A Highly Miniaturized Dissolved Oxygen Sensor Using a Nanoporous Platinum Electrode Electroplated on Silicon

Journal of the Korean Physical Society, Vol. 58, No. 5, May 2011, pp. 1505 1510 A Highly Miniaturized Dissolved Oxygen Sensor Using a Nanoporous Platinum Electrode Electroplated on Silicon Yi Jae Lee and Jae Yeong Park Department of Electronic Engineering, Kwangwoon University, Seoul 139-701, Korea (Received 11 July 2010, in final form 19 January 2011) A highly sensitive and miniaturized dissolved oxygen (DO) sensor with a nanoporous platinum (Pt) electrode on a silicon substrate was recently developed and characterized for use in environmental monitoring and bio/medical sensor applications. The proposed DO sensor was designed with four different layers: the electrodes (a nanoporous Pt working electrode, a plane Pt counter electrode, and a Ag/AgCl reference electrode), a poly(dimethylsiloxane) (PDMS) passivation & electrolyte reservoir, a PDMS film used as a gas permeable membrane, and a PDMS chamber for storing the DO sample. The fabricated DO sensor has a compact size and a volume of 15 mm 8 mm 8.05 mm. The utilized nanoporous Pt working electrode of the DO sensor showed a much larger roughness factor (RF), at 313, than the plane Pt electrode did. At various oxygen concentrations, the fabricated sensor exhibited distinctive response currents, a high oxygen sensitivity of 102.78 na/ppm, a fast response time of less than 100 sec at -0.8 V vs. Ag/AgCl, and a good reproducibility at room temperature and pressure. We confirmed these excellent results were caused by the highly-roughened catalytic nanoporous Pt electrode. PACS numbers: 81.15.Pq, 82.80.Fk, 83.85.Gk Keywords: Dissolved oxygen, Roughness factor, Catalytic, Nanoporous Pt, Environmental monitoring, Bio/medical devices DOI: 10.3938/jkps.58.1505 I. INTRODUCTION Dissolved oxygen (DO) in physiological and environmental systems is an indication of physical, chemical, and biochemical activity. Thus, the evaluation of the amount of DO in aqueous solutions is an essential monitoring task, and its scope ranges from measurements of natural and industrial waters to medical applications, such as the oxygen content in blood and tissues [1,2]. There are two methods commonly used to measure the DO. One is to use iodine titration; the other is to use an oxygen electrode. The iodine titration method is purely chemical, and needs complex chemistry and takes a long time to get results. In addition, since it is easily disturbed, it needs complex and expensive equipment and cannot be applied to real-time in-place measurements. The oxygen electrode method is an electrochemical approach [3]. It provides several benefits in the measurement of the DO concentration, such as in situ continuous measurement. However, since the oxygen electrodes currently used for this type of sensor are not all-solid-state devices, their volume is relatively large. In addition, the oxygen electrode s electrolyte and gas-permeable membrane need to be replaced frequently. Therefore, many researchers have E-mail: jaepark@kw.ac.kr; Fax: +82-2-942-1502 -1505- attempted to miniaturize the oxygen electrode by means of micromachining technology. However, they have not succeeded in making a working mechanism for the oxygen electrode and the membrane techniques. For these reasons, much research is currently being performed using various measuring methods, such as titration, colorimetry, fluorescence, chemi-luminescence, potentiometry, amperometry, and voltammetry [4 10]. Among these diversified fields, an electrochemical sensor, such as the Clark electrode, is the most popular type of sensor for analyzing DO in water or fluids. However, there are several disadvantages, such as low sensitivity, miniaturization limitations, and an increasing instability over its lifetime. In this study, a highly miniaturized and sensitive sensor with a nanoporous Pt electrode used for detecting DO in water or other fluids has been newly developed and characterized. The excellent catalytic characteristic of the nanoporous Pt electrode was reported in our previous work [11]. The proposed DO sensor has been designed and fabricated using a nanoporous Pt working electrode, a poly(dimethylsiloxane) (PDMS) reservoir, and a PDMS film as a gas permeable membrane on a silicon substrate by using electroplating and polymeric packaging techniques. The fabricated sensor is first characterized in a sulfuric acid solution in order to check its

