Formic acid concentration detection optical fiber sensor and detection method

Abstract

The invention discloses a formic acid concentration detection optical fiber sensor and a detection method. The sensor comprises a formic acid detection optical fiber, a coupler coupled with the formic acid detection optical fiber, and a formic acid sensing unit arranged in the coupler, the formic acid sensitive unit comprises a light reflecting mirror directly facing the detection end of the formic acid detection optical fiber during use and a formic acid sensitive film arranged on the surface of the reflecting surface of the light reflecting mirror; and the formic acid sensitive film is an MOF-802 / Ph-g-C3N4 / polyurethane film which is prepared from an MOF-802 compound, Ph-g-C3N4 and polyurethane. After the formic acid sensitive film absorbs formic acid, the refractive index is increased, the absorption intensity of light with the wavelength of 292 nm is enhanced, the reflectivity is reduced, the intensity of light received by the conical receiving optical fiber is reduced, and the concentration of formic acid in the environment can be accurately measured by measuring the relative variable of the output light intensity of the formic acid detection optical fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of formic acid concentration detection, and particularly to an optical fiber sensor and a detection method for formic acid concentration detection. Background Art

[0002] Formic acid (HCOOH), as an important organic compound, has wide applications in fields such as chemical industry, pharmaceuticals, food processing, leather, textile printing and dyeing, etc. At the same time, formic acid has great prospects as a hydrogen storage material, which also increases the necessity of its use. However, when the concentration of formic acid is too high and exceeds the safety threshold, it will corrode production equipment and cause irritation or even poisoning to the eyes, skin, respiratory system, digestive system, etc. of the human body. Formic acid is an organic acidic gas. In the study of the atmospheric environment, it may not be regarded as a major pollutant due to its relatively low toxicity and concentration. However, in the closed microenvironment of cultural relic display and storage, low-concentration formic acid will accumulate over time and be adsorbed by cultural relics, causing damage to the cultural relics.

[0003] Cultural relics in museums are non-renewable precious cultural resources. In recent years, museums at home and abroad have found that organic acids can cause phenomena such as weathering on the surfaces of copper alloys and lead objects, corrosion and embrittlement of cultural relics such as enamel and pottery, fading of pigments and dyes, and deterioration of textile fibers. Therefore, in order to effectively prevent the damage caused by corrosive acidic gases such as formic acid in the museum collection environment to the cultural relics in the collection, it is extremely important to detect the formic acid concentration information online and in-situ.

[0004] Currently, the measurement methods for formic acid concentration mainly include offline methods and online measurement methods. Among them, the offline measurement methods mainly include titration, gas chromatography, mass spectrometry, chromatography-mass spectrometry coupling method, etc. Although the offline measurement method can accurately identify the formic acid components and content information, the measurement process will disrupt the stable state of the environment, the measurement and analysis process is complex and time-consuming, and the cost of instrument equipment is high, so it is impossible to realize the online in-situ detection of formic acid concentration in the environment. The online measurement methods include sensing technologies such as electrochemical sensing, fluorescence spectroscopy, and optical fiber sensing. Although technologies such as electrochemical sensing and fluorescence spectroscopy can realize the online in-situ measurement of formic acid concentration in the environment, the service life of electrochemical sensors is short and they are easily affected by electromagnetic interference; for the fluorescence spectroscopy technology, the selectivity is relatively low, and it is difficult to accurately distinguish formic acid in a complex environment.

[0005] However, fiber optic sensing technology can be used for in-situ measurement of target gases in harsh and small spaces with toxic and harmful gases or strong electromagnetic interference environments. At the same time, the sensing unit has a simple structure, low fiber transmission loss and can be transmitted over long distances, making it easy to achieve long-distance on-site stable and reliable online measurements. Due to the outstanding advantages of fiber optic sensing technology, current research on fiber optic gas sensors has received widespread attention and has been applied to the detection of various gases such as carbon dioxide, hydrogen, formaldehyde, hydrogen sulfide, and methane. However, there are no literature reports or commercial products on fiber optic sensors for formic acid concentration detection. Therefore, it is very necessary to invent a fiber optic sensor for in-situ online detection of formic acid in the environment. Summary of the invention

