Patent Publication Number: US-2010121210-A1

Title: Method and device for detection ofanalyte in vapor or gaseous sample

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/105,788, filed on Oct. 15, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Various aspects of the invention generally relate to a method and system for detection of an analyte, for example, a hydrophilic analyte, in a vapor or gaseous sample, and more specifically to a corresponding method and system for detecting  Streptococcus pneumoniae  infection. 
     BACKGROUND OF THE INVENTION 
     Electrochemistry is a branch of chemistry that deals with the current produced by an electron transfer reaction of an electrochemically active species at the surface of a conductive electrode driven by an externally applied potential. Electrochemistry is a powerful tool in analytical chemistry to detect various compounds, for example, hydrogen peroxide, but traditionally this tool has been limited to detection of compounds present in an aqueous solution. 
     It has been known that  Streptococcus pneumoniea  bacterium present in the upper respiratory system of human beings and other mammals produce hydrogen peroxide. The amount of hydrogen peroxide produced by  Streptococcus pneumoniea  is a direct indicator of the population of  Streptococcus pneumoniea  present in a human body. Electrochemical analysis can give detailed information about the concentration of hydrogen peroxide in solution, but traditionally has not been used to study gaseous or vapor phase hydrogen peroxide present, for example, in an exhaled breath of a patient. Although  Streptococcus pneumoniea  is a common, innocuous inhabitant of the upper respiratory system of healthy humans, excessive undesirable amounts of this bacterium is the leading cause of infectious disease in young children and the elderly as documented, for example, in Tomasz, “ Streptococcus pneumoniae : Molecular Biology and Mechanisms of Disease,” ISBN 0-913113-85-9, Mary Ann Liebert, Inc., Larchmont, N.Y. (2000). A healthy immune system keeps the bacteria population safely under control, but if the immune system is weakened, the bacteria can become invasive and cause diseases like pneumonia, otitis media, and meningitis. 
     Conventional treatments for  Streptococcus pneumoniae  infection are very harsh and invasive where a person infected with an over-population of this bacterium has his or her immune system effectively shutdown. Risk of infection in patients with this weakened immune state is high, and without the body&#39;s natural defenses to check the invading  Streptococcus pneumoniae , even a mild infection can become dangerous to the point of fatality as documented in Tuomanen, “The Biology of Pneumococcal Infection,”  Pediatric Research  42:253-258 (1997). Further, because  Streptococcus pneumoniae  is able to adapt and mutate to become resistant to antibiotics, it is not desirable to administer this treatment unless infection becomes a problem as documented in Schmidt, “Genes and Antibiotic Resistance,” Genome New Network (2000). 
     Conventional systems and methods for monitoring and diagnosis of  Streptococcus pneumoniae  levels require clinical symptoms to first become apparent, and then the diagnosis involves the use of invasive techniques to detect presence of  Streptococcus pneumoniae.    
     Unfortunately, there is no direct method or system to detect amount of hydrogen peroxide produced by the  Streptococcus pneumoniae.    
     What is needed, therefore, is a non-invasive technique that can rapidly detect an analyte, for example, hydrogen peroxide produced by  Streptococcus pneumoniae , in a vapor or gas phase sample, such as a patient&#39;s breath. 
     The present invention is directed to overcoming these and other deficiencies in the art. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention relates to a method for electrochemical detection of an analyte in a vapor or gas sample, the method including exposing a vapor or gas sample to an electrochemical sensor comprising one or more electrodes and a coating that surrounds the one or more electrodes, which coating is capable of partitioning the analyte directly from the vapor or gas sample. The method includes detecting an oxidation/reduction current during the exposing, wherein the detected current relates to a concentration of analyte in the vapor or gas sample. 
     A second aspect of the present invention relates to a method of detecting a  Streptococcus pneumoniae  infection in a patient, the method including exposing a patient breath sample to an electrochemical sensor including one or more electrodes and a coating that surrounds the one or more electrodes, which coating is capable of partitioning hydrogen peroxide directly from the breath sample; and detecting an oxidation/reduction current during said exposing, wherein the detected current relates to a concentration of hydrogen peroxide in the patient breath sample, the concentration of hydrogen peroxide indicating the extent of the  Streptococcus pneumoniae  infection. 
     A third aspect of the present invention relates to an electrochemical sensor that includes two or more electrodes, and a coating that surrounds the two or more electrodes and is capable of selectively partitioning an analyte, e.g., hydrogen peroxide, from a vapor or gas phase such that an oxidation/reduction current within the coating can be measured. 
