DEVICE

This application is a continuation-in-part application of U.S. Patent Application Serial No. 07/632,273, filed December 21 , 1990, which is a continuation-in-part application of U.S. Patent Application Serial No.07/205,136, filed June 10, 1988, both of which enjoy common ownership and are incorporated by reference herein.

1 0

Field of the Invention

The present invention relates to selectively permeable membranes for the separation of gases and other substances. 1 5 In particular, the present invention relates to membranes which are permselective for oxygen and to oxygen sensing devices employing such membranes.

Background of the Invention

20

The use of membranes for the separation of one or more components of a mixture is based upon the selective permeability of a membrane material. The terms "selective permeability" or "permselective" as used herein refer to

25 different chemical components, such as different gases, which permeate through a membrane at different rates. For example, a membrane which exhibits a greater rate of permeation for oxygen than for other gases, such as nitrogen, carbon dioxide or argon, is considered to be permselective for oxygen. In

30 particular, such terms are intended to mean that the differences in the permeability of a membrane toward the various components of a mixture is such that, upon passage through the membrane, the composition of the mixture , will be selectively enriched in the mixture components having 35 the highest permeabilities. It is to be understood, however, that these terms do not necessarily mean that the passage of one component occurs to the complete exclusion of others, even though such exclusivity is desirable for the purposes of

separating one component of a mixture from one or more other components of the mixture.

Permselective membranes have been developed for various purposes such as, for example, the preparation of high purity gases from impure feedstocks, the desalination of sea water, the separation of the isotopes of uranium, and the like. In cases where passage through a single membrane does not provide adequate enrichment of the component of interest, multiple permselective membranes may be applied in sequence to achieve the desired degree of separation. Gas permselective membranes have also been employed in devices for sensing the presence or concentration of a particular gas in a mixture, such as oxygen electrodes.

Oxygen electrodes are most commonly employed to determine the concentration of oxygen in a gaseous or liquid sample, such as for the in vivo measurement of oxygen in blood or other biological fluids. The output of such an electrode is typically in the form of an electrical current or voltage that is a function of the oxygen concentration in the sample. The output may also be in other forms such as, for example, an optical output.

Generally, an electrochemical oxygen sensor consists of two or more electrical conductors (often referred to as electrodes, either individually or in combination). Typically, a first conductor is constructed of a metal such as gold or platinum which is capable of catalyzing the electrochemical reduction of oxygen. Such first conductor is held, by external means, at an electrical potential which is sufficiently more negative relative to a second conductor, which may be of the same or different material, to cause the electrochemical reduction of oxygen reaching the surface of the first conductor. For example, where a circuit comprises a liquid test sample, the first and second conductors, and the external means of electrical potential, such electrochemical reduction causes an electrical current to flow in the circuit comprising In addition, means for measuring the magnitude and

polarity of the current may be included in the circuit, wherein the magnitude of the current flowing in the circuit is a function of the quantity of oxygen undergoing reduction according to the following reaction:

2e" 2e"

0 - — > 2H2O2 — --> 4 OH-

which occurs at the cathode, i.e., the conductor having the more negative electrical potential, and which requires that the cathode supply electrons to the reaction. Connection of the two electrodes in an electrical circuit results in the flow of current through the circuit, including any associated measuring instruments, as a function of the quantity of oxygen undergoing reduction at the cathode and which is related to the concentration of oxygen in the test sample.

Where the optical output from such device is detected and measured, an optical analog, referred to as an optrode, can be employed which comprises an optical fiber coated with or appended to a matrix that contains a material that reversibly changes its optical properties upon exposure to oxygen. The sensitive material may be, for example, a dye that changes its absorbance, fluorescent wavelength, fluorescent intensity, or the like, upon exposure to oxygen.

