U.S. patent application number 10/109687 was filed with the patent office on 2002-09-19 for gas sensors.
This patent application is currently assigned to BRITISH GAS PLC. Invention is credited to Agbor, Napolean Enompagu, Monkman, Andrew Paul, Petty, Michael Charles, Scully, Margaret Teresa.
Application Number | 20020131901 10/109687 |
Document ID | / |
Family ID | 10728639 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020131901 |
Kind Code |
A1 |
Monkman, Andrew Paul ; et
al. |
September 19, 2002 |
Gas sensors
Abstract
A gas sensor for use in monitoring gases such as H.sub.2S,
NO.sub.2 or SO.sub.2 comprises a film or layer of non-protonated
polyaniline as the gas sensing material.
Inventors: |
Monkman, Andrew Paul;
(Stanhope, GB) ; Petty, Michael Charles;
(Stockton-on-Tees, GB) ; Agbor, Napolean Enompagu;
(South Road, GB) ; Scully, Margaret Teresa;
(Raynes Park, GB) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
BRITISH GAS PLC
Rivermill House 152 Grosvenor Road
London
GB
SW1V 3JL
|
Family ID: |
10728639 |
Appl. No.: |
10/109687 |
Filed: |
April 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10109687 |
Apr 1, 2002 |
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08589001 |
Jan 19, 1996 |
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08589001 |
Jan 19, 1996 |
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08284436 |
Sep 6, 1994 |
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5536473 |
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Current U.S.
Class: |
422/90 ; 422/83;
422/98; 436/106; 436/116; 436/118 |
Current CPC
Class: |
Y10T 436/177692
20150115; Y10T 436/179228 20150115; G01N 27/126 20130101; Y10T
436/17 20150115 |
Class at
Publication: |
422/90 ; 422/98;
422/83; 436/106; 436/116; 436/118 |
International
Class: |
G01N 007/00; B32B
027/04; B32B 005/02; G01N 021/00; G01N 027/00; G01N 031/00; B32B
027/12; G01N 033/00; G01N 027/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 1993 |
GB |
93000560.1 |
Jan 13, 1994 |
WO |
PCT/GB94/00075 |
Claims
1. A gas sensor comprising a film or layer of non-protonated
polyaniline as the gas sensing material.
2. A gas sensor as claimed in claim 1, in which the non-protonated
polyaniline is in the base emeraldine form having the formula:
3
3. A gas sensor as claimed in claim 1, in which the non-protonated
polyaniline is in the base leuco-emeraldine form having the
formula: 4
4. A gas sensor as claimed in any of the preceding claims, in which
the film or layer of non-protonated organic material is deposited
on a substrate which supports spaced electrodes, with the film or
layer bridging the spaced electrodes.
5. A method of making non-protonated polyaniline comprises treating
protonated polyaniline with an alkaline solution to de-protonate
the material.
6. A method of sensing a gas, comprising contacting a film or layer
of non-protonated polyaniline forming the gas sensing material with
the gas, determining the conductivity of the film or layer so
contacted, and using the determined conductivity as a measure of
the concentration of the gas contacting the film or layer.
7. A method for the preparation of a gas sensor substantially as
hereinbefore described with reference to the Examples.
Description
[0001] The present invention relates to gas sensors and to methods
of making them.
[0002] Applicants are particularly, though not exclusively,
interested in gas sensors for use in monitoring gases, such as acid
gases, e.g. H.sub.2S, NO.sub.2 and SO.sub.2, in connection for
example with industrial process control or environmental
protection.
[0003] It is well known that organic polymers may form the gas
sensing material in gas sensors. Such organic materials include
conducting polymers which are normally p-type semiconductors whose
conductivities are changed when exposed to oxidising gases such as
NO.sub.x or reducing gases such as NH.sub.3. Sensors using such
materials have been based on both electrical techniques and optical
techniques (e.g. surface plasmon resonance).
