- Email: [email protected]
poly(toluidine blue) composite
Synthetic Metals 245 (2018) 87–95
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Enhanced electrochemical sensing of dopamine based on carboxylic acid functionalized multi-walled carbon nanotubes/poly(toluidine blue) composite Venkata Narayana Palakollu, Rajshekhar Karpoormath
T
⁎
Department of Pharmaceutical Chemistry, College of Health Sciences, University of KwaZulu-Natal (Westville Campus), Durban 4000, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical determination Dopamine Toluidine blue Carboxylic acid functionalized MWCNTs Cyclic voltammetry
A highly sensitive and simple electrochemical method was developed for detection of dopamine (DA) based on carboxylic acid functionalized multi-walled carbon nanotubes (MWCNTs-COOH) and electrochemically synthesized polymer of toluidine blue (Poly(TB)) composite film modified glassy carbon electrode (GCE) (abbreviated as MWCNTs-COOH/Poly(TB)/GCE). The physicochemical and electrochemical characterizations were evaluated using FT-IR spectroscopy, field emission scanning electron microscope (FE-SEM) and electrochemical impedance spectroscopy (EIS). Further, the possible electrochemical polymerization mechanism of toluidine blue was proposed. The EIS responses confirmed that the least charge transfer rate at the composite film modified GCE. The developed MWCNTs-COOH/Poly(TB)/GCE demonstrated superior sensitivity towards DA over that of the other modified electrodes in a physiological pH. Under optimal experimental conditions, the limit of detection (LOD) was found to be 10.39 nM (3S/m) in the linear dynamic range from 1 to 300 μM. The modified electrode exhibited good stability, repeatability, reproducibility and also selectivity over the commonly interfering species. Finally, the developed method was applied for real sample analysis and this can be further extendable as a sensing platform for clinical laboratory applications.
1. Introduction Dopamine (DA), a biogenic catecholamine, is considered as one of the key neurotransmitters in the brain that is implicated in a number of human diseases. DA is administered intravenously and acts on the sympathetic nervous system causing high blood pressure and increase in heart rate [1] and is prescribed for the treatment/management of melancholia and Alzheimer's disease. An abnormal concentration of DA leads to neurological conditions such as Parkinson's disease and schizophrenia [2]. DA is crucial for the normal function of the brain, on the other hand, as a neurotransmitter, it is essential for the nervous system. The use of recreational or psychostimulant drugs (cocaine, nicotine, and heroin etc.) blocks the transportation of DA that hinders the reuptake followed by an upsurge in DA levels, thereby initiating an increased risk [3]. Therefore, an accurate and sensitive quantification of DA is of great importance in biological systems. Thus, there is high demand for a highly sensitive and reliable method that can be used for accurate quantification of DA in vitro and in vivo. Several approaches such as HPLC [4], mass spectrometry [4], capillary electrophoresis [5,6], spectrophotometry [7], ion-exchange chromatography [8] and
⁎
colorimetric detection [9] have been carried out for DA detection. However, above-mentioned methods have some drawbacks such as the need of skilled personnel, cost of equipment, high analysis time and complex sample pre-treatments etc. On the other hand, an electrochemical technique is a straightforward tool and has grabbed considerable attention over the recent years in sensing of the analytes. Moreover, it has cost-effectiveness, little or no sample pre-treatment, easier operation, rapid detection, feasibility for miniaturization, takes short time for analysis and does not require extremely skilled personnel. However, surface fouling and irreversible behaviour of analytes lead to poor performance at conventional electrodes. In order to enhance the catalytic activity of electrodes, many researchers have been developing a variety of sensing materials for electrode modifications [9]. Moreover, the development of sensing material involves numerous steps in the synthesis. An effective and efficient sensing material is desirable for the target analytes and on the other hand, the approach should be relatively very simple. In recent years, the interest in the synthesis and optimization of effective as well as efficient sensing materials for the surface modification of electrodes has increased. Carbon nanotubes (CNTs) were first discovered in 1991 and since
Corresponding author. E-mail address: [email protected] (R. Karpoormath).
https://doi.org/10.1016/j.synthmet.2018.08.012 Received 2 April 2018; Received in revised form 5 August 2018; Accepted 20 August 2018 0379-6779/ © 2018 Published by Elsevier B.V.
Synthetic Metals 245 (2018) 87–95
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Fig. 1. Successive CVs recorded during the electropolymerization of TB onto the surface of GCE.
Germany), MWCNTs-COOH (Techinstro, India), absolute ethanol (Merck, South Africa), NaCl (Sigma-Aldrich, USA), KCl (Merck, South Africa) and MgSO4 (RLFCI, India) were used as received without additional pre-treatment. Phosphate buffer solution (PBS) was used as a supporting electrolyte for this study. For the preparation of buffers, NaH2PO4·2H2O (Merck, South Africa) and Na2HPO4·2H2O (SigmaAldrich, Germany) were used. De-ionized water was used throughout the experiment.
then are superior representatives of the carbon family offering unique properties such as good mechanical strength, large surface area, acceptable biocompatibility and high conductance combined with their chemical stability [10]. Due to their hydrophobic nature, the functionalization of CNTs is often required prior to their use, in order to ensure a homogeneous dispersion [11]. Particularly, carboxylic acid functionalized multi-walled CNTs (MWCNTs-COOH) show good dispersion ability, binding activity for molecular recognition and COOH groups play redox activity on the surface of MWCNTs [12]. Organic dyes have been extensively using as mediators for electrochemical determinations. Toluidine blue (TB) is an organic dye with good biocompatibility and characteristic charge transfer properties and can effectively be used for electrochemical sensing [13–16]. Further, electro-polymerization of dyes results in cross-linked oligomers with improved electrocatalytic activity [17,18]. The development of electropolymers is made simple by the electropolymerization of dye molecules, by changing only the electrochemical parameters. Moreover, electropolymer/CNTs based sensing hybrids have widely been investigated since the integration of CNTs with electropolymers make new composite materials holding the properties of individual component composites through a synergistic effect, which have shown outstanding electrochemical sensing capacity for analysis of biomolecules [12]. Herein, by exploiting the advantages offered by electropolymer/ CNTs composites, we report on the fabrication of MWCNTs-COOH and polymer of TB (Poly(TB)) composite film onto a glassy carbon electrode (GCE) (hereinafter called as MWCNTs-COOH/Poly(TB)/GCE). To the best of our knowledge, electrochemical sensing of DA at this developed composite film of MWCNTs-COOH/Poly(TB) has not yet been investigated. The new composite film displayed high electrochemical sensitivity by synergistic effect, which was confirmed by examining ampoule containing DA sample (injection dosage form) and human urine samples at nano molar concentrations (nM).
2.2. Instrumentation All electrochemical measurements were performed using CHI 660E electrochemical workstation (CH Instruments, Austin, USA). The measurements were executed in a cell with a three-electrode system, a platinum wire and a silver/silver chloride (3.0 M NaCl) electrode served as an axillary and reference electrodes, respectively. A GCE or a modified GCE was used as a working electrode. All the potentials reported, in this work, were versus an Ag/AgCl (3.0 M NaCl) electrode. FE-SEMZEISS® FE-SEM Ultra Plus was used to investigate the surface morphology of developed composites. Fourier transform infrared (FT-IR) spectra were recorded using a Bruker® ATR–FT-IR (Alpha-P). 2.3. Preparation of Poly(TB)/GCE At first, the GCE (3 mm diameter) was carefully polished with aqueous slurries of 0.3 and 0.05-micron γ-alumina powder on a polishing pad until a mirror-like finish surface was achieved. Then, it was washed gently with distilled water and was allowed for dryness at an ambient temperature (22 ± 3 °C). The pre-treated GCE was then allowed for electrochemical polymerization by potential sweeping from -0.3 to 1.3 V in a solution of 0.1 M PBS (pH 7.0) having 1 mM TB at a scan rate of 50 mV s−1. From CV measurements (Fig. 1), we can ratify that a thin TB polymer layer was successfully formed. After electropolymerization, the electrode was gently washed with de-ionized water and was dried at an ambient temperature (22 ± 3 °C).
