M KCl (Fig. 7A) leads to the corresponding calibration graph in a concentration range between 20 and 220 nM DA (Fig. 7B). A sensitivity of 0.302 μA nM DA−1 was determined from the slope of the linear regression and detection limit of 0.87 nM DA was calculated from 3 times the noise level of the background current, clearly outperforming previously reported graphene-supported cobaltbased sensors for the electrochemical detection of DA, which exhibit sensitivities in the μM range [22–25].
3.7. Interference and reproducibility It is well known that DA, ascorbic acid (AA) and uric acid (UA) 20
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Fig. 6. Optimization of accumulation potential (Ea) and accumulation time (ta) for DA oxidation of 10 μM DA in 0.1 M PBS (pH 7.3) at a CoTSPc/Gr-GC modified electrode. A) DPSVs at a scan rate of 5 mV s−1, pulse height of 50 mV, pulse width: 0.1 s. B) Peak currents as function of Ea for ta = 20 s. C) DPSV peak current as a function of ta at Ea = −0.8 V vs. Ag/AgCl/3 M KCl. Fig. 7. A) DPSVs at a CoTSPc/Gr-GC modified electrode upon successive addition of DA in 0.1 M PBS (pH 7.3): (a) 0 nM, (b) 20 nM, (c) 40 nM, (d) 60 nM, (e) 100 nM, (f) 140 nM, (g) 180 nM, and (h) 220 nM of DA. B) Calibration curve for DA oxidation at a CoTSPc/Gr-GC modified electrode (scan rate = 5 mV s−1; pulse height = 50 mV; pulse width = 0.1 s).
Fig. 8. Cyclic voltammograms of: (a) 10 mM AA, (b) 10 mM UA, and (c) a mixture of 0.3 mM DA, 10 mM AA and 10 mM UA at CoTSPc/Gr-GC modified electrodes in 0.1 M PBS (pH 7.3, scan rate = 100 mV s−1).
Fig. 9. DPSVs of 5 μM DA at CoTSPc/Gr-GC electrodes: (a) in the absence of AA and UA, (b) in presence of 1 mM AA and 1 mM UA, and (c) in presence 2 mM AA and 2 mM UA, in 0.1 M PBS (pH 7.3, Ea = −0.8 V, ta = 70 s, scan rate = 5 mV s−1, pulse height = 50 mV, pulse width = 0.1 s).
coexist in the extra cellular fluid of the central nervous system [34]. They are oxidized at very close potentials at bare electrodes resulting in overlapped voltammetric responses [35]. Therefore, AA and UA were chosen as potentially interfering compounds to demonstrate the selectivity of the CoTSPc/Gr-GC modified electrodes toward DA (Fig. 8). The cyclic voltammograms for AA (Fig. 8; trace a) and UA (Fig. 8; trace b) showed oxidation signals at 570 mV and 477 mV vs. Ag/AgCl/ 3 M KCl, respectively, whereas, the oxidation peak of DA is located at 295 mV vs. Ag/AgCl/3 M KCl (Fig. 2). The CV recorded in the presence of 0.3 mM DA, 10 mM AA and 10 mM UA at the CoTSPc/Gr-GC modified electrode (Fig. 8; trace c) shows two pronounced oxidation peaks with peak potentials of 338 mV and 567 mV, respectively. Although the
first peak can be attributed mainly to the oxidation of DA, it appears at higher potentials in the presence of AA and UA (Fig. 8) compared to DA alone (Fig. 2), clearly indicating that the DA oxidation peak overlaps with those of AA and UA oxidation. The second peak (567 mV) is attributed mainly to an overlap of AA and UA oxidation peaks. Hence, cyclic voltammetry is not a suitable technique to selectively determine DA in the presence of AA and UA. DPSV was therefore used to demonstrate the selectivity of the modified CoTSPc/Gr-GC electrode for oxidation of DA alongside AA and UA. The oxidation current of DA was examined in the presence of various concentrations of AA and UA. Fig. 9 shows the obtained DPSVs 21
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Fig. 10. A) DPSVs of various concentrations of DA in the presence of 10 mM AA and 10 mM UA at CoTSPc/Gr-GC modified electrodes: (a) 0.5 μM, (b) 0.6 μM, (c) 0.7 μM, (d) 0.8 μM, (e) 0.9 μM, and (f) 1.0 μM DA. B) Corresponding plot of the peak current at 270 mV vs. Ag/ AgCl/3 M KCl as a function of the DA concentration in 0.1 M PBS (pH 7.3, Ea = -0.8 V, ta = 70 s, scan rate = 5 mV s−1, pulse height = 50 mV, pulse width = 0.1 s).
