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Homogeneous Platinum-Deposited Screen-Printed Edge Band Ultramicroelectrodes for Amperometric Sensing of Carbon Monoxide Chih-Hung Chou, Jen-Lin Chang, Jyh-Myng Zen* Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan *e-mail: [email protected] Received: August 8, 2008 Accepted: September 30, 2008 Abstract We report here the fabrication and application of an electrochemical carbon monoxide (CO) gas sensor based on the deposition of Pt nanoparticles on a screen-printed edge band ultramicroelectrode (SPUME) with Nafion as the solid polymer electrolyte. Homogeneous size and distribution of Pt nanoparticles is stably deposited on the SPUME without either protective or capped agents. The edge diffusion effect at the SPUME, to even out the generation rate of hydrogen and to speed up the mass transfer of Pt solution, is believed to play a key role in achieving the deposition result. The obvious advantage of the proposed ultramicroelectrode system is that no supporting electrolyte (i.e., internal electrolyte) is required in the sensor scheme. The current – time curve recorded under conditions of þ 0.45 V vs. pseudo Ag reference electrode and various CO concentrations suggests that current response depends linearly on CO concentration up to 1000 ppm (correlation coefficient ¼ 0.994) with a sensitivity of 3.76 nA/(ppm · cm2). This report demonstrates potential application of the disposable CO gas sensor. Keywords: Carbon monoxide, Platinum nanoparticles, Ultramicroelectrode, Screen-printed electrode DOI: 10.1002/elan.200804376

Carbon monoxide (CO) is a deadly gas that human senses cannot detect because it is both colorless and odorless. It is produced by the incomplete combustion of fossil fuels, including natural gas, propane, kerosene, gasoline and heating oil. Potential CO sources include fireplace, woodstove, furnace, water heater, gas dryer or stove, vehicles, grill and space heater. It can inhibit the bloods ability to carry oxygen by combining with the oxygen carrying hemoglobin of the blood to form carboxyhemoglobin. Death is caused when the carboxyhemoglobin concentration reaches 50%. Different type and brand of CO detectors, including semiconductor, colorimetric, infrared and electrochemical sensors, are readily available commercially for industrial applications [1 – 3]. Nonetheless, besides a low price, such devices should have a low detection limit, high stability and be convenient to use. In this regard, electrochemical sensors hold advantages over other types of CO sensors in terms of their linear output, low power requirement, quick response, high sensitivity and capable of working at room temperature without a heater [4 – 8]. In this study, we report an electrochemical CO gas sensor based on the deposition of Pt nanoparticles on a screenprinted edge band ultramicroelectrode (SPUME) with Nafion as the solid polymer electrolyte. Platinum is employed as the working electrode since it is highly catalytic to CO oxidation and is chemically stable [9, 10]. Nafion thin film solid polymer electrolyte, on the other hand, is common used in gas sensor because of its hygroscopicity to conduct Electroanalysis 2009, 21, No. 2, 206 – 209

proton and to permit the permeation of the gases to the electrodes [11]. Previously, our group reported a simple method for fabricating disposable SPUME with a built-in three-electrode pattern of alternating printed layer of carbon, silver and insulator on a nonconducting polypropylene substrate [12, 13]. The edge of the carbon and/or metal-sandwiched films between the insulator layers can serve as a band type ultramicroelectrode. This study demonstrates that the edge diffusion effect [14] at ultramicroelectrode allows us to deposit homogeneous size and distribution of Pt nanoparticles without either protective or capped agents. Indeed, the most efficient strategy to control the extent of nucleation is to reduce the electroactive area of the substrate electrodes and electrodes with very small electroactive areas simplify the nucleation and growth mechanism involved in electrodeposition [15 – 18]. In this paper, we present results on the electrodeposition of Pt on the SPUME and its application as an amperometric CO gas sensor. The main advantage of this method over CO gas sensors already commercially available is that the in-built three-electrode configuration can be fabricated by a simple procedure. As illustrated in Figure 1, the establishment of a simple and quick method for the deposition of metal nanoparticles onto a SPUME surface also would contribute as a useful approach to sensor applications. Because it can be used for in situ measurement of CO and for continuous monitoring, it is suitable for environmental monitoring and control. In the future, the cheap SPUME can open a  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 1. A) Illustration of the electrodeposition of Pt nanoparticles at the SPUME. B) The structure of the SPUME assembly with a built-in three-electrode configuration (a), the SEM picture of Pt nanoparticles deposited on surface of carbon working electrode (b), and a cartoon representation of the electrode structure (c).