2. Experimental 2.1. Chemicals
2.4. Preparation of MWCNTs-COOH/Poly(TB)/GCE Dopamine hydrochloride (Merck, South Africa), uric acid (Sigma, USA), ascorbic acid (Sigma-Aldrich, USA), potassium ferrocyanide (Sigma-Aldrich, China), potassium ferricyanide (Saarchem, South Africa), glucose (ACE, South Africa), L-lactose (Sigma-Aldrich,
For the preparation of MWCNTs-COOH/Poly(TB)/GCE, 5.0 mg of MWCNTs-COOH were dispersed in 5 mL of absolute ethanol and allowed for ultrasonic agitation to make it a homogeneous suspension. 88
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Scheme 1. Proposed mechanism during the electrochemical polymerization of TB.
Then, 5 μL of suspension of MWCNTs-COOH was drop-casted onto the surface of Poly(TB)/GCE and permitted for dryness for 10 min. at an ambient temperature (22 ± 3 °C). The developed electrode was abbreviated as MWCNTs-COOH/Poly(TB)/GCE.
phenothiazine and phenoxazine compounds [21]. 3.2. Characterization of MWCNTs-COOH/Poly(TB) composite The surface morphology of Poly(TB), MWCNTs-COOH and MWCNTs-COOH/Poly(TB) composite film was examined using FE-SEM, and are presented in Fig. 2. The MWCNTs-COOH has a large net-like and tube-like interlaced structure that offers a large surface area. The morphology of MWCNTs-COOH/Poly(TB) composite in which, the nanotubes appear to be highly dispersed and is also similar to the morphology of MWCNTs-COOH. The FT-IR spectra were presented in Fig. S1 (Supplementary data). As can be seen in the Fig. S1 (C), two strong bands were identified near 3200 cm−1 (2981, 3191 cm−1) attributed to a NeH distention and an intense band at 1595 cm−1 was attributed to an aromatic ring breathing mode NeH deformation (primary amine characteristic). The above-said band appeared in FT-IR spectra of TB also Fig. S1(A). The absorption bands appeared at 1222 cm−1 (CeN), 1475 cm−1 (C]C stretching) 1664 cm−1 (C]O symmetric stretching in carboxylic acid), 824, 894 cm−1 (aromatic CeH bending) and 2310 cm−1 (CeS) also confirm the formation of MWCNTs-COOH/Poly (TB) composite film from the observation of Fig. S1 (B).
3. Results and discussion 3.1. Electrochemical polymerization of TB The electrochemical polymerization of TB was carried out from the potential window of -0.3 to 1.3 V in 0.1 M PBS (pH 7.0) containing 1 mM TB and it was presented in Fig. 1. As can be seen in the Fig. 1, one anodic peak (I) and one cathodic peak (Iˈ) were observed in the TB reduction region when the potential sweep started from -0.3 V. This can be ascribed as redox reaction of TB, which is corresponding to conversion of structure A and B in the Scheme 1 . However, another anodic peak (II) appeared around 1.0 V when the sweep allowed to 1.3 V, which attributes the oxidation of amine group and leads to the formation of radical cation [19] (structure of A to C in the Scheme 1). Further, the delocalization in the radical cation can be addressed with the help of canonical forms (structure of C and D in the Scheme 1). Moreover, no parallel reverse peak was perceived suggesting that the radical cation undergoes the following chemical reaction. Therefore, radical dimerization may take place via carbon-nitrogen coupling directions. Moreover, the peak current augments with increasing successive scans at peak II signifying the formation of more number of radical cations. Furthermore, there was a broad couple (a couple of III and IIIˈ) in which the formal potential was little more than the monomer reaction of a couple of I and I´. There has also been noticed that the peak currents of a couple of III and IIIˈ increased continuously with successive scans [20]. This can be attributed to the redox reaction of the polymer (structure of E and F in the Scheme 1). Such kind of phenomenon was also observed in the electrochemical polymerization of some
3.3. Impedance study of modified electrodes Electrochemical impedance spectroscopy (EIS) is considered as one of the most effective and reliable tools to investigate the interfacial electronic properties of surface-modified electrodes. The conductivity and electron transfer properties of different electrodes were examined by EIS in 0.1 M KCl containing 2.5 mM [Fe(CN)6]3−/4−. Usually, it is used to characterize and evaluate the material which is adsorbed onto the surface of the electrode, since it expresses the resistance of a charge transfer between the electrode and the electrolyte [22]. The charge transfer resistance considerably varies based on the electrode surface 89
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Fig. 2. FE-SEM images of MWCNTs-COOH (A, B) and MWCNTs-COOH/Poly(TB) composite (C, D) of low magnification (A, C) and high magnification (B, D).
Rct value was 57.18 Ω at the surface of MWCNTs-COOH/GCE. This observation addresses that MWCNTs-COOH has good electron carrying property that boosts the rate of charge transfer. However, the Rct value at the surface of MWCNTs-COOH/Poly(TB)/GCE was found to be 15.78 Ω. This can be attributed to the excellent conductivity and large surface area of MWCNTs-COOH and redox polymer. Due to the combination of the excellent catalytic properties of both components, allows the selective and sensitive electrochemical sensing applications.
3.4. Electrocatalytic activities of bare and modified GCEs towards DA detection Fig. 3. EIS spectra of 0.1 M KCl containing 2.5 mM [Fe(CN)6]3−/4− at (a) GCE, (b) Poly(TB)/GCE, (c) MWCNTs-COOH/GCE and (d) MWCNTs-COOH/Poly (TB)/GCE. Inset-Randles equivalent circuit model.
In order to test the potential electrocatalytic activity of proposed sensor, the cyclic voltammograms (CVs) were recorded using cyclic voltammetry (CV) technique in 0.1 M PBS (pH 7.0) containing 1 mM DA at a potential scan rate of 0.05 V s−1. The electrochemical sensing performance towards oxidation of DA at various modified electrodes was presented in Fig. 4. It can be observed here that no oxidation or reduction peaks were detected without DA (system ‘a’) at bare GCE. Nonetheless, System ‘b’ shows over redox potentials with redox peaks of DA at bare GCE. However, the peak currents of CV curves of Poly(TB)/ GCE was considerably increased with peak to peak separation (ΔEp) of 0.049 V as compared to that of GCE (system ‘c’). Similarly, the redox peaks of DA occurred with greatly increased peak currents (ipa/ipc of 1.3) and with ΔEp of 0.057 V at MWCNTs-COOH/GCE (system ‘d’), which is a characteristic behaviour of reversible process at MWCNTs-
modifications. The Nyquist impedance spectra of bare and modified GCEs are depicted in Fig. 3. The Randles equivalent circuit model presented in the Fig. 3 (inset) was used for all Nyquist impedance spectra. The results obviously demonstrate that the charge transfer resistance (Rct) value decreased upon varying from a naked surface to various modifiers. From Fig. 3, Rct value (the semicircle) was 242.2 Ω at the GCE and describes least interfacial charge transfer. The Rct value at Poly(TB)/GCE decreased (182.5 Ω) relative to the bare GCE. This is mainly due to the conducting nature of Poly(TB), which would promote the charge transfer rate on the surface of Poly(TB)/GCE. Whereas the
90
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Table 1 Apparent diffusion coefficient values of DA at different modified electrodes. S. No.
Electrode
Dapp values (cm2 s−1)
1 2 3 4
GCE Poly(TB)/GCE MWCNTs-COOH/GCE MWCNTs-COOH/Poly(TB)/GCE
3.4759 × 10−6 3.8227 × 10−6 4.3228 × 10−6 5.03298 × 10−6
Fig. 4. CVs at (a) GCE without 1 mM DA (b) GCE, (c) Poly(TB)/GCE, (d) MWCNTs-COOH/GCE and (e) MWCNTs-COOH/Poly(TB)/GCE in 0.1 M PBS (pH 7.0) containing 1 mM DA.
