Fabrication of Ru–Pd bimetallic monolayer on nanoporous gold film electrode with excellent electrocatalytic performance towards captopril oxidation

Accepted Manuscript Title: Fabrication of Ru-Pd bimetallic monolayer on nanoporous gold film electrode with excellent electrocatalytic performance towards captopril oxidation Author: Nahid Tavakkoli Nasrin Soltani Elahe Khorshidi PII: DOI: Reference:

S0013-4686(15)00373-4 http://dx.doi.org/doi:10.1016/j.electacta.2015.01.219 EA 24366

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

21-11-2014 25-1-2015 25-1-2015

Please cite this article as: Nahid Tavakkoli, Nasrin Soltani, Elahe Khorshidi, Fabrication of Ru-Pd bimetallic monolayer on nanoporous gold film electrode with excellent electrocatalytic performance towards captopril oxidation, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.01.219 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of Ru-Pd bimetallic monolayer on nanoporous gold film electrode with excellent electrocatalytic performance towards captopril oxidation

Nahid Tavakkoli*, Nasrin Soltani, Elahe Khorshidi Chemistry Department, Payame Noor University, 19395-4697 Tehran, I. R. of IRAN Corresponding author. Fax. +98-313-7381002 *[email protected]

Abstract

This paper describes a simple and novel method to construct ruthenium-palladium (RuPd) bimetallic thin films by coating thin layer of RuPd metals on the nanoporous gold film (NPGF) electrode. The codeposition of Ru and Pd was done through oxidation of copper underpotential deposition (UPD) layer by Ru and Pd ions. This low RuPd-loading electrode (RuPdNPGF) behaved as the nanostructured bimetallic RuPd for the detection of captopril (CAP). Whereas at the surface of the bare electrode an electrochemical activity for CAP cannot be observed, a very sharp anodic peak of the potential of -0.295 V (vs. Ag/AgCl) in pH = 7.0 is obtained using the prepared RuPdNPGF electrode. RuPdNPGF exhibited an excellent performance toward electrochemical oxidation of CAP without any additional mediator showing a significant decrease in the anodic over potential, a high sensitivity and a low detection limit (1.25 × 10-9 M) for CAP. Under the optimized conditions, the amperometric of CAP showed two linear ranges for determination of CAP: 2.50 × 10-9 to 4.75 × 10-7 M CAP and 2.5 × 10-6 to 3.25× 10-5 M CAP. The results show that the RuPdNPGF electrode exhibited a selective, rapid response, good stability with excellent precision (RSD=2.19%).

Keywords: Nanoporous gold film, Bimetallic, RuPd, Underpotential deposition, Captopril.

1. Introduction

Nanostructure electrodes with high surface area have received considerable attention in recent years [1-4]. These materials are conductive and have surface areas that are higher than a planar electrode of similar size. They consist of oriented, well-defined or random pore morphology. A subset of nanoporous material is nanoporous metals, which are of special interest due to their catalytic activity, high electrical conductivity and mechanical properties [5]. Nanoporous gold is a kind of nanoporous metals, which has three-dimensional connective frames and pores in nanometer scale. Nanoporous gold has been widely used in the fields of sensor, catalysis, separation, filter, biomaterial and fuel cell [6-9]. Because of its unique porous structure, nanoporous gold is an ideal substrate material on the area of analytical chemistry [10-12]. Porous structure nanometer scale and metallic conductivity make some substances have high catalytic activity [13, 14]. Bimetallic nanoparticles exhibit interesting electronic, optical, and chemical or biological properties due to new bi-functional synergistic effects [15-18]. From the catalytic point of view, bimetallic nanoparticles are composed of two different metal elements, having drawn a greater interest than monometallic ones. Because bimetallization would make it possible not only to obtain an improved catalytic activity but also to create a new property, which may not be achieved by monometallic catalysts [19]. Bimetallic nanoporous–assembly systems are of recent origin and are of particular importance in catalysis and electrocatalysis. The addition of a second metal contributed to the variations in particle size, shape, composition, surface morphology, physical and chemical properties and such variations included the catalytic activity and chemical selectivity of the material as compared to the single metallic nanocatalyst [20, 21]. There is a strong interest in bimetallic nanostructure system offering highly dispersed and maximized atomic contacts of constituent elements, and consequent catalytically large and electrochemically active surface areas that promote both bifunctional and electronic effect [22].With the advent of new synthetic routes, surface analyzing techniques and surface science modeling facilities, it has become possible to design and prepare tailor-made bimetallic nanoporous with desired properties. Pd nanoparticles are one of the noble metals, which possess their own special properties for the electrode modification process [23, 24]. Previously, the Pd nanoparticles modified electrodes have been used for various types of electrochemical sensors. Ruthenium is a transition metal, and its complex compounds have received attention in organic chemistry as catalysts in synthesis.

