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SiNW electrode
Sensors and Actuators B 155 (2011) 592–597
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Study of an amperometric glucose sensor based on Pd–Ni/SiNW electrode Shichao Hui, Jian Zhang ∗ , Xuejiao Chen, Huhua Xu, Dianfei Ma, Yanli Liu, Bairui Tao Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electrical Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 9 October 2010 Received in revised form 31 December 2010 Accepted 10 January 2011 Available online 19 January 2011 Keywords: Glucose sensor Pd–Ni/SiNW electrode Electroless co-plating Cyclic voltammetry Amperometry
a b s t r a c t An amperometric glucose sensor based on Pd–Ni/SiNW electrode has been investigated. The silicon nanowire (SiNW) electrodes were first fabricated by chemical etching, and then nickel and palladium particles were deposited onto the surfaces of SiNWs via electroless co-plating technique followed by annealing in nitrogen atmosphere at 350 ◦ C for 300 s. The morphology of Pd–Ni/SiNW electrode was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The sensor performance was characterized by cyclic voltammetry (CV) and fixed potential amperometry techniques. In 0.1 M KOH alkaline medium with different glucose concentrations, the sensor shows an excellent sensitivity of 190.72 A mM−1 cm−2 with the detection limit (S/N ratio = 3) of 2.88 M. And it also exhibits superior anti-interference properties to the species including ascorbic acid (AA), uric acid (UA) and 4acetamidophenol (AP). All results demonstrate that this Pd–Ni/SiNW electrode is a candidate with great potential for glucose detection. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, more and more investigations have focused on the development of fast and reliable methods for glucose detection, since glucose monitoring is very important in lots of fields such as clinical diagnostics, food industry, and biotechnology. Among the detection methods, electrochemical oxidation approaches have been widely used due to the advantages of low detection limit, high sensitivity and excellent selectivity. In the electrochemical detection, catalysts play a crucial role in glucose oxidation. So far, the catalysts which have the potential for application in commercial glucose sensors can be divided into two groups: enzymatic [1–3] and non-enzymatic [4–7]. Although enzyme based glucose sensors have high sensitivity and selectivity, the activity of enzyme is easily affected by temperature, pH, humidity and toxic chemicals [1]. Thus these sensor stability are greatly limited by the loss of enzyme’s activity. In addition, the enzyme based sensors suffer from interference caused by other electro-oxidizable species such as ascorbic acid (AA), uric acid (UA) and 4-acetamidophenol (AP) present in the blood samples, which has also been an inevitable problem. However, via the electrooxidation method, glucose detection without enzyme exhibits many advantages such as high stability, simple structure, and superior anti-interference properties. Hence, recent studies have focused on the development of non-enzymatic catalysts, and many metallic and alloying electrodes have been
∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345203. E-mail address: [email protected] (J. Zhang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.015
employed as the enzymeless electrode in glucose sensors [8–12]. Actually, it is difficult for these electrodes to put into commercial application. The metallic electrodes always suffer from surface poisoning by adsorbed intermediates as well as interference from chloride, while alloying catalysts are hindered by toxicities of heavy metal elements [13]. Therefore, it is important to develop a nonenzymatic sensor which has high sensitivity, good stability, and anti-interference property, for detection of glucose by electrooxidation method. Fortunately, the development of nanostructured materials has compensated for the poor electrochemical activity of nonenzymatic glucose sensors due to their unique physical and chemical properties [14]. These nanostructured materials, including nanoparticles (NPs), nanowalls/nanowires (NWs), and carbon nanotubes (CNTs), with high surface to volume ratio, have been extensively used as the backbones of catalysts [13,15–19]. For example, Pt–Pb nanoparticles on MWCNTs [13], silver oxide nanowalls on copper [15], Ni nanowire arrays modified glassy carbon [16] and copper deposited on CNTs [5], have been used as glucose sensors and exhibit excellent performance. Therefore, using nanostructured materials as the backbone of sensing electrode is well worth studying. Recently, the micro-fabrication technology for integrated circuits (ICs) has been developed as a promising technique for portable sensor applications. And, in the previous work [19,20], Pd–Ni modified silicon nanowire (SiNW) electrodes have been prepared and used for amperometric methanol and ethanol sensors which exhibit excellent sensitivity and stability. The sensor backbone material: nanostructured silicon is the principal component of most