COOH/GCE. On the other hand, the improvement in the peak currents and reduction in peak to peak separation can be attributed to inherent materials properties such as, remarkable conductivity, electron transfer capability and high surface-to-volume ratio of MWCNTs-COOH [23,24]. In addition, MWCNTs-COOH have binding activity for molecular recognition, high dispersion quality and COOH groups play redox activity on the surface of MWCNTs [12]. However, a pair of well-defined redox systems with dramatic enhancement in the peak currents was noticed with redox potentials of 0.171 and 0.124 V (vs Ag/AgCl) respectively at the MWCNTs-COOH/Poly(TB)/GCE (system ‘e’). The redox potential of DA at MWCNTs-COOH/Poly(TB)/GCE was evidently reversible with ΔEp of 0.047 V (vs Ag/AgCl). Moreover, the ratio of ipa and ipc at MWCNTs-COOH/Poly(TB)/GCE was almost equal to one and it can be reviewed as a key feature to confirm the stability of redox peaks formed at MWCNTs-COOH/Poly(TB)/GCE under the experimental conditions [25]. Moreover, this noticeable electrocatalytic response is strongly attributes that the integration of MWCNTs-COOH and Poly(TB) would evidently boost up the peak currents at the modified electrode and increase the sensitivity of the sensor for the quantification of DA through a synergistic effect by creating even higher surface area as well as contact of DA molecule. These results remarkably demonstrates that the GCE was modified successfully with the MWCNTs-COOH/Poly(TB) composite. Therefore, it concludes that MWCNTs-COOH/Poly(TB)/GCE facilitated enrich electron transfer at the interface of electrode and the analyte. The apparent diffusion coefficient (Dapp) of DA at various modified electrodes have been calculated from the following Randles-Sevcik equation [26].
Fig. 5. (A) CVs of 1 mM DA in 0.1 M PBS of pH 7.0 at different scan rates (a to h, 0.06– 0.13 V s−1) at MWCNTs-COOH/Poly(TB)/GCE. (B) Calibration plot of the redox peak currents vs. square root of scan rate.
evidence of electrocatalytic activity of MWCNTs-COOH/Poly(TB)/GCE [27]. 3.5. Effect of scan rate The CV studies have been performed in 0.1 M PBS of pH 7.0 containing 1 mM DA at the MWCNTs-COOH/Poly(TB)/GCE as a function of scan rate from 60 to 130 mV s−1 (Fig. 5(A)). Generally, potential scan rate can possibly influence the electrochemical response of an analyte. The magnitudes of anodic and cathodic peak currents have been found to vary linearly with increasing scan rate. The redox peak potentials are found to be shifted as the scan rate increases from 60-130 mV s−1. The arisen slight deviation from ideal behaviour at lower scan rates could be ascribed to the limitations connected with charge transfer at the surface of MWCNTs-COOH/Poly(TB)/GCE. The linear increase in the redox peak currents as a function of square root of scan rate describes a diffusion-controlled nature (Fig. 5(B)). The resultant linear regression
ip = (2.69 × 105) ADapp1/2 n3/2ν1/2 C0 Where ‘ip’ is the peak current (A), ‘n’ is the number of electrons involved, ‘ν’ is the scan rate (V s−1), ‘A’ is the geometrical surface area of the electrode and ‘C0’ is the concentration of DA (mol/cm3). The calculated Dapp valued are presented in Table 1. From Table 1, it was observed that the Dapp value of DA was higher at MWCNTs-COOH/Poly (TB)/GCE in comparison with bare GCE, Poly(TB)/GCE and MWCNTsCOOH/GCE. This higher apparent diffusion coefficient of DA at MWCNTs-COOH/Poly(TB)/GCE might be due to the fast electron transfer rate in the electro-oxidation of DA at MWCNTs-COOH/Poly (TB)/GCE. This observation is also an additional support for the 91
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Fig. 6. (A) DPVs of DA at different pH. (B) The plot of pH (5.0–8.5) vs Epa and ipa.
peak current increased with improvement in pH from 5.0 to 7.0 and then declined with further increase in pH. Moreover, the peak potentials of DA shifted negatively with the increase of pH signifying that protons participate actively in the electrode processes by the electrocatalytic oxidation of DA. Therefore, the linear dependence of peak potential (Ep) on pH can be presented as
equations are: ipa (μA) = (35.08578 ± 5.3599) – (5.8599 ± 0.54993)ν ½ R2 = 0.99385
½
(mV s
−1
)
ipc (μA) = (-33.059 ± 10.57055) + (16.85132 ± 1.08454)ν½ (mV s−1) ½ R2 = 0.97174
Ep pH
Additionally, the steady change in the shape of the voltammograms from lower to higher scan rates specifies a non-linear diffusion at MWCNTs-COOH/Poly(TB)/GCE.
(V) = (0.5274 ± 0.02562) R2 = 0.97588.
−
(0.0565 ± 0.000362)
The obtained slope value in the above equation can now be used for calculating the number of electrons and protons by using the following Nernst equation [28].
3.6. Effect of solution pH
dEp/dpH = 2.303 mRT/nF
The solution pH usually effects the electrochemical response by shifting the redox potential towards either more positive or negative directions with varying redox peak currents. It is an important parameter in determining the performance of electrocatalytic activity of the sensor. The influence of pH on the electro-oxidation of DA at MWCNTsCOOH/Poly(TB)/GCE was studied by differential pulse voltammetry (DPV) technique using PBS in the pH range of 5.0 to 8.5 (Fig. 6(A)). The differential pulse voltammograms (DPVs) of DA at MWCNTs-COOH/ Poly(TB)/GCE displayed a strong dependence on the pH of PBS. The
where m and n are the number of protons and electrons respectively. The ratio of m and n was found to be 1.04 for the oxidation of DA. Since the plot of Ep (V) vs. pH (Fig.6(B)) provided a slope of 0.0565 V/pH, and is close to the estimated value of 0.059 V pH−1. It specifies that the numbers of protons and electrons involved in the oxidation of DA were almost unity. Hence, according to Nernst equation, it can be proposed that an equal number of protons and electrons are involving in the oxidation of DA at the electrode process. Therefore, DA oxidation at 92
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Table 2 Comparison of electrocatalytic performance of DA with recently reported sensors.
Scheme 2. Electrochemical redox reaction mechanism of DA at the proposed electrode.
MWCNTs-COOH/Poly(TB)/GCE is likely to undergo a two-electron and a two-proton process as outlined in the Scheme 2 . The peak current responses provided the highest value at pH 7.0 and best peak response may be due to the charge of neutrality of DA at pH 7.0. Thus, the PBS of pH 7.0 was selected for further analytical experiments.
Sensor
Technique
Linear Range (μM)
LOD (nM)
Ref.
ERGO-P/GCE Fe2O3/GO/GCE Graphene/GCE PILs/PPy/GO/GCE PEDOT:PSS/ITO Pyrolytic carbon film ACh/GCE MWCNTs-COOH/Poly(TB)/ GCE
DPV DPV CV DPV DPV DPV DPV DPV
0.1– 500 0.5–100 2.5–100 4–18 1–50 18–270 0.7– 5 1– 300
35 700 500 73 6840 2300 300 10.39
[29] [30] [31] [32] [33] [34] [35] Present work
ERGO-P/GCE: electrochemically reduced graphene oxide-porphyrin/glassy carbon electrode; Fe2O3/GO/GCE: iron oxide/graphene/glassy carbon electrode, PILs/PPy/GO/GCE: poly(ionic liquids)/polypyrrole/graphene oxide nanosheets/glassy carbon electrode; PEDOT:PSS/ITO: poly(3,4-ethylenedioxythiophene): polystyrene sulfonate/indium tin oxide; Poly(A-AA)-MIP composite: poly(aniline-co-anthranilic acid)–molecularly imprinted polymer composite; ACh/GCE: acetate/choline/glass carbon electrode; SWV: square wave voltammetry.