Due to its high chemical resistivity, Ru is applied in the chloro-alkali industry as electrode. Rupd, one of the most widely- studied bimetal catalysts, has attracted much attention in the field of heterogeneous catalysis. There is a growing number of publications on studies on RuPd systems used as electrocatalysts for reactions such as ethanol [25] and formic acid oxidation [26,27]. Wang et al. proved that Pd-Ru/C has better activity and stability for hydrogen peroxide electroreduction than those of monometallic Pd/C and Ru/C [28]. This suggests that Ru can improve the electrocatalytic properties of Pd. A series of titanium- supported binary PdRu electrocatalyst for ethanol oxidation in alkaline media reported that adding a small amount of Ru to Pd could improve the electrocatalytic activity of Pd [29]. Awasthi and Singh prepared a highactivity RuPd catalyst supported on graphene for methanol electrooxidation in alkaline media [30]. They attributed the high performance of RuPd to the electronic structure of Pd, modified by the presence of Ru. In this work, the NPGF were used simply as a template, and an ultrathin layer electrocatalytically active bimetals, such as Pd and Ru, were coated on the NPGF. Studies of palladium

(Pd) show that it would be a useful sensor for compounds containing thiol group because it is highly reactive towards sulfur [31]. Captopril (CAP), (S)-1-(3-mercapto-2-methyl-1-oxopropyl)-1-prolin, is an oral medication that inhibits angiotensin-converting enzyme, the conversion of angiotensin I to angiotensin II is prevented. Angiotensin II is a potent mediator, which causes blood vessels to narrow and sodium and water retention of the body. Captopril alone or in combination with other drugs, is prescribed to treat hypertension. Captopril plays an important biological role in the treatment of myocardial infarction and heart failure [32]. Several analytical methods have been proposed for the determination of captopril .These methods include chemiluminescence [33-34], spectrometry [35], fluorimetry [36], and chromatographic techniques [37, 38], however, spectrophotometric method need thiol derivatization. Although the results obtained from chromatographic method are reliable with respect to selectivity, their separations are time consuming and they need expensive instruments. Captopril with its thiol group can undergo electrochemical oxidation at the surface of various electrodes. Electrochemical methods have proved useful for the sensitive and selective determination of various analysts. These methods do not require tedious pretreatment and only use limited pre separation, and consequently, they reduce the cost of measurements [39-40]. Several modified electrodes have been successfully developed for the

detection of captopril using graphite [41, 42], a selective membrane electrode [41] and a boron– doped diamond thin film [43] as the working electrode. To the best of our knowledge, there is no study on using a Ru-Pd bimetallic modified layer on an NPGF electrode. In this work, a novel and highly sensitive electrochemical sensor based on the bimetallic system was designed by nanoporous gold electrode coated with Ru and Pd, and then it was used for determination of captopril. The procedure consisted of the UPD of copper on the gold nanoporous film, with subsequent replacement of the copper by Ru and Pd at open circuit potential in a Ru and Pd containing solution. The electrocatalytic activity of RuPdNPGF electrode for CAP oxidation using voltammetric, amperometry and electrochemical impedance spectroscopic (EIS) methods was investigated. The results showed the RuPdNPGF electrode has excellent electrocatalytic activity in detecting captopril and it acts well in the direct estimation of captopril at a low applied potential with good selectivity, stability, fast response time, high sensitivity and good reproducibility.

2. Experimental All electrochemical experiments were performed using an electrochemical workstation (Autolab, PGSTAT302N). Cell used in this study is made of Teflon, which consists of three electrodes

as follows: an Ag/AgCl electrode with a double junction chamber and a platinum

wire served as the reference and counter electrodes, respectively. Gold recordable compact disk (CD) was used as working electrode [44]. Thus a small piece of gold CD is cut and fixed to the bottom cell with an O-ring. NPGF Electrode can be prepared by a method reported throughout the literature [45]. The fabrication process of NPGF electrode was carried out in two steps. In the first step gold electrode is anodic in a solution of 0.1 M phosphate buffer (pH=7.0) for 3 min by applying a step potential from the open circuit potential (OCP) to 5 V versus Ag/AgCl. Gas bubbles were produced from the gold surface during anodization. The growth of oxide film was accompanied by gas evolution at all times until the anodization was terminated. Because only oxygen evolution can result in gas bubbles during anodic processing under these conditions, the bubbles produced at the anode must have been oxygen even though it was not detected. In the second step to reduce the gold oxide to metallic gold, ascorbic acid was used as a non-toxic and cheap price reducing agent. Thus the anodized gold substrate was in contact with a solution of 1.0 M

