Sensitive Dopamine Biosensor Based on Polypyrrole-Coated

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A sensitive dopamine biosensor based on polypyrrole coated palladium silver nanospherical composites Mohammad Reza Mahmoudian, Wan Jeffrey Basirun, and Yatimah binti Alias Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00570 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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Industrial & Engineering Chemistry Research

A sensitive dopamine biosensor based on polypyrrole coated palladium silver nanospherical composites Mohammad Reza Mahmoudiana,b*, Wan Jeffrey Basiruna, , Yatimah Binti Aliasa,c*, a Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia. b Department of Chemistry, Shahid Sherafat, University of Farhangian, 15916, Tehran, Iran. c University of Malaya Centre for Ionic Liquids (UMCiL), Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur. *E-mail: [email protected], Tel: +610173928320 * E-mail: [email protected], Tel: +6179674184 Abstract We report a facile synthesis of polypyrrole coated palladium silver nanospherical (PdAg NSPs-PPy) composites via direct reduction of Pd2+ and Ag+ in the presence of pyrrole monomers from an aqueous solution of Pd (CH3COO)2, AgNO3 and NaOH. The nanocomposites were synthesized with different ratios of Pd and Ag (2:1, 1:1 and 1:2). X-ray diffraction shows that the Pd2+ and Ag+ were completely reduced to Pd and Ag respectively, during the formation of PdAg NSPs-PPy. The modified glassy carbon electrode with the Pd2Ag1 NSPs-PPy demonstrated excellent electrochemical activity towards dopamine oxidation compared with the bare GCE, Pd1Ag1 NSPs-PPy and Pd1Ag2 NSPs-PPy. From the DPV results, the estimated limit of detection (S/N = 3) and limit of quantification (S/N = 10) for the linear segment (0.001-200 µM of dopamine) are 0.0258 µM and 0.0860µM respectively.

Key words: Composite materials; Nanostructures; Electrical properties

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1. Introduction A review on the recent progress of electroactive polymers has shown that it is still considered an important and a wide area of research due to their excellent properties. Some recent works have demonstrated that these materials or their metal oxide composites with metals such as silver and palladium are useful for the fabrication of different type of devices .1-4 Among the group of conducting polymers, Polypyrrole (PPy) and its composites of noble metal nanoparticles have been broadly utilized for this purpose, due to its excellent electrical conductivity and environmental stability.5, 6 Moreover, recent investigations have shown that the catalytic properties of newly synthesized nanocomposites are influenced by their size.7, 8

Therefore, based on this motivation, most researchers are focused on developing new

synthetic methods for the fabrication of nanoparticles and nanocomposites of electroactive polymers with noble metals or metal oxides. On the other hand, the choice of the noble metal or metal oxide for the synthesis of nanocomposites as catalysis is another important factor. Dopamine (DA) is a catecholamine neurotransmitter that is widely present in the brain and the peripheral nervous system. DA is one of the most important catecholamine neurotransmitters in the mammalian central nervous system.9 Abnormalities in DA concentration may induce several neurological disorders. Consequently, the measurement and monitoring of DA levels in the body has been the major focus in the healthcare industry. The recent shift towards the development of electrochemical sensors for the detection of biomolecules is due to its economic cost, high sensitivity and portability10-13. DA is electroactive and therefore the oxidation of DA can be monitored using electrochemical techniques. 14 Moreover, the sensor electrodes can be miniaturized and suitably placed in the living organism for real time analysis. This is one of the important advantages of the electrochemical method compared to non-electrochemical techniques. However, one of the major problems encountered in the electrochemical monitoring of DA is the interference from 2 ACS Paragon Plus Environment

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other compounds especially in biological samples. These interfering compounds are usually present at concentrations much higher than DA and are oxidized at similar potentials with DA at most electrodes. This is particularly true of ascorbic acid (AA), the main interfering compound in the determination of DA. So far, different types of electrodes have been fabricated for the detection of DA such as polymer modified electrodes, monolayer modified electrodes

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and surfactant modified electrodes.