The optical output of such device, which is typically measured at the end of the fiber opposite to that coupled to the sensitive material, is a function of oxygen concentration. However, in certain environments the cathode of an electrochemical sensor, or the sensitive region of an optical sensor, can be reduced or otherwise lose its ability to interact with the oxygen present in the sample. For example, certain ions or proteins in a biological test sample may be adsorbed onto the sensitive regions of the sensors to thereby block those sites at which the interaction with oxygen occur. Interferences can also occur in which another component in the test sample undergoing analysis may interact with the

sensitive region of the sensor in a manner which results in a sensor output that is indistinguishable from that caused by the component of interest to thereby impair the performance, reliability and reproducibility of sensor operation. In order to reduce such effects, a permselective membrane applied to the sensitive regions of a sensor can be employed. Such permselective membrane separates the sensitive region from the environment in which it is used. The permselectivity of the membrane permits the component of interest to reach the sensitive region while, at the same time, prevents unwanted sample components from fouling or otherwise interfering with sensor operation.

Permselective membranes are conventionally made by casting and dip coating methods known in the art. According to a typical casting method, an appropriate solvent is used to dissolve a membrane precursor material, wherein the resulting membrane precursor solution is spread or cast onto a solid casting surface and the solvent caused to evaporate to leave the membrane on the solid casting surface. The membrane can then be removed from the casting surface and attached to a sensing device. Alternatively, the membrane may be cast directly upon the sensing device. The dip coating method is similar to the casting method except that the solid surface upon which the membrane is to be formed is dipped into and removed from the coating solution one or more times to build up a membrane layer of the desired thickness.

In addition to the solvent, the membrane precursor solution employed in dip and casting methods previously described generally incorporates multiple components such as, for example, a polymer such as PVC to provide a porous membrane matrix, a plasticizer such as Tris(2- ethylhexyl)phosphate to provide a fluid phase within the pores of the membrane matrix and which may provide some measure of permselectivity to the final membrane, and a carrier material such as an ionophore (e.g., Valinomycin) which is free to migrate within the fluid filled pores of the membrane and which imparts the primary permselectivity to the membrane.

Single component membranes are also known which are typically hydrophobic to facilitate partitioning of the test sample component between the test sample and the membrane and to minimize dissolution or extraction of the membrane constituents into the test sample.

However, cast and dip coated membranes present several disadvantages in the fabrication of sensors. For example, membranes which are prepared as independent entities and then mounted onto a sensor device will not necessarily be conformal to the surface of the sensor, nor will they exhibit strong adhesion to this surface. In addition, since sensor membranes are generally made as thin as possible to minimize sensor response time, such thin membranes are delicate, difficult to handle and mount and, in the case of small or miniature sensors, are particularly difficult to cover with separately fabricated membranes. Moreover, dip coated or cast in place membranes typically exhibit poor adhesion to the sensor surface and may entrap voids within their structure or at the membrane-sensor interface. In both cases, the thickness and uniformity of the membrane are difficult to control and chemical compatibility between the membrane and the sensor must be considered, as well as bleeding or leaching of the membrane constituents.

Permselective membranes prepared by glow discharge (plasma) polymerization processes have also been described for use in reverse osmosis, large scale purification of gases, and the like. Generally, such polymerization processes are performed by establishing a glow discharge in a gaseous atmosphere of suitable composition to result in the deposition of a permselective coating on a porous support. Although aliphatic hydrocarbons have been used with glow discharge polymerization processes, such permselective membranes exhibit poor permselectivity for oxygen. In another glow discharge polymerization process, the pores of a porous substrate are filled with a material, the surface (or bulk) of which can undergo polymerization when exposed to a glow discharge established in an unpolymerizable gas. Plasma

deposition may then be employed as previously described to deposit a contiguous membrane on the surface of this composite. When the pores of the substrate are filled with a silicone oil and the overlying deposited membrane is formed from a tertiary organosilicone compound, the resulting composite membrane can be permselective for oxygen.

A membrane or other coating deposited using a properly formulated plasma polymerization process exhibits a very high degree of conformity and adhesion to the underlying surface. Although such membranes and coatings may also be applied as extremely thin, highly uniform and defect free layers which are durable, the advantages of the plasma process are lost when such a membrane is formed on a porous subcarrier prior to assembly into a sensor.

