GB2438423A

GB2438423A - Optical sensor for detecting an analyte

Info

Publication number: GB2438423A
Application number: GB0610308A
Authority: GB
Inventors: Dr Emil Jw List; Dr Christian Slugovc; Evelyn Fisslthaler; Stefan Sax; Kerstin Waich; Thorsten Mayr
Original assignee: Technische Universitaet Graz
Current assignee: Technische Universitaet Graz
Priority date: 2006-05-24
Filing date: 2006-05-24
Publication date: 2007-11-28
Also published as: GB0610308D0; AT10020U1

Abstract

An optical sensor for detecting an analyte of interest in a fluid medium (preferably gaseous medium), comprising an organic light emitting device (OLED) having at least two electrodes 2, 5 and an organic electroluminescent layer 4 that is sensible to an analyte contained in said fluid medium, said organic electroluminescent layer 4 being adapted to change its electroluminescence emission properties upon contact with said analyte for generating an optically detectable response. The optical sensor may be realised in a sandwich structure or may be in the form of a surface cell device. Changes in luminescence intensity, lifetime of the excited sates or emission wavelength may be monitored.

Description

OIrIcAL SENSOR FOR DETECTING AN ANALYTE The present invention relates

to an optical sensor for detecting an analyte of interest in a fluid medium (preferably gaseous medium), comprising an organic light emitting device having at least two electrodes and an organic electroluminescent layer. The invention is set in the field of organic light emitting devices (OLED) used as a sensor device, in particular to such devices that are built from semi-conducting organic materials. Such sensor devices are used to detect miscellaneous analytes and in particular gaseous analytes, e.g. oxygen, bases, acids, olefins or aromatic compounds.

The most widespread and straightforward OLED configuration is the so called "sandwich-structure": A transparent bottom electrode, for instance made of indium tin oxide (ITO), is covered with a layer of electroluminescent organic material and a metallic top electrode.

Several intermediate layers, such as hole and electron transport layers, can be inserted into this structure to improve the electronic properties of the devices.

The US 6,331,438 (Aylott et al.) presents a passive optical sensor, which reflects one of the recent attempts of using OLEDs in integrated monolithic functional units. It comprises a sensing layer with an indicator agent which can react optically with an analyte and a thin film OLED comprising a luminescent layer which is optically coupled to the sensing layer but is physically separated from the latter, in order to allow access of the analyte to the sensing layer. Another optochemical sensor (oxygen sensor) is disclosed in US 6,432,363.

Details and discussion of OLED structure can be found in US 6,995,519.

Another possible approach is the "surface cell structure". Here the electrodes are located alongside the polymer film. This structure is more suitable for ink jet printing, so the entire device could be printed on one substrate. Besides, the film is directly exposed to the atmos-phere, thus bypassing a major problem of the sandwich structure, namely the necessity to get the analyte to the sensing layer via permeable or structured electrodes. An OLED with a surface cell structure is, e.g., disclosed in WO 99/16039 A2.

It is a goal of the present invention to simplify the architecture of a sensor device and further integrate the functional units of an OLED sensor device, aiming at what may be called an active organic light emitting sensor device which enables fabrication of cheap, integrated, reliable, sensitive and selective sensors. P11

This aim is met by an optical sensor for detecting an analyte of interest in a fluid medium, comprising an organic light emitting device having at least two electrodes and an organic electroluminescent layer that is sensible to an analyte contained in said fluid medium, said organic electroluminescent layer being adapted to change its electroluminescence emission properties upon contact with said analyte for generating an optically detectable response.

The invention is primarily directed at gaseous or volatile analytes, which may be transported by itself (then the analyte represents the gaseous medium) or borne in a gaseous medium. If a liquid medium is used, the analyte may be a dissolved substance.

