SIMULTANEOUS DETERMINATION OF EQUILIBRIUM AND KINETIC PROPERTIES

Abstract: Method of using a sensor array for simultaneously determining a partition coefficient and a diffusion coefficient of analytes. At least two sensors comprising the same polymer composition, but with differing predetermined polymer thicknesses, are used to measure the time lag difference between the sensors. This measurement is used to determine both equilibrium or steady state parameters as well as kinetic or diffusion coefficient information. The steady-state data can be used for analyte determination, whereas the kinetic data can be used for sensor array optimization and design.

Description: SIMULTANEOUS DETERMINATION OF EQUILIBRIUM AND KINETIC PROPERTIES CROSS-REFERENCES TO RELATED APPLICATIONS The present application claims priority to U.S. Provisional Patent Application No. 60/108,674, filed November 16, 1998, and U.S. Provisional Patent Application No. 60/108,915, filed November 17, 1998, the disclosures of which are hereby incorporated by reference in their entirety for all purposes. STATEMENTAS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCHAND DEVELOPMENT The research embodied in the present application was supported in part by Grant No. DAAK60-97-K-9503 awarded by DARPA and Grant No. DAAG55-97-1-0187 awarded by the United States Army. The government may have rights in any patent issuing on this application. BACKGROUND OF THE INVENTION Polymer-based chemical sensors exist in a large variety of forms and types, differing in application, sensitivity, operating principles, types of response and stability. Conducting polymer devices are popular and operate under the principle of a change in response upon exposure to different volatiles. A wide variety of polymers are used for such devices including substituted pyrroles and anilines. Such organic polymer-based sensors have found use in a variety of different applications and devices including, for example, devices that function as analogs of the mammalian olfactory system (see, U.S. Patent No. 5,571,401, which issued to Lewis et al, Lundstrόm et al, Nature 352:47-50 (1991) and Shurmer and Gardner, Sens. Actuators B 8:1-11 (1992)), bulk conducting polymer films (Barker et al, Sens. Actuators B 17: 143 (1994) and Gardner et al, Sens. Actuators B 18:240 (1994)), surface acoustic wave devices (Grate etal., Anal. Chem. 67:2162 (1995), Grate eta , Anal. Chem. 65:A987 (1993) and Grate et al, Anal. Chem. 65:A940 (1993)), fiber optic micromirrors (Hughes et al, J. Biochem. and Biotechnol 41 :77 (1 993)), quartz crystal microbalances (Chang et al, Anal. Chim. Ada 249:323 (1991)) and dye impregnated polymeric coatings on optical fibers (Walt et al, Anal. Chem. 68:2191 (1996)). Studies have shown that conducting polymers are sensitive to a wide range of gases and vapors and can be used in gas-sensing microelectronic devices. In certain instances, a basic model for a polymer gas sensor consists of a thin uniform polymer film lying on top of a pair of coplanar electrodes supported by an insulating substrate. It is assumed that the gas, or vapor, diffuses into the film and is adsorbed at sites randomly distributed throughout the film. The diffusion and adsorption equations can be presented in terms of several fundamental dimensionless parameters which describe the underlying chemical and physical properties of the system, (see, Gardner et al, IEEProc, Circuits Devices Syst. 142, 321-33, (1995)). The concept of an electronic nose that mimics the mammalian olfactory sense is now well established. An essential feature of an electronic nose is an array of chemical sensors such as polymer based sensors, coupled with computerized multivariate statistical processing tools. These sensors respond to a wide variety of analytes giving rise to a unique signature or pattern for a given analyte. The pattern is interpreted using pattern recognition algorithms to identify or quantify the analyte of interest. Using pattern recognition algorithms, the output signature, such as an electrical response, can be correlated and compared to the known output signature of a particular analyte or mixture of substances. By comparing the unknown signature with the stored or known signatures, the analyte can be detected, identified and quantified. Examples of such detection devices can be found in U.S. Patent Numbers 5,571,401, which issued to Lewis et al; 5,675,070, which issued to Gelperin; 5,697,326, which is issued Mottram et al; 5,788,833 which issued to Lewis et al; 5,807,701, which issued to Payne et al; and 5,891,398, which issued to Lewis et al, all disclosures of which are incorporated herein by reference. Using the combination of an array of non-specific gas sensors and suitable pattern recognition techniques, it is possible to build electronic systems that mimic the ability of the mammalian olfactory system for the discrimination and classification of odors. Potential applications for such systems include quality control in the food and beverage industries, as well as environmental monitoring and mammalian health. Recently, the availability of first generation commercial instrumentation has stimulated considerable interest in sensor development. The need to have arrays of sensor elements is necessary because the power of discrimination increases with the diversity of the type of sensors in the array. The variation or differentiation in the sensor array can be achieved in various fashions. For instance, in bulk conducting polymer sensors, a different polymer can be used in the fabrication of each sensor element. Using this fabrication technique, a 32 sensor array would requires 32 different polymers. What is needed in the art is a polymer based sensor system that shows intra-array variation without necessarily changing the polymer itself. Such a system would allow simultaneous determination of kinetic and equilibrium properties of an analyte. The present invention fulfills these and other needs. SUMMARY OF THE INVENTION The discrimination of sensor arrays increases with the number of sensors in the array. In certain polymer based sensors, it is desirable to have intra-array variation while maintaining the polymer constant or by using relatively fewer polymers. As such, the present invention provides a sensor array for detecting an analyte in a fluid, comprising: a first sensor having a first predetermined polymer thickness and a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness. The thickness of the sensors ranges from about 0.01 μm to about 20 μm, and preferably between about 0.05 μm to about 2 μm. Using the sensor arrays of the present invention it is possible to detect and identifying various analytes. Moreover, using the sensor arrays of the present invention