[0006] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is: how to provide a formic acid concentration detection optical fiber sensor, when formic acid in the external environment is transmitted to the formic acid sensitive membrane (MOF-802 / Ph-gC 3 N 4 / polyurethane film), the refractive index of the formic acid sensitive film increases after absorbing formic acid, the absorption intensity of light with a wavelength of 292nm is enhanced, and the reflectivity is reduced, causing the light intensity received by the tapered receiving optical fiber to decrease, making it possible to accurately measure the formic acid concentration in the environment by measuring the relative variable of the output light intensity of the formic acid detection optical fiber.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The formic acid concentration detection optical fiber sensor comprises: a formic acid detection optical fiber, a coupler coupled to the formic acid detection optical fiber, and a formic acid sensitive unit arranged inside the coupler;

[0009] The formic acid sensitive unit includes a light reflector facing the detection end of the formic acid detection optical fiber when in use and a formic acid sensitive film arranged on the reflection surface of the light reflector; the formic acid sensitive film is formed by MOF-802 compound, Ph-gC 3 N 4 MOF-802 / Ph-gC composed of polyurethane 3 N 4 / polyurethane film, of which Ph-gC 3 N 4 It is a graphite carbon nitride material modified by phenyl functionalization.

[0010] Preferably, the formic acid sensitive membrane is prepared by the following steps:

[0011] 1) dissolving zirconium oxychloride octahydrate and 1h-pyrazole-3,5-dicarboxylic acid in a mixed solvent of DMF and formic acid, stirring to completely dissolve them, and transferring the mixed solution into a polytetrafluoroethylene-lined autoclave;

[0012] 2) heating the polytetrafluoroethylene-lined autoclave, cooling it to room temperature, and then taking it out; washing it with ethanol and drying it, and grinding the obtained MOF-802 into powder for later use;

[0013] 3) Mixing cyanuric acid and 2,4-diamino-6-phenyl-1,3,5-triazine in the presence of ethanol and grinding, transferring the obtained powder to a crucible, and heating it in a tube furnace; cooling to room temperature after heating, and ultrasonically treating the obtained material in deionized water; centrifuging the colloidal suspension solution obtained by ultrasonic treatment to remove large particles, and then discharging the aqueous solution, placing it in a vacuum drying oven, and drying it to obtain dry Ph-gC 3 N 4 , the obtained Ph-gC 3 N 4 Grind into powder and set aside;

[0014] 4) Add MOF-802 powder and Ph-gC to polyurethane 3 N 4 The powder was stirred to obtain MOF-802 / Ph-gC 3 N 4 / Polyurethane sol, i.e. formic acid selective sensitive sol;

[0015] 5) The formic acid selective sensitive sol is coated on the surface of the light reflecting lens by a spin coating method and dried to obtain a formic acid sensitive film.

[0016] 3. The formic acid concentration detection optical fiber sensor as claimed in claim 2 is characterized in that before coating the formic acid selective sensitive sol, the light reflecting lens is first cleaned with alcohol and deionized water, and then the cleaned light reflecting lens is blown dry with nitrogen.

[0017] Preferably, the formic acid detection optical fiber includes an incident optical fiber, a plurality of tapered receiving optical fibers and an optical fiber fixture; the detection ends of the incident optical fiber and the plurality of tapered receiving optical fibers are fixed together by the optical fiber fixture and the plurality of tapered receiving optical fibers are distributed around the periphery of the incident optical fiber;

[0018] The detection ends of the incident optical fiber and a plurality of tapered receiving optical fibers are the detection ends of the formic acid detection optical fibers.

[0019] Preferably, the detection end of the tapered receiving optical fiber is tapered by a fusion taper method to form a truncated cone-shaped structure with a taper angle.

[0020] Preferably, the coupler is a hollow Teflon tube with an opening at one end and a seal at the other end;

[0021] The formic acid detection optical fiber is inserted into the opening of the Teflon tube through an optical fiber fixture to achieve coupling with the coupler, and the incident optical fiber of the formic acid detection optical fiber and the detection ends of several tapered receiving optical fibers face the inside of the Teflon tube.

[0022] Preferably, an opening area is provided on the side wall of the Teflon tube to connect the interior of the Teflon tube with the outside, so that the formic acid-containing air can enter the interior of the Teflon tube through the opening area.

[0023] A formic acid concentration detection method based on an optical fiber sensor is implemented based on the formic acid concentration detection optical fiber sensor of the present invention, comprising:

[0024] S1: The incident optical fiber of the optical fiber sensor transmits the light beam emitted by the light source;

[0025] S2: The detection end of the incident optical fiber of the optical fiber sensor radiates the light beam into the interior of the Teflon tube containing formic acid air, and the light beam is transmitted to the formic acid sensitive unit after passing through the formic acid air;

[0026] S3: The detection end of each tapered receiving optical fiber of the optical fiber sensor receives the light beam reflected by the formic acid sensitive unit; the output light intensity of the optical fiber sensor is calculated based on the light intensity output by each tapered receiving optical fiber;

[0027] S4: Calculate the corresponding output signal based on the output light intensity of the optical fiber sensor;

[0028] S5: Calculating the formic acid concentration of the formic acid-containing air by using the output signal of the optical fiber sensor and a predetermined calibration relationship between the output signal and the formic acid concentration.

