Background to the Invention

[0001] This invention relates to a coating for an optical sensor, an optical sensor and a method of making the sensor.

[0002] A review of fibre-optic chemical sensors is given by Pospisilova, M.; Kuncova, G.; Trogl, J. "Fiber-optic chemical sensors and fiber-optic bio-sensors", Sensors 2015, 15, 25208-25259, doi: 10.3390/ S151025208. Because of their unique properties, such as small size, immunity to electromagnetic interference, and biocompatibility, they facilitate remote and real-time measurements with high sensitivity and selectivity. Most importantly, an optical fibre sensor (OFS) can be deployed in a harsh corrosive environment owing to the chemically inert silica substrate of the fibre. See Kharaz, A; Jones, B.E. "A distributed optical-fibre sensing system for multi-point humidity measurement." Sens. Actuators A Phys. 1995, 47, 491-493, doi.org/ 10.1016/ 0924-4247(94)00948-H.

[0003] Various optical fibre geometries are possible. US 7263246 Bl and US 8703505 B2 describe U-shaped fibres, which have several advantages. Firstly, their evanescent wave absorbance sensitivity is high, due to conversion of the lower- order modes into higher-order modes. Secondly, they are less fragile than other geometries. Thirdly, fabrication of the probes is easy and repeatable.

[0004] JP 5388309 B2 describes an optical fibre sensor bearing a composite thin film that comprises at least one layer of silica particles and at least one organic compound film. When an amine gas such as ammonia or pyridine is to be detected, the organic compound film is of polyacrylic acid or polyglutamic acid.

[0005] Korposh, S.; Kodaira, S.; Selyanchyn, R.; Ledezma, F.H.; James, S.W.; Lee, S.-W. "Porphyrin-nanoassembled fiber-optic gas sensor fabrication: Optimization of parameters for sensitive ammonia gas detection." Opt. Laser Technol. 2018, 101, 1-10, doi.org/ 10.1016/j.optlastec.2017.10.027 describes an ammonia gas sensor. Layers of poly(diallyldimethylammonium chloride) (PDDA) and tetrakis(4- sulfophenyl)porphine (TSPP) were deposited onto the surface of the core of a hard- clad multimode fibre that was stripped of its polymer cladding. The sensor was able to detect ammonia at a limit of 0.5 ppm. However, it was sensitive to humidity owing to the hydrophilicity of the porphyrin. Also, the immobilized polymer swelled.

Summary of the Invention

[0006] It is an aim of the present invention to provide an optical sensor for detecting ammonia and similar analytes, with improved stability.

[0007] From a first aspect, the present invention provides a coating for a transmissive optical component of an optical sensor, the coating comprising TSPP crosslinked with a diazo resin (DAR).

[0008] We have found that DAR improves the stability of the TSPP to a surprising extent.

[0009] The coating may comprise alternate layers respectively comprising DAR and TSPP.

[0010] For even greater stability, the coating may also include poly(styrene sulfonate) (PSS), and may comprise alternate layers of (a) diazo resin and (b) a mixture of TSPP and PSS.

[0011] The DAR may have the formula where R ¹ is hydrogen or an alkyl group and R ² is hydrogen, 2-SO3H or 3-OCH3. [0012] For example, the DAR may have the formula

[0013] From a second aspect, the invention provides an optical sensor for detecting an analyte that modifies the absorption spectrum of tetrakis(4- sulfophenyl)porphine (TSPP), the sensor comprising a transmissive optical component, a light source at an input side of the optical component and a detector for detecting light at at least one frequency at an output side of the optical component, and the optical component having a coating as described above.

[0014] For detecting ammonia dissolved in water, the sensor may include at least one gas permeable, waterproof membrane that shields the coating on the optical component. The membrane(s) may be of expanded polytetrafluorethylene. One embodiment includes two such membranes, in two respective planes with an operative part of the optical component therebetween.

[0015] In a specific embodiment, the transmissive optical component comprises an optical fibre, for example a U-shaped optical fibre. However, an alternative optical component, such as a prism or a planar waveguide, is within the scope of the invention.

[0016] The optical fibre may be housed in a holder for manual use, the operative part of the optical fibre extending from the holder. The holder can have an aperture into which the optical fibre extends. The membrane(s) can be lodged in a periphery of the aperture. [0017] In one example, the sensor includes a temperature sensor, and is arranged to compensate for a temperature-dependent variance in measurement of the analyte.

[0018] From a third aspect, the invention provides a method of making an optical sensor, comprising immersing at least a part of a transmissive optical component in TSPP and DAR, and crosslinking the TSPP and diazo resin to form a coating.

[0019] The immersion may comprise separate immersions into aqueous solutions of diazo resin and TSPP, for example repeated alternate immersions into the respective solutions. In an embodiment, the solution of TSPP also contains PSS.

[0020] The crosslinking may be performed with ultraviolet light.

[0021] The method may include a subsequent step of protecting that part of the component having the coating with at least one waterproof, gas-permeable membrane.

[0022] In an embodiment of the invention, the transmissive optical component comprises an optical fibre. The part that is immersed may be a U-shaped bend in the optical fibre and the method may comprise a preliminary step of forming the U-shaped bend in the optical fibre.

