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Electrochimica Acta 170 (2015) 337–342
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three-dimensional nanoporous Au films as high-efficiency enzyme-free electrochemical sensors Xi Ke a , Zhihao Li a , Lin Gan c , Jie Zhao d , Guofeng Cui a,b, * , William Kellogg e, Daniel Matera e, Drew Higgins f , Gang Wu e, * a
Electronic Packaging Electrochemistry Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China SYSU-CMU Shunde International Joint Research Institute, Shunde, 528300, China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education of China, Sun Yat-sen University, Guangzhou, 510275, China d School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China e Department of Chemical and Biological Engineering, University at Buffalo, the State University of New York, Buffalo, New York 14260, United States f Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 February 2015 Received in revised form 25 April 2015 Accepted 25 April 2015 Available online 28 April 2015
Herein we develop an enzyme-free electrochemical sensor for hydrogen peroxide (H2O2) and hydrazine (N2H4) by preparing three-dimensional nanoporous gold (NPG) supported on three dimensional Ni foam. The NPG@Ni foam hybrid electrode was prepared via electrodeposition of an AuSn alloy film on a Ni foam substrate, followed by a chemical dealloying process to leach the Sn component. The morphology and structure of the as-prepared NPG@Ni foam hybrid were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrum (EDS). The as-prepared hybrid electrodes showed high sensitivity and selectivity for the detection of both H2O2 and N2H4. The non-enzymatic sensor exhibits a linear response for H2O2 and N2H4 in the range of 20–9740 and 0.2–110.1 mM, with a sensitivity of 2.88 and 10.687 mA mM 1 cm 2, and a detection limit (S/N = 3) of 10 mM and 33 nM, respectively. The excellent sensitivity and selectivity make the NPG@Ni foam electrodes very attractive for electrochemical sensing applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: nanoporous gold film Ni foam electrochemical sensors hydrogen peroxide hydrazine
1. Introduction Hydrogen peroxide (H2O2) is not only a common analyte in several important sectors including pharmaceutical research, food production and environmental analysis, but is also an important species that participates in many biological processes and is active in cell signaling and growth. However, the presence of excess H2O2 in cells can induce biological damage, leading to the development of several human diseases such as cancer [1], neurodegenerative diseases [2] and cardiovascular disorders [3]. Hydrazine (N2H4) has been widely used in various application areas such as agriculture, industry, medicine and the military. It is also extensively employed as an important chemical intermediate in the production of pesticides, antioxidants and polymers [4]. However, hydrazine is also extremely toxic and can be readily absorbed orally, dermally or by inhalation. This in turn leads to detrimental effects in humans, including severe liver damage and disturbances in the central
* Corresponding authors. E-mail addresses: [email protected] (G. Cui), [email protected] (G. Wu). http://dx.doi.org/10.1016/j.electacta.2015.04.144 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
nervous system [5,6]. Therefore, the rapid determination of H2O2 and N2H4 in biological environments is regarded as highly significant and has attracted considerable attention. To date, a number of available techniques such as titrimetry, fluorescence, chemiluminescence and electrochemical methods have been employed for the detection of H2O2 [7–10] and N2H4 [11–14]. Among these analytical techniques, electrochemical approaches are considered favourable owing to several advantages that include high sensitivity, fast response and low operating cost. Electrodes modified with nanoparticles of metals and oxides, conducting polymer and metal hexacyanoferrates have been explored as electrochemical catalysts for H2O2 [15–18] or N2H4 detection [19–22]. Among them, the metal hexacyanoferrates are commonly used as electrocatalysts for such analysis owing to their excellent electrochemical activity. For example, Guadagnini et al. fabricated a sensor based on the intercalation of Cu2+ into copper hexacyanoferrate for H2O2 detection with a high sensitivity as large as 31.7 mA M 1 cm 2. Nanoporous metallic structures have recently received increasing attention owing to their monolithic bicontinuous porous structures and intriguing catalytic and optical properties [23–29].