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semiconductor devices, most importantly integrated circuits and microchips. For the reasons above, Pd–Ni modified SiNWs are compatible with integrated circuits (ICs) and micro-fabrication process. Therefore, Pd–Ni modified SiNW is one of the best materials that may well repay the investigation on glucose sensor. In this work, an amperometric non-enzyme glucose sensor based on Pd–Ni/SiNW electrodes had been investigated. As the backbone of the electrode, SiNWs were fabricated by chemical etching. Then, the electrode was formed by electroless co-plating of nickel and palladium nanoparticles. The electrochemical characteristics of the electrode are analyzed by cyclic voltammetry (CV), and the glucose detection is examined by fixed potential amperometry technology. 2. Experimental 2.1. Preparation of the Pd–Ni/SiNW electrode Chemicals such as hydrofluoric acid (HF), silver nitrate (AgNO3 ), nickel sulfate hexahydrate (NiSO4 ·6H2 O), palladium chloride (PdCl2 ), ammonium sulfate (NH4 )2 SO4 , ammonium fluoride (NH4 F), sodium citrate (Na3 C6 H5 O7 ·2H2 O), potassium hydroxide (KOH) were used as-received without further purification. Two-inch 1 0 0-orientated double-polished P-type silicon wafers (1–10 cm) were used as the substrate of Pd–Ni/SiNW fabrication. These wafers were cut into 0.5 cm × 0.5 cm chips for application. Before preparation of the SiNWs, one side of these chips was first protected by photoresist and used for making electrode pad. Next, the silicon nanowires were fabricated according to Ref. [17] using a chemical etching procedure. The chemical solution of HF (20%):AgNO3 (35 mM) = 1:1 (volume ratio) was used as the etchant for SiNW preparation. After the formation of SiNWs, palladium and nickel were deposited onto the SiNWs by electroless coplating and this method is similar to the one previously reported [19,20]. After palladium and nickel were coated on SiNWs, the photoresist was removed and aluminum was evaporated on the same side to form electrode pad. Finally, the as-prepared Pd–Ni/SiNW electrode was annealed in nitrogen atmosphere at 350 ◦ C for 300 s to stabilize the structure and form good ohmic contact. 2.2. Electrode characterization The crystalline phase of SiNWs and Pd–Ni/SiNW electrode is examined by X-ray diffraction (XRD) system with Cu K␣ radiation ( = 0.15405 nm). The morphologies of the SiNWs and Pd–Ni/SiNW electrode are also characterized by scanning electron microscope (SEM). The electrochemical characterization of the electrode is carried out by a three-electrode system provided by LK3200A electrochemical workstation (Tianjin, China), with platinum wire as the counter electrode and saturated Ag/AgCl as the reference electrode, respectively. The working electrode is constructed by sticking a copper leading wire onto the aluminum (Al) film of Pd–Ni/SiNW electrode. All experiments were processed at the room temperature (25 ◦ C).
Fig. 1. The scanning electron micrographs of (a) top-view of the SiNWs and (b) cross-sectional view of Pd–Ni/SiNWs.
particles ranges from tens to hundreds of nanometer. Besides, from Fig. 1(b), it can be observed that palladium and nickel particles were deposited onto the silicon nanowires all over, even including the root section. Fig. 2 is the XRD pattern of the SiNWs and Pd–Ni/SiNWs. From the pattern of SiNWs, only one diffraction peak at 69.13◦ corresponds to Si (4 0 0), which indicates that SiNWs prepared by chemical etching are single-crystalline [17]. After Pd–Ni co-plating, three new peaks emerge, corresponding to the single phase of (1 1 1), (2 0 0) and (2 2 0). It implies that Pd–Ni has formed a solid solution of face-centered cubic (FCC) structure. The inset picture in Fig. 2 is partial enlarged view of Pd–Ni/SiNWs at 2 = 39–46◦ . From the image we can see that the phase of Pd (1 1 1) (the dashed line) is not visible. The slight shift of Ni (1 1 1) towards lower angle proves the existence of Pd–Ni alloy, which is in good agreement with Haridoss’s work [6]. And, the X-ray diffraction shows that Pd and Ni particles were successfully coated on the silicon nanowires.
3. Results and discussion 3.2. The glucose electro-oxidation on the Pd–Ni/SiNW electrode 3.1. SiNW characterization Fig. 1(a) and (b) is the top-view and cross-sectional SEM images of Pd–Ni/SiNWs, respectively. Fig. 1(a) shows that there are many palladium (Pd) and nickel (Ni) particles uniformly coating on the bundle-like silicon nanowires (SiNWs). The diameter of Pd and Ni
To verify the electro-catalytic behavior of Pd–Ni modified electrode, the SiNW electrode with/without modification was first studied in an alkaline solution without glucose. Then, the electrooxidation activity of Pd–Ni/SiNW electrode towards glucose was also investigated.
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Fig. 2. The XRD pattern of (a) the SiNWs and Pd–Ni/SiNWs and (b) the enlarged portion of the Pd–Ni/SiNWs between 39◦ and 46◦ (2).