[0004] Various organic polymers, such as polyaniline, which appear
to be suitable as gas sensing materials are commonly deliberately
`doped` to improve the specificivity and/or sensitivity of the
materials towards particular gases. Such `doping` also increases
the electrical conductivity and facilitates the detection of change
in conductivity of the polymer on exposure to and interaction with
the gases being sensed; the change in conductivity being used as a
measure of the concentration of the sensed gases.
[0005] With some organic polymers, such as polyaniline, in order
for doping to be achieved the cationic part of the dopant can only
be a hydrogen ion. In such a case the doped form of the polymer can
be regarded as the protonated form.
[0006] Applicants investigations have revealed that there is a
disadvantage with gas sensors comprising such protonated polymers
as the sensing material. Applicants have found that the protonated
polymers are unstable as gas sensing materials in the sense that
they have a tendency to produce inconsistent conductivity readings;
thus unreliable results may be obtained. It is believed that this
disadvantage is a consequence of the mechanism of the interaction
between the gases being sensed and the polymer which involves a
direct de-doping or de-protonating process resulting in undesired
degradation of the material.
[0007] An object of the invention is to overcome or alleviate the
disadvantages associated with protonated polymeric materials as
mentioned above.
[0008] Accordingly, the present invention provides a gas sensor
comprising a film or layer of non-protonated polyaniline as the gas
sensing material.
[0009] In this specification the term "non-protonated" means that
less than 1% of the protenatable imine nitrogen in the polyaniline
is protonated.
[0010] The non-protonated polyaniline may be in the base (neutral)
emeraldine form as depicted by the formula: 1
[0011] or may be in the base (neutral) leuco-emeraldine form (that
is the fully reduced form), as depicted by the formula: 2
[0012] Alternatively, the non-protonated polyaniline may be formed
of a mixture of base emeraldine and base leuco-emeraldine
forms.
[0013] The base emeraldine form appears to be stable up to about
200.degree. C. in air while the base leuco-emeraldine form appears
to be stable up to about 200.degree. C. in an inert atmosphere such
as argon or nitrogen.
[0014] According to another aspect of the invention a method of
making non-protonated polyaniline comprises treating protonated
polyaniline with an alkaline solution to de-protonate the
material.
[0015] The alkaline solution may be an ammonium hydroxide
solution.
[0016] It will be appreciated that the non-protonated material
should not be contacted with a proton-donating species, such as an
inorganic acid before being incorporated into a gas sensor.
[0017] Dry, solid non-protonated material may be obtained by
separating the material from the alkaline solution, for example by
filtering, washing the filter cake with water, and. optionally in
addition with an organic solvent such as isopropanol, after which
the filter cake may be suitably dried, for example under vacuum at
room temperature.
[0018] The dried non-protonated material may be stored in a dry
inert atmosphere, for example in a vacuum desiccator.
[0019] Where the gas sensor is based on an electrical technique,
the film or layer of non-protonated polyaniline may conveniently be
deposited on a substrate already supporting spaced electrodes, so
that the deposited film or layer bridges the spaced electrodes. For
example, the substrate may have thereon a pair of interdigitated
electrodes.
[0020] The thickness of the film or layer may be from about 5
nanometers to about 5 micrometers.
[0021] The substrate may be any suitable insulating, inert material
such as the material forming a printed circuit board or glass or
quartz.
[0022] The electrodes may be made of any suitable inert conductor,
such as cold, platinum, or gold-plated copper.