-1506- Journal of the Korean Physical Society, Vol. 58, No. 5, May 2011 Fig. 2. (Color online) Conceptual drawing of the miniaturized DO sensor with the nanoporous Pt working electrode. (RE: Ag/AgCl; CE: plane Pt) Fig. 1. (Color online) Schematic illustration for the net reaction mechanism of the proposed DO sensor with a nanoporous Pt working electrode. enlarged surface activation area, and is then evaluated at various dissolved oxygen concentrations. II. FABRICATION 1. Reagents and Instrumentation All the chemicals used were of analytical reagent grade. All the solutions were prepared with deionized water (resistivity 18 MΩ-cm). The electroplating mixture for the nanoporous Pt was comprised of 42% (w/w) C 16 EO 8 (octaethylene glycol monohexadecyl ether, 98% purity, Fluka), 29% (w/w) deionized water (18 MΩ-cm) and 29% (w/w) HCPA (hexachloroplatinic acid hydrate, 99.9% purity, Aldrich) [11]. The surface roughness of the fabricated electrode was measured in a 1 M sulfuric acid (H 2 SO 4, 95-98%, ACS, Sigma-Aldrich) solution by using cyclic voltammograms. The 1 M H 2 SO 4 solution and the 0.1 M KCl (potassium chloride, 99%, Sigma) used as an electrolyte were prepared by dilution in deionized water. The dissolved oxygen concentration was varied by adding Na 2 SO 3 (anhydrous sodium sulfite, YAKURI Pure chemicals, Japan), and the concentration was simultaneously measured with an oxygen meter (DO-24P, TOA-DKK Co., Japan). The electrochemical experiments were performed on the fabricated sensor at room temperature with the use of an electrochemical analyzer (Model 600B series, CH Instruments Inc., USA) at room temperature and pressure. 2. Design and Fabrication of the DO sensor Figure 1 shows a schematic illustration for the reaction mechanism of a DO sensor. A Clark-type DO sensor consists of electrodes, electrolyte, an oxygen gas per- meable film, and a reservoir for the electrochemical detection, the selective oxygen permeation, and the storing of a sample, respectively. The working mechanism of this sensor is the measurement of the oxygen reduction current that permeates through a membrane from a molecule-containing solution to an electrolyte at the working electrode. The reduction of the oxygen at the cathode is a two-step process. As shown in Fig. 1, the working electrode reactions are summarized by; O 2 + 2H + + 2e H 2 O 2 (1) H 2 O 2 + 2H + + 2e 2H 2 O (2) The reaction occurring at the anode, also known as the counter electrode, converts the products from the cathode back into reactants. This is summarized by the equation [12]. 2H 2 O O 2 + 4H + + 4e (3) Figure 2 shows a schematic drawing of the miniaturized DO sensor with the nanoporous Pt working electrode. The DO sensor is separated into four parts, which are the reservoir (9 mm 4 mm 5 mm, PDMS) for the dissolved oxygen sample, the gas-permeable membrane (50 µm thick PDMS), the electrolyte reservoir (9 mm 4 mm 2.5 mm, PDMS), and the electrodes (the working electrode (nanoporous Pt), the counter electrode (plane Pt), and the reference electrode (Ag/AgCl)) layers. Figure 3 shows the fabrication sequence of the miniaturized DO sensor with the nanoporous Pt electrode on the silicon substrate (upper) and a photograph of the fabricated DO sensor (lower). The nanoporous Pt was fabricated by the use of a nonionic surfactant and an electroplating technique. The detailed fabrication sequence of the nanoporous Pt electrode was discussed in our previous work [11]. In the fabrication steps shown in Fig. 3, the PDMS passivation & the electrolyte layer and the chamber for storing the sample solution were fabricated by the use of Teflon molds. In addition, the PDMS film need as a gas-permeable membrane was fabricated by spin coating on a dummy Si wafer. The prepared electrode with the nanoporous Pt, the PDMS passivation & the electrolyte reservoir layer, the PDMS gas permeable film, and the PDMS sample chamber layer were bonded

A Highly Miniaturized Dissolved Oxygen Sensor Using a Nanoporous Yi Jae Lee and Jae Yeong Park -1507- Fig. 4. (Color online) Comparison of the cyclic voltammograms of the nanoporous Pt and the plane Pt working electrodes in a 1 M sulfuric acid solution to check their electrochemical roughness factors with a scan rate of 200 mvs 1. Fig. 3. (Color online) Fabrication sequence of the miniaturized DO sensor with the nanoporous Pt electrode (upper) and a photograph of the fabricated DO sensor (lower). using silicon adhesive (Silicon RTV composite, Shin-Etsu Chemical Co., Ltd). III. EXPERIMENTAL RESULTS AND DISCUSSION Figure 4 shows a comparison of the cyclic voltammograms of the fabricated DO sensors with the nanoporous Pt and plane Pt working electrodes in a 1 M sulfuric acid solution for checking their electrochemical roughness factors. The cyclic voltammogram was obtained at a scan rate of 200 mvs 1, and the potential ranged from -0.4 to 1.2 