Colorimetric sensor and use thereof

Abstract

A luminescence-based colorimetric sensor comprising a first, oxygen-sensitive, lumophore and a second, reference lumophore, each lumophore being dispersed in a polymer matrix; and a substrate; where

Description

Technical Field The present invention relates to a luminescence-based colorimetric sensor and to methods of use thereof. The sensor can be used to assess oxygen concentration, in particular in controlled oxygen atmospheres such as food packaging. In embodiments, the invention also relates to thin-film oxygen sensors, in particular for food packaging applications. Background In recent years, the use of protective atmosphere packaging has increased substantially due to a number of factors, including stringent food safety requirements, increased demand for fresh produce, the importation of non-native and non-seasonal foodstuffs, and the drive towards reducing food waste and associated CO2 and CO2-equivalent emissions. Outside of food supply chains, protective atmosphere packaging is often used for disposable sterile medical products and electronic components, further fuelling the increased demand. Protective atmosphere packaging generally includes gas-free (i.e. vacuum packaging) and modified atmosphere packaging (MAP). A gas-free environment is made by expelling all of the air, and therefore the oxygen, from the packaging under vacuum. Gas-free environments are favoured for the packaging of products that are susceptible to oxidative degradation, which include electronic components, pharmaceuticals, and foodstuffs. MAP is created by altering the normal gas composition of air (78% nitrogen, 21% oxygen, 1% trace gases) within the packaging. These processes provide atmospheres that inhibit the primary food spoilage processes, thereby extending shelflife of the products and enhancing food quality and safety. Oxygen (O2) and carbon dioxide (CO2) are the most commonly used gases for MAP. Generally, the modified atmosphere is tailored to the product to be packaged, with fruit and vegetables as well as cooked meats typically being stored under low oxygen atmospheres (0-20%) and raw red meat and poultry products typically being stored under higher oxygen atmospheres (20-80%). The partial pressures of these gases (pCO2, pO2) within the packaging headspace can fluctuate over time and are influenced by factors such as the product type, microbial activity, storage conditions, and the packaging material and integrity. Since any leaks or damage to the packaging will induce loss of the modified atmosphere and a return to ambient air conditions, and improper gas flushing can lead to an incorrect atmosphere being created within the packaging, the composition of the gas in the packaging headspace, and in particular the O2 concentration, can provide a useful indication of the packaging integrity and reliability of the gas flush, as well as of the freshness and safety of the packaged foodstuff. Thin film oxygen sensors for the assessment of oxygen in food packaging are known. In these known sensors, the sensor response is typically based on a change in an optical property, such as absorbance or luminescence, as a function of oxygen concentration. Luminescence-based sensors can be particularly advantageous, as they provide a nondestructive means of quantitative detection. Incorporating these sensors into supply chains can allow individual damaged or spoiled goods to be removed from the supply chain, without necessitating quarantine and potential disposal of the entire batch, thereby retaining any remaining fresh and safe to eat produce, or undamaged sterile components. On a commercial scale, this can result in increased efficiencies and lead to significant reductions in food and other waste, as well as a reduction in waste packaging. However, in order for sensors to monitor real time conditions within a closed environment, such as modified atmosphere packaging, the sensor has to come into direct contact with the atmosphere, i.e., it needs to be positioned within the packaging headspace, and in direct contact with the foodstuffs or other ingestible products the MAP is designed to preserve. This can be problematic, as many known lumophores tend to exhibit various toxicities, including organ toxicity, cytotoxicity, and / or genotoxicity, and are therefore inherently unsuitable for applications where they come into direct contact with foodstuffs or other ingestible products. While non-toxic lumophores are known, these often suffer from poor fluorescence and / or formulation characteristics, leading to disadvantages such as low levels of brightness, poor colour response, and poor photostability. It is an aim of the invention to obviate or mitigate one or more of the disadvantages associated with the prior art. Ideally, it would be advantageous to provide a colorimetric oxygen sensor with improved colour response, and / or improved brightness, and / or improved photostability. A colorimetric oxygen sensor which, in use, can be positioned in direct contact with foodstuffs and / or other ingestible products, would also be advantageous. A tuneable colorimetric sensor which can deliver a colour change over a relevant oxygen concentration range, for instance the oxygen levels required for preservation of specific food items such as meat or vegetables in modified atmosphere packaging, would be particularly advantageous. Summary of the Invention According to the present invention there is provided a colorimetric oxygen sensor comprising a first, oxygen-sensitive lumophore, and a second, reference lumophore, each lumophore being dispersed in a polymer matrix; and a substrate; wherein the first and second lumophores are on opposing sides of the substrate, and wherein the substrate is glass or a polymer film. The inventors have surprisingly determined that a colorimetric oxygen sensor configured with the first and second lumophores on opposing sides of a substrate exhibit enhanced brightness over conventional luminescent sensors in which the oxygen-sensitive and reference lumophores are printed in a contiguous lumophore layer configuration. Furthermore, colorimetric oxygen sensors according to the invention have been demonstrated to have improved colour response and improved photostability compared with such conventional colorimetric oxygen sensors. Such advantages are unexpected as a skilled person would expect the intermediate substrate to have a deleterious effect on the operation of the sensor, when the opposite has been demonstrated in this invention. A practical, but significant, advantage of this configuration is that it allows for a wide selection of references lumophores to be used, even for packaging of foodstuffs or other ingestible items, as the reference lumophore is separated from the oxygencontaining atmosphere by the presence of the substrate which can act as a functional barrier. In another aspect of the present invention there is provided a method of determiningthe oxygen content of an atmosphere, the method comprising exposing the colorimetric sensor described above to the atmosphere; applying a source of UV or UV-visible excitation to the sensor, and observing the colour of the sensor. The atmosphere may be a food packaging atmosphere. Advantageously, the sensor of the invention is suitable for direct contact with foodstuffs and other ingestible products, and, in embodiments, can be used as a lidding film or flow wrap film forfood packaging. Alternatively, the atmosphere may be any environment in which a vacuum or controlled oxygen concentration is preferred; such as in the packaging of pharmaceutical or veterinary products, disposable medical products or electronic components; or in packaging or storage in the areas of conservation / preservation (e.g. artworks or antiquities etc.). For instance, the substrate could be a glass vial. In a further aspect of the present invention there is provided a lidding or flow wrap film for modified atmosphere packaging, wherein the lidding or flow wrap film comprises a first, oxygen-sensitive, lumophore and-a second, reference lumophore, each lumophore being dispersed in a polymer matrix, wherein the oxygen-sensitive lumophore and the reference lumophoreare on opposing sides of the lidding orflow wrap film. Various further features and aspects of the invention are defined in the claims. Brief Description of the Drawings Embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings where like parts are provided with corresponding reference numerals and in which: Figure 1 (a) shows the configuration of a sensor according to the invention, while Figure 1 (b) shows a comparative sensor; Figure 2 shows colour response of oxygen sensors (a) PtOEP: Coumarin 1 (1:2.8) (Example 1.1); (b) PtTFPP:BPEA (1:1.0) (Example 1.2); (c) [Ru(dpp)3]2+:Curcumin-Al (1:2.8) (Example 1.3); (d) [Ru(dpp)3]2+:coumarin 314 (1:0.8) (Example 1.5); and (e) [Ru(dpp)3]2+:stilbene 3 (1:1.4) (Example 1.5). Detailed Description According to the present invention there is provided a colorimetric oxygen sensor comprising a first, oxygen-sensitive lumophore, and a second, reference lumophore, each lumophore being dispersed in a polymer matrix, wherein the first and second lumophores are on opposing sides of a glass or polymer film substrate. According to the invention, the first and second lumophores are on opposing sides of the substrate. The first and second lumophores may be coated on opposing sides of the substrate. The technique of applying the first and second lumophore to the substrate is not particularly limited once it results in a layer or coating of lumophore-containing polymer matrix on each side. In an embodiment, the first and second lumophores are printed on opposing sides of a polymer substrate film. Suitable printing techniques include, for instance, inkjet printing such as piezo-based inkjet or thermal inkjet, flexographic printing, gravure printing, offset lithography, pad printing, screen-printing and letterpress. The printing may be performed by inkjet printing, flexographic printing or gravure printing. Alternatively, the first and second lumophores may be applied to a polymer substrate film by techniques such as dip coating, slot-dye coating, blade coating, bar coating, spray coating or spin coating. Alternatively, the first and second lumophores may be printed on opposing sides of a glass substrate, for example using inkjet printing or screen printing. As another alternative, the first and second lumophores may be printed or coated separately on a label, such as a self-adhesive polymer label to form sensor labels, which are adhered to either side of the substrate; such as, for example, by printing or coating the polymer on a polymer label, and adhering the polymer label using an adhesive; or by heat-sealing or lamination to the substrate with a further polymer film; or by coating the polymer onto a self-adhesive polymer label and adhering the label to the substrate. The thickness of the lumophore-containing polymer layers is not particularly limited, but is typically in the region of from 1 pm to 5 pm. At this thickness, sufficient lumophores can be incorporated to impart sufficient brightness to the sensor, while allowing the illumination to passthrough the entire layer so that lumophores throughout the layer can absorb and subsequently emit light. The lumophore is dispersed in the polymer matrix, optionally along with any solvents used in the preparation process. Alternatively, the lumophore may be dispersed in the polymer matrix along with additional lumophores of the same type (i.e. more than one oxygen-sensitive lumophore or more than one reference lumophore) and / or additional components. In embodiments, more than one reference lumophore can be used, once the one or more reference lumophores are on the opposingside of the polymer substrate film to the oxygen-sensitive lumophore. For instance, the polymer matrix could comprise two or more reference lumophores. Alternatively, two contiguous layers of reference lumophore could be used. Thus, the polymer and any solvents can act as vehicles for the lumophores to facilitate application to the substrate, for example by printing. When the lumophores are printed on opposing sides of the substrate, the lumophore may be dispersed in the polymer matrix along with one or more printing additives. Such additives