ascorbic acid for 3 min. The color of the gold surface became dark due to its high surface area of nonporous structure [46].To prepare the Cu-decorated NPGF (CuNPGF) electrode, a thin layer of copper was deposited onto the nano-electrode, which was used for the deposition of copper UPD method. The UPD of Cu was performed in a solution of 0.10 M H2SO4 containing 1.0 mM CuSO4 by holding the potential from the open circuit potential (OCP) to 0.30 V versus Ag/AgCl at 30 s. At the next step, an aqueous solution containing 21 mM RuCl3 and 7 mM Pd (NO3)2 was used as the Ru and Pd metal ion source. The replacement reactions were carried out by immersing the Cu UPD layer coated nonporous gold film into a solution containing Pd and Ru (schematic 1). Scanning electron microscopy (SEM) (model Vega-Tescan) was used to investigate the change of surface after each step. 3. Result and discussion 3.1 Preparation of the NPGF A nanoporous gold film (NPGF) electrode was prepared by the described method. In order to compare the electrochemical active surface electrode after and before anodization process, cyclic voltammograms (CV) of 0.5 M H2SO4 solution was recorded on smooth gold (SG) and nanoporous gold (NPGF) electrode under the same condition (Fig.1). A voltammogram of smooth gold shows a symmetric cathodic peak at 0.853V, resulting from the electrochemical reduction of gold oxide at positive potential scan. Cyclic voltammogram of NPGF electrode shows a similar electrochemical behavior with the smooth gold electrode except for the fact that both the anodic and cathodic peak currents increase substantially. The increase in peak current is resulted from the increased surface area of NPGF electrodes, and is thought possibly to reflect the nanoporous nature on the electrode surface. The apparent surface area of smooth gold was 0.38 cm2. The actual surface area is estimated 7.05 cm2 for NPGF electrode that exhibits created porous in surface of NPGF electrode. Considering a specific charge of 386 μc/cm2 required for gold oxide reduction [47], the roughness factor (ratio of apparent surface area over actual surface area) was determined 18.5 for NPGF electrode. The evidence shows the effectiveness of the present method for preparation of a highly porous NPGF electrode. For depositing a monolayer of copper on the NPGF electrode surface, UPD was used. The UPD provides a way to obtain uniform coverage of foreign metal on the surface of porous film

[40]. Appropriate potential of UPD was estimated by CV of 1.0 mM CuSO4 in 0.1 M H2SO4 solution on the NPGF electrode. Fig. 2 shows electrochemical behavior smooth gold electrode at 1.0 mM CuSO4 + 0.1 M H2SO4 solution. The presence of two cathodic peaks A1 and B1 expressed the fact that deposition of Cu on smooth gold electrode occurs in two steps. Peak A1 at 0.185 V is related to UPD Cu, where the monolayer of Cu deposit on the surface of smooth gold electrode, while the peak B1 at -0.09 V shows OPD (over potential deposition), where Cu overlayers deposit on Cu metal. The CV of 0.5 M H2SO4 solution was recorded on NPGF electrode after deposition of Cu to investigate surface electrochemistry of CuNPGF electrode. Fig 3 shows an anodic peak at 0.33 V vs. Ag/AgCl in positive scan which is relative to the oxidation of Cu, and corresponds to cathodic peak observed during the negative scan, at 0.262 V vs. Ag/AgCl. This evidence proved the presence of copper on the NPGF electrode surface after underpotential deposition. So the potential of 0.30 V was selected for UPD Cu on the surface of NPGF electrode. To obtain RuPdNPGF electrode, spontaneous replacements of Ru and Pd with Cu monolayer UPD was used. The UPD-replacement method is used instead of the direct electrochemical deposition to design Ru and Pd overlayer on NPGF electrode. The direct electrochemical deposition on the surface of CuNPGF electrode at thermodynamic or more negative potential occurred with various speeds at the different sites, resulting in non uniform coating coverage, while the UPD-replacement method leads to formation of uniform Ru and Pd overlayer on the surface of CuNPGF electrode. In the spontaneous replacement process, the UPD monolayer is oxidized by noble metal (Ru, Pd) cation, which are simultaneously reduced and deposited on the substrate gold. As suggested by Adzic and coworkers [48], the redox process can be described schematically as: MUPD + (m/z) NZ+→Mm++ (m/z) N Where MUPD is a UPD metal adatom on the gold electrode surface and NZ+ is the cation of noble transition metal. In this work: CuUPD + Pd2+→ Cu2++ Pd and 3CuUPD + 2Ru3+→ 3 Cu2++ 2Ru. For the systems employed here, the pertinent standard redox potential (E0) for Pd (NO3) 2/ Pd is 0.915 V [49] and RuCl3/Ru is 0.68V [50]. These values are more positive than the values for Cu2+/CuUPD couple on gold =0.20-0.25V [51], so the above reactions are thermodynamically favorable. Therefore, the CuNPGF electrode was immersed into a solution containing Pd and Ru

