Highly Selective and Sensitive Electrochemical ... - IEEE Xplore

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IEEE TRANSACTIONS ON NANOBIOSCIENCE, VOL. 10, NO. 4, DECEMBER 2011

Highly Selective and Sensitive Electrochemical Detection of Dopamine Using a Nafion Coated Hybrid Macroporous Gold Modified Electrode With Platinum Nanoparticles Yi-Jae Lee and Jae-Yeong Park

Abstract—A coral-like macroporous Au electrode with electroplated Pt nanoparticles (hybrid macroporous Au-/nPts) coated with Nafion has been fabricated for the first time and used for highly selective and sensitive determination of dopamine (DA). The physically characterized results indicated that the electroplated Pt nanoparticles were dispersed uniformly on the macroporous Au electrode. The porosity and window pore size of the fabricated macroporous Au electrode were 50% and 100–300 nm, respectively. Also the electroplated Pt nanoparticles size was approximately 10–20 nm. The cyclic voltammograms results showed that the hybrid macroporous Au-/nPts exhibited a much larger surface activation area, a roughness factor (RF) of 2024.7, much higher than that of the macroporous Au electrode, which is 46.07. The electrochemical experimental results showed that the hybrid macroporous Au-/nPts coated with Nafion exhibited a dramatic electrocatalytic effect on the oxidation of DA. At 0.1 V, it responded linearly to DA concentrations ranging from 20 M to 160 M with a detection sensitivity of 90.9 A mM cm . Furthermore, it showed wide detection ranging from 20 nM to 900 M. At the same time, the interference of ascorbic acid (AA) was effectively avoided because of the Nafion film coated on the surface of the hybrid electrode. Index Terms—Macroporous Au, Pt nanoparticles, hybrid, electroplating, Nafion, dopamine, ascorbic acid.

I. INTRODUCTION

D

OPAMINE (DA) IS an important neurotransmitter that performs a key role in the mammalian central and peripheral nervous systems [1]. DA is widely distributed in mammalian central nervous systems with high amounts (50 nmol/g) in a region of the brain tissues [2]. Furthermore, DA is involved in the regulation of cognitive functions such as attention, stress, rewarding behavior, and reinforcing effects of certain stimuli [3]. Also, DA has been implicated in the pathogeneses and treatment of a variety of psychiatric disorders, such as schizophrenia, Manuscript received October 23, 2010; revised August 27, 2011; accepted November 09, 2011. Date of publication November 22, 2011; date of current version January 20, 2012. This work was supported by the Ministry of Knowledge & Economy, Republic of Korea, under the International Collaborative R&D Program. Asterisk indicates corresponding author. Y.-J. Lee is with the Department of Electronic Engineering, Kwangwoon University, 447-1, Wolgye-Dong, Nowon Gu, Seoul, 139-701, Korea. *J.-Y. Park is with the Department of Electronic Engineering, Kwangwoon University, 447-1, Wolgye-Dong, Nowon Gu, Seoul, 139-701, Korea (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNB.2011.2176348

Parkinsons’s disease, depression, and addiction [4]. Thus, quantitative determination of DA is very important and has been attracted interest among neuroscientists and chemists. Recently, the identification and determination of DA with electrochemical procedures have attracted much attention. However, it is difficult to determine DA by direct oxidation at bare electrodes because of the high overpotential [5] and the fouling effect by its oxidation products [6]. Moreover, the oxidation waves of ascorbic acid (AA) and DA are at nearly the same potential and overlapped, which results in poor selectivity and reproducibility. The ability to determine AA or DA selectively in the presence of each other has been a major goal of electroanalytical research [7]. Especially, the coexistence of AA with a concentration of 100–1000 times higher than that of DA provides a great challenge for the electrochemical strategy to be used for DA determination. Various methods, mainly based on the chemical modification of traditional electrode materials, have been developed to resolve the problem. Various modified electrodes such as a poly (3,5-dihydroxy benzoic acid) film modified electrode [8], a carbon nanotube modified electrode [9], poly (Evans Blue) film modified electrode [10], pyridine bromide/chitosan composite film modified electrode [11], a mesoporous silica modified electrode [12], nanoparticle [13], a Nafion modified glassy carbon electrode [14], a nanoporous gold modified electrode [15], a Pd nanoparticles-loaded carbon nanofibers modified electrode [16], and a Pt-Au hybrid film modified electrode [2] have been developed for the determination of DA. Also, in order to selectively determine the DA concentration in the presence of a large excess of AA or to perform their simultaneous determination, some electroanalytical techniques such as differential pulse voltammetry (DPV), square wave voltammetry, and fast scan voltammetry have been reported in the literature. Meanwhile, amperometric technique also has been extensively studied for DA detection. The advantage of the amperometric technique is that it permits the simple and fast gathering of data. The time scale of the measurement is limited only by the speed of the electronics used to record the current. There are no timing limitations associated with the pulse or sweep waveforms. Also, the electrode responds extremely quickly to changes in analyte concentration, and high sampling rates can be used. Therefore, amperometric technique can offer the best temporal resolution among the available techniques. It is appropriate for measurement of electrically evoked DA release in anesthetized

