OPTICAL SENSOR

Abstract

The present invention provides a component for use in an optical sensor, said component comprising a substrate, a surface of the substrate being coated with a layer of a composition comprising: (i) carbon nano-tubes; (ii) an optically-active substance and (iii) a matrix material.

Claims

1. A component for use in an optical sensor, said component comprising a substrate, a surface of the substrate being coated with a layer of a composition comprising: (i) carbon nano-tubes; (ii) an optically-active substance and (iii) a matrix material.

2. The component of claim 1 wherein said optically-active substance is a fluorophore.

3. The component of claim 1 or claim 2 wherein said optically-active substance is Platinum-Octaethyl-Porphyrin.

4. The component of any preceding claim wherein said matrix material is a polymer.

5. The component of any preceding claim wherein said composition further comprises a solvent.

6. The component of any preceding claim wherein the weight ratio of carbon nano-tubes to matrix material is 1:50 to 1:1.

7. The component of any preceding claim wherein said substrate is an optical fibre.

8. The component of any preceding claim wherein said substrate is a polymeric optical fibre.

9. The component of any preceding claim wherein said component is a sensor tip, a probe, or part thereof.

10. A composition as described in any one of claims 1 to 6.

11. A coated substrate as described in any one of claims 1 to 8.

12. A method for measuring a parameter or the concentration of an analyte in a sample, said method comprising using a composition, substrate or component as claimed in any one of the preceding claims.

13. The method of claim 12 comprising applying a component as claimed in any one of claims 1 to 9 to a sample, supplying light to the optically-active substrate via the substrate, measuring the optical output of the optically-active substance and using the result to calculate the parameter or the concentration of the analyte.

14. The method of claim 12 or claim 13 wherein the analyte is oxygen.

15. The method of any one of claims 12 to 14 wherein said sample is blood.

Description

[0069] The invention will now be described in more detail with reference to the accompanying figures, in which:

[0070] FIG. 1A shows a schematic of a measurement apparatus, focussing on the sensor tip.

[0071] FIG. 1B is a schematic illustration of the experimental set-up for evaluating sensor sensitivity in the gas phase.

[0072] FIG. 2 shows the variations of luminescent life time as a function of oxygen concentration for two fibre optic oxygen sensors with different sensing matrices.

[0073] FIG. 3 shows Stern-Volmer plots (representing sensitivity of the sensors) for two fibre optic oxygen sensors with different sensing matrices.

[0074] FIG. 4 shows the signal responses from a piezo pressure sensor and the fibre optic sensors to the change in pressure of a test chamber from 19 kPa to 100 kPa and the pO.sub.2 step change from 3 kPa to 21 kPa in the test chamber.

[0075] FIG. 5 shows the responses of a piezo pressure sensor and the fibre optic sensors to the change in pressure from 100 kPa to 19 kPa and the pO.sub.2 step change from 21 kPa to 3 kPa in the test chamber.

[0076] FIG. 6A-L are sensor sensitivity results.

[0077] FIG. 7A-D are response time results.

[0078] As noted above, FIG. 1A shows an example of a typical measurement set-up according to the present invention. The sensing layer, comprising matrix material, CNTs and optically-active substance, is applied to the substrate by any suitable means. For an optical fibre, the area to be coated may be decladded using standard techniques if desired, e.g. to remove any coating or to prepare the surface such that it has a stronger bond with the sensing layer to be added. For example, FIG. 1A shows a PMMA based polymer (matrix material) sensing film (about 2 m thickness), containing CNTs and an optically-active substrate (PtOEP). FIG. 1A shows the sensing film, i.e. layer of the composition of the present invention coated at the decladded end section of an optical fibre to form a sensor head or tip. Typically such coatings may be formed by dipping the fibre into a solution of polymer, luminophore and CNTs in a solvent such as dichloromethane.

[0079] Prior to the dipping and coating process, the substrate is optically decladded, cleaned and/or dried. The section of the substrate to be coated is then dipped into the luminophore-doped polymer solution and withdrawn from it sufficiently slowly for some of the solution to adhere to the substrate surface. As the solvent evaporates, a sensing film is formed on the substrate. The coated substrate may be dried for some time prior to its use in a measurement method.

[0080] The sensing layer of composition as herein described should be conveniently placed on the substrate such that it is accessible to the sample in which the analyte, e.g. oxygen, is to be measured. For applications where the sensor is being used as a probe, e.g. for dipping into water or blood samples, or intravenous measurements, the layer will be on or near the part of the substrate that is furthest from the excitation light source, e.g. towards the tip of an optical fibre.

[0081] The compositions of the present invention are advantageously applicable to known optical sensors (e.g. by replacing the existing sensing matrix with the layer of the invention) and thus, for measurement of analytes in, e.g., blood (ex vivo and in vivo), breath, industrial fluids and environmental applications, standard procedures and apparatus known in the relevant field can be applied. Due to the improved time response, sensors comprising the compositions of the present invention may be inserted into the blood vessel of a patient for real time in vivo measurements. For intravenous applications, the sensor can be attached to the measurement apparatus using standard connectors.

[0082] When the substrate coated with the composition of the invention, e.g. the optical fibre of FIG. 1A is in contact with the sample, e.g. a blood sample, analyte concentration can be measured using standard optical techniques based on luminescence. In the measurement method, a pulse of excitation light, typically from a LED is transmitted through the substrate, e.g. along the optical fibre to the part which is coated with the composition and in contact with the sample. The light excites the luminophore, thus causing it to luminesce. The emission of luminescent light travels back up the fibre and is detected by a detector. The presence of the analyte alters the emission in a way that enables the concentration of analyte to be determined. For example, for oxygen measurement, the emission is quenched in the presence of oxygen molecules such that the lifetime and intensity of the emitted light are inversely proportional to the concentration of gaseous or dissolved oxygen. Using the Stern-Volmer relation, the concentration of oxygen in the sample may be determined.

[0083] A measurement apparatus thus typically comprises, in addition to the component or coated substrate of the invention, an excitation light source, e.g. an LED and a detector. In order that the light may pass from the LED to the sensor tip and back to the detector, these may be linked using a Y-type optical fibre coupler such as that shown in FIGS. 1A and 1B. Suitable detectors include fluorescent lifetime measurement systems such as those obtainable from Neofox Ocean Optics.

[0084] Measurements may conveniently be carried out at room temperature and atmospheric pressure, e.g. for clinical applications, but the invention is also applicable to non-ambient conditions, such as those encountered in industrial settings.

[0085] The oxygen concentration result is typically expressed as pO.sub.2 (e.g. in sensor time response tests in a pressure change chamber).

[0086] The present invention is further illustrated by the following Examples, in which parts and percentages are by weight and degrees are Celsius, unless otherwise stated. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, various modifications of the invention in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

EXAMPLE 1

[0087] The influence of adding CNTs into polymer material on the permeability of the polymer and the time response of the oxygen sensors was investigated by using CNTs as nano-filler to form a nano-composite in the oxygen sensing matrix. The performance of the CNT based nano-composite was evaluated by testing the sensitivity and time response of the fibre optic oxygen sensors.

[0088] A PMMA based polymer sensing film (about 2 m thickness), which contained CNTs and PtOEP, was coated at the decladded end section of a silica fibre to form a sensor head. This structure enhances a high luminescence excitation and produces a strong luminescent emission. The luminescent light emitted from the luminophore was collected by the same fibre and its lifetime was measured using a phase measurement system (NeoFox, Ocean optics) in the experimental set-up (see FIG. 1A).

[0089] Poly(methyl methacrylate) (PMMA), single-walled CNTs and dichloromethane (as solvent) were obtained from Sigma-Aldrich (USA). The luminophore, Platinum-Octaethyl-Porphyrin (PtOEP), was purchased from Porphyrin (USA). For the preparation of the nano-composite sensing films, two luminophore-doped polymer solutions were made firstly by mixing and dissolving 0.5 mg PtOEP and 50 mg PMMA material in 1 ml dichloromethane, then 10 mg of sonicated CNTs were added to one of polymer solutions to form a CNT-based nano-composite polymer solution. The remaining solution did not contain CNTs. The two polymer solutions were then capped and stirred to ensure complete dissolution of the polymer and the luminophore. Prior to the dipping and form coating process, the 10 mm end section of a pigtailed silica optical fibre (2 m length and 200 m core diameter) was decladded and cleaned with IPA (iso-propyl alcohol), and dried for 10 minutes. The decladded section of the optical fibre was dipped into the luminophore-doped polymer solutions and then withdrawn from it slowly. An oxygen sensing film was thus formed on the end section of the optical fibre as the solvent quickly evaporated. After coating, the optical fibre was dried for at least 24 hours. The process was carried out at room temperature. In this way a total of six fibre optic oxygen sensors were made. Three had a PMMA polymer sensing film (without CNTs), and the other three had a CNT-based nano-composite sensing film. The sensitivities and time responses of the sensors were evaluated using the method of luminescence life-time measurement.

[0090] FIG. 1B shows a schematic illustration of the experimental set-up for evaluating sensor sensitivity in the gas phase. During the experiment, nitrogen and oxygen gas from cylinders were mixed and the mixing ratio was controlled using a precision gas mixing pump (Wostoff, Germany) before flowing into the gas testing chamber. The oxygen concentration was also measured using an oxygen analyzer (Servomex OA570).

[0091] During the experiment, the fibre optic oxygen sensor was inserted into the testing chamber and the lifetime of the luminescent light from the sensor head was measured using phase measurement system (NeoFox, Ocean optics). Luminescence excitation of the sensor was provided by a LED (LS-450, Ocean Optics) with a central wavelength of 450 nm attached to a fluorescent lifetime measurement system (Neofox Ocean Optics). The fibre optic oxygen sensing system consisted of a fibre optic oxygen sensor (inserted in the testing chamber) and a Y-type optical fibre coupler. The excitation light from the LED was fed to the sensor, and the emitted luminescent light from the sensor was transmitted to the lifetime measurement system the fibre coupler. The lifetime of the luminescence was measured by the system. The experiment was carried out at room temperature and one atmosphere pressure.