The electrochemical behavior of the electrodes was characterized by cyclic voltammetry (CV). Cyclic voltammograms of the SiNWs (curve A) and Pd–Ni/SiNW (curve B) electrode in 0.1 M KOH without glucose addition are displayed in Fig. 3(a) and the voltammogram of Pd–Ni/SiNW electrode in 0.1 M KOH solution with 10 mM glucose is depicted in Fig. 3(b). From curve A in Fig. 3(a), no visible oxidation or reduction peaks can be observed implying that SiNWs without modification have poor catalysis in alkaline medium. However, Pd–Ni modified SiNWs showed different properties comparing with unmodified SiNWs. Four oxidation and three reduction peaks can be observed (in curve B) and the shape of voltammogram is analogous to the previously reports [21–24]. In the voltammogram, four oxidized peaks are represented as A1 (−0.63 V), A2 (0.10 V), A3 (0.43 V) and A4 (0.60 V) and three reduction peaks are C1 (−0.21 V), C2 (0.2 V) and C3 (0.50 V), respectively. The current in the potential region from −0.95 V to −0.63 V is mainly due to the adsorption/desorption of hydrogen ad-atoms and the current in the potential range from 0.65 V to 0.50 V ascribes to the oxygen evolution. Peak A1 is attributed to the desorption of diffusional hydrogen absorbed in the Pd–Ni lattices and hydrogen/oxygen evolution occurred at the terminal end of the sweep potential range [21,22]. Thus, all the above analysis indicates that Pd–Ni/SiNW electrodes have good catalytic characterization in alkaline medium. In order to investigate the electrocatalytic behavior of Pd–Ni/SiNW electrode towards glucose, the Pd–Ni modified SiNW electrodes was also studied by cyclic voltammetry in 0.1 M KOH with 10 mM glucose and the relevant cyclic voltammogram is shown in Fig. 3 (b). The hydrogen and oxygen adsorption/desorption regions are similar to those in curve B which are displayed in Fig. 3(a). However, two different oxidation peaks due to glucose oxidation at −0.27 V and −0.07 V could be observed clearly. Thus, the glucose electrooxidation process on Pd–Ni/SiNW electrode could be characterized by these two well-defined oxide current peaks in the positive and negative scans [18]. In the forward sweeping, the oxidation current peak appears at ∼−0.07 V and it is due to the glucose oxidation. In the reverse sweeping, another oxidation peak turns up at ∼0.27 V which is attributed to the oxidation of glucose intermediates [8,11,18]. The cyclic voltammogram indicates that Pd–Ni/SiNW electrode is an excellent electro-catalyst for glucose detecting.
Fig. 3. CVs of (a) SiNWs (curve A) and Pd–Ni/SiNW electrode (curve B) without glucose and (b) Pd–Ni/SiNW electrode with 10 mM glucose in 0.1 M KOH.
3.3. Stability of Pd–Ni/SiNW electrode The stability of Pd–Ni/SiNW electrode was examined in 0.1 M KOH in the absence/presence of 5 mM glucose for 100 times with a scanning rate of 50 mV s−1 . The scanning window ranges from −0.65 to 0.4 V to avoid the interference of hydrogen and oxygen adsorption/desorption or evolution. Each 1st, 10th, 20th, . . ., 100th of the multi-cyclic voltammograms are chosen and the corresponding results are shown in Fig. 4. Fig. 4(a) is the multi-cyclic voltammograms of the Pd–Ni/SiNW electrode in 0.1 M KOH without glucose. As the scanning number increases, it can be found that the peak current decreases slightly and the peak voltages keep constant. After 100 times scanning, the maximum variation of the current is less than 0.65% per cycle, which shows that Pd–Ni/SiNW electrode is reproducible as the electrocatalyst in alkaline medium. Fig. 4(b) is the multi-cyclic voltammogram of Pd–Ni/SiNW electrode in 0.1 M KOH with 5 mM glucose. Compared with Fig. 4(a), two oxidation peaks can be clearly observed. And the potential that began to oxidize glucose is more negative than −0.6 V in the anodic sweep. These behaviors indicate that Pd–Ni/SiNW electrode is excellent for glucose monitoring [18]. The maximum oxidation current peak occurred at potential of −0.06 V and it reduces from 0.231 to 0.206 mA after 100 times scanning. The cur-
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Fig. 5. The current–time response of Pd–Ni/SiNW electrode measured at −0.06 V in 0.1 M KOH solution with successive additions of 2 mM glucose (2–20 mM), with successive additions in a smaller step of 0.2 mM glucose (0–1.8 mM) (left inset) and the corresponding calibration plots (right inset).
3.5. Effect of interfering species
Fig. 4. CVs of the Pd–Ni/SiNW electrode in 0.1 M KOH solution for 100 times (a) without glucose and (b) with 5 mM glucose.
The amperometric responses of some possible interfering species on Pd–Ni/SiNW electrode have also been studied. The interferents such as ascorbic acid (AA), uric acid (UA) and 4acetamidophenol (AP) could be easily oxidized at a relative positive potential [4]. The experiment was carried out by adding 0.2 mM ascorbic acid (AA), 0.1 mM uric acid (UA), 0.1 mM 4acetamidophenol (AP) interfering agents and 0.5 mM glucose successively into a constantly stirring 0.1 M KOH solution at a fixed potential of −0.06 V. The corresponding result is shown in Fig. 6. The Pd–Ni/SiNW electrode did not give any clear response to the interference species of AA, UA and AP. However, the sensor still displays strong current response to the subsequent glucose additions, which demonstrates that Pd–Ni/SiNW electrode has high sensitivity and selectivity for non-enzymatic glucose sensor.
rent decreases less than 0.1% per cycle. It demonstrates that this Pd–Ni/SiNW electrode has an outstanding stability. In addition, the Pd–Ni/SiNW electrode has not been poisoned by the oxidation products. Thus, the electrode can be used for repeated glucose detection. 3.4. Glucose detection with fixed potential amperometry The current–time response of Pd–Ni/SiNW electrode to glucose concentration was also studied by amperometry in 0.1 M KOH at a fixed potential of −0.06 V, with a step of 2 mM glucose added in the range from 2 to 20 mM and a smaller step of 0.2 mM in the range from 0 to 1.8 mM, respectively. The relevant results are illustrated in Fig. 5. The sensor presents a fast current response to glucose concentration in all the two ranges and reached steadystate within 8 s. Along with the addition of glucose, the oxidation current increases linearly and the corresponding calibration plot is shown in the right inset of Fig. 5. And the regression equation is I (A) = 7.70 + 47.68C (mM) with a correlation coefficient of 0.997. The sensitivity to glucose concentration is ∼190.7 A mM−1 cm−2 with a lowest detection limit of 2.88 M (S/N ratio = 3). The sensor exhibits excellent linearity in the wide range from 0 to 20 mM.