3.7. Analytical performance of DA DPV is one of the most sensitive techniques for determining the lowlevel detection of electrochemically active molecules. Thus, DPV was used to record voltammograms with the increasing amounts of DA under the optimal experimental conditions. The recorded voltammograms displayed linearity of the peak currents with the increasing concentration of DA at MWCNTs-COOH/Poly(TB)/GCE in 0.1 M PBS of pH 7.0 (Fig. 7(A)). It can be observed that there was no prominent peak
appeared in the absence of DA. The oxidation peak current of the DA was plotted against the concentration of DA and presented in Fig. 7(B). The fitted linear regression equation can be demonstrated as follows ip (μA) = (1.0468 ± 0.46203) μM R2 = 0.98671
+ (0.6783 ± 0.02271)
[DA]
The limit of detection (LOD) was calculated by using 3S/m, where ‘S’ is the standard deviation of the voltammetric blank responses and ‘m’ is the slope of the calibration plot and was found to be 10.39 nM. It is worth pointing out that the analytical performance of the proposed sensor is even better than the recently reported similar kind of work and the comparison has been presented in Tables 2 and 3. It is obviously showed an improvement over the reported values. 3.8. Repeatability, reproducibility and stability Repeatability, reproducibility and stability are the key elements of sensor performance. The repeatability of the developed MWCNTsCOOH/Poly(TB)/GCE was verified by performing distinct 30 CV cycles in 0.1 M PBS (pH 7.0) containing 100 μM of DA (Fig. 8). The observation reveals that, there was no much difference in the peak current as well as peak potential of the system for these 30 individual cycles and relative standard deviation (RSD) was found to be 0.41%. The RSD Table 3 Comparison of electrocatalytic performance of DA at MWCNTs based sensor material. MWCNTs based sensor material
Method
Linear Range (μM)
LOD (nM)
Ref.
RuO2/MWCNTs GR/Poly(DA)/MWCNTs MIP-Polypyrrole/MWNTs HRP–MWCNTs–SiSG/Poly(Gly) MWNTs-SiO2-chit Nafion/Ni(OH)2NPs/MWCNTs MWCNTs-COOH/Poly(TB)
DPV DPV DPV DPV SWV DPV DPV
0.6–3600 7–297 0.625–100 15–865 1–20 0.05–25 1–300
60 1000 60 600 200 15 10.39
[36] [37] [38] [39] [40] [41] Present work
RuO2/MWNTs: RuO2 nanoparticles/multi-walled carbon nanotubes; GR/Poly (DA)/MWCNTs: Graphene coated by polydopamine/multi-walled carbon nanotubes; MIP-Polypyrrole/MWNTs: Molecularly imprinted polymer of polypyrrole/multi-walled carbon nanotubes; HRP-MWCNTs-SiSG/Poly(Gly): Horseradish peroxidase/multi-walled carbon nanotubes-sol–gel/Poly(Glycine); MWNTs-SiO2-chit: multi-walled carbon nanotubes-SiO2-chitosan; Nafion/Ni (OH)2NPs/MWNTs: Nafion/Ni(OH)2 nanoparticles/multi-walled carbon nanotubes.
Fig. 7. (A) DPVs of DA with the different concentrations (from top to bottom) (a to p) in 0.1 M PBS of pH 7.0 at MWCNTs-COOH/Poly(TB)/GCE. (B) Calibration plot of DA concentration vs peak current. 93
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Table 5 Determination of DA in human urine sample (n = 3). Sample
S. No
Added (μM)
Found (μM)
Recovery (%)
Bias (%)
Urine
1 2 3 4 5
0.0 5.0 10.0 20.0 30.0
Not detected 4.95 ± 0.05 9.62 ± 0.12 19.43 ± 0.45 29.51 ± 0.72
– 99.00 96.20 97.10 98.36
– 1.00 3.80 2.90 1.64
Table 6 Determination of DA in injection sample (n = 3).
Table 4 Selectivity of DA in various interfering compounds at MWCNTs-COOH/Poly (TB)/GCE. Interferent
Molar ratio of [interferent] and [DA]
% of Id/Id inf
1 2 3 4 5 6
MgSO4 NaCl Glucose L-lactose UA Ascorbic acid
100 : 100 : 100 : 100 : 1:1 1:1
96.23 100.85 102.52 102.11 117.26 113.42
1 1 1 1
S. No
Added (μM)
Found (μM)
Recovery (%)
Bias (%)
Injection
1 2 3 4 5
0.0 5.0 10.0 20.0 30.0
Not detected 5.01 ± 0.14 10.14 ± 0.77 19.78 ± 0.26 30.15 ± 0.06
– 100.2 101.4 98.9 100.5
– 0.2 1.4 1.1 0.5
urine samples. An ampoule having an aliquot of 4 mg mL−1 was further diluted with 0.1 M PBS of pH 7.0 so as to get the linear range. The recovery tests of DA were carried out using DPV under optimized conditions through the standard addition method in order to avoid any matrix issues. Likewise, standard addition method was also applied to the determination of DA in healthy human urine. The calculated recovery results have been presented in Tables 5 and 6. The results display that the percentage recoveries are very acceptable and good agreement between the spiked and calculated concentrations. Moreover, the results exhibit that the probable interference effect caused by the matrix was negligible. Therefore, the proposed sensing platform can be employed to quantify the DA in real samples.
Fig. 8. A bar chart showing peak current for individual CVs of without DA and with 100 μM DA in 0.1 M PBS of pH 7.0 at MWCNTs-COOH/Poly(TB)/GCE.
S. No.
Sample
4. Conclusions
Id: current strength before interferent addition; Id inf: current strength after interferent addition.
In summary, a facile and effective MWCNTs-COOH/Poly(TB) composite film was successfully synthesized onto the surface of GCE. The surface morphology of MWCNTs-COOH/Poly(TB) composite film was successfully characterized using FE-SEM and FT-IR. The possible mechanism for the electrochemical polymerization of TB at GCE was proposed. The electron transfer property of MWCNTs-COOH/Poly(TB)/ GCE was successfully verified by means of EIS. Moreover, the developed electrochemical sensor displayed good sensitivity, selectivity, a linear concentration range, low LOD and stable response for the quantification of DA. The newly developed composite sensor made a good analytical tool for the accurate analysis of DA in real samples. A recovery of more than 95% was observed. Due to good stability and repeatability of the composite film-based sensor, it has prospective for the fabrication of nanosensors for diagnosis applications.
for measurements in 0.1 M PBS (pH 7.0) containing 100 μM of DA using CV at six different electrodes prepared under the same experimental condition was 3.2%. This indicates that the MWCNTs-COOH/Poly(TB)/ GCE has good reproducibility. The stability of the electrode was also verified for two weeks and RSD for the measured responses was only 3.52%. From these results, it is obvious that the proposed electrode is having good stability, repeatability and reproducibility. 3.9. Interference study The selectivity of the electrochemical sensor towards DA was investigated at the proposed electrode in the presence of some possible foreign species in biological fluids and/or in pharmaceutical formulations and presented in Fig. S2. The tested interfering species includes Mg2+, Na+, Cl−, SO42-, glucose, L-lactose, UA and Ascorbic acid. The tolerance limit was specified as the maximum concentration of foreign compounds that affected an error of less than ± 5% in the quantification of DA [42]. From the results (Table 4), 100 folds concentration of Mg2+, Na+, SO42-, Cl−, L-lactose and glucose had no interference with the peak current of DA which suggests good selectivity of the method. Their reduction or oxidation peaks could evidently be separated from each other.
