FIBRE OPTIC CHEMICAL SENSOR

Abstract: A sensor (1) for detecting changes in or monitoring the level of a chemical parameter of interest, comprising an optical fibre cable (2) having a sensing segment (5) that is a film coating of a support matrix (7) on the core (6) of the optical fibre cable with any sheathing and cladding removed, on which support matrix is immobilized an indicator compound having spectral characteristics sensitive to the parameter of interest, wherein the support matrix consists of a material permeable to the species of interest. Embodiments wherein the sensor is configured as a probe are also disclosed.

Description: FIBRE OPTIC CHEMICAL SENSOR This invention relates to fibre optic chemical sensors . There have been two major trends in the development of these sensors. One is to immobilize a chemical indicator in a matrix, at the end of a fibre optic light guide, and to observe and process the signal reflected from the matrix. A second approach is to bind a suitable matrix containing the immobilized indicator on to the surface of a fibre optic light guide, and to observe and process the transmitted signal. In some cases this signal may be purposefully reflected back along the same fibre optic light guide for observation. It is also possible to bind the indicator to the fibre optic light guide surface, either directly, or by using an intermediary binding agent. A certain amount of success has been achieved using both of these approaches e.g. in U.S. patent application No. 855,397 (Peterson, 1980), in I.C.I. PLC' s European patent applications No. 82301495.6, No. 82301496.4, and No. 82303803.9, and in British patent applications No. 8313527 and No. 8314519 by Elf U.K. PLC. In all of these applications the fibre optic probe was used in conjunction with a porous material. Description of the Invention This invention relates to a sensor for monitoring a change in, or measuring the level of, a chemical parameter of interest, e.g. a chemical parameter such as the nature or concentration of a chemical species in solution or in the gaseous phase. Various types of electrical sensors are already known. These sensors do however have a number of disadvantages. Firstly, the sensors rely on the use of electrical signals, which may be undesirable if the sensor is to be used in an inflammable environment. Secondly, there may be impedance effects in the electrical sensor which adversely affect the measurement being made. The problems associated with impedance effects become particularly acute when miniaturized sensors are required. In such a case, it may be necessary to operate the sensor within a Faraday cage to avoid the impedance effects. It is an object of this invention to provide a sensor which obviates or mitigates the above disadvantages, and which also overcomes the problems of increased reflectance and scatter of light, and of insufficiency in the amount of light collected, which have been experienced using fibre optic sensors with a porous coating. It is a further object of the invention to provide a fibre optic sensor which resists leaching of the indicator material from the surface of the senso . According to the present invention there is provided a sensor for detecting changes in or monitoring the level of a chemical parameter of interest, which sensor comprises an optical fibre cable having a sensing segment that is a film coating of a material (henceforth known as the support matrix) on the core of the optical fibre cable with any sheathing and cladding removed, and on this said material is immobilized a compound (henceforth known as the indicator compound) having spectral characteristics sensitive to the parameter of interest. The support matrix consists of a material permeable to the species of interest. Further, when configured as a probe a tip is incorporated, this tip being either reflective, absorptive or a combination of both. Also a second outer film coating of a different material to the support matrix may be incorporated. The sensor of the invention is suitable for use in an aqueous environment, in non-aqueous liquid environments, and in gaseous environments. For long-term use in the latter two cases, a constant minimum water content of the environment is preferred. The optical fibre cable provides the means for transmitting light to the indicator compound from a light source, and for transmitting light from the said indicator compound to a detector. The detector may be set to detect a change or level in a property of the light affected by the indicator compound; e.g. a property such as the intensity or wavelength of the light. A single fibre means is preferred to form the core of the sensing segment, although a multiple fibre bundle may be employed to conduct light to the single fibre from the source and from the single fibre to the detector. This method simplifies the support instrumentation required when the sensor is configured as a probe. Multiple fibre bundles with a bundle size comparable with the single fibre means, are physically abutted to the single fibre in a snugly fitting tube arrangement. Gel or fluid of refractive index matching that of the fibre may be employed at the joint to reduce undesirable reflections and losses. The support matrix is a polymer material chosen to have an index of refraction similar to that of the core material of the optical fibre cable. In this way the light interaction with the immobilized indicator compounds is maximized, and only short lengths of sensing segment are required. A second and important function of the support matrix is to act as a permeable membrane which allows or excludes certain parameters from interaction with the indicator compound, e.g. a hydrophobic support matrix such as poly (methyl methacrylate) will exclude at its surface ionized species, thus hydrogen (hydronium) ions are not able to affect a colorimetric pH indicator when this is the immobilized indicator compound. Another material (henceforth known as the sensor cladding material, which may be different from the original cladding material used on the optical fibre means) may be used to provide both optical and chemical isolation of the sensor segment from the environment under investigation. Optical isolation is best achieved when the index of refraction of the sensor cladding material is similar to that of the cladding material used on the optical fibre means. The cladding material may also be based on the same material as the support matrix, but fabricated in such a way as to produce a gradient refractive index change radially from the core material. The degree of optical isolation obtained is similar whether the sensor cladding material is incorporated as a step refractive index change or a gradient refractive index change. It is possible to have multiple film coatings, using films with different properties, although such multiple coatings will result in a slower response. The use of a sensor cladding material as described is not always preferred. All the coating layers are desirably as thin as is practicable, in order to attain the most rapid access by diffusion of the species of interest to the indicator material and hence to attain the most rapid response time possible. The indicator compound is at least one reagent which undergoes a change in its spectral characteristics in response to a change in the parameter of interest, e.g. a change in the nature or concentration of a particular chemical species. The spectroscopic change may be one of luminescence or absorption. Suitable indicator compounds are the colorimetric and fluorimetric indicator dyestuffs, chosen with regard to the parameter of interest, e.g. bromothymol blue or bromocresol purple may be used to determine ammonia concentration. The indicator may be physically contained within the sensor support matrix or held by non-covalent interactions. •* When configured as a probe it is important to define accurately the boundary conditions at the tip of the sensor. The primary purpose of this is to ensure that the light reflection at that extremity is constant and independent of the environment under investigation. An absorptive tip ensures that light escaping at the end of the probe does not reflect back into the probe regardless of the environment under investigation. A reflective tip enables a substantial proportion of the light that would normally escape the probe to be reflected back into the probe. This may be advantageous, especially when the indicator compound used in the sensor is of the absorptive type. It is preferred in practice to use an absorptive, or an absorptive and reflective tip, e.g. a metal-loaded material or a two-layer combination, to reduce environmental effects. The invention will be further described by way of examples incorporating the features so far outlined, and with reference to the accompanying drawings in which: Figure 1 represents a sectional view of the sensor in one embodiment of the probe in accordance with the invention; Figure 2 represents the response time of the probe to NH 4 OH; Figure 3 represents a sectional view of an alternative embodiment of the probe in accordance with the invention; and Figure 4 represents a sectional view of an embodiment in which the sensor of the invention is configured in line with the detector. The probe (1) shown in Figure 1 comprises an optical fibre cable (2) (including sheathing (3) and original cladding (4)) and a sensing segment (5) at one extremity of the optical fibre cable (2). The sensing segment itself comprises the core (6) of the optical fibre cable (2) with a film coating of the support matrix (7) containing the immobilized indicator compound, and an outer film coating of a cladding material (8), and with a combination reflective (9) and absorptive (10) tip. In use, the probe of the invention is associated with a light source and a light detector. Light from the source is transmitted along the multiple fibre bundle (11) to the optical fibre cable (2) and on to the indicator compound, through the abutted joint (12). Reflections and losses at the joint are reduced by index-matching gel or fluid (13). The two types of cable are held concentric and in close proximity by a snugly fitting and clamped tube (14), providing a quickly and easily separable joint. Light from the indicator compound is transmitted along the optical fibre cable (2), and a proportion of the light is collected by about half of the • multiple fibre bundle (11) at the abutted joint (12) and transmitted on to the detector. To give an idea of the size, the sensing segment (5) of the probe (1) may be of length about 10 mm whereas the optical fibre cable (2) may have a core (6) diameter of 1 mm. These dimensions are given purely by way of an example and may of course be different to suit particular applications for the probe (1). In particular, miniaturization or encasement in a hypodermic needle would allow use of the probe in clinical and other biomedical applications. Example 1 The sensor configured as a probe as shown in Figure 1 may be used to determine the concentration of ammonia in aqueous solution. In this application it is preferred that a sensing segment cladding (8) is not employed unless immiscible oils and solvents are likely to be present in the aqueous solution. As has already been mentioned, an indicator compound such as bromothymol blue or bromocresol purple may be used, immobilized in a support matrix (7) of poly (methyl methacrylate) which is coated on an optical fibre core (6) which may be of the same material. A combination or metallised tip (9) and (10) as illustrated is preferred in this particular application. Light of wavelength in the mid-visible region (about 590 nm) is strongly absorbed by the two indicator compounds mentioned, in their base form. In the presence of ammonia these indicator compounds exist in two conjugate forms, the acid form and the base form. The higher the concentration of ammonia present, the greater the proportion of the base form of the indicator compound in the conjugate combination. If the source used to provide light to interact with the indicator compound emits in the wavelength region about 590 nm, then the proportion that is absorbed by the indicator compound may be related to the concentration .of ammonia. The detector is used to detect the level of light absorbed with reference to another wavelength region. In this particular example, the reflective nature of the tip greatly enhances the light received at the detector and therefore assists in the determination of the proportion of light absorbed. The probe may thus be used simply to indicate a change in ammonia concentration. Alternatively the probe may previously have been calibrated so that the level of, or change in intensity of the partially absorbed light gives a quantitative indication of the concentration of, or change in concentration of ammonia. * The polymer materials used are permeable to ammonia, methylamine, and ethylamine, although the response time with ethylamine is somewhat prolonged. It appears that the size limit of molecules able to penetrate the polymer is of the o order of 3A. The sensor function is relatively insensitive to the temperature and ionic strength of the environment, and can operate in the short term in a water-free environment. The response to NH^OH of a probe using bromothymol blue as indicator with the wavelength of light provided by the source using 1 L water to which different volumes of 30% NH., solution had been added, has its maximum at approximately 584 nm. To determine the response time of the probe, NH-, (as a 30% solution in water) was added to a reservoir of 1.0 L of distilled water at 24°C. The results are shown in Fig. 2, which represents a chart recording of the response of a bromothymol blue coated probe with reflective tip monitored at 580 nm. Example 2 The sensor configured as a probe as shown in Figure 1 may be used to determine the pH of a solution. In this application, the sensing segment cladding (8) is not required to achieve optical isolation. Fabrication methods may be employed that enable a gradient index to form from the fibre core (6) to the support matrix (7) outer surface. This gradient index appears when the probe is in an aqueous operational environment and the support matrix (7) has become hydrated. The support matrix (7) is designed to have a diffusion profile into the surface of the core material (6), so that when this matrix becomes hydrated it will cause a gradient in the radial index, rather than an abrupt or step change. Because of this gradient index, total internal reflection within the probe will occur before the outermost surface boundary, thus providing optical isolation. The preferred materials used as the support matrix (7) are the hydroxylated acrylates and ethacrylates . To ensure permanent immobilisation of the indicator on the support matrix (7) under conditions where the matrix swells- considerably and extreme values of pH can occur, it is also preferred to covalently bond the indicator. With the preferred hydroxylated matrixes, reactive indicators have been used, some of which are commercially available, i.e. Procion Brilliant Red. However there are numerous other bonding techniques recorded in the literature. Example 3 Alternative Configuration as a Probe In another preferred embodiment of the invention, shown in Figure 3, the probe (1) comprises an optical fibre cable (2) (including the core (3) and cladding (4) and optionally including sheathing (5)), and a sensing segment (6) at one extremity of the optical fibre cable (2). The sensing segment (6) itself comprises a thin coating of the support matrix (7) permanently bound on the exposed face of the fibre cable (2) at the distal end. The support matrix (7) contains an immobilised indicator compound. In use, the probe of the invention is associated with a light source and a light detector. Light from the source is transmitted along the optical fibre cable (2) and is incident upon the support matrix (7) . Although some of the light maybe reflected at the boundary with the support matrix (7), that which is not reflected will be transmitted through the support matrix (7) and will interact with the immobilised indicator. Some of the backscattered or re-emitted light from the immobilised indicator is collected by the optical fibre and transmitted on to the detector. It is not important that the coating covers the sheathing and cladding, but it is important that it covers the core material completely, and that the cladding covers the core and is adjacent to the coating. The 'distal end" of the optical fibre is the end furthest from a connection point to the associated instrumentation. • In contrast, the "proximal end' is the end of the optical fibre attached to the instrumentation. For a 'probe' of this second configuration, it is a necessary condition that the coating is at the distal end of the transmitting fibre, although a second optical fibre might be employed to pick up the transmitted optical signal and conduct it back to the detector. In a 'sensor' of the original configuration, the sensing segment does not necessarily have to appear at any end of the optical fibre and may be any distance along it. However when termed a 'probe' it is implied that the sensing segment is at the distal end of the fibre. A 'probe' of this second configuration may have a support matrix of either a translucent or transparent nature depending on the immobilised indicator compound employed. An absorptive indicator would be best bound in a scattering translucent matrix, whereas a fluorescent indicator would prove more effective in a transparent matrix. Example 4 Inline Sensor Configuration As illustrated in Figure 4, the sensor (1) comprises an optical fibre cable (2) (including the core (3) and cladding (4) and optionally including sheathing (5)), and a sensing segment (6) at any point along the length of the optical fibre (2) . The sensing segment (6) itself comprises a film coating of the support matrix (7) on the core (3) of the optical fibre cable (2) with the cladding (4) and sheathing (5) removed. The support matrix (7) contains an immobilised indicator compound. A further film coating (8) may be applied over the support material to act as cladding or as a membrane permeable to a specific chemical species. In use, the probe of the invention is associated with a light source and a light detector. Light from the source is transmitted along the optical fibre cable (2) to the sensing segment where it will interact with the indicator compound immobilised on the support matrix (7). An indicator modified light signal is then transmitted further along the optical fibre cable (2) to the detector. Advantages 1. The indicator material of the present invention is permanently bound within the permeable polymer coating. 2. The indicator is not subject to losses by leaching into the medium. 3. The polymer coating does not scatter light and so does not diffuse the signal. 4. The sensor of the present invention can be used in a variety of different environments. 5. The probe according to the invention is robust, with a long lifetime. 6. The probe can be miniaturized, or configured in a hypodermic needle or a catheter for biomedical or clinical use. Possible applications include: 1. Measurement of ammonia levels (using for example bromothymol blue or bromocresol purple as indicator) . 2. Measurement of oxidation/reduction reactions. 3. Measurement of pH. 4. Measurement of fluorescence or other luminescence. 5. Remote sensing of a variety of parameters. 6. Measurement of a variety of parameters in vivo in humans or animals. 7. Measurement of metal ion levels using metallochromic indicators. 8. Measurement of organic functional groups using appropriate indicators. It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims: _ C_L__A_I_M_S_ . 1. A sensor for detecting changes in or monitoring the level of a chemical parameter of interest, which comprises an optical fibre cable having a sensing segment that is a film coating of a support matrix on the core of the optical fibre cable with any sheathing and cladding removed, on which support matrix is immobilized an indicator compound having spectral characteristics sensitive to the parameter of interest, wherein the support matrix consists of a material permeable to the species of interest. 2. A sensor according to Claim 1 further incorporating a second outer film coating of a material different to that of the support matrix. 3. A sensor according to Claim 1 or Claim 2 wherein the optical fibre cable comprises a single fibre means. 4. A sensor according to Claim 1 or Claim 2 wherein a multiple fibre bundle is employed to conduct light to the single fibre from the source and from the single fibre to the detector . 5. A sensor according to Claim 4 wherein multiple fibre bundles with a bundle size comparable with the single fibre means, are physically abutted to the single fibre in a snugly fitting tube arrangement, and gel or fluid of refractive index matching that of the fibre is contained in the tube surrounding the joint. 6. A sensor according to any preceding claim wherein the support matrix is a polymer material chosen to have an index of refraction similar to that of the core material of the optical fibre cable. 7. A sensor according to Claim 6 wherein the support matrix is hydrophobic. 8. A sensor according to Claim 6 wherein the support matrix is poly(methyl methacrylate). 9. A sensor according to Claim 6 wherein the support matrix is a hydroxylated aσrylate. 10. A sensor according to-Claim 6 wherein the support matrix is a hydroxylated methacrylate. 11. A sensor according to any previous claim wherein a sensor cladding material, which may be different from the original cladding material used on the optical fibre means is used to provide both optical and chemical isolation of the sensor segment from the environment under investigation. 12. A sensor according to Claim 11 wherein the refractive index of the sensor cladding material is similar to that of the cladding material used on the optical fibre means. 13. A sensor according to Claim 11 wherein the cladding material is based on the same material as the support matrix, but fabricated in such a way as to produce a gradient refractive index change radially from the core material. 14. A sensor according to any one of Claims 11 to 12 wherein the sensor cladding material produces a step refractive index change radially from the core material. 15. A sensor according to any previous claim wherein multiple film coatings are used, characterized in that the films have different properties. 16. A sensor according to any previous claim wherein the indicator compound is at least one reagent which undergoes a change in its spectral characteristics in response to a change in the parameter of interest. 17. A sensor according to Claim 16 wherein the spectral change is one of luminescence. 18. A sensor according to Claim 16 wherein the spectral change is one of absorption. 19. A sensor according to Claim 16 wherein the indicator compound is held within the sensor support matrix by covalent bonding. 20. A sensor according to Claim 16 wherein the indicator compound is physically contained within the sensor support matrix. 21. A sensor according to Claim 16 wherein the indicator compound is held within the sensor support matrix by non-covalent interactions. 22. A sensor according to any one of Claims 16 to 21 wherein the indicator is sensitive to pH, redox potential or ammonia. 23. A probe characterized in that a tip is incorporated into a sensor according to any previous claim. 24. A probe according to Claim 23 wherein the tip is reflective and/or absorptive. 25. A probe according to Claim 23 further characterized in that it is incorporated into a hypodermic needle, endoscope or catheter. 26. A probe according to Claim 23 comprising an optical fibre cable, including sheathing and original cladding and a sensing segment at one extremity of the optical fibre cable, wherein the sensing segment itself comprises the core of the optical fibre cable with a film coating of the support matrix containing the immobilized indicator compound, and an outer film coating of a cladding material, and with a combination reflective and absorptive tip. 27. A probe according to Claim 23 comprising an optical fibre cable including the core and cladding and optionally including sheathing, and a sensing segment at one extremity of the optical fibre cable, wherein the sensing segment itself comprises a thin coating of the support matrix permanently bound on the exposed face of the fibre cable at the distal end, and the support matrix contains an immobilised indicator compound. 28. An inline sensor comprising an optical fibre cable including the core and cladding and optionally including sheathing, and a sensing segment at any point along the length of the optical fibre, wherein the sensing segment itself comprises a film coating of the support matrix on the core of the optical fibre cable with the cladding and sheathing removed, and the support matrix contains an immobilised indicator compound. 29. An inline sensor according to Claim 28 wherein a further film coating is applied over the support material to act as cladding or as a membrane permeable to a specific chemical species. 30. Apparatus for detecting changes in a chemical parameter which incorporates a sensor or probe according to any previous claim. 31. A method for estimation of ammonia, methylamine or ethylamine characterized by the use of a sensor or probe according to the invention. 32. A method for determination of pH characterized by the use of a sensor or probe according to the invention. 33. A method for determination of redox potential characterized by the use of a sensor or probe according to the invention. 34. Products and processes as hereinbefore defined with reference to the examples and drawings.

Inventors: WELTI NEAL ARTHUR, WELTI, NEAL, ARTHUR
Applicants: UNIV MONASH, MONASH UNIVERSITY
Application year: 1986
Main classification: G01N21
IPC codes: G01N21