platform to varied applications by the deposition of diverse metal nanoparticles. The electrochemical behavior of Pt nanoparticles on the SPUME is first evaluated. To demonstrate the advantage of using an ultramicroelectrode for Pt deposition, the electrodeposition of Pt on an UME is studied. Figure 2 clearly shows the SEM graphs along with cyclic voltammograms of Pt nanoparticles deposited on the SPUME. As can be seen, the size of Pt nanoparticles (with 100  5 nm in diameter) is homogeneously deposited on the surface of SPUME. Note that the edge diffusion at the SPUME indicates hemicylindrical diffusion of the analyte on the carbon microband electrode/electrolyte interface. It is believed that the edge diffusion effect at the SPUME to even out the generation rate of hydrogen and to speed up the mass transfer of Pt solution should play a key role in achieving this deposition result. As reported earlier [19], the electrochemical reaction of the reduction of proton is sluggish at a carbon UME but is more rapid at Pt. In the presence of Pt on the electrode  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

surface and adequate potential in acidic medium, the protons were catalytically reduced to hydrogen. Proton reduction (i.e., hydrogen evolution) does not occur at a carbon electrode at potentials positive of  0.5 V vs. SHE (standard hydrogen electrode). For a carbon electrode covered with Pt particles or a pure Pt electrode, proton reduction gives rise to a steady-state current at potentials more negative than  0.3 V. Bards group also reported current amplification through the catalytic behavior of Pt nanoparticle for proton reduction at a carbon fiber electrode [20]. In addition to the increasing current, intense gas production could be observed with bubbles at the working electrode in Pt nanoparticle-modified SPE [21]. In other words, our observation of proton reduction only at the Pt nanoparticles deposited SPUME but not at a bare SPUME (carbon working electrode) is in consistent with the above studies. The H2 bubbles can then avoid the agglomeration between each Pt nanoparticle and the evolution rate of H2 is expected to affect the distribution of Pt nanoparticles.

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Fig. 2. Cyclic voltammetric responses with respective SEM pictures at a bare SPUME (a) and Pt nanoparticle-deposited SPUME (b) in pH 6, 0.1 M PBS at a scan rate of 20 mV/s.

With Nafion as solid polymer electrolyte to permit the permeation of CO gas and to retain humidity, the catalytic oxidation of CO at the NPt-SPUME is next evaluated. Figure 3 shows the gas phase cyclic voltammograms in the absence/presence of CO (47 – 1000 ppm) in N2 stream. Note that the obvious advantage of the proposed ultramicroelectrode system is that no supporting electrolyte (i.e., internal electrolyte) is required in the sensor scheme. As can be seen, the gas sensor developed has a good catalytic oxidative activity in the determination of CO. The catalytic activity is enhanced substantially because of the Pt nanoparticles deposited homogeneously on the electrode. An increasing catalytic oxidation peaks were obtained with increasing concentration of CO at ca. þ 0.45 V vs. pseudo Ag reference electrode. There was good linear correlation between CO oxidation current and CO concentration. The proper function of the proposed gas sensor is thus confirmed in this study. To further improve the sensitivity, amperometric determination of CO is studied. The response of CO at the NPtSPUME with the constant potential amperometry was measured based on the current before and after the CO injection to the air-captor. As shown in Figures 4A and B, at a detection potential of þ 0.45 V vs. pseudo Ag, the CO current responses are related to the CO concentration. It is apparent that the sensor responds rapidly to CO concentration changes. The linearity is up to 1000 ppm and the sensitivity of the sensor is 3.76 nA/(ppm cm2) of electrode area with a detection limit of 28.6 ppm (S/N ¼ 3). Most importantly, the current response can quickly and fully come back to the background level after purged with N2 and the response was reproducible (RSD ¼ 3.7%, n ¼ 6), as shown in Figure 4C. The reproducibility on signal/electrode variability was also checked by measuring 200 ppm CO with six different piece of NPt-SPUME and the response with RSD ¼ 5.7% (n ¼ 6) was acceptable. Of course, the use of Electroanalysis 2009, 21, No. 2, 206 – 209