     The accompanying examples set forth herein demonstrate the development and testing of a hydrogen peroxide electrochemical sensor and detector device using a coated electrochemical sensor whose coating is capable of selectively partitioning hydrogen peroxide from a vapor or gas phase (e.g., patient breath sample). Because the partitioned analyte concentration in the coating is in equilibrium with the vapor or gas phase, i.e., the analyte concentration in the coating is proportional to the concentration in the vapor or gas phase, a reliable assessment can be made concerning the analyte concentration in the vapor or gas phase. In addition, by correlating the vapor or gas phase analyte concentration to the severity of  Streptococcus pneumoniae , it is possible to assess the extent of the  Streptococcus pneumoniae  infection in the patient from whom a sample was obtained. Further, detecting presence of the analyte in the vapor or gas sample (rather than in a liquid sample) improves the selectivity of the sensor against dissolved analyte that does not equilibrate with the vapor or gas phase sample. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic and functional block-diagram of an exemplary setup and environment in which various aspects of the invention can be used; 
         FIG. 2  illustrates an exemplary electrochemical sensor used for detection of hydrogen peroxide according to various aspects of this invention; 
         FIG. 3  illustrates an exemplary experimental setup for testing the electrochemical sensor of  FIG. 2  according to various aspects of this invention; 
         FIGS. 4A and 4B  illustrate graphs of current versus voltage for detection of hydrogen peroxide according to the experimental setup of  FIG. 3 ; 
         FIG. 5A  illustrates a graph of current versus voltage for detection of hydrogen peroxide with ohmic drop reduced in the electrochemical sensor and  FIG. 5B  illustrates a logarithmic plot of the voltage output by the electrochemical sensor versus the concentration of hydrogen peroxide, according to various aspects of this invention; 
         FIG. 6A  illustrates a graph of current versus voltage for detection of hydrogen peroxide with an acetate buffer added to the gelatinous coating on the electrochemical sensor and  FIG. 6B  illustrates a logarithmic plot of the voltage output by the electrochemical sensor versus the concentration of hydrogen peroxide according to various aspects of this invention; and 
         FIG. 7  is a flow chart of a method for detection of hydrogen peroxide according to various aspects of this invention. 
     
    
    
     While these examples are susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail various aspects of the invention, with the understanding that the present disclosure is to be considered as an exemplification and is not intended to limit the broad aspect of the embodiments illustrated in the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to electrochemical sensors and their use for the detection of an analyte present in a vapor or gas phase. As used herein, the term “vapor or gas phase” can be any such fluid sample, for example, a sample of exhaled breath that includes both vapor and gaseous components. The sample can be unmodified prior to analysis in accordance with the aspects of the present invention. As used herein, the “analyte” can be any electrochemically active analyte that partitions from the gas/vapor phase into a coating on the electrochemical sensor. Exemplary analytes include, without limitation, hydrogen peroxide, carbon dioxide, and ammonia. 
     Referring to  FIG. 1 , an exemplary setup and environment  100  for detection of an analyte in a vapor or a gas phase is illustrated. In the setup and environment  100 , a detector device  140  includes an inlet  104  through which a vapor or a gas sample is introduced into detector device  140  and an outlet  106  through which the inlet vapor or gas exits detector device  140 . For example, through inlet  104 , a patient P introduces expelled breath into detector device  140 . To facilitate exposure to the vapor or gas sample, detector device  140  can include a mouthpiece that defines inlet  104  and/or outlet  106  through which patient P exhales, thereby causing the vapor or gas sample to pass over an electrochemical sensor  102  coupled to inlet  104 . Alternatively, detector device  140  can be incorporated into a portion of an intubation tubing such that sensing of the exhaled vapor or gas can be achieved passively, i.e., without patient P actively participating in introducing the vapor or gas sample. According to yet another alternative aspect of the invention, detector device  140  may include only a single inlet/outlet but also includes expandable reservoir (e.g., a balloon) that receives and collects the expelled breath. After patient P fills the reservoir, the expelled breath is subsequently released through the single inlet/outlet while or after the received expelled breath is analyzed by detector device  140 . It is to be noted that all example arrangements of inlet  104  and outlet  106  disclosed immediately above will allow hydrogen peroxide in the expelled breath to reach electrochemical sensor  102 , and the various aspects of the invention are not limited by type of inlet  104  and/or outlet  106  used. 
     Electrochemical sensor  102  coupled to inlet  104  of detector device  140  receives the vapor or the gas sample in the exhaled breath. In response to the vapor or gas sample passing over electrochemical sensor  102 , an output current is produced from a reduction-oxidation (redox) reaction at electrochemical sensor  102  as explained in more detail with respect to  FIGS. 2 and 3  below. The construction and structure of electrochemical sensor  102  is described in more detail below in relation to  FIG. 2 . The amount of output current produced is in direct correlation to an amount of hydrogen peroxide present in the vapor or gas sample. The output current from electrochemical sensor  102  is coupled to a current/voltage detector  108  configured to filter unwanted frequencies from the output current, i.e., background current/voltage caused by ambient air. Optionally, depending upon specific applications, current/voltage detector  108  can convert the detected current output from electrochemical sensor  102  into a corresponding calibrated value, as will be apparent to those skilled in the art in view of this disclosure. Further, output from electrochemical sensor  102  may be directly fed to controller  130 . The output of current/voltage detector  108  is a conditioned current substantially free from noise and other undesirable frequencies. 