may be selected based on the substrate and printing technique to be used and include, for instance, rheology modifiers, dispersants, wetting agents, plasticisers etc., as would be well known to a person skilled in the art. Throughout this specification, the term “lumophore” is used synonymously with “luminophore” to mean a chemical species which spontaneously emits light upon radiative relaxation (luminescence) from a higher energy electronic excited state (unstable) to its lowest energy ground state (stable). The higher energy electronic state is formed when the lumophore absorbs radiation, which is usually, but not always, at an energy greater than that of the subsequently emitted light. The colorimetric sensor is oxygen-sensitive, i.e. it exhibits an observable colour change in response to a change in the oxygen concentration of an atmosphere or local environment to which the oxygensensitive lumophore is exposed. The colorimetric sensor is luminescence-based. The colour change arises due to preferential quenching of the light emitted by at least one of the lumophores in the sensor. The colour change is visible to the naked eye only when the sensor is exposed to UV or UV-Visible light. This can have advantages in terms of monitoring consumer products, as the sensors can be concealed from general view, if preferred. The colorimetric sensor is an oxygen sensor. Throughout this specification, “oxygen sensor” is intended to mean a single parameter sensor which is sensitive to oxygen as an analyte. According to the invention there is provided a colorimetric oxygen sensor for assessing a change in the oxygen concentration of an oxygen-containing atmosphere, the sensor comprising a first, oxygen-sensitive lumophore, and a second, reference lumophore, each lumophore being dispersed in a polymer matrix, and a substrate, wherein the first and second lumophores are on opposing sides of the substrate such that, in use, the oxygen-sensitive lumophore is in contact with the oxygen-containing atmosphere. The sensor changes colour in response to a change in the oxygen concentration in an atmosphere or local environment to which it is exposed due to a preferential and quantitative reduction (quenching) in the luminescence of the oxygen-sensitive lumophore. Advantageously, the colour change is reversible, enabling colour changes to proceed in the forward and reverse directions of the response range in line with an increase or decrease in the local oxygen concentration. This affords the oxygen sensor broad applicability for commercial applications. The emission of each of the lumophores is typically insensitive to pH and temperature, allowing the sensor to be applied directly in a wide variety of environments and ensuring that the colorimetric change is directly attributable to the change in oxygen concentration. The first lumophore is an oxygen-sensitive lumophore, i.e. its emission is quenched, preferably significantly, in the presence of oxygen. The emission of the first lumophore is insensitive to pH and temperature. Suitable oxygen-sensitive lumophores for use as the first lumophore in the present invention include platinum(ll) octaethylporphyrin (PtOEP), palladium(ll) octaethylporphyrin (PdOEP), ruthenium (II) 4,7-diphenyl-1,10’-phenanthroline ((Ru(dpp)3]2+), platinum(ll) meso-tetraphenylporphyrin (PtTPP), pyrene, erythrosine B; ruthenium (II) 1,10-phenanthroline ([Ru(phen)3]2+), osmium (II) 1,10-phenanthroline ((Os(phen)3]2+), osmium (II) 4,7-diphenyl-1,10’-phenanthroline complex ([Os(dpp)3]2+), meso-tetraphenylporphyrin (TPP), platinum(ll) 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (PtTCPP), platinum(ll) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (PtTFPP), platinum(ll) coproporphyrin (PtCP), platinum coproporphyrin tetraethyl ester) (PtCPTEE), platinum(ll) meso-tetra(2,6-dichlorophenyl)porphyrin (PtTDCPP), platinum(ll) meso-tetra(3,5-bis(trifluoromethyl)phenyl)-porphyrin (PtTFMPP), platinum(ll) meso-tetrakis(4-A / -methylpyridyl)porphyrin (PtTMPyP4+), palladium(ll) meso- tetraphenyltetrabenzoporphyrin (PdTPTBP), palladium(ll) 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) (PdTCPP), palladium(ll) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (PdTFPP), palladium meso-tetraphenylporphyrin (PdTPP), fac-Tris(phenylpyridine)iridium(lll) ([Ir(PPy)s)]), acetylacetonatobis(2- phenylpyridine)iridium(lll), ([lr(ppy)2(acac)]), tris[2-(p-tolyl)pyridine]iridium(lll) ([lr(mppy)3]), tris(2-(benzo[b]thiophen-2-yl)pyridine)iridium(lll) ([lr(btpy)3]), tris(2-thenoyltrifluoroacetonate)gadolinium(lll) ([Gd(tta)3]), tris(2- thenoyltrifluoroacetonato)(1,10-phenanthroline)europium(lll) ([Eu(tta)3Phen]), tris(acetylacetonato)(1,10-phenanthroline)terbium(lll) ([Tb(acac)3Phen]). In an embodiment, the first, oxygen-sensitive, lumophore is selected from PtOEP, [Ru(dpp)3]2+, [lr(ppy)3] and PtTFPP. In an embodiment, the first, oxygen-sensitive, lumophore is [Ru(dpp)3]2+or [lr(ppy)3]. These are particularly suitable as the first, oxygen-sensitive lumophore, for assessing oxygen changes in a medium to high O2 atmosphere (i.e. 20-80% O2). In an embodiment, the first, oxygen-sensitive, lumophore is PtOEP or PtTFPP. These are particularly suitable as the first, oxygen-sensitive lumophore, for assessing oxygen changes in a low O2 atmosphere (i.e. below «10% O2). The second lumophore is a reference, or oxygen-insensitive lumophore, i.e. its emission is not quenched significantly in the presence of oxygen. The second lumophore typically absorbs UV and / or UV-visible light substantially in the same spectral region as the first lumophore. This is advantageous as it allows a single excitation source to be used. However, if the first and second lumophores do not absorb in substantially the same spectral region, multiple excitation sources can be used. The second lumophore emits substantially in a different spectral region to the first lumophore. By ‘substantially’ it is meant that the maximum emission of each lumophore should differ by at least ~ 20 nm in order to allow a colour change to be observed. The emission of the second lumophore is also insensitive to pH and temperature. A significant advantage of the sensor of the invention is that, in use, the reference lumophore is separated from the oxygen-containing atmosphere by an intermediate substrate. When the substrate is a functional barrier, migration of substances from outside the substrate is prevented. As the reference lumophore is not in direct contact with the food or other ingestible items, this allows a broad range of reference lumophores to be used, based on their formulation and photophysical properties, without necessarily needing to meet the rigorous toxicity testing requirements for food or ingestible contact materials. Further information about functional barriers for polymer films such as those used in food packaging are set out in Commission regulation (EU) No 10 / 2011. For the purposes of this application, a “functional barrier” is one that prevents migration of substances, and in particular migration of the reference lumophore layer material through the substrate. Suitable second lumophores for use as reference lumophores include coumarins, xanthenes, diarylheptanoids, acridines, polycyclic aromatic hydrocarbons (PAHs), benzoxazoles and benzoxadiazoles, alkaloids, stilbenes, naphthalimides, cyanines, phenoxazines, and metal complexes with Zn(ll), with Mg(ll), or with Al(lll). In an embodiment, the reference lumophore is selected from coumarins, xanthenes, diarylheptanoids, acridines, polycyclic aromatic hydrocarbons, benzoxazoles and benzoxadiazoles, alkaloids, stilbenes, and metal complexes with Zn(ll), with Mg(ll) or with Al(lll). Suitable coumarins include, but are not limited to, 10-acetyl-2,3,6,7-tetrahydro-1 H.5H.11 H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one (coumarin 334); 7- (dimethylamino)-4-methyl-2H-1 -benzopyran-2-one (coumarin 311); 2,3,6,7-tetrahydro-9-methyl-1 H,5H,11 H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one (coumarin 102); 2,3,6,7-tetrahydro-9-(trifluoromethyl)-1 H,5H,11 H-[1 ]benzopyrano[6,7,8-ij]quinolizin-11-one (coumarin 153); 3-(2-benzothiazolyl)-7-(diethylamino)-2H-1-benzopyran-2-one (coumarin 6); 2,3,6,7-tetrahydro-11 -oxo-1 H,5H,11 H-[1 ]benzopyrano[6,7,8-ij]quinolizine-10-carboxylic acid, ethyl ester (coumarin 314); 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one (coumarin 1); 7-(ethylamino)-4,6-dimethyl-2H-1-benzopyran-2-one (coumarin2); 7-hydroxy-4-methyl-2H-1-benzopyran-2-one(4-MU); 2,3,6,7,10,11-hexahydro-1 H,5H-cyclopenta[3,4][1 ]benzopyrano[6,7,8-ij]quinolizin-12(9H)-one (coumarin 106); 7-(diethylamino)-3-(1 -methyl-1 H-benzimidazol-2-yl)-2H-1 -benzopyran-2-one (coumarin 30); 7-amino-4-(trifluoromethyl)-2H-1-benzopyran-2-one (coumarin 151); 3-(1 H-benzimidazol-2-yl)-7-(diethylamino)-2H-1 -benzopyran-2-one (coumarin 7); 2,3,6,7-tetrahydro-11 -oxo-1 H,5H,11 H-[1 ]benzopyrano[6,7,8-ij]quinolizine-1 O-carboxylic acid (coumarin 343); and 7-amino-4-methyl-2H-1-benzopyran-2-one (coumarin 120). In an embodiment, the coumarin is selected from 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one (coumarin 1); 7-(ethylamino)-4,6-dimethyl-2H-1-benzopyran-2-one (coumarin 2); 3-(2-benzothiazolyl)-7-(diethylamino)-2H-1-benzopyran-2-one (coumarin 6); 7-(diethylamino)-3-(1 -methyl-1 H-benzimidazol-2-yl)-2H-1 -benzopyran-2-one (coumarin 30); 2,3,6,7-tetrahydro-9-(trifluoromethyl)-1 H,5H,11 H-[1]-benzopyrano-[6,7,8-ij]quinolizin-11-one (coumarin 153); and 2,3,6,7-tetrahydro-11-oxo-11-1,51-1,11H-[1]benzopyrano[6,7,8-ij]quinolizine-1 O-carboxylic acid, ethyl ester (coumarin 314). Suitable xanthenes include, but are not limited to, 3,6-diamino-9-(2-carboxyphenyl)-xanthylium, chloride (rhodamine 110); 9-[2-(ethoxycarbonyl)phenyl]-3,6-bis(ethylamino)-2,7-dimethyl-xanthylium, chloride (rhodamine 6G); 3',6'-dihydroxy- spiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3-one (fluorescein); . 2',4',5',7'-tetrabromo-3',6'-dihydroxy-spiro[isobenzofuran-1 (3H),9'-[9H]xanthen]-3-one (Eosin Y); 3,6-diamino-9-[2-(methoxycarbonyl)phenyl]-xanthylium, chloride (rhodamine 123); 4,5,6,7-tetrachloro-3',6'-dihydroxy-spiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3-one (tetrachlorofluorescein); 2',7'-dichloro-3',6'-dihydroxy-spiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3-one (dichlorofluorescein); 4',5'-dibromo-3',6'-dihydroxy-2',7'-dinitro-spiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3-one, sodium salt (Eosin B); 9-(2-carboxyphenyl)-3,6-bis(diethylamino)-xanthylium, chloride (rhodamine B); and 9-(2-carboxyphenyl)-2,3,6,7,12,13,16,17-octahydro-1 H,5H,11 H,15H-xantheno[2,3,4-ij:5,6,7-i'j']diquinolizin-18-ium, chloride (rhodamine 101). In an embodiment, the xanthene is selected from 3,6-diamino-9-(2-carboxyphenyl)-xanthylium chloride (rhodamine 110) and 3',6'-dihydroxy-spiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3-one (fluorescein). Suitable diarylheptanoids include, but are not limited to, (1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (curcumin); (1 E,6E)-1 -(4-hydroxy-3- methoxyphenyl)-7-(4-hydroxyphenyl)-1,6-heptadiene-3,5-dione (demethoxycurcumin); and (1 E,6E)-1,7-bis(4-hydroxyphenyl)-1,6-heptadiene-3,5-dione (bisdemethoxycurcumin). In an embodiment, the diarylheptanoid is (1 E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (curcumin). Suitable acridines include, but are not limited to, 3,6-diamino-10-methyl-acridinium, chloride, mixt. with 3,6-acridinediamine (acriflavine hydrochloride); 9(10H)-acridinone (acridone); 3,6-acridinediamine, hydrochloride (proflavine hydrochloride); N3,N3,N6,N6-tetramethyl-3,6-acridinediamine, hydrochloride (acridine orange); 2,7-dimethyl-3,6-acridinediamine, hydrochloride (acridine yellow); and 9-acridinamine (9-aminoacridine). In an embodiment, the acridine is selected from 3,6-diamino-10-methyl-acridinium chloride mixture with 3,6-acridinediamine (acriflavine hydrochloride); 9(10H)-acridinone (acridone); 2,7-dimethyl-3,6-acridinediamine, hydrochloride (acridine yellow); and 9-acridinamine (9-aminoacridine). Suitable polycyclic aromatic hydrocarbons (PAHs) include, but are not limited to, perylene; 9,10-bis(2-phenylethynyl)-anthracene (BPEA); 9,10-diphenyl-anthracene (9,10-DPA); 2-[2,6-bis(1-methylethyl)phenyl]-1 H-perylo[3,4-cd]pyridine-1,3(2H)-dione (Perylene PMI); 2,9-bis[2,5-bis(1,1-dimethylethyl)phenyl]-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone (perylene PDI); 4,10-dicyano-3,9-perylenedicarboxylic acid, 3,9-bis(2-methylpropyl) ester (Lumogen F Yellow 083); 2,9-bis[2,6-bis(1-methylethyl)phenyl]-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline- 1,3,8,10(2H,9H)-tetrone (Lumogen F Orange 240); and 2,9-bis[2,6-bis(1-methylethyl)phenyl]-5,6,12,13-tetraphenoxy-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone (Lumogen F Red 300). In an embodiment, the polycyclic aromatic hydrocarbon is selected from perylene; 9,10-bis(2-phenylethynyl)anthracene (BPEA), 2,9-bis[2,5-bis(1,1-dimethylethyl)phenyl]-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone (perylene PDI); 4,10-dicyano-3,9-perylenedicarboxylic acid, 3,9-bis(2-methylpropyl) ester (Lumogen F Yellow 083); 2,9-bis[2,6-bis(1-methylethyl)phenyl]-anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone (Lumogen F Orange 240); and 2,9-bis[2,6-bis(1 -methylethyl)phenyl]-5,6,12,13-tetraphenoxy-anthra[2,1,9-def :6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetrone (Lumogen F Red 300). Suitable benzoxadiazoles include, but are not limited to, 7-nitro-N-(phenylmethyl)-2,1,3-benzoxadiazol-4-amine (BBD); N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-|3-alanine (N-propylamino-NBD); 6-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-hexanoic acid (hexylamino-NBD); 11 -[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-undecanoic acid (undecylamino-NBD); and 12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-dodecanoic acid (laurylamino-NBD). Suitable benzoxazoles include, but are not limited to, 2,2'-(2,5-thiophenediyl)bis[5-(1,1 -dimethylethyl)-benzoxazole (BBOT); 2,2'-(1 E)-1,2-ethenediylbis[5-methyl-benzoxazole (Fluorescent Brightener 135); 2,2'-(1,4-naphthalenediyl)bis-benzoxazole (Fluorescent Brightener 367); 2,2'-(1,2-ethenediyldi-4,1-phenylene)bis-benzoxazole (Fluorescent Brightener 393); and 2-[4-[2-[4-(2-benzoxazolyl)phenyl]ethenyl]phenyl]-5-methyl-benzoxazole (Fluorescent Brightener KSN). Suitable alkaloids include, but are not limited to, (8a,9R)-6'-methoxy-cinchonan-9-ol, sulfate (quinine sulphate); 9H-pyrido[3,4-b]indole (norharmane); 1-methyl-9H-pyrido[3,4-b]indole (harmane); 7-methoxy-1-methyl-9H-pyrido[3,4-b]indole (harmine); and 7-methoxy-1,2-dimethyl-2H-pyrido[3,4-b]indole (2-methylharmine). Suitable metal complexes include, but are not limited to, tris(8-quinolinolato)-aluminum(lll) (Alq3); bis(8-quinolinolato)-zinc(ll) (Znq2); bis[2-(2- benzothiazoly)phenolato]-zinc(ll) ([Zn(BTZ)2]); zinc(ll) phthalocyanine (ZnPc); and magnesium(ll) meso-tetraphenylporphyrin (MgTPP). Suitable stilbenes include, but are not limited to, 2,2'-([1,1 '-biphenyl]-4,4'-diyldi-2,1 -ethenediyl)bis-benzenesulfonic acid (stilbene 3); 2,2'-(1,2-ethenediyl)bis[5-[[4-[bis(2-hydroxyethyl)amino]-6-(phenylamino)-1,3,5-triazin-2-yl]amino]-benzenesulfonic acid, sodium salt (Fluorescent Brightener 28); 1,1'-(1E)-1,2-ethenediylbis-benzene ((E)-stilbene); 2,2'-(1,2-ethenediyl)bis[5-amino-benzenesulfonic acid (DAS); and N,N-dimethyl-4-[2-(4-nitrophenyl)ethenyl]-benzenamine (DANS). Other reference lumophores which could be used include cyanine dyes, such as 2-[2-[4-(dimethylamino)phenyl]ethenyl]-1-methyl-pyridinium, iodide (DASPMI); and 2-[2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile (DCM); naphthalimide dyes, such as Lumogen F Violet 570; and phenoxazine dyes such as 9-(diethylamino)-5H-benzo[a]phenoxazin-5-one (Nile Red). In an embodiment, the reference lumophore is a coumarin, a polycyclic aromatic hydrocarbon, a benzoxadiazole, a stilbene, an acridine, a diarylheptanoid or a metal complex with Zn(ll), with Mg(ll) or with Al(lll). In an embodiment, the reference lumophore is selected from coumarin 1, coumarin 2, coumarin 6, coumarin 30, coumarin 153, coumarin 314, curcumin, acriflavine, Alq3, perylene, BPEA, stilbene 3, BBD and Lumogen F red 300. In an embodiment, the molar ratio of lumophore 1: lumophore 2 is from 1:0.5 to 1:5.0. In an embodiment, the molar ratio of lumophore 1: lumophore 2 is from 1:1.0 to 1: 3.0. In an embodiment, the sensor comprises the following oxygen -sensitive:reference lumophores: PtOEP:coumarin 1 PtTFPP: BPEA [Ru(dpp)3]2+:curcumin-Al [Ru(dpp)3]2+: Rhodamine 110 [Ru(dpp)3]2+: coumarin 2 [Ru(dpp)3]2+: stilbene 3 [Ru(dpp)3]2+: coumarin 153 [Ru(dpp)3]2+: coumarin 314 [Ru(dpp)3]2+: perylene PtOEP: coumarin 30 [Ru(dpp)3]2+: coumarin 6 [Ru(dpp)3]2+: BBD [Ru(dpp)3]2+: curcumin [Ru(dpp)3]2+: acriflavine [Ru(dpp)3]2+: Alq3 In the colorimetric sensor of the invention, each of the oxygen-sensitive and reference lumophores is dispersed in a separate polymer matrix. The polymers making up the polymer matrices may be the same or different. The oxygen-sensitive and reference lumophores are dispersed directly in their respective polymer matrix and are not encapsulated or otherwise embedded (e.g. in a gel) before dispersion in the polymer matrix. The lumophores are not incorporated on the surface of microbeads. The lumophores are not incorporated within mesoporous structures. The lumophores are not incorporated into microspheres at any point in time during the formation of the polymer matrix. The lumophores are not emulsified within the polymer matrix. The lumophores are directly dispersed in their polymer matrix. The polymer may be selected from polystyrenes, polyvinyls, polyamides, polyurethanes, polyesters, acrylates, shellac, rosin, rosin esters, celluloses and cellulose-derivatives, silicones; and mixtures thereof. The polymer may be, for example, a cellulose-based polymer or polymers. Suitable cellulose-based polymers include but are not limited to ethyl cellulose (EC), cellulose acetate (CA), cellulose acetate propionate (CAP), cellulose acetate butyrate (CAB), nitrocellulose (NC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC) or carboxymethyl cellulose (CMC). The polymer may be, for example, a polystyrene (PS). The polymer may be, for example, a polyurethane (PU). The polymer may be a vinyl polymer. Suitable examples include polyvinyl chloride (PVC), polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), polyvinylidene chloride (PVdC), and polyvinyl acetate (PVA). The polymer may be a polyolefin. Suitable examples include polyethylene (PE), and polypropylene (PP). The polymer may be a polycarbonate (PC), for example polycarbonate diol (PCD). The polymer may be an acrylate. Suitable examples include polymethylmethacrylate (PMMA), and polyethylmethacrylate (PEMA). The polymer may be a polyester. The polymer may be a polyamide (PA). Suitable examples include Nylon 6 (PA 6). The polymer may be polysiloxane (silicone). A suitable example is polydimethylsiloxane (PDMS). A mixture of polymers may be used. In an embodiment, the polymer is one or more of EC, CAP, CAB, PDMS, PU, HPC, PVC, PS, PVA, PMMA, PVOH, and mixtures thereof. For the oxygen-sensitive lumophore, a polymer which is highly permeable to oxygen (i.e. PO2> 10 barrers) can be advantageous, as it results in a faster, and often brighter, O2 response. Polymers having a high permeability to oxygen include, but are not limited to, EC, NC, CAB, CAP, and PDMS. However, polymers having a moderate permeability to oxygen (i.e. 1 <PO2< 10 barrers) will also provide a sufficiently bright and rapid response. Polymers having a moderate permeability to oxygen include, but are not limited to, PS, polyurethanes, PE, PP, polycarbonate. In an embodiment, the oxygen-sensitive lumophore is dispersed in a polymer having a permeability of oxygen PO2 or Oxygen Transmission Rate (OTR) >1 barrer as measured by ASTM-D3985. In an embodiment, the oxygen-sensitive lumophore is dispersed in one or more of EC, NC, CAP, PDMS, PS, PU, PE, PP or PC, and mixtures thereof. In an embodiment, the oxygen-sensitive lumophore is dispersed in a cellulose-based polymer. In an embodiment, the oxygen-sensitive lumophore is dispersed in ethyl cellulose. For reference lumophores, the permeability of the polymer to oxygen does not affect the functioning of the sensor, and the polymer can be chosen based on factors such as ease of dispersion of the lumophore, etc. In some embodiments, polymers such as PS, PVC, PVdC, PVA, PMMA and PU may be beneficial due to their high resistance to damage. As the reference lumophore layer may be positioned in use external to the modified atmosphere, higher resistance to damage such as scuffing and peeling may be advantageous to prevent damage due to handling, both on the production line and by consumers. In an embodiment, the reference lumophore is dispersed in one or more of PS, PVC, PVdC, HPC, PVA, PMMA, PU, PE, PP and mixtures thereof. Each of the oxygen-sensitive lumophore and the reference lumophores are dispersed in polymer matrices. Additives may also be included in the polymer matrices, for instance to increase the photostability of the lumophore, or to increase the durability or abrasion resistance of the polymer layer. In an embodiment, the additive is an opacity agent. Opacity agents are insoluble particles which can be included in the polymer matrix to scatter and / or redirect light, thereby increasing brightness. Suitable opacity agents include metal oxides, metal carbonates, metal sulphates and inorganic fillers. Suitable metal oxides include titanium dioxide (TiO2), zinc oxide (ZnO), aluminium oxide (Al2O3) and silicon dioxide (SiO2). Suitable carbonates include calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Suitable sulphates include barium sulphate (BaSO4), zinc sulphate (ZnSO4) and calcium sulphate (CaSO4). Suitable inorganic fillers include kaolin (Al2SiO5(OH)4), talc (Mg3Si4Oi0(OH)2) and aluminium hydroxide (Al(OH3)). In an embodiment, the opacity agent is added to the polymer matrix comprising the reference lumophore. In an embodiment, the additive is an optical brightener, a UVabsorber or a light stabiliser. The first, oxygen-sensitive, lumophore and the second, reference, lumophore are on opposing sides of a transparent substrate. Throughout this application a material is transparent if it has a haze <30% as measured in accordance with ASTM D1003. The substrate may be a transparent polymer substrate film. The polymer substrate film may itself be a laminate film and may comprise a plurality of layers or may be a mono layer film. When the film has a plurality of layers, the layers may be of the same material or may be different materials. The layers may be co-extruded or laminated, or a combination thereof. Multi-material multi-layer films are particularly useful, as the different materials can impart different properties on the film. For instance, polyolefins such as polypropylene and polyethylene are commonly incorporated as the modified atmosphere / food contact side of a multi-layer polymer substrate film, due to their foodsafe nature and excellent heat-sealing properties, while polymers such as PET and PA often make up the external layers of such multi-layer films due to their good functional barrier properties (i.e. resistance to transmission of the modified atmosphere), their robustness, mechanical strength, and the fact that they can be printed on easily, for example to provide consumer information or branding on the outside of the packaging. Multi-layer films can also include barrier layers, such as an ethylene vinyl alcohol (EVOH) layer or an aluminium oxide (alox) coating. The polymer substrate films with which this technique can be used include, but are not limited to, polyamide (PA) / polyethylene (PE) films, polyethylene terephthalate (PET) / polyethylene (PE) films, polyethylene terephthalate (PET) / ethylene vinyl alcohol (EVOH) / polyethylene (PE) films, polyethylene terephthalate (PET) / aluminium oxide (alox) / polyethylene (PE) films, polypropylene (PP)Zethylene vinyl alcohol (EVOH) / polyethylene (PE) films, biaxially oriented polypropylene (OPP) / polypropylene (PP)Z ethylene vinyl alcohol (EVOH) / polypropylene (PP), and polypropylene (PP) / polyamide (PA) / polyethylene (PE) films. For any of the polymer films, they may be biaxially oriented or not biaxially oriented. Polymer substrate films can also include biopolymer films or films containing biopolymer layers, formed of biopolymers such as chitosan, starch, polylactic acid (PLA) or polyhydroxyalkanoates (PHAs). The film may be printed or coated on opposing sides with the lumophore layers to form the sensor, or the layers may be affixed or adhered to the