The important advantage of the concept underlying the invention is the incorporation of the sensing material into the active region of the light emitting device, which dramatically reduces the production effort of a sensor element, eases fabrication and simplifies the struc-ture of such a sensing element. To date, no one has suggested to exploit a light emission resulting from electroluminescence of the sensing film. To the contrary, the effect of con-tamination of the active layer by chemical substances was seen as an unwanted degradation effect impairing the effectivity of the OLED device. The invention, however, uses this effect which turned out to be astonishingly reproducible (and reversible), thus turning a previously undesirable side effect into a useful application.

While previous approaches only considered the variations in photoluminescence behavior where the sensing material was excited with an external light source, such external excitation is dispensable with the invention.

A sensor according to the invention may resemble an OLED with a common sandwich or surface cell structure. The electroactive layer comprises an electroluminescent material that is sensitive to analytes, in particular gaseous analytes, and reacts by a change of its emission wavelengths, by change of emission intensity or by a change of the lifetime of the excited state of the emitter. If a sandwich structure is used, a part of the OLED (e.g. one of the elec-trodes or the substrate) is fabricated in a way that allows gaseous analytes to rapidly diffuse into the electroluminescent sensing layer which is also the emitting layer.

The electroluminescent film is at the same time the sensing and light emitting area, combin-ing the sensitive material and the active layer of the OLED in one emissive material (which can be a small molecule or a polymer), or by blending the sensitive material into a non-sensitive but active matrix. In this case, the sensitive material is excited via energy transfer.

The only light emitting process in a sensor according to the invention is therefore electrolu-minescence, and it is the modification of aforementioned that enables the detection of the P102 analyte (by changes of luminescence intensity, lifetime of the excited states and emission wavelength). The change in the EL properties may also by recorded in the form of an electri-cally detectable response (e.g. via the characteristic curve) indicating the presence of the analyte and/or allowing for a quantitative analysis.

The structure of the sensor can be the standard OLED "sandwich" structure (usually with adapted electrode structures for optimized response time), or it can be carried out as a "surface cell" device.

Compared to a classical luminescent sensor system, which comprises an additional light source, a photoluminescent sensing layer and a detector system, an active organic light emitting sensor device according to the invention combines the light source with the sensing element into one layer, which advantageously reduces the fabrication effort of such a sensor element -the sensor is incorporated in the organic layer, instead of requiring a bipartite structure. The resulting system is a purely electroluminescent assembly -the use of elec-troluminescence (EL) effects instead of photoluminescence represents the core idea of the invention. Therefore, an additional light source for external excitation to trigger the lumines-cence (as in sensors that use photoluminescence) is redundant. Notably, when using a phosphorescent sensing material (e.g. PtOEP, see Example 1), the EL quantum yield is enhanced by taking advantage of the luminescent triplet recombination of such materials.

In particular a sensor according to the invention can reversibly detect miscellaneous analytes (e.g. oxygen, bases, acids, olefins or aromatic compounds) contained in the liquid or gaseous medium by changing its EL properties.

The invention should be seen in contrast to other sensor systems based on organic light emitting devices which have been proposed earlier and where the light emitting device is only used as the light source for an external sensing material, sensing differences in the photoluminescent behavior of the sensor layer. The invention differs from these hitherto existing sensors by the basic excitation principle it uses and by the structure, which is simpli-fied. What is more, the structure is adaptable for ink jet printing processes.

In order to match different analytes and applications it is possible to tailor special sensing materials with exactly the properties needed.

The invention also lends itself to applications for multiple analyte detection, by combining a number of sensors which are adapted for sensing different analytes into an array of sensors.

Another possibility is the blending of two or more different types of sensing electrolumines-PlC' cent molecules in one single sensor layer, using the different reactions of the materials to their respective analytes to distinguish the sensor response. When using solution processable materials like polymers large scale production processes like ink jet printing are especially efficient.