[0023] Various methods or techniques may be used to deposit the
film or layer on the substrate. For example, in one method the
polyaniline may be dissolved in a suitable solvent, such as
N-methyl-2-pyrrolidinone (NMP), to produce a solution, for example
of 0.01 to 10 Wt/wt concentration, from which the film or layer may
be cast or spun onto the substrate. In one such a method it is
preferred to subject the solution to centrifuging which, in effect,
serves as a spin filtration step which filters out undissolved
particles of polyaniline, before `spreading` a film on a
horizontally disposed substrate employing a spinning operation. In
the spinning operation a predetermined amount of the solution is
deposited on the substrate which is supported on a horizontal
spinning surface arranged to spin about a vertical axis. Initially
the spinning surface is slowly accelerated to a relatively slow
speed which is maintained for a period to distribute the
polyaniline over the substrate, after which the speed is increased
to a maximum in order to produce a substantially uniform thin
film.
[0024] In another method the polyaniline may be deposited by vacuum
evaporation or sublimation onto the substrate in accordance with
procedures generally known per se. In a further method the
polyaniline film or layer may be formed by using Langmuir-Blodgett
techniques generally known per se.
[0025] In order that the invention may be more readily understood
reference will now be made, by way of example, to the accompanying
drawings, in which:
[0026] FIG. 1 illustrates in schematic and block diagram form,
apparatus for measuring the resistance/conductivity of a film of
non-protonated polyaniline deposited on an interdigitated electrode
structure to form a gas sensor,
[0027] FIG. 2 is an ultra-violet/visible absorption spectrum of
non-protonated emeraldine base form of polyaniline which the
inventors made and heat treated at 120.degree. C. for ten minutes
in vacuum,
[0028] FIG. 3 shows a current against voltage characteristic of the
non-protonated emeraldine base form of polyaniline in air at room
temperature when using gold-plated copper interdigitated
electrodes,
[0029] FIG. 4 illustrates the effect that the presence of N.sub.2
on non-protonated emeraldine base form of polyaniline has on the
conductance of the film,
[0030] FIG. 5 shows the conductivity response curve of the sensor
when the non-protonated polyaniline film is exposed to 8 ppm of
H.sub.2S at room temperature,
[0031] FIGS. 6, 7 and 8 illustrate how the conductivity changes in
a film of non-protonated polyaniline in the base emeraldine form
(obtained by spin-coating) on exposure at room temperature to
different concentrations of NO.sub.x (i.e. NO.sub.2O.sub.4 etc.),
SO.sub.2 and H.sub.2S, respectively,
[0032] FIGS. 9, 10 and 11 illustrate how the conductivity changes
in a film of non-protonated polyaniline in the leuco-emeraldine
form (obtained by vacuum evaporation) on exposure at room
temperature to different concentrations of NO.sub.2, SO.sub.2 and
H.sub.2S, respectively,
[0033] FIGS. 12 and 13 illustrate how the conductivity changes in a
film of non-protonated polyaniline in the base emeraldine form
(obtained by Langmuir-Blodgett deposition technique) on exposure at
room temperature to different concentrations of NO.sub.x and
H.sub.2S, respectively,
[0034] FIG. 14 is an infra-red absorption spectrum of a sample of
non-protonated emeraldine base form of polyaniline made by the
inventors,
[0035] FIG. 15 is an infra-red absorption spectrum of a sample of
slightly protonated form of polyaniline, and
[0036] FIGS. 16 and 17 show on an enlarged scale portions of
spectra similar to those shown in FIGS. 14 and 15,
respectively.
[0037] A Method of Preparing Non-Protonated Polyaniline in Powder
Form
[0038] The "emeraldine base" form of polyaniline was prepared as
follows. Aniline (0.1 moles) was added to hydrochloric acid
solution (100 ml, 3.5%, ca. 0.1 moles HCl) in a 250 ml beaker, and
mixed using a magnetic stirrer to give a solution with a final pH
between l and 2, as measured by indicator paper. Ammonium
persulphate (0.1 moles) to act as the oxidising agent to effect
polymerisation of the aniline was dissolved in distilled water (60
ml), and this was added to the stirred reaction mixture. The
mixture turned a dark blue/green colour and the reaction was
observed to be slightly exothermic over a period of about 10
minutes. The mixture was left to stir for a total of about 6 days,
after which it was filtered and washed with water, then methanol
(to remove any low molecular weight species), and finally with more
water. The filter cake was added to an ammonia solution (100 ml,
35%) and stirred for 7 hours before filtering and repeatedly
washing with water, occasionally interspersed with washing with
isopropanol. At this point the filtrate was colourless, indicating
that the filtrate had been washed sufficiently. The filter cake was
then dried under vacuum at 20.degree. C. for 24 hours to give a
brown/purple product which was crushed using a mortar and
pestle.