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14

self-assembled

However, various

other modified electrodes have also been used. On the other hand, other researchers reported that electrodes modified with metal nanoparticles usually demonstrate high electrocatalytic activities towards compounds with slow redox process at bare electrodes17,18. Chiniforoshan et al19 reported a new polymer which has the ability to detect DA. They showed that the potential difference between the oxidation peaks of AA and DA was more than 200 mV, which was sufficient for the selective detection of DA. Saha. et al20 fabricated a new electrode based on chitosan-stabilized silver nanoparticles and p-toluene sulfonic acid-doped ultrathin polypyrrole film for the detection of DA. They showed that the presence of interfering compounds such as AA, UA, EP and glucose was negligible, due to the shift in the oxidation potentials, as shown by the DPV results. Recently, palladium nanoparticles (Pd NPS) were also used as modified electrodes for the detection of DA18. Madhusudana et al18 reported a novel chemically modified electrode based on the electrodeposition of Pd NPs. The electrode exhibited strong electrocatalytic activity towards the oxidation of DA in the presence of AA and UA. The modified electrode with Pd NPs showed three well resolved oxidation peaks for DA, AA and UA with lower oxidation potentials. In this work, we report a facile and new synthetic method for the fabrication of polypyrrole coated palladium silver nanospherical (PdAg NSPs-PPy) composites with different ratios of Pd and Ag via a direct reduction of palladium and silver cations in the presence of pyrrole monomers. The performance of these new materials toward the DA detection is investigated.



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2. Experimental methods

2.1. Synthesis of polypyrrole coated palladium silver alloy bimetallic nanospherical composites All chemicals (Palladium (II) acetate (98%) and Silver nitrate, (99.99%)) were procured from Sigma-Aldrich (St. Louis, MO, USA). Pure pyrrole monomer used in these experiments was stored in the dark prior to synthesis. The nanocomposites were synthesized with different ratios of Pd and Ag (2:1, 1:1 and 1:2). In a typical process, 1 mL of 0.1 M (CH3COO)2Pd and AgNO3 in the desired ratio was added to 30 mL, 7M NaOH solution in a reaction vessel; the reaction was allowed to occur at room temperature with continuous stirring at 500 rpm with a mechanical stirrer. After 20 minutes, 0.5 mL pyrrole monomer was added and the colour of the solution turned from brown to light grey. The reaction between the pyrrole monomer, Pd2+ and Ag+ occurred for 30 minutes. This was followed with the addition of 0.01 mL hydrazine monohydrate into the reaction mixture in the solvothermal reduction process and increasing the reaction temperature to 60 °C at a rate of 1.5 °C min-1. This process was allowed to occur for another 60 minutes for the completion of the reaction. Finally the reaction mixture was centrifuged at 4000 rpm for 10 minutes to separate the PdAg NSPs-PPy from the solution, followed by drying in a vacuum oven at 60 °C for 24 h. The whole process was repeated for the synthesis of Pd-PPy and Ag-PPy for the comparison of different ratios of PdAg NSPs-PPy nanocomposites. 2.2. Electrode preparation The fabricated PdAg NSPs-PPy (with different ratios of Ag and Pd), Pd-PPy and AgPPy (1 mg) were dispersed separately in DMF (1 ml) with ultra-sonicated for 1 h to obtain a


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homogenous suspension. Then, 5 µL of each homogenous suspension were dropped on the surface of a polished glassy carbon electrode (GCE) (MF 2012, Electical system West Lafayette, in USA) separately and dried at room temperature.

2.3. Apparatus and characterizations The morphology of the Pd-PPy and Ag-PPy were investigated using field emission scanning electron microscopy (FESEM, Quanta 200F). The sample was dropped on the ITO surface and dried at room temperature before the FESEM images were taken. The morphology of the PdAg NSPs-PPy (with different ratio of Ag and Pd), were investigated using transmission electron microscopy (TEM) (Philips CM200, at an operating voltage of 200 kV). X-ray diffraction (Siemens D5000) with Cu Kα radiation measurements was used to analyze the structures and surface morphologies of the prepared PdAg NSPs-PPy (with different ratios of Ag and Pd), Pd-PPy and Ag-PPy. The FT-IR spectra were obtained using a Spectrum 400 (FT-IR / FT-FIR spectrometer). Electrochemical impedance spectroscopy (EIS) measurements were performed at 0 V potential in 1 mM Fe(CN)6