In one preferred embodiment, the optical sensor is realized in a sandwich structure compris-ing a substrate, at least one electroluminescent organic layer, as well as a top electrode. The other electrode may be formed by the substrate or be present as additional structure compo-nent. Preferably, the sandwich structure further comprises a transparent electrode positioned beneath one of the at least one electroluminescent organic layer. Furthermore, the sandwich structure may further comprise additional organic or metallic intermediate layers, which may facilitate the charge carrier transport, enhance wetting properties or serve other pur- poses supporting the sensor function. Moreover, the top electrode may be made of alumi-num.

In the context of this disclosure, transparent' denotes a substrate or layer fabricated from a material that has a low absorption coefficient for light in the relevant region, allowing a substantial amount of light to be transmitted through the substrate/layer.

In a suitable layout of the sensor according to the invention, at least one of the electrodes may be positioned above the organic electroluminescent layer and be structured laterally, allowing optimized access of the analyte to the organic electroluminescent layer. As an alternative or in combination, at least one of the electrodes may be positioned above the organic electroluminescent layer and be permeable to the analyte.

In another preferred embodiment based on the surface cell architecture, the sensor may be realized in a structure comprising, located on a substrate, at least two electrodes covering different portions of the substrate and leaving a gap between them, and an organic elec-troluminescent layer bridging the gap between said electrodes.

This structure may further comprise a controlling electrode that is separated from the active layer by a dielectric layer.

The electrodes may be shaped according to various geometric arrangements, and e.g. are shaped in an inter-digital structure.

In order to enable detection of the optical signal, such a sensor may be supplemented by a photodetector adapted to detect light emitted by the organic light emitting device. pir

In order to allow the emission of light through the substrate side, the sensor may advanta-geously be assembled on a transparent substrate. The transparent substrate may also be made of a flexible material.

At least one transparent electrode may be positioned between the transparent substrate and the organic electroluminescent layer.

For applications where the analyte reaches the sensor from the substrate side, it is necessary that the sensor is assembled on a substrate and bottom electrode that are both permeable to the analyte.

The optical sensor may further comprise a semi-permeable membrane covering at least one of the sensor surfaces, in order to protect the sensor from unwanted substances, e.g. con-taminants, or to enable access of selected analytes from a mixture of several substances.

The electroluminescent layer may be made of an electroluminescent organic polymer blended with a sensing molecule. In a variant, the electroluminescent layer may be made of an electroluminescent organic polymer incorporating the sensing unit into the polymer backbone or incorporating the sensing unit into at least one side chain. In both latter cases, the sensing unit is chemically or physically bonded to the polymer. Furthermore, the elec-troluminescent layer may be made of a electroluminescent small molecule, where the term electroluminescent small molecule refers to a non polymeric emitter material. Also in this case, the sensing molecule can be either blended into the electroluminescent small molecules or can be attached to the electroluminescent small molecule. Finally, the electroluminescent layer can be made from a material, which combines the sensing and electroluminescence properties in one.

As one way of functionality of the sensor, the organic electroluminescent layer is made of an organic material adapted to change, upon contact with an analyte, its electroluminescence emission properties relating to the lifetime of the exited state, i.e. the triplet excitation life-time of the organic material. The organic electroluminescent layer may also change, upon contact with an analyte, its electroluminescence emission properties relating to the elec-troluminescence intensity or to the electroluminescence spectrum (resulting in a detectable brightness or color change).

In a special variant using the sensor according to the invention as an electro-optical sensor, the organic electroluminescent layer is adapted to change, upon contact with an analyte, its electric properties relating to at least one of the current-voltage and the current-luminescence characteristic. plc

In order to facilitate diffusion of the analyte into the layer, thus enhancing response time, the luminescent layer of the optical sensor may have a structured topography, which offers an enhanced permeation topography for the analyte due to increased adsorptive surface and! or enhanced absorption and diffusion (within the layer) of the analyte.