(7mM Pd (NO3) in 0.5 M HNO3 + 21 mM RuCl3 in 0.5 M HClO4) for the optimum time 10 min at open circuit [52]. Then RuPdNPGF electrode was formed. The differences in electrode morphology were observed by means of scanning electron microscopy (SEM). Fig. 4 showed different SEM images of NPGF, CuNPGF, RuPdNPGF electrodes. The SEM image of RuPdNPGF showed that there was little or no change in morphology after coating with Cu, Ru and Pd. The observed parallel channels in SEM images are the grooves on recordable CD. The approximate elemental composition of the RuPdNPGF electrode was determined by energy dispersive X-ray spectroscopy (EDX) on a scanning electron microscope with an integrated EDX apparatus. These results confirm not only the presence of the Ru and Pd at the surface of NPGF electrode after underpotential deposition but also Cu replacement with Ru and Pd atoms. EDX spectrum showed no detectable traces of Cu in RuPdNPGF electrode (Fig 5). Some parameters must be optimized to achieve maximum Cu monolayer coverage without

bulk Cu deposition; UPD potential and UPD time. For optimizing the UPD potential, the NPGF electrode was subjected to different potentials (between 0.20 to 0.35 V vs. Ag/AgCl) 1.0 mM CuSO4 + 0.1M H2SO4 solution, for a constant period of 30 s. After each deposition step and replacement Cu UPD with Ru and Pd, anodic peak of oxidation of 10 µM captopril was recorded at the surface of RuPdNPGF electrodes in the 0.1 M phosphate buffer (pH=7.0) by cyclic voltammetry. The results showed that the maximum current was observed at potential 0.30 V. Therefore, 0.30 V was chosen as the UPD potential. Deposition time is the second important parameter that affects the amount of Cu UPD. The effect of this parameter was investigated by carrying out the Cu UPD at an applied potential of 0.30 V for different times (between 20 to 35 s). After each deposition and replacement Cu UPD with Ru and Pd, the corresponding cyclic voltammogram of oxidation of 10 µM captopril was recorded at the surface of RuPdNPGF electrodes. The anodic peak current of captopril oxidation reaches a plateau after 30s. Therefore, an applied potentiial of 0.30 V and deposition time of 30 s were chosen as the optimal values for the Cu UPD process in present work. Also the effect of temperature was investigated in range of 20- 40 °C. According to the results temperature does not affect the current oxidation of captopril. The voltammetric behavior of NPGF, RuNPGF, PdNPGF and RuPdNPGF electrodes in 0.5 M H2SO4 solutions are also investigated in the potential range of 0.5 to 1.5 V (Fig 6). It is known that on the RuNPGF electrode surface, a rapid rise in the oxidation current between 1.0 and 1.2 V is observed that is consistent with electrochemical formation of surface RuO2, considered a

reversible oxide at E < 1.2 V (vs. Ag/AgCl) as noted elsewhere [53]. This observation confirms the presence of ruthenium on the electrode surface. At PdNPGF and RuNPGF electrodes, oxide reduction starts at potential lower than on NPGF electrode. This effect of Ru and Pd is visible well on the cathodic scan, where the cathodic peak of surface oxide reduction of RuNPGF and PdNPGF electrodes appear at ca. 0.83V, while for NPGF electrode, cathodic peak occurs in 0.86V. This observation indicated that Pd and Ru act as catalysts. In the cyclic voltammogram of RuPdNPGF bimetallic electrode the surface oxide reduction peak is placed at potential lower than that for PdNPGF or RuNPGF under identical experimental conditions. Also, both the anodic and cathodic peak currents increase significantly. According to the literature [54], this difference indicated that, on surface, alloy formation happened between Pd and Ru surface atoms.

3.2. Cyclic voltammetry Cyclic voltammetry was used to compare the electroactivity between the electrodes, SG, NPGF and RuPdNPGF, using Fe (CN)6 3-/4- system as a redox marker (Fig. 7). CVs were recorded in the potential range between -0.4 and 0.9 V at the scan rate of 20 mV/ s. A pair of well defined [Fe (CN)6

3-/4-

] was observed at these electrodes, however, with distinct variations in the values of

anodic and cathodic peak positions and the separation in peak potentials (ΔEp) between anodic and cathodic processes. SG, NPGF and RuPdNPGF electrodes showed ΔEp as ~ 490 mV, 312 mV and 140 mV, respectively. A high value for the anodic peak current (Ip) was noticed at RuPdNPGF as compared with that of the NPGF and SG electrodes. Thus, RuPdNPGF electrode provides a good electron conduction pathway as well as electron transfer kinetics that is suited for electrochemical sensor application.