[0029] Preferably, in step S3, the output light intensity of the optical fiber sensor is calculated by the following steps:

[0030] S301: Define the light intensity of the incident optical fiber radiation as I in ;

[0031] S302: Light intensity is I in The light beam passes through the formic acid-containing air and is transmitted to the surface of the formic acid sensitive film, and then refracted into the formic acid sensitive film. The incident light intensity on the formic acid sensitive film surface is I 1 ;

[0032] The formula is:

[0033] I 1 =I in exp(-σ 1 Cd / cosθ);

[0034] Where: 1 is the intrinsic absorption coefficient of formic acid to a light beam under a certain wavelength condition; d is the distance between the end face of the incident optical fiber and the surface of the formic acid sensitive film; θ is the incident angle of the light beam on the interface between the formic acid sensitive film and the air containing formic acid; C represents the formic acid concentration;

[0035] S303: Light intensity is I1 The light beam is refracted on the surface of the formic acid sensitive film and enters the formic acid sensitive film. Then, the light beam is reflected at the interface between the formic acid sensitive film and the light reflector and enters the formic acid sensitive film again. Then, the light beam is refracted at the interface between the formic acid sensitive film and the air and enters the air medium containing formic acid again. The light intensity of the light beam after attenuation by the formic acid sensitive film is I 2 ;

[0036] The formula is:

[0037] I 2 =I 1 exp(-σ 2 2l / cosθ 1 );

[0038] σ 2 =2πε / (λn 2 );

[0039] Where: 2 is the attenuation coefficient of the formic acid sensitive film to the light beam; l is the film thickness of the formic acid sensitive film; ε is the imaginary part of the complex dielectric constant of the formic acid sensitive film; λ is the wavelength of the light beam; n 2 is the refractive index of formic acid sensitive film; θ 1 is the refraction angle of the light beam;

[0040] S304: Light intensity is I 2 The light beam passes through the formic acid-containing air medium and reaches the detection end of the tapered receiving optical fiber. The light intensity output by the tapered receiving optical fiber is I 3 ;

[0041] The formula is:

[0042] I 3 =I 2 exp(-σ 1 Cd / cosθ);

[0043] S305: Based on the light intensity I output by each tapered receiving optical fiber 3 Calculate the output light intensity I of the optical fiber sensor out ;

[0044] The formula is:

[0045]

[0046] in:

[0047]

[0048] I out =k 1 I in exp(-σ 1Cd / cosθ)exp(-σ 2 2l / cosθ 1 )exp(-σ 1 Cd / cosθ);

[0049] Where: r 1 r is the waist radius of the conical receiving optical fiber detection end; 2 is the end face radius of the conical receiving optical fiber detection end; β is the cone angle of the conical receiving optical fiber detection end; k 1 is a constant; n represents the number of receiving optical fibers.

[0050] Preferably, in step S4, the output light intensity I′ is calculated when the formic acid sensitive unit of the optical fiber sensor does not contain formic acid sensitive material. out ; Based on the output light intensity I out And the output light intensity I′ out Calculate the output signal K of the optical fiber sensor;

[0051] The formula is:

[0052] I′ out =k 1 I in exp(-σ 1 C2d / cosθ);

[0053]

[0054] Compared with the prior art, the formic acid concentration detection optical fiber sensor of the present invention has the following beneficial effects:

[0055] The present invention designs an optical fiber sensor for formic acid concentration detection, comprising a formic acid detection optical fiber, a formic acid sensitive unit and a coupler. The formic acid sensitive unit comprises a light reflector and a formic acid sensitive film, wherein the formic acid sensitive film is a formic acid sensitive material coated on the light reflector, wherein the formic acid sensitive material is MOF-802 / Ph-gC having a selective adsorption function for formic acid. 3 N 4 / Polyurethane composite material composition: MOF-802 has the function of selective adsorption of formic acid, and can also improve the adsorption capacity of the material for formic acid, increase the sensor's measurement range for formic acid and the sensitivity of the sensor; Ph-gC 3 N 4 It is used to enhance the selective adsorption capacity of formic acid, and can effectively eliminate the interference of other gases on the formic acid concentration detection results; polyurethane is used to enhance the MOF-802 / Ph-gC 3 N 4 / The adhesion strength between the polyurethane film and the light reflective lens and the mechanical strength of the film improve the repeatability and service life of the sensor.

