Anal. Chem. 1992, 64, 1777-1704
1777
Reversible Potentiometric Oxygen Sensors Based on Polymeric and Metallic Film Electrodes Hyoung-Sik Yim and Mark E. Meyerhoff Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
Varlous materials and sensor conflguratlons that exhlblt reverdble potentlometric responses to the partlal pressure of oxygen at room tmperature In neutral pH solutlon are examlned. I n one arrangement, platlnum electrodes are coated wlth plastlclzd poly(vlnyl chlorlde) films dopad wlth a cobalt( I I ) tetraethylene pentamlne complex. For such sensors, potentlometrlc oxygen response Is attrlbutd to a mlxed potontlal orlglnatlng from the underlylng platlnum electrode surface as well as a change In redox potential of the Co( I I)-tetrendoped flhn as the complex blnds oxygen reverdbly. The response due to the platlnum surface Is prolonged by the presence of the Co( II)-tetren/PVC flhn. Alternately, thln fllms of metalllc copper, electrochmkally depodtedon platlnum and/or sputtered or vapor deporlted on a dngk crystal dllcon substrate, may be used for reversible oxygen senslng. The long-term reverdblllty and potentlometrlc stablltty of such copper fllm-bared sensors Is enhanced (up to 1 month) by preventlngthe fonnatlon of cuprous oxide on the surfaces vla the appllcatlon of an external nonpolarlzlng cathodk current through the worklng electrode or by speclflcally udng sputtered copper flhns that have [loo] preferredcrystalstructures as determlnedby X-ray dMractlon. The Impllcatlona of these flndlngs In relatlon to fabrlcatlng analytlcally useful potentlometrlc oxygen sensors are dls cussed.
Oxygen gas is a very important target for chemical sensing because ita presence controls various processes in the environment, in vivo, and in numerous industrial processes. Existing oxygen sensors can be grouped broadly into two categories: (a) electrochemical and (b) optical. The electrochemical type include classical Clark style amperometric/ polarographic devices,l galvanic sensors,2 and solid-state electrolyte (e.g., calcia/yttrium-doped zirconia) potentiometric sensors3 that operate only at high temperatures. Newer optical fiber oxygen sensors are generally equilibrium devices that are based on changes in the absorbance of molecules that bind oxygen reversibly or on the ability of oxygen to quench the fluorescence or phosphorescence of a wide variety of aromatic species (including anthracene and pyrene derivatives, porphyrins, e t ~ . ) . ~ Almost all current biomedical measurements of dissolved oxygen in blood are made with the amperometric Clark style oxygen probe.5 In this design, oxygen diffuses through a gaspermeable membrane and is reduced a t a platinum cathode polarized a t 4 6 to-O).8VvsAg/AgClreference. Theresultant
* Author to whom all correspondence should be addressed.
(1)Clark, L. C., Jr. Trans. Am. SOC.Ant. Int. Organs 1956,2,41-57. (2) Mancy, K.H.; Okun, D. A,; Reilley, C. N. J. Electroanal. Chem. Interfacial Electrochem. 1962,4,65-92. (3) Siebert, E.;Fouletir, J. Ion-Selective Elect. Reu. 1986,8,133-151. (4) Wolfbeis, 0.S.Oxygen Sensors, in Fiber Optic Chemical Sensors and Bioaensors; Wolfbeis, 0. S., Eds.; CRC Press: Boca Raton, 1991; Chapter 10, pp 19-54. ( 5 ) Hitchman, M. L. Measurement ofDissolued Oxygen; Wiley: New York, 1978. 0003-2700/92/0364-1777$03.00/0
current flow is linearly related to the partial pressure of oxygen (POz) in the sample. Changes in membrane permeability, cathode surface area, etc., however, can cause considerable drift in sensor response. Optical oxygen sensors offer prospects for improved stability, since the measurement is based on an equilibrium interaction of oxygen with an appropriate indicator species. By analogy, equilibrium potentiometric detection of oxygen should, in principle, offer advantages over existing amperometric devices in terms of long-term sensor stability. Indeed, the solid electrolyte type high-temperature (>600 OC) potentiometric oxygen sensors (gas phase) do operate for long periods within automobile engines, smokestacks, etc., with relative little drift in signal output. Unfortunately, no commercialpotentiometric oxygen sensors are available that are operative in aqueous solution a t room temperature. Thus, efforts to devise new potentiometric oxygen sensors that can function in this ambient environment seem worthwhile, both in terms of fundamental science and potential bioanalytical applications. Recently, Yamazoe et aL6J suggested that solid electrolyte oxygen sensors prepared by sputtering LaF3 on Pt can operate at room temperature, although the exact mechanism of 02 response appears to be complex and has yet to be resolved. The original purpose of this study was to investigate various materials and sensor configurations that may be amenable to the design of new and analytically useful potentiometric oxygen sensors that operate at room temperature. Among a wide range of materials and arrangements initially examined, electrode systems based on polymer films doped with oxygenbinding Co(I1)-polyamine complexes and various metallic copper films deposited on platinum and/or silicon surfaces exhibited the most promise for use in oxygen sensing. This report summarizes, in detail, electrochemical and spectroscopic studies of these two systems in relation to their use in fabricating potentiometric sensors for dissolved oxygen that operate in aqueous media at room temperature.