Summary of the Invention

The present invention relates to a single layer membrane that is permselective for oxygen, wherein the membrane comprises a siloxane monomer of Formula I or Formula II or Formula III:

R2 R4 I I

R-| -Si-0-Si-R5 I I

R2 R2 I I Rl -Si-0-Si-Ri

R3 R3

TTT

wherein R-| , R2, R3, R4, R5 and Rβ, which can be the same or different, can be:

-H; -CH3; -0-(CH2)t ₇ -CH3, where n = 0-5;

-CH2-C6H5;

-C ₆ H ₅ ;

-(CH2)A7-COOH, where n = 0-5;

-CO-(CH2)t ₇ -CH3, where n = 0-5; or -(CH2)n-CH3, where n = 0-5,

and wherein such siloxane monomer is polymerized directly onto a non-porous support material by glow discharge polymerization to form a membrane thereon. According to the present invention, the resultant membrane can be polymerized

onto the support material contiguously with the active area or areas of one or more electrodes to thereby provide an electrochemical sensing device. According to a preferred embodiment of the present invention, such electrochemical sensing device or sensor comprises the permselective membrane and three electrical conductors or electrodes, referred to herein as a working electrode, a counter electrode, and a reference or auxilliary electrode, which are contiguous with the permselective membrane. The resulting membrane has an oxygen content of between about 10% and 60% and is highly permselective for oxygen. The membrane can be deposited upon electrochemical or optical sensors, thereby forming an oxygen sensing device that is resistant to fouling, contamination and interferences. Such a membrane deposited upon a sensor exhibits high conformity and adhesion to the sensor, as well as excellent uniformity, freedom from defects, and durability. The plasma deposition process further permits the precisely controlled formation of very thin permselective membranes having typical thicknesses of less than one micron. When applied to an oxygen sensor, such thin membranes result in a sensor that exhibits very fast responses to changes in oxygen concentration. In addition, such membranes may readily be deposited upon very small sensors or upon selected areas of sensors, thereby enabling the formation of microminiature oxygen sensors and similar devices. By appropriate plasma treatment of the exposed surface of such a membrane deposited upon a sensor, and the subsequent application of additional membrane layers by glow discharge plasma polymerization or other techniques known in the art, a sensing device according to the present invention can also be prepared which is responsive to components other than oxygen.

Brief Description of the Drawings

Figure 1 illustrates a sensing device according to the present invention.

Description of the Invention

The present invention is based upon the discovery that the vapor of an organo-siloxane compound can be used to produce an ultrathin permselective membrane coating upon a non-porous solid support by glow discharge polymerization. It was further discovered that when such membranes were deposited upon a non-porous substrate incorporating electrical conductors to form an electrochemical cell, the resulting combination can function as an oxygen sensor of the amperometric or polarographic type.

Amperometric or polarographic membrane covered oxygen sensors previously described require the presence of a layer of an electrolyte between the membrane and the conductors of the underlying electrochemical cell in order to support the flow of electrical current between the conductors and thus electrochemical reduction of oxygen at the cathodic conductor. According to the present invention, it has been unexpectedly and surprisingly found that the presence of such discrete electrolyte layer is not necessary when preparing a sensing device according to the method of the present invention. In particular, the membrane according to the present invention performs the function of such electrolyte layer whereby a membrane deposited according to the present invention is hydrophyllic and undergoes hydration sufficient to acquire adequate electrical conductivity for this purpose. It is to be understood that optical sensors do not require the presence of such electrolyte layer, nor do electrochemical sensors of the Field Effect Transistor (FET) type, as such devices sense the electrostatic fields generated by the presence of, for example, an analyte at the surface of the sensing device rather than sensing the flow of electrical current associated with the electrochemical reduction of the analyte of interest. Accordingly, the membranes according to the present invention may be employed with optical sensors and FET sensors, although such devices function by

mechanisms different from those sensing devices described herein.