3-/4-

(1:1) solution

with 0.1 M KCl supporting electrolyte (Potassium ferricyanide K3Fe(CN)6, 99.2%, SigmaAldrich and potassium ferrocyanide K4Fe(CN)6.3H2O, Fischer Scientific). The impedance spectra were run over a frequency range of 100 kHz–0.1 Hz, with an acquisition of 10 points per decade and signal amplitude of 5 mV around the open circuit potential. The analysis of the impedance spectra was done by fitting the experimental results to equivalent circuits using the non-linear least-square fitting procedure. A potentiostat/ galvanostat model PGSTAT-302N from Autolab (Ecochemie, Netherlands), controlled by a USB IF030 (Metrohm Autolab) interface card with the FRA.EXE software (version: 409.007, distributor: Metrohm Malaysia) installed in a PC was used to perform these experiments. A glassy carbon



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electrode (GCE), calomel and a platinum wire (Pt) were the working, reference and counter electrodes, respectively. 3. Results and discussions 3.1. Materials characterizations The FESEM images of the Ag-PPy and Pd-PPy are shown in Figure 1(a) and (b) respectively. The FESEM images confirm the morphologies of the polypyrrole coated silver nanostrip bundles (Ag NBs-PPy) and polypyrrole coated palladium nanoclusters (Pd NCsPPy). The FESEM images in Figure 1(a) and (b) show a high surface area of the synthesized Ag NBs-PPy and Pd NCs-PPy. The TEM images of the polypyrrole coated palladium silver nanospherical composites (PdAg NSPs-PPy) with different ratios of Pd and Ag are shown in Figure 2(a, Ag2Pd1 NSPs-PPy), (b, Ag1Pd1 NSPs-PPy) and (c, Ag1Pd2-NSPs PPy). The results clearly show a spherical morphology of the PdAg NSPs-PPy nanocomposites, where the Ag and Pd nanoparticles are coated and dispersed in the PPy. Moreover, the TEM result confirms the small size of the nanoparticles, and the available surface area is suitable for the interaction with the analyte. A comparison between the nanospherical composites with the Ag-PPy and Pd-PPy shows that the nanospherical morphology is affected by the ratio of Ag or Pd, but the ratio of Ag or Pd did not influence the homogeneity of the AgPd-PPy. The XRD pattern of the Ag NBs-PPy, Pd NCs-PPy, Ag2Pd1 NSPs -PPy, Ag1Pd1 NSPs -PPy and Ag1Pd2- NSPs PPy are shown in Figure 3(a), (b), (c), (d) and (e) respectively. Figure 3 (a) shows the intensity of the (1 1 1), (2 0 0), (220) and (311) peaks attributed to Ag NBs-PPy (Ref. code: 00-001-1164). Moreover, the intensity of the (1 1 1), (2 0 0) and (220) peaks in the diffractogram are related to the Pd-PPy (00-001-1201). The XRD pattern of Ag2Pd1NSPs-PPy, Ag1Pd1 NSPs-PPy and Ag1Pd2-NSPs PPy indicates that the Ag and Pd peaks have different intensities..The peaks marked with (♦) and (Ο) in Figure 3 are due to Ag 6 ACS Paragon Plus Environment

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and Pd respectively. In addition, a broad amorphous diffraction peak can be seen at 2θ = 5– 25° range in the XRD diagram of the Ag NBs-PPy, Pd NCs-PPy, Ag2Pd1 NSPs-PPy, Ag1Pd1 NSPs-PPy and Ag1Pd2-NSPs PPy which is attributed to the scattering from the bare polymer chains at the interplanar spacing. 21, 22 The XRD result clearly confirm that Ag+ and Pd2+ were converted to Ag and Pd element in the presence of pyrrole monomers during the synthesis of Ag NBs-PPy and Pd NCs-PPy. On the other hand, the pyrrole monomer is a reducing agent for the Ag+ and Pd2+ cations. The pyrrole monomers are polymerized to PPy in the presence of Ag+ and Pd2+ cations and acts as a protector to initiate the PPy primary layer on the Ag or Pd particles, as nucleus for the nanoparticles growth, via the Ostwald ripening process.