polymer substrate film, for instance by printing the layers onto labels which are affixed or adhered to either side of the substrate. In an embodiment, the polymer substrate film is a liddingfilm or flow wrap film. A lidding film is a flexible packaging film which can be used to seal packaging, such as modified atmosphere packaging. Lidding films are mono-layer or multi-layer coextruded and / or laminated polymer films, and are typically heat-sealable to facilitate their use in packaging. Liddingfilms typically have a number of properties makingthem particularly suitable in packaging applications, and are usually co-extruded and / or laminate films, with the different layers imparting desired properties on the film. These desired properties include, for example, transparency, to allow consumers to see the products inside the packaging. High gas and water barrier properties (to retain the modified atmosphere within the packaging and avoid ingress / egress of water or moisture) are also a feature of these lidding films-some films comprise a layer of EVOH or aluminium oxide (alox) to impart these barrier properties. The sealing properties of the lidding film are often imparted by the presence of a polyethylene (PE) or polypropylene (PP) sealing layer. Lidding films are known in the art and a skilled person could select one based on the modified atmosphere needed and the type of product (e.g. O2 sensitivity, respiration etc.) Commercial polymer lidding films are known to those skilled in the art and include commercial products such as Univel M film from Tecnopack Univel, LINtop PE HB film from Klbckner Pentaplast, Toplex Locked Seal films from Plastopil, PurePP Ecopol Flow Pack film from Sudpack, Cryovac(R) film from Sealed Air, PrimeSeal™ film from AMCOR. Flow wrap films are mono or multi-layer films used with vertical and horizontal form-fill-seal (VFFS and HFFS) packaging machines that automate the process of bag making, filling products into the bags, and sealing them vertically / horizontally with the required modified atmosphere. Flow wrap films are mono-layer or multi-layer coextruded and / or laminated polymer films. These films are typically mono-material films, consisting of multiple layers of the same polymer, that can be easily sealed to facilitate their use in packaging. Unlike lidding films, which are typically used to seal themselves to a packaging tray, flow wrap films are typically sealed to themselves. Flow wrap films typically present the same key properties (gas and water vapour barriers, transparency, mechanical strength) as lidding films making them particularly suitable in packaging applications. These properties can be further enhanced by the introduction of additives or additional layer (EVOH) or coating (alox). Commercial polymer flow wrap films are known to those skilled in the art and include commercial products such as PurePP Ecopol Flow Pack, a mono-material PP laminated and coextruded film from Sudpack (oriented polypropylene (OPP) / polypropylene (PP) / ethylene vinyl alcohol (EVOH) / polypropylene (PP)), and EcoLamHighPlus, a mono-material PE laminate film from Constantia Flexibles (oriented polyethylene (OPE) / metallised oriented polyethylene (met OPE) / polyethylene (PE)). The polymer substrate film is typically from 20 to 80 pm thick. In an embodiment, the polymer substrate film is from 30 to 70 pm thick. When the polymer substrate film is a lidding film or flow wrap film, the lumophore layers (oxygen-sensitive; reference) can be applied to opposing sides of the polymer substrate film, so that when the lidding film is used to seal the modified atmosphere, the sensor is fully integrated into the packaging. The polymer substrate film is preferably a commercial lidding film or flow wrap film. In use, the sensor can be formed by printing or coating a layer of the oxygen-sensitive lumophore dispersed in a polymer matrix on one (the MAP / food-contact side) of the polymer film, and a layer of the reference lumophore dispersed in a polymer matrix on the opposite side of the polymer film. The polymer film can then be used to seal the packaging in the normal way, meaning that the sensor is fully integrated into the food packaging production line. Alternatively, the polymer substrate film is a rigid film. Suitable rigid films include, for instance, rigid PVC, PMMA and PET films. In an embodiment, the substrate is glass. The oxygen-sensitive and reference lumophore layers are substantially aligned on opposing sides of the substrate. The invention also relates to a method of determining the oxygen content of an atmosphere. This method comprises the steps of exposing the colorimetric sensor described above to the atmosphere; applying a source of UV or UV-visible excitation to the sensor, and observing the colour of the sensor. The step of exposing the colorimetric sensor to the atmosphere may include positioning the sensor within modified atmosphere packaging, such that the side with the oxygen-sensitive lumophore is in contact with the modified atmosphere. In this embodiment, the substrate may be a lidding film or flow wrap film, such that the film can be used to seal the packaging with the oxygen-sensitive lumophore containing side positioned in contact with the modified atmosphere, and the reference lumophore-containing side of the film on the outside. Alternatively, the substrate may be glass. Advantageously, due to its non-toxic nature, embodiments of the invention include positioning the sensor where the oxygen-sensitive lumophore side can come into direct contact with food or other ingestible products, such as, for instance, on the internal surface of food packaging. As would be generally understood by a skilled person, the UV region is from 190 nm to 400 nm and the visible region is from 400 nm to 700 nm. In an embodiment, the excitation wavelength is from 340 nm to 465 nm. The step of observing the colour of the sensor can be performed via the naked eye, via the use of spectrometry, or via the use of a digital imaging apparatus / machine vision system. Advantageously, the colour change is visible to the naked eye only upon the application of a UV / UV-visible light source to the sensor. This means that the sensor can be concealed within food packaging, for example, which can be beneficial when supplying consumer products. The non-toxic nature of the sensor makes it particularly suitable for such applications, as for instance when the substrate is a lidding film, the sensor can be used to seal the headspace of modified atmosphere packaging or to check for seal integrity. As another advantage, the colour change takes place very rapidly, and typically within milliseconds of exposure to the MAP. This allows the sensor to be used for real-time monitoring. The colour of the sensor when exposed to the atmosphere can be compared with a reference colour to determine the oxygen content. For instance, the colour of the sensor can be visibly observed by the naked eye and compared with a calibrated colour chart for the specific sensor being used, which can give a rapid, semi-quantitative result about the oxygen concentration in the atmosphere. Alternatively, it can be observed using a camera and compared with a calibration curve for a fully quantitative result. In embodiments, therefore, the method further comprises comparing the colour of the sensor with a reference colour to make an assessment of the oxygen content. Advantageously, the sensor can be tailored to the specific atmosphere being monitored, with, for example PtOEP:coumarin 1 and PtTFPP:BPEA being suitable for low to medium oxygen concentration atmospheres (e.g. those typically used to package fresh fruit and vegetables, cooked meats, bakery, and fresh pasta), and [Ru(dpp)3]2+: curcumin-Al(lll), [Ru(dpp)3]2+: coumarin 314, and [Ru(dpp)3]2+: stilbene 3 being suitable for high oxygen concentration atmospheres (e.g., those typically used to package raw red meat (beef, veal, pork, venison) and poultry). Alternatively, fully quantitative analysis can be achieved using fluorescence spectroscopy. The invention also relates to automated methods of assessing the oxygen concentration of an atmosphere using the sensor described above. For example, in an embodiment of the invention, a camera can be used to obtain a digital image of the colour signature of the sensor. The digital image can then be processed, and the colour converted into a numericalvalue, which can be automatically correlated with a pre-measured calibration curve, to yield an oxygen content value. The software can be designed to locate the sensor (for example in the packaging of a product or in the headspace of food product packaging), and to identify and eliminate optical variations (e.g. reflections, scattering etc.) in the image before the image is processed against the empirical calibration file for the specific sensor combination and attributed an oxygen percentage. This approach can enable real-time, non-destructive, high throughput and remote analysis of multiple samples, for instance each packaged product on a manufacturing line. The oxygen percentage determined following this approach can then be used to eliminate packages which fall outside the prescribed specifications. For instance, the manufacturing / packaging line can incorporate a linked rejection mechanism which can remove such eliminated packages from the line, thereby preventing these products from moving down the supply chain. This has significant advantages in terms of food-waste and food-packaging waste reduction, as it can isolate individual damaged or spoiled food products and remove them from the supply chain, without removing the unspoiled or undamaged remainder of the batch. Advantageously, the sensor of the present invention allows a colour change in many cases to be observed visually, i.e. the colour change is visible to the naked eye once exposed to UV / UV-visible light. This can negate the need for expensive external detection equipment (such as a spectrometer) and also allows the detection to be performed by a lay person (i.e. no specific technical skills required). A further advantage is that the detection can be performed using standard vision systems. In many cases, these are already present in manufacturing production lines, for instance for use in label inspection, damage inspection, product colour etc., thereby avoiding the need in these cases to install expensive detection systems. Comparison of the colour with a database of calibration curves or colour charts, specific to the lumophore pair (or lumophore trio etc.) being used, allows a quantitative determination to be made. The use of a camera and comparison of the colour change with a calibration curve provides a sensitive methodology which can allow even small changes in the oxygen concentration to be determined. This may be particularly beneficial for the monitoring of oxygen concentration in modified atmosphere packaging, where, as an example, the difference between 70% and 80% oxygen may be critical to determining if a packaged food item is in-specification or out-of specification when it comes to its headspace gas mixture. However, the colour change can be monitored via spectrometry, if required, and a full quantitative analysis performed. Embodiments of the invention also relate to electronic means of detecting the colour change, i.e. via the use of software, which may be coded into an “app”, and which can detect the colour of the atmosphere and compare it with a pre-programmed database, to yield a result on the freshness or safety of the packaged item or the packaging integrity. Such a result can be binary (i.e. fresh = yes / no; safe = yes / no) or it can be a quantitative value, indicating the degree of freshness of the packaged product (for example to indicate to a seller to "maintain stock as normal”, “reduce the product price”, “discard product”, etc.). In an embodiment, the atmosphere is a food packaging atmosphere. As indicated above, the sensor of the invention is particularly suitable for use in the context of food packaging atmospheres, and is ideally suited for the packaging of foodstuffs and other ingestible products due to its non-toxic nature. However, the invention is not specifically limited thereto, and it may find applicability in other areas in which oxygen concentrations are monitored, such as in pharmaceutical