for a constant concentration of 5 μM DA at different concentrations of AA and UA. The results clearly demonstrate that DA can be quantitatively determined in the presence of substantially higher concentrations of AA and UA with a relative standard deviation (RSD) of 4.8%. Determination of DA in the 0.5–1.0 μM concentration range was performed in the presence of high concentrations of both AA (10 mM) and UA (10 mM) by DPSV (Fig. 10A). Two clear peaks are observed at 270 mV vs. Ag/AgCl/3 M KCl assigned to DA oxidation and at 508 mV vs. Ag/AgCl/3 M KCl assigned to UA and AA oxidation. Increasing DA concentration leads to an increase in the intensity of the first peak, similar to the response shown in Fig. 7A, whereas just a minor random change was observed for the second peak (RSD = 4.6%). The oxidation peak current was directly proportional to the DA concentration (Fig. 10B). The separation between the DA oxidation peaks (276 mV) and AA and UA oxidation (512 mV) was sufficiently large to allow quantification of DA in presence of more than 10 times higher concentrations of AA and UA. We attribute this high sensitivity in the detection of DA even in the presence of high concentrations of potentially interfering compounds as revealed by the CoTSPc/Gr-GC modified electrodes to the positive charged amino group of DA at the measuring pH value of 7.3 which facilitates the adsorption of DA on the negatively charged CoTSPc surface. Simultaneously, the negatively charged CoTSPc prevents the adsorption of negatively charged AA and UA on the electrode surface. Moreover, the electrochemical behavior of DA may be facilitated by π−π interaction between DA and graphene in contrast to AA [9], whereas only weak π–π interactions occur between UA and graphene [36]. Cobalt as central metal atom may allow axial coordination between CoPc and DA [37] which may be responsible for the observed high sensitivity. The reproducibility of DA determination at CoTSPc/Gr-GC modified electrodes were examined using DPSV at Ea = −0.8 V vs. Ag/AgCl/3 M KCl and ta = 70 s in 0.1 M PBS. A satisfying reproducibility was obtained with a RSD of 4.2% (n = 5) and of 5.2% (n = 5) at DA concentrations of 2 μM and 0.1 μM, respectively. Differences in current intensity among sensitivity and selectivity investigations as shown in Figs. 7A and 10 A, respectively, can be attributed to the difference in DA concentration. Since accumulation parameters were optimized for nM concentrations, it is likely that at larger concentrations a more pronounced accumulation of analyte molecules at the modified electrode surface occurs, leading to a decrease in sensitivity. Hence, the proposed CoTSPc/Gr-GC sensor is more suitable for the quantification of low DA concentrations.
sensitivity for DA determination in the nM range, with a low detection limit of 0.87 nM. Moreover, the electrodes showed an excellent selectivity that could be used for quantitation of low concentrations of DA in the presence of high concentrations of AA and UA, suggesting the applicability of these materials for determination of DA in complex solutions. We aim towards the development of corresponding CoTSPc/ Gr modified microelectrodes for potential in-vivo determination of DA in future. Declarations of interest None. Acknowledgments N. Diab and M. Masoud acknowledge the financial support from the Scientific Research Deanship at Arab American University, Jenin (Palestine). N. Diab acknowledges the financial support from Welfare Association (UK) through the Zamala Fellowship Program. D. M. Morales is grateful to Deutscher Akademischer Austauschdienst (Germany) and to Consejo Nacional de Ciencia y Tecnología (Mexico) for the financial support. The authors are grateful for financial support by the Deutsche Forschungsgemeinschaft (Germany) in the framework of the project FLAG-ERA JTC 2015 “Graphtivity” (Schu929/14-1). References [1] K.A. Jennings, ACS Chem. Neurosci. 