Anal. Chem. 2002, 74, 6126-6130
Correspondence
Photoelectrochemical Oxygen Sensor Using Copper-Plated Screen-Printed Carbon Electrodes Jyh-Myng Zen,* Yue-Shian Song, Hsieh-Hsun Chung, Cheng-Teng Hsu, and Annamalai Senthil Kumar
Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan
We report here an efficient photocatalytic amperometric sensor for the determination of dissolved oxygen (DO) in phosphate buffer solution using a disposable copperplated screen-printed carbon electrode (CuSPE). The photoelectrochemical activity toward DO of the CuSPE was related to the formation of a p-type semiconductor CuI2O. The solution pH and biased potential (Ebias) were systematically optimized as pH 8 PBS and -0.7 V vs Ag/ AgCl, respectively. Under optimized conditions, the calibration plot was linear in the range of 1-8 ppm with sensitivity and regression coefficient of 23.51 (µA cm2)-1 ppm-1 and 0.9982, respectively. The reproducibility of the system was good with seven successive measurements of DO yielding a RSD value of 1.87%. Real sample assays for groundwater and tap water were also consistent with those measured by a commercial DO meter. The principle used in DO measurement has an opportunity to extend into various research fields. Dissolved oxygen (DO) measurement is important in the fields of biochemical, fermentation control, food production and storage, environmental monitoring, and industrial applications.1-5 Among the various detection routes, electrochemical methods have received much attention in numerous applications since the introduction of the Clark sensor.6 Various detection systems based on oxide catalysts and macrocylic complex (metal-porphyrins or metal-phthalocyanines)-oriented mediators and organic and inorganic dye-based photochemical-quenching systems were reported for oxygen sensing.7-12 Nevertheless, most of the approaches are relatively expensive or have unsatisfactory elec* To whom correspondence should be addressed. Phone: (+886) 4-22854007. Fax: (+886) 4-2286-2547. E-mail:
[email protected]. (1) Chaudhury, R. R.; Sorbrinho, J. A. H.; Wright, R. M.; Sreenivas, M. Water Res. 1998, 32, 2400-2412; Abstr. 5565329. (2) Ohashi, M.; Osamu, A. Biotechnol. Adv. 1997, 15, 463. (3) Hua, Q.; Shimizu, K. J. Biotechnology 1999, 68, 135-147. (4) Culberson, S. D.; Peidrahita, R. H. Ecol. Modell. 1996, 89, 231-258. (5) Numata, M.; Funazaki, N.; Ito, S.; Asano, Y.; Yano, Y. Talanta 1996, 43, 2053-2059. (6) Clark, L. C. Electrochemical Device for Chemical Analysis. U.S. Patent 2,913,386, 17 November, 1959. (7) Zen, J.-M.; Wang, C.-B. J. Electroanal. Chem. 1994, 368, 251-256 and references therein. (8) Oritz, J.; Gautierm, J. J. Electroanal. Chem. 1995, 391, 111-118. (9) D’Souza, F.; Hsieh, Y.-Y.; Deviprasad, G. R. J. Electroanal. Chem. 1997, 426, 17-21.