[0056] When the optical fiber sensor is working, the intensity of light radiated into the coupler by formic acid detection optical fiber. When formic acid in the external environment is transmitted to the formic acid sensitive membrane (MOF-802 / Ph-gC 3 N 4 / polyurethane film), the refractive index of the formic acid sensitive film increases after absorbing formic acid, the absorption intensity of light with a wavelength of 292nm is enhanced, and the reflectivity is reduced, causing the light intensity received by the tapered receiving optical fiber to decrease, making it possible to accurately measure the formic acid concentration in the environment by measuring the relative variable of the output light intensity of the formic acid detection optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0058] Figure 1 Schematic diagram of the structure of the optical fiber sensor for detecting formic acid concentration: Figure 1 (a) is a schematic diagram of the sensor structure in which the formic acid detection optical fiber is not coupled to the coupler; Figure 1 (b) is a schematic diagram of the sensor structure after the formic acid detection optical fiber is coupled to the coupler.

[0059] Figure 2 This is a schematic diagram of the structure of each component of the Y-type optical fiber: Figure 2 (a) is a schematic diagram of the incident optical fiber end face structure. Figure 2 (b) is a schematic diagram of the structure of the incident optical fiber end. Figure 2 (c) is a schematic diagram of the coupling structure of the incident optical fiber, the tapered receiving optical fiber and the optical fiber holder. Figure 2 (d) is a schematic diagram of the coupling end face structure of the incident optical fiber, the tapered receiving optical fiber and the optical fiber holder. Figure 2 (e) is a schematic diagram of the optical fiber structure for formic acid detection.

[0060] Figure 3 Schematic diagram of the formic acid sensitive unit structure.

[0061] Figure 4 Schematic diagram of the optical transmission path of the fiber optic sensor.

[0062] Figure 5 The response characteristics of the sensor to formic acid concentration are: Figure 5 (a) is the sensor output spectrum, Figure 5 (b) is the sensor output signal K.

[0063] Figure 6 The selectivity of the optical fiber sensor for formic acid.

[0064] The figure marks in the drawings of the specification include: incident optical fiber 1, tapered receiving optical fiber 2, optical fiber protective cover 3, SMA optical fiber connector 4, optical fiber collimator 5, optical fiber holder 6, UV shadowless glue 61, external thread 7, coupler 8, internal thread 9, opening area 10, formic acid sensitive film 11, light reflector 12. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0066] It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. In the description of the present invention, it should be noted that the orientation or position relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inside", "outside", etc. is based on the orientation or position relationship shown in the drawings, or the orientation or position relationship in which the invention product is usually placed when used, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance. In addition, the terms "horizontal", "vertical", etc. do not mean that the components are required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted. In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0067] The following is a further detailed description through specific implementation methods:

[0068] Embodiment 1:

[0069] This embodiment discloses a formic acid concentration detection optical fiber sensor.

[0070] like Figure 1 As shown, a formic acid concentration detection optical fiber sensor includes: a formic acid detection optical fiber, a coupler coupled to the formic acid detection optical fiber, and a formic acid sensitive unit arranged inside the coupler;

[0071] The formic acid sensitive unit comprises a light reflector facing the detection end of the formic acid detection optical fiber when in use and a formic acid sensitive membrane arranged on the reflective surface of the light reflector; the formic acid sensitive membrane is a MOF-802 compound having a formic acid selective adsorption function and improving the material's adsorption capacity for formic acid, a Ph-gC for enhancing the formic acid selective adsorption capacity, and a 3 N 4 MOF-802 / Ph-gC composed of polyurethane for enhancing the adhesion strength between the formic acid sensitive film and the light reflective lens and the mechanical strength of the formic acid sensitive film 3 N 4 / Polyurethane membrane.

[0072] in:

[0073] MOF-802 compound is a metal-organic framework material formed by the coordination of carboxylic acid ligands and Zr(IV) ion clusters.

[0074] Ph-gC 3 N 4 It is a graphite carbon nitride material modified by phenyl functionalization, and is a functional material prepared by introducing phenyl into the graphite carbon nitride structure.

[0075] Polyurethane is a polymer compound made by the reaction of isocyanate and polyol.

