Anal. Chem. 2004, 76, 4579-4582
Technical Notes
Smart Polyoxometalate-Based Nitrogen Monoxide Sensors Shaoqin Liu,† Dirk Volkmer,‡ and Dirk G. Kurth*,†
Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany, and Department of Inorganic Chemistry 1, University of Bielefeld, P.O. Box 100 131, D-33501 Bielefeld, Germany
An electrochemical sensor design for selective NO detection is presented based on a polyoxometalate (POM) cluster immobilized on an electrode through a polyelectrolyte matrix. It is suggested that the POM can electrocatalyze the reduction of NO. The reduction current is proportional to the NO concentration in the investigated concentration window ranging from 1 nM to 10 µM. The sensitivity of the device can be adjusted by the number of immobilized layers. The response to possible interfering reagents such as nitrate and nitrite can be controlled through the multilayer design. By a predominant negatively charged outer surface, the response to these ions is markedly reduced. Monitoring the concentration of nitrogen monoxide is an important aspect of industrial processing and pollution control. Moreover, it is now established that NO plays a critical role in biochemical processes.1,2 To develop an understanding for the unique biochemical and physicochemical significance of this “simple” molecule in biological systems, further research is needed for highly selective (bio)chemical materials with specific interaction with NO, as well as designing reliable, sensitive, and selective, as well as low-cost sensors, which permit subsequent recognition and in situ real-time analysis. The exertion of physiological or pathophysiological effects of NO in a biological system involves the ready and rapid coordiantion of NO with the metal center of metalloenzymes such as ironporphyrin-based cytochrome P-450, cytochrome oxidase, and nitrite hydratase, and therefore, the selective interaction between NO and transition metal compounds is often used for fabricating (electro)chemically based sensors for in situ NO detection.3,4 On the other hand, polyoxometalate clusters (POMs) and especially their transition-metal-substituted derivatives exhibit a remarkably * Corresponding author. Tel.: +49-(0)331-567 92 11. Fax: +49-(0)331-567 92 02. E-mail:
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ University of Bielefeld. (1) (a) Nathan, C. FASEB J. 1992, 6, 3051-3064. (b) Lewis, R. S.; Tamir, S.; Tannenbaum, S. R.; Deen, W. M. J. Biol. Chem. 1995, 270, 29350-29355. (2) Palmer, R. M. J.; Ferrige, A. G.; Moncada, S. Nature 1987, 327, 524-526. (3) Wu, D.; Ashkenasy, G.; Shvarts, D.; Ussyshkia, R.; Maaman, R.; Shanzer, A.; Cahen, D. Angew. Chem., Int. Ed. 2000, 39, 4496. (4) Malinski, T.; Taha, Z. Nature 1992, 358, 676-678. 10.1021/ac0495283 CCC: $27.50 Published on Web 06/17/2004
© 2004 American Chemical Society
rich redox chemistry, with stable redox states, and multiple electron-transfer steps.5 The properties of these uniquely welldefined transition metal polyoxoanions can be controlled and finetuned through the heteroions and addenda ions that are incorporated into the structural framework and are, therefore, attractive candidates for selective, long-lived, and tunable redox-active devices.6,7 In this context, we report on POM-based electrodes suitable for selective and sensitive detection of NO. Herein, we have chosen a cobalt(II)-substituted sandwich complex, [CoII4(H2O)2P4W30O112]16- (Co-POM, Figure 1A), derived from the well-known Wells-Dawson ion R-[P2W18O62].6-8 Like many other POMs, Co-POM is a reversible catalyst, capable of undergoing repeated cycles of reduction and reoxidation.9 The electrochemical potentials for the two two-electron reduction of CoPOM at pH 5.0 in aqueous solution (-0.53 and -0.705 V versus Ag/AgCl electrode) lie below that of NO reduction, thus making this particular cluster a attractive candidate for electrocatalytic NO detection. Due to the negative charge of the polyoxoanion, the preparation of the sensor is based on the electrostatic layer-by-layer selfassembly (ELSA) technique introduced by Decher,10 which is based on alternating adsorption of Co-POM anions and oppositely charged polyelectrolytes (PEs). The simplicity, economy, and modularity of film fabrication by ELSA have contributed to the widespread popularity of this method.11 The resulting multilayers show controlled permeability toward redox-active ions and molecules,12,13 a property required for separation and sensing technologies. Our previous studies on these composite multilayers have demonstrated that the electrochemical response can be (5) (a) Pope, M. T. In Mixed Valence Compounds; Brown, D. B., Ed.; D. Reidel: Dordrecht, 1980; p 365. (b) Pope, M. T. Prog. Inorg. Chem. 1991, 39, 181-257. (6) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 219-237. (7) (a) Neumann, R.; Dahan, M. Nature 1997, 388, 353-355. (b) Weinstock, I. A.; Barbuzzl, E. M. G.; Wemple, M. W.; Cowan, J. J.; Reiner, R. S.; Sonnen, D. M.; Heintz, R.; Bond, J. S.; Hill, C. L. Nature 2001, 414, 191-195. (8) Na16[CoII4(H2O)2P4W30O112] was prepared according to literature procedures: Finke, R. G.; Droege, M. W. Inorg. Chem. 1983, 22, 1006-1008. (9) Liu, S.; Kurth, D. G.; Volkmer, D. Chem. Commun. 2002, 976-977. (10) Decher, G. Science 1997, 277, 1232-1237. (11) (a) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430-442. (b) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32-39. (c) Knoll, W. Curr. Opin. Colloid Interface Sci. 1996, 1, 137-143.