Acknowledgements This work was supported by UKZN-Nanotechnology platform and College of Health Sciences, University of KwaZulu-Natal, South Africa. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2018.08. 012. References
3.10. Real sample analysis The analytical practicability of the MWCNTs-COOH/Poly(TB)/GCE sensor was validated by quantifying the content of DA in a commercially available pharmaceutical (injection dosage form) and human
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Enhanced electrochemical sensing of dopamine based on carboxylic acid functionalized multi-walled carbon nanotubes/poly(toluidine blue) composite Venkata Narayana Palakollu, Rajshekhar Karpoormath
T
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Department of Pharmaceutical Chemistry, College of Health Sciences, University of KwaZulu-Natal (Westville Campus), Durban 4000, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical determination Dopamine Toluidine blue Carboxylic acid functionalized MWCNTs Cyclic voltammetry
A highly sensitive and simple electrochemical method was developed for detection of dopamine (DA) based on carboxylic acid functionalized multi-walled carbon nanotubes (MWCNTs-COOH) and electrochemically synthesized polymer of toluidine blue (Poly(TB)) composite film modified glassy carbon electrode (GCE) (abbreviated as MWCNTs-COOH/Poly(TB)/GCE). The physicochemical and electrochemical characterizations were evaluated using FT-IR spectroscopy, field emission scanning electron microscope (FE-SEM) and electrochemical impedance spectroscopy (EIS). Further, the possible electrochemical polymerization mechanism of toluidine blue was proposed. The EIS responses confirmed that the least charge transfer rate at the composite film modified GCE. The developed MWCNTs-COOH/Poly(TB)/GCE demonstrated superior sensitivity towards DA over that of the other modified electrodes in a physiological pH. Under optimal experimental conditions, the limit of detection (LOD) was found to be 10.39 nM (3S/m) in the linear dynamic range from 1 to 300 μM. The modified electrode exhibited good stability, repeatability, reproducibility and also selectivity over the commonly interfering species. Finally, the developed method was applied for real sample analysis and this can be further extendable as a sensing platform for clinical laboratory applications.
1. Introduction Dopamine (DA), a biogenic catecholamine, is considered as one of the key neurotransmitters in the brain that is implicated in a number of human diseases. DA is administered intravenously and acts on the sympathetic nervous system causing high blood pressure and increase in heart rate [1] and is prescribed for the treatment/management of melancholia and Alzheimer's disease. An abnormal concentration of DA leads to neurological conditions such as Parkinson's disease and schizophrenia [2]. DA is crucial for the normal function of the brain, on the other hand, as a neurotransmitter, it is essential for the nervous system. The use of recreational or psychostimulant drugs (cocaine, nicotine, and heroin etc.) blocks the transportation of DA that hinders the reuptake followed by an upsurge in DA levels, thereby initiating an increased risk [3]. Therefore, an accurate and sensitive quantification of DA is of great importance in biological systems. Thus, there is high demand for a highly sensitive and reliable method that can be used for accurate quantification of DA in vitro and in vivo. Several approaches such as HPLC [4], mass spectrometry [4], capillary electrophoresis [5,6], spectrophotometry [7], ion-exchange chromatography [8] and
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colorimetric detection [9] have been carried out for DA detection. However, above-mentioned methods have some drawbacks such as the need of skilled personnel, cost of equipment, high analysis time and complex sample pre-treatments etc. On the other hand, an electrochemical technique is a straightforward tool and has grabbed considerable attention over the recent years in sensing of the analytes. Moreover, it has cost-effectiveness, little or no sample pre-treatment, easier operation, rapid detection, feasibility for miniaturization, takes short time for analysis and does not require extremely skilled personnel. However, surface fouling and irreversible behaviour of analytes lead to poor performance at conventional electrodes. In order to enhance the catalytic activity of electrodes, many researchers have been developing a variety of sensing materials for electrode modifications [9]. Moreover, the development of sensing material involves numerous steps in the synthesis. An effective and efficient sensing material is desirable for the target analytes and on the other hand, the approach should be relatively very simple. In recent years, the interest in the synthesis and optimization of effective as well as efficient sensing materials for the surface modification of electrodes has increased. Carbon nanotubes (CNTs) were first discovered in 1991 and since
Corresponding author. E-mail address: [email protected] (R. Karpoormath).
https://doi.org/10.1016/j.synthmet.2018.08.012 Received 2 April 2018; Received in revised form 5 August 2018; Accepted 20 August 2018 0379-6779/ © 2018 Published by Elsevier B.V.
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Fig. 1. Successive CVs recorded during the electropolymerization of TB onto the surface of GCE.
Germany), MWCNTs-COOH (Techinstro, India), absolute ethanol (Merck, South Africa), NaCl (Sigma-Aldrich, USA), KCl (Merck, South Africa) and MgSO4 (RLFCI, India) were used as received without additional pre-treatment. Phosphate buffer solution (PBS) was used as a supporting electrolyte for this study. For the preparation of buffers, NaH2PO4·2H2O (Merck, South Africa) and Na2HPO4·2H2O (SigmaAldrich, Germany) were used. De-ionized water was used throughout the experiment.
then are superior representatives of the carbon family offering unique properties such as good mechanical strength, large surface area, acceptable biocompatibility and high conductance combined with their chemical stability [10]. Due to their hydrophobic nature, the functionalization of CNTs is often required prior to their use, in order to ensure a homogeneous dispersion [11]. Particularly, carboxylic acid functionalized multi-walled CNTs (MWCNTs-COOH) show good dispersion ability, binding activity for molecular recognition and COOH groups play redox activity on the surface of MWCNTs [12]. Organic dyes have been extensively using as mediators for electrochemical determinations. Toluidine blue (TB) is an organic dye with good biocompatibility and characteristic charge transfer properties and can effectively be used for electrochemical sensing [13–16]. Further, electro-polymerization of dyes results in cross-linked oligomers with improved electrocatalytic activity [17,18]. The development of electropolymers is made simple by the electropolymerization of dye molecules, by changing only the electrochemical parameters. Moreover, electropolymer/CNTs based sensing hybrids have widely been investigated since the integration of CNTs with electropolymers make new composite materials holding the properties of individual component composites through a synergistic effect, which have shown outstanding electrochemical sensing capacity for analysis of biomolecules [12]. Herein, by exploiting the advantages offered by electropolymer/ CNTs composites, we report on the fabrication of MWCNTs-COOH and polymer of TB (Poly(TB)) composite film onto a glassy carbon electrode (GCE) (hereinafter called as MWCNTs-COOH/Poly(TB)/GCE). To the best of our knowledge, electrochemical sensing of DA at this developed composite film of MWCNTs-COOH/Poly(TB) has not yet been investigated. The new composite film displayed high electrochemical sensitivity by synergistic effect, which was confirmed by examining ampoule containing DA sample (injection dosage form) and human urine samples at nano molar concentrations (nM).
2.2. Instrumentation All electrochemical measurements were performed using CHI 660E electrochemical workstation (CH Instruments, Austin, USA). The measurements were executed in a cell with a three-electrode system, a platinum wire and a silver/silver chloride (3.0 M NaCl) electrode served as an axillary and reference electrodes, respectively. A GCE or a modified GCE was used as a working electrode. All the potentials reported, in this work, were versus an Ag/AgCl (3.0 M NaCl) electrode. FE-SEMZEISS® FE-SEM Ultra Plus was used to investigate the surface morphology of developed composites. Fourier transform infrared (FT-IR) spectra were recorded using a Bruker® ATR–FT-IR (Alpha-P). 2.3. Preparation of Poly(TB)/GCE At first, the GCE (3 mm diameter) was carefully polished with aqueous slurries of 0.3 and 0.05-micron γ-alumina powder on a polishing pad until a mirror-like finish surface was achieved. Then, it was washed gently with distilled water and was allowed for dryness at an ambient temperature (22 ± 3 °C). The pre-treated GCE was then allowed for electrochemical polymerization by potential sweeping from -0.3 to 1.3 V in a solution of 0.1 M PBS (pH 7.0) having 1 mM TB at a scan rate of 50 mV s−1. From CV measurements (Fig. 1), we can ratify that a thin TB polymer layer was successfully formed. After electropolymerization, the electrode was gently washed with de-ionized water and was dried at an ambient temperature (22 ± 3 °C).