One particularly useful application of the invention is a sensor device comprising a plurality of optical sensors according to the invention, wherein the optical sensors are adapted to detect different species of analytes, respectively. In such a device, the optical sensors may be fabricated employing different sensor materials, each having different specifics of sensibility to analytes, or may be realized with a common electroluminescent material individually modified so as to exhibit different analyte sensing functionality.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional schematic diagram of the structure of the invention with a structured top electrode configuration; Fig. 2 is a cross sectional schematic diagram demonstrating the functionality of the sensor including a detector; Fig. 3 shows a plan view of a second embodiment having the structure of a surface cell sensor; Fig. 4 displays EL spectra of an oxygen sensor, each with one spectrum measured in an argon atmosphere, and one measured after 10 mm of exposure to air; Fig. 5 is a cross-sectional schematic diagram of the structure of a further top-electrode sensor according to the invention, including a semi-permeable membrane shielding the organic sensing layer from the headroom; Fig. 6 is a cross-sectional schematic diagram of the structure of a top-electrode sensor according to the invention, featuring a structured electroluminescent layer; and Fig. 7 is a cross sectional diagram of the structure of the second embodiment having the structure of a surface cell sensor, featuring an additional controlling electrode and a dielectric layer.

For realizing a sensor structure according to the invention, common OLED technology is used. There is no necessity for an implementation of new production processes. The organic materials can be solution processed (e.g. spin coating, ink jet printing) or vapor deposited, and the carrier material can be both rigid (e.g. glass) or flexible (e.g. PET). In comparison to a P1O2 common OLED, the invention requires certain modifications concerning the contact between the analyte and the sensor layer (e.g. structured or gas permeable top electrode), and the organic materials that have to be carefully chosen to match the relevant application, since the active electroluminescent sensing material has to be sensitive for the analyte. Selectivity towards the analyte can be achieved by chemical modification of the sensing material, the electroactive material or by applying semi-permeable membranes covering the analyte permeable sites of the device.

For instance, when using a "sandwich"-structure the top (or alternatively the bottom) elec-trode has to be fabricated in a way that allows the analyte to reach the sensing molecules.

This can be realized by -Using a gas permeable material for one of the electrodes, in case of the bottom electrode the substrate must be gas permeable -Structuring one of the electrodes to allow the analyte to diffuse into the sensing layer within a reasonable time span The top electrodes can be either vapour deposited, sputtered, laminated, applied via a soft lithography based process or ink jet printed; however, they have to be fabricated in such a way to allow the analyte to reach the sensing layer (by using gas permeable electrode mate-rial or by appropriate structuring).

The sensing material or molecule is an integral part of the electroluminescent organic layer of the light emitting device upon which the sensor architecture is based. This can be achieved by appropriate chemical design of the electroluminescent molecules or by blending of sens-ing molecules (as a guest) with an electro-active host material. In both cases no external light source is necessary to excite the luminescent sensing material. The EL properties of this layer are dependent on the presence and the amount of the analyte in the medium present in the headspace of the sensor. Detectable changes of these properties include variations in lumi-nescence intensity, lifetime of the excited states and spectral or emission wavelength.

For detecting different analytes, it will be suitable to exploit different molecules with sensing capabilities (e.g. changes in luminescence intensity, lifetime of the excited states or emission wavelength in the presence of the analyte) that are covalently attached or blended in. Elec-troluminescent organic materials can be tailor made for a broad spectrum of uses.

Sensor arrays composed of sensors according to the invention, containing a plurality of (different) sensor molecules, can be used to detect an assortment of different analytes simul-taneously.

We have combined the active material in an OLED with an integrated receptor molecule for different volatile analytes such as 02 by making use of a combination of electroluminescent sensor materials which have been integrated into gas-permeable optical sensor configura-tions. A sensor of this kind is discussed below as Example 1.

A possible solution for a sensor for acidic or alkaline gaseous analytes is the application of polymers that protonate or deprotonate upon exposition, which results in a change of the spectral characteristics (e.g. color). An example of this kind of sensor is discussed below as

Example 2.

The sensing event (i.e. the effect the sensor according to the invention exhibits when getting in contact with the appropriate analyte) can be made to be reversible: When, for instance, the oxygen is removed from a sensor for oxygen sensing, the pristine state of the sensor and thus its initial emission characteristics (in terms of luminescence intensity, lifetime of the excited states and/or emission wavelength) is restored. A similar approach works for the restoration of a sensor for acidic or alkaline gaseous analytes -exposure to an alkaline gas will refresh a sensor that has reacted to an acidic gas, and vice versa.

Referring to the sectional view of Fig. :1, one typical implementation of a sensor according to the invention is based on a transparent substrate 1, which can be either rigid (e.g. glass) or flexible (e.g. PET). This substrate I is covered with a transparent electrode 2, for example made of indium tin oxide (ITO) that serves as bottom electrode -typically the anode. The substrate I and the bottom electrode 2 can also be made of gas permeable materials or realized as permeable structure (e.g. channels). Further components are an electrolumines-cent layer 4 and a top electrode 5; the space above the layers 4, 5 serves as headspace for the medium to be sensed for the presence of an analyte.

The substrate I and the electrode(s) 2 below the electroactive layer 4 are preferably transpar-ent in order to allow the emission of light to the space below the sensor, where a detector (not shown) may be positioned, or the light emission may be monitored directly (e.g. by a human eye) from below.

Additional intermediate layers 3 may be added to enhance, for example, charge injection and charge transport. In the sensor of Fig. I, for example, the lower one of the two intermediate layers 3 may consist of PEDOT:PSS. Depending on the effect that is intended either or both of these intermediate layers can consist of organic or metallic materials. P102

The electroluminescent layer 4 may consist of a variety of organic materials, including, but not limited to: -an electroluminescent polymer, -an electroluminescent polymer blended with sensing molecule, - an electroluminescent polymer incorporating a sensing functionality, -a small electroactive molecule, -a small electroactive molecule blended with a sensing molecule, -a small electroactive molecule incorporating a sensing functionality.

The term electroluminescent small molecule refers to a non polymeric emitter material.

This organic layer can be applied via solution processing (e.g. spin coating, ink jet printing) or vapour deposition. The electroluminescent layer is not limited to one single film -it may be composite and consist of one or of more electroluminescent layers.

The top electrode 5 serves as the second electrode -typically the cathode. Fabrication meth-ods for the top electrode 5 may be deposition via thermal evaporation, sputtering, e-beam evaporation, a solution based process such as ink jet printing, lamination or via a soft lithog-raphy based process. One suitable material for the top electrode is aluminum. The top electrode 5 is preferably -gas permeable to allow contact of the analyte and the sensing layer, or -structured in a way that permits the gaseous analyte to diffuse into the sensing layer (e.g. by use of a shadow mask).

While it is not implicitly necessary to structure a non-gas-permeable top electrode, it is in general beneficial because an optimized electrode structure eases diffusion of the gaseous analyte into the organic sensing layer, shortening the response time of the sensor.

Advantageously, all the materials that constitute the sensor according to the invention, with the exception of the sensing electroluminescent layer, are insensitive to the analyte (and to other substances the sensor will get in contact with). This wifi ensure that the sensor re- sponse upon exposure to the analyte in the headspace originates solely from the reac- tion/interaction of the sensing material and the analyte. One notable example is the other-wise widespread usage of calcium as a cathode in OLEDs to enhance the performance of the OLED -this is not possible for an oxygen sensor since calcium will corrode very quickly upon contact with oxygen.

Depending on the material that is used in the electroluminescent sensing layer vola-tile/gaseous analytes can he detected. If a liquid medium is used, the analyte may be a PlO' dissolved substance. This includes, but is not limited to specific molecules such as oxygen, ammonia, nitrogen, carbon monoxide or carbon dioxide, substances with specific chemical properties such as acidic substances (proton donators), in particular acidic gases, or ba-sic/alkaline substances (proton acceptors), in particular basic gases, or substance groups such as olef ins or aromates.