The glow discharge polymerization process according to the present invention permits the deposition of a contiguous or continuous, ultrathin permselective membrane upon any surface that has an unbroken, although not necessarily smooth, surface. Continuous membranes having thicknesses ranging from between less than about 100 Angstroms and about several microns can be deposited according to the present invention. Although the present invention utilizes only a single membrane of homogeneous composition, the glow discharge deposition process can also be conveniently employed for the formation of membranes consisting of multiple distinct layers, each having a unique composition, and for single or multiple component membranes in which the properties of the membranes are graded in a continuous fashion across the thickness of the membrane, particularly for the formation of sensors for the determination of analytes including, but not intended to be limited to, oxygen, glucose, cholesterol, and the like, in biological test samples.

The single layer permselective membranes of the present invention utilize both size exclusion and diffusion rate as molecular selection processes. Small molecules such as oxygen can move through the interstitial voids of the membrane while larger molecules such as proteins cannot.

The diffusion rate is a function of not only the molecular size and weight of the diffusing component, but also of the composition and solubility of this component (in the membrane) as well as, to some extent, the concentration of molecules in the membrane. In particular, the atomic composition of the membrane affects the solubilities of various components in the membrane and thereby affects the transfer of those molecular components through the membrane. The membranes of the present invention are formed from siloxane compounds, i.e., organic groups bonded together by

silicon-oxygen-silicon subunits, and are of the family of compounds of Formula 1 or Formula II or Formula III:

R2 R4 I I

R-| -Si-0-Si-R5 I I

R3 R6

R2 R2 I I Rl -Si-0-Si-Ri I I

R3 R3

TTT

wherein R-| , R2, R3, R4, R5 and Re, which can be the same or different, can be:

-H;

-CH3;

-0-(CH2)t ₇ -CH3, where n = 0-5;

-CH2-C6H5;

-C6H5;

-(CH2)n-COOH, where n = 0-5; -CO-(CH2)n-CH3, where n = 0-5; or

-(CH2)t ₇ -CH3, where n = 0-5. The silicon atoms in siloxane compounds are tetravalent, having single bonds to four (4) substituents, wherein both symmetrical (R1 = R5, R2 = R4. R3 = R6) and assymmetrical siloxanes may be used, wherein membranes formed from such compounds are highly permselective to oxygen according to the present invention. It should be understood that such membrane precursors are generally considered not to be polymerizable by conventional techniques but, however, are readily polymerizable by the glow discharge method according to the present invention. According to the glow discharge method of the present invention, an electric field causes the membrane precursor to break down into molecular and atomic fragments including both ions and neutral species such as free radicals. These fragments react, particularly upon surfaces, to form unique polymers. Polymers deposited in this manner have no discernible repeating units, are highly crosslinked, and in many cases have compositions markedly different from that of the precursor. These features distinguish polymers formed by a plasma process from those formed by other means. The composition and structure of the resulting membrane are sensitive functions of the reaction conditions employed. The properties of these membranes may be controlled by selecting and adjusting the reaction conditions. A typical reaction sequence begins by loading the non- porous support material, typically a support material upon which one or more electrical conductors have been formed, into the glow discharge reaction chamber. The chamber is then evacuated to its base pressure, which is defined as the lowest pressure that can be achieved after sustained evacuation. This step removes the majority of the volatile contaminants from the chamber. As an optional step,

additional contaminants may be removed and the surface of the support material conditioned to receive the deposited membrane by introducing a controlled amount of an inert gas, such as, for example, argon or helium, into the chamber and establishing a glow discharge plasma in the gas by the application of an electrical field.