Fig. 6. Current–time response curves in 0.1 M KOH with successive additions of 0.2 mM AA, 0.1 mM UA, 0.1 mM AP and 0.5 mM glucose under the electrode potential −0.06 V.
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Table 1 Comparison of published non-enzymatic glucose sensors. Working electrode
Detection technology
Pd–Ni/SiNWs
Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry
CuO/CuOx/Cu NiNWAs Cu–Ag2 O NWS/GCE Pt–Ir/Ti Pd–Ni/Si-MCP Pt–PbNPs/MWCNTs Cu–CNTs–GCE Pt–Pb/Ti Pt-NTAE/AAM
Sensitivity
Detection limit (M)
190.7 A mM−1 cm−2
Response time (s)
Reference
2.88
8
This work
1890 A mM−1 cm−2
0.05
–
[4]
1043 A mM−1 cm−2
0.1
10
[16]
10
–
[15]
–
–
[25]
81.4 A mM−1
5
10
[18]
17.8 A mM−1 cm−2
1.8
–
[13]
0.21
5
[5]
10.8 A mM−1 cm−2
–
–
[7]
0.1 A mM−1 cm−2
1
–
[8]
298.2 A mM−1 93.7 A mM
−1
17.76 A mM
−2
cm
−1
3.6. Discussion A comparison of some basic parameters between the sensor in our work and the other developed non-enzymatic glucose sensors is listed in Table 1. The sensitivity of Pd–Ni/SiNW electrode is higher than that of the literature for Cu–CNTs–GCE [5] and Pt–Pb nanoparticles on MWCNTs [13]. The detection limit is lower than that of the literature for Cu–Ag2 O NWs/GCE [15]. It can be attributed to the following points. First, high volume ratio of Pd–Ni/SiNW electrode increases the activity surface, accelerating the electron-transfer reaction, lowering the activation energy. Second, after annealed at 350 ◦ C for 300 s, the Ni–Si layer located between the Ni and silicon nanowire (SiNW) interface has been formed, which prevents the electrode from etching by OH− ions and improves the electrode stability [20,26]. Third, the Pd–Ni particles are deposited onto the SiNWs with high uniformity and good dispersion, indicating that a large amount of Pd–Ni nanoparticles participate in the reaction and result in high activity. 4. Conclusion In this paper, an electrochemical non-enzymatic glucose sensor based on Pd–Ni/SiNW electrode has been prepared. The resulting materials were characterized by SEM, XRD, CV and fixed potential amperometry. The sensing electrode is easily fabricated, low-cost and compatible with IC industry. The non-enzymatic sensor exhibits good linearity, high sensitivity, outstanding stability, and excellent selectivity for it is interference free with the species such as AA, UA, and AP. The sensitivity of this sensor is 190.7 A mM−1 cm−2 with a low detection limit of 2.88 M (S/N = 3). All the above results demonstrate Pd–Ni/SiNW electrode is a candidate with great potential for glucose detection. Acknowledgements This work is supported by the National Natural Science Foundation of China (grant no. 60672002, 61076070) and Innovation Program of Shanghai Municipal Education Commission (grant no. 09ZZ46). We deeply appreciate the financial support. References [1] Y. Shimizu, K. Morita, Microhole array electrode: as a glucose sensor, Anal. Chem. 62 (1990) 1498–1501.
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Biographies Shichao Hui received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, fuel cells, super-capacitors and nano-sensors. Jian Zhang received his master degree from Changchun Institute of Optics and Fine Mechanics, and PhD degree from Shanghai Institute of Metallurgy, Chinese Academy of Sciences (CAS), China, in 1994 and 1997, respectively. From 1997 to 1998, he worked as the assistant professor in State Key Laboratory of Transducer, CAS, China. From 1998 to 2000, he worked as a Research Fellow in Nanyang Technological Uni-
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versity, Singapore. From August 2000 to December 2003, he worked as a Research Fellow in Institute of Materials Research and Engineering, Singapore. From January 2004, he worked as a professor in Department of Electrical Engineering, East China Normal University. His current research interests include micro-sensors and arrays, micro-fabrication, gas sensors and biosensors. Xuejiao Chen was born in Shanghai, China, on December 3, 1984. She received her BS degree from the Microelectronics, East China Normal University (ECNU), Shanghai, China in 2007. She is currently a graduate student in ECNU. Her research interest includes nanomaterials, humidity sensors and biosensors. Huhua Xu received his BS degree in Microelectronics from East China Normal University, Shanghai, China, in 2009. Since September 2009, he has been working towards his MS degree in Integrated Circuit Engineering from East China Normal University. His current research interests include fuel cells, solar cells, nano-sensors and semiconductor materials. Dianfei Ma received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, schottky barrier contact and temperature sensor. Yanli Liu received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, dielectrophoresis. Bairui Tao received his MS and PhD degree in Electrical Engineering from East China Normal University, China in 2007 and 2010, respectively. Currently, he works as an associate professor in College of Communications and Electronic Engineering, Qiqihar University.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Study of an amperometric glucose sensor based on Pd–Ni/SiNW electrode Shichao Hui, Jian Zhang ∗ , Xuejiao Chen, Huhua Xu, Dianfei Ma, Yanli Liu, Bairui Tao Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electrical Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 9 October 2010 Received in revised form 31 December 2010 Accepted 10 January 2011 Available online 19 January 2011 Keywords: Glucose sensor Pd–Ni/SiNW electrode Electroless co-plating Cyclic voltammetry Amperometry