it is possible to determine both equilibrium or steady-state parameters as well as kinetic or diffusion coefficient information. In certain instances, the steady-state data can be used for analyte determination whereas the kinetic data can be used for sensor array optimization and design. As such, the present invention provides a method for detecting the presence of an analyte in a fluid, comprising: providing a sensor array comprising a first sensor having a first predetermined polymer thickness; a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness; contacting the sensor array with the analyte to generate a response; detecting the response with a detector that is operatively associated with each sensor, and thereby detecting the presence of the analyte. In certain other aspects, the present invention provides a method for fabricating an array of sensors for detecting an analyte in a fluid, comprising: depositing a polymer layer onto a first sensor forming a first predetermined polymer thickness; and depositing a polymer onto a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness. Suitable fabrication techniques include, but are not limited to, spin casting, vacuum deposition, sputtering, MAPLE, air brushing and chemical vapor deposition. In yet another aspect, the present invention provides a method for measuring a diffusion coefficient of an analyte, comprising: contacting a first sensor having a first predetermined polymer thickness to elicit a first response; contacting a second sensor having a second predetermined polymer thickness to elicit a second response; comparing the first response to the second response to calculate a time lag and thereafter measuring the diffusion coefficient of the analyte. The diffusion coefficient is an important parameter in designing and optimizing sensor performance. Moreover, in certain instances, the present invention provides a method for simultaneously determining a partition coefficient and a diffusion coefficient, comprising: providing a sensor array comprising a first sensor having a first predetermined polymer thickness; a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness; contacting the sensor array with the analyte to generate a response; detecting the response with a detector that is operatively associated with each sensor, and thereby simultaneously determining a partition coefficient and a diffusion coefficient. These and other embodiments will become more apparent when read with the detailed description and accompanying drawings that follow. BRD F DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a response of a sensor array of the present invention. Figure 2 illustrates various responses as a function of sensor thickness and analyte volatility. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS I. SENSORS In certain embodiments, the present invention provides a sensor array for detecting an analyte in a fluid, comprising: a first sensor having a first predetermined polymer thickness and a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness. Sensors suitable for use in the present invention include, but are not limited to, surface acoustic wave (SAW) sensors; quartz microbalance sensors; conductive composites; chemiresitors; metal oxide gas sensors, such as tin oxide gas sensors; organic gas sensors; metal oxide field effect transistor (MOSFET); conductive polymers with conductive fillers; conductive polymers with conductive fillers and an insultaing polymer; piezoelectric devices; infrared sensors; sintered metal oxide sensors; Pd-gate MOSFET; metal FET structures; CHEM FET; metal oxide sensors, such as a Tuguchi gas sensors; phthalocyanine sensors; electrochemical cells; conducting polymer sensors; catalytic gas sensors; organic semiconducting gas sensors; solid electrolyte gas sensors; temperature sensors, humidity sensors, piezoelectric quartz crystal sensors; and Langmuir-Blodgett film sensors. As used herein the term "predetermined" means that the polymer layer associated with the sensor has a predefined or predetermined thickness which is measured within about 1 nm using atomic force microscopy, scanning tunneling microscopy, profilometry or other suitable method known by those of skill in the art. In a preferred embodiment, the sensors of the present invention are disclosed in U.S. Patent No. 5,571,401. Briefly, the sensors described therein are conducting materials and nonconducting materials arranged in a matrix of conducting and nonconducting regions. The nonconductive material can be a nonconducting polymer such as polystyrene. The conductive material can be a conducting polymer, carbon black, an inorganic conductor and the like. The sensor arrays comprise at least two sensors, typically about 32 sensors and in certain instances 1000 or more sensors. The array of sensors can be formed on an integrated circuit using semiconductor technology methods, an example of which is disclosed in PCT Patent Application Serial No. WO 99/08105, entitled "Techniques and Systems for Analyte Detection," published February 19, 1999, and incorporate herein by reference. Another preferred sensor is disclosed in WO 99/27357 entitled "Materials, Method and Apparatus for Detection and Monitoring Chemical Species," published June 3, 1999. Preferably, the sensor arrays of the present invention comprise at least one sensor selected from the following group of sensors, organic conducting polymers such as polymers of pyrrole and aniline, mass sensitive piezoelectric sensors such as bulk acoustic wave and surface acoustic wave sensors, quartz crystal microbalances, dye impregnated polymeric coatings on optical fibers, insulating polymers with conductive fillers and nonconducting/conducting regions sensors. In another embodiment, the present invention includes sensors as disclosed in U.S. Patent No. 5,512,490, which issued to Walt et al, on April 30, 1996. An optical detection and identification system is disclosed therein that includes an optic sensor, an optic sensing apparatus and methodology for detecting and evaluating one or more analytes or ligands of interest, either alone or in mixtures. The optic sensor of the system is comprised of a supporting member and an array formed of heterogeneous, semi- selective polymer films which function as sensing receptor units and are able to detect a variety of different analytes and ligands using spectral recognition patterns. The optic sensors of the present invention comprise semi-selective polymer films of predetermined varying thickness. In this manner, it is possible to assemble an optic sensor system or apparatus having intra-array variation i.e., varying predetermined polymer thickness. In one embodiment, the sensors are those described in U.S. Patent No. 5,945,060, issued August 31, 1999 to Buehler. As described therein, the sensor or resistor element has a plurality of different electrode configurations. These electrode configurations include: U-bend, comb and serpentine shapes. The resistor elements are then exposed to predetermined known gases. Patterns of characteristic changes in the conductance of the resistor elements in response to exposure to the predetermined gases is then measured and stored. As further described in the patent issued to Buehler, a gas sensor can be fabricated on a substrate that includes a plurality of electrodes wherein each electrode has a plurality of shapes or dimensions. Polymer films are deposited over the electrodes. The polymer films have a conductivity that changes when the film is exposed to various gases as measured across the electrodes. As used in the present invention, the polymer films have varying predetermined thicknesses to generate an array of sensors. Thereafter, a current measuring circuit is connected to the electrodes to measure conductivity changes. An optimal geometric configuration for the electrode is selected based on prior tests of the film's response to various gases of interest. By having a electrode geometry as well as other factors that optimizes the polymer film's response to a particular gas, the gas sensor exhibits an improved ability to distinguish among different gases. The sensor permits the optimization of numerous other factors such as polymerization process by control of the polymerization heat using the heater elements, polymer film doping and other factors. The sensor devices, methods and systems of the present invention can detect a wide range of differential responses. Examples of differential responses that can be detected include, but are not limited to, electrical responses, such as resistance, impedance or capacitance, optical, magnetic, surface acoustic and fluorescence responses. H. POLYMERS In certain aspects, a sensor of the present invention have a predetermined polymer thickness. In the presence of an analyte, a sensor comprising a predetermined polymer thickness, will sorb the analyte, and produce a detectable response. A detector operatively associated with the sensor detects the response. In certain embodiments, the polymer is nonconductive and is mixed with an electrically conductive material. In other instances, the sensor comprises nonconductive regions of organic polymer, such as insulating organic polymer, interspersed with regions of electrically conductive material. In still other instances, the polymer is a conducting polymer, an insulating polymer with conductive fillers or a plasticized polymer. As the polymer of the sensor sorbs an analyte, the properties of the sensor changes. Many different polymers can be used. Suitable insulating organic polymers include, but are not limited to, main-chain carbon polymers such as poly(dienes), poly(alkenes), poly(acrylics), poly(methacrylics), poly(vinyl ethers), poly(vinyl thioethers), poly( vinyl alcohols), poly( vinyl ketones), poly(vinyl halides), poly(vinyl nitriles), poly(vinyl esters), poly(styrenes), poly(arylenes), and the like, main- chain acrylic heteroatom organic polymers such as poly(oxides), poly(carbonates), polyesters), poly(anhydrides), poly(urethanes), poly(sulfonates), poly(siloxanes), poly(sulfides), poly(thioesters), poly(sulfones), poly(sulfonamides), poly(amides), poly(ureas), poly(phosphazenes), poly(silanes), poly(silazanes), and the like, and main- chain heterocyclic polymers such as poly(furan tetracarboxylic acid diimides), poly(benzoxazoles), poly(oxadiazoles), poly(benzothiazinaphenothiazines), poly(benzothiazoics), poly(pyrazinoquinoxalines), poly(pyromellitimides), poly(quinoxalines), poly(benzimidazoles), poly(oxindoles), poly(oxoisoindolines), poly(dioxoisoindolines), poly(triazines), poly(pyridazines), poly(piperazines), poly(pyridines), poly(piperidines), poly(triazoles), poly(pyrazoles), poly(pyrrolidines), poly(carboranes), poly(oxabicyclononanes), poly(dibenzofarans), poly(phthalides), poly(acetals), poly(anhydrides), carbohydrates, and the like. In a preferred embodiment, the polymers employed are poly(vinyl acetate) (PVA) and poly (methacrylate) (PMMA) and polystyrene (PS). Each of the above organic polymers, and the monomer units that polymerize to form these polymers, are well known to those of skill in the art. One or more of a variety of electrically conductive materials can be employed as the electrically conductive material. In some embodiments, the conductive material is a conducting polymer, or an insulating polymer with conductive fillers. Moreover, in certain other embodiments, the conductive material is a metal such as gold, silver, platinum or other conductive metals well known to those of skill in the art. In other embodiments, the conductive material is a carbonaceous material such as carbon blacks, graphite, coke, C 6 o, and the like. In a preferred embodiment, the conductive material is carbon black. In an equally preferred embodiment, the conductive material can be a particle, such as a gold nanoparticle with a capping ligand shell (see, WO 99/27357 incorporated herein by reference). In certain embodiments, the polymers of the present invention comprise a compatible molecule of low volatility (a plasticizer) as disclosed in U.S. Serial Number 09/338,339, filed June 22, 1999. In another embodiment, the organic mixture comprises a plasticizer combined with an organic polymer blend of a first organic polymer and a second organic polymer. In certain embodiments, the first or second polymers are both insulating polymers. In still another embodiment, the organic mixture comprises a plasticizer combined with an organic polymer formed from a first organic monomer and a second monomer. In yet another embodiment, the organic mixture comprises a plasticizer combined with an interpenetrating network comprising a first organic polymer and a second organic polymer formed from an organic monomer polymerized in the presence of the first organic polymer. In another embodiment, the present invention provide polymer based sensors comprising the plasticized polymeric electrolytes (PPE) (see, K. Buhlmann et al, Sensors and Actuators B, 49, 156-165 (1998). As disclosed therein, the PPE comprises three components: a polymer; a plasticizer; and an organic salt. The polymers included polyvinylchloride (PVC), celluloseacetate, (CA), poly(epichlorohydrin) (PECH), poly(ethyleneoxide) (POX), poly(vinylpropionate) (PVP), poly(-isobutylene) (PLB) from Sigma/ Aldrich GmbH (Germany) and polyurethane from Elastogran BASF (Germany). Suitable plasticizers include, but are not limited to, bis-(2-ethyl- hexyl)phthalate (BEHP), bis-(2-ethylheyxl)sebacate (DOS), dibutylsebacate (DBS), 2- fluorophenyl-2'-nitro-phenylether (2F2NE), ort/rø-nitrophenyloctylether (o-NPOE), tris- (2-ethylhexyl)phosphate (TEHP), tris-(2-ethylhexyl)trimelliate (TEHT) were from Fluka. Triphenylphosphate (TPP) was from Sigma/ Aldrich. Organic salts used included potassiumtetrakis-(4-chlorophenyl)borate (KTpCPB), tridodecylmethylammoniumchloride (TD-MAC), tridodecylmethylammoniumnitrate (TDMAN-03), tetradodecylammonium-(4-chlorophenyl)borate (TDApCPB) and tetraoctylammoniumbromide (TOAB) were all from Fluka. In certain instances, the polymer suitable for use in the present invention comprises a chiral center. For instance, European Patent Application No. 0 794 428, published September 10, 1997, describes sensors capable of distinguishing between enantiomers. The sensor comprise a pair of spaced apart contacts and a conducting polymer material spanning the gap. The polymer has chiral sites in the polymer material formed by incorporating optically active counter ions such as camphor sulfonic acid. Moreover, WO 99/40423, published August 12, 1999, discloses sensor arrays of that are capable of distinguishing or differentiating between chiral compounds, such as enantiomers, i.e., optical isomers. That publication relates to a device for detecting the presence or absence of an analyte in a fluid, the device comprises a sensor, the sensor comprising a chiral region. The sensor comprises a conductive region and a nonconductive region, wherein at least one of the conductive and nonconductive regions is chiral, and wherein the analyte generates a differential response across the sensor. WO 99/00663, published January 7, 1999, discloses a combinatorial approach for preparing arrays of chemically sensitive polymer-based sensors which are capable of detecting the presence of a chemical analyte in a fluid contact therewith. The described methods and devices comprise combining varying ratios of at least first and second organic materials which, when combined , form a polymer or polymer blend that id capable of absorbing a chemical analyte, thereby providing a detectable response. The detectable response of the sensors prepared by this method is not linearly related to the mole fraction of at least one of the polymer-based components of the sensor. In still yet another embodiment, the conductive region can be a conductive particle, such as a colloidal nanoparticle. In this embodiment, the term "polymer" means nanoparticles comprising a central core conducting element and an insulating attached ligand optionally in a polymer matrix. These metallic nanoparticles can be synthesized using a variety of methods. In a preferred method of synthesis, a modification of the protocol developed by Brust et al. can be used, (see, Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D.J.; Whyman, R. J. Chem. Soc, Chem. Commun., 1994, 801-802.) As used herein the term "nanoparticle" refers to a conductive cluster, such as a metal cluster, having a diameter on the nanometer scale. Examples of colloidal nanoparticles for use in accordance with the present invention are described in the literature (see, Templeton et al J. Am. Chem. Soc. (1998) 120 : 1906-1911; Lee et al, Isr. J Chem. (1997) 37: 213-223 (1997); Hostetler et al. LANGMUIR (1998) 14: 17-30; Ingram et al, J. Am. Chem. Soc, (1997) 119 :9175-9178; Hostetler et al, J. Am Chem. Soc.(l996) 118 :4212-4213; Henglein J. Phys. Chem. (1993) 97 :5457-5471; Zeiri, J. Phys. Chem.(l992) 96 :5908-5917; Lef et al, LANGMUIR (1996) 4723-4730. Another preferred nanoparticle is disclosed in WO 99/27357 entitled "Materials, Method and Apparatus for Detection and Monitoring Chemical Species," published June 3, 1999. In another preferred embodiment, the polymers are disclosed in WO 99/31494, which published on June 24, 1999. As disclosed therein, the sensors comprise substituted polythiophenes. One preferred embodiment is poly (3,3"-dihexyl-2-2":5',2"- terthiophene). HI. ANALYTE DETECTION AND APPLICATIONS A wide variety of analytes can be detected using the devices, methods and systems of the present invention. Suitable analytes include, but are not limited to, alkanes, alkenes, alkynes, dienes, alicyclic hydrocarbons, arenes, alcohols, ethers, ketones, aldehydes, carbonyls, carbanions, heterocycles, polynuclear aromatics, organic derivatives, biomolecules, off-gasses of microorganisms, microbes, bacteria and viruses, sugars, nucleic acids, isoprenes, isoprenoids, fatty acids and their derivatives. Moreover, in certain embodiments, the methods and systems of the present invention can be used for monitoring medical conditions and disease processes. For instance, WO 98/29563, published July 9, 1998, and incorporated herein by reference, discloses a method for monitoring conditions in a patient wherein a sample is obtained from a patient over a period of time. The samples are then flowed over a gas sensor and a response is measured. Thereafter, the response is correlated with known responses for known conditions. The conditions include, but are not limited to, the progression and or regression of a disease state, bacterial infections, viral, fungal or parasitic infections, the effectiveness of a course of treatment and the progress of a healing process. In another embodiment, the methods and systems of the present invention can be used for monitoring medical conditions in a respiring subject. For instance, WO 98/39470, published September 11, 1998, and incorporated herein by reference, discloses a method for detecting the occurrence of a condition in a respiring subject. The method comprises introducing emitted respiratory gases to a gas sensing device, detecting certain species present in the gas and correlating the presence of the species with certain conditions. A wide variety of conditions can be ascertained using this aspect of the present invention. These conditions include, but are not limited to, halitosis, ketosis, yeast infections, gastrointestinal infections, diabetes, alcohol, phenylketonuria, pneumonia, and lung infections. Those of skill in the art will know of other conditions and diseases amenable to the methods and systems of the present invention. In yet another embodiment, the methods and systems of the present invention can be used for monitoring conditions and disease processes in female patients. For instance, WO 99/09407, published February 25, 1999, and incorporated herein by reference, discloses a method for detecting the occurrence of a condition in a female patient comprising obtaining a sample of gaseous or volatile substance from the vaginal region of the patient, detecting the gas and correlating the detection with the occurrence of a condition. A wide variety of conditions can be ascertained using this aspect of the present invention. These