storage, packaging of disposable sterile medical products, conservation of artworks and cultural works or packaging of electronic components. For all of these applications an oxygen-free or controlled oxygen environment may be favoured (i.e. 0% O2). In addition, the luminescent-based colorimetric sensors of the invention could find application in the monitoring of gas supply products, such as in pre-mixed gas cylinders, where controlled oxygen concentrations (-0-100%) are required. As noted above, the colorimetric sensor can be tuned to the application of interest, that is, by controlling the lumophore selection and their molar ratio, a colorimetric response can be obtained within the O2 concentration ranges of interest. In an embodiment, the colorimetric sensor is configured to provide a colour change across O2 concentration ranges of from 0-20% (applicable to fruit, vegetables, and oxygen-sensitive foodstuffs such as cooked meats, nuts, fish, and cheese) or from 20-80% (applicable to meat products and other foodstuffs requiring high O2 MAP). Other defined concentrations ranges can be tailored based on the application of interest. The substrate may be subjected to surface treatment priorto printing in order to improve the wettability of the sensor ink or inks on the substrate. Such surface treatment steps include, for example, corona discharge, flame activation, low-pressure oxygen plasma, laser beam, focused ion beam or electron beam processing, as well as chemical treatment via the application of an organic or inorganic coatings. The integration of the sensor into packaging is a key component of the invention and allows the sensor to be provided within the packaging for in-situ detection and / or 5 monitoring of oxygen concentration within the controlled or modified atmosphere. The invention will now be described by way of reference to the following examples, which are intended to be illustrative only. Examples: Polymer substrate films were purchased from Tecnopack Univel (TU). Toplex HB44 was sourced from Plastopil, LINTOP PE HB film sourced from Klbckner Pentaplast; Ecopol Pure PP flow wrap film was sourced from SudPack. Example 1: Preparation of sensors according to the invention Sensors accordingto the invention were prepared as outlined in Example 1. As shown in Figure 1, the configuration of sensors according to the invention comprises a substrate(i), sandwiched between a layer (iii) comprising an oxygen-sensitive lumophore in a polymer matrix; and a layer (ii) on the opposing side of the substrate, which comprises at least one reference lumophore in a polymer matrix. The lumophore layers are positioned on the opposing sides of the substrate such that they are substantially aligned with each other. 1.1 PtOEP:Coumarin 1 (1:2.8 molar ratio) A colorimetric oxygen sensor accordingto the invention was prepared as set out below. The configuration of the sensor is shown in Figure 1 (a). Firstly, stock solutions of the polymer matrices were prepared as follows: 7.2 g of dry ethyl cellulose polymer (4 cP) was added to a 69:31 (v / v) mixture of ethanoLethyl acetate (35.2 ml ethanol; 16.0 ml ethyl acetate) and stirred overnight to ensure full dissolution of the EC polymer stock solution (P1). Separately, 10.0 g of cellulose acetate butyrate (CAB) was added to a 90:10 (v / v) mixture of acetone:ethyl acetate (45.0 ml acetone; 5.0 ml ethyl acetate) and stirred until full dissolution. Tributyl 2-acetyloxypropane-1,2,3-tricarboxylate (ATBC, 0.50 g) was then added to the polymer solution as a printing aid to yield a CAB polymer stock solution (P2). 19.8 mg (2.714x105 mol) of platinum(ll) octaethylporphyrin, PtOEP, oxygen-sensitive Lumophore 1) was first dissolved in tetrahydrofuran (THF) before being added to the EC polymer solution P1 and the solution stirred until complete homogenisation was achieved and no sedimentation of the lumophore was observed. Separately, 17.5 mg (7.578x10-5 mol) of 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one (Coumarin 1, reference Lumophore 2) was first dissolved in ethanol (EtOH) before being added to the CAB polymer solution P2. In order to assess sensor performance, the lumophore solutions were then printed separately on to a self-adhesive polyethylene (PE) substrate film (EZ-Pierce™from Excel Scientific) using a 20 pm spiral coating bar from Elcometer™. The coated substrates were then dried for 2 hours at 40 °C in a thermostated oven, before being cut into 1 cm x 1 cm squares and stored in the dark at room temperature until needed. The dry ink film thickness was calculated by multiplying the wet film thickness by the solid wt% of that formulation (see Table 1). The lumophore 1 printed polyethylene film squares (6 squares) were adhered on the inner side (product-contact side) of a commercial lidding film (PET / alox / PE laminate from Tecnopack Univel; 49 microns thickness), while the lumophore 2 printed film squares (6 squares) were adhered on the opposite side of that lidding film, with both P1 and P2 sensors aligned to be superimposed. The colour change response was measured as outlined in Example 2 below. 1.2 PtTFPP:BPEA (1:1.0 molar ratio) Firstly, stock solutions of the polymer matrices were prepared as follows: 7.2 g of dry ethyl cellulose polymer (4 cP) was added to a 69:31 (v / v) mixture of ethanoLethyl acetate (35.2 ml ethanol; 16.0 ml ethyl acetate) and stirred overnight to ensure full dissolution of the EC polymer stock solution (P1). Separately, 7.5 g of polystyrene (PS) was added to THF (47.8 ml) and stirred until full dissolution. Tributyl 2-acetyloxypropane-1,2,3-tricarboxylate (ATBC, 0.50 g) was then added to the polymer solution as a printing aid to yield a PS polymer stock solution (P2). 31.8 mg (2.719x10-5 mol) of platinum(ll) 5,10,15,20-tetrakis(2,3,4,5,6- pentafluorophenyl)porphyrin (PtTFPP, oxygen-sensitive Lumophore 1) was first dissolved in THF before being added to the EC polymer solution P1 and the solution stirred until complete homogenisation was achieved and no sedimentation of the lumophore was observed. Separately, 10.3 mg (2.715x105 mol) of 9,10-bis(2-phenylethynyl)anthracene (BPEA, reference Lumophore 2) was first dissolved in THF before being added to the PS polymer solution P2. A sensor was prepared using the same methodology as described in Example 1.1, but with a multilayer lidding film from Plastopil (Toplex HB44; 45 microns thickness) as the substrate. The colour change response was assessed as outlined in Example 2 below. 1.3 [Ru(dpp)3]2+: Curcumin-Al (1:2.8 molar ratio) Firstly, a stock solution of the polymer matrix was prepared as follows: 56.6 g of dry ethyl cellulose polymer (4cP) was added to a 69:31 (v / v) mixture of ethanol:n-propyl acetate (254.4 ml ethanol; 115.2 ml n-propyl acetate) and stirred mechanically to ensure full dissolution of the polymer. Tributyl 2-acetyloxypropane-1,2,3-tricarboxylate (ATBC, 2.0 g) and polyethylene wax, 40% dispersion in isopropanol (4.0 g), were then added to the polymer solution as printing additives. The polymer solution was then separated equally into two different flasks, to be used as the polymer matrix for each of the first, oxygen-sensitive lumophore, and the second, reference lumophore. 144.1 mg (1.087x10-4 mol) of ruthenium (II) 4,7-diphenyl-1,10’-phenanthroline ([Ru(dpp)3]2+, oxygen-sensitive Lumophore 1) was freshly prepared in EtOH and added to the first EC solution (flask 1). While lumophore 1 was left stirring, (1 E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione e (curcumin, reference Lumophore 2) was first coordinated to Al(lll) before being added to the second polymer solution (flask 2). In brief, a 10 ml stock solution of curcumin (20.4 g / l in ethyl acetate:ethanol 2:1 (v / v), 5.543x104 mol) was mixed with 1 equivalent of NaOH 1N (0.550 ml, 5.543x10-4 mol), followed by the addition of 134.0 mg of AlCl3.6H2O (1 eq., 5.550x10-4 mol). The solution was stirred vigorously to ensure full coordination of Al(lll). Finally, 5.50 ml (3.046 x 10'4mol curcumin) curcumin-Al(lII) solution was added to the second polymer solution (flask 2). DL-a-tocopherol (2 eq., 6.087x104 mol) was added as a photostabiliser. Both EC-based formulations in flasks 1 and 2 were stirred until complete homogenisation was achieved and no sedimentation of the lumophores was observed before being printed and adhered to the polymer substrate film from Plastopil (Toplex HB44; 45 microns thickness) using the same methodology as described in Example 1.2. The colour change response was assessed as outlined in Example 2 below. 1.4 [Ru(dpp)3]2+: Rhodamine 110 (1:5.0 molar ratio) Firstly, stock solutions of the polymer matrices were prepared as follows: 7.2 g of dry ethyl cellulose polymer (4 cP) was added to a 69:31 (v / v) mixture of ethanoLethyl acetate (35.2 ml ethanol; 16.0 ml ethyl acetate) and stirred overnight to ensure full dissolution of the EC polymer stock solution (P1). Separately, 9.0 g of cellulose acetate propionate (CAP) was added to a 90:10 (v / v) mixture of acetone:ethyl acetate (45.9 ml acetone; 5.1 ml ethyl acetate) and stirred until full dissolution. Tributyl 2-acetyloxypropane-1,2,3-tricarboxylate (ATBC, 0.50 g) was added as a printing aid to yield CAP polymer stock solution (P2). 36.0 mg (2.717x105 mol) of ruthenium (II) 4,7-diphenyl-1,10’-phenanthroline ([Ru(dpp)3]2+, oxygen -sensitive Lumophore 1) was first dissolved in EtOH before being added to the EC polymer solution P1. Separately, 49.8 mg (1.360x10 4 mol) of 3,6-diamino-9-(2-carboxyphenyl)-xanthylium, chloride (Rhodamine 110, reference Lumophore 2) was dissolved in EtOH before being added to the CAP polymer solution P2. Both formulations were printed and the sensor prepared using the same methodology as described in Example 1.1, using a commercial lidding film from Tecnopack Univel (PET / alox / PE laminate; 49 microns thickness). The colour change response was assessed as outlined in Example 2 below. 5 1.5 Further Formulations Sensors comprising [Ru(dpp)3]2+:Coumarin 2 (1:1.9 molar ratio) in EC / EC; [Ru(dpp)3]2+:Stilbene 3 (1:1.4 molar ratio) in EC / CAB; [Ru(dpp)3]2+:Coumarin 153 (1:1.4 molar ratio) in EC / EC; [Ru(dpp)3]2+:Coumarin 314 (1:0.8 molar ratio) in EC / PVC; 10 [Ru(dpp)3]2+:Perylene (1:1.4 molar ratio) in EC / PMMA; PtOEP:Coumarin 30 (1:1.3 molar ratio) in EC / PS; [Ru(dpp)3]2+:Coumarin 6 (1:0.7 molar ratio) in EC / PVA; [Ru(dpp)3]2+:BBD (1:1.2 molar ratio) in EC / EC; [Ru(dpp)3]2+:Curcumin (1:2.8 molar ratio) in EC / PS; [Ru(dpp)3]2+:Acriflavine (1:1.4 molar ratio) in EC / PVOH-HPC 1:1; [Ru(dpp)3]2+:Alq3 (1:1.5 molar ratio) in EC / PVA and [Ru(dpp)3]2+:Alq3 (1:2.0 molar ratio) in EC / PMMA were 15 prepared using the methodology described in Example 1.1. The sensors prepared in Examples 1.1 to 1.5 are summarised in Table 1 below: Example Lumophore LI / Polymer Matrix Pl Lumophore L2 / Polymer Matrix P2 L2 emission colour* Molar Ratio L1:L2 Additive(s) Substrate / thickness, pm** Dry ink thickness Pl / P2, pm*** Working O2% range 1.1 PtOEP / EC Coumarin 1 / CAB Blue 1:2.80 ATBC TU / 49 2.9 / 4.0 0-20% 1.2 PtTFPP / EC BPEA / PS Turquoise 1:1.00 ATBC Toplex HB44 / 45 2.9 / 3.0 0-20% 1.3 [Ru(dpp)3]2+ / EC Curcumin-AI / EC Green 1:2.80 ATBC, PE wax, TP Toplex HB44 / 45 3.2 / 3.2 0-80% 1.4 [Ru(dpp)3]2+ / EC Rhodamine 110 / CAP Orange 1:5.00 ATBC Lintop PE HB / 32 2.9 / 3.6 0-80% 1.5 2+ [Ru(dpp)3] / EC Coumarin 2 / EC Blue 1:1.90 ATBC TU / 49 2.9 / 2.9 0-80% 1.5 [Ru(dpp)3]2+ / EC Stilbene 3 / CAB Blue 1:1.38 ATBC Toplex HB44 / 45 2.9 / 4.0 0-80% 1.5 [Ru(dpp)3]2+ / EC Coumarin 153 / EC Turquoise 1:1.39 ATBC Toplex HB44 / 45 2.9 / 2.9 0-80% 1.5 [Ru(dpp)3]2+ / EC Coumarin 314 / PVC Turquoise 1:0.83 ATBC Lintop PE HB / 32 2.9 / 1.5 0-80% 1.5 [Ru(dpp)3]2+ / EC Perylene / PMMA Turquoise 1:1.41 ATBC Toplex HB44 / 45 2.9 / 3.9 0-80% 1.5 PtOEP / EC Coumarin 30 / PS Turquoise 1:1.33 ATBC TU / 49 2.9 / 3.0 0-20% 1.5 [Ru(dpp)3]2+ / EC Curcumin / PS Green 1:2.80 ATBC Toplex HB44 / 45 2.9 / 3.0 0-80% 1.5 [Ru(dpp)3]2+ / EC Coumarin 6 / PVA Green 1:0.65 - TU / 49 2.9 / 3.0 0-80% 1.5 [Ru(dpp)3]2+ / EC BBD / EC Green 1:1.20 ATBC Toplex HB44 / 45 2.9 / 2.9 0-80% 1.5 [Ru(dpp)3]2+ / EC Acriflavine / PVOH-HPC Green 1:1.42 ATBC Ecopol Pure PP / 60 2.9 / 1.6 0-80% 1.5 [Ru(dpp)3]2+ / EC Alq3 / PVA Green 1:1.45 ATBC Toplex HB44 / 45 2.9 / 3.8 0-80% 1.5 [Ru(dpp)3]2+ / EC Alq3 / PMMA Green 1:2.00 ATBC Toplex HB44 / 45 2.9 / 3.8 0-80% Emission colour