     The conditioned current at the output of current/voltage detector is provided to an analog to digital converter (ADC)  110  inside controller  130 . ADC  110  converts the analog output of current/voltage detector  108  to a corresponding digital value for processing by controller  130 . The digital value of the detected current is provided to central processing unit (CPU)/processor  112  via an internal bus  138 . By way of example only, ADC  110  can be an 8-bit ADC, although other types of ADCs may also be used as known to those skilled in the art. 
     CPU/processor  112  receives and processes the digital current from ADC  110 . CPU/processor  112  can be a single board computer which includes one or more microprocessors or CPUs. Controller  130  may be conveniently implemented using one or more general purpose computer systems, microprocessors, digital signal processors, and micro-controllers, programmed according to the teachings described and illustrated herein. For example, CPU/processor  112  can be an Intel Core Duo® processor provided by Intel Corporation of Santa Clara, Calif. Alternatively, CPU/processor  112  may be a special purpose processor designed and fabricated to carry out various aspects of this invention. For example, CPU/processor  112  may be an application specific integrated circuit (ASIC) chip. 
     CPU/processor  112  is coupled to a memory  114  that stores various settings for detector device  140 . For example, memory  114  stores a threshold value of the output current from electrochemical sensor  102 . Memory  114  can be a random access memory (RAM) and/or read only memory (ROM), along with other conventional integrated circuits used on a single board computer as are well known to those of ordinary skill in the art. Alternatively or in addition, memory  114  may include a floppy disk, a hard disk, CD ROM, or other computer readable medium which is read from and/or written to by a magnetic, optical, or other reading and/or writing system that is coupled to one or more processors. Memory  114  can include instructions written in a computer programming language or software package for carrying out one or more aspects of the present invention as described and illustrated herein, although some or all of the programmed instructions could be stored and/or executed elsewhere. For example, instructions for executing steps outlined in  FIG. 7  can be stored in a distributed storage environment where memory  114  is shared between one or more controllers similar to controller  130 . 
     Controller  130  can include an input/output (I/O) device  116  (e.g., an I/O card) coupled to CPU/processor  112  and a display  118  via internal bus  138 . I/O device  116  includes a bi-directional port  122  for communication to/from other computing and/or electronic devices via a link  136 . By way of example only, I/O device  116  can be a keypad/keyboard that is capable of providing user inputs. Port  122  can also be used for charging detector device  140  when used in a mobile or wireless mode. By way of example only, port  122  can be a Universal Synchronous Bus (USB) port, although other types of communication and input/output ports may also be used, as known to those skilled in the art. 
     Internal bus  138  is designed to carry data, power and ground signals, as known to one skilled in the art. By way of example only, internal bus  138  can be a Peripheral Component Interconnect (PCI) bus, although other types of local buses (e.g., Small Computer System Interface or “SCSI”) may also be used, as known to those skilled in the art. 
     Display  118  can be a suitable display panel on which instructions and data are presented to a user in both textual and graphic format. In addition, display  118  can include a touch screen also coupled to I/O device  116  for accepting input from a user (e.g., a medical professional or patient P). Display  118  can display the concentration of the analyte in the vapor or gas sample based on the output current or voltage that is generated by electrochemical sensor  102 . Further, display  118  may be substituted by or used in conjunction with an audio device (e.g., a speaker, a buzzer, or a beeper alarm) controlled by CPU/processor  112  to indicate various conditions resulting from patient P exhaling into inlet  104 . 
     Controller  130  receives power from a power supply  120 . Power supply  120  can be a battery or a direct pluggable outlet to a main power-line. 
     Alternatively, power supply  120  may be a switched mode power supply (SMPS) commonly used in computer systems, although other forms for powering controller  130  using power supply  120  may also be used, as known to those skilled in the art. 
     Using electrochemical sensor  102  of the present invention in combination with state of the art techniques for assessing  Streptococcus pneumonia  load, it is possible to generate empirical data that correlates detected conditioned current levels with the  Streptococcus pneumonia  counts. This empirical data can be used to form a model. 
     Based upon a model stored in memory  114  that correlates hydrogen peroxide concentration in the vapor/gas sample, i.e., detected signal levels from electrochemical sensor  102 , to  Streptococcus pneumonia  infection, detector device  140  can also monitor and/or semi-quantitatively identify the severity of an infection in patient P. Output related to the severity of infection can also be displayed on display  118 , for example. Alternatively, detector device  140  may sound an alarm or a beep to indicate that patient P has an infection or the severity of infection. That is, a higher measured hydrogen peroxide concentration, which correlates to a higher  Streptococcus pneumonia  load, may signal an alarm that differs in kind from a signal that corresponds to a lower measured hydrogen peroxide concentration. 
     In addition, various aspects of the present invention can also be used to monitor the sufficiency of a treatment of  Streptococcus pneumoniae  infection by assessing the concentration of hydrogen peroxide in exhaled breath both during and following a course of antibiotic treatment. Where an antibiotic has no effect even during a mid-course of a multi-day treatment regimen, it remains possible to monitor patient P&#39;s response and, if desired, switch therapies prior to completion of the particular course of therapy. 