a b s t r a c t An amperometric glucose sensor based on Pd–Ni/SiNW electrode has been investigated. The silicon nanowire (SiNW) electrodes were first fabricated by chemical etching, and then nickel and palladium particles were deposited onto the surfaces of SiNWs via electroless co-plating technique followed by annealing in nitrogen atmosphere at 350 ◦ C for 300 s. The morphology of Pd–Ni/SiNW electrode was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The sensor performance was characterized by cyclic voltammetry (CV) and fixed potential amperometry techniques. In 0.1 M KOH alkaline medium with different glucose concentrations, the sensor shows an excellent sensitivity of 190.72 A mM−1 cm−2 with the detection limit (S/N ratio = 3) of 2.88 M. And it also exhibits superior anti-interference properties to the species including ascorbic acid (AA), uric acid (UA) and 4acetamidophenol (AP). All results demonstrate that this Pd–Ni/SiNW electrode is a candidate with great potential for glucose detection. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, more and more investigations have focused on the development of fast and reliable methods for glucose detection, since glucose monitoring is very important in lots of fields such as clinical diagnostics, food industry, and biotechnology. Among the detection methods, electrochemical oxidation approaches have been widely used due to the advantages of low detection limit, high sensitivity and excellent selectivity. In the electrochemical detection, catalysts play a crucial role in glucose oxidation. So far, the catalysts which have the potential for application in commercial glucose sensors can be divided into two groups: enzymatic [1–3] and non-enzymatic [4–7]. Although enzyme based glucose sensors have high sensitivity and selectivity, the activity of enzyme is easily affected by temperature, pH, humidity and toxic chemicals [1]. Thus these sensor stability are greatly limited by the loss of enzyme’s activity. In addition, the enzyme based sensors suffer from interference caused by other electro-oxidizable species such as ascorbic acid (AA), uric acid (UA) and 4-acetamidophenol (AP) present in the blood samples, which has also been an inevitable problem. However, via the electrooxidation method, glucose detection without enzyme exhibits many advantages such as high stability, simple structure, and superior anti-interference properties. Hence, recent studies have focused on the development of non-enzymatic catalysts, and many metallic and alloying electrodes have been
∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345203. E-mail address: [email protected] (J. Zhang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.015
employed as the enzymeless electrode in glucose sensors [8–12]. Actually, it is difficult for these electrodes to put into commercial application. The metallic electrodes always suffer from surface poisoning by adsorbed intermediates as well as interference from chloride, while alloying catalysts are hindered by toxicities of heavy metal elements [13]. Therefore, it is important to develop a nonenzymatic sensor which has high sensitivity, good stability, and anti-interference property, for detection of glucose by electrooxidation method. Fortunately, the development of nanostructured materials has compensated for the poor electrochemical activity of nonenzymatic glucose sensors due to their unique physical and chemical properties [14]. These nanostructured materials, including nanoparticles (NPs), nanowalls/nanowires (NWs), and carbon nanotubes (CNTs), with high surface to volume ratio, have been extensively used as the backbones of catalysts [13,15–19]. For example, Pt–Pb nanoparticles on MWCNTs [13], silver oxide nanowalls on copper [15], Ni nanowire arrays modified glassy carbon [16] and copper deposited on CNTs [5], have been used as glucose sensors and exhibit excellent performance. Therefore, using nanostructured materials as the backbone of sensing electrode is well worth studying. Recently, the micro-fabrication technology for integrated circuits (ICs) has been developed as a promising technique for portable sensor applications. And, in the previous work [19,20], Pd–Ni modified silicon nanowire (SiNW) electrodes have been prepared and used for amperometric methanol and ethanol sensors which exhibit excellent sensitivity and stability. The sensor backbone material: nanostructured silicon is the principal component of most