4 (2013) 704. [2] X. Xia, X. Shen, Y. Du, W. Ye, C. Wang, Sens. Actuator B Chem. 237 (2016) 685. [3] G.E. de Benedetto, D. Fico, A. Pennetta, C. Malitesta, G. Nicolardi, D.D. Lofrumento, F. de Nuccio, V. La Pesa, J. Pharm. Biomed. Anal. 98 (2014) 266. [4] A. Naccarato, E. Gionfriddo, G. Sindona, A. Tagarelli, Anal. Chim. Acta 810 (2014) 17. [5] H. Duan, L. Li, X. Wang, Y. Wang, J. Li, C. Luo, Spectrochim. Acta A 139 (2015) 374. [6] L. Liu, S. Li, L. Liu, D. Deng, N. Xia, Analyst 137 (2012) 3794. [7] M. Mamiński, M. Olejniczak, M. Chudy, A. Dybko, Z. Brzózka, Anal. Chim. Acta 540 (2005) 153. [8] Y.H. Park, X. Zhang, S.S. Rubakhin, J.V. Sweedler, Anal. Chem. 71 (1999) 4997. [9] Y. Wang, Y. Li, L. Tang, J. Lu, J. Li, Electrochem. Commun. 11 (2009) 889. [10] S.-M. Chen, W.-Y. Chzo, J. Electroanal. Chem. Lausanne (Lausanne) 587 (2006) 226. [11] V. Parra, Á.A. Arrieta, J.A. Fernández-Escudero, H. García, C. Apetrei, M.L. Rodríguez-Méndez, J.A.d. Saja, Sens. Actuator B Chem. 115 (2006) 54. [12] J.H. Zagal, S. Griveau, J.F. Silva, T. Nyokong, F. Bedioui, Coord. Chem. Rev. 254 (2010) 2755. [13] F. Bedioui, S. Griveau, T. Nyokong, A.J. Appleby, C.A. Caro, M. Gulppi, G. Ochoa, J.H. Zagal, Phys. Chem. Chem. Phys. 9 (2007) 3383. [14] A. Kutluay, M. Aslanoglu, Anal. Chim. Acta 839 (2014) 59. [15] L. Yang, X. Li, Y. Xiong, X. Liu, X. Li, M. Wang, S. Yan, L.A.M. Alshahrani, P. Liu, C. Zhang, J. Electroanal. Chem. Lausanne (Lausanne) 731 (2014) 14. [16] K. Wang, J.-J. Xu, H.-Y. Chen, Biosens. Bioelectron. 20 (2005) 1388. [17] V. Mani, S.-T. Huang, R. Devasenathipathy, T.C.K. Yang, RSC Adv. 6 (2016) 38463. [18] H. Wang, Y. Bu, W. Dai, K. Li, H. Wang, X. Zuo, Sens. Actuator B Chem. 216 (2015) 298. [19] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. Engl. 48 (2009) 7752. [20] H. Kim, A.A. Abdala, C.W. Macosko, Macromolecules 43 (2010) 6515.
4. Conclusions Graphene sheets decorated with cobalt tetrasulfonated phthalocyanine were electrochemically-induced deposited on GC electrodes using a sequence of potential pulses. The modified electrodes were employed for the electrocatalytic oxidation of dopamine by means of CV and DPSV. After optimization of the accumulation potential and the accumulation time the CoTSPc/Gr-GC modified electrodes exhibited high 22
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N. Diab et al. [21] V. Mani, B. Dinesh, S.-M. Chen, R. Saraswathi, Biosens. Bioelectron. 53 (2014) 420. [22] A. Numan, M.M. Shahid, F.S. Omar, K. Ramesh, S. Ramesh, Sens. Actuator B-Chem. 238 (2017) 1043. [23] K. Deng, J. Zhou, X. Li, Electrochim. Acta 114 (2013) 341. [24] Q. Wang, Q. Tang, Microchim. Acta 182 (2015) 671. [25] J. Yang, Mu Di, Y. Gao, J. Tan, A. Lu, D. Ma, J. Nat. Gas Chem. 21 (2012) 265. [26] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [27] Y. Wei, Z. Sun, Curr. Opin. Colloid Interface Sci. 20 (2015) 311. [28] C.G. Claessens, U. Hahn, T. Torres, Chem. Rec. 8 (2008) 75. [29] D. Parviz, S. Das, H.S.T. Ahmed, F. Irin, S. Bhattacharia, M.J. Green, ACS Nano 6 (2012) 8857. [30] A.D. Adler, F.R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970) 2443.
[31] D.M. Morales, J. Masa, C. Andronescu, Y.U. Kayran, Z. Sun, W. Schuhmann, Electrochim. Acta 222 (2016) 1191. [32] W. Schuhmann, C. Kranz, H. Wohlschläger, J. Strohmeier, Biosens. Bioelectron. 12 (1997) 1157. [33] G. Alarcón-Angeles, B. Pérez-López, M. Palomar-Pardave, M.T. Ramírez-Silva, S. Alegret, A. Merkoçi, Carbon 46 (2008) 898. [34] A.J. Downard, A.D. Roddick, A.M. Bond, Anal. Chim. Acta 317 (1995) 303. [35] S.S. Shankar, B.E.K. Swamy, U. Chandra, J.G. Manjunatha, B.S. Sherigara, Int. J. Electrochem. Sci. 4 (2009) 592. [36] C. Cuadrado, L. Ibarra, J. Hurtado, O. García-Beltrán, E. Nagles, Anal. Bioanal. Electrochem. 8 (2016) 910. [37] J. Oni, T. Nyokong, Anal. Chim. Acta 434 (2001) 9.
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