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trode modification procedures not to mention the problem associated with extension into practical application under physiological conditions. The instability of the mediator-based modified electrodes in the working matrixes can easily cause error in practical measurements. Even the Clark sensor design needs further improvement in performance to meet the new requirement in DO measurement.13,14 Recently, we noticed a profound electrochemical activity toward the oxygen reduction reaction in the detection assays of H2O2 at a copper-plated screen-printed carbon electrode (designated as CuSPE).15-17 Because of the very insoluble behavior of copper ions with phosphate (Ksp ) 1.39 × 10-37),17,18 the CuSPE was found to be stable and free from electrode fouling. This is indeed a highly desirable condition for application in phosphate buffer at physiological pH.18,19 The specific oxide redox couples of CuI2O/CuIIO (C1/A1) and Cu0/CuI2O (C2/A2) are responsible for effectively mediating the current signals.15-18 It is well known that CuI2O is a p-type semiconductor with an Eg value of 1.9 eV and has good photochemical behavior.20-23 In this work, the CuSPE is utilized for the photoelectrochemical determination of DO, which offers a simple route for the quantitative detection of DO. To our knowledge, so far there is no report for the photochemical quantitative assays of DO using a semiconducting system. (10) Wijnoltz, A. L. B.; Visscher, W.; van Veen, J. A. R. Electrochim. Acta 1998, 43, 3141-3152. (11) Lalande, G.; Cote, R.; Tamilzhmani, G.; Guay, D.; Dobelet, J. P.; DignaruBailey, L.; Wengs, L. T.; Bertrands, P. Electrochim. Acta 1995, 19, 26352646. (12) Gobi, K. V.; Ramaraj, R. J. Electroanal. Chem. 1998, 449, 81-89. (13) Nei, L.; Compton, R. G. Sens. Actuators, B 1996, 30, 83-87 and references therein. (14) Sohn, B.-K.; Kim, C.-S. Sens. Actuators, B 1996, 34, 435-440. (15) Zen, J.-M.; Chung, H.-H.; Senthil Kumar, A. Analyst 2000, 125, 16331637. (16) Zen, J.-M.; Chung, H.-H.; Senthil Kumar, A. Anal. Sci. 2001, 17, i287i290. (17) Senthil Kumar, A.; Zen, J.-M. Electroanalysis 2002, 14, 671-678. (18) Zen, J.-M.; Chung, H.-H.; Senthil Kumar, A. Anal. Chem. 2002, 74, 12021206. (19) CRC Handbook of Chemistry and Physics, 72 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1991. (20) De Jongh, P. E.; Vamaekelbergh, D.; Kelly, J. J. Chem. Commun. 1999, 1069-1067. (21) Tennakone, K.; Kumarsinghe, A. R.; Sirimanne, P. M. J. Photochem. Photobiol. A 1995, 88, 39-41. (22) Richardson, T. J.; Slack, J. L.; Rubin, M. D. Electrochim. Acta 2001, 46, 2281-2284. (23) Nair, M. T. S.; Guerrero, L.; Arenas, O. L.; Nair, P. K. Appl. Surf. Sci. 1999, 150, 143-151. 10.1021/ac020058r CCC: $22.00
© 2002 American Chemical Society Published on Web 10/29/2002
Preparation of the electrode is simple and low cost and thus allows for mass production as disposable electrodes.15-18 Systematic investigation and discussion regarding solution pH and biased potential (Ebias) were carried out to optimize the photocurrent produced. Finally, real sample application was demonstrated for the measurement of DO in water samples, and satisfactory results were obtained in comparison to the values measured by a commercial Clark-type electrode (i.e., DO meter). EXPERIMENTAL SECTION Chemical and Reagents. All chemicals were obtained from Merck (Darmstadt, Germany) in analytical grade. A 1000 ppm Cu(II) solution in 0.1 M nitric acid was used for the platting experiments. The other standard solutions used in interference studies were also obtained from Merck. Unless otherwise stated, a pH 8 phosphate buffer solution (PBS) of ionic strength of 0.1 M was used in all experiments. Working solutions were prepared using double-filtered deionized water. Real water samples were collected from the campus of Chung-Hsing University, Taiwan. Apparatus. Voltammetric and photoelectrochemical measurements were carried out with a CHI model 660 electrochemical workstation (Austin, TX). The three-electrode system consisted of the CuSPE working electrode, an Ag/AgCl reference electrode, and a platinum auxiliary electrode. In situ photochemical experiments were performed using a BAS working cell in combination with a 250-W light source of an overhead projector. All instruments were turned on 10 min before start of the experiments to attain equilibrium with