Abstract:
A method of avoiding or delaying degradation of a transduction molecule in a trace gas sensor by controlling oxygen exposure is disclosed. Degradation of the gas sensor can be avoided by storage of the sensor in a low-oxygen or substantially oxygen-free environment.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to methods for controlling the degradation of transduction molecules in trace gas sensors.  
         [0003]     2. General Background  
         [0004]     Trace gas analysis is a promising tool for many applications. For instance, in the medical field, changes in exhaled nitric oxide (NO) concentration in exhaled breath can indicate a change in the level of inflammation in the airway of an asthmatic, indicating an increase in the likelihood of an asthma attack. Trace gas analysis may also be useful in measuring other trace constituents of exhaled breath, such as carbon monoxide. Also, there is a need for means to measure trace gases in the atmosphere, for environmental assessment, and to measure trace gases in manufacturing and industrial settings.  
         [0005]     To quantify the concentration of trace gases, various sensors have been developed. Some of these sensors detect and measure changes in substances in response to the trace gas analyte. For instance, a sensor developed by the present inventors measures the optically-quantifiable changes in xerogel (stabilized sol-gel) encapsulated cytochrome-c in response to nitric oxide (NO). This sensor and related technology are disclosed in the following U.S. patent applications, the disclosures of which are hereby incorporated herein by reference: Ser. No. 10/334,625, filed 30 Dec. 2003, and Ser. No. 10/767,709, filed 28 Jan. 2004.  
         [0006]     However, previous work with transduction molecule trace gas sensors has revealed a potential vulnerability: rapid degradation of the sensor. In particular, the applicants have found that in normal circumstances (when attempting to measure trace concentrations) their cytochrome-c sensor would degrade rapidly. The degradation time depends on many parameters, such as temperature, etc., but this unpredictability would make such a sensor impracticable for commercial use, since the sensor may not be usable by the time it reached the end user.  
         [0007]     For purposes of this patent, degradation is defined to include any loss in the sensor&#39;s functionality, including the sensor&#39;s loss in responsivity to the analyte of interest (e.g. NO) in both magnitude and time-course. It is also defined in the case of cytochrome-c/NO as the loss in the soret peak, which is the spectral peak of the iron porphyrin (the active part of the heme-protein). Thus, one can measure the loss in reactivity to NO or the loss in the magnitude of the soret peak centered around 400 nm. The technology of this application is designed to work with sensors that have sensing elements with transduction molecules, where such molecules undergo optical or electrical changes in response to the analyte.  
         [0008]     The cause or causes of this degradation were previously unknown, but the applicants have now discovered the primary mechanism that causes the degradation, and therefore have created the present invention which preserves transduction molecule trace gas sensors for a sufficient period of time to allow for the creation of a viable commercial product.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is a method of reducing the degradation of a transduction molecule in a trace gas sensor by controlling the exposure of the protein to oxygen. Through their research, the applicants have discovered that oxidation is responsible for the rapid degradation of the sensor. To combat this degradation, the sensor can be stored in a low-oxygen or a substantially oxygen-free environment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a graph showing the degradation of a cytochrome-c NO sensor over time. The y-axis on this graph represents the change in absorbance in a NO sensor in 90 seconds when the humidity surrounding the sensor is fixed at 200 ppm water and the sensor is reacted with 500 ppb NO.  
         [0011]      FIG. 2  is a graph comparing the NO response after 7 days at 70° C. for sensors aged in an ambient environment, and those aged in an oxygen free environment. 
     
    
     DETAILED DESCRIPTION  
       [0012]     The applicants discovered that oxidation was causing the unacceptable degradation of the sensor. The applicants made this discovery in the context of developing a transduction sensor comprised of cytochrome-c encapsulated in a sol-gel matrix. The applicants left sensors in the ambient air, and the sensors had a detectable loss in optical density within 24 hours. When the sensors were left in a nitrogen purged environment, the sensors retained all optical density and substantially all responsivity. This indicated that a constituent in the atmosphere besides nitrogen caused the degradation.  
         [0013]     Next, the applicants performed experiments to isolate the cause of the degradation. The applicants measured percentage of degradation in various environmental conditions, as shown below.  
                                                   Experimental Groups   Stability (% degradation)                           1. Initial @ 6% RH −&gt;   −78%           O2, salt @ 6% RH (control)           2. Initial @ 6% RH −&gt;   −10%           No O2, 16% 3A @ &lt;0.1% RH                      
 
         [0014]     In the first experimental group, which is the control, a sensor was taken from an initial ambient environment with 6% relative humidity and then placed in an environment with ambient oxygen and salt at 6% relative humidity for a period equivalent to 220 days at room temperature. The 6% RH was maintained by a saturated solution of LiBr. Under these storage conditions a degradation of 78% in sensor performance was observed, meaning that this sensor was 78% less sensitive to NO after exposure to the tested environment. To determine percentage of degradation, the applicants first made a baseline measurement of the reactivity of the sensor to 500 ppb NO in air, and then measured the degree to which the reactivity was lost after exposure to the testing environment.  
         [0015]     In the second experimental group, a sensor was placed in an environment with no oxygen and a relative humidity of 0.1% and a 3 A molecular sieve for the equivalent to 220 room temperature days. This sensor experienced a 10% degradation rate. This confirms that oxygen is the primary cause of the degradation, and that RH also contributes to the problem.  
         [0016]     The discovery that oxygen is a major cause of degradation is surprising, since the applicants are aware of no prior art teaching that protein-based gas sensors need to be stored in an oxygen-deprived environment. For instance, to the applicants&#39; knowledge, previous protein based sensors have not been stored in oxygen-deprived environments, but instead typically only require removal of moisture for storage.  
         [0017]     The applicants believe that their sensing element is especially sensitive to oxygen degradation because it is has a high surface area, and this increases the susceptibility of the device to oxygen. In one embodiment, the applicant&#39;s sensing element is cytochrome-c in a sol-gel with a surface area of approximately 400 m2/g.  
         [0018]     A number of different techniques can be used to control the degradative effects of oxygen. In one embodiment of the present invention, nitrogen or another suitable substance can be used to purge oxygen from the sensor housing, and then the sensor housing can be sealed in an oxygen-free (i.e. oxygen-purged) packaging environment. For instance, the purging can be accomplished with five cycles of nitrogen, based on sensor volume and sensor housing volume. Or a vacuum can be created within the housing, either with our without nitrogen purging.  
         [0019]     In a second embodiment, an oxygen absorber can be used to remove oxygen from a sealed sensor housing. The oxygen absorber could be OS film from Cryovac of Cerritos, Calif., or one of the oxygen absorbers (such as PharmaKeep®) from Sud-Chemie of Belen, New Mexico, or any other suitable oxygen absorber. In this embodiment, the oxygen absorber could be placed in the packaging with the sensor. The sealed sensor housing could be made of permeable material that allows the exit of oxygen into the packaging environment and from there into the oxygen absorber. Purging of oxygen from the sensor housing is optional in this embodiment.  
         [0020]     In a third embodiment, an oxygen absorber can be used to remove oxygen from an unsealed sensor housing. This embodiment is similar to the second embodiment, except that the sensor housing is unsealed to facilitate diffusion of oxygen to the absorber. Purging of oxygen from the sensor housing is also optional in this embodiment.  
         [0021]     One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments, which are presented for purposes of illustration and not of limitation.