V vs. Ag/AgCl. The roughness factor (RF) can be calculated as follows [13]: f r = A r /A g, (4) where A r is the real (true, actual) surface (interface) area and A g is the geometric surface (interface) area. The determination of the RF of the Pt electrode is based on the formation of a hydrogen monolayer electrochemically adsorbed at the electrode surface [14]. In Fig. 4, the first cathodic peak is the reduction of oxygen, which is quickly followed by a second dual peak related to the hydrogen reduction (shadow area). Since charge is necessary to form a monolayer of adsorbed hydrogen and the electrode area is covered by hydrogen atoms, the electrochemical RF is easily calculated. The RF of nanoporous Pt is the value of the adsorption charge divided by 0.21 mc cm 2 [15]. The calculated RF was 313, which is a highly improved value over that of the plane Pt electrode. The surface morphological analysis of the nanoporous Pt was reported in our previous studies [11, 16]. The main characteristics of such nanoporous Pt electrodes in the electroanalytical domain are their high surface/volume ratio, thereby favoring their interaction with external reagents and providing excellent conductivity and interconnectivity between the pores, which makes them highly attractive for miniaturized electrochemical devices. Furthermore, the much enhanced electro-oxidation current on the porous Pt electrode is influenced by the porous structures because the porous structure can enhance the mass transport of molecules to the active electrode sites. If they are electrically conducting, such materials deposited as thin films on electrode surfaces can contribute to increasing the electro-active surface area by several orders of magnitude and, hence, to the sensitivity of the resulting device. The electrochemical behavior of the oxygen reduction in the fabricated DO sensor was investigated. Figure 5 shows the cyclic voltammograms of the oxygen reduction occurring in the DO sensor with an electrolyte solution of 0.1 M KCl at the full-oxygen and the zero-oxygen state. The cyclic voltammograms were obtained at scan rate of 50 mvs 1 ; the potential ranged from 0 to -0.9 V vs. Ag/AgCl. The limiting current for the oxygen reduction was observed to lie in the potential region of -0.7 to -0.9 V, indicating that the reduction current measured in the region can be used to estimate the oxygen concentration. Figure 5 shows the cyclic voltammograms for the oxygen

-1508- Journal of the Korean Physical Society, Vol. 58, No. 5, May 2011 Fig. 5. (Color online) Cyclic voltammograms of the miniaturized DO sensor with the nanoporous Pt working electrode at the full-oxygen and the zero-oxygen states with a scan rate of 50 mvs 1. Fig. 6. (Color online) Cyclic voltammogram of the miniaturized DO sensor with the nanoporous Pt working electrode at different oxygen concentrations with a scan rate of 50 mvs 1. in the sensor with or without the Na 2 SO 3 solution in the membrane separated chamber (zero-oxygen). Since the Na 2 SO 3 solution in the chamber effectively removed the oxygen in the measurement space, little reduction current for the oxygen was observed because of the oxygen remaining in the electrolyte reservoir. On the other hand, a well-defined current for the oxygen was seen in the air-filled chamber (full-oxygen). Cyclic voltammetry of the miniaturized DO sensor with the nanoporous Pt working electrode was performed using different oxygen concentrations (1.33, 2.88, 4.3, and 6 ppm). The cyclic voltammogram was obtained at a scan rate of 50 mvs 1, and the potential ranged from 0 to -0.9 V vs. Ag/AgCl. As shown in Fig. 6, the fabricated sensor showed distinctive response currents correspond- Fig. 7. (Color online) Amperometric responses of the miniaturized DO sensor with a nanoporous Pt working electrode at different oxygen concentrations (a) and comparison of its calibration curve with that of a plane Pt electrode (b). The applied potential was -0.8 V vs. Ag/AgCl at room temperature. ing to the different oxygen concentrations at the potential of -0.8 V. The oxygen reduction current increased as the oxygen concentration in the PDMS chamber for storing DO solution increased. Therefore, we selected -0.8 V as an applied potential for the subsequent amperometric detection. Figure 7 shows a typical response current curve for the fabricated DO sensor with the nanoporous Pt electrode when the oxygen concentration was varied (left). In Fig. 7(a), water with various oxygen concentrations were injected into the sample storing chamber every 100 sec. Afterwards, the saturation currents were recorded. As shown in Fig. 7(a), the response currents were distinguished by their different oxygen concentrations. Their relatively large background currents might be caused by the enlarged surface activation area of the nanoporous Pt electrode. The 90% response time of the miniaturized DO sensor was less than 100 sec. This might