[0039] Elemental analysis showed that the material contained small
residual amounts of chlorine (0.50 wt %) and sulphur (0.38 wt %).
4.69 wt % was not accounted for and, inventors believe, is ascribed
to oxygen, possibly associated with the chlorine or sulphur, or
with trapped solvent species (water, methanol, or isopropanol).
Isopropanol was subsequently positively identified in .sup.13C
spectra. Chlorine and sulphur residues are normally found in the
products of such preparations and they remain largely unchanged by
attempts to wash them out suggesting that they may be present as
ring substituents.
[0040] Preparation of the Gas Sensor
EXAMPLE 1
Using Spin-Coating to Form a Thin Film of Non-Protonated
Polyaniline.
[0041] Polyaniline powder obtained from the above described method
of preparation was dissolved in N-methyl-2-pyrrolidinone (NMP) at
20.degree. C. in an amount to produce a 5% (by wt.) solution (blue
in colour) of polyaniline.
[0042] The solubility of the polyaniline in the solvent was found
to improve with time and after 48 hours the solution was further
prepared (prior to forming a thin layer on the electrode structure)
in either of the two following ways:
[0043] 1. By centrifuging the solution and then decanting,
Applicants obtained satisfactory results by repeating these
operations several times, for example three times, with the
centrifuge operating each time at a speed of about 4000 rpm for a
period of about 30 minutes.
[0044] 2. By homogenising the solution, for example at a speed of
about 20500 rpm for about 10 minutes.
[0045] The centrifuging process is used, in effect, as a filtration
like step to remove relatively larger particles/aggregates from the
solution whilst the homogenising process is used to break up and
disperse the relatively larger particles/aggregates in the
solution.
[0046] Samples from the resulting centrifuged or homogenised
solution are then used in a spin-coating operation which deposits
or forms a thin film of the polyaniline on the surface of the
cleaned electrode structure.
[0047] Spin-coating techniques are known per se and in the present
operation the parameters were controlled so that the resulting thin
film on the electrode structure had a substantially constant
thickness of, for example, about 1 micrometer (.mu.m) (.+-.0.01
.mu.m).
[0048] In the spin-coating operations conducted by the inventors it
was possible to program specific spinning conditions into the
spinner control apparatus in relation to the initial acceleration
phase, the constant speed spinning periods and the deceleration
phase so that duration times, spinning speeds and ramp were
controlled.
[0049] As an example, the inventors found that the following
spinning conditions involving essentially four steps produced
satisfactory films:
[0050] (i) The spinner surface supporting the cleaned electrode
substrate structure was accelerated from zero in a few seconds to
520 rpm which was maintained for 60 secs to distribute the
polyaniline over the electrode structure.
[0051] (ii) At the end of the 60 secs at 520 rpm, the speed was
accelerated to 1500 rpm (within a few tenths of a second) which was
maintained for 60 secs in order to produce the thin film of
substantially uniform thickness.
[0052] (iii) At the end of the 60 secs at 1500 rpm, the spinner was
decelerated to 300 rpm (within a few tenths of a second) which was
maintained for 20 secs to aid solvent removal so that the film does
not shrink before it is `dry`.
[0053] (iv) At the end of the 20 secs at 300 rpm, the spinner was
decelerated to zero speed (within a few tenths of a second).
[0054] Thus the spinning operation only a little over 2 minutes 20
seconds.