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LEE AND PARK: HIGHLY SELECTIVE AND SENSITIVE ELECTROCHEMICAL DETECTION OF DOPAMINE

animals or brain slices, and its fast sampling rate makes it ideal to analytical DA detection [17], [18]. Nafion, as a cation exchange polymer, is known for its ability to incorporate positively charged ions and reject the negatively charge species, due to the presence of anionic sites in its structure. MWCNTs-Nafion modified glass carbon (GC) not only improves the redox peak currents but also makes the redox reaction of DA more reversible [19]. A poly (aniline boronic acid) (PABA)/Nafion composite GC modified electrode was employed to detect DA in the presence of AA, using cyclic voltammetry. The authors have demonstrated that the composite electrode can sense DA in the presence of excess of AA [20]. In order to achieve a truly highly catalytic DA detection electrode, the fabrication of porous structured electrodes is very promising in electrochemical sensing applications. The main characteristics of such porous electrodes in the electroanalytical domains are their high surface/volume ratio, favoring thereby the interaction with external reagents, excellent conductivity, and the interconnectivity between the pores, which makes them very attractive for miniaturized electrochemical devices [21]. If they are electrically conducting, such materials deposited as thin films on the other electrode surfaces can result in an increase of the electroactive surface by several orders of magnitude and a commensurate increase in the sensitivity of the resulting device. In addition, the surface modification of electrodes with metal nanoparticles has led to some recent developments of electrochemical sensors. Especially, Pt nanoparticles have evoked increasing interest in the design of sensors [22], and some reports have demonstrated that Pt nanoparticles can facilitate the electron transfer and increase the surface activation area with enhanced mass transport characteristics [2]. Nanostructured Pt modified electrodes were prepared by the electrodeposition of Pt on a Nafion film coated GC electrode and electrocatalytic oxidation of DA and serotonin effected by GC/Nafion/Pt nanoparticles electrodes in the presence of interfering molecules such as AA [23]. The bimetallic Au-Pt hybrid film modified electrodes have been the subject of important studies in this new era of electrode development. However there are no previous reports concerning bimetallic macroporous Au electrode modified with Pt nanoparticles in the development of sensor for the determination of DA. The present paper reports the fabrication of macroporous Au with Pt nanoparticles (hybrid macroporous Au-/nPts)/Nafion modified 3-D hybrid electrode, its application in the amperometric determination of DA in the presence of excess of AA, and its remarkable sensitivity and selectivity, compared with the case of a macroporous Au electrode/Nafion. The macroporous Au electrode is fabricated by the use of the templating method. The Pt nanoparticles are then formed onto the macroporous Au electrode by the use of a nonionic surfactant and electroplating technique. We check the surface morphology and compositions of the fabricated hybrid macroporous Au-/nPts by the use of a field emission scanning electron microscope (FESEM) and an energy dispersive X-ray (EDX) spectroscopy. Also, it is characterized in a sulfuric acid solution for the purposes of checking its surface roughness by cyclic voltammetry. After that stage, the highly roughened electrodes are coated with Nafion for the selective detection of DA. Although there have