2. Experimental 2.1. Chemicals
2.4. Preparation of MWCNTs-COOH/Poly(TB)/GCE Dopamine hydrochloride (Merck, South Africa), uric acid (Sigma, USA), ascorbic acid (Sigma-Aldrich, USA), potassium ferrocyanide (Sigma-Aldrich, China), potassium ferricyanide (Saarchem, South Africa), glucose (ACE, South Africa), L-lactose (Sigma-Aldrich,
For the preparation of MWCNTs-COOH/Poly(TB)/GCE, 5.0 mg of MWCNTs-COOH were dispersed in 5 mL of absolute ethanol and allowed for ultrasonic agitation to make it a homogeneous suspension. 88
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Scheme 1. Proposed mechanism during the electrochemical polymerization of TB.
Then, 5 μL of suspension of MWCNTs-COOH was drop-casted onto the surface of Poly(TB)/GCE and permitted for dryness for 10 min. at an ambient temperature (22 ± 3 °C). The developed electrode was abbreviated as MWCNTs-COOH/Poly(TB)/GCE.
phenothiazine and phenoxazine compounds [21]. 3.2. Characterization of MWCNTs-COOH/Poly(TB) composite The surface morphology of Poly(TB), MWCNTs-COOH and MWCNTs-COOH/Poly(TB) composite film was examined using FE-SEM, and are presented in Fig. 2. The MWCNTs-COOH has a large net-like and tube-like interlaced structure that offers a large surface area. The morphology of MWCNTs-COOH/Poly(TB) composite in which, the nanotubes appear to be highly dispersed and is also similar to the morphology of MWCNTs-COOH. The FT-IR spectra were presented in Fig. S1 (Supplementary data). As can be seen in the Fig. S1 (C), two strong bands were identified near 3200 cm−1 (2981, 3191 cm−1) attributed to a NeH distention and an intense band at 1595 cm−1 was attributed to an aromatic ring breathing mode NeH deformation (primary amine characteristic). The above-said band appeared in FT-IR spectra of TB also Fig. S1(A). The absorption bands appeared at 1222 cm−1 (CeN), 1475 cm−1 (C]C stretching) 1664 cm−1 (C]O symmetric stretching in carboxylic acid), 824, 894 cm−1 (aromatic CeH bending) and 2310 cm−1 (CeS) also confirm the formation of MWCNTs-COOH/Poly (TB) composite film from the observation of Fig. S1 (B).
3. Results and discussion 3.1. Electrochemical polymerization of TB The electrochemical polymerization of TB was carried out from the potential window of -0.3 to 1.3 V in 0.1 M PBS (pH 7.0) containing 1 mM TB and it was presented in Fig. 1. As can be seen in the Fig. 1, one anodic peak (I) and one cathodic peak (Iˈ) were observed in the TB reduction region when the potential sweep started from -0.3 V. This can be ascribed as redox reaction of TB, which is corresponding to conversion of structure A and B in the Scheme 1 . However, another anodic peak (II) appeared around 1.0 V when the sweep allowed to 1.3 V, which attributes the oxidation of amine group and leads to the formation of radical cation [19] (structure of A to C in the Scheme 1). Further, the delocalization in the radical cation can be addressed with the help of canonical forms (structure of C and D in the Scheme 1). Moreover, no parallel reverse peak was perceived suggesting that the radical cation undergoes the following chemical reaction. Therefore, radical dimerization may take place via carbon-nitrogen coupling directions. Moreover, the peak current augments with increasing successive scans at peak II signifying the formation of more number of radical cations. Furthermore, there was a broad couple (a couple of III and IIIˈ) in which the formal potential was little more than the monomer reaction of a couple of I and I´. There has also been noticed that the peak currents of a couple of III and IIIˈ increased continuously with successive scans [20]. This can be attributed to the redox reaction of the polymer (structure of E and F in the Scheme 1). Such kind of phenomenon was also observed in the electrochemical polymerization of some
3.3. Impedance study of modified electrodes Electrochemical impedance spectroscopy (EIS) is considered as one of the most effective and reliable tools to investigate the interfacial electronic properties of surface-modified electrodes. The conductivity and electron transfer properties of different electrodes were examined by EIS in 0.1 M KCl containing 2.5 mM [Fe(CN)6]3−/4−. Usually, it is used to characterize and evaluate the material which is adsorbed onto the surface of the electrode, since it expresses the resistance of a charge transfer between the electrode and the electrolyte [22]. The charge transfer resistance considerably varies based on the electrode surface 89
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Fig. 2. FE-SEM images of MWCNTs-COOH (A, B) and MWCNTs-COOH/Poly(TB) composite (C, D) of low magnification (A, C) and high magnification (B, D).
Rct value was 57.18 Ω at the surface of MWCNTs-COOH/GCE. This observation addresses that MWCNTs-COOH has good electron carrying property that boosts the rate of charge transfer. However, the Rct value at the surface of MWCNTs-COOH/Poly(TB)/GCE was found to be 15.78 Ω. This can be attributed to the excellent conductivity and large surface area of MWCNTs-COOH and redox polymer. Due to the combination of the excellent catalytic properties of both components, allows the selective and sensitive electrochemical sensing applications.
3.4. Electrocatalytic activities of bare and modified GCEs towards DA detection Fig. 3. EIS spectra of 0.1 M KCl containing 2.5 mM [Fe(CN)6]3−/4− at (a) GCE, (b) Poly(TB)/GCE, (c) MWCNTs-COOH/GCE and (d) MWCNTs-COOH/Poly (TB)/GCE. Inset-Randles equivalent circuit model.
In order to test the potential electrocatalytic activity of proposed sensor, the cyclic voltammograms (CVs) were recorded using cyclic voltammetry (CV) technique in 0.1 M PBS (pH 7.0) containing 1 mM DA at a potential scan rate of 0.05 V s−1. The electrochemical sensing performance towards oxidation of DA at various modified electrodes was presented in Fig. 4. It can be observed here that no oxidation or reduction peaks were detected without DA (system ‘a’) at bare GCE. Nonetheless, System ‘b’ shows over redox potentials with redox peaks of DA at bare GCE. However, the peak currents of CV curves of Poly(TB)/ GCE was considerably increased with peak to peak separation (ΔEp) of 0.049 V as compared to that of GCE (system ‘c’). Similarly, the redox peaks of DA occurred with greatly increased peak currents (ipa/ipc of 1.3) and with ΔEp of 0.057 V at MWCNTs-COOH/GCE (system ‘d’), which is a characteristic behaviour of reversible process at MWCNTs-
modifications. The Nyquist impedance spectra of bare and modified GCEs are depicted in Fig. 3. The Randles equivalent circuit model presented in the Fig. 3 (inset) was used for all Nyquist impedance spectra. The results obviously demonstrate that the charge transfer resistance (Rct) value decreased upon varying from a naked surface to various modifiers. From Fig. 3, Rct value (the semicircle) was 242.2 Ω at the GCE and describes least interfacial charge transfer. The Rct value at Poly(TB)/GCE decreased (182.5 Ω) relative to the bare GCE. This is mainly due to the conducting nature of Poly(TB), which would promote the charge transfer rate on the surface of Poly(TB)/GCE. Whereas the
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Table 1 Apparent diffusion coefficient values of DA at different modified electrodes. S. No.
Electrode
Dapp values (cm2 s−1)
1 2 3 4
GCE Poly(TB)/GCE MWCNTs-COOH/GCE MWCNTs-COOH/Poly(TB)/GCE
3.4759 × 10−6 3.8227 × 10−6 4.3228 × 10−6 5.03298 × 10−6
Fig. 4. CVs at (a) GCE without 1 mM DA (b) GCE, (c) Poly(TB)/GCE, (d) MWCNTs-COOH/GCE and (e) MWCNTs-COOH/Poly(TB)/GCE in 0.1 M PBS (pH 7.0) containing 1 mM DA.