[0076] The formic acid detection optical fiber (Y-shaped optical fiber) includes an incident optical fiber 1, a plurality of tapered receiving optical fibers 2 and an optical fiber holder 6; the first ends (i.e., detection ends) of the incident optical fiber 1 and the plurality of tapered receiving optical fibers 2 are fixed together by the optical fiber holder 6 and the plurality of tapered receiving optical fibers 2 are distributed around the incident optical fiber 1, the second ends of the incident optical fiber 1 and the plurality of tapered receiving optical fibers 2 are separated and the second ends of the plurality of tapered receiving optical fibers 2 are fixed together;

[0077] The first ends of the incident optical fiber and the plurality of tapered receiving optical fibers are detection ends of the formic acid detection optical fibers.

[0078] In this embodiment, one incident optical fiber and six tapered receiving optical fibers are included.

[0079] The first end of the tapered receiving optical fiber is tapered by a fusion taper method to form a truncated cone structure with a taper angle.

[0080] The coupler 8 is a hollow Teflon tube with an opening at one end and a sealed end at the other end; the formic acid detection optical fiber is inserted into the opening of the Teflon tube through the optical fiber holder 6 to achieve coupling with the coupler 8, and the first ends of the incident optical fiber 1 of the formic acid detection optical fiber and several tapered receiving optical fibers 2 are facing the inside of the Teflon tube.

[0081] The working logic of the optical fiber sensor is:

[0082] 1) The second end of the incident optical fiber transmits the light beam emitted by the light source;

[0083] 2) The first end of the incident optical fiber radiates the light beam into the interior of the Teflon tube containing formic acid air, and the light beam is transmitted to the formic acid sensitive unit after passing through the formic acid air;

[0084] 3) The first end of each tapered receiving optical fiber receives the light beam reflected by the formic acid sensitive unit and transmits it to the second end, and the output light intensity of the optical fiber sensor is calculated based on the light intensity output by the second end of each tapered receiving optical fiber;

[0085] 4) Calculate the corresponding output signal based on the output light intensity of the optical fiber sensor;

[0086] 5) Calculating the formic acid concentration of the formic acid-containing air by using the output signal of the optical fiber sensor and a predetermined calibration relationship between the output signal and the formic acid concentration.

[0087] The present invention designs an optical fiber sensor for formic acid concentration detection, including a formic acid detection optical fiber, a formic acid sensitive unit and a coupler. The formic acid detection optical fiber is composed of an incident optical fiber, six tapered receiving optical fibers and an optical fiber holder; the incident optical fiber and the tapered receiving optical fiber are fixed in the optical fiber holder, the tapered receiving optical fiber is distributed around the incident optical fiber, and the optical fiber and the holder are fixed and packaged with UV shadowless glue. The formic acid sensitive unit includes a light reflector and a formic acid sensitive film, wherein the formic acid sensitive film is a formic acid sensitive material coated on the light reflector, wherein the formic acid sensitive material is made of MOF-802 / Ph-gC having a selective adsorption function for formic acid. 3 N 4 / Polyurethane composite material composition: MOF-802 has the function of selective adsorption of formic acid, and can also improve the adsorption capacity of the material for formic acid, increase the sensor's measurement range for formic acid and the sensitivity of the sensor; Ph-gC 3 N 4 It is used to enhance the selective adsorption capacity of formic acid, and can effectively eliminate the interference of other gases on the formic acid concentration detection results; polyurethane is used to enhance the MOF-802 / Ph-gC 3 N 4 / The adhesion strength between the polyurethane film and the light reflective lens and the mechanical strength of the film improve the repeatability and service life of the sensor. The coupler is a hollow Teflon tube with an opening at one end and a seal at the other end. The opening area set on the Teflon tube is convenient for formic acid in the environment to pass through the Teflon tube freely. The formic acid sensitive unit is fixed to the end of the coupler with UV shadowless glue; the coupler has an internal thread structure and is fixed with the external thread on the optical fiber holder to form an optical fiber sensor for formic acid measurement.

[0088] When the optical fiber sensor is working, the intensity of light radiated into the coupler by formic acid detection optical fiber. When formic acid in the external environment is transmitted to the formic acid sensitive membrane (MOF-802 / Ph-gC 3 N 4 / polyurethane film), the refractive index of the formic acid sensitive film increases after absorbing formic acid, the absorption intensity of light with a wavelength of 292nm is enhanced, and the reflectivity is reduced, causing the light intensity received by the tapered receiving optical fiber to decrease, so that the formic acid concentration in the environment can be accurately measured by measuring the relative variable of the output light intensity of the formic acid detection optical fiber; and the formic acid measurement result is not affected by other gas components and temperature changes, there is a linear relationship between the output signal of the optical fiber sensor and the formic acid concentration, and it can be widely used in the in-situ online detection of formic acid concentration in various environments.

