GB2186087A - Heterocyclic semiconductor gas sensors - Google Patents

Abstract

An organic chemical semi-conductor gas sensor comprises a film of 1 to 50 molecules thick of a compound of formula I: <IMAGE> in which one of R2 and R3 is hydrophilic and the other is hydrophobic, other R's X's and Y's are specified radicals, and M is a specified metal, preferably nickel, or two hydrogen atoms. The sensors may be incorporated into NOx detectors.

Description

SPECIFICATION Improvements in gas sensors This invention concerns improvements in gas sensors, and more especially it concerns improved organic sensors for gases such as NOx.

Reference is made to UK Published Patent Application No. 2,111,987A which discloses certain symmetric heterocyclic compounds as having semi-conductor properties which are effective as gas sensors.

We have now discovered different classes of compounds offer the possibility of improved performance as gas sensors, and novel gas sensors utilising such compounds.

The present invention provides an organic chemical semi-conductor gas sensor comprising a film of 1 to 50 molecules thick of a compound of general formula

in which each of A and B, which may be the same or different, is a carbon atom or a nitrogen atom, Each R1, R1,, Re, R6,, which may be the same or different, is a hydrogen atom, methyl group or forms part of a condensed six membered ring system, R2 and R3 are different and one is selected from hydrophilic atoms and groups and the other is selected from hydrophobic atoms and groups.

Each pair of X's and Y's, which may be the same or different, together form an unsaturated ring system selected from phenyl, napthalene, anthracene, 2,3-pyridine and 3,4-pyridine, and phenyl groups which together with adjacent nitrogen atoms and adjacent R1, R,', R6, R6, radicals, form part of a condensed double or triple six-membered ring system, or are hydrogen atoms.

Each R4, R4,, R5, B5,, which may be the same or different, is a hydrogen atom, chlorine, bromine or iodine atom, or a nitro, carboxy, alkyl of 1 to 4 carbon atoms, cyano or ester group, and M is selected from manganese, iron, cobalt, copper and zinc atoms or may be two hydrogen atoms.

Exemplary molecular structures for use in the present invention include the following:

In the compounds of formula I, it is preferred that each pair of X's and Y's are identical one with another, including substituents, and if substituents are present, it is preferred that these are symmetrically arranged.

The radicals R2 and R3 may be selected from hydrophilic substituents such as acid, including especially carboxy groups, amides, carboxylic acid esters and alcohols, and from hydrophobic groups such as straight or branched chain alkyl, including especially isopropyl, and aralkyl or alkyl aryl, for example, paramethyl phenyl.

It is preferred that the R,, R,', R6, B6,, substituents are identical and are hydrogen.

Certain of the compounds and their preparation have been described in the literature, although they have not been suggested for use in this film semiconductor gas sensors, and the novel compounds may be prepared by analogous methods or by syntheses within the ability of the competent chemist. Reference may be made to Mueller and Woehrle, Markromol. Chem 176 2775-95(1975); Cutler and Dolphin J. Coord. Chem. 6, 59-61 (1976); Guendel and Bohnert, Z.Nat. 376, 1648-1651 (1982); Phace et al, J. Hetero. Chem. 17, 439-443 (1980), Honeybourne, Inorg. Synth. 1849(1978), etc.

It is preferred to use nickel as the metal M in the selected compound of formula I. Metals may be exchanged or inserted, for example by refluxing using the appropriate metal acetate in a dilute alcoholic solution.

Preferably, the film of the compound of general formula ' is deposited upon a suitable substrate by Langmuir-Blodgett techniques to give a film which is substantially completely ordered on the molecular level. Ordering may be by stacking the largely planar molecules edge-on with respect to the substrate, or parallel to the substrate. The substrate may be an insulator, semi-conductor or conductor, and conductors may be applied to the film by any suitable means to enable a current to be passed across or through said film. The present invention also lends itself to application to an insulated gate field effect transitor, thus to chemically modify its performance in the presence of gases being sensed. Deposition of the film by other techniques such as from the vapour phase, may be used.

It is preferred to use as the substrate a crystal face or optical flat of a material having a highly ordered lattice or a lattice of highly reproducible structure. Silicon or gallium arsenide are particularly preferred substrate materials.

The preferred deposition method by Langmuir-Blodsett technique is suitably carried out by a method derived from that described by Tredgold (Thin Solid Films 113, 115-128). For example, the selected compound of formula 1, in dilute solution in chloroform, is spread on an aqueous sub-phase and compressed. This monolayer is then deposited on the substrate in order to produce an ordered monolayer. Additional ordered layers may be built up by dipping and withdrawing the substrate through the spread organic solution which is supported by a buffered aqueous sub-phase of pH in the region 5-0-6.0 or 7.0 to 8.0, at below room temperature, e.g.

about 10 C, at a sufficiently slow rate to enable additionally deposited monolayers to order up.