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Among them, nanoporous gold (NPG) is of particular interest because of its excellent chemical stability, high electrical conductivity and fascinating catalytic activity [30–33]. NPG structures are typically fabricated in free-standing and supported thin film formats by chemical dealloying of Au-based binary alloys such as Au-Ag [30,34,35] and Au-Sn alloys [36–38]. Recently, NPG electrodes have also been studied as a sensor to detect H2O2 [39] and N2H4 [40,41]. The bicontinuous open porosity of NPG electrodes is beneficial for fast mass transport of reactants at the solid-liquid interface, and the large effective surface area can promote the electrochemical reactions. However, the ultrathin free-standing NPG films commonly used in these studies are very difficult to operate and require complicated transfer methods during the electrode fabrication process. Additionally, it is also necessary to increase the surface area of the catalysts in order to enhance the detection sensitivity. Therefore, the development of robust and highly efficient NPG sensors with three-dimensional (3D) interconnected structures is highly desirable for overcoming these issues. In our previous study [36] we developed an approach to fabricating NPG films through chemically dealloying electrodeposited Au-Sn alloy films. This method is simple and easy to be reproduced. Importantly, the NPG films can be prepared on various conductive substrates including copper, stainless steel and Ni foam. Among these substrates, 3D Ni foam is a low cost commercial material with high surface area and excellent electrical conductivity. Its application in electrochemical sensors is however rarely reported in the literature. Herein we demonstrate the first use of NPG films grown on Ni foams as a novel enzyme-free sensor
for electrochemical detection of H2O2 and N2H4. This new electrode configuration provides a fast response time, high sensitivity and excellent selectivity. The newly developed 3D NPG@Ni foam sensor also allows a fast electron transfer rate, with significantly reduced interfacial resistance between the catalysts and the collectors. Moreover, the interconnected 3D hierarchical micro/nanoporosity of the hybrid sensor provides substantially increased effective surface area that is capable of facilitating mass transport. Thus, the NPG@Ni foam holds great promise to be an efficient electrochemical sensor for the detection of H2O2 and N2H4.
2. Experimental section 2.1. Preparation of NPG@Ni foam hybrids The NPG@Ni foam hybrids were prepared by a two-step route developed in our initial work [36]. In brief, an AuSn alloy film was electrodeposited onto a clean Ni foam (approximately 1 cm 7 cm, purchased from MTI Corp. in the United States (Richmond, CA)) by cathodic electrodeposition, applying a current density of 5 A dm 2 for 10 min at 45 C. This was facilitated using a DC sourcemeter and an Au-Sn alloy plating solution (Huizhou Leadao Electronic Material Co., Ltd., Huizhou, China). Then, the Ni foam modified with the Au-Sn alloy film was immersed into an etching solution (5 M NaOH + 1 M H2O2) under ambient conditions and left for 3 days. In doing so, Sn was dissolved from AuSn coatings and the NPG films remained.
Fig. 1. SEM images with different magnifications (a-d), EDX spectrum (e) and XRD pattern of the NPG@Ni foam hybrid (f).
X. Ke et al. / Electrochimica Acta 170 (2015) 337–342
Fig. 2. CV curves of the NPG@Ni foam hybrid electrode in 0.1 M PBS without and with 2 mM H2O2 at a scan rate of 50 mV s 1.
2.2. Characterization and electrochemical measurements The surface morphology was examined by scanning electron microscopy (SEM, JEOL, JSM-6700F), and elemental analysis was carried out using an energy-dispersive X-ray spectrometer (EDX). The crystal structure was analyzed using the Rigaku D/max-2200/ PC X-ray diffractometer with Cu Ka radiation. Cyclic voltammetry (CV) and amperometric measurements were conducted using the Gamry Reference 600 potentiostat with a conventional threeelectrode cell. A prepared NPG@Ni foam electrode, Pt mesh electrode and an Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. Continuous stirring was maintained for all amperometric measurements. 3. Results and discussion As described in our previous reports [36–38], we synthesized NPG on Ni foams by dealloying the AuSn alloy film electrodeposited on Ni foam substrates. Fig. 1a to 1d shows SEM images of the NPG@Ni foam hybrid at various magnification, whereby an interconnected 3D porous network is clearly visible. The average pore size of the Ni foam is about 100–200 mm, and the width of the Ni skeleton is about 50 mm. A higher-resolution image of NPG (Fig. 1d) shows the presence of bicontinuous nanoporous structure composed of nanoporous channels and gold ligaments. The ligament size ranges from ca. 40-150 nm, and the nanopore size ranges from 20-100 nm. The presence of 3D micro/nanopores can not only provide a sufficiently large surface area for rapid electrochemical reactions, but can also promote mass transfer at the electrode/electrolyte interface efficiently. The elemental analysis of the as-prepared samples was further performed using
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EDX. A typical EDX spectrum of the NPG@Ni foam (Fig. 1e) clearly shows that the hybrid is composed of Au and Ni, indicating that the less-noble Sn component has been nearly completely depleted from the AuSn alloy film during the chemical dealloying process. XRD analysis was done to further examine the phase structures of the as-prepared hybrids. From the XRD pattern (Fig. 1f), the sharp diffraction peaks located at 2u = 38.3 , 64.7, 77.6 and 81.8 were assigned to diffraction from the (111), (2 2 0), (3 11) and (2 2 2) planes of face-centered cubic (fcc) gold (PDF no. 04-0784), respectively. The other three peaks at 44.7, 52.0 and 76.6 can be identified to the (111), (2 0 0) and (2 2 0) planes of the Ni foam substrate. These results further demonstrated the successful construction of NPG structures on the Ni foam substrate. The catalytic activity of the electrode materials were investigated towards the electroreduction of H2O2. CV polarization plots obtained in 0.1 M phosphate buffer saline (PBS, pH = 7.4), both with and without 2 mM H2O2, are shown in Fig. 2. Compared to the pure electrolyte, an obvious current response occurs starting from 0 V vs. Ag/AgCl and showing a peak at -0.42 V vs. Ag/AgCl. This behavior reveals that the NPG@Ni foam hybrid electrode is electrocatalytically active towards H2O2 reduction. The electrochemical responses of the NPG@Ni foam electrode to different concentrations of H2O2 (0.5–3.5 mM) are compared in Fig. 3a. Upon increasing the concentration of H2O2, an associated increase in current density is observed. The corresponding plot of the peak current density versus H2O2 concentration is shown in Fig. 3b, indicating a linear relationship up to 3.5 mM of H2O2 (R2 = 0.995). Thus, the NPG@Ni foam hybrid electrode exhibits excellent catalytic activity for the electrocatalytic reduction of H2O2. Chronoamperometry was further used to study the H2O2 detection sensitivity of the NPG@Ni foam hybrid electrode. Fig. 4a presents a typical current-time response for the NPG@Ni foam hybrid electrode held at -0.4 V vs. Ag/AgCl, obtained after stepwise additions of H2O2. By steadily increasing the H2O2 concentration from 20 mM to 2 mM, the reduction current increased stepwise, reaching the 95% steady state currents in 3 seconds. This highlights a rapid amperometric response to H2O2 concentration. By using a signal-to-noise ratio of 3, a detection limit of 10 mM can be achieved. A current versus H2O2 concentration plot is also shown in Fig. 4b. The NPG@Ni foam hybrid sensor exhibits an excellent linear relationship in the range of 20 to 9740 mM (R2 = 0.998) with a sensitivity of 2.88 mA mM 1 cm 2, demonstrating the capability of the NPG@Ni foam hybrid electrode to be a new type of electrochemical sensor for H2O2 detection. To evaluate the selectivity of the NPG@Ni foam hybrid sensor for H2O2 detection, the amperometric response of H2O2 was investigated in the presence of 1 mM methanol, 1 mM ethanol, 0.5 mM glucose and 0.2 mM ascorbic acid (AA). Fig. 5 exhibits the result which shows that these interfering species result in negligible current changes,
Fig. 3. CV curves of the NPG@Ni foam hybrid electrode in 0.1 M PBS in the presence of H2O2 over a concentration range from 0.5 to 3.5 mM at a scan rate of 50 mV s the corresponding calibration curve of the current responses vs. H2O2 concentration (b).
1
(a), and
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Fig. 4. The amperometric response of the NPG@Ni foam hybrid electrode at -0.4 V with stepwise addition of H2O2 (a), and the corresponding calibration curve of the current responses vs. H2O2 concentration (b).
compared to that of just H2O2. This further attests to the excellent selectivity of the developed electrode towards H2O2. In addition, we investigated the stability and reproducibility of the as-prepared sensors. The current response of the sensor to 0.1 mM H2O2 had a decrease of about 3.5% after 40 days of storage at room temperature in an inverted beaker to prevent contamination with airborne particulates. The electrode-to-electrode reproducibility was examined between five different sensors, and the responses of the five electrodes to 0.1 mM H2O2 were measured with a relative standard deviation (RSD) of 5.4%. These results indicate the excellent stability and reproducibility of the NPG@Ni foam sensors for H2O2 detection. The NPG@Ni foam hybrids were also employed as working electrodes to evaluate the electrochemical performance for the electrooxidation of hydrazine. Fig. 6 presents the CVs for the NPG@Ni foam hybrid electrode in 0.1 M PBS, with and without 5 mM hydrazine. It is shown that the CV curve with the presence of hydrazine exhibits an apparent current response for anodic oxidation of hydrazine starting from -0.2 V vs. Ag/AgCl and forming a peak at 0.2 V vs. Ag/AgCl, indicating the electrocatalytic activity of the NPG@Ni foam hybrid electrode toward hydrazine oxidation. The electrochemical responses of the NPG@Ni foam hybrid electrode to various concentrations of hydrazine (0.2–15 mM) are shown in Fig. 7a. The oxidation current densities increase with the increased hydrazine concentrations. The corresponding calibration curve of the peak current density on the hydrazine concentrations is depicted in Fig. 7b, displaying a good linear relationship up to 15 mM of hydrazine (R2 = 0.991).