The presence of the corresponding analyte changes the electroluminescent properties of the materials, which can be detected with the corresponding detection methods. Such changes can be: -Reduction/variation of the lifetime of the exited state of the organic material, -Changes in the electroluminescence intensity, -Variation in the electroluminescence spectrum of the sensor (a detectable colour change), -Changes in the current-voltage and/or in the current-luminescence-characteristic.

Sensor arrays composed of sensors containing different sensor molecules can be used to detect an assortment of different analytes simultaneously.

A further approach to detect several analytes at the same time would be a extended sensor with a multianalyte sensor layer: one single electroluminescent layer fabricated from a blend of different sensor materials (of the kinds and for the analytes mentioned above), or from one electroluminescent material with different sensing functionalities, responding to various analytes with the same or distinguishable reactions and interactions.

Different modes of operation are possible to drive a sensor according to the invention, namely continuous mode or pulsed operation. Pulsed operation (i.e., occasional operation with possible intervals and periods of operation from a few split seconds up to even days or weeks) is expected to enhance the duration of the sensor considerably.

The sensor may further comprise a photodetector to pick up the optically detectable and recognizable response indicating the presence of the analyte.

Fig. 2 illustrates the operation principle of a sensor according to the invention. The analyte, for instance in gaseous form arrives at the surface of the sensor and diffuses into the sensing organic layer 25 between the (or through the) top electrode 24 (cathode). There it reacts with the sensitive molecule 22, 23 (wherein 22 refers to the sensing guest and 23 to the host mole-cule when a blend system is used), modifying its light emission 28. The light then passes through the transparent electrode 26 and substrate 27, and is received in a detector 29. P102

Unlike a sandwich structure as shown in Figs. 1 and 2, having the electroluminescent sensing layer positioned between anode and cathode, a surface cell structure architecture may be employed as shown in Fig. 3. The surface cell sensor has inter-digital electrodes realized on a substrate 31, which may be transparent; both anode and cathode 33 are e.g. comb-shaped and positioned on top of the structure. The light emitting areas 32 are located into and/or on the surface between anode and cathode digits. The sensing layer is directly exposed to the atmosphere; electrodes 33 and the electroactive sensing material 32 can be deposited by ink jet printing. Primary advantage of this architecture is the very short response time of the system.

A sensor assembly according to the invention can also be realised with a structure as shown in Fig. 7: It shows a cross section of a surface cell structure sensor, having an additional controlling electrode 72 that is located on the substrate 71. This electrode is separated from the inter-digital electrodes 75 (namely, anode 75-1 and cathode 75-2) and the electrolumines-cent sensing layer 74 by a layer 73 that is fabricated from a dielectric.

The sensing layer is directly exposed to the atmosphere, and therefore bypassing the draw-back of the sandwich structure: the necessity to get the analyte to the sensing layer via gas permeable or structured electrodes which takes a certain amount of time, depending on the gas permeability of the organic layer for the gaseous analyte.

One remarkable feature and major advantage of the invention is that the sensing effect is reversible: using appropriate procedures the contaminated sensor can be refreshed again (e.g. for an oxygen sensor by storing in a vacuum chamber, or for a sensor for acidic gases by atreatment with an alkaline gas) and can be used for further measurements.

As an example, Fig. 4 illustrates the functionality and the restoration of an oxygen sensor. It shows two electroluminescence spectra of an oxygen sensor as described in Example I below, each with one spectrum measured in an argon atmosphere, and one measured after mm exposure to air. Between the left diagram (Fig. 4a) and the right diagram (Fig. 4b), a vacuum restoration cycle was performed.

The complete sensor may be also built on or enclosed in a semi-permeable membrane (which may be used as a substrate or covering), in order to allow selective contact with the desired analyte in a headspace that comprises a mixture of two or more gaseous substances. An example is shown in Fig. 5. A semipermeable membrane 56 covers the upper surface of the sensor shielding the organic sensor layer from the headroom. The membrane 56 may cover F1O the entire surface of the sensor or, as shown in Fig. 5, only the windows left by the top electrode layer 55. The layers 51 to 55 correspond to the layers I to 5 of Fig. 1, respectively.

Furthermore, the organic layer may be structured (for example with grooves or channels, formed by a soft lithography process) to facilitate absorption and/or diffusion of the analyte into the electroluminescent sensing layer, thus enhancing response time of the sensor, as illustrated in Fig. 6. As can be seen, the electroluminescent layer 64 (and, correspondingly, the covering intermediate layer 63 as well) has grooves which largely augments the surface of the layer. In other respects, the layers 61 to 65 correspond to the layers I to 5 of Fig. 1, respectively.

Example 1

We have combined the active material in an OLED with an integrated receptor molecule for different volatile analytes such as 02 by making use of a combination of electroluminescent sensor materials which have been integrated into a gas-permeable sensor configuration. An active sensor for the perception of oxygen was realised in conventional thin film sandwich geometry: 1. A transparent anode of indium tin oxide coated glass was cleaned in an ultrasonic bath in isopropyl alcohol and toluene. It was exposed to oxygen plasma for about 15 minutes.

2. A layer of Bayer PEDOT:PSS was spin coated onto the substrate and dried in vacuum at 120 C.

3. A polymer layer fabricated from poly(9-vinylcarbazloe) (PVK) blended with 1%wt of (2,3,7,8,l2,l3,17,18-Octaethyl-21-, 23H-porphyrin, platinum(lI)) (PtOEP) was spin coated from a chloroform solution as organic electroluminescent sensor layer and dried in vac-uum at 70 C.

4. Thin aluminium stripes were vapour deposited on top of the structure through a shadow mask as a cathode.

The sensor was encapsulated in an air tight specimen holder in an argon atmosphere.

The polymer blend used for the sensing layer is a guest host system of PVK, a deep blue fluorescing polymer, doped with 1% of PtOEP for the sensing of oxygen. The electrolumines-cent layer emits red light with a peak wavelength of 650 nm.

The measurements were performed in argon and in air. When the sensor is exposed to air the atmospheric oxygen causes a decime of the intensity of the red electroluminescence emission that becomes more significant in the course of time. This is called a quenching process. It is due to a decrease of the triplet lifetime of the PtOEP, which can also be observed by appro-p-ir -13 -priate appliances. Like all quenching processes the variation in time proceeds according to the Stern-Volmer theory.

The blend was fabricated from PVK (poly(9-vinylcarbazole)) with 1% of PtOEP. The percent-age of PtOEP is variable; blends in the range of 0,1% to 5% were found to be suitable, and the I %-blend turned out to be particularly efficient.

As already mentioned, the sensor effect is reversible: When the oxygen is removed again (for example by exposing the sensor to a vacuum) the fresh state of the sensor is restored, as indicated by the recovery of luminescence intensity and life-time of the device before expo-sure to oxygen.

It will be clear to those skilled in the art that other materials and blends from two or more materials are possible, especially for other analytes.

Example 2

An active sensor for the perception of acidic and alkaline gases can be realized by using a conjugated polymer as the active luminescent sensing layer that protonates or deprotonates, depending on the type of gas (acidic or alkaline). This leads to a change in the EL emission spectrum, resulting in a colour change of the sensor emission. This color change may be detected by a suitable detector device or may be visible to the eye, allowing a direct assess-ment of the presence of one or more analytes. Possible applications vary with the sensitive material used.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

-14 -Claims: 1. An optical sensor for detecting an analyte of interest

in a fluid medium, comprising an organic light emitting device having at least two electrodes and an organic electrolumines- cent layer that is sensible to an analyte contained in said fluid medium, said organic elec-troluminescent layer being adapted to change its electroluminescence emission properties upon contact with said analyte for generating an optically detectable response.

2. The optical sensor according to claim 1, being realised in a sandwich structure compris-ing -a substrate, -at least one electroluminescent organic layer, -a top electrode.