[0055] The inventors found that high quality films could be
reproducibly formed using the above described procedure. The
quality of the films was judged using optical microscopy, with high
quality films being considered to be ones which were not only of
substantially uniform thickness but were relatively free from
`pin-holes`.
[0056] The interdigitated electrode structure comprised a pair of
gold-plated copper electrodes which had been photolithographically
developed onto a substrate of printed circuit board or glass. Each
electrode had 32 digits or fingers. The overlap distance of the
electrode digits or fingers was about 14.3 mm whilst the finger
width and spacing was about 0.381 mm.
[0057] The electrode structure now carrying the thin film was then
heated in a vacuum oven at about 10.sup.-2 mbar to a temperature of
120.degree. C. for 10 minutes to remove NMP solvent from the film
substantially completely.
[0058] Both uncoated and coated electrode structures were subjected
to experiments involving the recording of current passed
therethrough against applied voltage at room temperature
(20.degree. C.) in the dark by using the apparatus as shown in FIG.
1.
[0059] Gas sensing measurements were performed on the gas sensor
structure by exposing the sensor to different gases (diluted in
N.sub.2) in a chamber through which the gases were passed--as will
be described below. (The gases were all obtained from British
Oxygen Company--Special Gases Division with purities up to 99% and
were introduced into the chamber via a Signal Instrument Series 850
gas blender).
[0060] With the object of producing thin films of the polyaniline
for analysis purposes, the thin films were produced by spinning the
polyaniline onto glass substrates at a speed of about 3000 rpm for
30 seconds using solutions of the polyaniline powder (as prepared
above) dissolved in NMP as solvent, in solvent:solute weight ratios
of 10:1, 100:1 and 1000:1. In each case the solutions obtained were
dark blue indicating that the polyaniline is in a non-protonated
form. Protonated polyaniline does not dissolve in the NMP solvent
to any significant extent unless specific surfactant counter ion
acids are used to protonate the material.
[0061] Again the thin films, this time supported by the glass
substrates, were transferred to a vacuum oven and heated at
10.sup.-2 mbar to a temperature of 120.degree. C. for 10
minutes.
[0062] The thickness of the thin films of polyaniline formed by the
spinning process on the glass substrates was found to be of the
order of 0.1 .mu.m as measured using a surface profiling
Talystep.
[0063] FIG. 2 is a representative example of the UV-VIS spectrum of
the heat treated polyaniline film, obtained using a Perkin-Elmer
Lambda 19 spectrophotometer. Two adsorption bands are clearly
evident at about 320 nm and 635 nm and these are considered to be
characteristic of the non-protonated emeraldine base form of
polyaniline.
[0064] The inventors interpret the above findings as confirmation
that no significant thermally induced chemical degradation took
place during the vacuum heating process to remove solvent.
EXAMPLE 2
Using an Evaporation Deposition Method for Forming a Thin Film of
Non-Protonated Polyaniline.
[0065] An evaporation method carried out in a vacuum chamber was
used to produce a thin film of non-protonated polyaniline in the
base leuco-emeraldine form on a substrate such as a glass substrate
or on an electrode structure located in the chamber. Incorporated
into the system was a temperature controller which maintained a
substantially constant source temperature, an adjustable shutter to
control the deposition/growth time and a mask to define the
specific area of deposition. Initially a source boat or container,
located within the chamber but separated from the substrate, was
heated up to 300.degree. C. to drive off surface contamination. 40
mg of non-protonated polyaniline powder obtained from the earlier
described method were then introduced into the source boat and the
system was evacuated to a background pressure of 10.sup.-3 mbar.
With the shutter closed, the temperature was raised to and
maintained at 400.degree. C. by means of a heater. When a stable
pressure was reached, the shutter was opened and evaporated
polyaniline was thereby allowed to pass into the region containing
the substrate and to deposit through the mask onto the substrate.