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The surface energy and the capping effect are considered as the major driving force for the nanoparticle formation. 26 The FTIR spectra of Ag NBs-PPy, Pd NCs-PPy, Ag2Pd1 NSPs-PPy, Ag1Pd1 NSPsPPy and Ag1Pd2-NSPs PPy are shown in Figure 4(a), (b), (c), (d) and (e) respectively. The peaks between 3440-3455cm−1 in the FT-IR spectra of the composites are attributed to the NH bond. The peaks between 1655 -1687 cm−1 are attributed to the C-N-C bond and the strong bands between 2960 - 3027cm−1 are due to the aliphatic C-H vibration. The peaks for C–N and C–H groups in the composites spectra appear in the range of 1170-1178 cm−1 and 1056 1070 cm−1 respectively. On the other hand, the peaks between 1400 - 1439 cm−1 are characteristic of a typical PPy ring vibrations. The appearance of these peaks confirms the completion of the polymerization process and confirms the existence of PPy in the composites.

3.2. Electrochemical behavior of DA



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The cyclic voltammograms (CVs) of Ag1Pd2 NSPs-PPy (1), Ag1Pd1 NSPs-PPy (2) , Ag2Pd1 NSPs-PPy (3) and PPy (inset) in 0.1 M phosphate buffer solution (pH 7) and 100 µM of dopamine are shown in Figure (5a). The sweep potential was from −0.2 V to 0.4 V at 50 mV s-1. Figure (5a) clearly shows the specific peak at 0.18 V which due to oxidation of DA. The results show that the peak shifts to positive region with the increase of the ratio of Pd to Ag. This is also confirmed from the comparison of Figure 5(b) with Figure 5(c) which is related to the CVs of Ag NBs-PPy and Pd NCs-PPy, in 0.1 M phosphate buffer solution (pH 7) and 100 µM of DA, respectively. The peak at 0.3 V is related to the oxidation of Ag which is overlapped with the oxidation peak of PPy (inset of Figure 5(a)). The peak at 0.1V is related to the reduction of dopamine oxide, while the peak at 0.04 V is related to the reduction of Ag oxide which is overlapped with the reduction peak of PPy. Moreover, in Figure 5(a), the anodic peak current of DA in the phosphate buffer (pH 7) solution is larger than the reduction peak current, which shows a quasi-reversible electrode process. The comparison of Figure 5(a) with Figure 5(b) and Figure 5(c) can confirmed the above phenomenon. Figure 5(d) shows the CVs of Ag1Pd2 NSPs-PPy (1), PPy (2), GCE (3) in 0.1 M phosphate buffer solution (pH 7) in the absence of 100 µM DA. The comparison of Figure 5(d) and Figure 5(a) clearly confirms the electroactivity of the Ag1Pd2 NSPs-PPy GCE for the detection of dopamine. The higher electroactivity of the Ag1Pd2-NSPs PPy is attributed to the following reasons. First, the special morphology of the AgPd-NSPs PPy has a large surface area, compared to the Pd NCs-PPy. Therefore, with the presence of Ag, the morphology change of the Pd–PPy increases the active surface area for the electrocatalytic reaction. But it should be noted that the increase of the Ag ratio in the AgPd-NSPs PPy decreases the catalytic performance due to the inactivity of Ag and oxidation of its surface. The cathodic peak at 0.04V in the CV of Ag1Pd2 NSPs-PPy in Figure 5(a)(1) clearly confirms the reduction of the Ag oxide. On the other hand, the existence of Ag can enhance the


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activation of the surface with the adsorption of OHads and –NH2 through a synergistic effect. 27

This claim supports the d-band centre theory. 28, 29 It is believed that in the presence of Ag,

a tensile strain occurs on the Pd surface due to the smaller lattice constant of Pd compared to Ag. This phenomenon leads to a shift in the d-band centre of the Pd and can enhance the adsorption of OHads and -NH2 on the surface of the Pd. Second, the resistance of the modified GCE with Ag1Pd2 NSPs-PPy is lower than the modified GCE with Ag1Pd1 NSPs-PPy and Ag2Pd1 NSPs-PPy. This interpretation can be confirmed by the EIS results.