Brief Description of the Drawings

[0023] Embodiments of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0024] Figure 1 schematically shows a coating on an optical fibre according to the invention;

[0025] Figure 2 shows a sensor according to the invention, including internal components;

[0026] Figure 3 is a fragmentary view of a distal end of the sensor of Figure 2, including internal components; [0027] Figure 4 is a cut-away view of the distal end shown in Figure 3, including internal components;

[0028] Figure 5 is a simplified view of a printed circuit board of the sensor;

[0029] Figure 6 shows transmission spectra of light obtained when an optical fibre of a sensor according to the invention is used to detect ammonia in solution; and

[0030] Figure 7 shows normalised absorption spectra of light obtained when an optical fibre of a sensor according to the invention is used to detect ammonia in solution.

Detailed Description of Particular Embodiments

[0031] According to the invention, a coating on an optical component includes a DAR, for example synthesised as described by Zhao, C.; Chen, J.Y.; Cao, W.X. "Synthesis and characterization of diphenylamine diazonium salts and diazoresins." Angew. Makromol. Chem. 1998, 259, 77 -82.

[0032] In this particular example, in order to form a sensor according to the invention, a multimode optical fibre of a suitable length such as 40 cm can be used. A short section, e.g. 1 cm, of the plastic cladding of the optical fibre is removed, for example by burning. This section is then bent to a U-shape, for example using a burner. The exposed section of the silica core is prepared, for example by rinsing in ethanol and DI water several times and treating with a desiccant such as 1 wt% ethanolic KOH (ethanol/ water = 3:2, v/v). This forms a negatively charged evanescent region.

[0033] Next, an electrostatic film is deposited by alternately immersing the negatively charged activated core of the optical fibre into an aqueous solution of a positively charged DAR (e.g. 1 wt%) and a mixture of TSPP (e.g. 1 mM in H2O) and optionally PSS (e.g. 0, 0.025, and 0.1 wt% in H2O), in this example for 15 min each. Each cycle results in a DAR/ TSPP or DAR/TSPP+PSS bilayer. Between the immersions, the optical fibre can be washed with water and dried by flushing with nitrogen gas. Multilayer assemblies can be formed by cyclic repetition of these two steps. Figure 1 shows just 3.5 bilayers comprising DAR 100 and TSPP+PSS 102. However, 5, 8.5 or more bilayers can be formed. The films are deposited in the dark to avoid the decomposition of the DAR.

[0034] The deposited bilayers are cured by exposing the optical fibre to UV light. In one example, the wavelength did not exceed 365 nm and the exposure was at a distance of 10 cm for 15 min. Consequently, the ionic bonds be-tween the diazonium cationic ions in DAR and the sulfonate anionic residues in TSPP [and PSS] are converted into crosslinked covalent bonds by UV light irradiation.

[0035] Figures 2 to 5 show a sensor incorporating an optical fibre prepared as described above. This sensor is suitable for handheld use. However, sensors of different geometries and sizes can be made according to the invention, with an optical fibre of any reasonable length.

[0036] The first and second clad parts 10, 12 of the optical fibre are partly contained within a handle section 14 of the sensor, and extend through a neck 15, with the exposed section 16 of the optical fibre seated in a head 18 of the sensor.

[0037] In the head 18, the U-shaped exposed section 16 extends into a window 20, which in this example is circular. The window is defined by first and second frames 22, 24 on opposite sides of the head 18. First and second gas-permeable, waterproof membranes 26, 28 are lodged in the respective frames 22, 24 to prevent water from entering the window 20 and contacting the DAR/TSPP+PSS. The membranes can be of expanded polytetrafluorethylene, e.g. Gore-Tex ®.

[0038] Figure 5 shows a PCB 2 is housed in a base of the sensor, omitting components not discussed here. The PCB is fitted with a cluster 4 of LEDs of different specific wavelengths, all coupled to the first clad part 10, and at least one photodetector 6 coupled to the second clad part 12. Each LED can be operated individually to transmit its light to the optical fibre via the first clad part 10 to the exposed section 16. The PCB may also have a microcontroller arranged to activate each of the LEDs and record the activity of the photodetector(s).

[0039] A temperature sensor 29 such as a thermocouple can be included in the sensor to measure temperature at the head. The connection of the temperature sensor to the PCB is item 30 in Figure 2.

[0040] When ammonia contacts the exposed section, the TSPP attenuates the light in the optical fibre at certain frequencies. Figure 6 shows transmission spectra at different concentrations of ammonia, and Figure 7 shows normalised absorption spectra (i.e. with the spectrum of the sensor itself subtracted).

[0041] These spectra show (a) the frequencies at which the effect of the ammonia is pronounced, and (b) those at which there is essentially no effect. It is possible to produce a sensor according to the invention with a single LED at one of the (a) frequencies and calibrate the sensor with known ammonia concentrations. However, for repeatability, and in view of the wavelength spread and manufacturing tolerance of the LEDs, the PCB can include LEDs emitting at more than one of the (a) frequencies. It can also include one or more LEDs emitting at at least one of the (b) frequencies for use as a reference.

[0042] To measure the concentration of ammonia in a solution, such as of seawater, the head of the sensor is immersed in the solution, so that gaseous ammonia enters the window. To measure the concentration of ammonia e.g. in an atmosphere, the head of the sensor is exposed to that atmosphere. Thus, in a sensor for use purely in dry, gaseous environments, the waterproof membranes can be omitted.

[0043] The LEDs are operated in sequence and the response at the photodetector is monitored for each LED. The microcontroller, or a connected computer, then calculates the concentration of ammonia and corrects for temperature changes sensed by the temperature sensor. The sensor can be provided with its own miniature display to show the result.