     Various components of controller  130  (e.g., CPU/processor  112  along with memory  114 ) embody a computer readable medium having stored thereon instructions for determining an amount of an analyte (e.g., hydrogen peroxide) in a vapor or gas sample (e.g., patient P&#39;s exhaled breath). The instructions can include machine executable code which when executed by CPU/processor  112  causes CPU/processor  112  to perform steps of flowchart  700  described in  FIG. 7  below and carry out the various methods disclosed herein. In addition, the computer readable medium can have instructions for making various decisions for operation of different aspects of the invention, including correlating the detected output current from electrochemical sensor with one or more values stored in memory  114  to determine whether patient P is infected with  Streptococcus pneumoniae  or not. Further, controller  130  can include other numbers and types of components, parts, devices, systems, and elements in other conventional components. 
     It is to be noted that although electrochemical sensor  102  is shown within detector device  140 , various aspects of the present invention may equally be realized using a standalone electrochemical sensor  102  externally coupled to controller  130 , as will be apparent to those skilled in the art in view of this disclosure. For example, one skilled in the art after reading this disclosure may modify the detector for trace level detection of analytes using artificial olfactometry as described in U.S. Pat. No. 6,244,096 to Lewis et al., which is hereby incorporated by reference in its entirety, to make a suitable detector that can include electrochemical sensor  102  of the present invention. 
     In addition, two or more computing systems or devices can be substituted for any one of the systems described above. Accordingly, principles and advantages of distributed processing, such as redundancy and replication, also can be implemented, as desired, to increase the robustness and performance of the devices and systems described above. The embodiments of the present invention may also be implemented on computer system or systems that extend across any suitable network using any suitable interface mechanisms and communications technologies, including, by way of example only, telecommunications in any suitable form (e.g., voice and modem), wireless communications media, wireless communications networks, cellular communications networks, G3 communications networks, Public Switched Telephone Networks (PSTNs), Packet Data Networks (PDNs), the Internet, intranets, and combinations thereof. 
     Referring to  FIG. 2 , an exemplary structure of electrochemical sensor  102  is illustrated. Element  210  illustrates a bottom plan view of electrochemical sensor  102 . Electrochemical sensor  102  includes three micro-electrodes fabricated inside a borosilicate capillary structure with three compartments, although other numbers and types of electrodes, for example, two or four electrodes, may also be used. More specifically, according to one aspect of the invention, electrochemical sensor  102  includes a reference electrode  202 , a counter electrode  204  and a working electrode  206 , each of which have a tip at least one end surrounded with a coating  208  over which one or more molecules of an analyte pass. Alternatively, coating  208  may cover or surround more than the tip of reference electrode  202 , counter electrode  204  and working electrode  206 , for example, the whole of electrochemical sensor  102  could be embedded in the coating material. 
     Coating  208  is capable of selectively partitioning an electrochemically active analyte directly from the vapor or gas phase sample such that an oxidation/reduction current within coating  208  can be measured by two or more electrodes among reference electrode  202 , counter electrode  204  and working electrode  206 . 
     According to various aspects of the invention, a suitable structural component can be utilized in coating  208 . The structural component can be polymeric or non-polymeric. Exemplary structural components include, without limitation, polyvinylchloride (PVC), silicone rubber, polyurethane, (meth)acrylate polymer, polypyrrole, polythiophene, polyoctylthiophene, polyanaline, polyvinyl pyrrolidone, agarose, hydrogel, (meth)acrylate gels, sol-gel materials, and combinations thereof. 
     According to other aspects of the invention, a suitable water immiscible organic solvent can be utilized in coating  208 . The organic solvent is responsible for assisting in the partitioning of the analyte of interest from the vapor or gas sample into coating  208 . Exemplary water immiscible organic solvents include, without limitation, 2-nitrophenyl octyl ether (o-NPOE), dioctyl sebacate (DOS), bis(2-ethylhexyl) sebacate, benzyl s-nitrophenyl ether, bis(1-butyipentyl) adipate, bis(2-ethylhexyl)adipate, bis(2-ethylhexyl)phthalate, 1-chloronaphthalene, chloroparaffin, 1-decanol, dibutyl phthalate, dibutyl sebacate, dibutyl-dilaurate, dodecyl 2-nitrophenyl ether, and combinations thereof. 
     According to another aspect, a suitable charge transfer agent can be utilized in coating  208 . Exemplary charge transfer components include, without limitation, tetradecylammonium tetrakis(pentofluorophenyl)borate (TDATPFPB), tetrahexylammonium perchlorate, and combinations thereof. 
     According to another aspect, a suitable membrane resistance controlling agent can be utilized in coating  208 , when desired. Exemplary membrane resistance controlling agents include, without limitation, lipophilic electrolytes, tetradodecyl ammonium-tetrakis(4-chlorophenyl) borate (ETH500), bis(triphenylphoranylidene) ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (BTPPATFPB), and combinations thereof. 