3.3. EIS results The interfacial properties of the GCE electrodes modified with AgPd NSPs-PPy (6a) (with different ratios of Ag and Pd) were studied by EIS. Figure 6 shows the Nyquist plots of Ag1Pd1 NSPs-PPy (6b), Ag1Pd2 NSPs-PPy (6c) , Ag2Pd1 NSPs-PPy (6d), Ag-PPy(6e), PdPPy (6f), bare GCE (6g) and PPy (6h) and their equivalents circuits in 1 mM Fe(CN)6

3−/4−

(1:1) in 0.1 M KCl supporting electrolyte. The Nyquist plots of all three electrodes modified with different ratios of AgPd NSPs-PPy show semicircles which are due to the high interfacial resistance (Rct). The behavior of Ag1Pd1 NSPs-PPy (6b), Ag1Pd2 NSPs-PPy (6c) and Ag2Pd1 NSPs-PPy (6d) are approximately similar but the results clearly show that the Ag1Pd2 NSPs-PPy has smaller semicircle diameter. Some of the Nyquist plots indicate a “depressed semi-circle” with the center of the circle is below the ZR axis, which is due to the deviation from the double layer capacitance. A constant phase element (CPE) was introduced instead of a pure capacitor in the fitting procedure to obtain a good agreement between the simulated and experimental data. The impedance of CPE is defined as ZCPE=Q−1 (jω)−n; where Q is the combination of properties related to the surface and electroactive species independent of frequency; “n” is related to the slope of the log Z vs. log f. The parameters obtained by the simulation of the EIS results are given in Table 1. The total Rct values of the 9 ACS Paragon Plus Environment


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modified electrodes with different ratios of AgPd NSPs shows that the total resistance of Ag1Pd2 NSPs –PPy is the lowest. It is reasonable that the conductivity of the composite increases with the increase Pd percentage. This increases the current response of the electrode and a much faster interfacial electron transfer process of the electrochemical sensing platform could take place. The decrease of the CPE (Q) and C1 from Ag1Pd2 NSPs –PPy /GCE to Ag2Pd1 NSPs –PPy / GCE, is attributed to the decrease of the surface roughness. From the comparison of these results, the optimum ratio of Pd to Ag which can increase the surface roughness of the synthesized composite is 2:1. It is noteworthy that the available surface area of the composite increases with increase of the roughness, and this significantly increases the sensitivity of the material for the detection of the analyte. The surface roughness also has a large influence on the CPE behavior. The fractal dimension (D) is around 2-3 for a rough surface. The electrode surface can be considered between 2 dimensions (flat) and 3 dimensions (similar to a porous cube.) Other researchers believe that for these type of electrodes, the interfacial impedance is modified by an exponent, n = 1/(D-1). 30 For a smooth surface, the fractal dimension (D) is 2.0 and n=1, while for a highly contorted surface, D is 3 and n=0.5. The decrease of “n” from Ag1Pd2 NSPs –PPy /GCE to Ag2Pd1 NSPs –PPy / GCE shows a decrease in the surface roughness. 3.4 Effects of the pH of the solution The effect of pH on the electrochemical response of the Ag2Pd1 NSPs –PPy / GCE on the addition of 100 µM DA was investigated using CV. The change in peak current with pH (pH range of 3–7.4) is shown in Figure 7. It can be observed that the anodic peak current increases with pH until pH 7. Consequently, the buffering at pH 7, which is close to the physiological pH, is used in the rest of the work. Smaller currents were detected when the pH of the solution is either lower or higher than 7. 3.5. Effects of scan rate 10 ACS Paragon Plus Environment

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Figure 8(a) shows the CVs of Ag2Pd1 NSPs –PPy / GCE at different scan rates in 0.1 M phosphate buffer solution (pH 7) and 100 µM of dopamine. The linear increase of the oxidation and reduction peak currents with the scan rate shows that the electrode reaction of the immobilized Ag2Pd1 NSPs –PPy redox couple is a surface-confined electrochemical process. The linear regression equation can be expressed as Ipa = 1.797υ (mV s−1) + 76.15(R2 = 0.993) and Ipc =-2.369(mVs-1) -38.55 (R2=0.991) (Figure 8(b)).

