Semiconducting organic polymer gas sensor

Details Semiconducting organic polymer gas sensor Semiconducting organic polymer gas sensor

1997-03-11

2001-01-30

Snay, Jeffrey (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S098000

Reexamination Certificate

active

06180064

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to semiconducting organic polymers which may be used in gas sensors.
BACKGROUND OF THE INVENTION
It is known that certain electrochemically prepared semiconducting polymers such as polypyrrole may be employed in sensors in order to detect gases, vapors and odors. Such a sensor may comprise a pair of electrodes mounted on a substrate, with a layer of the semiconducting organic polymer deposited on and between the electrodes in order to produce an electrical connection between the electrodes. The semiconducting organic polymer may be sensitive to the presence of a gas or, more likely, to a range of gases, to the extent that adsorption of the gas onto the polymer surface affects the electrical properties of the polymer. Hence the presence of gas may be detected by monitoring, for example, the change in DC resistance of the sensor on exposure to the gas. For instance, Maisik et al (Maisik, J J, Hooper, A and Tofield, B C) JCS Faraday Trans. 1, 1986, 82, 1117-26 demonstrated a polypyrrole gas sensor displaying a DC resistance which was sensitive to the presence of nitrous oxide and hydrogen sulphide. GB-2,203,553-B discloses an improved method of detection wherein various AC impedance characteristics are measured at different AC frequencies.
A given semiconducting organic polymer will typically be sensitive to a range of compounds. Clearly this lack of selectivity is a major problem if one wishes to develop a sensor which is specific to a particular gas. Conversely, a sensor which employs a given semiconducting organic polymer may not be sufficiently sensitive to such a broad range of gases that it may be considered a general purpose device.
SUMMARY
A solution to these problems is a device which employs a plurality of sensors, wherein each sensor incorporates a different polymer and each polymer possesses differing gas response profiles. Thus a suite of polymers may be selected which possess broadly overlapping responses, but which are individually chemically tailored to enhance differences in response to certain molecules or classes of molecules. Often the variation of a substituent group on the monomer unit is sufficient to enable such “fine tuning” of response. A multi-sensor device detects gases and odors as a characteristic pattern of individual responses across the array of sensors.
The present invention relates to a class of semiconducting organic polymers based on 1, 2, 5 substituted five membered heterocyclic aromatic ring monomer units. The substituent groups at the 2 and 5 positions are preferably aromatic rings; in particular, thienyl or thienyl derivatives appear to confer enhanced stability to the resulting polymer. Further substitution at the 3 position of the ring may be desirable. By judicious variation of the substituent groups, a range of semiconducting polymers for use in gas sensors of the type described above can be produced, wherein the response profile of a polymer can be “fine tuned” to a molecule or a class of molecules.
U.S. Pat. No. 5,210,217 discloses a family of 3-substituted 2,5-di(2-thienyl)pyrrole polymers. These polymers are functionalised at the 3 position with an analyte specific receptor such as an enzyme and are employed in conductimetric liquid phase analyte detection.
According to the present invention there is provided a gas sensor which comprises:
a pair of electrodes;
one or more semiconducting organic polymers deposited onto and between the pair of electrodes in such manner as to effect a semiconducting electrical connection between said electrodes; and
transduction means;
characterized in that one or more of the semiconducting organic polymers is polymerized from a monomer comprising a five membered heterocyclic aromatic ring with substituent groups at the 1, 2 and 5 positions.
The transduction means may comprise means for applying electric signal across the electrodes and detection means for detecting a chosen electrical property in the presence of a gas. For instance, if a DC electric signal is applied the change in polymer resistance on exposure of the sensor to a gas may be monitored; if an AC electric signal is applied, the change in an impedance characteristic, such as the capacitance, may be detected at a specific AC frequency.
There may be a substituent group at the 3 position.
In any of the aforementioned cases, the substituted five membered heterocyclic aromatic ring may be pyrrole.
The substituent groups at the 2 and 5 positions may be aromatic rings and, in particular, may be thienyl or derivatives thereof.
Any of the polymers described above may be polymerised electrochemically from a solution containing the monomer and a counter-ion. This counter-ion may be BF
4
−
, PF
6
−
, CLO
4
−
, C
8
H
17
SO
3
−
, Fe(CN)
6
3−
or CH
3
C
6
H
4
SO
3
−
.
A gas sensor in accordance with the invention will now be described with reference to the accompanying drawings.


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Ferraris, J.P. et al “Poly(N-isopropyl-2,5-di-(2-thienyl)pyrrole): a sterically hindered pyrrole-thiophene copolymer” New Polymeric Materials, vol. 2, No. 1, pp. 41-65 (1990).
Ferraris, J.P. et al “Substitutional alloys of organic polymeric conductors” Polymer, vol. 28, No. 2, pp 179-182 (1987).
K.C. Persaud et al, “Sensor Arrays Using Conducting Polymers for an Artificial Nose”, ((Instituto di Industrie Agrarie, University of Pisa, Italy), pp. 237-257, 1992.
Julian W. Gardner et al, “A Brief History of Electronic Noses*”, (Centre for Nanotechnology and Microengineering, Depto. Of Engineering, University of Warwick, Coventry, U.K.), pp. 211-220, 1994.
B.A. Gregory, “An Introduction to Electrical Instrumentation and Measurement Systems”, 1982 (MacMillen).
Maisik et al., JCS Faraday Trans. 1, 1986 82, 1117-26.

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