Fig. 3. Cyclic voltammograms at the NPt-SPUME in a nitrogen stream containing CO at concentrations of a) 0, b) 200, c) 500, and d) 1000 ppm.

machine cutting (instead of manual cutting) can result in a better precision. We propose that the edge diffusion effect is the main factor for homogeneous electrodeposition of Pt nanoparticles on the SPUME. Furthermore, the hydrogen evolution induced by Pt nanoparticles can avoid extensive agglomeration. The function of both effects was verified from SEM graphs. A good linearity and sensitivity was attained by the proposed gas sensor. The establishment of a simple and quick method for fabricating sensor by screen-printing technology has the strong advantage to create a large number of near identical electrodes that can be used in a single shot context at a low cost. The obvious advantage of the ultramicroelectrode system is that no supporting electrolyte (i.e., internal electrolyte) is required in the sensor scheme. The establishment of gas sensor for CO detection can offer a new platform to diverse applications. Experimental Nafion (5 wt% in mixture of lower aliphatic alcohols and water, Aldrich), H2PtCl6 (Sigma), Na2HPO4, NaH2PO4 and nitric acid were of ACS certified reagent grade. Nitrogen and CO were obtained from Air Products and Chemicals (Taipei, Taiwan). Aqueous solutions were prepared with deionized water purified using Millipore-Q purification system. Carbon and silver inks were purchased from Acheson (Tokyo, Japan). The SPUME (20 mm in width and 0.18 mm in length of working electrode) in threeelectrode configuration, which consists of carbon working, silver quasireference and silver counter electrodes, was prepared as reported earlier [12]. The Pt nanoparticles were direct deposited on the SPUME at  0.6 V (vs. Ag/AgCl) for 190 s in 50 ppm H2PtCl6 aqueous solution. After electrodeposition of Pt nanoparticles, 3 mL Nafion solution was dipcoated on the SPUME and dried in air (designated as NPt-

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buffer) and finally connected to the glass bottle with electrochemical detector and a humidity meter. During the detection of CO, a constant flux of pure N2 (200 mL/min) with various flux of 1000 ppm CO was controlled by a fluid meter to obtain different concentration of CO. Acknowledgements The authors gratefully acknowledge financial support from the National Science Council of Taiwan. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References

Fig. 4. A) Current – time diagram recorded using the NPtSPUME in a nitrogen stream containing CO at concentration of a) 47, b) 111, c) 200, d) 500, e) 1000, and f) 0 ppm. B) Calibration curve. C) The repeatability of the current response of 200 ppm CO after purged with N2 for six measurements at the same piece of NPt-SPUME.

SPUME), as illustrated in Figure 1. All experiment conditions are at 25  3 8C and 60% relative humidity in N2 during the detection of CO. Electrochemical experiments were performed on a CHI832 electrochemical workstation (Austin, TX, USA). The Pt nanoparticles were characterized by scanning electron microscopy (SEM). The experimental device of air-captor for determination of CO was prepared as follow. Two glass bottles of 81 mL were parallel connected, then cascaded with the other glass bottles (as the gas mixing

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