3.6. Determination of DA using differential pulse voltammetry A typical amperometric response of the Ag2Pd1 NSPs –PPy /GCE to consecutive DA concentration changes was studied with differential pulse voltammetry (DPV) in 0.1 M phosphate buffer solution at pH 7. Figure 9 (a) shows the dependence of the anodic peak current (Ipa) on the DA concentration. The sensor was calibrated three times and the standard deviations were calculated. The calibration curve for the Ag2Pd1 NSPs –PPy /GCE shows a linear segment from 0.001 to 200 µM with a linear regression equation of Ipa=1.574(µA µM−1 cm-2) +79.52 (R2=0.994) (Figure 9(b)).The limit of detection (LOD) and the limit of quantification (LOQ) of the Ag2Pd1 NSPs –PPy /GCE were calculated from the following equations: 31 LOD = 3SB / b

(1)

LOQ = 10SB / b

(2)

Where SB is the standard deviation of the blank solution and b is the slope of the analytical curve, as shown in Figure 9(b). The estimated limit of detection (S/N = 3) and limit of quantification (S/N = 10) for the linear segment (0.001-200 µM of dopamine) are 0.0258 µM and 0.0860µM respectively. In addition, the sensitivity for this linear segment is 1.574 µA µM−1 cm-2 .



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3.6. Interference effects Ascorbic acid (AA) and uric acid (UA) are common compounds found together with DA in real samples, such as biological fluids. The oxidation potentials of these three compounds are close to each other, therefore the overlap of the oxidation peaks due to the interferences from AA and UA are the major problems in the detection of DA. The effects of AA and UA on the analytical determination of DA were examined by the amperometric response of successive additions of 100 µM DA and 100 µM of AA and UA in a phosphate buffer solution (pH 7.0) at 200 mV (Figure 9c). The results indicate that the presence of AA and UA does not significantly affect the DA response. On the other hand, the Ipa sharply increases when 100 µM DA is spiked into the phosphate buffer solution with AA and UA. In addition, DPV in the presence of AA and UA was investigated to determine the selectivity of Ag2Pd1 NSPs –PPy /GCE toward the detection of DA. Figure 9(d) and 9(e) show the DPV results of 0.1 µM DA in the presence of 10, 50 and 100 µM AA and UA in phosphate buffer solution (pH 7.0), respectively. The DPV results confirm that the presence of AA and UA is negligible toward the detection of DA by the modified electrode. These results demonstrate that the Ag2Pd1 NSPs –PPy /GCE has a high selectivity towards DA detection, even in the presence of some common interfering compounds that are normally found in biological samples. In a basic solution, the counter anions in PPy are substituted by OH- from solution. 32

Although the charge on the OH- groups may be balanced by an equal and opposite charge

from the oxidized polypyrrole backbone (PPy+), the –OH- pendants will provide a highly negative local charge. AA exists to some extent as an anion at pH 7.This is also true for UA, due to the existence of a lone pair of electrons on the nitrogen atoms, which are repelled from the surface by the negatively charged OH- groups, as the counter anion in the PPy. In addition, the neutral AA should not be attracted to the negatively charged interface.


