Proceedings of the 3rd IEEE Int. Conf. on Nano/Micro Engineered and Molecular Systems January 6-9, 2008, Sanya, China
Long-term Stabled Non-enzymatic Glucose Sensor for Continuously Monitoring System Applications Dae-Joon Park, Yi-Jae Lee, Jae-Yeong Park*, and Dae Heum Kim+, Member, IEEE Department of Electronic Engineering, Kwangwoon University, Seoul, Korea +Department of Chemical Engineering, Kwangwoon University, Seoul, Korea Abstract— In this paper, two different non-enzymatic glucose sensors with Ag/AgCl and Pt reference electrodes (RE) are fabricated, characterized, and compared for continuously monitoring system applications. The fabricated non-enzymatic sensors are comprised of three electrodes, nanoporous platinum working electrode (WE), platinum count electrode (CE), and Ag/AgCl or Pt REs, respectively. An optimum applied potential for the detection of glucose was determined as 0.4V by using glucose cyclic voltammetry graph. Although the sensor with Pt RE has larger response current in glucose solution than the other, it has too much noise and no repeatability. On the other hand, the sensor with Ag/AgCl RE has good repeatability and improved selectivity over interfering species such as ascorbic acid (AA) and acetaminophen (AP). In addition, it has excellent long-term stability. These results indicate that fabricated non-enzymatic glucose sensor with Ag/AgCl RE is strongly applicable for continuously monitoring systems. Keywords: nanoporous platinum, continuously monitoring system, Non-enzymatic, glucose sensor
I.
INTRODUCTION
Many studies in electrochemical glucose sensors to obtain good sensitivity, selectivity, and long-term stability, have been performed. The long-term stability is very important issue, especially for continuously monitoring system applications. However, most of commercialized electrochemical glucose sensors are disposable because of using enzymes, which are living creatures [1], [2]. Therefore, they are limited to use in continuously monitoring glucose sensor system applications. To overcome these drawbacks, nanoporous (nano-hole arrayed) Pt film was fabricated and characterized on a Pt rod [3] and on silicon substrate [4]. Since the diameter of the nanoporous Pt is much smaller than the chronoamperometric diffusion field, ascorbic acid (AA) and acetaminophen (AP) which are rapidly oxidizable and reducible reactants are very speedily exhausted in the diffusion layer. However, since glucose which is slowly reacted reactant is distributed and reacted along with the largely expanded surface area of the nanoporous Pt electrode, extremely large response current to glucose is produced unlike interfering species such as AA and AP [5]. In this paper, non-enzymatic glucose sensors with two different reference electrodes, Ag/AgCl and Pt, have been designed, fabricated, and characterized. In particular, long-term stability test has been performed for continuously monitoring glucose sensor system applications.
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Figure 1. Surface morphology of fabricated nanoporous Pt electrode (analyzed with AFM)
This work was supported by the IT R&D program of MIC/IITA [2005-S093-03, Development of Implantable System Based on Biomedical Signal Processing]. *Contact author: Tel. +82-2-940-5113; Fax. +82-2-942-1502; e-mail.
[email protected]
978-1-4244-1908-1/08/$25.00 ©2008 IEEE.
EXPERIMENTAL PROCEDURES
The nanoporous Pt electrode was previously fabricated on silicon substrate for non-enzymatic electrochemical glucose sensor applications [6]. 42 % (w/w) C16EO8, 29 % (w/w) distilled water (18 Mǡ cm) and 29 % (w/w) HCPA were well mixed and heated up to 85 ͠, at which the mixture became transparent and homogeneous. Silicon substrate was inserted into the mixture and the temperature was lowered to 25 ͠. At this step, the liquid crystalline structure was formed on top of Pt film deposited on the silicon substrate. And then, Pt ions were electrodeposited into the hexagonally packed cylindrical nano-molds on top of the Pt electrode by constant potential (0.12 V vs. Ag/AgCl). Finally, the mesoporous Pt electrode was soaked in deionized water for several hours to remove the C16EO8. The fabricated nanoporous Pt electrodes are confirmed to have the hexagonal cylindrical nanoporous structure with approximately 2.5 nm in a diameter and approximately 5 nm in a pore to pore distance. Fig. 1 shows surface morphology of fabricated nanoporous Pt electrode analyzed with AFM (atomic force microscopy, XE-100, Park Systems, Korea). As shown in Fig. 1, RMS (root mean square) values, which are revealed the surface roughness, are approximately 3.041 nm.
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Fig. 2 shows schematic drawing (a) and photomicrograph (b) of proposed non-enzymatic glucose sensor on silicon substrate. As shown in Figure 2 (a), the proposed nonenzymatic glucose sensors have three electrodes which are comprised of working electrode (WE; nanoporous Pt electrode), counter electrode (CE; plane Pt electrode), and reference electrode (RE; Ag/AgCl electrode or plane Pt electrode).
solution and 0.1 M PBS solution containing 0.1 M glucose solution at the scan rate of 20mV/s. This experiment was performed in order to find an appropriate potential applied for the detection of glucose. This graph shows that the optimum potential is around 0.4V and the sensor with Pt RE has larger response current than the other one. 100
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Figure 4. Cyclic voltammetry of fabricated non-enzymatic glucose sensors in 0.1 M PBS solution (dashed line) and 0.1 M PBS solution containing 0.1 M glucose solution (solid line) at the scan rate of 20mV/s
For evaluation of the fabricated non-enzymatic glucose sensors with Ag/AgCl and Pt reference electrodes (RE), current responses were measured and compared in various solutions. Fig. 5 shows cyclic voltammetry in 1 M sulfuric acid solution of fabricated non-enzymatic glucose sensors. This experiment was performed in order to check their surface roughness. As shown in Fig. 5, the sensor with Ag/AgCl RE has slightly larger surface activation area than the other one. 2.5
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(f) Figure 3. Fabrication steps of proposed non-enzymatic glucose sensor.
III.
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Figure 2. Schematic drawing (a) and photomicrograph (b) of proposed nonenzymatic glucose sensor on silicon substrate.
Fig. 3 shows fabrication steps of proposed non-enzymatic glucose sensor. Firstly, SiO2 layer was firstly deposited on top of silicon substrate. Next, Ti and Pt films were sputtered on top of the SiO2 layer. And then, it was patterned by using conventional photo lithography and metal dry-etching. Finally, mesoporous Pt film was formed on the working electrode, and Ag/AgCl was screen printed on top of the platinum reference electrode.
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EXPERIMENTAL RESULTS AND DISCUSSIONS
Fig. 4 shows comparison of cyclic voltammetry of the fabricated non-enzymatic glucose sensors in 0.1 M PBS
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Voltage [V] Figure 5. Comparison of cyclic voltammetry in 1M sulfuric acid of fabricated non-enzymatic glucose sensors with two different reference electrodes (RE).
Fig. 6 shows current responses of the fabricated nonenzymatic glucose sensors to various glucose concentrations with PBS solution. As shown in Fig. 6, the sensor with Pt RE
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has larger response current than the other. This data was not matched with the above experimental results shown in Fig. 5. Several samples were made and tested at the same conditions. While the sensor with Pt RE showed too much noises and bad repeatability, the sensor with Ag/AgCl RE displayed extremely low noises and excellent repeatability and uniformity. Thus, amperometric test and long-term stability test were performed in interference solutions by using the non-enzymatic glucose sensor with Ag/AgCl RE. 0
Fig. 8 shows long-term stability test of the fabricated nonenzymatic glucose sensors with Ag/AgCl RE in 10 mM glucose solution. The long-term stability was performed for 6 days. As shown in Fig. 8, the current responses were not changed in a noticeable way during 6 days. These data indicate that it has excellent repeatability and reliability. Thus, the nonenzymatic sensor with Ag/AgCl RE is strongly applicable for continuously monitoring systems. 0 Ag/AgCl RE
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Fig. 7 shows amperometric response of the fabricated nonenzymatic glucose sensor with Ag/AgCl RE to the successive additions of 5 mM glucose solutions, 0.1 mM AA and 0.1 mM AP in PBS solution. As shown in Fig. 7, the response of nonenzymatic glucose sensor with Ag/AgCl RE was linearly proportional to the glucose concentrations. On the other hand, there was negligible interference in the response current signal at the addition of AA and AP into the solution. These data indicate that the non-enzymatic sensor with Ag/AgCl RE has excellent sensitivity and selectivity.
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Figure 8. Long-term stablity test of fabricated non-enzymatic glucose sensor with Ag/AgCl RE in 10mM glucose solution.
IV.
CONCLUSION
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Fully integrated non-enzymatic glucose sensors with Ag/AgCl and Pt reference electrodes have been fabricated and characterized for finding out an appropriate reference electrode. Optimum applied potential for the detection of glucose was determined as 0.4V. While the sensor with Pt reference electrode demonstrated larger response current in glucose solution than the other, it showed too much noises and bad repeatability. The sensor with Ag/AgCl reference electrode had excellent sensitivity, selectivity, repeatability, and reliability. In addition, it demonstrated good long-term stability. These results indicate that fabricated non-enzymatic glucose sensor with Ag/AgCl RE is promising for continuously monitoring system applications.
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ACKNOWLEDGMENT
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This work was supported by the IT R&D program of MIC/IITA [2005-S-093-03, Development of Implantable System Based on Biomedical Signal Processing]. The authors acknowledge MiNDaP group members for their technical supports and discussions.
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REFERENCES -12
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Time [sec] [2] Figure 7. Amperometric responses of fabricated non-enzymatic glucose sensor to the successive additions of 5 mM glucose solutions, 0.1 mM AA and 0.1 mM AP in PBS solution.
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