WO1992015876A1

WO1992015876A1 - Oxygen sensor

Info

Publication number: WO1992015876A1
Application number: PCT/GB1992/000369
Authority: WO
Inventors: Andrew Mills; Hamilton Neil Mcmurray
Original assignee: Abbey Biosystems Limited
Priority date: 1991-03-01
Filing date: 1992-03-02
Publication date: 1992-09-17
Also published as: GB9104432D0

Abstract

An oxygen monitor which comprises a substrate provided with an oxygen-permeable film comprising a polymeric vehicle and at least one luminescent compound immobilised within the polymeric vehicle. The luminescent compound emits luminescence upon irradiation of the film with visible light, the luminescence being quenched by any molecular oxygen present in the film. The oxygen monitor can be used for measuring oxygen by locating the monitor in a position for receiving oxygen from an oxygen-containing medium, irradiating the oxygen permeable film of the monitor with light having a wavelength of at least 400 nm and monitoring luminescence emitted by the luminescent compound or compounds present in the oxygen-permeable film.

Description

Oxygen Sensor

The present invention is concerned with a monitoring device and method for measuring oxygen concentrations in oxygen-containing media.

The idea of employing monitoring devices using optical fluorescence technology in the detection of oxygen is already known in the biomedical and biotechnology fields. However, the choice of a suitable chromophore for use in such monitors has previously been relatively limited and has been dominated by a narrow range of organic materials, most notably polynuclear aromatics which are susceptible to photooxidative degradation.

We have now developed an oxygen monitoring device using alternative compounds in place of the previously used chromophores which together with appropriate film forming media provide our monitoring device with improved properties (such as visible light excitation, resistance to photodegradation, rapid response time, long term physical and chemical stability).

According to the present invention there is provided a monitoring device suitable for measuring oxygen concentrations in oxygen-containing media, which device comprises a substrate having thereon an oxygen-permeable film comprising a polymeric vehicle and having immobilised therein at least one luminescent compound which can be induced to emit luminescence upon irradiation of said film with visible light, which luminescence can be quenched by any molecular oxygen present in said film.

By measuring the intensity, lifetime or polarisation of the luminescence emitted from the excited luminescent compound it is possible to obtain an estimate of the level of oxygen dissolved in the film and hence its concentration in the medium being measured. The oxygen present in the medium of interest diffuses between the medium and the film until equilibrium is attained.

Irradiating the film with visible light causes the luminescent compound to luminesce. This luminescence can then be quenched by oxygen dissolved in the film. This occurs via a rapid diffusion controlled bimolecular process. ___>

The efficiency with which quenching takes place is determined by the permeability of the film to oxygen and the natural luminescence lifetime of the luminescent compound . Although luminescent quenching can result in the generation of oxygen in an electronically excited state this relaxes thermally and no significant consumption of oxygen by chemical reaction takes place within the film.

Typically, the device according to the invention exhibits a fully equilibrated response to change in the oxygen content of a gas contacting the film in less than one second.

Suitable luminescent compounds include porphyrin and polyheterocyclic complexes of transition metals, such as ruthenium, palladium or osmium, or lanthanides, and thiocarbonyl compounds. These complexes are preferably substantially resistant to photodegradation and radiation (particularly gamma ray) sterilisation and exhibit long term physical and chemical stability.

Preferably, the oxygen permeable film is of about 20 microns in thickness and comprises a substantially water insoluble, non-polar polymer vehicle. Typically, the polymer vehicle has high permeability to oxygen, which ensures rapid and sensitive response in measuring the oxygen concentration, and it is often preferred that a non-volatile liquid plasticiser is intimately mixed with the polymer to improve this permeability. Suitable plasticisers include phosphoric acid esters, phthalate esters, sebacate esters, adipate esters or the like. Such plasticisers are generally used with polymers such as polyvinyl chloride, polyvinylbutyral, polyvinyl isobutylether, polyvinyl acetate, polyethyl acrylate, polymethyl methacrylate, cellulose acetate, or ethyl cellulose. However, such a plasticiser is often not necessary when elastomeric polymers with an intrinsic high oxygen permeability are used, for example, silicone rubber, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer or polyurethane rubber.

It is often preferred that a solubilising moiety is present in the oxygen permeable film. This solubilising moiety is often an organophilic counterion, which may be included when it is necessary to improve the compatibility of ionic luminescent compounds with the polymer vehicle. Typically the solubilising moiety binds (either covalently or electrostatically) to the luminescent compound and this helps to hold the compound in the non-polar film. Cationic metal complexes can be ion-paired with quaternary borate anions, such as tetraphenyl borate, tetra-p-chlorophenyl borate, tetra-p-fluorophenyl borate, or anions derived from carboxylic acids and incompletely substituted esters of phosphoric, sulphonic, or phosphonic acids, such as di (2-ethylhexyl) phosphate, di-decylphosphate, naphthalene sulphonic acid, naphthenic acid. Anionic metal complexes can be ion-paired with quaternary ammonium or phosphonium ions such as tetra-octyl ammonium, tetra-butyl ammonium, tricapryl methyl ammonium or tetraphenyl phosphonium.