     According to another aspect, a suitable biocompatibility enhancing component can be utilized in coating  208 , when desired. Exemplary biocompatibility enhancing components include, without limitation, nitric-oxide releasing sol-gel materials, N-(6-aminohexyl)aminopropyltrimethoxysilane, balanced isobutyltrimethoxysilane diazeniumdiolate, and combinations thereof. 
     According to another aspect, coating  208  is formed from a composition including about 15 to about 67 weight percent PVC, about 33 to about 85 wt percent o-NPOE, and about 0.001 to about 15 weight percent TDATPFPB. 
     According to another aspect, coating  208  can contain a structural component, a water and glycerol mixture, and salt. The mixture of glycerol and water is intended to reduce the evaporative loss of water. Coating  208  may optionally contain one or more further additives including, without limitation, a charge transfer component, a membrane resistance controlling component, and a biocompatibility enhancing component as described above. Any of the above-identified structural components can be utilized, preferably polyvinylchloride (PVC), (meth)acrylate gels, agarose, hydrogel, sol-gel materials, and combinations thereof. By way of example only, the glycerol-to-water ratio is at least about 1:3, more preferably about 1:4 up to about 1:20, and most preferably about 1:5 up to about 1:10, although other ratio values may also be used, depending upon specific applications, as known to those skilled in the art. 
     Coating  208  can be of a suitable dimension that affords effective partitioning while allowing for sufficient oxidation/reduction current within coating  208 . For example, and not by limitation, coating  208  is less than about 200 μm thick, more preferably less than about 100 μm thick. According to one embodiment, coating  208  has a sub-micron thickness. According to another embodiment, coating  208  is between about 1 to about 25 μm thick. According to various aspects of the invention, the thickness of coating  208  can be optimized (e.g., by maintaining a constant salt concentration) for replication and for keeping the peak output current constant. 
     Reference electrode  202 , counter electrode  204  and working electrode  206  can be formed out of a suitable conductive material including, without limitation, carbon, gold, platinum, palladium, ruthenium, rhodium or combinations thereof. Although only three microelectrodes—reference electrode  202 , counter electrode  204  and working electrode  206  are described with respect to  FIG. 2 , according to certain embodiments four electrodes can be present. Further, various aspects of the invention are not limited by specific arrangement and structure of reference electrode  202 , counter electrode  204  and working electrode  206  shown in  FIG. 2 , and one skilled in the art after reading this disclosure may devise other arrangements and structures. Exemplary electrode functions include, working electrode, auxiliary or counter electrode, and reference electrode. The particular function and number of electrodes will depend upon the type of electrochemical sensor  102  that is employed, and aspects of the present invention are not limited by specific formation(s) of electrochemical sensor  102 . 
     Exemplary electrochemical sensor  102  types include, without imitation, voltammetric sensors, potentiometric sensors, conductometric sensors, and coulometric sensors. A voltammetric sensor can include, without limitation, one or more working electrodes (e.g., working electrode  206 ) in combination with reference electrode  202 , or one or more working electrodes (e.g., working electrode  206 ) in combination with reference electrode  202  and counter electrode  204 , as shown in  FIG. 2 . In voltammetry, the potential applied to working electrode  206  is varied over time to measure the current through coating  208 . 
     Alternatively, a conductometric sensor can include two or four electrodes, which measure the impedance of the coating; a potentiometric cell can include two electrodes, in which the potential of the indicator electrode is measured at zero current; and a coulometric sensor can include two or more electrodes. The design and principles surrounding these types of electrochemical sensors are described, for example, in Toth et al., “Electrochemical Detection in liquid Flow Analytical Techniques: Characterization and Classification,”  Pure Appl. Chem.  76(6):1119-1138 (2004), which is hereby incorporated by reference in its entirety. 
     According to a further aspect of the invention, the electrochemical sensor can be incorporated into a microfluidic sensor that includes a microfluidic channel and coated electrode(s) positioned with coating  208  in communication with the microfluidic channel through which the vapor or gas sample passes during the detection procedure. Such a microfluidic sensor can be used to assess the presence or quantity of the analyte of interest in the sample. 
     EXAMPLES 
     The Examples set forth below are for illustrative purposes only and are not intended to limit, in any way, the scope of the present invention. 
     Example 1 
     Construction of Sensors 
     A prototype sensor according to  FIG. 2  was fabricated using working electrode  206  formed of a 25 μm diameter platinum wire, counter electrode  204  formed of a cluster of 8 μm diameter carbon fiber, and reference electrode  202  formed of a 75 μm silver wire. According to an exemplary fabrication technique for electrochemical sensor  102 , a capillary holding working electrode  206 , counter electrode  204 , and reference electrode  202  was sealed at one end over a Bunsen burner. Electrode wires were then placed in separate compartments, and the capillary placed in a heating coil under vacuum to collapse the glass and seal the electrode wires. Each of the working electrode  206 , counter electrode  204 , and reference electrode  202  was polished using fine sandpaper and a polishing pad with silicon paste to expose a surface of the three electrodes. Connections to working electrode  206 , counter electrode  204 , and reference electrode  202  were made using electrical wire held by silver epoxy and the electrodes were tested for connectivity in a 0.1 M phosphate buffer solution. 
     Coating  208  was optimized to maximize salt content, spreading, and solidity, and to minimize moisture loss. Agar gel (provided, for example, by Fischer Scientific of Pittsburgh, Pa., which has a low gelation temperature) in water (acting as a solvent) was used to form coating  208 . To increase drying time of the gel for better charge flow, a mixture of glycerol and water was used in different ratios (2:1, 1:1, 1:2, 1:3, and 1:5). The glycerol:water mixtures were saturated with sodium nitrate to determine their loading capacity prior to adding Agar gel in powder form. As demonstrated by the results below, of these mixtures tested, a ratio of 1:5 was preferred, because the higher water content supports maximum salt loading, the mixture is substantially fluid, and is easy to spread on the electrode surface. A higher concentration of salt used for coating  208  facilitates a lower resistance to the redox current and consequently provides a higher observed current output by electrochemical sensor  102 . Unfortunately, a high concentration of salt can also interfere with gelation of coating  208 . By way of example only, an unsaturated solution of 3M sodium nitrate in the 1:5 glycerol to water solvent can be used to make a substantially effective gel for purposes of various aspects of this invention. During gelation, some water may be lost, creating a higher salt concentration as compared to a nominal value of salt concentration in the gel before coating  208 , on which the gel resides, can be tested. 
     Upon the oxidation of H 2 O 2  to O 2  on working electrode  206  (made of platinum, for example), H +  ions are generated. To prevent the continuous acidification of coating  208  in repeated and/or continuous measurements, coating  208  was buffered. Therefore, according to one aspect, a supporting electrolyte can be introduced into the gel in the form of a 1M phosphate buffer or an acetate buffer. An acetate buffer can be introduced in the gel (in addition to the 3M NaNO 3 ) by adding 0.01M acetic acid and 0.005M sodium hydroxide. 
     In an alternative construction, a PVC membrane can also be used to determine the effect of a more hydrophobic membrane on hydrogen peroxide detection. According to one aspect of the invention, the PVC membrane was cast over the surface of working electrode  206  from a diluted tetrahydrofuran (THF) solution. A few microliters of THF solution can be dispensed over the surface of the planar electrochemical cell using, for example, a microsyringe. By way of example only, the composition of the THF solution used was 60 mg PVC, 120 mg (2-nothrophenyl octylether (o-NPOE) as plasticizer, in 2.0 mL THF. The PVC membrane formed over the surface of working electrode  206 , counter electrode  204 , and/or reference electrode  202  after the THF evaporated. 
     Example 2 
     Experimental Detection Chamber 
     Referring to  FIG. 3 , an experimental setup  300  to determine whether the agar or PVC coating provides a medium where electrochemical experiments can be performed is illustrated. The experimental setup  300  included electrochemical sensor  102  inserted into a beaker  308  filled at least partially with a hydrogen peroxide solution  306 . Electrochemical sensor  102  passes through a parafilm cover  304  covering an open top of 20 mL beaker  308  such that coating  208  of electrochemical sensor  102  is exposed to vapors  310  emanating from hydrogen peroxide solution  306 . 
     The concentration of the hydrogen peroxide solution  306  was varied between the cyclic voltammetric scans by diluting with distilled, deionized water and stirring with a magnetic stir bar. 
     Example 3 
     Detection of Hydrogen Peroxide 
     A cyclic voltammetric scan to a negative potential was performed to observe the signal from the reduction of atmospheric oxygen in vapors  310  over coating  208 . Once charge transport was detected from electrochemical sensor  102  (e.g., by current/voltage detector  108  coupled to electrical leads  302 ), cyclic voltammetric scans were performed with potentials ranging from 0 to 1.4 V and 0 to 1.2 V with a scan rate of 0.02 V/s and a sample interval of 0.001 V. 
     Measurements can be made inside a Faraday cage to insulate experimental setup  300  from external and/or undesirable static charges (e.g., using a potentiostat provided by CH Instruments of Austin, Tex.). In addition to cyclic voltammetry other voltammetric techniques can also be used, e.g., chronoamperometry, pulse voltammetry, differential pulse voltammetry, square wave voltammetry. Although in these techniques the calculation of the Faradic current is different, it does not affect the various aspects of this invention. In addition, the background scan can be taken in air and subtracted from the collected data to account for Faraday currents in working electrode  206 , counter electrode  204 , and reference electrode  202 , without affecting the various aspects of this invention. 
     Thereafter, voltammetric scans were performed in the gas phase, above solutions having varying concentrations of hydrogen peroxide. Referring to  FIGS. 4A ,  4 B,  5 A,  5 B,  6 A, and  6 B, results of different experiments conducted using experimental setup  300  are illustrated. It is to be noted that one skilled in the art can contemplate other conditions for testing electrochemical sensor  102  apart from the condition described above. A voltammogram for experimental setup  300  results in a background current output from electrical leads  302  when a potential was applied to working electrode  206 , both with the agar and the PVC membrane forming coating  208 . Exposure to hydrogen peroxide in vapors  310  surrounding coating  208  causes an increase in the current measured by electrochemical sensor  102 . 
     The results demonstrate that the current observed in the agar gel is higher than the current in the PVC membrane due to the hydrophobicity of the PVC. As a result, agar was used primarily as the medium for a supporting electrolyte for the remainder of the experiments. The results presented herein demonstrate that as the concentration of hydrogen peroxide in vapors  310  around and below electrochemical sensor  102  increased, the measured current from electrical leads  302  also increased. 
     Referring to  FIG. 4A  results from experimental setup  300  with a 2:1 glycerol/water ratio with 1M NaNO 3  for coating  208 , over 0.89 M and 0.089 M stock hydrogen peroxide solution  306  corrected for baseline are shown. In  FIG. 4B , results from experimental setup  300  with an optimized agar gel including a 1:5 glycerol/water ratio with 3M NaNO 3  over 8.9e-2M, 8.9e-3M, 8.9e-4M, and 8.9e-5M, in a top to bottom arrangement, stock hydrogen peroxide solution  306  corrected for baseline (blue) are shown. As can be seen from the plots in  FIGS. 4A and 4B , current output by electrical leads  302  of electrochemical sensor  102  increases as concentration of hydrogen peroxide in hydrogen peroxide solution  306  increases. 
     Referring more specifically to  FIGS. 5A and 5B , results using experimental setup  300  with a different value for concentration of hydrogen peroxide solution  306  are shown.  FIG. 5A  illustrates a scenario with coating  208  comprising 3M NaNO 3  over 0.089 M, 8.9e-3 M, 8.9e-4 M, 8.9e-5 M, 8.9e-5 M, 8.9e-7 M, and 8.9e-8 M concentrations of hydrogen peroxide solution  306 . As seen in  FIG. 5A , current output from electrical leads  302  of electrochemical sensor  102  varies as potential across reference electrode  202  and working electrode  206  is varied. 
       FIG. 5B  illustrates a graph showing direct correlation between the current output by electrical leads  302  and the concentration of hydrogen peroxide in solution  306  when a logarithm of the current was plotted as a function of a logarithm of the concentration of hydrogen peroxide solution  306  at a fixed voltage potential of 0.7 V, although other values of voltage may also be used as known to those skilled in the art. 
     Referring to  FIGS. 6A and 6B , the effects of acetate buffer on coating function is illustrated (agar coating contained 3M NaNO 3  and acetate buffer). Because the electron transfer half reaction of hydrogen peroxide with platinum of working electrode  206  produces two charged hydroxide ions, the pH inside parafilm cover  304  can be variable during the electrochemical scan. In  FIG. 6A , the resulting current following exposure to hydrogen peroxide concentrations of 0.089M, 8.9e-3 M, 8.9e-4 M, 8.9e-5M, 1.1e-6 M, and 1.1e-7 M are illustrated. As shown in  FIG. 6A , current output from electrical leads  302  of electrochemical sensor  102  varies as potential across reference electrode  202  and working electrode  206  is varied.  FIG. 6B  illustrates a graph where a logarithm of current output by electrical leads  302  versus a logarithm of concentration of hydrogen peroxide solution  306  at a fixed voltage of 0.7 V. Similar to the graph of  FIG. 5B , the dependence of output current in direct proportion to the concentration of hydrogen peroxide in hydrogen peroxide solution  306  can be observed. 
     Based on the current profile measured in air, it is apparent from  FIGS. 4A-6B  that the agar and the PVC are viable substitutes for a conventional supporting electrolyte medium for electrochemistry. When hydrogen peroxide was present in vapors  310 , a significant current was detected in the potential range for hydrogen peroxide, and the analyte in the vapor or gas phase could partition into and move through coating  208  to the surface of working electrode  206 , reference electrode  202  and counter electrode  204 , allowing for electron transfer to take place and cause charge flow. 
     Because the partitioned analyte concentration in coating  208  is in equilibrium with the vapor (or gas) phase, i.e., the analyte concentration in coating  208  is proportional to the concentration in the vapor or gas phase, a reliable assessment can be made concerning the analyte concentration in the vapor or gas phase. In addition, by correlating the vapor or gas phase analyte concentration to the severity of  Streptococcus pneumoniae , it is possible to assess the extent of the  Streptococcus pneumoniae  infection in patient P from whom a sample was obtained. 
     Referring to  FIG. 7 , a flowchart  700  illustrates exemplary steps for a method for electrochemical detection of an analyte in a vapor or gas sample, as described with respect to  FIGS. 1-6B  above, according to one aspect of the invention. According to another aspect of the invention, steps  702 - 710  can be used in a method for detecting a  Streptococcus pneumoniae  infection in patient P, as described with respect to  FIGS. 1-6B  above. Further, steps  702 - 710  discuss the operation of electrochemical sensor  102  used according to various aspects of the present invention. 
     In step  702 , electrochemical sensor  102  is exposed to a vapor or a gas sample (e.g., vapors  310 ). The exposing can be performed, for example, by patient P blowing or exhaling at a mouthpiece attached to inlet  104 . Alternatively, electrochemical sensor  102  may be exposed to an analyte (e.g., hydrogen peroxide) in other setups, for example, experimental setup  300  in  FIG. 3 . 
     In step  704 , based upon exposure to the analyte in the vapor or gas sample, electrochemical sensor  102  outputs a current (or equivalently, a voltage) from electrical leads  302  as a result of a redox reaction on coating  208 . By way of example only, the output current can be detected by current/voltage detector  108  of detector device  140 . Alternatively, the output current may be directly fed to controller  130  for further processing or detected by other current/voltage detection schemes, which are well known to those skilled in the art. 
     In step  706 , controller  130  compares the value of the output current or voltage detected with a preset threshold value stored in memory  114  of controller  130 . The preset threshold value may correspond to a desired level of analyte concentration in the vapor or gas sample and the amount of output current detected can be correlated to a corresponding amount of the analyte concentration by controller  130 . Based upon the correlating and comparison with the preset threshold, controller  130  determines if the value of analyte concentration is higher than the preset threshold value stored in memory  114 . 
     If the value of detected current/voltage is higher than the preset threshold, the flow proceeds to step  708  in which controller  130  displays a condition corresponding to a high concentration of the analyte. For example, controller  130  can show a presence of  Streptococcus pneumoniea  in the upper respiratory system of patient P if high amounts of hydrogen peroxide are present in the vapor or gas sample received by detector device  140 . Alternatively, controller  130  may display other conditions based upon the comparison, for example, presence of undesirable additives in a hydrogen peroxide sample. 
     However, if the value of detected current/voltage is lower than the preset threshold, the flow proceeds to step  708  in which controller  130  displays a condition corresponding to a low concentration of the analyte. For example, such a condition can be used as a diagnostic tool to test whether or not patient P is free from  Streptococcus pneumoniea  infection and to infer whether or not, even with a low concentration of hydrogen peroxide, patient P is developing a  Streptococcus pneumoniea  infection by monitoring patient P on a regular or a random basis. 
     It is to be noted that although display  130  is being used to show conditions corresponding to high or low concentrations of the analyte, other forms of indication such as a beep, a buzzer, or a flashing light may also be used, as known to one skilled in the art. 
     Various aspects of the present invention provide advantages over conventional detection of analytes in vapor phase, and more specifically over conventional methods and devices for detecting undesirable populations of  Streptococcus pneumoniea . For example, using experimental setup  300  it is possible to use electrochemical techniques to detect the presence of hydrogen peroxide in air by condensing the electrochemical cell to one unit and applying a membrane layer over the tip of the cell as the supporting electrolyte. In addition, as shown in  FIGS. 4A-6B , the current measured with this technique has a clear linear dependence on the concentration of the ambient hydrogen peroxide in the air surrounding electrochemical sensor  102 . Using various aspects of the present invention, it is possible to detect the hydrogen peroxide concentration in the breath of patient P produced by  Streptococcus pneumoniea  in the upper respiratory system. Detection of hydrogen peroxide in this way provides an efficient noninvasive diagnostic tool to monitor  S. pneumoniea  levels in individuals at high risk for infection, for example, young children, elderly persons, and the immunocompromised. 
     The results of experimental setup  300  conclusively show that coating the surface of working electrode  206 , reference electrode  202  and counter electrode  204  with a hydro-gel or polymeric membrane allows electrochemistry to be used to detect the presence of hydrogen peroxide in the gaseous or vapor phase. In addition, a linear correlation between the concentration of hydrogen peroxide in air surrounding electrochemical sensor  102  and the observed current shows that electrochemical sensor  102  is sensitive to the concentration as well as the presence of hydrogen peroxide. This indicates that electrochemical sensor  102  can be used to quantify gaseous or vapor phase hydrogen peroxide in a sample, and particularly for purposes of detecting the presence of hydrogen peroxide in the exhaled breath of patients having a  Streptococcus pneumoniea  infection. It is also possible to use the concentration of hydrogen peroxide in breath as an indirect measure of the severity of infection—i.e.,  Streptococcus pneumoniea  population size, for example, in an  Streptococcus pneumoniea  culture, or otherwise. 
     In addition, variations in the materials for working electrode  206 , coating  208  and dimensions thereof can be assessed as part of the optimization process. Due to the delicate properties of agar gel used in exemplary coating  208 , additional materials can also be used, such as sol gels, polymeric substances like polymethylmethacrylate, and other hydrogel materials. While the PVC membrane is functional, modifications to the PVC membrane with variations of electrode materials to can be made to optimize sensitivity to analyte concentrations in vapor or gas phase without undue experimentation by those skilled in the art. 
     All of the features and aspects of the present invention described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims and equivalents thereto.