Such results indicate that the electrocatalytic oxidation of hydrazine on the NPG@Ni foam hybrid electrode exhibits an outstanding sensitivity to the changes of hydrazine concentrations. Chronoamperometry was used to assess the sensitivity of the NPG@Ni foam hybrid electrode for hydrazine detection. Fig. 8a shows the current-time response for the NPG@Ni foam hybrid electrode obtained after successive additions of hydrazine into 0.1 M PBS at 0 V vs. Ag/AgCl. With the additions of 0.2 mM to up to 20 mM hydrazine, the oxidation current increases rapidly and reaches the 95% steady state currents within 3 seconds, revealing fast amperometric response of the NPG@Ni foam hybrid electrode. With a signal-to-noise ratio of 3, the detection limit is determined to be 33 nM. The current vs. hydrazine concentration calibration plot is displayed in Fig. 8b. The NPG@Ni foam hybrid electrode exhibits a good linear relationship in the range of 0.2 to 110.1 mM (R2 = 0.9995) with a sensitivity of 10.687 mA mM 1 cm 2, indicating the potential of the NPG@Ni foam hybrid electrode to serve as a highly efficient electrode material for electrochemical detection of hydrazine. The interferences for the determination of hydrazine at the NPG@Ni foam hybrid electrode were studied by adding various common species into the solution. Fig. 9 displays the amperometric response of the NPG@Ni foam sensor for 5 mM hydrazine (a), 200-fold concentration of Cl (b), NO3 (c), SO42 (d), NH4+ (e), Na+ (f), K+ (g), Mg2+ (h) and Ca2+ (i). A rapid amperometric response is observed for 5 mM hydrazine at the NPG@Ni foam electrode, and the common ions had no interference on hydrazine detection, suggesting an excellent selectivity of the sensor. The amperometric response of the NPG@Ni foam electrode toward 20 mM hydrazine
Fig. 5. The amperometric response of the NPG@Ni foam hybrid electrode in 0.1 M PBS with stepwise additions of H2O2, methanol, ethanol, glucose and AA.
Fig. 6. CVs of the NPG@Ni foam hybrid electrode in 0.1 M PBS without and with 5 mM hydrazine at a scan rate of 50 mV s 1.
X. Ke et al. / Electrochimica Acta 170 (2015) 337–342
Fig. 7. CV results of NPG@Ni foam hybrid electrode in the presence of hydrazine over a concentration range from 0.2 to 15 mM at a scan rate of 50 mV s corresponding calibration curve of the current response with changes of hydrazine concentration (b).
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1
(a), and the
Fig. 8. The amperometric response of NPG@Ni foam hybrid electrode at 0 V with stepwise additions of hydrazine (a), and the corresponding calibration curve of the current response vs. hydrazine concentration (b).
Fig. 9. Amperometric i-t response at the NPG@Ni foam hybrid electrode for 5 mM hydrazine (a), 200 fold concentration of Cl (b), NO3 (c), SO42 (d), NH4+ (e), Na+ (f), K+ (g), Mg2+ (h) and Ca2+ (i).
The high sensitivity of the NPG@Ni foam hybrid electrode for H2O2 and hydrazine detection can be attributed to the following advantages of the 3D hierarchical porous architecture. First, the microporous Ni foam substrate possesses high mechanical stability and allows rapid diffusion of reacting species onto the surface of the electrocatalysts. Second, the NPG structures can be fabricated directly on the surface of Ni foams, eliminating the tedious process of mixing active materials with binders, along with the reduced internal resistance. Finally, the bicontinuous open porosity of NPG structures can efficiently increase the surface area of the electrocatalysts, which enhances rapid electrochemical reactions. This low-cost and robust 3D electrode holds great potential as a new type of enzyme-free electrochemical sensors for the detection of H2O2 and N2H4. Acknowledgements
remained 93.7% of its initial value after storage at 4 C for 4 weeks, indicating the long-term stability of the sensor. The relative standard deviation (RSD) for 20 mM hydrazine detection at three independently prepared electrodes was also less than 4.2%, revealing a relatively good reproducibility of the sensor.
4. Conclusions A novel hybrid electrode structure based on NPG supported on Ni foam was developed and studied as an enzyme-free electrochemical sensor for the detection of H2O2 and N2H4. The NPG@Ni foam hybrid electrode possesses a 3D hierarchical porosity, and exhibits excellent sensing capability to detect H2O2 and N2H4 with a fast response time, high sensitivity and excellent selectivity.