3.3. Electrochemical impedance spectroscopy Electrochemical impedance spectra (EIS) were recorded to investigate the interfacial properties of SG, NPGF, RuPdNPGF electrodes. Fig. 8 displays the typical Nyquist plots (Z" vs. Z' ) of the electrodes recorded in 1 mM K3Fe(CN)6/ K4Fe(CN)6 in 0.1 M phosphate buffer solution at pH 7.0 with frequency ranging from 104 to 10-1 and at open circuit potential. The diameter of semi-circle part infers the electron transfer resistance (Rct). The electron transfer resistance (Rct) of SG (102.9 Ω) was higher than the NPGF (25.1 Ω) electrode (Fig. 8). Thus, the conductivity of NPGF electrode increased after anodizing of the bulk gold CD electrode and

converting that to nanoporous form. This observation also corroborates with the increased peak current Fe (CN)6

3-/4-

at NPGF electrode and decreased ΔEP over the SG electrode (Fig. 7).

Whereas the RuPdNPGF electrode showed almost a straight line, indicating a facile electron transfer from the redox probe towards the electrode surface. This could be definitely attributed to the good electron transfer ability of Ru and Pd monolayer onto the surface of NPGF electrode, which could then facilitate the electrocatalytical oxidation of CAP on the electrode surface. The results from EIS correlate with cyclic voltammetric data (Fig. 7).

3.4. Elecrtroxidation of captopril on the RuPdNPGF To investigate the electrocatalytic activity of the RuPdNPGF electrode for oxidation of captopril, the cyclic voltammogram of RuPdNPGF electrode in 0.1 M phosphate buffer solution with pH 7.0 was recorded in the absence and presence of 10µM captopril. Fig. 9 shows that the presence of captopril, electrochemical behavior of RuPdNPGF electrode change significantly and shows a very sharp anodic peak for oxidation of captopril at range from -0.630 to -0.018 V. This anodic peak start at -0.630V and at potential of -0.28V reaches the maximum height. At the negative scan cannot observe the cathodic peak for reduction of captopril. Such a voltammetric behavior implies an irreversible chemical reaction during the electro-oxidation of captopril. We propose the following mechanism for oxidation of captopril at the surface of RuPdNPGF electrode. According to the literature, this voltammetric behavior implies an EC mechanism with an irreversible following chemical reaction (dimerization of produced thiol radical) during the electro-oxidation of captopril [55]. The overall oxidative reaction process can therefore be attributed to first the initial one electron, one proton oxidation of captopril (RSH) to generate the radical species RS· which can undergo rapid dimerization to form the disulfide (RSSR). To illustrate the special effects of RuPdNPGF catalyst toward the oxidation of captopril, the characteristics of the oxidation of 10µM captopril at the smooth gold film (SG), NPGF, CuNPGF, PdNPGF and RuNPGF electrodes were also studied and compared with that obtained at the RuPdNPGF electrode in PBS (0.1 M, pH 7.0) with 10 µM captopril (Fig. 10).The electrocatalytic features of these electrodes are similar to that of the RuPdNPGF electrode. However, these results demonstrate that the electroacatalytic activity of RuPdNPGF electrode is much higher than activity

of NPGF, CuNPGF, PdNPGF and RuNPGF electrodes. The electrocatalytic activity of RuPdNPGF can be considered to result from the combination of two factors: one is the catalytic activity of the bimetallic nanostructures, and the other one is good electrical communication between the RuPd film and the NPGF and synergistic effect. The anodic peak potentials observed at SG, NPGF, CuNPGF, PdNPGF and RuNPGF electrodes are similar to that obtained the RuPdNPGF electrode. However, by carefully analyzing the curves in Fig. 10, it is obvious that the electrocatalytic current at RuPdNPGF electrode is ca. 163.8, 19.4, 41.4, 2.4, 1.8 times of those obtained at the SG, NPGF, CuNPGF, PdNPGF and RuNPGF electrodes, respectively. Moreover, the oxidation of captopril occurred at a much lower potential (-0.295 V) as compared to PdNPGF (-0.234 V) and RuNPGF (-0.155 V) electrodes. RuPd catalyst in RuPdNPGF diminishes the overpotential for the oxidation captopril. It also enhances the catalytic oxidation current. Thus it can be stated that the more negative potential of this peak than that for pure Ru or Pd suggests mutual interactions between Ru and Pd surface atoms leading to their different electrochemical properties as compared to pure Ru and Pd. The synergistic effect of Ru and Pd on NPGF should play the crucial role in enhancing the overall catalytic activities of this unique bimetallic catalyst. Moreover, in RuPdNPGF, Ru and Pd exist in a nearly two dimensional dispersed state, which means that almost all Ru and Pd atoms are exposed on the NPGF surface which itself is a high-surface-area electrode. In comparison with its monometallic components, a bimetallic system is also expected to display not only a combination of the properties associated with two distinct metals, but also new or unexplored properties due to a possible synergistic effect between the two metals.[56, 57]. The position of the d-band center has been found to depend on both the strain and electronic coupling presented in catalyst [58]. According to Nørskov and co-workers, a compressive strain tends to down-shift the energy of the d-band center, causing adsorbents to bind less strongly to the catalyst, whereas a tensile strain has the opposite effect [59]. The electrochemical properties of Ru-Pd bimetallic towards CAP can be explained taking into account the possible modifications of the electronic structure of Pd when interacting with Ru. Wicke et al. [60, 61] reported that Ru possess virtual bound states around Pd Fermi level and in RuPd alloys Ru atom is embedded in the Pd matrix with part of its virtual bound states above the Fermi level. The electrochemical