COOH/GCE. On the other hand, the improvement in the peak currents and reduction in peak to peak separation can be attributed to inherent materials properties such as, remarkable conductivity, electron transfer capability and high surface-to-volume ratio of MWCNTs-COOH [23,24]. In addition, MWCNTs-COOH have binding activity for molecular recognition, high dispersion quality and COOH groups play redox activity on the surface of MWCNTs [12]. However, a pair of well-defined redox systems with dramatic enhancement in the peak currents was noticed with redox potentials of 0.171 and 0.124 V (vs Ag/AgCl) respectively at the MWCNTs-COOH/Poly(TB)/GCE (system ‘e’). The redox potential of DA at MWCNTs-COOH/Poly(TB)/GCE was evidently reversible with ΔEp of 0.047 V (vs Ag/AgCl). Moreover, the ratio of ipa and ipc at MWCNTs-COOH/Poly(TB)/GCE was almost equal to one and it can be reviewed as a key feature to confirm the stability of redox peaks formed at MWCNTs-COOH/Poly(TB)/GCE under the experimental conditions [25]. Moreover, this noticeable electrocatalytic response is strongly attributes that the integration of MWCNTs-COOH and Poly(TB) would evidently boost up the peak currents at the modified electrode and increase the sensitivity of the sensor for the quantification of DA through a synergistic effect by creating even higher surface area as well as contact of DA molecule. These results remarkably demonstrates that the GCE was modified successfully with the MWCNTs-COOH/Poly(TB) composite. Therefore, it concludes that MWCNTs-COOH/Poly(TB)/GCE facilitated enrich electron transfer at the interface of electrode and the analyte. The apparent diffusion coefficient (Dapp) of DA at various modified electrodes have been calculated from the following Randles-Sevcik equation [26].
Fig. 5. (A) CVs of 1 mM DA in 0.1 M PBS of pH 7.0 at different scan rates (a to h, 0.06– 0.13 V s−1) at MWCNTs-COOH/Poly(TB)/GCE. (B) Calibration plot of the redox peak currents vs. square root of scan rate.
evidence of electrocatalytic activity of MWCNTs-COOH/Poly(TB)/GCE [27]. 3.5. Effect of scan rate The CV studies have been performed in 0.1 M PBS of pH 7.0 containing 1 mM DA at the MWCNTs-COOH/Poly(TB)/GCE as a function of scan rate from 60 to 130 mV s−1 (Fig. 5(A)). Generally, potential scan rate can possibly influence the electrochemical response of an analyte. The magnitudes of anodic and cathodic peak currents have been found to vary linearly with increasing scan rate. The redox peak potentials are found to be shifted as the scan rate increases from 60-130 mV s−1. The arisen slight deviation from ideal behaviour at lower scan rates could be ascribed to the limitations connected with charge transfer at the surface of MWCNTs-COOH/Poly(TB)/GCE. The linear increase in the redox peak currents as a function of square root of scan rate describes a diffusion-controlled nature (Fig. 5(B)). The resultant linear regression
ip = (2.69 × 105) ADapp1/2 n3/2ν1/2 C0 Where ‘ip’ is the peak current (A), ‘n’ is the number of electrons involved, ‘ν’ is the scan rate (V s−1), ‘A’ is the geometrical surface area of the electrode and ‘C0’ is the concentration of DA (mol/cm3). The calculated Dapp valued are presented in Table 1. From Table 1, it was observed that the Dapp value of DA was higher at MWCNTs-COOH/Poly (TB)/GCE in comparison with bare GCE, Poly(TB)/GCE and MWCNTsCOOH/GCE. This higher apparent diffusion coefficient of DA at MWCNTs-COOH/Poly(TB)/GCE might be due to the fast electron transfer rate in the electro-oxidation of DA at MWCNTs-COOH/Poly (TB)/GCE. This observation is also an additional support for the 91
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Fig. 6. (A) DPVs of DA at different pH. (B) The plot of pH (5.0–8.5) vs Epa and ipa.
peak current increased with improvement in pH from 5.0 to 7.0 and then declined with further increase in pH. Moreover, the peak potentials of DA shifted negatively with the increase of pH signifying that protons participate actively in the electrode processes by the electrocatalytic oxidation of DA. Therefore, the linear dependence of peak potential (Ep) on pH can be presented as
equations are: ipa (μA) = (35.08578 ± 5.3599) – (5.8599 ± 0.54993)ν ½ R2 = 0.99385
½
(mV s
−1
)
ipc (μA) = (-33.059 ± 10.57055) + (16.85132 ± 1.08454)ν½ (mV s−1) ½ R2 = 0.97174
Ep pH
Additionally, the steady change in the shape of the voltammograms from lower to higher scan rates specifies a non-linear diffusion at MWCNTs-COOH/Poly(TB)/GCE.
(V) = (0.5274 ± 0.02562) R2 = 0.97588.
−
(0.0565 ± 0.000362)
The obtained slope value in the above equation can now be used for calculating the number of electrons and protons by using the following Nernst equation [28].
3.6. Effect of solution pH
dEp/dpH = 2.303 mRT/nF
The solution pH usually effects the electrochemical response by shifting the redox potential towards either more positive or negative directions with varying redox peak currents. It is an important parameter in determining the performance of electrocatalytic activity of the sensor. The influence of pH on the electro-oxidation of DA at MWCNTsCOOH/Poly(TB)/GCE was studied by differential pulse voltammetry (DPV) technique using PBS in the pH range of 5.0 to 8.5 (Fig. 6(A)). The differential pulse voltammograms (DPVs) of DA at MWCNTs-COOH/ Poly(TB)/GCE displayed a strong dependence on the pH of PBS. The
where m and n are the number of protons and electrons respectively. The ratio of m and n was found to be 1.04 for the oxidation of DA. Since the plot of Ep (V) vs. pH (Fig.6(B)) provided a slope of 0.0565 V/pH, and is close to the estimated value of 0.059 V pH−1. It specifies that the numbers of protons and electrons involved in the oxidation of DA were almost unity. Hence, according to Nernst equation, it can be proposed that an equal number of protons and electrons are involving in the oxidation of DA at the electrode process. Therefore, DA oxidation at 92
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V.N. Palakollu, R. Karpoormath
Table 2 Comparison of electrocatalytic performance of DA with recently reported sensors.
Scheme 2. Electrochemical redox reaction mechanism of DA at the proposed electrode.
MWCNTs-COOH/Poly(TB)/GCE is likely to undergo a two-electron and a two-proton process as outlined in the Scheme 2 . The peak current responses provided the highest value at pH 7.0 and best peak response may be due to the charge of neutrality of DA at pH 7.0. Thus, the PBS of pH 7.0 was selected for further analytical experiments.
Sensor
Technique
Linear Range (μM)
LOD (nM)
Ref.
ERGO-P/GCE Fe2O3/GO/GCE Graphene/GCE PILs/PPy/GO/GCE PEDOT:PSS/ITO Pyrolytic carbon film ACh/GCE MWCNTs-COOH/Poly(TB)/ GCE
DPV DPV CV DPV DPV DPV DPV DPV
0.1– 500 0.5–100 2.5–100 4–18 1–50 18–270 0.7– 5 1– 300
35 700 500 73 6840 2300 300 10.39
[29] [30] [31] [32] [33] [34] [35] Present work
ERGO-P/GCE: electrochemically reduced graphene oxide-porphyrin/glassy carbon electrode; Fe2O3/GO/GCE: iron oxide/graphene/glassy carbon electrode, PILs/PPy/GO/GCE: poly(ionic liquids)/polypyrrole/graphene oxide nanosheets/glassy carbon electrode; PEDOT:PSS/ITO: poly(3,4-ethylenedioxythiophene): polystyrene sulfonate/indium tin oxide; Poly(A-AA)-MIP composite: poly(aniline-co-anthranilic acid)–molecularly imprinted polymer composite; ACh/GCE: acetate/choline/glass carbon electrode; SWV: square wave voltammetry.