[0089] In order to better introduce the technical solution of the present invention, this embodiment is described through the following parts.

[0090] 1. Formic acid detection optical fiber (Y-shaped optical fiber)

[0091] In order to improve the sensitivity of the sensor in detecting formic acid, seven quartz optical fibers with a length of 0.2 to 5 m and a diameter of 100 to 2000 μm were first cut, and the end faces of the optical fibers were polished with optical fiber polishing paper. The optical fiber surfaces were rinsed with deionized water and dried at 50 to 70 ° C for later use. The schematic diagram of the optical fiber end face structure is shown in Figure 1. Figure 2 (a) is shown. Then, the first ends of six of the optical fibers are tapered by the fusion taper method. The taper angle after taper is a truncated cone structure of 55 to 85 degrees. The tapered optical fiber is used as a tapered receiving optical fiber to enhance the light collection capability of the tapered receiving optical fiber. Then, the tapered optical fiber is cleaned with alcohol and deionized water in turn to remove surface cutting debris, dust, grease and other contaminants for use. The schematic diagram of the end face structure of the tapered receiving optical fiber is shown in FIG. Figure 2(b) As shown. Secondly, an incident optical fiber and six tapered receiving optical fibers are fixed in an optical fiber holder made of Teflon, with a length of 10 to 100 mm and a diameter of 5 to 25 mm, and one end of which has an external thread 7. The diameter of the seven holes in the holder is 110-2100 μm, and the length of the external thread 7 is 5 to 50 mm. The tapered receiving optical fibers are distributed around the incident optical fiber, and the distance between the core of the tapered receiving optical fiber and the core of the incident optical fiber is 2 to 5 mm. UV shadowless glue 61 is used to fill and fix the optical fiber and the holder to achieve coupling between the incident optical fiber and the tapered receiving optical fiber, as shown in FIG. Figure 2 (c) and Figure 2 (d) As shown. The second ends of the incident optical fiber and the tapered receiving optical fiber are fixed in the optical fiber protective sleeve 3 again with UV shadowless glue 61 for later use. Finally, the second end of the incident optical fiber is connected to the SMA optical fiber connector 4, and the second end of the tapered receiving optical fiber is connected to the optical fiber collimator 5 and the SMA optical fiber connector 4 in sequence to form a formic acid detection optical fiber (Y-shaped optical fiber), as shown in FIG. Figure 2 (e) as shown.

[0092] During the specific implementation process, the preparation of formic acid detection optical fiber (Y-shaped optical fiber) is as follows:

[0093] The length of the quartz optical fiber is 0.7m, the diameter of the incident optical fiber is 1000μm, the diameter of the six tapered receiving optical fibers is 500μm, the cone angle of the tapered receiving optical fiber is 60°, and the core spacing between the incident optical fiber and the tapered receiving optical fiber is 3mm. The optical fiber holder with external threads is 50mm long and 10mm in diameter, the diameter of the middle hole of the holder is 1010μm, the diameter of the other six holes is 510μm, and the length of the external threads is 30mm. The first end of the tapered receiving optical fiber is inserted into the hole in the middle of the optical fiber holder, and the first ends of the six tapered receiving optical fibers are respectively inserted into the other six holes of the optical fiber holder. UV shadowless glue is used to fill and fix the package between the optical fiber and the holder, and an SMA optical fiber connector is connected to the second end of the incident optical fiber, and an optical fiber collimator and an SMA optical fiber connector are connected to the second end of the tapered receiving optical fiber in sequence to form a formic acid detection optical fiber (Y-shaped optical fiber).

[0094] 2. Formic acid sensitive unit

[0095] The preparation steps of the formic acid sensitive unit include:

[0096] 1. Preparation steps of MOF-802:

[0097] 1) Dissolve 500-800 mg of zirconium oxychloride octahydrate and 200-400 mg of 1h-pyrazole-3,5-dicarboxylic acid in a mixed solvent of 20-40 mL of DMF and 10-30 mL of formic acid, stir for 1-3 hours to completely dissolve, and after uniform mixing, transfer the resulting solution into a polytetrafluoroethylene-lined autoclave;

[0098] 2) The lid of the autoclave is closed and heated at 120-140° C. for 48-96 hours, and the polytetrafluoroethylene-lined autoclave is cooled to room temperature and then taken out; the MOF-802 is washed with ethanol and dried at 40-70° C., and the obtained MOF-802 is ground into powder for standby use; in order to improve the selective adsorption capacity of the sensitive membrane material for formic acid and improve the selectivity of the sensor for detecting formic acid, a certain amount of phenyl-modified carbon nitride Ph-gC is added to the MOF-802 3 N 4 .