properties of Pd–Ru bimetallic towards CAP can be explained taking into account the possible modifications of the electronic structure of Pd when interacting with Ru. Since the smallest changes in the metallic nanoparticle structure and composition might have ’

a decisive effect on the particle s catalytic properties, the electro-oxidation of captopril on a codeposited and a sequential deposited Pd/Ru thin overlayer formed on a NPGF surface was compared. At first, the replacement reaction was carried out in the presence of different molar ratios of Pd and Ru ions in order to control surface Ru/Pd composition. Fig. 11 shows the captopril oxidation CV profiles on the Ru/Pd metal ion mixture solutions with different ion concentration ratios, e.g. 1:1, 1:3 and 3:1. The highest electrocatalytic activity on the electrode surface is relevant when the Ru/Pd codeposited overlayer was formed from a Ru/Pd metal ion ratio of 3:1. Secondly, the similar replacement reactions were carried out sequentially by repeating the replacement, first forming a Pd overlayer coating by immersing CuNPGF into a solution containing Pd metal ions for 10 min. Then PdNPGF was coated by a Ru overlayer by keeping the electrode into a solution containing Ru metal ions for 10 min, to form Ru-PdNPGF. Furthermore, the spontaneous replacement reactions were performed sequentially by repeating the replacements, first Ru overlayer-coating, followed by a Pd overlayer-coating to form PdRuNPGF. The influence of deposition methods on electrochemical oxidation of captopril is demonstrated in Fig.12. The CVs of Pd-RuNPGF, Ru-PdNPGF and RuPdNPGF electrodes were recorded for a solution of captopril (10 µM) in pH 7.0. The peak current values for the oxidation of captopril at Pd-RuNPGF, Ru-PdNPGF and RuPdNPGF were 0.147 mA, 0.138 mA and 0.278 mA, respectively. Compared to other electrodes, RuPdNPGF (Codeposition of Ru and Pd) exhibited a prominent electro-oxidation for captopril at potential of -0.295V. The anodic peak current value at RuPdNPGF is about two times higher than that observed at Pd-RuNPGF and Ru-PdNPGF. This behavior might be due to the fact that Pd electronic structure can be affected by the presence of adjacent Ru layers, when a Ru over layer was coated onto the NPGF. One of the parameters that affect the electrode response in biological sample determination is pH. Fig. 13 shows the electrochemical behavior of CAP at 0.1 M phosphate buffer solution with various pH, in the pH range 4.0-9.0, at the RuPdNPGF electrode using cyclic voltammogram. It can be seen that the anodic peak current of CAP reaches a maximum value at pH = 7.0, and then decreases gradually as pH increases. Therefore, the pH 7.0 was selected as an optimum condition

is the order to obtain maximum sensitivity in quantitative analytical measurements. Moreover, the anodic peak potential of CAP at the surface of RuPdNPGF electrode shifted to negative values with increasing pH of the buffer solution. The calibration curve Ep versus pH, has a slope of -0.0755 mV/pH indicated that electron transfer was accompanied by an equal of proton in electrode reaction of CAP. This result demonstrates that the proposed mechanism for oxidation of captopril. The cyclic voltammogram of RuPdNPGF electrode was recorded in 0.1 M phosphate buffer solution whit pH 7.0 in the presence of 10µM captopril at the scan rate 1-50 mV/s. Fig 14 shows that peak current for anodic oxidation of captopril increased whit the rise of ν. The curve of current versus ν showed a linear correlation between peak current and scan rate in rang 1-50 mV/s. This result, indicated that electrocatalyst oxidation of CAP is adsorption-controlled at the surface of RuPdNPGF electrode.

The amperometric method was used as a very sensitive and fast method for determination of captopril. For practical application, an amperometric sensor is more desirable than a voltammetric one. To determine the optimum potential for amperometric detection of captopril, amprograms of the RuPdNPGF electrode were recorded in phosphate buffer solution (pH=7.0, 0.1 M) whit 10 μM captopril at the different potentials. The best result in terms of sensitivity and S/N was obtained at E = -0.30V. According to this, potential was chosen for evaluation of sensing properties of our electrode. Fig 15(a) displays the amperometric responses of the suggested electrode for the successive additions of certain concentration of captopril at an applied potential of -0.3V. Upon injection of captopril the RuPdNPGF amperometric sensor electrode responded rapidly and achieved 95% of the steady-state current within 6 s, such a fast response implies a fast electron transfer on the surface of RuPdNPGF electrode. The results showed two linear segments with different slopes for captopril concentration: for 2.50 × 10-9 to 4.75 × 10-7 M CAP and 2.5 × 10-6 to 3.25× 10-5 M CAP (Fig15 b). The detection limit (according to the definition of YLOD = YB + 3s, the average blank signal plus three times of its standard deviation (n=10)) was obtained 1.25×10−9 M CAP. The reproducibility of the developed biosensor were further determined. The response reproducibility of six RuPdNPGF electrodes, prepared in the optimum condition was estimated in phosphate buffer solution (pH=7.0, 0.1 M) at -0.30 V by the response to 10μM captopril, and

yielded a relative standard deviation (RSD) of 1.5%. Repeatability of the determination was also investigated by detecting the response of 10 μM captopril for five successive amprometric measurements. Relative standard deviation (RSD) was 2.19% in the phosphate buffer (pH=7.0, 0.1 M) indicating that the electrode had good repeatability. The long term stability of the RuPdNPGF electrode was also examined using cyclic voltammetry. The results showed that the RuPdNPGF electrode response remained stable with no significant change in the voltammetric signal during one month. When cyclic voltammograms were recorded after the electrode was stored in a dry atmosphere at room temperature, the peak potential for the oxidation of captopril was unchanged, and the oxidation current shows a 1.03% decrease relative to the initial response. The presence of active electrochemical species which can be found in biological sample containing captopril may be influence on the oxidation peak of captopril. The interference effect of these foreign compounds were investigated by using solution containing10µM of CAP and adding various concentration of the interfering compound under the optimum conditions. The results are reported in the Table 1, which shows the peak current of CAP is not affected by all studied interfering species. These results prove that the RuPdNPGF electrode can operate as a sensor for determination of captopril in the biological sample. To investigation the applicability of the proposed sensor for electrocatalytic determination of captopril in the pharmaceutical and real sample, we selected tablet and human serum for the analysis of their captopril content. The standard addition method was applied to drawing the current data for captopril concentration added to the prepared sample. The RuPdNPGF electrode was used for obtaining the recovery result of the captopril added to the tablet, human serum and urine samples. The urine sample was centrifuged and diluted five-times with buffer solution without any further pretreatment. These results in the phosphate buffer (pH=7.0, 0.1 M) were shown in Table 2. The excellent recovery results indicate that the complex matrix of the human serum or pharmaceutical sample does not interfere with the detection of the minor amount of captopril. To evaluate the performance of the proposed electrode as a sensor for determination of captopril, the results of this study were compared with results obtained previously from sensors of determination of captopril (Table 3). The RuPdNPGF electrode has obviously lower detection limit and a marked lowering in the anodic over potential than those reported previously.

Conclusion In this work, a RuPdNPGF electrode was fabricated in a very simple, efficient and costeffective method at room temperature in water as a green solvent. Also, it does not involve any toxic reducing reagents, stabilizer and surfactant, which are commonly used during the fabrication of catalysts. The Ru and Pd loading of this electrode are also very low, and its activity is better than a NPGF electrode. The observed results showed that galvanic replacement provides a simple and facile method to form a nanoporous bimetallic Ru-Pd film with good electrical communication between the Ru-Pd and the NPGF and synergistic effect. Thus, the best electroanalytical performance happens as a result of combined advantages like low working potential, simple and rapid approach for preparation and high selectivity with elimination interference from glucose, ascorbic acid and uric acid. Thus the RuPdNPGF electrode can provide an efficient and new sensor with the minimal use of precious metal components. Finally, the mechanical stability of the prepared electrode makes it a potential candidate for electrochemical devices such as fuel cells and sensors.

Acknowledgement The authors gratefully acknowledge financial support of the Research Council of Payame Noor University.

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Figures captions

Scheme 1. Schematic representation of the preparation of RuPdNPGF. Fig 1. Cyclic voltamograms for (a) SG electrode and (b) a NPGF electrode in 0.5 M H2SO4 solution at scan rate 20 mV/s. Fig 2. Cyclic voltamograms for SG electrode in 1.0 mM CuSO4 + 0.1M H2SO4 solution at scan rate 20mV/s. Fig 3. Cyclic voltamograms for CuNPGF electrode in 0.5 M H2SO4 solution at scan rate 20 mV/s.