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3.7. Analysis of real samples The practical feasibility of the sensor has been evaluated in pharmaceutical samples. A commercially available dopamine hydrochloride injection of 8.44 mM concentration was acquired . The concentration of the injection sample has been diluted to the final concentrations of 0.5 and 20 and 80 µM by using 0.1 M phosphate buffer (pH 7). The recovery was obtained by using DPV to evaluate the accuracy of the method. Based on the replicates (n = 4), the relative standard deviation (RSD) of this method is presented in Table 2. The satisfactory recoveries of DA at the Ag2Pd1 NSPs –PPy /GCE in the range of 0.001– 200 µM demonstrate the efficiency and reliability of this method. Table 3 shows that the LOD of DA for Ag2Pd1 NSPs –PPy /GCE is lower compared to nanostructured Au,

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molecular imprinted polymer/GCE 34 and molecular imprinted polymer/MWCN/GCE. 35

4. Conclusions Polypyrrole coated palladium silver nanospherical (PdAg NSPs-PPy) composites with different ratios of Pd and Ag (2:1, 1:1 and 1:2) were synthesized by a direct reduction of Pd2+ and Ag+ in the presence of pyrrole monomers in an aqueous solution. The results demonstrated that the surface modification of the glassy carbon electrode with Pd2Ag1 NSPsPPy resulted in superior electrocatalytic activity toward DA electrooxidation in buffer solution (pH 7). The higher electroactivity of the Ag1Pd2-NSPs PPy was due to the following reasons. First, with the presence of Ag, the morphology change of the Pd–PPy increases the active surface area for the electrocatalytic reaction. But the catalytic performance decreases due to the inactivity of Ag and oxidation of its surface, with the increase of the Ag ratio in the AgPd-NSPs PPy. Second, the resistance of the modified GCE with Ag1Pd2 NSPs-PPy was lower than the modified GCE with Ag1Pd1 NSPs-PPy and Ag2Pd1 NSPs-PPy. The estimated LOD (S/N=3), LOQ (S/N = 10) for the two linear segments (low and high concentration of 13 ACS Paragon Plus Environment


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dopamine) were 0.930 nM, 0.199 µM and 3.110 nM, 0.665 µM, respectively. In addition, the sensitivity of these two linear segments was 0.428 mA µM−1 cm-2 and 0.002 mA µM−1 cm-2 respectively.

Acknowledgments The authors wish to thank Mojdeh Yeganeh for valuable discussion. This research is supported by High Impact Research MoE Grant UM.C/625/1/HIR/MoE/SC/04 from the Ministry of Education Malaysia, PRGS grant PR002-2014A, GC001C-14SBS, RP038C15HTM and University Malaya Centre for Ionic Liquids (UMCiL).


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Figure and table captions Figure 1. FESEM images of: (a) Ag NBs-PPy and (b) Pd NCs-PPy. Figure 2. TEM images of: (a) Ag2Pd1 NSPs –PPy, (b) Ag1Pd1 NSPs –PPy and (c) Ag1Pd2NSPs PPy. Figure 3. XRD pattern of: (a)Ag NBs-PPy, (b)Pd NCs-PPy, (c) Ag2Pd1 NSPs -PPy, (d) Ag1Pd1 NSPs -PPy and (e) Ag1Pd2- NSPs PPy. Figure 4. FTIR spectra of: (a) Ag NBs-PPy, (b) Pd NCs-PPy, (c) Ag2Pd1 NSPs -PPy,(d) Ag1Pd1 NSPs -PPy and (e) Ag1Pd2- NSPs PPy. Figure 5. The cyclic voltammograms (CVs) of: (a) Ag1Pd2 NSPs-PPy (1), Ag1Pd1 NSPs-PPy (2) , Ag2Pd1 NSPs-PPy (3) and PPy (inset) in 0.1 M phosphate buffer solution (pH 7) and 100 µM of DA.(b) the CV of Ag NSPs-PPy and (c) Pd NSPs-PPy in 0.1 M phosphate buffer solution (pH 7) and 100 µM of DA respectively. (d) the CVs of Ag1Pd2 NSPs-PPy (1), PPy (2) , GCE (3) in 0.1 M phosphate buffer solution (pH 7) and in the absent of 100 µM of DA. The sweep potential is from −0.2 V to 0.35 V at 50 mV s-1. Figure 6. The Nyquist plots of AgPd NSPs-PPy (with different ratios of Ag and Pd) in comparison with the Nyquist plots of Ag/PPy and Pd/PPy (a). The Nyquist plot and related equivalent circuit of: Ag1Pd1 NSPs-PPy (b), Ag1Pd2 NSPs-PPy (c) , Ag2Pd1 NSPs-PPy (d), Ag-PPy (e), Pd-PPy (f), bare GCE (g) and PPy (h) in 1 mM Fe(CN)6 3−/4− (1:1) in 0.1 M KCl supporting electrolyte. Figure 7. The change in peak current with pH (pH range of 3–7.4).