conditions include, but are not limited to, cervical cancer, ovarian or uterine cancer, HJN, sexually transmitted diseases, cytomegalovirus, yeast infections, pregnancy and Chlamydia. In still yet another embodiment, the methods and systems of the present invention can be used for monitoring conditions and disease processes using a device that affixes to a portion of the skin on a subject. For instance, WO 99/09408, published February 25, 1999, and incorporated herein by reference, discloses a method for detecting a condition of a subject with a device that is adapted to be affixed to the subject and having a gas sensing means disposed so as to detect gases and volatile species emanating from a portion of the skin. A wide variety of conditions can be ascertained using this aspect of the present invention. These conditions include, but are not limited to, skin cancer, diabetes, heart disease, heavy metal in the subject and drugs. In addition to the foregoing applications, a wide variety of commercial applications are available for the sensors arrays and electronic noses including, but not limited to, environmental toxicology and remediation, biomedicine, materials quality control, food and agricultural products monitoring, heavy industrial manufacturing, ambient air monitoring, worker protection, emissions control, product quality testing, oil/gas petrochemical applications, combustible gas detection, H 2 S monitoring, hazardous leak detection and identification, emergency response and law enforcement applications, illegal substance detection and identification, arson investigation, hazardous spill identification, enclosed space surveying, explosives detection, utility and power applications, emissions monitoring, transformer fault detection, food/beverage/agriculture applications, freshness detection, fruit ripening control, fermentation process monitoring and control applications, flavor composition and identification, product quality and identification, refrigerant and fumigant detection, cosmetic/perfume, fragrance formulation, product quality testing, patent protection applications, personal identification, chemical/plastics/pharmaceutical applications, fugitive emission identification, leak detection, solvent recovery effectiveness, perimeter monitoring, product quality testing, hazardous waste site applications, fugitive emission detection and identification, leak detection and identification, perimeter monitoring, transportation, hazardous spill monitoring, refueling operations, shipping container inspection, diesel/gasoline/aviation fuel identification, building/residential natural gas detection, formaldehyde detection, smoke detection, automatic ventilation control applications (cooking, smoking, etc.), air intake monitoring, hospital/medical anesthesia & sterilization gas detection, infectious disease detection and breath applications, body fluids analysis, pharmaceutical applications, drug discovery and telesurgery. In one embodiment, the sensor arrays of the present invention are used to evaluate the progress of chemical reactions, such as asymmetrical synthesis, in a quantitative or qualitative fashion. In this aspect, the arrays provide quick assays for example, the enantiomeric excess or purity of compound, such as in a combinatorial library of catalysts forming the compounds of interest. The methods and devices described herein allow detection and selection of promising leads in a rapid fashion from multiwell plates, and also allow for evaluation of certain compounds for chiral similarity and enantiomeric purity. IV. SENSOR FABRICATION Methods of sensor fabrication are numerous and certain of these methods represent other aspects of the present invention. In certain embodiments, the present invention provides a method for fabricating an array of sensors for detecting an analyte in a fluid comprising: depositing a polymer layer onto a first sensor forming a first predetermined polymer thickness; depositing a polymer onto a second sensor having a second predetermined polymer thickness wherein the first predetermined polymer thickness is different than the second predetermined polymer thickness. In certain embodiments, the sensors of the present invention can be fabricated using a spin casting process. In certain aspects, the spin casting process involves use of a thin-film material dissolved in a volatile liquid solvent. The solution is poured on the sample and the sample is rotated at high speed. Centrifugal force spreads the material and after the solvent evaporates, a thin layer of film remains on the sample. In this aspect, the thickness depends on the solubility of the deposited material and the spin film, and typically is in the range from about 0.01 μm to about 20 μm, and preferably between about 0.05 μm to about 2 μm. Using spin casting, it is possible to fabricate provide an array of sensor elements with various predetermined thicknesses. In another aspect, the sensors of the present invention can be fabricated using sputtering techniques. Sputtering is performed in a vacuum chamber. After evacuation of air, an inert gas, such as argon or helium, is introduced into the chamber at about 2X10 '6 to 5X10 "6 Torn An external high voltage power supply is attached to the cathode which is fabricated of the material to be deposited on the sample. The sample is attached to the anode at some distance from the cathode. High voltage ignites a plasma of the inert gas and the gas ions bombard the target. The kinetic energy of the bombarding ions is sufficiently high to free some atoms from the target surface. Hence, the escaped sputtered atoms deposit on the surface of the sample. Using this technique, very uniform polymer layers are possible. Moreover, materials from more than one target can be deposited at the same time (co-sputtering) permitting a controlled ratio of materials. In another aspect, a chemical vapor phase deposition process is used for the fabrication of sensor array having varying predetermined thicknesses. In this technique, the substrates are positioned on a stationary or rotating table and thereafter the temperature is elevated to the required level by means of the heating elements. The chamber lid has an inlet for the carrier H 2 gas, that can be added by various precursors and dopants. These additives, while being carried over the heated surface of the substrate, form a polymer film layer. Using this technique, sensor arrays can be fabricated having varying predetermined thicknesses. In another embodiment, sensors of the present invention can be coated with a thin layer of chemoselective material providing highly sensitive chemical sensors for the detection and monitoring of vapors and gases using a thin film deposition technique such as