as observed in ethanol solutions *** Dry film thickness calculated based on formulations solid wt%. Table 1: Preparation of oxygen sensors 5 33 Example 2: Measurement of colour response The colour response of the sensors of Example 1 to changes in oxygen concentration was determined as follows: A vision system equipped with a CMOS camera and 2 LED bar lights was used to assess the sensor response (i.e. colour response). The sensors of Examples 1.1 to 1.5 consisted of: (i) 1 cm2 square of PE self-adhesive film printed with the oxygen-sensitive lumophore-containing polymer matrix was adhered to the underside (i.e. the oxygen-containing atmosphere side) of a commercial multilayer lidding or flow wrap film and (ii) 1 cm2 square of PE self-adhesive film printed with the reference lumophore-containing polymer matrix was adhered to the opposing / external side of the commercial multilayer lidding or flow wrap film, with care taken to ensure that both sets of squares were aligned and superimposed. The sensors were then placed in a measuring chamber. Typically, six sensors were used to prevent against sensor failure and provide an average sensor response. The gas atmosphere in the measuring chamber can be continuously modified by flowing IN the desired gas mixture using a computer-controlled gas blender and confirming the modified atmosphere within the chamber by analysing the OUT flow using a gas analyser. Once the required oxygen within the chamber is reached, a manual trigger strobes the LED bar lights, and the camera captures an RGB image of the sensors during the illumination pulse. The gas is then modified within the chamber and the image capture is repeated once the gas atmosphere set on the gas blender matches the gas reading from the gas analyser. In this experiment, for low O2 sensors (i.e. operating at oxygen levels <21%), images were captured at 0, 1,3, 5, 10, 21% O2. For high O2 sensors (i.e. operating at oxygen levels >35%), images were recorded at 0, 20, 35, 50, 60, 70 and 80% O2. The RGB and HSI outputs from the camera were then analysed to obtain the colour response of the various formulations tested. To standardise the reporting of the colour response, the relative percentage changes are presented as follows: For formulations having their application for high O2 atmospheres, the absolute difference in hue is indicated using the 80% O2 hue value as reference (AHue = Hue80-Hue,, where Hue,, is the value measured at i% O2), so that the relative percentage change is obtained by dividing the hue difference by the hue value at 80% O2, i.e. AHue / Hue80. Similar reporting is performed for formulations working at lowO2, but in that case, the 0% O2 Hue value is used as reference, so that the relative percentage change is given by (Huei-Hue0) / Hue0. The results of these measurements for PtOEP: Coumarin 1 (1:2.8 molar ratio) in EC / CAB (Example 1.1), PtTFPP: BPEA (1:1.0 molar ratio) in EC / PS (Example 1.2), [Ru(dpp)3]2+: Curcumin-Al (1:2.8 molar ratio) in EC / EC (Example 1.3), for [Ru(dpp)3]2+: Coumarin 314 (1:0.8) in EC / PVC (Example 1.5), and [Ru(dpp)3]2+: Stilbene 3 (1:1.4) in EC / CAB (Example 1.5) are shown in Figure 2. Figure 2(a) shows the change in colourforthe sensor of Example 1.1 (PtOEP: Coumarin 1 [(1:2.8) in EC / CAB) as the oxygen concentration increases. This sensor shows a discernible colour change from an initial plum colour to a final dark blue colour between 0-21% O2. The three-colour transition from plum to purple to blue takes place between 0-10% O2 with a much steeper response in the 0-3% range, and illustrates that this sensor is particularly useful for products that are kept or packaged under an oxygen-free or low oxygen atmosphere. Table 2 shows the average hue and % change for this sensor and demonstrates that the change of colour resulted in an absolute change in Hue between 0 and 3% O2 of-83.1 units, which translates in a relative change (Hue3-Hueo / Hueo) of 25.2 % for this PtOEP:Coumarin 1 sensor. o2, % Avg Hue Std dev. (± 1o) AHue = Huei-Hue0 % change (AHue / Hue0) 0 330.1 0.4 0.0 0.0% 1 294.0 0.7 -36.1 -10.9% 3 247.0 1.6 -83.1 -25.2% 5 229.9 3.1 -100.2 -30.3% 10 218.5 7.6 -111.6 -33.8% 20 213.1 1.2 -117.0 -35.5% Table 2: Hue change for PtOEP: Coumarin 1 sensor [(1:2.8) in EC / CAB, Example 1.1], Figure 2(b) shows that for the PtTFPP: BPEA [(1:1.0) in EC / PS, Example 1.2] sensor, a 5 violet to purple to blue colour change is evident between 0-10% O2, before the colour changed to a lighter blue / turquoise at 20% O2. The average hue and % change were measured, and the results are included in Table 3 below. The relative change in Hue between 3 and 10% O2 (A%change)) reached 29%, or an absolute difference of 103.2 units of Hue. The oxygen range where the colour change takes place is relevant to 10 products that require to be kept under a low to medium oxygen atmosphere (5-15% O2), required for fresh vegetables and fruits. O2, % Avg Hue Std dev. (± 1o) AHue = Huei-Hue0 % change (AHue / Hue0) 0 356.7 4.3 0.0 0.0% 1 354.4 2.9 -2.3 -0.6% 3 348.0 2.1 -8.7 -2.4% 5 334.4 1.6 -22.3 -6.2% 10 244.8 1.3 -111.9 -31.4% 20 203.5 1.0 -153.2 -42.9% Table 3: Hue change for PtTFPP: BPEA [(1:1.0) in EC / PS, Example 1.2]. Figure 2(c) shows that for [Ru(dpp)3]2+: Curcumin-Al [(1:2.8) in EC / EC (Example 1.3)] a dark red to brown to dark green colour change is produced in a modified atmosphere containing 0-80% O2. The average hue and % change were measured, and the results are included in Table 4 below. The change in colour resulted in a Hue difference between 80 and 50% O2 of 13.7 or a relative change (Hue8o-Hue5o / Hue8o) of 21 %. The colour transition occurs in a range of oxygen suitable for packaging or applications where high oxygen levels are required. o2, % Avg Hue Std dev. (± 1g) AHue = HuerHue8o % change (AHue / Hue80) 0 22.9 0.9 42.5 65.0% 20 35.2 1.8 30.3 46.2% 35 43.7 2.1 21.8 33.3% 50 51.7 2.8 13.7 20.9% 60 56.6 3.0 8.8 13.5% 70 61.4 3.3 4.0 6.2% 80 65.4 2.8 0.0 Table 4: Hue change for [Ru(dpp)3]2+: Curcumin-Al [(1:2.8) in EC / EC (Example 1.3)] Figure 2(d) shows that for [Ru(dpp)3]2+: Coumarin 314 [(1:0.8) in EC / PVC (Example 1.5)] an orange to dark turquoise colour change is produced in a modified atmosphere containing 0-80% O2. The average hue and % change were measured, and the results are included in Table 5 below. The change in colour resulted in a Hue difference between 80 and 50% O2 of 38.0 or a relative change (Hue8o-Hue5o / Hue8o) of 55%. The colour transition occurs in a range of oxygen suitable for packaging or applications where high oxygen levels are required. o2, % Avg Hue Std dev. (± 1o) AHue = HuerHue8o % change (AHue / HueSo) 0 11.3 0.4 71.1 102.8% 20 21.2 1.0 61.2 88.5% 35 31.0 1.8 51.4 74.3% 50 44.4 3.3 38.0 54.9% 60 57.6 6.6 24.8 35.8% 70 69.2 6.5 13.2 19.1% 80 82.4 7.5 0.0 Table 5: Hue change for [Ru(dpp)3]2+: Coumarin 3141 [(1:0.8) in EC / PVC (Example 1.5)] Figure 2(e) shows that for [Ru(dpp)3]2+: Stilbene 3 [(1:1.4) in EC / CAB (Example 1.5)] a red to pink to purple colour change is produced in a modified atmosphere containing 0-80% 5 O2. The average hue and % change were measured, and the results are included in Table 6 below. The change in colour resulted in a Hue difference between 80 and 50% O2 of 33.9. The colour transition occurs in a range of oxygen suitable for packaging or applications where high oxygen levels are required. O2, % Avg Hue Std dev. (± 1o) AHue = HuerHue8o % change (AHue / HueSo) 0 359.5 3.7 -66.3 -22.6% 20 349.1 8.6 -55.9 -19.1% 35 339.7 13.1 -46.5 -15.9% 50 327.1 19.2 -33.9 -11.6% 60 316.9 21.8 -23.7 -8.1% 70 305.5 22.8 -12.3 -4.2% 80 293.2 22.0 0.0 10 Table 6: [Ru(dpp)3]2+: Stilbene 3 [(1:1.4) in EC / CAB (Example 1.5)] The results of Examples 1.1 to 1.5 demonstrate that excellent colour changes were observed for the sensors of the invention across both low and high oxygen concentration 15 ranges. Example 3: Effect of sensor configuration To investigate the effect of the sensor configuration on the sensor performance, and in particular the effect of physical separation of the oxygen-sensitive and reference lumophore layers, a comparative sensor was prepared, and the colour response, brightness, and photostability of the sensors according to the invention, and of the comparative sensors, were measured and compared. The comparative sensor configuration is shown in Figure 1(b). This configuration comprises a substrate (i), which has been printed on one side (i.e. the oxygen-containing atmosphere side) with two contiguous layers comprising (ii) a reference lumophore dispersed in a polymer matrix; and (iii) an oxygen-sensitive lumophore dispersed in a polymer matrix. In this configuration, the two lumophore-containing layers are contiguous, i.e. in direct contact with each other, with the reference lumophore-containing layer sandwiched between the oxygen-sensitive lumophore containing layer and the substrate. The comparative sensors were prepared using the same lumophore-containing polymer solutions as prepared in Examples 1.1 to 1.5 and applied to a polymer substrate film by printing, using spiral coating bars, firstly a layer of the reference lumophore-containing polymer matrix on to a self-adhesive polyethylene (PE) substrate film (EZ-Pierce™from Excel Scientific). The layer was allowed to dry for 30 minutes to avoid redissolution of the reference lumophore layer and consequential mixing of the layers. Once dry, a layer of the oxygen-sensitive lumophore-containing polymer was printed on top of the reference layer to form the comparative sensor label. The reference layer was printed before the oxygen-sensitive layer for more direct comparison with the configuration of the invention in which the reference-lumophore containing polymer layer is the first to be subject to exposure to the LED lights, and the oxygen-sensitive lumophore-containing polymer layer, directly exposed to the MAP headspace. The effect of the sensor configuration on the colour response was investigated by fitting the curves obtained by plotting (Huei / Hue8o or Huei / Hue2o for high and low O2 sensing systems, respectively) as a function of the %O2 for both configurations. The normalisation of the data is necessary considering the significant differences in Hue observed ,within the O2 range tested, depending on the configuration. The slopes obtained from the linear fits are then compared and the %enhancement provided by the configuration of the invention (I) over the comparative (C) configuration is determined as follows: %enhancement = (Slope I - Slope C) / Slope C. The colour response enhancement determined is summarised in Table 7 below. Unexpectedly, the brightness of the sensing system was significantly enhanced for the sensors of the invention versus the comparative configuration, despite the physical separation of the lumophore layers. While reabsorption of the reference emission by the oxygen-sensitive lumophore was observed in some cases, a sizeable increase in brightness was still observed for the sensors of the invention. To determine the extent of the brightness increase between the two configurations, measurements were taken using identical camera acquisition parameters. The perceived brightness was calculated for both configurations as follows: Perceived Brightness = ^ / (0.299 x R2 + 0.587 x G2 + 0.114 x B2) The %brightness increase, summarised in Table 7, was then obtained by dividing the difference between the invention (I) and comparative (C) configurations by the comparative sensor brightness ([Brightness I - Brightness C] / Brightness C). Example Lumophore LI / Polymer Matrix Pl Lumophore L2 / Polymer Matrix P2 Colour response Enhancement -Linear regression Brightness increase 1.1 PtOEP / EC Coumarin 1 / CAB +49.9% +30.2% 1.2 PtTFPP / EC BPEA / PS +165.0% +66.5% 1.3 [Ru(dpp)3]2+ / EC Curcumin-AI / EC +65.2% +28.6% 1.4 [Ru(dpp)3]2+ / EC Rhodamine 110 / CAP +32.6% +17.5% 1.5 [Ru(dpp)3]2+ / EC Coumarin 2 / EC +4.3% +51.0% 1.5 [Ru(dpp)3]2+ / EC Stilbene 3 / CAB +14.3% +35.2% 1.5 [Ru(dpp)3]2+ / EC Coumarin 153 / EC +6.4% +22.3% 1.5 [Ru(dpp)3]2+ / EC Coumarin 314 / PVC +82.8% +15.6% 1.5 [Ru(dpp)3]2+ / EC Perylene / PMMA +12.0% +27.7% 1.5 PtOEP / EC Coumarin 30 / PS +138.2% +26.2% 1.5 [Ru(dpp)3]2+ / EC BBD / EC +3.3% +56.3% 1.5 [Ru(dpp)3]2+ / EC Curcumin / PS +6.7% +15.2% 1.5 [Ru(dpp)3]2+ / EC Acriflavine / PVOH-HPC +10.1% +23.3% 1.5 [Ru(dpp)3]2+ / EC Alq3 / PVA +34.7% +113.4% Table 7: Effect of the printing configuration on the colour response and brightness of the sensing system. 