This invention relates to gas sensing devices, "gas" in this specification being taken

to include vapour.

Gas sensing devices are known, of a kind in which a piezo-electric component is

coated with a sensor material having characteristics of mass, stiffness and viscosity. The

sensor material interacts with gaseous materials, in this context analytes, selectively, to

produce a change in one or other of the characteristics such as to affect the oscillation frequency of the piezo-electric component when incorporated in an oscillator circuit.

Some sensor materials are relatively non-discriminating, causing a similar reaction to

a wide range of gases, while others are fairly selective. Other problems with known sensor

materials include inadequate repeatability of response, insufficiently rapid response to

stimulation, and limited 'designability' of response to suit specific gases.

An object of the present invention is to provide a sensor material for a gas sensing

device which at least partly overcomes one or more of these disadvantages.

According to one aspect of the present invention a gas sensing device comprises a

piezo-electric component connected in an oscillator circuit to oscillate at the component

resonant frequency, the piezo-electric component being loaded with a polysiloxane sensor

material which interacts with a range of gaseous analyte materials so as to alter the oscillation frequency response resulting from such interaction being indicative of the analyte material, and the sensor material being a polysiloxane having the general formula:

wherein M is a mesogenic group having a general structure:

and wherein:

Me is methyl;

D and E are either alkyl, typically methyl or ethyl, or alkyl cyanides, (CH~)_ CN, the rings P & Q are selected from phenyl, trans-cyclo hexyl, pyridyl, pyrimidyl, dioxanyl, and bicyclo (2.2.2) octyle;

m is 0 or 1 ;

Y is R„ OR,, Hal (F,Cl,Br,I), CF ₃ , CO ₂ R, or CN when = 1

and R„ OR,, Hal (F,Cl,Br,I), CO ₂ R, or CN when m = 0;

where R,is alkyl, open chain or branched, containing 1-12 carbon atoms;

A is COO, OOC or CH ₂ .CH ₂ ;

X and Z are selected independently from H, methyl and Hal such that one at least of

X, Y and Z contains fluorine;

the ratio b:a is less than 3:1 and > 0.

Preferably the piezo-electric oscillating component is coated with a polymer selected

from the following polymers described in GB Patent Specification Numbers: 2195646,

2221690, 2249794 and 2249795:

a. A liquid crystal polymer having a general Formula 1 :

(CH ₂ )n O

wherein X is esogenic group having a general structure

wherein

may carry lateral methyl, fluoro or chloro substituents, and are selected from phenyl,

trans-cyclohexyl, pyridyl, pyri midyl, dioxanyl and bicyclo-(2,2,2)-octyl;

wherein Y is COO, OOC or CH ₂ CH ₂ , C is 0 or 1; wherein n is an integer between 4 and 9 inclusive;

wherein R is F, CF ₃ , CN, R', OR ¹ or COOR ¹ where R ¹ is alkyl, and (L) indicates that a lateral methyl or fluorosubstituent may be present; in cases other than when R is CN and Y is COO and n=5 and C=l, L=CH ₃ : wherein the ratio bra is less than 3 and greater than 0;

The polymer being random or substantially random polymer, b. Liquid crystal polymers, of the general formula:

(CH ₂ )nO- M

wherein the mesogenic grouping M has a general structure

wherein the rings P and Q are phenyl, trans-cyclohexyl, pyridyl, pyrimidinyl, dioxanyl, or

bicyclo (2,2,2) octyl; A is COO, OOC, CH ₂ CH ₂ ; m is 0 or 1; X, Z are H, Me hal; and Y is alkyl, alkoxy, hal, CF ₃ ,CN or CO ₂ alkyl.

Homopolymers wherein b=0 and at least one of x, y and z is F or CF ₃ or wherein b=0 and R is CH ₂ Me, and copolymers wherein R ⁴ is (CH ₂ ) ₃ CN are preferred materials, c. Liquid crystal polymers of the general formula:

wherein the mesogenic grouping M has a general structure

wherein the rings P and Q are selected from phenyl, trans-cyclohexyl, pyridyl, pyrimidinyl,

dioxanyl, and bicyclo (2,2,2) octyle, A, x, y and z are standard mesogenic substitutuents and

b/a < 3 and > 0. d. Liquid crystal polymers of the general formula:

(CH2) _n O— M

wherein the mesogenic grouping M has a general structure

wherein the rings P and Q are selected from phenyl, trans-cyclohexyl, pyridyl, pyrimidinyl,

dioxanyl, and bicyclo (2,2,2) octyl; and X, Y, Z, A and m are standard mesogenic groups.

A gas sensing device in accordance with the invention will now be described by way of example with reference to the accompanying drawings, Figure 1-8, showing frequency response characteristics for a number of specific sensor materials and analytes.