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been reports on the electrochemical behavior of Nafion and its determination [24], there were no reports on a composite modified electrode with macroporous Au or Au-Pt nanoparticles coated with Nafion as presented in this paper. In order to evaluate their electrochemical catalytic characteristics and applicability, the fabricated electrodes coated with Nafion are characterized in various concentrations of DA and AA. II. EXPERIMENTAL A. Reagents and Apparatus All solutions were prepared with deionized water (resistivity 18 M -cm). The aluminum precursor (aluminum sec-butoxide), surfactants (stearic acid and magnesium stearate), gold precursor (HAuCl ), acid etchant (mixture of 11.8 M H PO and 0.6 M HNO ), and sodium tetrahydridoborate (NaBH ) were prepared for the achievement of a coral-like macroporous Au electrode [25]. The electroplating mixture for the Pt nanoparticles consisted of 42% (w/w) C EO (octaethylene glycol monohexadecyl ether, 98% purity, Fluka), 29% (w/w) deionized water (18 M -cm) and 29% (w/w) HCPA (hexachloroplatinic acid hydrate, 99.9% purity, Aldrich) [26]. The fabricated electrode was measured in order to check its surface roughness factor (RF) in a 1 M sulfuric acid (H SO , 95%–98%, ACS, Sigma-Aldrich) solution using cyclic voltammetry. A 1 M H SO solution was prepared by dilution in deionized water (100 mL volume). The ascorbic acid (AA, 98%), dopamine (DA, 95%), and Nafion (5 wt.% solution) were obtained from Aldrich. All electrochemical experiments were performed with an electrochemical analyzer (Model 600B series, CH Instruments Inc., USA) at room temperature. A three electrode system was used, including a fabricated electrode as the working electrode, a flat Pt bar employed as a counter electrode, and an Ag/AgCl as a reference electrode, respectively. The surface morphology and compositions of the fabricated electrodes were characterized with a Hitachi S-4300 field emission scanning electron microscope and Horiba EX-200 energy dispersive X-ray spectroscopy. B. Fabrication of Macroporous Au and Hybrid Macroporous Au-/nPts Electrodes Fig. 1 represents simplified fabrication sequences of corallike macroporous Au and hybrid Au-/nPts electrodes. As shown in Fig. 1(a), the coral-like macroporous Au was formed by the use of aggregated Au nanoparticles. A similar phase separation was found in the preparation of the monolithic TiO via a template-free sol-gel process [27]. The aluminum precursor (Al sec-butoxide) and surfactants (stearic acid and magnesium stearate) were dissolved separately in sec-butyl alcohol. An Au precursor (HAuCl ) was added to a solution of the dissolved surfactant. These two solutions were mixed and followed by the slow addition of water at the rate of 1 ml/min. The molar ratio of this reaction mixture was 1 Al(sec-BuO) : 0.09 HAuCl : 0.2 surfactant: 10sec-BuOH: 7 H O. NaBH was used as reducing agent for the Au precursor. Also the mixture was stirred continuously for 24 h. The mixture was then dried at 80 C and calcined at 550 C in air. At this calcination step, the surfactant was easily removed, and the resultant material had a nanoporous

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Fig. 1. Schematic diagrams for fabrication process of: (a) macroporous au electrode by the sol-gel process (this contains largely mixing, calcinations, and etching process) and (b) macroporous Au electrode with Pt nanoparticles (hybrid macroporous Au-/nPts) by the use of templating method and electroplating technique (This contains largely mixing for forming surfactant mold, electroplating, and removing surfactant process).

alumina structure with a sintered Au network. The alumina network with nanopores was etched selectively by etching with acid etchants (11.8 M H PO and 0.6 M HNO ). After etching, additional heat processing was conducted at 150 C for 20 min in order to remove the defects from the Au nanoparticles. As a result, we obtained macroporous Au, namely, the reverse phase of the alumina network [25]. The fabricated macroporous Au was pressed at room temperature into a circular mold in order to form a sensor working electrode (applied pressure; 10 kgf cm , mold radius; 3.5 mm). Fig. 1(b) shows fabrication sequences of the hybrid macroporous Au electrode with Pt nanoparticles. The electroplating mixture for forming the Pt nano-particles was well mixed and heated up to 85 C in the water-jacketed vial, after which the mixture became transparent and homogeneous. The macroporous Au electrode was dipped into the mixture and the temperature was lowered to 25 C. At this stage, a liquid crystalline structure of nonionic surfactant C EO was formed on the surface of the macroporous Au electrode. Then, Pt nanoparticles were electroplated on to the macroporous Au electrode V vs. Ag/AgCl) and by the use of a constant potential ( charge condition of 70 mC. The electrode was then soaked in distilled water for several hours in order to remove the C EO [28]. Finally, the hybrid macroporous Au electrode with Pt nanoparticles was formed. Finally, Nafion was drop coated on the macroporous Au electrode and the hybrid macroporous Au-/nPts. The Nafion layer was used for the elimination of negatively charged species, if any, by its negatively charged sulfonate groups. The proposed electrode with only Au and Pt bimetallic structure has some unique advantages compared to previous reported bimetallic particles based electrode on separate supporting materials, single material based porous electrode, and carbon based electrode. Macroporous Au was directly used supporting material for forming Pt nanoparticles. Thus, the surface area was extremely improved. It was also fully bio-compatible and processing-compatible with all the substrates including silicon and flexible polymer. Furthermore,