After cleaning and conditioning the support, the chamber is brought back to base pressure to remove any residual material. The membrane precursor is then admitted to the chamber in vaporous form and a glow discharge plasma is initiated and sustained by electrical means. The glow discharge plasma reduces the membrane precursor to molecular and atomic fragments which recombine on the support and chamber surfaces to form a membrane, or which are removed from the system by the vacuum pump. In this process, the membrane precursor vapor may be combined with other inert or reactive gases to facilitate the plasma process or to control the composition of the deposited membrane. Gases such as oxygen, hydrogen, ammonia, nitrous oxide and argon are particularly useful according to the method of the present invention, although other gases including, but not intended to be limited to, helium, nitrogen, xenon forming gas, neon, and the like, may also be used. Deposition of the membrane polymer occurs on all exposed surfaces within the reaction chamber. If certain areas of the support are not to be coated, they can be masked prior to the reaction by methods known by those skilled in the art. Upon completion of the deposition process, the discharge is extinguished, the reaction chamber is returned to atmospheric pressure by the introduction of air or an inert gas, and the coated supports are removed. The exposed surface of such a plasma deposited membrane may incorporate high concentrations of free radicals which may, if desired, be caused to undergo further chemical reactions such as the grafting of conventionally polymerized materials.

The properties of the resulting membrane are highly sensitive to, and will depend upon, the selected reaction

conditions, which include, but are not intended to be limited to, the monomer selected, the discharge frequency, power, monomer flow rate, residence time, and the like. For example, a continuous thin film deposited from a 20 KHz discharge has a smooth, featureless surface morphology and a preponderance of silicon-carbon bonding. Under similar conditions of monomer, flow rate, pressure and power, a film exhibiting a columnar growth pattern and consisting predominantly of Siθ2 is formed at a frequency of 13.56 MHz. Accordingly, one skilled in the art apprised of the foregoing considerations and teachings herein can generate the desired membrane composition and morphology.

The deposition rate of the polymer is an important process variable, especially when coating microminiature support materials. This variable can be monitored during deposition by an oscillating quartz crystal balance or an optical interferometer. Monitoring of the deposition rate and deposited thickness is necessary for several reasons. The response time of a sensor is intimately related to the thickness of the overlying membrane. One of the advantages of the present invention is the ability to reproducibly deposit ultra thin membranes to thereby permit the fabrication of sensors having consistent, short response times. It is to be understood that consistent, short response times are highly desirable for most types of sensors. Although plasma deposited membranes can be of high quality, a small number of localized defects nevertheless may occur wherein the rate of defect production is related to the rate of deposition, and is lowest over a limited range of deposition rates. According to the present invention, the optimum deposition rate is from between about 0.5 A/sec and about 3.0 A/sec. For most applications, the optimum final membrane thickness was found to be particularly useful when the thickness thereof was from between about 0.5 microns and about 1.0 microns. The polymer membrane according to the present invention is capable of serving as a permselective membrane for small molecules such as oxygen and the like. Suitable

substrates upon which such membranes can be deposited include, but are not intended to be limited to, metals, semiconductors, polymers, ceramics, glasses, and the like. When electrical conductors are formed upon a non-porous, non-conducting substrate, and a permselective membrane is deposited thereon according to the present invention, the resulting device may be used as an electrochemical sensor for the small molecules for which the membrane is permselective. According to one embodiment of the present invention, an electrochemical oxygen sensing device incorporates a plasma deposited permselective membrane as described above and three electrical conductors or electrodes. Referring to Figure 1 , the sensing device 10 comprises a working electrode or conductor 12, a counter electrode or conductor 14, and a reference or auxiliary electrode or conductor 16, which are formed contiguously with a membrane 18 by glow discharge polymerization on a non-porous support member 20 according to the method of the present invention. It is to be understood, of course, that the configuration of a sensing device according to the present invention is not intended to be limited to the configuration shown in Figure 1 , and that various other configurations are contemplated and can be prepared by one skilled in the art apprised of the considerations and teachings contained herein. The working electrode is the conductor at which the electrochemical reduction of oxygen occurs. If an electrical potential is set between the working electrode and the counter electrode, with the potential at the working electrode being negative with respect to that at the counter electrode, an electrical current that is proportional to the concentration of oxygen will pass between the the working electrode and the counter electrode. The reference electrode serves primarily to sense the electrical potential at the working electrode. Maximum measurement accuracy is obtained when the current passing through the reference electrode is minimized.