The main alternative method of film formation is by vacuum sublimation; it is rather more difficult to control the thickness of the film than with the Langmuir-Blodgett technique, but this can be achieved by suitable batch sizes and quality control. Other methods of film formation may be tried to achieve the best sensor efficiency and stability for any given compound of formula I, including deposition from solution, admixtures with polymers, such as bis-phenyl-apolycarbonate, polymethyl methacylate, or attaching the compound by a polysiloxane.

As previously indicated, a variety of sensor cells may be produced within the scope of the invention. For example in a sandwich cell, the substrate for film deposition is a suiable metal conductor, for example a clean aluminium foil or gold foil. A thin metal top electrode, porous to enable gas diffusion to the film, may be deposited on the film of compound of formula I.

Alternatively, the film of compound I may be deposited upon a silica (eg. quartz) optical flat which has been cleaned by boiling isopropanol and immersion in an acid etch bath, yielding a hydrophilic surface. Electrodes, e.g. of aluminium, copper, platinum or gold, may be applied prior to, or subsequent to film deposition, to contact the film and provide an electrical connection. It is envisaged that the thin film sensors of the present invention may also be manufactured by deposition/etching technology derived from that used for making silicon chip microprocessors.

The sensors of the invention demonstrate significant changes in electrical conductivity upon exposure to gas containing NOx. The gas induced increase in DC current is desirably amplified, preferably using a noise reduction design, and preferably an identical sensor protected from the gas is used as a reference in order to substantially eliminate or compensate for "drift" due to temperature. The sensor may be incorporated into an instrument of hand-held or larger dimensions and NOX concentrations may be displayed by LED, LCD or meter, and/or a visual or audible alarm may be triggered at an appropriate level; alternatively, or in addition, a signal derived from the sensor may be transmitted to a remote monitoring station.

The sensors of the invention incorporate different compounds to those previously disclosed for use in semi-conductor sensors, and the film thickness is a small fraction of that previously recommended, namely about 1 micron. The sensors of the invention offer faster response times because of reduced diffusion times and improved release of the gas being sensed from the molecular matrix.

While reference has been made extensively herein to the sensing of NOx, selection of appropriate compounds of formula I, for example by choosing metals other than nickel, may lead to sensors capable of quantitatively detecting gases such as CO, CH4, H2, N2 04, N2 2, CO2, NH3, H2S, H2O and halogens. Specificity may be improved by incorporation of a filter of passive or active type to remove potentially interfering gases.

Claims (9)

1. An organic chemical semi-conductor gas sensor containing a film of 1 to 50 molecules thick of a compound of general formula, I

in which each of A and B, which may be the same or different, is a carbon atom or a nitrogen atom, Each R1, B1,, R6, Rue', which may be the same or different, is a hydrogen atom, methyl group or forms part of a condensed six membered ring system, R2 and R3 are different and one is selected from hydrophilic atoms and groups and the other is selected from hydrophobic atoms and groups.

Each pair of X's and Y's which may be the same or different, together form an unsaturated ring system selected from phenyl, naphthalene, anthracene, 2,3-pyridine and 3,4-pyridine, and phenyl groups which together with adjacent nitrogen atoms and adjacent R1, B1,, Be, Re' radicals, form part of a condensed double or triple six-membered ring system, or are hydrogen atoms.

Each R4, B4,, Be, Rue', which may be the same or different, is a hydrogen atom, chlorine, bromine or iodine atom, or a nitro, carboxy, alkyl of 1 to 4 carbon atoms, cyano or ester group, and M is selected from manganese, iron, cobalt, copper and zinc atoms or may be two hydrogen atoms.

2. A sensor according to claim 1, wherein the compound of general formula I is of the structure la, Ib or Ic, hereinbefore defined.

3. A sensor according to claim 1, wherein in the compound each pair of X's and Y's are identical one with another.

4. A sensor according to claim 1, 2 or 3, wherein in the compound the radicals R2 and R3 are selected from carboxy groups, amides, carboxylic acid esters and alcohols and from straight or branched chain alkyl, aralkyl and alkyl aryl.

5. A sensor according to any one of claims 1 to 4, wherein in the compound, each R1, B1,, Re and B6, is hydrogen.

6. A sensor according to any one of claims 1 to 5, wherein in the compound, M is nickel.

7. A sensor according to any one of the preceding claims, wherein the film is deposited on a substrate by the Langmuir-Blodgett technique.

8. A NOx sensing instrument incorporating an organic sensor according to any one of the preceding claims.

9. An organic sensor according to claim 1, substantially as hereinbefore described.