5 Similarly, the effect of the sensor configuration on the photostability of the sensing system, was investigated as follows. The inventive (I) and comparative (C) sensors prepared above were continuously illuminated with the 405 nm LED bar lights (FWHM = 20 nm) at full power for 10 min under (i) a 80% O2 atmosphere for sensors with a 0-80% O2 working range; (ii) a 20% O2 atmosphere for sensors with a 0-20% O2 working range. An image was captured every minute and changes in RGB and Intensity, which equals to the third of the sum of R, G and B values (I = (R+G+B) / 3), were compared. To quantify the effect of the configuration on the photostability of the sensing system, l0 / l plots of the intensity (where l0 is the intensity at t = 0 min after illumination) as a function of the time for both configurations were graphed and fitted. The slopes obtained from the linear regressions over the full illumination period (0 to 600 s) were then compared and the %stabilisation provided by the inventive configuration (I) over the comparative configuration (C) was determined as follows: %stabilisation = (Slope C - Slope l) / Slope C. The stabilisation determined as well as changes in R, G and B are shown in Table 8 below and represent the stability improvement for the same illumination exposure (i.e. assuming the intensity changes observed between C sensor and I sensor configurations is directly correlated to the amount of light absorbed by the formulations). The values tabulated represent the difference in red, green and blue changes, AAR = AR(I)-AR(C), AAG = AG(I)-AG(C), and AAB = AB(I)-AB(C), for the sensing systems pre- and postillumination for the inventive and comparative configuration. The % stabilisation is then calculated by dividing AAR by AR(C) and same for the green and blue values. Example Lumophore LI / Polymer Matrix Pl Lumophore L2 / Polymer Matrix P2 %Stabilisation -Linear regression AAR (%stabilisation) AAG (%stabilisation) AAB (%stabilisation) 1.1 PtOEP / EC Coumarin 1 / CAB 61.4% -1.1% (84%) -3.0% (65%) -1.8% (60%) 1.2 PtTFPP / EC BPEA / PS 64.3% -1.0% (29%) -1.3% (72%) -1.2% (140%) 1.3 [Ru(dpp)3]2+ / EC Curcumin-AI / EC 21.2% X -1.0% (17%) X 1.4 [Ru(dpp)3]2+ / EC Rhodamine 110 / CAP 63.9% -1.5% (57%) -1.6% (65%) X 1.5 [Ru(dpp)3]2+ / EC Coumarin 2 / EC 57.1% X -1.0% (43%) -0.9% (157.3%) 1.5 [Ru(dpp)3]2+ / EC Coumarin 153 / EC 71.9% -2.2% (42%) -7.6% (78%) -4.9% (79%) 1.5 [Ru(dPP)3]2+ / EC Coumarin 314 / PVC 14.5% -1.0% (22%) X X 1.5 [Ru(dpp)3]2+ / EC Perylene / PMMA 45.5% -2.1%(55%) X X 1.5 PtOEP / EC Coumarin 30 / PS 42.1% -0.9% (50%) -0.6% (46%) -0.6% (52%) 1.5 [Ru(dpp)3]2* / EC BBD / EC 64.4% -1.0% (52%) -3.0% (73%) -0.34% (41%) 1.5 [Ru(dpp)3]2+ / EC Curcumin / PS 11.8% X -1.1% (8%) -0.6% (7%) 1.5 [Ru(dpp)3]2+ / EC Acriflavine / PVOH-HPC 45.6% -4.0% (58%) -4.5% (31%) X x AAR,G,B within experimental errors, i.e. <0.3%. Table 8: Effect of the printing configuration on the photostability of the sensing system. 5 As can be seen from the results in Table 7, the average enhancement observed when moving from a two contiguous layers configuration to the sensor of the invention is above 30% for both colour response and brightness (44.9% and +37.8%, respectively). Similarly, as shown in Table 8, the sensors of the invention demonstrated a significant improvement in photostability when compared with the comparative sensor configuration, with the average stabilisation close to 45% (44.9% stabilisation). The results demonstrate that the sensors of the invention exhibit surprisingly improved sensor characteristics when compared with conventional sensor (C) configurations. For instance, the PtOEP / EC and coumarin 30 / PS sensors of Example 1.5 showed significantly increased brightness (+26.2%) and colour response (+138.2%) compared with the comparative sensors comprising the same lumophore combinations. The change in Hue observed at oxygen concentrations between 0% and 3% was 34.2 units or 10% for the comparative configuration and 74.8 units or 22.4% for the sensor according to the invention. A significant 42% improvement in photostability was also observed for the sensor of the invention compared with the comparative version. This translates in half-lives (i.e. tv2, the time required for the sensing system intensity to drop to half of its initial intensity) under LED illumination to 164 hrs and 353 hrs for the comparative and inventive sensors, respectively. A similar photostability improvement was observed forthe sensors of the invention across the red, green and blue outputs (46-52%). Similarly, the sensing system comprising the lumophore pair PtTFPP:BPEA accordingto the invention showed much improved brightness (+66.5%) compared with the same lumophore pair printed on the polymer substrate film as contiguous layers. The sensor of the invention also exhibited an improved colour response, with a 165% increase in sensitivity (i.e. linear regression slopes) recorded for the inventive sensor versus its comparative counterpart. This translates in Hue changes over the 0-20% O2 range of 153 units for the sensor of the invention compared with 85 units for the comparative sensor. The photostability was tested and compared between the two configurations, with a 64.3% stabilisation observed for the sensor of the invention. Such a stabilisation results in the sensor of the invention allowing for a 4.8-fold increase in half-life of the sensing system; tv2 = 76 hrs (C), 362 hrs (I). For the ruthenium-based sensing system of Example 1.5, (i.e. [Ru(dpp)3]2+ / EC and Coumarin 153 / EC), the sensor of the invention exhibited both an increase in brightness (+22.3%) as well as an enhanced colour response, resulting in a 6.4% improvement in O2 sensitivity. While less significant improvements in brightness and sensitivity over the comparative sensors were demonstrated for this lumophore combination, this was compensated by a very significant increase in photostability, with a 72% stabilisation effect observed compared with its comparative counterpart. The largest stabilisation was observed in the green and blue outputs (78-79%), and thus in the photostability of the reference lumophore, coumarin 153. Such an improvement increases the half-life of the sensing system from 20 hrs (C) to 140 hrs (I), or a 7-fold increase. Example 4: Opacity agent The effect of the addition of an opacity agent on the brightness and photostability of the sensors according to the invention was investigated as follows: No opacity agent: Sensors were prepared as in Example 1, i.e. the polymer stock solution (P1) containing the oxygen-sensitive lumophore and the polymer stock solution (P2) containing the reference lumophore were printed onto self-adhesive PE substrates and adhered on either side of the polymer substrate film. The PE labels printed with the oxygen-sensitive lumophore containing polymer stock were adhered on the underside of the polymer substrate film (i.e. lidding film) (i.e. the side that comes in contact with the modified atmosphere), while the PE labels printed with the reference lumophore containing polymer stock were adhered on the outside of the lidding film. Opacity Agent 1 (OA1): The polymer stock solution (P1) containing the oxygen-sensitive lumophore was printed on the underside of the lidding film. Lidding films used were sourced from Tecnopack Univel (Univel M films) and Plastopil (Toplex Locked Seal films). Opacity agents were dispersed at 5-10wt.% directly in the polymer stock solution P2 containing the reference lumophore, before the solution was printed onto self-adhesive PE substrate and adhered to the outside of the lidding film. Opacity Agent 2 (OA 2): Polymer stock solutions P1 and P2 were printed as described for the “no opacity agent” sensor above. The reference layer was allowed to dry for 30 minutes before a further additive-containing layer, prepared by dispersing opacity agents at 5-10wt.% in ethyl cellulose (4 cP), was then printed on top of the reference lumophore layer. This experiment was repeated for multiple lumophore combinations from Examples 1.1 to 1.5, as shown in Table 9. The photostability of the sensors was then assessed by continuously illuminating the sensors with the 405 nm LED bar lights (FWHM = 20 nm) at full power for 10 min in an 80% O2 atmosphere. An image was captured every minute, and changes in RGB and HSI were compared. The brightness increase for the opacity agent-containing sensors compared with the sensors without an opacity agent, was determined as described in Example 3. The results are shown in Table 9 below. Lumophore LI / Polymer Matrix Pl Lumophore L2 / Polymer Matrix P2 LI :L2 Molar ratio Opacity agent (OA) OA (wt%) -additive / coating %Stabilisation - Linear regression Brightness increase 2+ [Ru(dpp)3] / EC BBD / EC 1:1.20 OA1 CaCO3 (5 wt%) -additive +5.6% +30.9% [Ru(dpp)3]2+ / EC BBD / EC 1:1.20 OA2 CaCO3 (5 wt%) -coating +33.3% +23.0% [Ru(dpp)3]2+ / EC BBD / EC 1:1.20 OA2 AI2O3 (10 wt%) -coating +22.2% +40.2% [Ru(dpp)3]2+ / EC Curcumin / EC 1:2.80 OA 1 ZnO (5 wt%) -additive +15.3% +46.7% [Ru(dpp)3]2+ / EC Curcumin / EC 1:2.80 OA2 SiO2 (10 wt%) -coating + 54.5% +31.5% [Ru(dpp)3]2+ / EC Alq3 / HPC 1:1.45 OA 1 SiO2 (10 wt%) -additive +11.0% +1.6% [Ru(dpp)3]2+ / EC Alq3 / HPC 1:1.45 OA 2 CaCO3 (5 wt%) -coating +48.8% +2.1% [Ru(dpp)3]2+ / EC Alq3 / PMMA 1:2.00 OA2 TiO2 (5 wt%) -coating +18.1% +62.6% [Ru(dpp)3]2+ / EC Stilbene 3 / CAB 1:1.38 OA 2 TiO2 (5 wt%) -coating +82.3% +43.3% Table 9: Effect of opacity agent on the photostability and brightness of sensors according to the invention. The results demonstrate that the incorporation of opacity agent into the sensors of the 5 invention leads surprisingly to further enhanced photostability, i.e. 32.5% on average, when compared with the sensors to which no opacity agent was added. The enhanced photostability is even more pronounced when the opacity agent is applied as a coating, i.e., in the configuration outlined as OA 2 above, with an average stabilisation close to 44% (+43.5%) being achieved. For instance, for the [Ru(dpp)3]2+:curcumin lumophore combination of Example 1.5 (1:2.8 molar ratio), the inventive sensor without opacity agent addition or coating displayed decrease in both RGB and HSI over 10 minutes illumination, as would be expected. Specifically, the green (G), blue (B) and hue (H) values decreased by 9.8, 4.9 and 15.2%, respectively, over the 10-minute illumination period. Conversely, when zinc oxide (ZnO, 5 wt.%) was added to the polymer stock P2 before printing (OA 1), the green (G), blue (B) and hue (H) values decreased by 0.8, 0.9 and 8.9%, respectively, over the same 10-minute illumination period. This suggests an improvement in photostability in the region of 60% for hue, 10% for green and 20% for blue when a metal oxide, such as ZnO, is incorporated within the reference lumophore layer. Significant improvements were also demonstrated when the opacity agent was applied in a separate layer, i.e. as OA 2. When silicon dioxide (SiO2) 10 wt% in ethyl cellulose was coated in a separate layer on top of the reference lumophore layer (OA 2), the green (G), blue (B) and hue (H) values decreased by 9.7, 3.4 and 5.3%, respectively, over the 10-minute illumination period. This suggests an improvement in photostability in the region of 70% for hue and 55% for green and blue when a metal oxide coating is applied onto the reference lumophore layer. From a brightness perspective, all of the sensors tested showed a further increase in brightness compared with the sensorto which no opacity agents were added. These results thus demonstrate that the performance of the sensor according to the invention, which is already improved over conventional sensors (comparative configuration), can be further enhanced by the addition of one or more opacity agents to the sensor, either as an additive in the reference lumophore layer, or as a separate layer applied on top of the reference layer. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., “a” and / or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations). It will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope of the present disclosure. Accordingly, the 5 various embodiments disclosed herein are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