[0092] Sensor Response Time

[0093] The gas pressure chamber system used in Chen et al., Sensors and Actuators B 222, 531-535, 2016 was used. The test chamber was connected to a second buffer chamber that was continuously evacuated by a vacuum pump (MZ 2C NT, Vacuubrand GMBH, Wertheim, Germany). The chamber had a controlled leak to atmosphere so that by switching the chamber to the buffer chamber or to atmosphere pO.sub.2 in the test chamber could be switched swiftly between any pre-set level between 21 and 0 kPa (vacuum). The dynamic change in total pressure was measured by a Honeywell piezo resistive pressure sensor with a time response of 1 ms (RS Components Ltd, UK). The response time of the oxygen sensor was tested in response to the above near step changes in pO.sub.2.

[0094] Sensor Sensitivity Evaluation

[0095] FIG. 2 shows the variations of luminescent lifetime as the function of oxygen concentration for the sensors with two different sensing matrices. FIG. 3 shows the Stern-Volmer plots (represented sensitivity of the sensors) of the sensors at room temperature setting, which revealed that the sensors all yield linear Stern-Volmer plots and the sensitivities (0/1001, where 0 and 100 are the excited state luminescence lifetimes in the absence and presence of oxygen respectively) of the sensors are around 1.75, which correlated to the oxygen concentration changing from 0% to 100%.

[0096] Response Time Evaluation

[0097] FIG. 4 shows the signal responses from the piezo pressure sensor and the fibre optic oxygen sensors to the change in pressure of test chamber from 19 kPa to 100 kPa and the pO.sub.2 change from about 3 kPa to 21 kPa in the test chamber; FIG. 5 shows the responses of the piezo pressure sensor and the fibre optic oxygen sensors to the change in pressure from 100 kPa to 19 kPa and the change of pO.sub.2 from 21 kPa to about 3 kPa in the test chamber.

[0098] The results showed that the response of the sensor was improved by adding CNTs into the polymer sensing matrix and the sensitivity was kept unchanged, which indicates that the CNTs only affect oxygen diffusivity and not solubility of the sensing matrix.

[0099] This experiment therefore demonstrates the feasibility of optimizing the time response of silica fibre optic oxygen sensors by using a CNT based nano-composite sensing matrix. The maximum sensitivity factor of the sensors (0/1) was approximately 1.75 with a faster response time than that with pure polymer sensing matrix. The CNT nano-composite allows for a thicker matrix film thickness to be used which provides a greater signal to noise ratio and is more physically robust, but without compromising the response time of the sensor.

EXAMPLE 2

Sensor Sensitivity Evaluation

[0100] Example 1 was repeated (without CNTs), with sensor sensitivity results similar to FIGS. 2 and 3 shown in FIGS. 6A to 6L.

TABLE-US-00001 FIG. PROBE CODE 6A O-AK001B-1 6B X-AA001B-1 6C X-AE001B-1 6D X-AC001B-1 6E Y-AG001B-1 6F Z-AO001B-1 6G O-AL001B-2 6H O-AN001B-2 6I Y-AB001B-2 6J Y-AF001B-2 6K Z-AD001B-2 6L Z-AP001B-2

EXAMPLE 3

Response Time Evaluation

[0101] Example 1 was repeated, with response time results similar to FIGS. 4 and 5 shown in FIGS. 7A-7D. FIGS. 7A (Sensor 1, PMMA only) and 7C (Sensor 3, PMMA only) show results for sensors which contained PMMA, but no CNTs. FIGS. 7B (Sensor 2, PMMA+CNTs) and 7D (Sensor 4, PMMA+CNTs) show results for sensors which contained PMMA and CNTs.

EXAMPLE 4

PEMA Matrix

[0102] A nanocomposite sensing matrix with PEMA as the polymer was used to fabricate fibre optic oxygen sensors and the time response of the sensors were evaluated in gas phase test chamber.

[0103] A schematic diagram of the gas phase test system is shown in FIG. 8A. A vacuum pump was used to extract air from the test chamber and an electrical switch valve was used to make a pressure stepping change in test chamber. The oxygen partial pressure (pO.sub.2) in the test chamber was changed with the total pressure step change in the chamber. Two polymer probes (one with PEMA matrix and one with PEMA+CNT matrix) were inserted into the chamber and tested separately. Data was recorded by using Ocean Optics-NewFox with 100 ms sample rate, the modulation frequency was set at 1.46 kHz for the tests.

[0104] During the experiments the total pressure in the chamber was changed several times and FIG. 8B shows the pO.sub.2 level changes with total pressure change in the test chamber measured by sensors. FIG. 8C shows the comparison result between the two probes while pressure step increasing. FIG. 8D shows the comparison result between the two probes while pressure step decreasing. The time response of the optical oxygen sensor has thus been shown to be improved by using poly ethyl methacrylate (PEMA) based nanocomposite sensing materials comprising CNTs.