Patent Application: US-201113024820-A

Abstract:
a method for detecting an organic carbonyl species involves exposing a metal oxide - free film of a polyaniline to an environment suspected of containing an organic carbonyl species , detecting a change in electrical conductivity and / or an optical or luminescent property of the polyaniline , and , correlating the change in electrical conductivity and / or optical or luminescent property to a presence of the organic carbonyl species in the environment . further , sensors for organic carbonyl species are disclosed having a metal oxide - free film of a polyaniline supported on an electrically insulating substrate . the method and sensors provide a good balance between response time and sensitivity , being considerably faster than metal oxide and metal oxide / polymer based sensors , while having greater sensitivity than other polymer - based sensors .

Description:
referring to fig1 a and 1b , two embodiments of sensor configurations are depicted each having a 4 - probe design of gold electrodes on a circular glass substrate over which a thin film of doped polyaniline is cast . the design of fig1 b provides a more stable signal that is less prone to interference due to mechanical vibration . the design of fig1 a may be optimized for greater sensitivity by decreasing the gap between the four electrodes , for example , by 100 - fold . electrical conductivity measurements are presented in fig2 for polyaniline homopolymer doped with polyvinylphosphonic acid ( pvpa ) at different doping levels . referring to fig2 a , electrical resistance as measured using the 4 - probe sheet sensor of polyaniline as illustrated in fig1 b was examined as a function of doping level with polyvinylphosphonic acid ( pvpa ). change in electrical resistance of the doped polyaniline thin films is illustrated for the 7 . 5 % ( mol / mol ) and 25 % ( mol / mol ) doping levels when plugs of formaldehyde are injected in air at a concentration of approximately 40 ppm for different lengths of time . it is evident from the graphs on the right in fig2 a that the micron thin films have a change in resistance , i . e . a response , on the order of about 20 %. the greatest response occurred at about 60 seconds after exposure to formaldehyde , but the time to a recordal response is as little as about 5 seconds . further , the greatest drop in resistance occurs at a fractional doping level of about 0 . 25 . referring to fig2 b , electrical resistance as measured using the 4 - probe sheet sensor of polyaniline as illustrated in fig1 b was examined at a polyvinylphosphonic acid ( pvpa ) doping level of 10 % ( mol / mol ) when plugs of formaldehyde were injected in air at a concentration of approximately 2 . 4 ppm for different lengths of time . its is evident that a response is obtained in as little as 5 seconds and that a maximum response of about 2 . 2 % occurs between about 30 and 60 seconds . fig2 b illustrates that the sensitivity of the polyaniline sensor can be as little as about 2 . 4 ppm or less , and from the magnitude of the response , it is evident that sub - ppm detection can be achieved . the 4 - probe sheet sensor depicted in fig1 a comprising a thin film of polyaniline was optimized by narrowing electrode gap by a factor of 100 . a set of resistance experiments was conducted in a manner similar to the ones described above . referring to fig2 c , electrical resistance as measured using the optimized 4 - probe sheet sensor was examined as a function of doping level with polyvinylphosphonic acid ( pvpa ). change in electrical resistance of the doped polyaniline thin films is illustrated for the 10 % ( mol / mol ) and 20 % ( mol / mol ) doping levels when plugs of formaldehyde are injected in air at a concentration of approximately 300 ppb under anhydrous conditions for different lengths of time . it is evident from the graphs on the right in fig2 c that the micron thin films have a change in resistance , i . e . a response , on the order of about 0 . 5 % to 1 . 1 %. the greatest response occurred at about 60 seconds after exposure to formaldehyde , but the time to a recordable response is as little as about 5 seconds . further , the greatest drop in baseline resistance occurs at a fractional doping level of about 0 . 2 . however , higher sensitivity is observed at a fractional doping level of 0 . 