G. F. C. gratefully acknowledges the financial support by National Natural Science Foundation of China (51271205, 50801070), Guangzhou Pearl Technology the Nova Special Project (2012J2200058), Excellent Young College Teachers Development Program in Guangdong Province (Yq2013006), Research and Application of Key Technologies Oriented the Industrial Development (90035-3283309, 90035-3283321), Science and Technology Plan Projects of Guangzhou city (2013Y2-00102), Huizhou city (2012B050013012), DaYa Gulf district of Huizhou city (20110108, 20120212) and SYSU-CMU Shunde International Joint Research Institute (20150209). X. K. acknowledges the support from China Postdoctoral Science Foundation (2014M562234). J. Z. acknowledges the financial support by National Natural Science Foundation of China (51101061). G. W. acknowledges the financial support from the start-up funding of University at Buffalo, SUNY.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three-dimensional nanoporous Au films as high-efficiency enzyme-free electrochemical sensors Xi Ke a , Zhihao Li a , Lin Gan c , Jie Zhao d , Guofeng Cui a,b, * , William Kellogg e, Daniel Matera e, Drew Higgins f , Gang Wu e, * a
Electronic Packaging Electrochemistry Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China SYSU-CMU Shunde International Joint Research Institute, Shunde, 528300, China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education of China, Sun Yat-sen University, Guangzhou, 510275, China d School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China e Department of Chemical and Biological Engineering, University at Buffalo, the State University of New York, Buffalo, New York 14260, United States f Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 February 2015 Received in revised form 25 April 2015 Accepted 25 April 2015 Available online 28 April 2015
Herein we develop an enzyme-free electrochemical sensor for hydrogen peroxide (H2O2) and hydrazine (N2H4) by preparing three-dimensional nanoporous gold (NPG) supported on three dimensional Ni foam. The NPG@Ni foam hybrid electrode was prepared via electrodeposition of an AuSn alloy film on a Ni foam substrate, followed by a chemical dealloying process to leach the Sn component. The morphology and structure of the as-prepared NPG@Ni foam hybrid were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrum (EDS). The as-prepared hybrid electrodes showed high sensitivity and selectivity for the detection of both H2O2 and N2H4. The non-enzymatic sensor exhibits a linear response for H2O2 and N2H4 in the range of 20–9740 and 0.2–110.1 mM, with a sensitivity of 2.88 and 10.687 mA mM 1 cm 2, and a detection limit (S/N = 3) of 10 mM and 33 nM, respectively. The excellent sensitivity and selectivity make the NPG@Ni foam electrodes very attractive for electrochemical sensing applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: nanoporous gold film Ni foam electrochemical sensors hydrogen peroxide hydrazine
1. Introduction Hydrogen peroxide (H2O2) is not only a common analyte in several important sectors including pharmaceutical research, food production and environmental analysis, but is also an important species that participates in many biological processes and is active in cell signaling and growth. However, the presence of excess H2O2 in cells can induce biological damage, leading to the development of several human diseases such as cancer [1], neurodegenerative diseases [2] and cardiovascular disorders [3]. Hydrazine (N2H4) has been widely used in various application areas such as agriculture, industry, medicine and the military. It is also extensively employed as an important chemical intermediate in the production of pesticides, antioxidants and polymers [4]. However, hydrazine is also extremely toxic and can be readily absorbed orally, dermally or by inhalation. This in turn leads to detrimental effects in humans, including severe liver damage and disturbances in the central
* Corresponding authors. E-mail addresses: [email protected] (G. Cui), [email protected] (G. Wu). http://dx.doi.org/10.1016/j.electacta.2015.04.144 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
nervous system [5,6]. Therefore, the rapid determination of H2O2 and N2H4 in biological environments is regarded as highly significant and has attracted considerable attention. To date, a number of available techniques such as titrimetry, fluorescence, chemiluminescence and electrochemical methods have been employed for the detection of H2O2 [7–10] and N2H4 [11–14]. Among these analytical techniques, electrochemical approaches are considered favourable owing to several advantages that include high sensitivity, fast response and low operating cost. Electrodes modified with nanoparticles of metals and oxides, conducting polymer and metal hexacyanoferrates have been explored as electrochemical catalysts for H2O2 [15–18] or N2H4 detection [19–22]. Among them, the metal hexacyanoferrates are commonly used as electrocatalysts for such analysis owing to their excellent electrochemical activity. For example, Guadagnini et al. fabricated a sensor based on the intercalation of Cu2+ into copper hexacyanoferrate for H2O2 detection with a high sensitivity as large as 31.7 mA M 1 cm 2. Nanoporous metallic structures have recently received increasing attention owing to their monolithic bicontinuous porous structures and intriguing catalytic and optical properties [23–29].