3. The optical sensor according to claim 2, wherein the sandwich structure further com-prises -a transparent electrode positioned beneath one of the at least one electroluminescent organic layer.

4. The optical sensor according to claim 2 or 3, wherein the sandwich structure further comprises additional organic or metallic intermediate layers.

5. The optical sensor according to any one of claims I to 5, wherein at least one of the electrodes is positioned above the organic electroluminescent layer and is structured later-ally, allowing access of the medium to the organic electroluminescent layer.

6. The optical sensor according to any one of claims I to 6, wherein at least one of the electrodes is positioned above the organic electroluminescent layer and is permeable to the analyte.

7. The optical sensor according to claim 1, being realized in a structure comprising, located on a substrate, at least two electrodes covering different portions of the substrate and leaving a gap between them, and an organic electroluminescent layer bridging the gap between said electrodes.

8. The optical sensor according to claim 7, wherein the structure further comprises a controlling electrode that is separated by a dielectric layer from the active layer.

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9. The optical sensor according to claim 7 or 8, wherein the electrodes are shaped in an inter-digital structure 10. The optical sensor according to any one of claims 1 to 9, further comprising a photodetector adapted to detect light emitted by the organic light emitting device.

11. The optical sensor according to any one of claims ito 10, wherein the substrate is made of a flexible material.

12. The optical sensor according to any one of claims I to 11 being assembled on a trans-parent substrate.

13. The optical sensor according to claim 12, comprising at least one transparent electrode positioned between the transparent substrate and the organic electroluminescent layer.

14. The optical sensor according to any one of claims 1 to 13, wherein the detector layer is provided with a structured topography.

15. The optical sensor according to any one of claims I to 14, being assembled on a sub-strate and bottom electrode that are both permeable to the analyte.

16. The optical sensor according to any one of claims I to 15, further comprising a semi-permeable membrane covering at least one of the sensor surfaces.

17. The optical sensor according to any one of claims 1 to 16, wherein the electrolumines-cent layer is made of an electroluminescent organic polymer blended with at least one type of sensing molecules.

18. The optical sensor according to any one of claims 1 to 16, wherein the electrolumines-cent layer is made of an electroluminescent organic polymer with a covalently attached sensing functionality.

19. The optical sensor according to any one of claims I to 16, wherein the electrolumines-cent layer is made of a small electroluminescent molecule blended with at least one type of sensing molecules. P1O

20. The optical sensor according to any one of claims 1 to 16, wherein the electrolumines-cent layer is made of a small electroluminescent molecule with a covalently attached sensing functionality.

21. The optical sensor according to any one of claims 1 to 16, wherein the organic elec-troluminescent layer is made of an organic material adapted to change, upon contact with an analyte, its electroluminescence emission properties relating to the lifetime of the exited state of the organic material.

22. The optical sensor according to any one of claims 1 to 16, wherein the organic elec- troluminescent layer is adapted to change, upon contact with an analyte, its electrolumines-cence emission properties relating to the electroluminescence intensity.

23. The optical sensor according to any one of claims 1 to 16, wherein the organic elec- troluminescent layer is adapted to change, upon contact with an analyte, its electrolumines-cence emission properties relating to the electroluminescence spectrum.

24. The optical sensor according to any one of claims 1 to 16, wherein the organic elec- troluminescent layer is adapted to change, upon contact with an analyte, its electric proper-ties relating to at least one of the current-voltage and the current-luminescence characteristic.

25. A sensor device comprising a plurality of optical sensors according to any one of claims I to 16, the optical sensor being adapted to detect different species of analytes, respec-tively.

26. A sensor device according to claim 25, wherein the optical sensors are realized with a common electroluminescent material individually modified so as to exhibit dif-ferent analyte sensing functionality.

27. An optical sensor substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

28. A sensor device comprising a plurality of optical sensors and as substantially here-inbefore described with reference to the accompanying drawings.