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semiconductor devices, most importantly integrated circuits and microchips. For the reasons above, Pd–Ni modified SiNWs are compatible with integrated circuits (ICs) and micro-fabrication process. Therefore, Pd–Ni modified SiNW is one of the best materials that may well repay the investigation on glucose sensor. In this work, an amperometric non-enzyme glucose sensor based on Pd–Ni/SiNW electrodes had been investigated. As the backbone of the electrode, SiNWs were fabricated by chemical etching. Then, the electrode was formed by electroless co-plating of nickel and palladium nanoparticles. The electrochemical characteristics of the electrode are analyzed by cyclic voltammetry (CV), and the glucose detection is examined by fixed potential amperometry technology. 2. Experimental 2.1. Preparation of the Pd–Ni/SiNW electrode Chemicals such as hydrofluoric acid (HF), silver nitrate (AgNO3 ), nickel sulfate hexahydrate (NiSO4 ·6H2 O), palladium chloride (PdCl2 ), ammonium sulfate (NH4 )2 SO4 , ammonium fluoride (NH4 F), sodium citrate (Na3 C6 H5 O7 ·2H2 O), potassium hydroxide (KOH) were used as-received without further purification. Two-inch 1 0 0-orientated double-polished P-type silicon wafers (1–10 cm) were used as the substrate of Pd–Ni/SiNW fabrication. These wafers were cut into 0.5 cm × 0.5 cm chips for application. Before preparation of the SiNWs, one side of these chips was first protected by photoresist and used for making electrode pad. Next, the silicon nanowires were fabricated according to Ref. [17] using a chemical etching procedure. The chemical solution of HF (20%):AgNO3 (35 mM) = 1:1 (volume ratio) was used as the etchant for SiNW preparation. After the formation of SiNWs, palladium and nickel were deposited onto the SiNWs by electroless coplating and this method is similar to the one previously reported [19,20]. After palladium and nickel were coated on SiNWs, the photoresist was removed and aluminum was evaporated on the same side to form electrode pad. Finally, the as-prepared Pd–Ni/SiNW electrode was annealed in nitrogen atmosphere at 350 ◦ C for 300 s to stabilize the structure and form good ohmic contact. 2.2. Electrode characterization The crystalline phase of SiNWs and Pd–Ni/SiNW electrode is examined by X-ray diffraction (XRD) system with Cu K␣ radiation ( = 0.15405 nm). The morphologies of the SiNWs and Pd–Ni/SiNW electrode are also characterized by scanning electron microscope (SEM). The electrochemical characterization of the electrode is carried out by a three-electrode system provided by LK3200A electrochemical workstation (Tianjin, China), with platinum wire as the counter electrode and saturated Ag/AgCl as the reference electrode, respectively. The working electrode is constructed by sticking a copper leading wire onto the aluminum (Al) film of Pd–Ni/SiNW electrode. All experiments were processed at the room temperature (25 ◦ C).
Fig. 1. The scanning electron micrographs of (a) top-view of the SiNWs and (b) cross-sectional view of Pd–Ni/SiNWs.
particles ranges from tens to hundreds of nanometer. Besides, from Fig. 1(b), it can be observed that palladium and nickel particles were deposited onto the silicon nanowires all over, even including the root section. Fig. 2 is the XRD pattern of the SiNWs and Pd–Ni/SiNWs. From the pattern of SiNWs, only one diffraction peak at 69.13◦ corresponds to Si (4 0 0), which indicates that SiNWs prepared by chemical etching are single-crystalline [17]. After Pd–Ni co-plating, three new peaks emerge, corresponding to the single phase of (1 1 1), (2 0 0) and (2 2 0). It implies that Pd–Ni has formed a solid solution of face-centered cubic (FCC) structure. The inset picture in Fig. 2 is partial enlarged view of Pd–Ni/SiNWs at 2 = 39–46◦ . From the image we can see that the phase of Pd (1 1 1) (the dashed line) is not visible. The slight shift of Ni (1 1 1) towards lower angle proves the existence of Pd–Ni alloy, which is in good agreement with Haridoss’s work [6]. And, the X-ray diffraction shows that Pd and Ni particles were successfully coated on the silicon nanowires.
3. Results and discussion 3.2. The glucose electro-oxidation on the Pd–Ni/SiNW electrode 3.1. SiNW characterization Fig. 1(a) and (b) is the top-view and cross-sectional SEM images of Pd–Ni/SiNWs, respectively. Fig. 1(a) shows that there are many palladium (Pd) and nickel (Ni) particles uniformly coating on the bundle-like silicon nanowires (SiNWs). The diameter of Pd and Ni
To verify the electro-catalytic behavior of Pd–Ni modified electrode, the SiNW electrode with/without modification was first studied in an alkaline solution without glucose. Then, the electrooxidation activity of Pd–Ni/SiNW electrode towards glucose was also investigated.
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Fig. 2. The XRD pattern of (a) the SiNWs and Pd–Ni/SiNWs and (b) the enlarged portion of the Pd–Ni/SiNWs between 39◦ and 46◦ (2).