[0099] 2. Phenyl-modified carbon nitride Ph-gC 3 N 4 Preparation steps:

[0100] 3) 0.3-0.7 g of cyanuric acid and 0.8-1.2 g of 2,4-diamino-6-phenyl-1,3,5-triazine were uniformly mixed and ground for 1-2 hours in the presence of ethanol (1-3 mL); the resulting powder was transferred to a crucible and heated in a tube furnace at N 2 The method comprises the following steps: heating the obtained material at 350-550° C. for 2-4 hours at a heating rate of 2-5° C. / min under an atmosphere; cooling the obtained material to room temperature, dispersing the obtained material in deionized water at a mass ratio of 1:2-2:1, ultrasonically treating the obtained colloidal suspension solution for 20-24 hours, and centrifuging the obtained colloidal suspension solution at 7000-10000 rpm to remove large particles; discharging the aqueous solution, placing the solution in a vacuum drying oven, and drying the solution at 60-70° C. for 24-48 hours to obtain a dry Ph-gC 3 N 4 , the obtained Ph-gC 3 N 4 Grind into powder and set aside;

[0101] 3. Preparation of formic acid selective sensitive sol:

[0102] 4) Add a mass of M to 0.2-2 mL of polyurethane (1,6-hexamethylene diisocyanate) 1 MOF-802 and mass M 2 Ph-gC 3 N 4 , where M 1 :M 2 The ratio of the mixture is 80:20 to 50:50. After stirring for 12 to 24 hours, MOF-802 / Ph-gC is obtained. 3 N 4 / Polyurethane sol, i.e. formic acid selective sensitive sol;

[0103] 4. Preparation of formic acid sensitive unit:

[0104] 5) In order to construct a fiber optic reflective sensor probe and realize formic acid concentration selective adsorption and light reflection, a formic acid sensitive unit was prepared using a light reflector and a formic acid selective sensitive sol. First, in order to enhance the adhesion strength between the formic acid selective sensitive sol and the light reflector, alcohol and deionized water were used to clean the light reflector lens with a diameter of 5 to 20 mm to remove surface dust and other organic pollutants; the cleaned light reflector lens was blown dry with nitrogen and stored in a clean room for later use;

[0105] 6) The formic acid selective sensitive sol is evenly coated on the surface of the light reflective lens by a spin coating method; and dried at 50 to 70° C. for 24 to 48 hours to obtain a formic acid sensitive film with a thickness of 40 to 160 μm. The light reflective mirror coated with the formic acid sensitive film is used as a formic acid sensitive unit, such as Figure 3 shown.

[0106] In the specific implementation process, for the preparation of formic acid sensitive units:

[0107] First, 0.644 g zirconium oxychloride octahydrate, 0.348 g 1h-pyrazole-3,5-dicarboxylic acid, 20 mL formic acid, and 30 mL DMF were used to prepare MOF-802 at 130 °C for 72 h. Then, 0.5 g cyanuric acid, 1.0 g 2,4-diamino-6-phenyl 1,3,5-triazine, and 1 mL ethanol were used to prepare MOF-802 at 450 °C for 72 h. 2 Ph-gC prepared by heating at a heating rate of 2℃ / min for 4h 3 N 4 Then 66 mg MOF-802 and 34 mg Ph-gC 3 N 4 Add to 300 μL of polyurethane and stir for 24 h to obtain MOF-802 / Ph-gC 3 N 4 / polyurethane sol, the sol was spin-coated onto the surface of a light reflector with a diameter of 10 mm, and dried at 60 ° C to obtain a formic acid selective sensitive film with a thickness of 80 μm (MOF-802 / Ph-gC 3 N 4 / polyurethane film), the light reflector coated with the formic acid selective sensitive film is the formic acid sensitive unit.

[0108] The above scheme is the optimal ratio of various components when preparing the formic acid sensitive unit (see Case 3 in Table 1). Through this optimal ratio, the light absorption intensity and reflectivity of the formic acid sensitive film can be best taken into account.