hybrid macroporous Au-/nPts can be easily used electrode for sensor device by using pressing or heating. III. RESULTS AND DISCUSSIONS A. Morphological Analysis Fig. 2 shows a photograph of the fabricated hybrid macroporous Au-/nPts and its coral-like shaped surface morphology and its compositions after electroplating with Pt nanoparticles. The photograph was obtained by the use of FESEM and EDX. As shown in Fig. 2, an FESEM image of the fabricated hybrid macroporous Au-/nPts had a coral-like structure. The aggregated Au branch (ca. 200–400 nm) is consisted of spherical Au particles with a 10–30 nm size. The porosity and window-pore size of the fabricated hybrid macroporous Au-/nPts were approximately 50% (Micrometrics ASAP-2010 model) and 100–300 nm, respectively. More detailed morphological information about macroporous Au has been previously reported [25]. An inset image of the FESEM shows apparently formed Pt nanoparticles on the macroporous Au electrode. The electroplated Pt nanoparticles were uniformly well deposited on the macroporous Au electrode and had a size of approximately 10–20 nm. The Pt nanoparticles density and size could be controlled by the electroplating charge condition, since the electroplating current can affect the formation of metal particles [28], [29]. The FESEM images demonstrate that the macroporous Au structure possesses a greatly enhanced surface roughness. EDX measurements were also used to calculate the atomic proportions of the elements presented in the fabricated hybrid electrodes. The EDX analyses revealed that the Pt nanoparticles were well deposited on the pure macroporous Au electrode. The elements presented in the surface are clearly indicated by the peaks corresponding to their energy levels. Moreover, the relative percentages of Au and Pt found in the surface are also exhibited as 92.05% and 7.95%, respectively.


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Fig. 2. Photograph, FESEM images (inset; close up view, white dots mean deposited Pt nanoparticles on the macroporous Au), and EDX spectrum of the fabricated coral-like shaped hybrid macroporous Au-/nPts (clockwise).

Fig. 3. Cyclic voltammograms of the fabricated (a) macroporous Au and (b) hybrid Au-/nPts electrodes in 1 M sulfuric acid solution for checking their surface roughness factors (accumulated charge density was calculated by the use of respective shadow area). Scan rate: 20 mV s .

B. Roughness Factors of Macroporous Au and Hybrid Au-/nPts Electrodes In order to electrochemically characterize the real surface of the fabricated electrode, a cyclic voltammogram of the electrode in a 1 M H SO solution was collected. The cyclic voltammograms were obtained at a scan rate of 20 mV s and the potential ranged from to 1.2 V vs. Ag/AgCl. The RF was then calculated as follows, [30] The roughness factor (RF): and mean the real surface (interface) area and the geometric surface (interface) area, respectively. Real surface area of the cyclic voltammogram of an Au electrode (before electroplating) and Pt electrode (after electroplating) are generally

based on the charge corresponding to the electrochemically adsorbed oxygen [31], [32] and hydrogen monolayer on the electrode surface, respectively. The amount of the formed surface oxide and hydrogen ad-atom on the electrode surface can be measured by integrating the Au oxide and hydrogen reduction peaks in the cathodic scan (shadow area in Fig. 3), respectively. Though the various previous researches, the 0.4 mC cm is usually considered as an oxygen monolayer charge density at the unit area for a regular polycrystalline Au electrode [33]. The accumulated charge density (18.42 mC cm ) in shadow area of Fig. 3(a) was divided by aforementioned charge density of the oxygen monolayer (0.4 mC cm ). Thus, the calculated RF of the fabricated macroporous Au electrode was approximately 46.07.