1. A colorimetric oxygen sensor comprising a first, oxygen-sensitive, lumophore and a second, reference lumophore, each lumophore being dispersed in a polymer matrix; and a substrate; wherein the oxygen-sensitive lumophore and the reference lumophore are on opposing sides of the substrate, and wherein the substrate is glass or a polymer film.

2. A colorimetric oxygen sensor as claimed in claim 1, wherein the first, oxygen -sensitive, lumophore is selected from platinum(ll) octaethylporphyrin (PtOEP), ruthenium (II) 4,7-diphenyl-1,10’-phenanthroline ([Ru(dpp)3]2+), fac-Tris(phenylpyridine)iridium(lll) ([lr(ppy)3)]), and platinum(ll) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (PtTFPP).

3. A colorimetric oxygen sensor as claimed in claim 1, wherein the reference lumophore is selected from coumarins, xanthenes, diarylheptanoids, acridines, polycyclic aromatic hydrocarbons, benzoxadiazoles, benzoxazoles, alkaloids, stilbenes, and metal complexes with Zn(ll), with Mg(ll), or with Al(l 11).

4. A colorimetric oxygen sensor as claimed in claim 3, wherein the reference lumophore is selected from coumarin 1, coumarin 2, coumarin 6, coumarin 30, coumarin 153, coumarin 314, curcumin, Rhodamine 110, acriflavine, Alq3, perylene, 9,10-bis(2-phenylethynyl)-anthracene (BPEA), stilbene 3 and 7-nitro-N-(phenylmethyl)-2,1,3-benzoxadiazol-4-amine (BBD) and Lumogen F red 300.

5. A colorimetric oxygen sensor as claimed in any preceding claim, wherein each polymer matrix is selected independently from polystyrenes, polyvinyls, polyamides, polyurethanes, polyesters, acrylates, shellac, rosin, rosin esters, celluloses and cellulose-derivatives, silicones; and mixtures thereof.

6. A colorimetric oxygen sensor as claimed in any preceding claim, wherein the first, oxygen-sensitive, lumophore and second, reference, lumophore, are dispersed in different polymer matrices.

7. A colorimetric oxygen sensor as claimed in any preceding claim, wherein the first,oxygen-sensitive, lumophore is dispersed in a polymer matrix selected from ethyl cellulose, nitrocellulose, cellulose acetate propionate, polydimethylsiloxane, polystyrene, polyurethane, polyethylene, polypropylene, or a polycarbonate, and mixtures thereof.

8. A colorimetric oxygen sensor as claimed in any preceding claim, wherein the second, reference, lumophore is dispersed in a polymer matrix selected from polystyrene, polyvinyl chloride, polyvinylidene chloride, hydroxypropyl cellulose, polyvinyl acetate, polymethylmethacrylate, polyurethane, a polycarbonate, polypropylene, and polyethylene.

9. A colorimetric oxygen sensor as claimed in any preceding claim, wherein the substrate is a mono-layer or multi-layer polymer film.

10. A colorimetric oxygen sensor as claimed in claim 9, wherein the polymer film is selected from polyamide (PA) / polyethylene (PE) films, polyethylene terephthalate (PET) / polyethylene (PE) films, polyethylene terephthalate (PET) / ethylene vinyl alcohol(EVOH) / polyethylene (PE) films, polypropylene (PP) / ethylene vinyl alcohol(EVOH) / polyethylene (PE) films, biaxially oriented polypropylene (OPP) / polypropylene (PP) / ethylene vinyl alcohol(EVOH) / polypropylene (PP), and polypropylene (PP) / polyamide(PA) / polyethylene (PE) films.

11. A colorimetric oxygen sensor as claimed in claim 9 or claim 10, wherein the polymer film has a thickness of from 20 to 80 pm.

12. A colorimetric oxygen sensor as claimed in claim 11, wherein the polymer film has a thickness of from 30 to 70 pm.

13. A colorimetric oxygen sensor as claimed in any preceding claim, wherein: (i) the oxygen-sensitive lumophore is platinum(ll) octaethylporphyrin (PtOEP), and the reference lumophore is coumarin 1 or coumarin 30 or curcumin; or(ii) the oxygen-sensitive lumophore is platinum(ll) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyljporphyrin (PtTFPP), and the reference lumophore is 9,10-bis(2-phenylethynyl)-anthracene (BPEA) or curcumin; or(iii) the oxygen-sensitive lumophore is [Ru(dpp)3]2, and the reference lumophore is selected from curcumin, Rhodamine 110, coumarin 2, stilbene 3, coumarin 153, coumarin 314, perylene, acriflavine and Alq3.

14. A colorimetric oxygen sensor as claimed in any preceding claim, wherein the sensor comprises an additive.

15. A colorimetric oxygen sensor as claimed in claim 14, wherein the additive is an opacity agent, an optical brightener, a UV absorber or a light stabiliser.

16. A colorimetric oxygen sensor as claimed in claim 15, wherein the additive is an opacity agent selected from a metal oxide, a metal carbonate, and a metal sulphate.

17. A colorimetric oxygen sensor as claimed in claim 16, wherein the opacity agent is selected from titanium dioxide, zinc oxide, aluminium oxide, silicon oxide, calcium carbonate, magnesium carbonate, barium sulphate, zinc sulphate, calcium sulphate, kaolin, talc and aluminium hydroxide.

18. A colorimetric oxygen sensor as claimed in any of claims 14 to 17, wherein the sensor comprises an additive layer sandwiching the reference-lumophore-containing polymer matrix between the additive layer and the substrate.

19. A colorimetric oxygen sensor as claimed in any of claims 9 to 18, wherein the polymer film is a lidding film or flow wrap film for modified atmosphere packaging.

20. A lidding or flow wrap film for modified atmosphere packaging, wherein the lidding or flow wrap film comprises a first, oxygen-sensitive, lumophore and-a second, reference lumophore, each lumophore being dispersed in a polymer matrix, wherein the oxygensensitive lumophore and the reference lumophore are on opposing sides of the lidding or flow wrap film.

21. A method of determining the oxygen content of an atmosphere, the method comprising exposing the colorimetric oxygen sensor of any of claims 1 to 20 to the5 atmosphere; applying a source of UV or UV-visible excitation to the sensor, and observing the colour of the sensor.55

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