Permselective membranes deposited in accordance with the present invention exhibit substantial hydrophyllic

character and undergo hydration upon contact with an aqueous sample or with humid air. Hydration of the membrane provides it with sufficient electrical conductivity to support the electrical current that must pass between the working electrode and the counter electrode. Conversely, membranes which have been previously described in the art are highly hydrophobic and, as a result, cannot support this current flow. Accordingly, previously described sensing devices require a layer of an electrolyte between the membrane and the electrical conductors in order to support such current flow. On the other hand, the elimination of the need for such layer of electrolyte in a sensing device according to the present invention offers substantial benefits in the manufacture, storage and use of such sensing devices. It is to be understood that optical and FET-based oxygen sensors operate on physical principles different from those identified above and, as a consequence, neither require the presence of this internal electrolyte for operation.

According to the present invention, any metal that is effective for catalyzing the cathodic reduction of oxygen can be used as the working electrode. Selection of the working electrode material is made upon the basis of compatibility between the metal and the environment in which the sensor will be used, as well as the efficiency of catalysis and manufacturing considerations. For example, platinum is a suitable selection for a sensor to be used in a biological environment. Other metals which can be employed include, but are not intended to be limited to, gold, silver, tungsten, palladium, tantalum, iridium, electrically conductive allotropes of the semi-metal carbon, and the like, and alloys and combinations thereof.

Although the counter electrode of the present invention is preferably made of the same metal as the working electrode, a different metal can instead be employed since the ability to catalyze the reduction of oxygen is not required for the counter electrode. The reference electrode is preferably formed of silver metal which has been coated with a thin,

contiguous coating of silver chloride, also known in the art as chloridized. The silver metal of the reference electrode is typically added to the substrate and then chloridized in situ by an electrochemical process. The non-porous support material can be selected from various materials and include, but are not intended to be limited to, materials such as metallic, semimetallic, polymeric, ceramic, glass, and semiconductor materials, and any combination thereof. The sensing devices according to the present invention can be employed for the determination of oxygen in liquid environments or test samples and gaseous environments or test samples. For example, such sensing devices may be used for the in vivo, ex vivo or in vitro measurement of oxygen in liquid test samples, such as whole blood and the like, or for the determination of oxygen in gaseous test samples, such as respired air. The oxygen sensing device according to the present invention may also be used by substitution of, or in conjunction with, other materials to create a composite sensing device that responds to analytes other than oxygen for the measurement or detection thereof in liquid test samples or gaseous test samples. Such other analytes include, but are not intended to be limited to, glucose, cholesterol, trigyclerides, sodium, potassium, calcium, urea, uric acid, creatinine, iron, lactic acid, albumin, phosphorous, magnesium, chloride, carbon dioxide, carbon monoxide, urea nitrogen, uric acid, lactose dehydrogenase, and the like. For example, a membrane containing the enzyme glucose oxidase can be overlaid on the plasma deposited membrane of the present invention, wherein the resulting composite sensing device is responsive to glucose. Similarly, a sensing device responsive to cholesterol can be made by adding cholesterol oxidase to the membrane. In such composite sensing devices, the enzyme catalyzed reaction which occurs on and within the added membrane consumes oxygen from this immediate vicinity, thereby reducing the amount of oxygen available for electrochemical reduction by the underlying sensor. The

decrease in sensed oxygen level is therefore proportional to the concentration of the target analyte.

The present invention will now be illustrated, but is not intended to be limited, by the following examples.

Materials And Methods

The sensing devices prepared according to the following examples were evaluated by tonometry using precisely prepared gas mixtures and aqueous solutions saturated with these mixtures. Comparison measurements were also made against an ABL-30 Blood Gas Analyzer (Radiometer, Copenhagen). Additional measurements of permselectivity were made by deposition of the membrane material on a porous carrier under conditions where the membrane bridged the pores of the carrier. Permeation rates were determined by differential pressure measurements using pure gases and, in some cases, by mass spectrometry. All of the membranes to be described were permselective for oxygen and produced functional electrochemical oxygen sensing devices.