The deposition was allowed to proceed for a predetermined or fixed
length of time for control purposes. After such deposition, the
system was allowed to cool to room temperature and return to
ambient pressure, and the substrate bearing the deposited film
removed. A typical thickness of the deposited film obtained by the
above described method was found to be about 120 nm for a shutter
time of 30 min.
EXAMPLE 3
Using a Langmuir-Blodgett Method for Forming a Thin Film of
Non-Protonated Polyaniline.
[0066] Preparation of a Solution of the Polyaniline
[0067] Using non-protonated polyaniline obtained from the earlier
described method, a solution of the polyaniline for use in LB film
deposition was prepared by first making a mixture of 1:10 by weight
of acetic acid:polyaniline (as an aid to spreading). The acid was
completely absorbed by the polymer and no change in colour from the
dark blue of the polyaniline was observed. 0.1 mg of the mixture
was then dissolved into 10 ml of solvent and sonicated for 30
minutes. The solvent used was a mixture of chloroform:n-methyl-2
pyrrolidinone in a 1:5 weight ratio.
[0068] LB Film Formation
[0069] The non-protonated polyaniline solution was added to a tank
of water and the solution was found to spread uniformly on the
water surface without any visible sign of aggregation. The surface
pressure versus area isotherm at 20.+-.2.degree. C. after multiple
(.sup.-4) compressions revealed the material to form a reasonably
condensed layer up to 40 mNm.sup.-1 surface pressure. On the
assumption that the area per molecule of the acetic acid was
negligible, the area per emeraldine base repeat was 0.20 nm.sup.2
at 30 mNm.sup.-1 surface pressure. It was thus concluded that the
polymer did not form a monomolecular film on the water surface. The
floating film was stable for several hours at a surface pressure of
30 mNm.sup.-1. The substrate on which the film was to be formed was
dipped into the floating film at speeds of 2 mmmin.sup.-1, allowing
at least 20 minutes between the first and second dips. Z-type
deposition with a transfer ratio of 1.0.+-.0.1 on all dip cycles
was observed. The films formed on the substrates were reasonably
uniform, by visual inspection, for up to 50 layers.
[0070] By way of illustration, the 10:1 sample which produced, via
the spin-coat method, good quality films was used to form the thin
film on the electrode structure.
[0071] With reference to the apparatus in FIG. 1, a sample of the
electrode structure bearing the thin film of non-protonated
polyaniline 1 is held in the dark and at constant temperature
(20.degree. C.) in a sample chamber 2. A voltage is applied to the
electrode structure via a D.C. voltage calibrator 3 (in effect, a
constant voltage source) while current through the film is
monitored via the picoammeter 4. A voltage output is derived from
the picoammeter 4. A voltage output is derived from the picoammeter
and applied to the y-axis of a y-t chart recorder 5. Thus the chart
recorder will reflect change in current through the film (and thus
the resistance/conductance) as a function of time. An offset unit 6
allows a constant voltage to be added (in parallel with the output
from the picoammeter) in order to `back-off` the output of the
picoammeter and allow small current changes to be monitored.
[0072] A test gas source 7 and a dilution gas source 8 (nitrogen in
present experiments) are connected to a gas blender 9 via
respective inlets 10 and 11. An outlet 12 from blender 9 is
connected to the sample chamber for introducing the blended gases
into the chamber 2.
[0073] FIG. 3 shows good linearity of the current versus voltage
characteristic in respect of the thin film of polyaniline on the
interdigitated electrode structure as a result of experiments
conducted in air at room temperature (20.degree. C.). Experiments
conducted on the uncoated electrode structure produced results (not
shown) that confirmed that in the case of the coated electrode
structure substantially all of the current which was flowing across
the structure was flowing through the polyaniline film rather than
the substrate. The good linearity of the current versus voltage
characteristic and the increase in slope of the line observed by
the inventors for thicker films (data not shown) indicate that
ohmic contacts were established between the electrodes and the
polymer film.