The electrochemical behavior of the electrodes was characterized by cyclic voltammetry (CV). Cyclic voltammograms of the SiNWs (curve A) and Pd–Ni/SiNW (curve B) electrode in 0.1 M KOH without glucose addition are displayed in Fig. 3(a) and the voltammogram of Pd–Ni/SiNW electrode in 0.1 M KOH solution with 10 mM glucose is depicted in Fig. 3(b). From curve A in Fig. 3(a), no visible oxidation or reduction peaks can be observed implying that SiNWs without modification have poor catalysis in alkaline medium. However, Pd–Ni modified SiNWs showed different properties comparing with unmodified SiNWs. Four oxidation and three reduction peaks can be observed (in curve B) and the shape of voltammogram is analogous to the previously reports [21–24]. In the voltammogram, four oxidized peaks are represented as A1 (−0.63 V), A2 (0.10 V), A3 (0.43 V) and A4 (0.60 V) and three reduction peaks are C1 (−0.21 V), C2 (0.2 V) and C3 (0.50 V), respectively. The current in the potential region from −0.95 V to −0.63 V is mainly due to the adsorption/desorption of hydrogen ad-atoms and the current in the potential range from 0.65 V to 0.50 V ascribes to the oxygen evolution. Peak A1 is attributed to the desorption of diffusional hydrogen absorbed in the Pd–Ni lattices and hydrogen/oxygen evolution occurred at the terminal end of the sweep potential range [21,22]. Thus, all the above analysis indicates that Pd–Ni/SiNW electrodes have good catalytic characterization in alkaline medium. In order to investigate the electrocatalytic behavior of Pd–Ni/SiNW electrode towards glucose, the Pd–Ni modified SiNW electrodes was also studied by cyclic voltammetry in 0.1 M KOH with 10 mM glucose and the relevant cyclic voltammogram is shown in Fig. 3 (b). The hydrogen and oxygen adsorption/desorption regions are similar to those in curve B which are displayed in Fig. 3(a). However, two different oxidation peaks due to glucose oxidation at −0.27 V and −0.07 V could be observed clearly. Thus, the glucose electrooxidation process on Pd–Ni/SiNW electrode could be characterized by these two well-defined oxide current peaks in the positive and negative scans [18]. In the forward sweeping, the oxidation current peak appears at ∼−0.07 V and it is due to the glucose oxidation. In the reverse sweeping, another oxidation peak turns up at ∼0.27 V which is attributed to the oxidation of glucose intermediates [8,11,18]. The cyclic voltammogram indicates that Pd–Ni/SiNW electrode is an excellent electro-catalyst for glucose detecting.
Fig. 3. CVs of (a) SiNWs (curve A) and Pd–Ni/SiNW electrode (curve B) without glucose and (b) Pd–Ni/SiNW electrode with 10 mM glucose in 0.1 M KOH.
3.3. Stability of Pd–Ni/SiNW electrode The stability of Pd–Ni/SiNW electrode was examined in 0.1 M KOH in the absence/presence of 5 mM glucose for 100 times with a scanning rate of 50 mV s−1 . The scanning window ranges from −0.65 to 0.4 V to avoid the interference of hydrogen and oxygen adsorption/desorption or evolution. Each 1st, 10th, 20th, . . ., 100th of the multi-cyclic voltammograms are chosen and the corresponding results are shown in Fig. 4. Fig. 4(a) is the multi-cyclic voltammograms of the Pd–Ni/SiNW electrode in 0.1 M KOH without glucose. As the scanning number increases, it can be found that the peak current decreases slightly and the peak voltages keep constant. After 100 times scanning, the maximum variation of the current is less than 0.65% per cycle, which shows that Pd–Ni/SiNW electrode is reproducible as the electrocatalyst in alkaline medium. Fig. 4(b) is the multi-cyclic voltammogram of Pd–Ni/SiNW electrode in 0.1 M KOH with 5 mM glucose. Compared with Fig. 4(a), two oxidation peaks can be clearly observed. And the potential that began to oxidize glucose is more negative than −0.6 V in the anodic sweep. These behaviors indicate that Pd–Ni/SiNW electrode is excellent for glucose monitoring [18]. The maximum oxidation current peak occurred at potential of −0.06 V and it reduces from 0.231 to 0.206 mA after 100 times scanning. The cur-
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Fig. 5. The current–time response of Pd–Ni/SiNW electrode measured at −0.06 V in 0.1 M KOH solution with successive additions of 2 mM glucose (2–20 mM), with successive additions in a smaller step of 0.2 mM glucose (0–1.8 mM) (left inset) and the corresponding calibration plots (right inset).
3.5. Effect of interfering species
Fig. 4. CVs of the Pd–Ni/SiNW electrode in 0.1 M KOH solution for 100 times (a) without glucose and (b) with 5 mM glucose.
The amperometric responses of some possible interfering species on Pd–Ni/SiNW electrode have also been studied. The interferents such as ascorbic acid (AA), uric acid (UA) and 4acetamidophenol (AP) could be easily oxidized at a relative positive potential [4]. The experiment was carried out by adding 0.2 mM ascorbic acid (AA), 0.1 mM uric acid (UA), 0.1 mM 4acetamidophenol (AP) interfering agents and 0.5 mM glucose successively into a constantly stirring 0.1 M KOH solution at a fixed potential of −0.06 V. The corresponding result is shown in Fig. 6. The Pd–Ni/SiNW electrode did not give any clear response to the interference species of AA, UA and AP. However, the sensor still displays strong current response to the subsequent glucose additions, which demonstrates that Pd–Ni/SiNW electrode has high sensitivity and selectivity for non-enzymatic glucose sensor.
rent decreases less than 0.1% per cycle. It demonstrates that this Pd–Ni/SiNW electrode has an outstanding stability. In addition, the Pd–Ni/SiNW electrode has not been poisoned by the oxidation products. Thus, the electrode can be used for repeated glucose detection. 3.4. Glucose detection with fixed potential amperometry The current–time response of Pd–Ni/SiNW electrode to glucose concentration was also studied by amperometry in 0.1 M KOH at a fixed potential of −0.06 V, with a step of 2 mM glucose added in the range from 2 to 20 mM and a smaller step of 0.2 mM in the range from 0 to 1.8 mM, respectively. The relevant results are illustrated in Fig. 5. The sensor presents a fast current response to glucose concentration in all the two ranges and reached steadystate within 8 s. Along with the addition of glucose, the oxidation current increases linearly and the corresponding calibration plot is shown in the right inset of Fig. 5. And the regression equation is I (A) = 7.70 + 47.68C (mM) with a correlation coefficient of 0.997. The sensitivity to glucose concentration is ∼190.7 A mM−1 cm−2 with a lowest detection limit of 2.88 M (S/N ratio = 3). The sensor exhibits excellent linearity in the wide range from 0 to 20 mM.
