Polyaniline-based optical ammonia detector

Details Polyaniline-based optical ammonia detector Polyaniline-based optical ammonia detector

2001-01-12

2002-06-18

Snay, Jeffrey (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing liquid or solid sample

C422S091000, C436S113000

Reexamination Certificate

active

06406669

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to apparatus for detecting ammonia in fluids and, more particularly, to the use of electronic absorption spectroscopy for determining the quantity of ammonia that has reacted with a conducting polymer film in contact therewith.
BACKGROUND OF THE INVENTION
Conducting polymers which can be prepared by a simple oxidative polymerization method have found use as chemical and biological sensors. They exhibit reversible pH-induced spectroscopic and gas-induced conductivity changes. Such materials also provide a suitable structure for immobilization of ligands, enzymes, and antibodies. See, e.g., “Immobilization Of Glucose-Oxidase In Ferrocene-Modified Pyrrole Polymers” by N. C. Foulds and C. R. Lowe, Analytical Chemistry 60, 2473-2478 (1988); “Pulsed Amperometric Detection Of Proteins Using Antibody Containing Conducting Polymers” by O. A. Sadik and G. G. Wallace, Analytica Chimica Acta 279, 209-212 (1993); and “Optical Sensing Of pH Based On Polypyrrole Films” by S. Demarcos and O. S. Wolfbeis, Analytica Chimica Acta 334, 149-153 (1996).
Conducting polymer gas sensors commonly rely on conductivity changes that occur when they are exposed to certain gases. For example, the dc conductivity of a polypyrrole film decreases with increasing ammonia gas concentration, and an ammonia gas sensor based on this property has been developed (See, e.g., “The Effect Of Initial Conductivity And Doping Anions On Gas Sensitivity Of Conducting Polypyrrole films to NH
3
” by M. Bile et al., Sensors and Actuators B37, 119-122 (1996)). At room temperature, the response time of such a sensor was found to be a few tens of minutes. By increasing the temperature from 20 to 100° C., the response time was shortened by a factor of five. After treatment with NO
2
, the response and sensitivity of the sensor deteriorated. The major problems of this polypyrrole ammonia gas sensor are slow response time, low sensitivity, irreversible response, and a controlled high temperature (100° C.) requirement.
The dc conductivity of polyaniline films also changes when the films are exposed to ammonia gas. For example, a polyaniline film containing nickel prepared by electrochemical oxidation can be used to detect ammonia gas in the range between 1 and 10,000 PPM at room temperature (See, e.g., “Polymer Film-Based Sensors For Ammonia Detection” by S. A. Krutovertsev et al., Sensors And Actuators B7, 492-494 (1992)). The response time was reported to be approximately two minutes which is much faster than that for a polypyrrole ammonia sensor; however, the regeneration of the polyaniline sensor was slow (See, e.g., “Ammonia Sensors Based On Sensitive Polyaniline Films” by A. L. Kukla et al., Sensors and Actuators B37, 135-140 (1996)). By heating the sensor layer to between 104 and 107° C., it was possible to completely regenerate the sensor within a short period of time. Polyaniline films are also sensitive to H
2
S, NO
x
, and SO
2
. Detection limits as low as 4 PPM can be achieved for H
2
S and NO
x
gases with polyaniline gas sensors (See, e.g., “Polyaniline Thin Films For Gas Sensing” by N. E. Agbor et al., Sensors and Actuators B28, 173-179 (1995)). In a variation of these measurements U.S. Pat. No. 5,252,292 for “Ammonia Sensor” which issued to Hirata et al. on Oct. 12, 1993 describes an ammonia sensor consisting of at least one pair of electrodes and an ammonia-sensing material comprising a polyaniline filling the space between the electrodes. Therein, the polyaniline changes its electric resistance in proportion to the ammonia concentration in an atmosphere such as air or other gas and accordingly the measurement of the electric resistance enables the detection of the ammonia concentration at a high sensitivity.
Recently, high-frequency and multi-frequency ac conductivity measurement techniques have been used for conducting polymer gas sensors (See, e.g., “High-Frequency a.c. Investigation Of Conducting Polymer Gas Sensors” by F. Musio et al., Sensors and Actuators B23, 223-226 (1995) and “Multi-Frequency Measurements Of Organic Conducting Polymers For Sensing Of Gases And Vapors” by M. E. H. Amrani et al., Sensors and Actuators B33, 137-141 (1996)).
The most important advantage of ac conductivity measurements is that it is possible to distinguish different chemical species with a single sensor. Organic vapors, such as methanol, acetone, and ethyl acetate, were detected by measuring a.c. conductivity changes of a polyaniline gas sensor at different frequencies (M. E. H. Amrani et al., supra). Another technique, which can differentiate different chemical species, is the frequency counting interrogation technique (See, e.g., “Frequency Counting Interrogation Techniques Applied To Gas Sensor Arrays” by M. E. H. Amrani et al., Sensors and Actuators B57, 75-82 (1999). In order to monitor characteristic resistance and capacitance changes simultaneously, a conducting polymer sensor was used as one of the arms of a four-channel Wien-bridge oscillator system. From the combined patterns of frequency changes in the four channels, it was possible to detect the vapor to which the system was exposed.
There are a few reports of conducting polymers being used for optical gas sensors (See, e.g., “An Optical Gas Sensor Based On Polyaniline Langmuir-Blodgett Films” by N. E. Agbor et al., Sensors and Actuators B41, 137-141 (1997)). Therein, a polyaniline optical sensor based on the surface plasmon resonance and sensitive to NO
2
and H
2
S with detection limits of approximately 50 vapor parts per million was described. However, the sensor response was slow, and total regeneration of the sensor after exposure to gases was impossible.
In U.S. Pat. No. 6,051,437 for “Optical Chemical Sensor Based On Multilayer Self-Assembled Thin Film Sensors For Aquaculture Process Control”, which issued to Luo et al. on Apr. 18, 2000 describes optical chemical probes having layers of anionic and cationic polyelectrolytes and one or more dyes incorporated into these layers. The probes are placed in the medium to be analyzed and the dye or dyes react in the presence of the corresponding chemical. Color changes may be observe manually or by a photodetector. A light source may be employed to increase the optical signal received from the probe.
In U.S. Pat. No. 6,117,686 for “Method For Detecting Harmful Gases Which Is Applicable To Broad Gas Concentration Range”, which issued to Tanaka et al. on Sep. 12, 2000, a method for detecting certain gases is described wherein a layer of matrix polymer which includes tetraphenylporphyrin (TPP) is used as a detector. When the concentration of tetraphenylporphyrin contained in the matrix polymer is increased, an absorption peak appears at approximately 718 nm; moreover, when the concentration of tetraphenylporphyrin is altered, the gas concentration at which the absorption peak appears also changes, as measured by transmittance or reflection of light from the detector. For example, the 718 nm feature begins to appear at a higher gas concentration when a detector containing a lower concentration of tetraphenylporphyrin is used, whereas the feature appears at a lower gas concentration when a detector containing a higher concentration of the pigment is employed. Tanaka et al. states that this phenomenon is observed specifically with tetraphenylporphyrin, and that it is possible to detect and quantify certain gases over a broader range of concentration using a plurality of detectors containing tetraphenylporphyrin in different concentrations in a matrix polymer.
Accordingly, it is an object of the present invention to provide an ammonia gas sensor based on polyaniline which is suitable for analytical chemistry use.
Another object of the invention is to provide an ammonia gas sensor based on polyaniline having rapid response time and capable of rapid regeneration.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination

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