matrix assisted pulsed laser evaporation (MAPLE) and MAPLE-Direct Write (MAPLE-DW). In a preferred aspect, the MAPLE technique is used to coat high quality polymer films on SAW devices, and conventional pulsed laser deposition is used to deposit a passivation layer of diamond-like-carbon on a SAW device surface to prevent water adsorption, (see, R. Andrew McGill, et al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 45: 1370-1380 (1998)). Those skilled in the art will know of other deposition techniques suitable for use in fabricating sensors of the present invention. In certain other aspects, the fabrication techniques of the present invention makes use of a common hobbyist's air-brush designed for decorative art. In certain aspects, a polymer solution or suspension is placed in a reservoir and this solution or suspension is stirred while the application is occurring. The apparatus is attached to a regulated supply or compressed air. The flowing air allows the solution or suspension to be drawn from the reservoir and is then ejected from the nozzle of the air brush. This flow of polymer, solvent, and, if present, suspended material is directed toward substrates that have been prepared to receive the films and thereafter become sensor elements. Advantageously, this air-brush method is amenable to automation. Many sensor elements can be fabricated in the time it takes to make one using some prior art methods and has the reproducibility of the best previous methods. A number of well known robotic systems have also been developed and can be adapted for use in the present invention. These systems include automated workstations like the automated apparatus developed by Takeda Chemical Industries, LTD. (Osaka, Japan) and many robotic systems utilizing robotic arms (Zymate IL Zymark Corporation, Hopkinton, Mass.; Orca, Hewlett-Packard, Palo Alto, Calif.) which mimic the manual synthetic operations performed by a person of skill in the art. The nature and implementation of modifications to these devices so that they can operate as discussed herein will be apparent to skilled artisans. Moreover, the air-brush method allows various and odd shaped substrates to be employed. The substrates do not need to be flat and without interruptions in the flat surface. The substrates can be set up such that a single pass of the airbrush coats a large number of sensors. This leads to a large number of very reproducible sensors. The reproducibility can be measured using the baseline resistance difference (BRD). The BRDs between sensors using the airbrush method were all within about 5% of each other. This technology can also be applied to small jets for the purpose of depositing into small wells or onto specific regions on an otherwise flat substrate. In certain aspects, the spray can be made to pass over the whole substrate surface or be made to be directed at only a small area. Custom jet sprayers can be made to direct conical sprays, linear sprays, planer sprays, or other spray geometries. V. EQUILIBRIUM CONSTANTS AND DIFFUSION COEFFICIENTS In another embodiment, the present invention provides systems, methods and devices for simultaneously determining an equilibrium constant (i.e., partition coefficient) as well as a diffusion coefficient. In certain instances, the thinner polymer layers are used to determine the partition coefficient and rapid identification of the analyte, whereas the thicker polymer layers can then be used to determine a diffusion coefficient. The partition coefficient, K, is defined as K = CJC V , wherein C s is the concentration of analyte (solute) in the sorbent phase and C v is the concentration of the analyte in the vapor phase at equilibrium (at steady-state). In the experimental protocol used herein, C v is constant since the vapor generation apparatus is continuously replenishing the vapor stream. Using the sensors of the present invention, it is possible to concurrently or subsequently determine the diffusion coefficient of an analyte with respect to a particular polymer. As such, the present invention provides a method for measuring a diffusion coefficient of an analyte, comprising: contacting a first sensor having a first predetermined polymer thickness to elicit a first response; contacting a second sensor having a second predetermined polymer thickness to elicit a second response; comparing the first response to the second response to calculate a time lag and thereafter measuring the diffusion coefficient of the analyte. In this aspect, it is assumed that the analyte and the polymer membrane or layer do not react. In the absence of a reaction between the analyte and the polymer, Flick's law applies i.e., d_ = D _C_ dt ~ dx 2 Equation I wherein C is the concentration of the analyte and D is the diffusivity. In order to measure the diffusivity experimentally in a polymer based sensor, the time lag procedure is used. Using this procedure, a plot of flux (Δ(ΔR/Ri)/ Δt) versus times yields a straight line whose intercept θ i.e., the time lag, on the t axis is represented by Equation II. L 2 D=— 6Θ Equation II From this graphical relationship the time lag θ is determined. ΔR is equal to Rt-Ri wherein R t is the resistance at time t and Ri is the initial resistance. L is the predetermined polymer thickness of the sensor, the time lag θ is the t intercept and D is the diffusion coefficient. The diffusion coefficient is an important parameter for a variety of reasons. By determining the diffusion coefficient of various polymer and analyte combinations, it is possible to design a very efficient sensor system. Thus, the design and efficiency of polymer based sensors depend in part on the diffusion coefficient. Moreover, the optimum number and kind of polymers in the sensor array depends in part on the diffusion coefficient. Thus, if the analyte is an aromatic polar analyte, certain polymers are better than others. Using the methods of the present invention, optimum polymers can be determined because the polymer /analyte pair can be better matched. In certain embodiments, an array of sensors comprise the same polymer, only the predetermined thickness of the polymer is different. In this aspect, the thinnest polymer sensor has the most resistance (i.e., fewer conductive paths to traverse) and the thickest polymer sensor shows the least resistance (i.e., the most conductive paths to traverse). However, the thinnest sensor will show steady-state conditions faster compared to the thickest sensor which reach steady state conditions at a slower pace. VI. EXAMPLES A. MATERIALS This example illustrates the use of an array of conductive/non-conductive regions sensors. In this example, carbon black is the conductive region material. The carbon black (Black Pearls 2000) is a furnace black material from Cabot Co. (Billerica, MA). The nonconductive region comprises a nonconductive polymer. To prepare the detector substrates, two parallel bands of gold, 50-100 nm thick and separated by 5 mm, are deposited onto conventional 7.5 cm x 2.5 cm glass slides. The slides are then cut into strips to produce 0.7 cm x 2.5 cm pieces of glass, with each strip of glass having one pair of Au leads spaced 5 mm apart. The detectors are made from a solution of polymer into which carbon black has been suspended. In this example, 125 mg of the polymer is dissolved in 10 mL of tetrahydrofuran, and carbon black (42 mg) is then suspended in this solution, to produce a composition of 75% polymer and 25% carbon black by weight of solids. One coating of this suspension is applied to each substrate yielding a film thickness of «1 micron as can be determined by atomic force microscopy. For larger sensor thicknesses, more coats are used. B. METHODS The dc resistance of each detector is determined as a function of time using a simple two-point resistance configuration. Contacts are made to the gold lines by pressure-contacting electrical leads using flat-jawed alligator clips. Resistance data is acquired using a Hydra 2620 A Data Acquisition Unit (John Fluke Mfg. Co.; Everett, WA) which is interfaced to a personal computer. All of the films had resistance values below the 10 MΩ limit of the Hydra 2620 A. To initiate an experiment, five copies of a given detector type are placed into the glass chamber and a background flow of nitrogen is introduced until the resistance of the detectors is stabilized. Solvent vapor streams are then passed over the detectors. The background and analyte flow rates are monitored using two flow meters (Gilmont Instruments, Inc.) which have limits of 0.2 L min "1 to 15.0 L min "1 and 0.0015 L min "1 to 0.310 L min "1 , respectively. In a typical experiment, resistance data on the detectors are collected for 150 s with just the background gas flowing (typically about 1 - 2 L min "1 ) to serve as a baseline. This is followed by a 10 s to 2 minutes data collection while the detectors are exposed to the analyte vapor stream (typically about 200 - 300 mL min "1 ). The detectors are then given 200 - 300 s to recover during which pure background gas was passed through the chamber. The exposure times varied, but steady state values of resistance change are always reached for any given exposure time. Resistances for all detectors in a given trial were monitored contemporaneously through the use of the multiplexing capabilities of the Hydra voltmeter. Results are obtained by running two trials of five exposures each, with the trials performed on different days. Each analyte is exposed to five copies of the detector simultaneously and the results. Example 1 This example illustrates a response of an array of sensors having predetermined polymer thicknesses of 100 nm and 500 nm to an analyte. With reference to Figure 1, a response of an array of detectors having predetermined polymer thicknesses of 100 nm and 500 nm to an analyte is illustrated. In this example, the polymer used was polycaprolactone and the analyte used was benzophenone. All the detectors displayed an increase in resistance upon exposure to the vapor, and returned to their baseline values after the vapor was removed. The change in resistance for the thinnest polymer showed steady-state conditions faster compared to the thickest sensor which reach steady-state conditions at a slower pace. The responses are analyzed by calculating the maximum differential response value, 4 R ma , observed during the exposure period and dividing it by the baseline value of the resistance, R, (taken as the resistance value just before the exposure began) and expressed as: ΔRmax/'Ri Equation III A plot of the response flux as a function of time can be used to calculate a diffusion coefficient. In order to measure the diffusivity experimentally in a polymer based sensor, the time lag procedure is used. The time lag θ is estimated from the time axis intercept and the diffusion coefficient is obtained using Equation II. Example 2 This example illustrates a response of an array of detectors having predetermined polymer thicknesses and analytes having different volatilities. With reference to Figure 2, the responses of an array of detectors having predetermined polymer thicknesses (one thin, one thick) and to different analytes is illustrated. The polymer used was polyethylene oxide and the analytes were hexane and decane. The results indicate that the thin sensor shows steady-state conditions faster compared to the thick sensor which reaches steady state conditions at a slower pace. In addition, the analyte that is more volatile, (hexane), reaches steady-state conditions faster than an analyte that is less volatile (decane). Table 1 shows the responses of the thin and thick sensors versus time as well as the responses of two different analytes. TABLE 1 By monitoring the temporal response lag between different film thicknesses to a response, sensors having advantageous features are realized. For example, when monitoring certain volitilities components, the lag time is an important component which can be monitored. False rates can be minimized if the lag profile does not correspond to the expected lag profile. All publications, patents and patent applications mentioned in this specification are herein incorporated by reference into the specification in their entirety for all purposes. Although the invention has been described with reference to preferred embodiments and examples thereof, the scope of the present invention is not limited only to those described embodiments. As will be apparent to persons skilled in the art, modifications and adaptations to the above-described invention can be made without departing from the spirit and scope of the invention, which is defined and circumscribed by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A sensor array for detecting an analyte in a fluid, said sensor array comprising: a first sensor having a first predetermined polymer thickness; and a second sensor having a second predetermined polymer thickness wherein said first predetermined polymer thickness is different than said second predetermined polymer thickness. 2. The sensors array according to claim 1, wherein said first sensor is a member selected from the group consisting of surface acoustic wave (SAW) sensors, quartz microbalances, organic semiconducting gas sensors, bulk conducting polymer sensors, polymeric coating on an optical fiber sensors, conducting/nonconducting regions sensors and conducting filler in insulating polymer sensors, dye impregnated polymeric coatings on optical fibers, polymer composites, micro-electro-mechanical system devices, micromachined cantilevers, and micro-opto-electro-mechanical system devices. 