environment. Analytical grade O2/N2 gas was used for the oxygenation/deaeration of the working solution, respectively. For the preparation of a saturated oxygen solution, O2 gas was continuously purged for at least for 30 min and immediately transferred to the working system. Design and Fabrication of the CuSPE. A semiautomatic screen printer was used to prepare disposable SPE as per our earlier report.15 The SPE has a working area of 0.13 cm2 with average film resistance of 85.64 ( 2.10 Ω/cm. As to the preparation of the CuSPE, a Cu layer was electrochemically plated on SPE in 200 mg/L Cu(NO3)2 aqueous solution at -0.7 V versus Ag/AgCl for 300 s. Procedure. The CuSPE was first washed thoroughly with deionized water and then dipped into working solution for subsequence electrochemical and photoelectrochemical experiments. Before the experiments, the CuSPE was pretreated in its base electrolyte in the potential region of -0.8 to +0.5 V at a scan rate of 20 mV/s for five continuous runs. The photocurrent experiments were performed by chronoamperometric (CA) technique. The CuSPE was equilibrated in the blank buffer at an optimized potential of -0.7 V until the current become constant. Normally it took ∼500 s. The quantification of oxygen was achieved by measuring the reduction photocurrent from the CA signals. All experiments were performed at room temperature (25 °C). RESULTS AND DISCUSSION Photoelectrochemical Behavior of DO at the CuSPE. The photoelectrochemical response of the CuSPE at Ebias ) -0.7 V in pH 8 PBS with various amounts of DO under light irradiation is shown in Figure 1. As can be seen, the photocurrents observed are proportional to the amount of DO. Our previous study reported
Figure 1. Photoelectrochemical response of the CuSPE in (a) N2saturated, (b) normal air, and (c) O2-saturated pH 8 PBS with a biased potential (Ebias) of -0.7 V vs Ag/AgCl.
Figure 2. Cyclic voltammetric response of the CuSPE in (a) N2saturated, (b) normal air, and (c) O2-saturated pH 8 PBS at v ) 5 mV/s.
the CuSPE for the determination of H2O2 by flow injection analysis (FIA) at ambient temperature without deoxygenation.15-17 The cyclic voltammogram of the CuSPE in pH 7.4 PBS showed the growth of CuIIO and CuI2O. A well-defined reduction signal corresponding to the mediations of CuIIO and CuI2O appeared in the presence of H2O2. The mechanistic study revealed that the reduction is a coupled chemical reaction mechanism with the operation of pseudo-first-order kinetics on the concentration of Cu2O.15 In this study, cyclic voltammograms, as shown in Figure 2, give evidence for the effect of DO and H2O2 on the CuSPE. For deaerated pH 8 PBS, an anodic shoulder at -0.12 V and a couple of distinct peaks at -0.2 (C1) and -0.3 V (C2) were observed at the CuSPE with a scan rate of 5 mV/s. Similar voltammetric behavior was observed for pH 8 PBS in normal air except with an increase in cathodic current at C1 and C2. A purge with oxygen to pH 8 PBS causes a further increase in current response, indicating the effective DO response on the CuSPE. The increase in current responses at C1 and C2 are due to the electron mediation by the specific redox reactions of CuI2O/CuIIO and Cu0/CuI2O, respectively.15 Meanwhile, the increase in current Analytical Chemistry, Vol. 74, No. 23, December 1, 2002
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Figure 3. (A) Photocurrent density (PCD) and anodic peak area against Ebias. (B) Cyclic voltammetrograms of starting light on at different potentials for anodic scan direction (v ) 50 mV/s). Inset: PCD against anodic peak area.
due to oxygen reduction reaction at -0.6 V again clearly indicates the increasing amount of DO in solution purging with oxygen. To understand the photoelectrochemical behavior, the effect of Ebias (-0.3 to -0.9 V) on the photocurrent response of DO at the CuSPE was first studied. Similar to the experiments in Figure 1, the results obtained by individual photoelectrochemical chronoamperometric measurements are given in Figure 3A. As can be seen, the photocurrent signal starts to occur at -0.3 V, increases regularly with the increase in Ebias negatively, and eventually gets saturated around -0.7 V. Meanwhile, it is interesting to note that by turning the light on at different Ebias at a constant anodic scan rate of 50 mV/s, the anodic peak generated on the CuSPE was also found to increase as the light-on potential was