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Among them, nanoporous gold (NPG) is of particular interest because of its excellent chemical stability, high electrical conductivity and fascinating catalytic activity [30–33]. NPG structures are typically fabricated in free-standing and supported thin film formats by chemical dealloying of Au-based binary alloys such as Au-Ag [30,34,35] and Au-Sn alloys [36–38]. Recently, NPG electrodes have also been studied as a sensor to detect H2O2 [39] and N2H4 [40,41]. The bicontinuous open porosity of NPG electrodes is beneficial for fast mass transport of reactants at the solid-liquid interface, and the large effective surface area can promote the electrochemical reactions. However, the ultrathin free-standing NPG films commonly used in these studies are very difficult to operate and require complicated transfer methods during the electrode fabrication process. Additionally, it is also necessary to increase the surface area of the catalysts in order to enhance the detection sensitivity. Therefore, the development of robust and highly efficient NPG sensors with three-dimensional (3D) interconnected structures is highly desirable for overcoming these issues. In our previous study [36] we developed an approach to fabricating NPG films through chemically dealloying electrodeposited Au-Sn alloy films. This method is simple and easy to be reproduced. Importantly, the NPG films can be prepared on various conductive substrates including copper, stainless steel and Ni foam. Among these substrates, 3D Ni foam is a low cost commercial material with high surface area and excellent electrical conductivity. Its application in electrochemical sensors is however rarely reported in the literature. Herein we demonstrate the first use of NPG films grown on Ni foams as a novel enzyme-free sensor
for electrochemical detection of H2O2 and N2H4. This new electrode configuration provides a fast response time, high sensitivity and excellent selectivity. The newly developed 3D NPG@Ni foam sensor also allows a fast electron transfer rate, with significantly reduced interfacial resistance between the catalysts and the collectors. Moreover, the interconnected 3D hierarchical micro/nanoporosity of the hybrid sensor provides substantially increased effective surface area that is capable of facilitating mass transport. Thus, the NPG@Ni foam holds great promise to be an efficient electrochemical sensor for the detection of H2O2 and N2H4.
2. Experimental section 2.1. Preparation of NPG@Ni foam hybrids The NPG@Ni foam hybrids were prepared by a two-step route developed in our initial work [36]. In brief, an AuSn alloy film was electrodeposited onto a clean Ni foam (approximately 1 cm 7 cm, purchased from MTI Corp. in the United States (Richmond, CA)) by cathodic electrodeposition, applying a current density of 5 A dm 2 for 10 min at 45 C. This was facilitated using a DC sourcemeter and an Au-Sn alloy plating solution (Huizhou Leadao Electronic Material Co., Ltd., Huizhou, China). Then, the Ni foam modified with the Au-Sn alloy film was immersed into an etching solution (5 M NaOH + 1 M H2O2) under ambient conditions and left for 3 days. In doing so, Sn was dissolved from AuSn coatings and the NPG films remained.
Fig. 1. SEM images with different magnifications (a-d), EDX spectrum (e) and XRD pattern of the NPG@Ni foam hybrid (f).
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Fig. 2. CV curves of the NPG@Ni foam hybrid electrode in 0.1 M PBS without and with 2 mM H2O2 at a scan rate of 50 mV s 1.
2.2. Characterization and electrochemical measurements The surface morphology was examined by scanning electron microscopy (SEM, JEOL, JSM-6700F), and elemental analysis was carried out using an energy-dispersive X-ray spectrometer (EDX). The crystal structure was analyzed using the Rigaku D/max-2200/ PC X-ray diffractometer with Cu Ka radiation. Cyclic voltammetry (CV) and amperometric measurements were conducted using the Gamry Reference 600 potentiostat with a conventional threeelectrode cell. A prepared NPG@Ni foam electrode, Pt mesh electrode and an Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. Continuous stirring was maintained for all amperometric measurements. 3. Results and discussion As described in our previous reports [36–38], we synthesized NPG on Ni foams by dealloying the AuSn alloy film electrodeposited on Ni foam substrates. Fig. 1a to 1d shows SEM images of the NPG@Ni foam hybrid at various magnification, whereby an interconnected 3D porous network is clearly visible. The average pore size of the Ni foam is about 100–200 mm, and the width of the Ni skeleton is about 50 mm. A higher-resolution image of NPG (Fig. 1d) shows the presence of bicontinuous nanoporous structure composed of nanoporous channels and gold ligaments. The ligament size ranges from ca. 40-150 nm, and the nanopore size ranges from 20-100 nm. The presence of 3D micro/nanopores can not only provide a sufficiently large surface area for rapid electrochemical reactions, but can also promote mass transfer at the electrode/electrolyte interface efficiently. The elemental analysis of the as-prepared samples was further performed using
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EDX. A typical EDX spectrum of the NPG@Ni foam (Fig. 1e) clearly shows that the hybrid is composed of Au and Ni, indicating that the less-noble Sn component has been nearly completely depleted from the AuSn alloy film during the chemical dealloying process. XRD analysis was done to further examine the phase structures of the as-prepared hybrids. From the XRD pattern (Fig. 1f), the sharp diffraction peaks located at 2u = 38.3 , 64.7, 77.6 and 81.8 were assigned to diffraction from the (111), (2 2 0), (3 11) and (2 2 2) planes of face-centered cubic (fcc) gold (PDF no. 04-0784), respectively. The other three peaks at 44.7, 52.0 and 76.6 can be identified to the (111), (2 0 0) and (2 2 0) planes of the Ni foam substrate. These results further demonstrated the successful construction of NPG structures on the Ni foam substrate. The catalytic activity of the electrode materials were investigated towards the electroreduction of H2O2. CV polarization plots obtained in 0.1 M phosphate buffer saline (PBS, pH = 7.4), both with and without 2 mM H2O2, are shown in Fig. 2. Compared to the pure electrolyte, an obvious current response occurs starting from 0 V vs. Ag/AgCl and showing a peak at -0.42 V vs. Ag/AgCl. This behavior reveals that the NPG@Ni foam hybrid electrode is electrocatalytically active towards H2O2 reduction. The electrochemical responses of the NPG@Ni foam electrode to different concentrations of H2O2 (0.5–3.5 mM) are compared in Fig. 3a. Upon increasing the concentration of H2O2, an associated increase in current density is observed. The corresponding plot of the peak current density versus H2O2 concentration is shown in Fig. 3b, indicating a linear relationship up to 3.5 mM of H2O2 (R2 = 0.995). Thus, the NPG@Ni foam hybrid electrode exhibits excellent catalytic activity for the electrocatalytic reduction of H2O2. Chronoamperometry was further used to study the H2O2 detection sensitivity of the NPG@Ni foam hybrid electrode. Fig. 4a presents a typical current-time response for the NPG@Ni foam hybrid electrode held at -0.4 V vs. Ag/AgCl, obtained after stepwise additions of H2O2. By steadily increasing the H2O2 concentration from 20 mM to 2 mM, the reduction current increased stepwise, reaching the 95% steady state currents in 3 seconds. This highlights a rapid amperometric response to H2O2 concentration. By using a signal-to-noise ratio of 3, a detection limit of 10 mM can be achieved. A current versus H2O2 concentration plot is also shown in Fig. 4b. The NPG@Ni foam hybrid sensor exhibits an excellent linear relationship in the range of 20 to 9740 mM (R2 = 0.998) with a sensitivity of 2.88 mA mM 1 cm 2, demonstrating the capability of the NPG@Ni foam hybrid electrode to be a new type of electrochemical sensor for H2O2 detection. To evaluate the selectivity of the NPG@Ni foam hybrid sensor for H2O2 detection, the amperometric response of H2O2 was investigated in the presence of 1 mM methanol, 1 mM ethanol, 0.5 mM glucose and 0.2 mM ascorbic acid (AA). Fig. 5 exhibits the result which shows that these interfering species result in negligible current changes,
Fig. 3. CV curves of the NPG@Ni foam hybrid electrode in 0.1 M PBS in the presence of H2O2 over a concentration range from 0.5 to 3.5 mM at a scan rate of 50 mV s the corresponding calibration curve of the current responses vs. H2O2 concentration (b).
1
(a), and
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Fig. 4. The amperometric response of the NPG@Ni foam hybrid electrode at -0.4 V with stepwise addition of H2O2 (a), and the corresponding calibration curve of the current responses vs. H2O2 concentration (b).
compared to that of just H2O2. This further attests to the excellent selectivity of the developed electrode towards H2O2. In addition, we investigated the stability and reproducibility of the as-prepared sensors. The current response of the sensor to 0.1 mM H2O2 had a decrease of about 3.5% after 40 days of storage at room temperature in an inverted beaker to prevent contamination with airborne particulates. The electrode-to-electrode reproducibility was examined between five different sensors, and the responses of the five electrodes to 0.1 mM H2O2 were measured with a relative standard deviation (RSD) of 5.4%. These results indicate the excellent stability and reproducibility of the NPG@Ni foam sensors for H2O2 detection. The NPG@Ni foam hybrids were also employed as working electrodes to evaluate the electrochemical performance for the electrooxidation of hydrazine. Fig. 6 presents the CVs for the NPG@Ni foam hybrid electrode in 0.1 M PBS, with and without 5 mM hydrazine. It is shown that the CV curve with the presence of hydrazine exhibits an apparent current response for anodic oxidation of hydrazine starting from -0.2 V vs. Ag/AgCl and forming a peak at 0.2 V vs. Ag/AgCl, indicating the electrocatalytic activity of the NPG@Ni foam hybrid electrode toward hydrazine oxidation. The electrochemical responses of the NPG@Ni foam hybrid electrode to various concentrations of hydrazine (0.2–15 mM) are shown in Fig. 7a. The oxidation current densities increase with the increased hydrazine concentrations. The corresponding calibration curve of the peak current density on the hydrazine concentrations is depicted in Fig. 7b, displaying a good linear relationship up to 15 mM of hydrazine (R2 = 0.991).