3. The sensors array according to claim 2, wherein said first sensor has a first sensor thickness of about 0.01 μm to about 20 μm. 4. The sensors array according to claim 3, wherein said first sensor has a first sensor thickness of about 0.05 μm to about 2 μm. 5. The sensors array according to claim 1, wherein said first sensor polymer is a member selected from the group consisting of a conducting polymer, a mixture of conducting polymer and insulating polymer, a plasticized polymer, and polymer having a chiral region. 6. The sensors array according to claim 5, wherein said first sensor polymer is a conducting polymer. 7. A method for detecting the presence of an analyte in a fluid, said method comprising: providing a sensor array comprising a first sensor having a first predetermined polymer thickness; providing a second sensor having a second predetermined polymer thickness wherein said first predetermined polymer thickness is different than said second predetermined polymer thickness; contacting said sensor array with said analyte to generate a response; and detecting said response with a detector that is operatively associated with each sensor, thereby detecting the presence of said analyte. 8. The method in accordance with claim 7, wherein said first sensor is a member selected from the group consisting of surface acoustic wave (SAW) sensors, quartz microbalances, organic semiconducting gas sensors, bulk conducting polymer sensors, polymeric coating on an optical fiber sensors, conducting/nonconducting regions sensors and conducting filler in insulating polymer sensors, dye impregnated polymeric coatings on optical fibers, polymer composites, micro-electro-mechanical system devices, micromachined cantilevers, and micro-opto-electro-mechanical system devices. 9. A system for detecting an analyte in a fluid, said system comprising: a sensor array comprising a first sensor having a first predetermined polymer thickness; a second sensor having a second predetermined polymer thickness wherein said first predetermined polymer thickness is different than said second predetermined polymer thickness; and a detector operatively associated with said sensor array. 10. The system in accordance with claim 9, wherein said detector is optimized to detect a member selected from the group consisting of electromagnetic energy, optical properties, resistance, capacitance, inductance, impedance and combinations thereof. 11. The system in accordance with claim 9, wherein said first sensor is a member selected from the group consisting of surface acoustic wave (SAW) sensors, quartz microbalances, organic semiconducting gas sensors, bulk conducting polymer sensors, polymeric coating on an optical fiber sensors, conducting/nonconducting regions sensors and conducting filler in insulating polymer sensors, dye impregnated polymeric coatings on optical fibers, polymer composites, micro-electro-mechanical system devices, micromachined cantilevers, and micro-opto-electro-mechanical system devices. 12. The system in accordance with claim 9, wherein said first sensor polymer is a member selected from the group consisting of a conducting polymer, a mixture of conducting polymer and insulating polymer, a plasticized polymer, and polymer having a chiral region. 13. A method for fabricating an array of sensors for detecting an analyte in a fluid, said method comprising: depositing a polymer layer onto a first sensor forming a first predetermined polymer thickness; and depositing a polymer onto a second sensor having a second predetermined polymer thickness wherein said first predetermined polymer thickness is different than said second predetermined polymer thickness. 14. The method according to claim 13, wherein said depositing is performed using a member selected from the group consisting of spin casting, vacuum deposition, sputtering, MAPLE, air brushing and chemical vapor deposition. 15. A method for measuring a diffusion coefficient of an analyte, said method comprising; contacting a first sensor having a first predetermined polymer thickness with said analyte to elicit a first response; contacting a second sensor having a second predetermined polymer thickness with said analyte to elicit a second response; comparing the first response to the second response to calculate a time lag and thereafter measuring the diffusion coefficient of said analyte. 16. The method according to claim 15, wherein said first sensor is a member selected from the group consisting of surface acoustic wave (SAW) sensors, quartz microbalances, organic semiconducting gas sensors, bulk conducting polymer sensors, polymeric coating on an optical fiber sensors, conducting/nonconducting regions sensors and conducting filler in insulating polymer sensors, dye impregnated polymeric coatings on optical fibers, polymer composites, micro-electro-mechanical system devices, micromachined cantilevers, and micro-opto-electro-mechanical system devices. 17. The method according to claim 15, wherein said first sensor has a first sensor thickness of about 0.01 μm to about 20 μm. 18. The method according to claim 17, wherein said first sensor has a first sensor thickness of about 0.05 μm to about 2 μm. 19. The method according to claim 15, wherein said first sensor polymer is a member selected from the group consisting of a conducting polymer, a mixture of conducting polymer and insulating polymer, a plasticized polymer, and polymer having a chiral region. 20. The method according to claim 15, wherein said first sensor polymer is a conducting polymer. 21. A method for simultaneously determining a partition coefficient and a diffusion coefficient of an analyte, said method comprising: contacting a first sensor having a first predetermined polymer thickness with said analyte to elicit a first response; contacting a second sensor having a second predetermined polymer thickness with said analyte to elicit a second response; comparing the first response to the second response thereby simultaneously determining said partition coefficient and said diffusion coefficient of an analyte. 22. The method according to claim 21, wherein said diffusion coefficient is determined using the lag time method.

Inventors: FREUND MICHAEL, LEWIS NATHAN S, MATZGER ADAM J, SEVERIN ERIK J, FREUND, MICHAEL, LEWIS, NATHAN, S., MATZGER, ADAM, J., SEVERIN, ERIK, J.
Applicants: CALIFORNIA INST OF TECHN, FREUND MICHAEL, LEWIS NATHAN S, MATZGER ADAM J, SEVERIN ERIK J, CALIFORNIA INSTITUTE OF TECHNOLOGY, FREUND, MICHAEL, LEWIS, NATHAN, S., MATZGER, ADAM, J., SEVERIN, ERIK, J.
Application year: 1999
Main classification: G01N15
IPC codes: G01N15, G01N27, G01N29, G01N33