Two basic types of glow discharge polymerization reactors were used. The first device was a inductively coupled reactor operating at a radio frequency of 13.56 MHz. Radio frequency power was provided by a crystal controlled oscillator through a linear radio frequency amplifier and a capacitance tuned matching network. The matching network was tuned for optimum power transfer into the plasma discharge as indicated by a VSWR (voltage standing wave ratio) meter. A net power input of 60 Watts was used in the following examples.

The reaction chamber consisted of a glass tube having an inside diameter of three inches and twenty inches long, and which was surrounded by a water cooled excitation coil consisting of 18 turns of 1/4" copper tubing. The matching network was connected across the ends of this coil. The tubular chamber was closed by two end plates, one of which

incorporated a gas dispersion manifold, and the other, an exhaust port. Capacitance manometer pressure gauges were provided at both end plates. The exhaust port was connected to a vacuum pump by way of a servo controlled automatic throttle valve which acted to maintain the chamber pressure at a constant predetermined value. The chamber pressure was typically selected to be between 0.200 and 0.600 Torr. The gas dispersion manifold was connected to the monomer reservoir by a precision metering valve. A heated mass flow controller was used in some cases. The monomer reservoir consisted of a round bottom flask situated within a temperature controlled electrical heating jacket. The temperature of the monomer was adjusted to maintain an adequate pressure of monomer vapor within the flask. A temperature of 40°C was adequate for most of the monomers employed. Provision was also made to bubble argon gas through the monomer at a controlled rate or to introduce argon at a point between the monomer reservoir and the gas manifold if desired. A quartz crystal deposition monitor was installed in the reaction chamber.

The second reactor was of the parallel plate type operating at audio frequencies. In this reactor, the output of a tunable audio oscillator, which was typically set to 20 KHz, was increased in power by a wideband amplifier and then increased in voltage by a transformer which also functioned to match the output impedance of the amplifier to the impedance of the plasma. The peak to peak excitation voltage at the reactor was typically 500 volts at a current of 0.2 Amps rms. The reaction chamber consisted of two parallel aluminum plate electrodes mounted vertically in a glass bell jar. The electrical connections, cooling water connections, reactant gas inlet, exhaust port and gauging ports were incorporated into an aluminum base plate upon which the bell jar sat. Objects to be coated were placed upon a rotating platen which passed between the electrodes and which could be left electrically isolated or connected to one of the electrodes or to an independently set DC or RF potential as

desired. The platen was electrically isolated in the examples to be presented. A quartz crystal deposition monitor was also installed in this chamber. The gas inlet, control and pumping systems were as previously described. In the following examples, the samples to be coated consisted of arrays of three electrode electrochemical cells deposited upon glass or alumina ceramic substrates. Each electrochemical cell included two platinum or gold electrodes and one chloridized silver electrode. In most cases, a layer of titanium tungsten was used to ensure adhesion of the electrode metal to the substrate. The electrode metals were deposited in such a manner as to insure that the titanium tungsten layer was completely covered. These arrays were separated into individual sensor devices after membrane deposition.

Example 1 Sputter Cleaning Of Samples

The samples to be coated were placed within the reaction chamber of the radio frequency reactor. The chamber pressure was reduced to base pressure and held at this level for four hours. Argon gas was then introduced into the chamber at a flow rate of 30 cc/min and the chamber pressure set at 0.300 Torr. A glow discharge was initiated in the argon atmosphere and maintained for 15 minutes to sputter clean the surface of the electrode to be coated and the interior of the chamber. This treatment promotes adhesion of the deposited membrane to the sample while reducing the number of defects caused by chamber contamination.