[0074] The conductivity of the sample was calculated to be
1.5.times.10.sup.-11 Scm.sup.-1 which was a good comparison with a
literature value of 1.times.10.sup.-11 Scm.sup.-1.
[0075] The conductivity was found to be relatively stable in air
but on exposure to a flow of nitrogen gas in the sample chamber it
decreased, rapidly at first, over a period of minutes to a lower
stable value. Such a change is shown in FIG. 4 in a case where 200
mV was applied across the electrode structure. The change in
conductivity is attributed to the removal of water molecules
trapped in the polymer matrix. The final stable conductivity in
N.sub.2 was considered to provide a good baseline for the gas
sensing experiments.
[0076] Gas sensing measurements were carried out at room
temperature (i.e. 20.degree. C.) using test gases diluted in
nitrogen, as mentioned above. For each experiment the polyaniline
coated electrode structure served as a chemiresistor and the effect
of exposing the chemiresistor to the different gas was studied. In
separate experiments, each test gas (diluted with N.sub.2) was
introduced into the sample chamber, so as to be in contact with the
polyaniline film, for a specific length of time which was chosen
having regard to the sensitivity of the device to the particular
test gas.
[0077] The test gases used were NO.sub.x, H.sub.2S, SO.sub.2, Co
and CH.sub.4. On exposure of the polyaniline film to any of the
gases NO.sub.x, H.sub.2S and SO.sub.2 a substantial increase in
current flow through the film, and thus an increase in
conductivity, was observed. In contrast, on exposure of the film to
CO or CH.sub.4 no measurable change in the conductivity was
observed.
[0078] FIG. 5 shows a typical response of the sensor when exposed
to 8 ppm of H.sub.2S. The response was found to be reversible with
a short delay time. In FIG. 5, .tau. represents the delay time,
i.e. the time between turning `on` the test gas and the first
measurable change in current through the polyaniline film sensor.
This delay time includes the time taken for the test gas to flow
from its source through connecting pipework to the sample chamber
and the time to interact with, and cause a resistance change in,
the film. Thus, although in FIG. 5 the delay time is shown as
approximately 30 seconds, the actual response of the sensor will be
much less.
[0079] Conductivity measurements which resulted by exposing
non-protonated base emeraldine form (obtained using the
spin-coating method), when used as the gas sensing material, to
different concentrations of NO.sub.x, SO.sub.2, and H.sub.2S are
shown respectively in FIGS. 6, 7 and 8. It can be seen that
H.sub.2S has the largest effect on the conductivity across the
sensor while NO.sub.x has the smallest effect. Substantially
reversible K responses were obtained using any of these three
gases.
[0080] Conductivity results obtained by exposing the non-protonated
base leuco-emeraldine form (produced via the vacuum evaporation
method), when used as the gas sensing material, to different
concentrations of NO.sub.2, SO.sub.2 and H.sub.2S are shown
respectively in FIGS. 9, 10 and 11. It will be noted that while
this form of the polyaniline was again also sensitive to exposure
to the gases, it led to different responses compared with those
shown in FIGS. 6, 7 and 8. In particular, with respect to sensing H
S an increase in concentration of the gas led to an increase in
current in FIG. 8 but a decrease in current in FIG. 11.
[0081] Illustrative conductivity measurements which resulted by
exposing non-protonated base emeraldine form (obtained using the
Langmuir-Blodgett preparation method), when used as the gas sensing
material, to different concentrations of NO.sub.x and H.sub.2S are
shown respectively in FIGS. 12 and 13. It will be noted that the
responses obtained here are different compared with the previously
illustrated responses.
[0082] Applicants have noted that the three different methods
described above for forming the films or layers of the organic
polymeric material enable a very wide range of film thicknesses to
be obtained and it is envisaged that appreciation of this may
enable the sensitivity of the films to particular gases to be
optimised.