Such results indicate that the electrocatalytic oxidation of hydrazine on the NPG@Ni foam hybrid electrode exhibits an outstanding sensitivity to the changes of hydrazine concentrations. Chronoamperometry was used to assess the sensitivity of the NPG@Ni foam hybrid electrode for hydrazine detection. Fig. 8a shows the current-time response for the NPG@Ni foam hybrid electrode obtained after successive additions of hydrazine into 0.1 M PBS at 0 V vs. Ag/AgCl. With the additions of 0.2 mM to up to 20 mM hydrazine, the oxidation current increases rapidly and reaches the 95% steady state currents within 3 seconds, revealing fast amperometric response of the NPG@Ni foam hybrid electrode. With a signal-to-noise ratio of 3, the detection limit is determined to be 33 nM. The current vs. hydrazine concentration calibration plot is displayed in Fig. 8b. The NPG@Ni foam hybrid electrode exhibits a good linear relationship in the range of 0.2 to 110.1 mM (R2 = 0.9995) with a sensitivity of 10.687 mA mM 1 cm 2, indicating the potential of the NPG@Ni foam hybrid electrode to serve as a highly efficient electrode material for electrochemical detection of hydrazine. The interferences for the determination of hydrazine at the NPG@Ni foam hybrid electrode were studied by adding various common species into the solution. Fig. 9 displays the amperometric response of the NPG@Ni foam sensor for 5 mM hydrazine (a), 200-fold concentration of Cl (b), NO3 (c), SO42 (d), NH4+ (e), Na+ (f), K+ (g), Mg2+ (h) and Ca2+ (i). A rapid amperometric response is observed for 5 mM hydrazine at the NPG@Ni foam electrode, and the common ions had no interference on hydrazine detection, suggesting an excellent selectivity of the sensor. The amperometric response of the NPG@Ni foam electrode toward 20 mM hydrazine
Fig. 5. The amperometric response of the NPG@Ni foam hybrid electrode in 0.1 M PBS with stepwise additions of H2O2, methanol, ethanol, glucose and AA.
Fig. 6. CVs of the NPG@Ni foam hybrid electrode in 0.1 M PBS without and with 5 mM hydrazine at a scan rate of 50 mV s 1.
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Fig. 7. CV results of NPG@Ni foam hybrid electrode in the presence of hydrazine over a concentration range from 0.2 to 15 mM at a scan rate of 50 mV s corresponding calibration curve of the current response with changes of hydrazine concentration (b).
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1
(a), and the
Fig. 8. The amperometric response of NPG@Ni foam hybrid electrode at 0 V with stepwise additions of hydrazine (a), and the corresponding calibration curve of the current response vs. hydrazine concentration (b).
Fig. 9. Amperometric i-t response at the NPG@Ni foam hybrid electrode for 5 mM hydrazine (a), 200 fold concentration of Cl (b), NO3 (c), SO42 (d), NH4+ (e), Na+ (f), K+ (g), Mg2+ (h) and Ca2+ (i).
The high sensitivity of the NPG@Ni foam hybrid electrode for H2O2 and hydrazine detection can be attributed to the following advantages of the 3D hierarchical porous architecture. First, the microporous Ni foam substrate possesses high mechanical stability and allows rapid diffusion of reacting species onto the surface of the electrocatalysts. Second, the NPG structures can be fabricated directly on the surface of Ni foams, eliminating the tedious process of mixing active materials with binders, along with the reduced internal resistance. Finally, the bicontinuous open porosity of NPG structures can efficiently increase the surface area of the electrocatalysts, which enhances rapid electrochemical reactions. This low-cost and robust 3D electrode holds great potential as a new type of enzyme-free electrochemical sensors for the detection of H2O2 and N2H4. Acknowledgements
remained 93.7% of its initial value after storage at 4 C for 4 weeks, indicating the long-term stability of the sensor. The relative standard deviation (RSD) for 20 mM hydrazine detection at three independently prepared electrodes was also less than 4.2%, revealing a relatively good reproducibility of the sensor.
4. Conclusions A novel hybrid electrode structure based on NPG supported on Ni foam was developed and studied as an enzyme-free electrochemical sensor for the detection of H2O2 and N2H4. The NPG@Ni foam hybrid electrode possesses a 3D hierarchical porosity, and exhibits excellent sensing capability to detect H2O2 and N2H4 with a fast response time, high sensitivity and excellent selectivity.
G. F. C. gratefully acknowledges the financial support by National Natural Science Foundation of China (51271205, 50801070), Guangzhou Pearl Technology the Nova Special Project (2012J2200058), Excellent Young College Teachers Development Program in Guangdong Province (Yq2013006), Research and Application of Key Technologies Oriented the Industrial Development (90035-3283309, 90035-3283321), Science and Technology Plan Projects of Guangzhou city (2013Y2-00102), Huizhou city (2012B050013012), DaYa Gulf district of Huizhou city (20110108, 20120212) and SYSU-CMU Shunde International Joint Research Institute (20150209). X. K. acknowledges the support from China Postdoctoral Science Foundation (2014M562234). J. Z. acknowledges the financial support by National Natural Science Foundation of China (51101061). G. W. acknowledges the financial support from the start-up funding of University at Buffalo, SUNY.
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