EXPERIMENTAL SECTION Apparatus. Potentiometric data were recorded in two fashions: multichannel experimentswere performed using a Compaq portable computer equipped with a Data Translations DT2805-5716analog/digitalinput/output board (Marlborough,MA). Electrodes were connected to the computer via a custom-built electrode interface module as previously described.8 Data acquisition was controlled by Labtech Notebook software (Laboratory Technologies Corp., Wilmington, MA). For singlechannel measurements,the signal from an Accumet Model 825 MP pH/mV meter (Fisher Scientific, Romulus,MI) was recorded on a Heath Model SR-204strip-chart recorder. For most studies, oxygen signals were measured relative to a common doublejunction Ag/AgCl reference electrode (Fisher). Measurements were made in thermostated double-walled beakers. A Fisher Model 80 thermostat provided controlled temperaturecirculating ~
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(6) Miura, N.; Yamazoe, N. Trends Anal. Chem. 1990,9, 170-175. (7) Miura, N.;Hisamoto, H.; Yamazoe, N.; Kuwata, S.; Salardenne, J. Sensors Actuators 1989, 26, 301-310. (8) Collison, M. E.;Aebli, G. V.; Petty, J.; Meyerhoff, M. E. Anal. Chem. 1989,62, 2365-2372. 0 1992 American Chemical Society
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interferences (Figure lb). Deposition of Cu on Platinum and Silicon Substrates. For the copper electrodeposition, Pt (0.33-cm2 disk) electrodes were immersed in 50 mL of 0.1 mol/L HCI containing 50 mg of a) b, CuCI2and cathodized at a constant potential of 4 . 4 V using a potentiostat for 60 min. Alternately, thin films of Cuo were deposited by a vacuum vaporizationmethod. Thiainvolvedapplyingheat (with tungsten wire) to the source of film material (Cu9 in a high vacuum environment (1 X 10-5 Torr). Copper films were deposited on metallic Pt or silicon [lo01 substrates placed into the chamber. The film growth rate was almost instantaneous (about 1 X 10' Urnin). Final film thicknesses were estimated to be 0.24.3 pm via scanning electron microscopy. Thin films of Cuo, about 0.5 pm thick. were also demited on a single-crystal silicon [lo01 substrate by de sputtering. The copper films were deposited by an Enerjet Sputter Coater using argon gas as a sputter gas at a power of 1A, 405 V, with the film growth rate of 350 Ahin. Pressure in the main chamber was typically below 7 X 10-3Torr. Construction of the CuO Film-Bawd 01Sensors. The p tentiometric 0%responses of Cuo films on Pt (Figure la) were measured M a common double-junction AgIAgCI reference 3 electrode in 5.10, or 25 mM MOPS and EPPS buffer solutions. 5 6 pH 7.W8.0. In the case of the oxygen sensors prepared with silicon substrates, copper wires were connected to the Cuo films 2 4 by soldering, and epoxy or parafilm was used to encapsulate the solder joints of the electrodes. After each set of measurements, Flpm 1. schematic aamm of varlaa potaniomarlc oxygen gas the electrodes were stored either in air or in buffer solution. wndw conflgvatkfmexamhed: (a)two slecwat fa d b . 3 socltlon meesuements:(b)two-elecb&~~aRB~~(c)thrt)OExternalCurrent Supply (Three-Electrode System). In electmde/constamcunmtfadtect~measuemems;(d)three order to minimize the irreversible oxidation of the copper films ebcw&/constant curent gas Mmsing amngsment: (1) R disk: (2) (insomeexperiments).aconstant non-polarizingcathodiccurrent Co(Il)-tetren complex doped PVC membrane a Cuofilm; (3)& u b b was passed through the working electrodes by using an external ]unCaarAg/AgCI referern ebctr&; (4) sRlcaw,r u b k gas parmeabb power supply, a 22-100 MR resistor, and an auxiliary platinum membrane; (5) AglAGl reference e W & ; (6) Pt wire electrode or another double-junction Ag/AgCI electrode (see Figurelc). Formostelectrodes, thearrangementshowninFigure water for experiments performed at these temperaturea. The ICyielded a constant current of 0.34.4 pA/cm2(with SCE). As sample solutions were tonometered to different oxygen levels with the two-electrodearrangements described above, potential withappropriate02/N2gasmixtures(1%.5%, 10%. 21%,and interferences from ionic speciesor pH changes in the sample can 100% OdN2, Air Products. Tamaqua. PA). A Cary 219 UV-vis be eliminated by using an outer gas permeable silicone rubber spectrophotometer was used to monitor the rate of oxidation of orTeflonmembranetoprotectthe threeelectrodecell (seeFigure Co(I1)-tetren within solution or membrane phases. Id). Reagents. Tris (tris(hydroxymethyl)aminomethane), MES Methods Used for Surface Characterization. ESCA (4-morpholinoethanesulfonicacid). MOPS (3-(N-morpholino)spectra were obtained with a Perkin-Elmer PHI Model 5400 Xpropanesulfonicacid),POPS0 (piperazine-Nfl-bis(2-hydroxyray photoelectron spectrometer operated in the fixed analyzer propanesulfonicacid)), and EPPS (N-(2-hydroxyethy1)piperatransmissionmode using monochromatic Mg Ka radiation (1253.6 zine-N-(3-propanesulfonicacid)) buffers were obtained from eV), at a power of 300 W. Pressure in the main chamber was Sigma Chemical Co. (St. Louis, MO). Ditridecyl phthalate was typically