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Figure 8 . (a) The influence of scan rate on the peak current of Ag1Pd2 NSPs-PPy. (b) The plot of anodic peak current (µA) vs. scan rate (mV s−1): 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120. Figure 9. (a) The DPV curves for different concentration of nitrate in 0.1 M phosphate buffer solution (0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 20, 30, 40, 50,60, 70, 80, 90, 100, 150 and 200 µM, at pH 7.(b) The calibration curve at concentration range. Error bars represent ± standard deviation. (c) The amperometric response of successive additions of 100 µM DA and 100 µM of AA and UA in a phosphate buffer solution (pH 7.0) at 200 mV. (d) DPV results of 0.1 µM DA in the presence of 10, 50 and 100 µM AA in phosphate buffer solution (pH 7.0). (e) DPV results of 0.1 µM DA in the presence of 10, 50 and 100 µM UA in phosphate buffer solution (pH 7.0). Table 1. Electrochemical parameters obtained by simulation of the EIS results of the bare GCE, Ag-PPy, Pd-PPy, Ag1Pd1 NSPs-PPy, Ag1Pd2 NSPs-PPy, Ag2Pd1 NSPs-PPy and PPy in 1mM Fe(CN)63−/4−(1:1) with 0.1 M KCl supporting electrolyte. Table 2. The detection of DA concentration in test samples (results based on six replicate determinations per sample). Table 3. A summary and comparison of the estimated LOD values of the present work and previous reports.


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Figure 1. M.R.Mahmoudian



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Page 26 of 34

Table. 1. Mahmoudian et al

Electrode

Rs Ohm.cm2

Rct1 Ohm.cm2

Y°(mOhm-1.sncm-2)

Q1

GCE Ag/PPy

1.12 1.25

3950.10 243.1

0.0013 0.0099

Pd/PPy

1.24

453

Ag1Pd1/PPy

1.21

Ag1Pd2/PPy

Rct2 Ohm.cm2

C1 mF

C2 mF

0.9902 0.5989

2672

0.997

-

0.1086 -

2.0100

0.2663

303.2

10.71

-

-

486

2.2620

0.7617

1058

4.00

-

-

1.17

124

2.3430

0.6629

1121

7.49

-

-

Ag2Pd1/PPy

1.11

10.55

-

-

4100

PPy

1.17

884.1

0.005970

0.7884

-

0.995× 10-3 -

n

0.439 -


Sample 1

Added/ µmol l-1 0.5

Found/ µmol l-1 0.4955

(RSD%) (n=6) 1.9663

(Recovery%) 99.11

2

20

20.127

2.3959

100.63

3

80

79.856

2.6204

99.82


W Y°(mSs1/2cm-2)

-

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Electrode

Technique

Linear range (µM) 0.2–600 0.05-10

Reference

Amperometric DPV

Detection limit 0.026µM 0.033µM

nanostructured Au Molecular imprinting polymer/GCE Molecularly imprinted polymer/MWCN/GCE Au@carbon dots– chitosan-modified GCE Pt/UltraPPy–GCE (patternable gold (Au) nanowire (NW)) NW/glass (Gold nanofilm)NPG/ITO

DPV

0.06 µM

0.6–100

35

DPV

1nM

0.01–100

36

DPV amperometry

0.67 nM 0.4 µM

0.01–400 0.4−250

37 38

DPV

1.5 µM

1.5−27.5

39

PdAg NSPs-PPy

DPV

0.025µM

0.001-200

This work


33 34


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Sensitive Dopamine Biosensor Based on Polypyrrole-Coated

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