[0083] Illustrations of results obtained from the experiments are
summarised in Tables 1, 2 and 3. In these Tables, "Normalised
change" relates to the sensitivity of the film to the various
gases; it is the fractional resistance change (.DELTA. R/R) divided
by the gas concentration (ppm). The greater the figure obtained,
the more sensitive is the film to the particular gas concerned.
[0084] It will be appreciated that by initially employing known
concentrations of gases the results can be used to calibrate the
gas sensor, such that subsequently when an unknown concentration of
gas is used the conductivity measurement can be used as a measure
of the gas concentration.
[0085] The results obtained from the above described experiments
indicated that non-protonated polyaniline, as opposed to protonated
forms of polyaniline is stable at room temperature and with respect
to the flow of gas thereover, and can be used as a selective,
reproducible and reversible sensing material in a gas sensor for
sensing the presence of e.g. H.sub.2S, NO.sub.x and SO.sub.2 in
very low concentrations (down to a few parts per million) at room
temperature. The selectivity to different gases can be determined
by the different values or the slope of the response curve.
[0086] To assess whether a sample of polyaniline is
"non-protonated" use can be made of infra-red absorption
spectographs. FIG. 14 shows an infra-red spectrum of a sample of
non-protonated emeraldine base form of polyaniline made in
accordance with the method of the invention, while FIG. 15 shows an
infra-red spectrum of slightly protonated (i.e. greater than 1%)
polyaniline. In FIG. 15, towards the high energy end of the
spectrum a large rising background is visable. This is due to a
charge transfer band of emeraldine caused by the protonation. The
extent of protonation can be determined by comparing the relative
peak heights of the peaks in about the 1594 cm.sup.-1 and 1512
cm.sup.-1 band regions. In the case of the slightly protonated
sample, see FIG. 17 where the peak in the 1594 cm.sup.-1 band
region is of higher intensity. In the case of the "non-protonated"
sample, it is the peak in the 1512 cm.sup.-1 band region which is
of higher intensity, albeit shifted slightly in energy--see FIG. 16
and the shift from about 1512 cm.sup.-1 to about 1501 cm.sup.-1.
The present inventors used this kind of data to estimate that the
degree of protonation of the samples of "non-protonated"
polyaniline was less than 1%.
[0087] It will be understood that the non-protonated polyaniline
film or layer may be incorporated into gas sensor arrangements
other than those based on a pair of interdigitated electrodes, e.g.
as shown in FIG. 1. For example, it is known that gas sensors can
be based on charge-flow transistors (CFT), in which case a thin
film of an electrically-resistive gas sensing polymer material is
deposited e.g. by spin coating, in a `gap` or `hole` deliberately
provided in the gate electrode of the CFT. The presence of the thin
film of resistive material in the `hole` in the gate structure
results in there being a time delay between the application of the
gate-to-source voltage and the appearance of a complete channel.
The time delay is dependent on the resistivity of the thin film. It
will be appreciated that the resistivity of the film is affected by
the gas or gases in the immediate vicinity of the film and that
such a response can be used as the basis for sensing gas.
[0088] Applicants have used spin-coating to fill such a `gap` or
`hole` in a CFT to produce a film of non-protonated polyaniline
approximately 100 nm thick; the `hole` being 35 .mu.m (microns) in
diameter. By way of illustration only, in the presence of 4 ppm
NO.sub.x, it was found that the turn on time of this modified CFT
was substantially decreased, resulting in a changed drain-source
current (I.sub.DS) versus time response as compared with that
obtained in the presence of a reference atmosphere of nitrogen.
[0089] It is again envisaged that by initially employing known
concentrations of a gas, the measurements can be used to calibrate
the sensor. Thus, when an unknown concentration of gas is sampled,
the measurements obtained can be used to determine the
concentration of the sampled gas.
* * * * *