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Fig. 4. Cyclic voltammograms of the macroporous Au [(a) and (b)] and hybrid macroporous Au-/nPts (C and D) electrodes to various AA (0, 1, 2, 3 mM) and DA (0, 10, 20, 30 M) concentrations (Inset of (b) and (d) clearly shows DA oxidation peaks as an increment of the DA concentration). Supporting electrolyte: 0.1 M PBS, pH 7.4; Scan rate: 20 mV s .

On the other hand, the RF of the electroplated Pt on the macroporous Au is calculated from the charge corresponding to the electrochemically adsorbed hydrogen monolayer at the electrode surface as shown in Fig. 3(b). In Fig. 3(b), the cyclic voltammograms of the fabricated hybrid macroporous Au-/nPts are similar to those of the other Pt electrodes. The V response currents in the applied potential range from to 0.2 V were caused by the adsorption/desorption of hydrogen ad-atoms. Since the charge is necessary to form a monolayer of adsorbed hydrogen and the electrode area is covered by each hydrogen atom (cathodic scan from 0.2 V to V), the electrochemical RF is easily calculated [31]. The RF of the hybrid macroporous Au-/nPts is determined as the calculation of the adsorption charge divided by 0.21 mC cm [34]. The accumulated charge density (425.18 mC cm ) in shadow area of Fig. 3(b) was divided by aforementioned charge density of the adsorbed hydrogen monolayer (0.21 mC cm ). The corresponding roughness factor was calculated to be 2024.7 7, which was the highest one in the previously reported ones. This might be caused by the synergistic effect of the macroporous Au structure and Pt nanoparticles. This result was also supported by the aforementioned FESEM images. Also these results implying that the second metal is a crucial factor affecting the electrochemical activity of the bimetallic nanocatalysts. C. Electrochemical Measurements and Determination of DA The voltammetric behaviors of the macroporous Au [Fig. 4(a) and (b)] and hybrid Au-/nPts electrodes [Fig. 4(c) and (d)] were obtained at a scan rate of 20 mV s and the potential ranged to 1 V vs. Ag/AgCl in a 0.1 M PBS (pH 7.4) solution from with the absence (PBS) and presence of AA (1, 2, 3 mM) and

DA (10, 20, 30 M). In Fig. 4, upon the addition of 1 mM AA and 10 M DA, changes of the oxidation current were observed and the voltammetric response also increased with increasing increments of AA and DA concentration. Although the initial redox current was relatively high in the absence of AA and DA due to the enlarged surface activation area, the distinct electrochemical reduction peaks caused by the AA and the DA concentration were observed, when the cyclic voltammetric scan was performed. However, in order to perform the selective detection of DA, the fabricated electrodes are required for the effective blocking from the AA oxidation. Therefore, voltammetric behaviors of the macrporous Au/Nafion [Fig. 5(a) and (b)] and hybrid macroporous Au-/nPts/Nafion [Fig. 5(c) and (d)] modified electrodes were measured at the same voltammetry condition. These figures showed oxidation peaks in the current for DA at about 0.1 V. On the other hand, the AA did not show oxidation current changes with increasing AA concentration. As shown in Fig. 5(a) and (c), the Nafion coated macroporous Au and hybrid macroporous Au-/nPts electrodes represented effective AA blocking characteristics. This can be explained by considering that the negatively charged Nafion repulses the negatively charged ascorbate anions. In the neutral solution, Nafion can attract the positively charged DA onto the coated layer. This is demonstrated in Fig. 5(b) and Fig. 5(d). As shown in Fig. 5(d), the electroplated Pt nanoparticles play a catalytic role in enhancing the sensitivity of macroporous Au electrodes for the detection of DA. The increase in initial background current in Fig. 5(b) and (d) might be caused by the Nafion coating. Although the coated Nafion has enhanced the selectivity of the electrodes for DA, it has the adverse effect of a large resistance to the diffusion of DA, thereby shifting its oxidation peak potential


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Fig. 5. Cyclic voltammograms of the macroporous Au/Nafion [(a) and (b)] and hybrid macroporous Au-/nPts/Nafion [(c) and (d)] to various AA (0, 1, 2, 3 mM) and DA (0, 10, 20, 30 M) concentrations. Inset of B and D shows DA oxidation peaks at 0.1 V potential area as an increment of DA concentration. Supporting electrolyte: 0.1 M PBS, pH 7.4; Scan rate: 20 mV s .