1 . this example illustrates the capability of the present sensors to detect formaldehyde even at the ppb level , demonstrating the remarkable sensitivity of the sensors . referring to fig2 d , electrical resistance as measured using the optimized polyaniline 4 - probe sheet sensor was examined at a polyvinylphosphonic acid ( pvpa ) doping level of 10 % ( mol / mol ) when plugs of formaldehyde were injected in air at a concentration of approximately 250 ppb at a relative humidity of about 44 % for different lengths of time . the sensor was equilibrated to the prescribed level of humidity . it is evident that a response is obtained in as little as 5 seconds and at a maximum response of about 0 . 32 %. fig2 d establishes that the polyaniline sensor can detect ppb levels of formaldehyde even under humid conditions . a relative humidity of 40 - 50 % is considered to be comfort level . electrical conductivity measurements are presented in fig3 a for thin films of polyaniline doped with 125 % ( mol / mol ) dodecylbenzenesulfonic acid ( dbsa ) and in fig3 b for thin films of poly ( o - anisidine ) doped with 75 % ( mol / mol ) dodecylbenzene sulfonic acid ( dbsa ) when exposed to acetone . the response for the polyaniline ( fig3 a ) is consistent with a solvation effect . the response for the poly ( o - anisidine ) ( fig3 b ) is consistent with a reversible chemical reaction between acetone and the poly ( o - anisidine ). the results indicate that altering nucleophilicity of the polymer as in poly ( o - anisidine ) as well as optimizing the doping level can permit tuning to specific organic carbonyl species or specific groups of organic carbonyl species . in this example , a sensor based on a polyaniline copolymer was tested for its selectivity towards acetaldehyde ( ch 3 cho ) and formaldehyde ( hcho ). referring to fig4 a , electrical resistance as measured using a poly ( aniline - co - 3 - nitroaniline ) 4 - probe sheet sensor was examined at a dodecylbenzenesulfonic acid ( dbsa ) doping level of 75 % ( mol / mol ) when plugs of acetaldehyde were injected in air at a concentration of approximately 10 ppm for different lengths of time . it is evident that a response is obtained in as little as 5 seconds and at a maximum response of about 24 . 5 %. referring to fig4 b , electrical resistance as measured using a poly ( aniline - co - 3 - nitroaniline ) 4 - probe sheet sensor was examined at a dodecylbenzenesulfonic acid ( dbsa ) doping level of 75 % ( mol / mol ) when plugs of formaldehyde were injected in air at a concentration of approximately 0 . 92 ppm for different lengths of time . it is evident that a response is obtained in as little as 5 seconds and at a maximum response of about 51 . 5 %. fig4 a and 4b that the response is much higher for formaldehyde than acetaldehyde at the same concentrations ( response to acetaldehyde at 0 . 92 ppm would be 2 . 3 %) making the device 22 times more responsive to formaldehyde . this is in contrast to polyaniline / pvpa system at 10 % doping level where the sensor is 10 times more responsive towards formaldehyde . this demonstrates the ability to tailor the chemical structure of the polymer to induce a higher degree of selectivity to a particular analyte . this will very useful in designing artificial noses . table 1 compares the response of the poly ( aniline - co - 3 - nitroaniline ) sensor to acetaldehyde and formaldehyde at doping levels of dbsa of 50 % ( mol / mol ), 75 % ( mol / mol ) and 100 % ( mol / mol ). in the third column of table 1 , the response to acetaldehyde is normalized to 0 . 92 ppm in order to compare it with the response to formaldehyde . it is evident from the selectivity factor that the poly ( aniline - co - 3 - nitroaniline ) sensor is well capable of resolving acetaldehyde and formaldehyde despite the chemical similarities between the two ( a most difficult system ). in the art , the acetaldehyde / formaldehyde system has hitherto been a difficult one to resolve , however , the present sensors remarkably have the ability to resolve that system . in this example and with reference to table 2 , a sensor based on a poly ( anilineboronic acid ) was tested for its ability to resolve acetaldehyde and formaldehyde . electrical resistance as measured using a poly ( anilineboronic acid ) 4 - probe sheet