The device includes a piezo-electric element incorporated in an oscillator circuit in known manner, the element resonant frequency determining the oscillation frequency, e.g.

10MHz. The frequency will depend upon the type of piezo-electric device, whether piezo-

electric crystal, surface or bulk acoustic wave or other mass-balace device. Means are

provided for monitoring the oscillation frequency. The piezo-electric element is coated with

a sensor material which interacts selectively with gaseous analytes to which it is exposed, the

characteristics - mass, stiffness or viscosity - of the sensor material being modified by this

interaction to cause a frequency response which is characteristic of the combination of piezo-

electric element, sensor material and analytes. Means are then provided for recording the

response and identifying it with predetermined analyte responses.

The present invention is based on the appreciation that materials, at least some of

which were previously known in connection with electro-optic liquid crystal display

technology, are admirably suited for use as sensor materials in piezo-electric gas sensing devices. Such materials, in their function as liquid crystal polymers, are described in GB Patent Specification Numbers: 2195646, 2221690, 2249794 and 2249795 to which attention

is directed for an explanation of the structure and preparation of such materials.

Particular examples of polysiloxanes suitable for use as sensor materials in gas sensing

devices are the following:

CH ₃ (A) I

(CH ₃ )3SiO--(SiO) ^h -Si(CH3)3

(CH ₂ )6 /θV Cc0o2 ₂ --/ V y- CN

CH ₃

CH3 CH3 I I (B)

(CH ₃ )3 SiO

CH ₃ CH ₃

I I (C)

(CH ₃ )3 SiO -

a:b = 21:19

o

Examples of frequency response obtained using the three polysiloxane examples (A), (B) and (C) with analytes toluene, di-butylsulphide and iso-amyl acetate are shown in Figures

1 to 8.

In Figure 1, a piezo-electric element coated with polysiloxane (A) is subjected to

toluene vapour at time zero, the toluene being purged at time 1600 seconds by replacement

with an unreactive gas. It can be seen that the frequency response undergoes an almost step

change by 700 Hz, both rising and falling edges being very steep.

The test was repeated three times, the three graphs being superimposed to show the

degree of repeatability.

Figure 2 shows the result of similar tests using toluene and polysiloxane (B), and it

may be seen that there is very little difference between the results for (A) and (B), (B) giving a slightly smaller frequency shift.

In Figure 3 however, using polysiloxane (C) and again toluene as the analyte, there

is a much greater frequency shift, of about 3200 Hz, although with a 100% instead of 50%

analyte concentration.

Figures 4, 5 and 6 show the response of the three example to di-butylsulphide analyte.

Again, there is very little difference between the results for polysiloxanes (A) and (B) but a

much larger frequency shift for (C).

Figures 7 and 8 show responses to iso-amyl acetate for polysiloxanes (B) and (C) and

again there is a considerable difference of frequency shift.

Many variations may be made to the polysiloxane examples given, particularly to the

pendant side chains, making the polymers effectively 'designable', the selectivity and

sensitivity being tunable to suit the particular analyte gas/vapour.

The polymer examples given above fall within the general formula:

(CH ₂ )n O- M

wherein M is a mesogenic group having a general structure:

In the above formula:

Me is methyl;

K is alkyl, typically methyl or ethyl;

the rings P & Q are selected from phenyl, trans-cyclo hexyl, pyridyl, pyrimidyl, dioxanyl, and

bicyclo (2,2,2) octyl;

m is 0 or 1 ;

y is R, OR,, Hal, CF ₃ , CO ₂ R, or Cn when m = 1

and R, OR,, Hal, CO ₂ R, or CN when m = 0;

R,is alkyl, open chain or branched, containing 1-12 carbon atoms;

A is COO, OOC or CH ₂ .CH ₂ ;

X and Z are selected independently form H and F such that one at least of X, Y and Z

contains fluorine the ratio b:a is less than 3:1.

The above mesogenic group M may be selected from the following structures (a) to (i):

^(b) - OVCOO-ZO CN

CH ₃

^(d) -/θ COO-ZO V-COO.CH2CH.C2H5

Advantages arising from the use of polymers described above are:

1) These materials demonstrate little visco-elastic change as a consequence of

vapour sorption yielding highly reproducible responses on exposure to gases

and vapours;

2) Rapid sorption and desorption of gases and vapours from these materials lead

to rapid responses upon stimulation;

3) Sensitivity of specific materials to specific vapours or gases is extremely high

as a result of considerable partitioning between the material and the gaseous

phase;

4) Selectivity for specific gases and vapours varies significantly between materials;

5) Designability of the materials for specific uses allows control of both the sensitivity and selectivity for specific gases and vapours.