Fig. 6. Current–time response curves in 0.1 M KOH with successive additions of 0.2 mM AA, 0.1 mM UA, 0.1 mM AP and 0.5 mM glucose under the electrode potential −0.06 V.
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Table 1 Comparison of published non-enzymatic glucose sensors. Working electrode
Detection technology
Pd–Ni/SiNWs
Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry Fixed potential amperometry
CuO/CuOx/Cu NiNWAs Cu–Ag2 O NWS/GCE Pt–Ir/Ti Pd–Ni/Si-MCP Pt–PbNPs/MWCNTs Cu–CNTs–GCE Pt–Pb/Ti Pt-NTAE/AAM
Sensitivity
Detection limit (M)
190.7 A mM−1 cm−2
Response time (s)
Reference
2.88
8
This work
1890 A mM−1 cm−2
0.05
–
[4]
1043 A mM−1 cm−2
0.1
10
[16]
10
–
[15]
–
–
[25]
81.4 A mM−1
5
10
[18]
17.8 A mM−1 cm−2
1.8
–
[13]
0.21
5
[5]
10.8 A mM−1 cm−2
–
–
[7]
0.1 A mM−1 cm−2
1
–
[8]
298.2 A mM−1 93.7 A mM
−1
17.76 A mM
−2
cm
−1
3.6. Discussion A comparison of some basic parameters between the sensor in our work and the other developed non-enzymatic glucose sensors is listed in Table 1. The sensitivity of Pd–Ni/SiNW electrode is higher than that of the literature for Cu–CNTs–GCE [5] and Pt–Pb nanoparticles on MWCNTs [13]. The detection limit is lower than that of the literature for Cu–Ag2 O NWs/GCE [15]. It can be attributed to the following points. First, high volume ratio of Pd–Ni/SiNW electrode increases the activity surface, accelerating the electron-transfer reaction, lowering the activation energy. Second, after annealed at 350 ◦ C for 300 s, the Ni–Si layer located between the Ni and silicon nanowire (SiNW) interface has been formed, which prevents the electrode from etching by OH− ions and improves the electrode stability [20,26]. Third, the Pd–Ni particles are deposited onto the SiNWs with high uniformity and good dispersion, indicating that a large amount of Pd–Ni nanoparticles participate in the reaction and result in high activity. 4. Conclusion In this paper, an electrochemical non-enzymatic glucose sensor based on Pd–Ni/SiNW electrode has been prepared. The resulting materials were characterized by SEM, XRD, CV and fixed potential amperometry. The sensing electrode is easily fabricated, low-cost and compatible with IC industry. The non-enzymatic sensor exhibits good linearity, high sensitivity, outstanding stability, and excellent selectivity for it is interference free with the species such as AA, UA, and AP. The sensitivity of this sensor is 190.7 A mM−1 cm−2 with a low detection limit of 2.88 M (S/N = 3). All the above results demonstrate Pd–Ni/SiNW electrode is a candidate with great potential for glucose detection. Acknowledgements This work is supported by the National Natural Science Foundation of China (grant no. 60672002, 61076070) and Innovation Program of Shanghai Municipal Education Commission (grant no. 09ZZ46). We deeply appreciate the financial support. References [1] Y. Shimizu, K. Morita, Microhole array electrode: as a glucose sensor, Anal. Chem. 62 (1990) 1498–1501.
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Biographies Shichao Hui received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, fuel cells, super-capacitors and nano-sensors. Jian Zhang received his master degree from Changchun Institute of Optics and Fine Mechanics, and PhD degree from Shanghai Institute of Metallurgy, Chinese Academy of Sciences (CAS), China, in 1994 and 1997, respectively. From 1997 to 1998, he worked as the assistant professor in State Key Laboratory of Transducer, CAS, China. From 1998 to 2000, he worked as a Research Fellow in Nanyang Technological Uni-
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versity, Singapore. From August 2000 to December 2003, he worked as a Research Fellow in Institute of Materials Research and Engineering, Singapore. From January 2004, he worked as a professor in Department of Electrical Engineering, East China Normal University. His current research interests include micro-sensors and arrays, micro-fabrication, gas sensors and biosensors. Xuejiao Chen was born in Shanghai, China, on December 3, 1984. She received her BS degree from the Microelectronics, East China Normal University (ECNU), Shanghai, China in 2007. She is currently a graduate student in ECNU. Her research interest includes nanomaterials, humidity sensors and biosensors. Huhua Xu received his BS degree in Microelectronics from East China Normal University, Shanghai, China, in 2009. Since September 2009, he has been working towards his MS degree in Integrated Circuit Engineering from East China Normal University. His current research interests include fuel cells, solar cells, nano-sensors and semiconductor materials. Dianfei Ma received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, schottky barrier contact and temperature sensor. Yanli Liu received her BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, dielectrophoresis. Bairui Tao received his MS and PhD degree in Electrical Engineering from East China Normal University, China in 2007 and 2010, respectively. Currently, he works as an associate professor in College of Communications and Electronic Engineering, Qiqihar University.