sensor was examined at various phosphoric acid doping levels when plugs of acetaldehyde ( ch 3 cho , 10 ppm ) or formaldehyde ( hcho , 2 ppm ) were injected in air for different lengths of time . in the undoped sample , no phosphoric acid was used to dope the poly ( anilineboronic acid ). in the fully doped case , the poly ( anilineboronic acid ) is doped with more than 100 % ( mol / mol ) of phosphoric acid . in the semi - doped case , the level of phosphoric acid doping varies in the transverse direction of the poly ( anilineboronic acid ) film . the maximum response at each doping level to each aldehyde is shown in table 2 . table 2 further compares the response of the poly ( anilineboronic acid ) sensor to acetaldehyde and formaldehyde at each doping level of phosphoric acid . in the third column of table 2 , the response to acetaldehyde is normalized to 2 ppm in order to compare it with the response to formaldehyde . it is evident from the selectivity factor that the poly ( anilineboronic acid ) sensor is well capable of resolving acetaldehyde and formaldehyde despite the chemical similarities between the two , further testifying to the remarkable ability of the present sensors to resolve the difficult acetaldehyde / formaldehyde system . prior art metal oxide and metal oxide / polymer based sensors ( hosono 2005a ; itoh 2007a ; itoh 2007b ; itoh 2007c ; itoh 2008a ; itoh 2008b ; itoh 2008c ; zheng 2008 ) generally have detection limits at the ppb level , however , they are typically plagued by extremely slow response times . japanese patent publication 2007 - 271598 ( itoh 2007c ), which discloses a sensor comprising a film of polyaniline intercalated between molybdenum oxide layers is typical of such sensors . it is evident from fig6 and 10 of itoh 2007c that the response time is on the order of 10 - 15 minutes , which is consistent with other metal - oxide - based sensors in which response is due to change in resistance of the metal oxide . as indicated above , response time for sensors of the present invention is less than 5 seconds . polymer - based sensors , such as polypyrrole - based sensors , polyaniline - based sensors and polythiophene - based sensors have been proposed ( flueckiger 2009 ), however , only polypyrrole - based sensors ( hosono 2005b ) have been characterized . as evident from hosono 2005b , their polypyrrole - based sensor has a response time of about 300 seconds ( page 397 , section 3 and fig1 of hosono 2005b ) and a detection limit of about 100 ppm ( fig4 of hosono 2005b ) for acetaldehyde . in an experiment by the current inventors ( fig5 ) a thin film of polypyrrole doped with phosphomolybdic acid was exposed to formaldehyde under the same conditions as in fig2 a of example 2 and the change in resistance was monitored as a function of time . as evidenced in fig5 , the response time was on the order of less than 5 seconds , comparable to the results for polyaniline - based sensors , but the maximum response ( at 60 seconds ) was less than about 8 %, which is considerably less than the at least 20 % response from polyaniline - based sensors of the present invention . further , a similar experiment with phosphomolybdic acid - doped poly ( 3 , 4 - ethylenedioxythiophene ) ( pdot ) was attempted but no response at all to formaldehyde was obtained ( fig6 ). clearly , polypyrrole - based sensors and polythiophene - based sensors do not function as well as the polyaniline - based sensors of the present invention . thus , sensors of the present invention based on metal oxide - free thin films of polyanilines demonstrate a good balance between response time and sensitivity , being considerably faster than metal oxide and metal oxide / polymer based sensors , while having greater sensitivity than other polymer - based sensors . the contents of the entirety of each of which are incorporated by this reference . deore b , yu i , freund m s . 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( 2008 ) “ polyaniline - tio 2 nano - composite - based trimethylamine qcm sensor and its thermal behavior studies ”. sensors and actuators b . 133 , 374 - 380 . other advantages that are inherent to the structure are obvious to one skilled in the art . the embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed . variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims .