SENSOR ASSEMBLY

Abstract: The present invention relates to a process for determining the absolute status (<i>eg</i> the absolute concentration) of a sensor component with a sensing layer or sensing waveguide and to a sensor assembly adapted to perform the process (for example at start-up).

Description: Sensor assembly The present invention relates to a process for determining the absolute status (eg the absolute concentration) of a sensor component with a sensing layer or sensing waveguide and to a sensor assembly adapted to perform the process (for example at start-up) . In order to obtain optimum precision from an integrated optical waveguide sensor, interferometric detection (which is itself a well-established technique) may be employed. It is intrinsic to this technique that the measured characteristics are cyclical and provide identical information every integer multiple of 2π radians in phase difference. If the history of the phase differences are not known or are lost during an interruption in the monitoring of the sensor assembly, the user cannot determine the absolute status of the sensor assembly in response to an analyte. Starting or resuming operation may lead to significant errors since signal loss or "aliasing" may have occurred. Typically this means that once a sensor assembly is calibrated, it is not possible to switch the sensor assembly off without the calibration being "lost" . By way of example, it will be appreciated that over the lifetime of a sensor component, the sensing layer or sensing waveguide will be exposed continuously to a changing localised environment and its absolute status will vary accordingly. For example, the level of moisture absorbed into the sensing waveguide or sensing layer will vary according to atmospheric conditions. In practice, this means that the instrument containing the sensor component will need to be calibrated on each occasion before it is used (typically with a calibrant gas) to determine the absolute concentration of moisture. This allows the measured phase shift caused by the introduction of or changes in a stimulus of interest to be related to a concentration of the stimulus of interest. The need to calibrate the instrument prior to each use is a great inconvenience to the user. The present invention provides a process for determining the absolute status ( eg the absolute concentration) of a sensor component at any arbitrary phase position. The advantages of this include a greatly reduced risk of aliasing and the removal of any need for calibration prior to use. Viewed from one aspect the present invention provides a process for determining the absolute status of a sensor assembly which is capable of detecting the introduction of or changes in a stimulus of interest in a localised environment, (said assembly comprising: a sensor component including either (1) one or more sensing layers capable of inducing a measurable response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest or (2) a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest) , said process comprising: (A) irradiating the sensor component with electromagnetic radiation; (B) measuring first movements in the interference pattern; (C) biassing the sensor component by applying a perturbation; (D) measuring second movements in the interference pattern following said perturbation; and (E) calculating the absolute status of the sensor component. The process of the invention allows the user to determine the absolute status of the sensor assembly regardless of the arbitrary phase position. The process may also be used to verify that a sensor assembly remains in its previously determined state after suspension of operation ( eg a power failure) has prevented continuous monitoring of the phase shift. This substantially eliminates the risk of the output of the sensor component being subjected to aliasing. Thermal perturbation is generally a function of "out- gassing" of the sensing layer or sensing waveguide and the intrinsic thermo-optic properties of the sensor component. Thus where the process comprises thermally biassing the sensor component by applying a thermal perturbation, the phase shift as a function of temperature (ie the thermo- optic response) is characteristic of a unique state of the sensor component (ie the 2π phase "quantum" which is "occupied" by the sensor at any given point in time) and so may be used to obtain the absolute concentration of a stimulus in the sensing layer or sensing waveguide. In this embodiment, the process may further comprise: deducing a measured thermo-optic response from the first and second movements in the interference pattern; and calculating the absolute concentration of the stimulus of interest from the measured thermo-optic response. Generally speaking, the measured thermo-optic response is compared against a reference thermo-optic response to calculate the absolute concentration. The reference thermo- optic response represents the phase shift/temperature relationship at each concentration and is typically stored in the sensor instrumentation. Alternatively, the measured thermo-optic response is compared against a reference thermo-optic response to calculate the absolute dimensions of the sensing waveguide or sensing layer. The reference thermo-optic response represents the phase shift/temperature relationship at each dimension and is typically stored in the sensor instrumentation. Alternatively, the measured thermo-optic response is compared against a reference thermo-optic response to calculate the absolute wavelength of the sensing waveguide or sensing layer. The reference thermo-optic response represents the phase shift/temperature relationship at each wavelength and is typically stored in the sensor instrumentation. Whilst the measured thermo-optic response of a sensor component is a unique signature of its characteristics in terms of refractive indices, layer thickness and wavelength of operation (and may therefore be modelled as a multi- parameter function as described hereinafter) , upon exposure to a stimulus of interest only the intrinsic refractive index and/or the dimensions of the sensing layer or sensing waveguide is changed (and these factors contribute to a change in the effective refractive index of the sensing layer or sensing waveguide) . Thus any changes in the measured thermo-optic coefficient can be ascribed exclusively to the effect of the stimulus of interest. In one embodiment of the process of the invention, steps (A) to (E) are carried out at start-up. This allows the absolute concentration of a stimulus in the sensing layer or sensing waveguide to be determined before introducing a stimulus of interest ( eg a chemical or biological stimulus of interest in an analyte) . The resulting phase change may be used to calculate the absolute amount of the stimulus of interest introduced into the localised environment. The process may comprise biassing the sensor component by applying a wavelength, frequency or polarisation perturbation. This embodiment is generally capable of measuring the absolute phase position to within a single 2π interval . Preferably the process further comprises: calculating a first phase shift from the first movements in the interference pattern and a second phase shift from the second movements in the interference pattern. Methods for performing this calculation will be familiar to those skilled in the art and will generally be carried out automatically within the instrumentation. Viewed from a further aspect the present invention provides a sensor assembly as hereinbefore defined adapted to determine the absolute status of the sensor component thereof . Preferably the sensor assembly is adapted to determine the absolute status of the sensor component at start-up. Particularly preferably the sensor assembly is adapted to determine the absolute status of the sensor component automatically at start-up. These embodiments advantageously dispense with the need for the user to calibrate the assembly prior to each use. Where the perturbation is a thermal perturbation, the sensor assembly further comprises a means for thermally perturbing the sensor component. The means for thermally perturbing the sensor component may be an active thermal assembly positioned in thermal contact with the sensor component such as a heater or a Peltier assembly. The Peltier assembly may comprise a Peltier mounted on a Peltier mount. Preferably, the Peltier mount has a concave underside to optimise thermal contact with the sensor component (or its base) . The Peltier and Peltier mount may be provided with suitable insulation as desired. In a preferred embodiment, the means for thermally perturbing the sensor component is an inner temperature controller and optional associated components of the type described specifically or generally in International Patent application no. GB00/03635 filed in the name of Farfield Sensors Limited. Preferably the perturbation is a wavelength perturbation. In this embodiment, the sensor assembly comprises a means for perturbing the source of electromagnetic radiation. This process enables similar information to that provided by thermal perturbation to be obtained. The response is a unique characteristic of the absolute status of the sensor component in terms of its layers, their thickness and their intrinsic refractive indices. Means for varying the wavelength of the excitation radiation will generally be familiar to those skilled in the art. For example, thermal perturbation of a laser source produces a well-characterised perturbation of output wavelengths. In a further embodiment, the perturbation is a wavelength perturbation . Preferably the sensor component is adapted so as to be usable in evanescent mode or whole waveguide mode. Thus in a first embodiment of the sensor assembly, the sensor component includes one or more sensing layers capable of inducing in a secondary waveguide a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest. In this first embodiment, the sensor assembly is advantageously adapted to optimise the evanescent component so as to induce in the secondary waveguide a measurable optical response. The sensor component may comprise a plurality of separate sensing layers to enable changes at different localised environments to be detected. In a preferred sensor assembly of the invention, the sensing layer comprises an absorbent material (eg a polymeric material such as polymethylmethacrylate, polysiloxane, poly- 4-vinylpyridine) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells) . The absorbent material may be capable of absorbing a gas, a liquid or a vapour analyte containing a chemical stimulus of interest. The bioactive material may be appropriate for liquid or gas phase biosensing. For example, the sensing layer may comprise a porous silicon material optionally biofunctionalised with antibodies, enzymes, DNA fragments, functional proteins or whole cells. In a preferred sensor assembly of the invention, the secondary waveguide comprises silicon oxynitride or silicon nitride. In a second embodiment of the invention, the sensor component includes a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest. In this second embodiment, the sensor assembly is adapted to minimise the evanescent component and may be used advantageously in a whole waveguide mode. In a preferred sensor assembly of the invention, the sensing waveguide comprises an absorbent material (eg a polymeric material such as polymethylmethacrylate, polysiloxane, poly- 4-vinylpyridine) or a bioactive material (eg containing antibodies, enzymes, DNA fragments, functional proteins or whole cells) . The absorbent material may be capable of absorbing a gas, a liquid or a vapour analyte containing a chemical stimulus of interest. The bioactive material may be appropriate for liquid or gas phase biosensing. For example, the sensing waveguide may comprise a porous silicon material optionally biofunctionalised with antibodies, enzymes, DNA fragments, functional proteins or whole cells. Where the sensor component of the sensor assembly of the invention comprises a sensing waveguide adapted for use in whole waveguide mode, an absorbent layer in the form of an overcoating may be present for use as a membrane (for example) to separate out stimuli of interest. To optimise the performance of the first embodiment, the sensor component may further comprise an inactive secondary waveguide in which the sensing layer is incapable of inducing a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest . The inactive secondary waveguide is capable of acting as a reference layer. It is preferred that the secondary waveguide and inactive secondary waveguide have identical properties with the exception of the response to the change in the localised environment caused by the introduction of or changes in the stimulus of interest. By way of example, the secondary waveguide and inactive secondary waveguide is made of silicon oxynitride. To optimise the performance of the second embodiment, the sensor component may further comprise an inactive (eg deactivated) waveguide substantially incapable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest. The inactive waveguide is capable of acting as a reference layer. The physical, biological and chemical properties of the sensing waveguide and inactive waveguide are as similar as possible (with the exception of the response to the change in the localised environment caused by the introduction of or changes in the stimulus of interest) . Typically the inactive waveguide is made of silicon oxynitride. Preferably each of the sensing waveguide or secondary waveguide (or any additional waveguides such as reference waveguides) of the sensor component is a planar waveguide (ie a waveguide which permits light propagation in any arbitrary direction within the plane) . Preferably, the sensor component of the sensor assembly of the invention constitutes a multi-layered structure (eg a laminated waveguide structure) . In this sense, the sensor assembly is simple to fabricate and fault tolerant in terms of construction errors. In a preferred embodiment, each of the plurality of layers in the multi-layered sensor component are built onto a substrate (eg of silicon) through known processes such as PECVD, LPCVD, etc. Such processes are highly repeatable and lead to accurate manufacture . Intermediate transparent layers may be added (eg silicon dioxide) if desired. Typically the sensor component is a multilayered structure of thickness in the range 0.2-10 microns. A layered structure advantageously permits layers to be in close proximity (eg a sensing waveguide and an inactive (reference) waveguide may be in close proximity to one another so as to minimise the deleterious effects of temperature and other environmental factors) . Preferably, the sensor component comprises a stack of transparent dielectric layers wherein layers are placed in close proximity. Preferably each layer is fabricated to allow equal amounts of optical radiation to propagate by simultaneous excitation of the guided modes in the structure. Particularly preferably, the amount of light in the sensing waveguide/inactive waveguide or in the secondary waveguide/inactive secondary waveguide is equal. Preferably the sensor assembly comprises: means for measuring the response of the sensor component to the perturbation. Electromagnetic radiation generated from a conventional source may be propagated into the sensor component in a number of ways. In a preferred embodiment, radiation is simply input via an end face of the sensor component (this is sometimes described as "an end firing procedure"). Preferably (but not essentially) , the electromagnetic radiation source provides incident electromagnetic radiation having a wavelength falling within the optical range. Propagating means may be employed for substantially simultaneously propagating incident electromagnetic radiation into a plurality of waveguides. For example, one or more coupling gratings or mirrors may be used. A tapered end coupler rather than a coupling grating or mirror may be used to propagate light into the lowermost waveguide. The incident electromagnetic radiation may be oriented (eg plane polarised) as desired using an appropriate polarising means. The incident electromagnetic radiation may be focussed if desired using a lens or similar micro-focussing means . Using electromagnetic radiation of different frequencies (either simultaneously or sequentially) varies the contributions of the various waveguides and may further enhance the utility of the sensor assembly. As a consequence of the introduction of or changes in a physical, biological and/or chemical stimulus in the localised environment (ie a change in the refractive index of material in the localised environment) , changes in the dielectric properties (eg the effective refractive index) of the sensing waveguide or sensing layer occur. This causes a measurable optical response (ie a change in the transmission of electromagnetic radiation down the sensing waveguide (or waveguides) in whole waveguide mode or the secondary waveguide in evanescent field mode) . An interference pattern may be generated when the electromagnetic radiation from the sensor component is coupled into free space and the pattern may be recorded in a conventional manner (see for example WO-A-98/22807) . In this embodiment, a measurable optical response of the sensor component to a change in the localised environment manifests itself as movement of the fringes in the interference pattern. The phase shift of the radiation in the sensor component ( eg induced in the secondary waveguide in evanescent field mode or exhibited in the sensing waveguide in whole waveguide mode) may be straightforwardly calculated from the movement in the fringes . The sensor component may be excited across its width and a two-dimensional photodiode array (or the like) may be used to effectively interrogate "strips" of the sensor component ( eg an array sensor) . This may be carried out across more than one axis simultaneously or sequentially to provide spatially resolved information relating to events on the surface of the sensor component . Movement in the interference fringes may be measured either using a single detector which measures changes in the electromagnetic radiation intensity or a plurality of such detectors which monitor the change occurring in a number of fringes or the entire interference pattern. The one or more detectors may comprise one or more photodetectors . Where more than one photodetector is used this may be arranged in an array. The electromagnetic radiation source and one or more detectors may be integrated with the assembly into a single assembly. A plurality of electromagnetic radiation detector units (eg in an array) and/or a plurality of electromagnetic radiation sources may be used to measure in discrete areas of the sensor component simultaneously the responses to changes in the localised environment. Alternatively, the position of the electromagnetic radiation detector and electromagnetic radiation source relative to the sensor component may be changed to provide information concerning responses in discrete areas of the sensor component. For example, discrete responses to a change in the localised environment caused by the amount of the same or different stimuli may be measured in discrete areas of the sensor component. In the first instance, concentration gradients of the same stimulus may be deduced. In the second instance, discrete responses to changes in the localised environment may be measured in different regions. For this purpose, the preferred assembly makes use of the versatility of the evanescent mode and comprises a plurality of separate sensing layers or regions. Conveniently, electrodes positioned in contact with a surface of the sensing layer or sensing waveguide enable capacitance to be measured simultaneously. The electrodes may take the form of either parallel plates laid alongside a plurality of planar waveguides or as an interdigitated or meander system laid down on the top and bottom surfaces of the sensing waveguide or sensing layer or adjacent to it. In the case of a meander system, the metal forming the electrode is responsible for absorbing excessive amounts of light and as such the capacitance is measured on an adjacent structure which is not utilised for optical measurement. In a biological or chemical sensor assembly, the interaction of the stimulus with the sensing waveguide or sensing layer may be a binding interaction or absorbance or any other interaction. Preferably the sensor assembly of the invention comprises means for intimately exposing at least a part of the (or each) sensing layer or the sensing waveguide of the sensor component to the localised environment. The sensor component may permit precise measurements to be made either across the entire architecture or at discrete areas thereby enabling effective integration with the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment. Thus in a preferred embodiment, the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment is integrated onto the sensor component . Preferably the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment is adapted to permit the continuous introduction of an analyte containing a chemical stimulus of interest (ie a dynamic system) . For example, it may permit the continuous introduction of the stimulus of interest in a discontinuous flow (eg as a train of discrete portions) into the localised environment. This may be achieved by capillary action or by a separate urging means. Preferably the means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment is adapted to induce chemical reactions in a static analyte containing a chemical stimulus of interest. In this sense, the system may be considered to be dynamic. Chemical reactions may be induced in any conventional manner such as by heat or radiation. The means for intimately exposing at least a part of the (or each) sensing layer or the sensing waveguide to the localised environment may be a part of a microstructure positionable on the surface of and in intimate contact with the sensor component . Preferably the microstructure comprises means for intimately exposing at least a part of the sensing layer or the sensing waveguide to the localised environment in the form of one or more microchannels and/or microchambers . For example, an analyte containing chemical stimuli may be fed through microchannels or chemical reactions may take place in an analyte located in a microchamber. An analyte containing chemical stimuli may be fed into the microchannels by capillary action or positively fed by an urging means. In a preferred embodiment, the means for intimately exposing at least a part of the (or each) sensing layer or the sensing waveguide to the localised environment is included in a cladding layer. For example, microchannels and/or microchambers may be etched into the cladding layer. The cladding layer may perform optical functions such as preventing significant discontinuities at the boundary of the sensing waveguide or sensing layer (s) or chemical functions such as restricting access of certain species to the sensing waveguide or sensing layer (s) . The cladding layer may be integrated onto the sensor component . Preferably, the whole of or a portion of any additional functionality may be included in the cladding layer. In one embodiment, the sensing layer itself may be incorporated in the cladding layer (for example in the form of an absorbent material) . Particularly preferably, the whole of the additional functionality may be provided in the cladding layer and include sensing devices such as for example quadrature electric field tracks or other microfluidic sensing devices. The cladding layer may incorporate an electromagnetic source (eg a laser) and/or means for detecting electromagnetic radiation (of the type detailed below) . The cladding layer may incorporate a chemical separating means ( eg an HPLC based device) . The present invention will now be described in a non- limitative sense with reference to the accompanying Examples in which: Example 1 The thermo-optic response of a sensor component to a stimulus of concentration (C) may be modelled in terms of a change in effective refractive index (Δn eff ) as a function of temperature (T) and related to the thermo-optic coefficients of the various parts of the sensor component and their relative weighted contributions by the following formula: d(Δn eff )/dT(C) = A dn 1 /dT(C) + p 2 dn 2 /dT(C) (wherein: p 2 is the fraction of light in the sensing waveguide n 2 is the refractive index of the sensing waveguide dn 2 /dT is the thermo-optic coefficient of the sensing waveguide p x is the fraction of light contained in additional layers dn^/dT is the fixed thermo-optic coefficient of the additional layers) The thermo-optic coefficient of the sensing waveguide and the fraction of light therein vary with absolute concentration. Small temperature perturbations may be used to measure the response and compare with calibration data which reflects the relationship between thermo-optic coefficient/temperature and power distribution/temperature. The effect of a small temperature perturbation can be assumed to have no effect on the power distribution and it can be assumed that there is no thermodynamic redistribution of the stimulus in the sensing waveguide. This leads to a calibration curve for the sensor component as a function of concentration. By way of example, the response to a change in the refractive index An 2 of 10 "4 RIU (refractive index units) of the sensing waveguide may be measured. In a typical integrated optical sensor assembly, this would correspond to a measurable phase change of around 3 radians . This is within the 2π repeat period and would give an adequate measurement of the absolute phase position of the sensor assembly in terms of periods of 2π. The internal option for fine phase resolution may then be employed to provide fine tuning to higher resolutions. Example 2 Where the perturbation is a wavelength perturbation, the wavelength (λ) can substitute the temperature (T) in the above equation. When using dispersion effects, the wavelength of the laser source may be tuned over a small range around its centre wavelength. For example, in a sensor assembly operating nominally at 650nm, the sensor assembly given in example 1 would give measurable dispersion differences for Δn s = 10 "4 . This technique is analogous to techniques of wavelength modulation described inter alia in Williams et al in J. Appl. Phys., 60, (1986), 1900.

Claims: CLAIMS 1. A process for determining the absolute status of a sensor assembly which is capable of detecting the introduction of or changes in a stimulus of interest in a localised environment, (said assembly comprising: a sensor component including either (1) one or more sensing layers capable of inducing a measurable response to a change in the localised environment caused by the introduction of or changes in a stimulus of interest or (2) a sensing waveguide capable of exhibiting a measurable response to a change in the localised environment caused by the introduction of or changes in the stimulus of interest) , said process comprising: (A) irradiating the sensor component with electromagnetic radiation; (B) measuring first movements in the interference pattern; (C) biassing the sensor component by applying a perturbation; (D) measuring second movements in the interference pattern following said perturbation; and (E) calculating the absolute status of the sensor component. 2. A process as claimed in claim 1 wherein the absolute status is the absolute concentration. 3. A process as claimed in claim 1 or 2 wherein step (C) comprises thermally biassing the sensor component by applying a thermal perturbation. 4. A process as claimed in any preceding claim further comprising : deducing a measured thermo-optic response from the first and second movements in the interference pattern; and calculating the absolute concentration of the stimulus of interest from the measured thermo-optic response. 5. A process as claimed in claim 4 comprising: comparing the measured thermo-optic response with a reference thermo-optic response to calculate the absolute concentration. 6. A process as claimed in any preceding claim wherein steps (A) to (E) are carried out at start-up. 7. A process as claimed in claim 1 wherein step (C) comprises: effectively biassing the sensor component by applying a wavelength perturbation to determine the absolute status . 8. A process as claimed in claim 1 wherein step (C) comprises: effectively biassing the sensor component by applying a polarisation perturbation to determine the absolute status. 9. A process as claimed in any preceding claim further comprising: calculating a first phase shift from the first movements in the interference pattern and a second phase shift from the second movements in the interference pattern. 10. A sensor assembly as defined in any preceding claim adapted to determine the absolute status of the sensor component thereof . 11. A sensor assembly as claimed in claim 10 adapted to determine the absolute status of the sensor component at start-up. 12. A sensor assembly as claimed in claim 10 or 11 adapted to determine the absolute status of the sensor component automatically at start-up. 13. A sensor assembly as claimed in any of claims 10 to 12 further comprising: means for thermally perturbing the sensor component . 14. A sensor assembly as claimed in claim 13 wherein the means for thermally perturbing the sensor component is an active thermal assembly positioned in thermal contact with the sensor component . 15. A sensor assembly as claimed in claim 13 or 14 comprising a Peltier assembly. 16. A sensor assembly as claimed in claim 15 wherein the Peltier assembly is a Peltier mounted on a Peltier mount. 17. A sensor assembly as claimed in claim 16 wherein the Peltier mount has a concave underside to optimise thermal contact with the sensor component (or its base) . 18. A sensor assembly as claimed in claim 10 wherein the sensor assembly further comprises a source of electromagnetic radiation capable of irradiating the sensor component and means for perturbing the source of electromagnetic radiation. 19. A sensor assembly as claimed in any of claims 10 to 18 wherein each of the waveguides of the sensor component is a planar waveguide. 20. A sensor assembly as claimed in any of claims 10 to 19 wherein the sensor component constitutes a multi-layered structure.

Inventors: CROSS GRAHAM, CROSS, GRAHAM
Applicants: CROSS GRAHAM, FARFIELD SENSORS LTD, CROSS, GRAHAM, FARFIELD SENSORS LIMITED
Application year: 2000
Main classification: G01N21
IPC codes: G01N21, G01N25