Example 2 Deposition Of Membrane By Glow Discharge Plasma

Polymerization

The samples to be coated were placed within the reaction chamber of the radio frequency reactor and cleaned

as described in Example 1. After cleaning, the argon flow was terminated and the chamber pumped to its base pressure. Hexamethyldisiloxa. e vapor was then introduced into the chamber at a rate of 1.0 cc/minute and the chamber pressure set to 0.100 Torr. The plasma discharge was initiated and maintained for four hours with the deposition rate as indicated by the quartz crystal monitor being between 0.5 and 3.0 A/sec. At the end of this time, the discharge and gas flow were terminated, the chamber pumped down to base pressure, and then brought to atmospheric pressure by the introduction of high purity nitrogen gas before the samples were removed for testing and analysis.

Example 3 Evaluation Of Membrane Coated Samples

The coated samples of Example 2 were evaluated by scanning electron microscopy (SEM) to determine the coating thickness and morphology. The membrane thickness was between 1800 A and 10800 A depending upon the deposition rate indicated by the quartz crystal monitor and the position of the sample in the chamber. The membrane thickness over a given sensor was very uniform. No pin holes were observed. The surface of the membrane was nodular indicating columnar growth.

Representative samples were evaluated by Electron Spectroscopy for Chemical Analysis (ESCA) to obtain information on the chemical composition and structure of the membrane. The membrane composition was determined to be:

Analysis of the peak energies revealed that the silicon was predominantly bonded to oxygen in the form of Siθ2 with a structure very similar to that found in silica glasses. The carbon was predominantly bonded to hydrogen as CH2, although traces of C-0 and C=0 were also present. The composition of the membrane deposited under these conditions can be described as a silicate glass contaminated with hydrocarbons. The monomer was extensively fragmented during deposition with some apparent loss of carbon containing fragments.

Example 4 Glow Discharge Polymerization Of Hexamethyldisiloxane

Membrane

Hexamethyldisiloxane was polymerized in the audio frequency reactor at 20 KHz under conditions identical to those in Example 2 with the following exceptions:

( i) the power level was 100 Watts RMS; and (ii) the samples were placed at 90 degree intervals on the platen which was rotated at ten revolutions per minute.

Example 5 Evaluation Of Hexamethyldisiloxane Membrane

The coated samples of Example 4 were evaluated by SEM.

The membrane thickness was extremely uniform. No pinholes were observed. The surface morphology was featureless.

Defects consisting of cracks, voids and gouges in the underlying metal layers were observed to be uniformly and conformally covered by the deposited membrane. ESCA analysis of these membranes gave the following composition:

The peak energies indicated a predominance of Si-O-Si, C-Si and C-H bonding indicating that the monomer was only partially fragmented during deposition.

The foregoing Examples demonstrate that a single siloxane compound can be plasma polymerized to form membranes having a range of compositions and structures. These membranes are permselective for oxygen and can be used as components in oxygen sensing devices. Plasma deposition of the membrane directly upon a substrate confers the advantages of conformity, uniformity and adhesion. If the substrate incorporates one or more arrays of electrical conductors disposed so as to form electrochemical cells, such membranes confer the additional advantage that the sensors thus formed are capable of functioning without the presence of a discrete internal electrolyte layer. The method of the present invention is particularly useful for the production of miniature and ultraminiature sensors as both discrete devices and as arrays of devices. The versatility of the plasma deposition process permits optimizing the performance of such sensing devices to the requirements of a specific target application.

The teachings of the present invention are applicable to silicon-containing permselective membrane coatings applied to a variety of non-porous supports including those comprising sensing devices. Variations of the present invention can be ascertained by one skilled in the art apprised of the foregoing considerations, including, but not limited to, those variations based upon other support materials, sensing devices, membrane forming substances, and mixtures of membrane forming substances to which the teachings of the present invention can be applied. In addition, sensor structures that incorporate additional membrane layers or external components, and membrane structures that vary in composition or structure in a controlled manner, are also contemplated by the present invention.

The embodiments contained herein are intended as examples rather than as limitations. Accordingly, the description of the invention is not intended to limit the invention to the particular embodiments disclosed herein, but is intended to encompass all equivalents and subject matter within the spirit and scope of the invention as described above and as set forth in the following claims.