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Figure 1. (A) Schematic representation of POM-based NO sensor. An ultrathin coating of a polyelectrolyte and a redox-active polyoxometalate (Co-POM) are used to sensitize an electrode surface toward electrocatalytic reduction of NO from solution. The selectivity toward NO is achieved by tailoring the permeability of the coating through the architecture of the topmost layers. The Co-POM cluster consists of four Co centers coordinated through two trivacent Dawson fragments. (B) CVs of a ITO electrode modified with (PSS/PAH/Co-POM/ PAH)3 at various concentrations of NO: 0, 40, 80, 120, 160, and 200 nM (from top to bottom). Inset: calibration curve corresponding to the electrochemical sensing of NO by the analysis of the second reduction peak of Co-POM anions. (All curves are recorded under Ar atmosphere in 0.2 M buffer solution, pH 7.0, scan rate 10 mV/s).
tailored through the layer architecture.9 It, therefore, becomes clear that the combination of POMs as redox-active and ELSAbased multilayers as structural components show great promise as electrochemical devices with tailored electrocatalytic properties and permselectivities. EXPERIMENTAL SECTION Fabrication of Ultrathin Film of Co-POM/PE. The indium tin oxide (ITO)-coated glass or quartz slides (on one side, ∼6 Ω‚m, 0.1256 cm2) were cleaned by a 15-min ultrasonic bath in ethanol, acetone, and pure water. The clean slides were functionalized with (3-aminopropyl)triethoxysilane according to literature procedures.14 Next, a layer pair of poly(styrenesulfonate) (PSS) (MW 70 000, Aldrich) and poly(allylamine hydrochloride) (PAH) (MW 15 000, Aldrich) were adsorbed on the amino-functionalized substrate. These polyelectrolyte layers were deposited from 10-3 mol/L aqueous solutions (pH 5-6), using an immersion time of 10 min, followed by rinsing with deionized water and drying after each second layer. Then, repeated layers of Co-POM (c ) 5 × (12) (a) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006-2013. (b) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Eradt, J.; Bruening, M. L. Langmuir 2001, 17, 931-937. (c) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192. (13) Liu, S.; Kurth, D. G.; Bredenkotter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279-12287. (14) Kurth, D. G.; Bein T. Langmuir 1995, 11, 3061-3067.
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10-4 mol/L in water with pH 5-6) and PAH were deposited on the surface using an immersion time of 10 min. Deposition of a final capping layer of PAH yields multilayers of the form PSS/ PAH(Co-POM/PAH)m and (PSS/PAH/Co-POM/PAH)m. The ionic strength of the solutions was adjusted with NaCl as mentioned in the text. NO gas was prepared by adding 5 M nitric acid to 5-10 g of copper under nitrogen atmosphere according to the reaction 3Cu + 8HNO3 f 2NO + 3Cu(NO3)2 + 4H2O.15 The collected NO gas was purified by passage through an ascarite column, two washing towers filled with 4 M NaOH, and one washing tower filled with distilled water. Standard saturated NO solutions were prepared by bubbling the purified NO gas through oxygen-free phosphate buffer solution (PBS, pH 7.0) for 30 min. Aliquots of the saturated solution were used to prepare solutions of known NO concentration, using a value of 1.9 mM for its concentration at saturation.16 For NO calibrations, 0.21-µL aliquots of a saturated NO solution were subsequently added to the electrochemical cell with a gastight syringe. The current due to the electrocatalytic reduction of NO by the Co-POM/PE multilayer was recorded after each addition. Electrochemical Measurements. The electrochemical response of Co-POM/PE multilayer-modified electrode was measured with a conventional three-electrode system using Ag/AgCl (3 M KCl) as a reference electrode and a Pt wire as an auxiliary electrode. A phosphate buffer solution (0.2 M, 10 mL, pH 7.0) was used for the electrochemical measurements. All measurements were carried out at a room temperature (∼20 °C) under argon atmosphere. RESULTS AND DISCUSSION The preparation of POM/PE multilayers is effected by alternating deposition of PAH, PSS, and Co-POM on the ITO substrate. Our previous study into the preparation of a POM-based ELSA multilayer showed that salt concentration and the assembly procedure have a significant effect on the structure and properties such as permeability of the ELSA multilayer.13 Here, all Co-POMbased multilayers are prepared from Co-POM aqueous solution with no salt, PAH solution containing 0.5 M NaCl, and pristine PSS solution. During the first several potential cycles, the peak currents drop by ∼10% and then remain unchanged. Probably loosely bound POM anions initially bleach out of the multilayer. CVs of (PSS/PAH/Co-POM/PAH)m multilayer show similar two reduction and subsequent oxidation waves as Co-POM anions in solution. The redox waves correspond to two two-electron processes. In addition, for less than 20 layers, the peak-to-peak separation (∆Ep ) Epa - Epc) is independent of the scan rate (