to the negative value (0.1 V). The detection potentials of DA to the macroporous Au electrode/Nafion [Fig. 5(b)] and hybrid macroporous Au-/nPts/Nafion [Fig. 5(d)] were 0.1 V, respectively, indicating that the fabricated electrodes have reduced the oxidation overpotential of DA. The small current change at V potential region in the Fig. 5(c) is not seemed to cause by increment of AA concentration. The current responses were not clearly distinguished and oxidation current was decreased as increment of AA concentration. Fig. 6 shows influence of 1 mM AA M DA compound to the fabricated hybrid macroporous Au-/nPts with and without Nafion coating in 0.1 M PBS (pH 7.4) solution. As shown in Fig. 6, Broad overlapped oxidation curve was obtained at hybrid macroporous Au-/nPts/Nafion. The inset of Fig. 6 depicts that the response current of the hybrid macroporous Au-/nPts without Nafion is affected in the anodic scan by the AA. This result indicates the Nafion coated hybrid electrode was not affected in the anodic scan by AA since the Nafion effectively blocks anionic species from reaching the electrode surface.

Fig. 6. Cyclic voltammograms of the porous Au-/nPts electrodes (w/Nafion and w/o Nafion) in PBS (pH 7.4) with 20 M DA solution for checking its blocking characteristic of anion species (1 mM AA). Scan rate: 200 mVs .

D. Effect of pH on Oxidation of DA on Hybrid Macroporous Au-/nPts/Nafion Electrode The effect of pH of the DA oxidation on hybrid macroporous Au-/nPts was evaluated by measuring the current response of the hybrid electrode to injection of 20 M of DA in 0.1 M PBS at pH values ranging from 3 to 11 (applied potential 0.1 V). The experimental results in Fig. 7 showed that the current responses of DA oxidation were increased to the increment of pH values, as expected. The current response increased in the pH values ranged from 3.0 to 9.0 and reached its maximum at the pH 9. The redox reaction might be affected by changes in pH because of the involvement of protons in the overall reaction [35]. Fig. 7

Fig. 7. Effect of pH on the current response for the oxidation of DA on hybrid macroporous Au-/nPts electrodes with Nafion.

shows the deprotonation of DA at a pH 11, which redox reaction of DA is no longer pH dependent as the DA is completely deprotonated. Since the DA exists in the protonated form at low pH values, the oxidation current extremely increased as increment

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Fig. 8. Amperometric response currents of the fabricated hybrid macroporous Au-nPts/Nafion (upper) and macroporous Au electrode/Nafion (lower) to various DA concentrations in 0.1 M PBS (pH 7.4) solution (a), calibration curve of the hybrid macroporous Au-/nPts/Nafion (b), and amperometric response of hybrid macroporous Au-/nPts/Nafion to consecutive addition of 10 M DA and 1 mM interfering species (AA) in a continuously stirring PBS (pH 7.4). Applied potential; 0.1 V.

of pH value from 3 to 9 [36]. When pH increased further, the peak current decreased instead. Although the current response versus pH value was maximized at pH 9, a pH 7.4 which is equivalent to a physiological level in human body was selected for further analytical characterization [35], [36]. E. Amperometric Measurements and Determination of DA Fig. 8 illustrates the electrocatalytic effect of the macroporous Au electrode/Nafion and the hybrid macroporous Au-/ nPts/Nafion towards oxidation of DA in PBS at 7.4 pH. Measurement was performed at a potential of 0.1 V after stabilizing the background current at a nearly constant value with 100 sec regular intervals. 10 M, 20 M, and 30 M DA solutions were consecutively injected at 100 sec regular intervals. As shown in Fig. 8(a), a steep increase in the DA concentration was made from 10 M to 220 M. On the other hand, the current response caused by the macroporous Au electrode/Nafion is relatively small compared to that of the hybrid macroporous Au-/nPts/Nafion. This indicates that the macroporous Au electrode might be relatively affected by DA diffusion through the resistance of the Nafion. Also, the noise of the amperometric signal becomes larger at higher concentration ranges, which can be attributed to the diffusion restrictions of DA. However, the response current of the hybrid macroporous Au-/nPts/Nafion for various DA concentrations was relatively large due to the relatively high electron transfer characteristic of the Pt nanoparticles. The electrocatalytic activity toward the oxidation of DA could be most likely caused by the enlarged surface activation area or surface roughness factor, which might provide many favorable sites for electron transfer. Fig. 8(b) shows the calibration curve plot of the DA response measured at the applied potential of 0.1 V. A good linear relationship could be observed between the current and the concentration of DA in the concentration range of 20–160 M and its correlation coefficient was greater than 0.998. The excellent electrocatalytic properties of the hybrid

macroporous Au-/nPts/Nafion were demonstrated more clearly in the corresponding calibration plot. The hybrid macroporous Au-/nPts/Nafion exhibited an extremely high sensitivity of 90.9 A mM cm in the concentration ranged from 20 M to 160 M. The reproducibility of the hybrid macroporous Au-/nPts/Nafion was also evaluated via the comparison of the currents of other electrodes prepared in the same experimental conditions. In the linear region ranged from 20 M to 160 M, they provided a maximum relative standard deviation (RSD) value of 3.72%. Fig. 8(c) shows amperometric responses of the fabricated hybrid macroporous Au-/nPts/Nafion to the consecutive addition of 10 M DA and 1 mM AA in a stirring PBS. One of the most important analytical factors for amperometric bio-molecules detection is the ability of the electrode to discriminate the interfering species having electroactivities similar to the target analyte. The oxidizable compounds such as AA normally coexist with DA in real samples. The AA concentration is 100–1000 times higher than that of DA at the physiological level [37]. We checked amperometric responses at 1 M DA and 1 mM AA. As shown in Fig. 8(c), the response current of the hybrid macroporous Au-/nPts/Nafion electrode was stable and linear without being affected by the interfering species (AA). Fig. 9 shows the specific DA sensing and AA repelling mechanism on the hybrid macroporous Au-/nPts/Nafion. Nafion, as a cation exchange membrane, is widely known for its ability to incorporate positively charged ions and reject negatively charged redox-active interferences from affecting sensor current reading due to the presence of anionic sites in its structure [38], [39]. These characteristics were also demonstrated in the previous works [19], [20]. As shown in Figs. 9, 5(c), (d), and 8(c), it demonstrates selective DA bio-molecules oxidation on the hybrid macroporous Au-/nPts/Nafion. In order to find out the detection limit, amperometric responses were also obtained by consecutive addition of various concentration of DA (10, 20, 40, 100, 200, 500, 1000, 10 000,


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TABLE I COMPARISON OF THIS WORK AND THE OTHER WORKS BASED ON METALLIC NANOPARTICLES MATERIALS

the inset of Fig. 10. It exhibited wide detection range and comparable detection limit with the previously reported electrodes such as grapheme modified electrodes [41], PtAu hybrid film modified electrodes [2], and glassy carbon electrode modified with MWCNTs, quercetin, and Nafion [24]. Table I shows comparison of this work and the other works based on metallic nanoparticles electrodes. IV. CONCLUSION

Fig. 9. Schematic drawing for selective DA detection principle of the hybrid macroporous Au-/nPts/Nafion.

A novel and simple strategy for the selective detection of DA using hybrid macroporous Au-/nPts/Nafion with a modified highly roughened coral-like was presented in this paper. The hybrid macroporous Au-/nPts was fabricated by the use of a templating method and electroplating technique. It was then characterized by the use of FESEM, EDX, and cyclic voltammetric methods. The electrode showed a highly improved surface roughness factor compared with conventional electrodes, higher than any previously reported. Unlike the Nafion coated macroporous Au electrode, the Nafion coated Pt nanoparticles exhibited a dramatic electrocatalytic effect on the oxidation of DA and presented a higher sensitivity and favorable selectivity for DA detection free of interfering species of excess AA. It also showed excellent stability and reproducibility. These attractive features provide potential applications for an enzyme free DA detection sensor. The electrochemically deposited Pt nanoparticles on the macroporous Au electrode led to a significant improvement in real surface activation area and it exhibited extremely high detection sensitivity for DA. ACKNOWLEDGMENT

Fig. 10. Amperometric response for detection limitation of the hybrid macroporous Au-/nPts/Nafion.

50 000, 100 000, 200 000 nM) as shown in Fig. 10. Although there were some signal noises in the obtained responses due to a small volume injection and a continuous magnetic stirring, the response change of the hybrid macroporous Au-/nPts/Nafion was monitored clearly at 20 nM DA concentration as shown in

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