Fig 4. SEM images for NPGF electrode, CuNPGF electrode and RuPdNPGFeledtrode. Fig 5. EDX pattern of RuPdNPGF electrode; atomic percent: 10.1% Ru, 87.9% Au, 2.0% Pd. Fig 6.Cyclic voltamograms for NPGF, PdNPGF, RuNPGF and RuPdNPGF electrodes in 0.5 M H2SO4 solution at scan rate 20 mV/s. Fig 7. Cyclic voltamograms for SG, NPGF and RuPdNPGF electrodes in 5mM K3[Fe(CN)6] + 5mM K4[Fe(CN) 6] contains 0.1M NaNO3 solution at scan rate 20 mV/s. Fig 8. Impedance spectroscopy for smooth gold electrode, NPGF and RuPdNPGF electrodes in 5mM K3[Fe(CN) 6] + 5mM K4[Fe(CN)6] contains 0.1M NaNO3 solution at frequency ranging from 104 to 10-1. Fig 9. Cyclic voltamograms of RuPdNPGF electrode (a) without and (b) with 10 µM captopril solution in 0.1 M phosphate buffer (pH = 7.0) solution at scan rate 20 mV/s. Fig 10. Cyclic voltammograms for SG , NPGF, CuNPGF, PdNPGF, RuNPGF and RuPdNPGF electrodes in 0.1M phosphate buffer (pH = 7.0) solution containing 10 µM captopril at scan rate 20 mV/s . Fig 11. Cyclic voltammograms for electro-oxidation of 10µM captopril at the surface of RuPdNPGF electrodes with different ratio of Ru:Pd at scan rate 20 mV/s . Fig 12. Cyclic voltammograms of Ru-PdNPGF electrode, Pd-RuNPGF electrode and RuPdNPGF electrode in 0.1 M phosphate buffer (pH =7.0) solution with 10µM captopril at scan rate 20 mV/s. Fig 13. Current-potential curve for electro-oxidation of 10µM captopril on the surface of RuPdNPGF electrode at various pH (4.0-9.0) in 0.1M phosphate buffer. Scan rate 20 mV/s. Fig 14. Cyclic voltammograms of RuPdNPGF electrode in the presence of 10µM captopril in 0.1M phosphate buffer (pH = 7.0) solution at various scan rate 1-50 mV/s. Fig 15. (a) Amperogram of different concentrations of captopril on RuPdNPGF electrode in 0.1M phosphate buffer (pH = 7.0) solution, at potential of -0.3V, (b) Calibration curves with different concentrations of captopril in 0.1M phosphate buffer (pH = 7.0) solution on the RuPdNPGF electrode.

Table 1 The influence of foreign compound in the determination of captopril

Species

Tolerant limits (w substance / w captopril )

Glucose, Fructose, Sucrose

1000.0

Ca2+, Mg2+

950.0

k+,Na+

1000.0

Uric acid, folic acid, ascorbic acid

750.0

Acetaminophen, dopamine

900.0

Atenolol, Prazosin

400

Hydrochlorothiazid

200

Cysteine, hemocysteine

150.0

Table 2 Determination of captopril in biological and pharmaceutical sample (n=3)

Sample

Captopril added

Captopril founded (µM/L)

Recovery (%)

(µM/L) Tablet

0.0

5.0

(±0.01)

2.0

7.13 (±0.02)

̲

101.85 Tablet

0.0

10.0

(±0.01)

4.0

14.32 (±0.03)

̲

102.28

Tablet

0.0

15.0

(±0.02)

6.0

21.05 (±0.01)

̲

100.23 Serum

0.0 2.0

̲

1.98

(±0.01) 98.5

Serum

0.0 4.0

-

̲

3.93 (±0.03) 98.25

Serum

0.0

-

6.0

6.07 (±0.02)

̲

101.16 urinea

urineb

urinec

0.0

5.62 (±0.39)

2.0

7.59 (±0.44)

0.0

10.71 (±0.60)

2.0

12.93 (±0.73)

0.0

3.82 (±0.25)

2.0

5.69 (±0.33)

a

Sampling was made after 1.0 h from a man who had heart problem and used captopril. Sampling was made after 2.5 h from a man who had heart problem and used captopril. c Sampling was made after 3.0 h from a man who had heart problem and used captopril. b

Table3 Linear range and detection limit for the determination of captopril at the modified electrodes

Linear range Electrode

Limit of detection (μmol/L)

Potential of anodic peak (V) vs. Ag/AgCl

Reference

(μmol/L)

GC

50-3000

25

+0.900

[43]

CPE

0.8- 65

0.3

+ 0.470

[62]

CPE

0.03- 2400

0.0096

+ 0.370

[63]

CPE

8.0- 1000

4.8

+ 0.625

[64]

0.15

+ 0.280

[65]

0.02

+ 0.200

[66]

0.00125

- 0.289

This work

0.5-12 CPE 12- 300

0.05- 0.5 CPE 0.5- 50

0.0025- 0.475 RuPdNPGF 2.5- 32.5

gr1 .

gr10 .

gr11 .

gr12 .

gr13 .

gr14 .

gr15 .

gr2 .

gr3 .

gr4 .

gr5 .

gr6 .

gr7 .

gr8 .

gr9 .