A Highly Miniaturized Dissolved Oxygen Sensor Using a Nanoporous Yi Jae Lee and Jae Yeong Park -1509- caused by the over-accumulation of hydrogen peroxide on the working electrode, as well as by the crosstalk between the byproducts from the counter electrode during continuous operation [18]. IV. CONCLUSION Fig. 8. (Color online) Reproducibility of the miniaturized DO sensor. The response was consecutively measured with the periodic injection and suction of the Na 2SO 3 solution from the membrane-separated chamber at -0.8 V vs. Ag/AgCl with a PDMS oxygen-permeable membrane. be caused by the highly catalytic characteristic of the nanoporous Pt electrode. The nanoporous Pt electrode allows not only a fast mass transport of ions through the electrolyte/electrode interface but also rapid electrochemical reactions because it has a short diffusion length and a considerable active surface activation area [17]. The calibration curve of the DO sensor for various oxygen concentrations is shown in Fig. 7(b). The miniaturized DO sensor with a nanoporous Pt working electrode exhibited an oxygen sensitivity of 102.78 na/ppm and a correlation coefficient of 0.957, which was much improved compared to that of the DO sensor with a plane Pt working electrode (18 na/ppm). A Na 2 SO 3 solution with zero-oxygen concentration was employed to repetitively test the reproducibility (- 0.8 V versus Ag/AgCl). This characteristic of the sensor is closely related to the potential stability of the reference electrode and the crosstalk effect between the three electrodes of the electrochemical detector. Figure 8 shows the response to periodic injection and suction of the Na 2 SO 3 solution from the membrane-separated chamber of the miniature DO sensor. Although the current baseline shifted slightly, the current response showed excellent reproducibility. In the full-oxygen state (without Na 2 SO 3 ) of the PDMS chamber for storing the DO solution, the mean current was 7.27 µa, and the standard deviation was 0.24 µa. In the zero-oxygen state with Na 2 SO 3 solution, the mean value was 5.67 µa, and the standard deviation was 0.24 µa. The baseline fluctuation might be attributed to a change in the surface status of the working electrode, such as hydrogen adsorption, and to a change in the potential shift of the reference electrode. The difference in the relative responses from the first response gradually increased. This result might be In this research, we fabricated and characterized a miniaturized DO sensor with a highly roughened and catalytic nanoporous Pt working electrode on a silicon substrate. Simple and low-temperature fabrication processes can be easily reproduced and integrated with other semiconductor elements and polymer structures in an ordinary laboratory. The DO sensor was comprised of four different layers: catalytic electrodes, a PDMS chamber for storing the sample solution, a gas permeable membrane, and a passivation & electrolyte reservoir layer. The fabricated DO sensor had an extremely small size and volume. The utilized nanoporous Pt electrode used as a working electrode exhibited an extremely high roughness factor, about 300 times better compared to the plane Pt electrode. The miniaturized DO sensor with the nanoporous Pt working electrode exhibited a high sensitivity, a fast response time at an applied potential of -0.8 V vs. Ag/AgCl, and a good reproducibility at room temperature and pressure. These results indicate that the highly miniaturized DO sensor with the nanoporous Pt working electrode is very promising for use in environmental monitoring and bio/medical sensor applications. ACKNOWLEDGMENTS This research was supported by the Seoul Research and Business Development Program (Grant No. 10583). The authors acknowledge MiNDaP (Micro/Nano Devices and Packaging Lab.) group members for their technical support and discussions. REFERENCES [1] X. Na, W. Niu, H. Li and J. Xie, Sens. Actuators, B 87, 222 (2002). [2] T. C. Chou, K. M. Ng and S. H. Wang, Sens. Actuators, B 66, 184 (2000). [3] H. Suzuki, H. Ozawa, S. Sasaki and I. Karube, Sens. Actuators, B 53, 140 (1998). [4] N. J. Bunce, Environmental Chemistry (Wure, Canada, 1991). [5] Y. Suzuki, H. Kishide and E. Tsuchida, Macromolecules 33, 2530 (2000). [6] R. T. Bailey, F. R. Cruickshank, G. Deans, R. N. Gillanders and M. C. Tedford, Anal. Chim. Acta 487, 101 (2003).

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