3.7. Analytical performance of DA DPV is one of the most sensitive techniques for determining the lowlevel detection of electrochemically active molecules. Thus, DPV was used to record voltammograms with the increasing amounts of DA under the optimal experimental conditions. The recorded voltammograms displayed linearity of the peak currents with the increasing concentration of DA at MWCNTs-COOH/Poly(TB)/GCE in 0.1 M PBS of pH 7.0 (Fig. 7(A)). It can be observed that there was no prominent peak
appeared in the absence of DA. The oxidation peak current of the DA was plotted against the concentration of DA and presented in Fig. 7(B). The fitted linear regression equation can be demonstrated as follows ip (μA) = (1.0468 ± 0.46203) μM R2 = 0.98671
+ (0.6783 ± 0.02271)
[DA]
The limit of detection (LOD) was calculated by using 3S/m, where ‘S’ is the standard deviation of the voltammetric blank responses and ‘m’ is the slope of the calibration plot and was found to be 10.39 nM. It is worth pointing out that the analytical performance of the proposed sensor is even better than the recently reported similar kind of work and the comparison has been presented in Tables 2 and 3. It is obviously showed an improvement over the reported values. 3.8. Repeatability, reproducibility and stability Repeatability, reproducibility and stability are the key elements of sensor performance. The repeatability of the developed MWCNTsCOOH/Poly(TB)/GCE was verified by performing distinct 30 CV cycles in 0.1 M PBS (pH 7.0) containing 100 μM of DA (Fig. 8). The observation reveals that, there was no much difference in the peak current as well as peak potential of the system for these 30 individual cycles and relative standard deviation (RSD) was found to be 0.41%. The RSD Table 3 Comparison of electrocatalytic performance of DA at MWCNTs based sensor material. MWCNTs based sensor material
Method
Linear Range (μM)
LOD (nM)
Ref.
RuO2/MWCNTs GR/Poly(DA)/MWCNTs MIP-Polypyrrole/MWNTs HRP–MWCNTs–SiSG/Poly(Gly) MWNTs-SiO2-chit Nafion/Ni(OH)2NPs/MWCNTs MWCNTs-COOH/Poly(TB)
DPV DPV DPV DPV SWV DPV DPV
0.6–3600 7–297 0.625–100 15–865 1–20 0.05–25 1–300
60 1000 60 600 200 15 10.39
[36] [37] [38] [39] [40] [41] Present work
RuO2/MWNTs: RuO2 nanoparticles/multi-walled carbon nanotubes; GR/Poly (DA)/MWCNTs: Graphene coated by polydopamine/multi-walled carbon nanotubes; MIP-Polypyrrole/MWNTs: Molecularly imprinted polymer of polypyrrole/multi-walled carbon nanotubes; HRP-MWCNTs-SiSG/Poly(Gly): Horseradish peroxidase/multi-walled carbon nanotubes-sol–gel/Poly(Glycine); MWNTs-SiO2-chit: multi-walled carbon nanotubes-SiO2-chitosan; Nafion/Ni (OH)2NPs/MWNTs: Nafion/Ni(OH)2 nanoparticles/multi-walled carbon nanotubes.
Fig. 7. (A) DPVs of DA with the different concentrations (from top to bottom) (a to p) in 0.1 M PBS of pH 7.0 at MWCNTs-COOH/Poly(TB)/GCE. (B) Calibration plot of DA concentration vs peak current. 93
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Table 5 Determination of DA in human urine sample (n = 3). Sample
S. No
Added (μM)
Found (μM)
Recovery (%)
Bias (%)
Urine
1 2 3 4 5
0.0 5.0 10.0 20.0 30.0
Not detected 4.95 ± 0.05 9.62 ± 0.12 19.43 ± 0.45 29.51 ± 0.72
– 99.00 96.20 97.10 98.36
– 1.00 3.80 2.90 1.64
Table 6 Determination of DA in injection sample (n = 3).
Table 4 Selectivity of DA in various interfering compounds at MWCNTs-COOH/Poly (TB)/GCE. Interferent
Molar ratio of [interferent] and [DA]
% of Id/Id inf
1 2 3 4 5 6
MgSO4 NaCl Glucose L-lactose UA Ascorbic acid
100 : 100 : 100 : 100 : 1:1 1:1
96.23 100.85 102.52 102.11 117.26 113.42
1 1 1 1
S. No
Added (μM)
Found (μM)
Recovery (%)
Bias (%)
Injection
1 2 3 4 5
0.0 5.0 10.0 20.0 30.0
Not detected 5.01 ± 0.14 10.14 ± 0.77 19.78 ± 0.26 30.15 ± 0.06
– 100.2 101.4 98.9 100.5
– 0.2 1.4 1.1 0.5
urine samples. An ampoule having an aliquot of 4 mg mL−1 was further diluted with 0.1 M PBS of pH 7.0 so as to get the linear range. The recovery tests of DA were carried out using DPV under optimized conditions through the standard addition method in order to avoid any matrix issues. Likewise, standard addition method was also applied to the determination of DA in healthy human urine. The calculated recovery results have been presented in Tables 5 and 6. The results display that the percentage recoveries are very acceptable and good agreement between the spiked and calculated concentrations. Moreover, the results exhibit that the probable interference effect caused by the matrix was negligible. Therefore, the proposed sensing platform can be employed to quantify the DA in real samples.
Fig. 8. A bar chart showing peak current for individual CVs of without DA and with 100 μM DA in 0.1 M PBS of pH 7.0 at MWCNTs-COOH/Poly(TB)/GCE.
S. No.
Sample
4. Conclusions
Id: current strength before interferent addition; Id inf: current strength after interferent addition.
In summary, a facile and effective MWCNTs-COOH/Poly(TB) composite film was successfully synthesized onto the surface of GCE. The surface morphology of MWCNTs-COOH/Poly(TB) composite film was successfully characterized using FE-SEM and FT-IR. The possible mechanism for the electrochemical polymerization of TB at GCE was proposed. The electron transfer property of MWCNTs-COOH/Poly(TB)/ GCE was successfully verified by means of EIS. Moreover, the developed electrochemical sensor displayed good sensitivity, selectivity, a linear concentration range, low LOD and stable response for the quantification of DA. The newly developed composite sensor made a good analytical tool for the accurate analysis of DA in real samples. A recovery of more than 95% was observed. Due to good stability and repeatability of the composite film-based sensor, it has prospective for the fabrication of nanosensors for diagnosis applications.
for measurements in 0.1 M PBS (pH 7.0) containing 100 μM of DA using CV at six different electrodes prepared under the same experimental condition was 3.2%. This indicates that the MWCNTs-COOH/Poly(TB)/ GCE has good reproducibility. The stability of the electrode was also verified for two weeks and RSD for the measured responses was only 3.52%. From these results, it is obvious that the proposed electrode is having good stability, repeatability and reproducibility. 3.9. Interference study The selectivity of the electrochemical sensor towards DA was investigated at the proposed electrode in the presence of some possible foreign species in biological fluids and/or in pharmaceutical formulations and presented in Fig. S2. The tested interfering species includes Mg2+, Na+, Cl−, SO42-, glucose, L-lactose, UA and Ascorbic acid. The tolerance limit was specified as the maximum concentration of foreign compounds that affected an error of less than ± 5% in the quantification of DA [42]. From the results (Table 4), 100 folds concentration of Mg2+, Na+, SO42-, Cl−, L-lactose and glucose had no interference with the peak current of DA which suggests good selectivity of the method. Their reduction or oxidation peaks could evidently be separated from each other.
Acknowledgements This work was supported by UKZN-Nanotechnology platform and College of Health Sciences, University of KwaZulu-Natal, South Africa. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2018.08. 012. References
3.10. Real sample analysis The analytical practicability of the MWCNTs-COOH/Poly(TB)/GCE sensor was validated by quantifying the content of DA in a commercially available pharmaceutical (injection dosage form) and human
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