Alternatively, the luminescent compound itself can be modified to improve its compatibility with the polymer film. This is equally applicable to both charged or neutral species. A preferred modification involves the covalent attachment of an organophilic moiety to the luminescent compound. This organophilic moiety is typically a side chain which branches from the luminescent compound to associate with the polymer but which does not appreciably influence the luminescent and energy transfer activities of the luminescent compound. Typical side chains may comprise groups such as alkyl, alkyl ester, aryl, aryl ester, ether, carbonyl or the like, which in some embodiments may be co-polymerisable with the polymer vehicle. Alternatively, the luminescent compound may have attached thereto selected monomers which are known to be co-polymerisable with the polymer vehicle.

It is preferred that the substrate is of a substantially transparent material (such as glass), or a material of refractive index not lower than that of the film. When films are coated on such substrates, the emitted luminescence tends to be collected in the substrate by total internal reflectance and can then be conducted to a suitable detector and a measure of the intensity, lifetime or polarisation of the emitted luminescence recorded. Typically, the emitted luminescence has a wavelength in the visible or near infra red region.

There is further provided by the present invention apparatus comprising a monitoring device suitable for measuring oxygen concentrations in oxygen containing media as hereinbefore described together with a light detector, wherein said light detector can be arranged to be located relative to said monitoring device so as to be capable of providing an estimate of the properties of the luminescence emitted by the luminescent compound. In a preferred embodiment, the light detector is connectable with the substrate of the monitoring device, so as to provide a direct measurement of the luminescence collected at the substrate by total internal reflectance.

The present oxygen monitoring device is capable of providing a 90% response to a change in oxygen concentration in less than about one second. The nature of tehe oxygen-permeable film is such that it is capable of exhibiting substantial long term stability in response to irradiation by the exciting light.

There is further provided by the present invention a method of measuring oxygen concentrations in oxygen containing media, which method comprises: a). locating a monitoring device as hereinbefore described in a position suitable for receiving oxygen from an oxygen-containing medium; b). irradiating the oxygen permeable film of said monitoring device with light having a wavelength of at least 400 nm; and c). monitoring the luminescence emitted by said luminescent compound or compounds present in the oxygen-permeable film of said monitoring device.

As previously described, the luminescence monitoring can involve monitoring of the lifetime, intensity or polarisation of the emitted luminescence; this emitted luminescence generally has a wavelength in the visible or near infra red region.

The present invention will now be illustrated by the following example which is not intended to limit the scope of the invention in any way.

A film composition comprising polyvinyl chloride plasticised with a trialkyl phosphate ester and in which the ruthenium (II) tris-bipyridyl cation has been solubilised by pairing with organophilic tetra-aryl-borate anions was prepared. A film of approximately 20 micron thickness was solution cast onto a glass substrate. Luminescence was excited by light at 430 nm directed perpendicular to the coated surface. Emitted light at 610 nm was trapped in the substrate by total internal reflection and conducted to a detector. The assembly exhibited a fully equilibrated response to change in the oxygen content of gas contacting the sensor film in less than 1 second. Figure 1 shows the relative intensity of luminescence as the composition of a nitrogen/oxygen mixture is varied between 0 and 100% oxygen. The sensitivity and dynamic range of the response closely approximates to the Stern Volmer relationship. Figure 2 shows the relative intensity of luminescence over a 24 hour period during which the example film was irradiated continuously in air with the light of 430 nm at an intensity of 10¹⁵ photons cm^"2 s^'1. The invariance of luminescent intensity illustrates the considerable photostability of the dye cation in the presence of oxygen.

Claims

Claims:

1. A monitoring device for measuring oxygen concentration in oxygen-containing media, which device comprises a substrate having thereon an oxygen-permeable film comprising a polymeric vehicle and having immobilised therein at least one luminescent compound which can be induced to emit luminescence upon irradiation of said film with visible light, which luminescence can be quenched by molecular oxygen present in said film.

2. A device according to claim 1, wherein said luminescent compound comprises a porphyrin or polyheterocyclic complex of a transition metal.

3. A device according to claim 1 or 2, wherein the oxygen-permeable film is of about 20 microns in thickness and comprises a substantially water-insoluble, non-polar polymer vehicle.

4. A device according to any of claims 1 to 3, wherein a solubilising moiety is present in the oxygen-permeable film.

5. A device according to claim 4, wherein the solubilising moiety is bonded covalently or electrostatically to the luminescent compound.

6. A device according to any of claims 1 to 5, wherein the substrate is of a substantially transparent material, or a material of refractive index not lower than that of the film.

7. Analytical apparatus which comprises a device according to any of claims 1 to 6, and a light detector arranged to be located relative to said monitoring device so as to be capable of providing an estimate of the properties of the luminescence emitted by the luminescent compound.

8. Apparatus according to claim 7, wherein the light detector is connectable to the substrate of the monitoring device, so as to provide a direct measurement of the luminescence collected at the substrate by total internal reflectance. A method of measuring oxygen concentrations in oxygen containing media, which method comprises: a) locating a monitoring device according to any of claims 1 to 6 in a position suitable for receiving oxygen from an oxygen-containing medium; b) irradiating the oxygen-permeable film of said monitoring device with light having a wavelength of at least 400 nm; and c) monitoring the luminescence emitted by said luminescent compound or compounds present in the oxygen-permeable film of said monitoring device.