[0109] In other preferred embodiments, formic acid sensitive units can also be prepared by the composition ratios of other cases in Table 1, but compared with Case 3, the light absorption intensity of the formic acid sensitive film in Case 1 and Case 2 is better, but the light reflectivity effect is worse, and the light absorption intensity of the formic acid sensitive film in Case 4 and Case 5 is better, but the light reflectivity effect is worse; only Case 3 can make the light absorption intensity and reflectivity of the formic acid sensitive film both excellent.

[0110] Table 1 Ratios of various components in the preparation of formic acid sensitive units

[0111]

[0112] 3. Coupler

[0113] In order to obtain a reflective formic acid concentration optical fiber sensor, firstly, a Teflon rod with a diameter of 10 to 30 mm and a length of 10 to 200 mm is cut. Then, a hole with a diameter of 5 to 22 mm and a depth of 8 to 190 mm is drilled at one end of the Teflon rod. Secondly, in order to facilitate the free entry and exit of the substances in the Teflon hole and the substances outside the Teflon, opening areas 10 are opened on both sides of the Teflon central area with the hole, and the width of the opening area 10 is 3 to 15 mm and the length is 4 to 150 mm, that is, the side wall of the Teflon round tube is opened to connect the inside of the Teflon round tube with the outside, so that the formic acid-containing air can enter the inside of the Teflon round tube through the opening area 10. Next, one end of the Teflon material with a hole drilled inside and an opening on the side is processed into an internal thread 9 by turning. The diameter of the internal thread 9 is 5 to 25 mm and the length is 5 to 50 mm. The internal thread 9 matches the structural dimensions of the external thread of the optical fiber holder. The Teflon material with the internal thread 9 is the coupler. Finally, the formic acid sensitive unit is fixed to the bottom of the coupler using UV shadowless glue. The coupler and the optical fiber holder of the formic acid detection optical fiber are coupled and fixed by thread rotation to form an optical fiber sensor for formic acid measurement. The structure of the reflective optical fiber sensor for selective detection of formic acid concentration is shown as follows. Figure 1 shown.

[0114] During the specific implementation process, for the preparation of the coupler:

[0115] First, a Teflon rod with a diameter of 18 mm and a length of 40 mm was cut. Then, a hole with a diameter of 10 mm and a depth of 30 mm was drilled in the center of the Teflon rod. Then, an opening area connected to the outside world was set on both sides of the central area of ​​the Teflon with a hole, and the width of the opening area was 7 mm and the length was 15 mm. Secondly, one end of the Teflon material with a hole drilled inside and an opening on the side was processed into an internal thread by turning. The diameter of the internal thread was 10 mm and the length was 15 mm. The Teflon material with the internal thread was a coupler. UV shadowless glue was used again to fix the formic acid sensitive unit at the bottom of the coupler. Finally, the coupler and the optical fiber holder of the formic acid detection optical fiber were fixed by threaded rotation coupling, and the distance between the optical fiber end face and the light reflection mirror was kept at 2 mm. The packaged and fixed sensor was the optical fiber sensor for selective measurement of formic acid concentration.

[0116] Embodiment 2:

[0117] This embodiment discloses a formic acid concentration detection method based on an optical fiber sensor, which is implemented based on the optical fiber sensor described in the first embodiment.

[0118] A method for detecting formic acid concentration based on an optical fiber sensor, comprising:

[0119] S1: The second end of the incident optical fiber of the optical fiber sensor transmits the light beam emitted by the light source;

[0120] S2: The first end of the incident optical fiber of the optical fiber sensor radiates the light beam into the interior of the Teflon tube containing formic acid air, and the light beam is transmitted to the formic acid sensitive unit after passing through the formic acid air;

[0121] S3: The first end of each tapered receiving optical fiber of the optical fiber sensor receives the light beam reflected by the formic acid sensitive unit and transmits it to the second end; the output light intensity of the optical fiber sensor is calculated based on the light intensity output from the second end of each tapered receiving optical fiber;

[0122] S4: Calculate the corresponding output signal based on the output light intensity of the optical fiber sensor;

[0123] S5: Calculating the formic acid concentration of the formic acid-containing air by using the output signal of the optical fiber sensor and a predetermined calibration relationship between the output signal and the formic acid concentration.

[0124] The formic acid concentration sensor in the first embodiment mainly utilizes the formic acid sensitive film to adsorb formic acid gas molecules, and the refractive index increases, resulting in the weakening of the light intensity transmitted to the tapered receiving optical fiber. By measuring the relative attenuation of the light intensity at the output end of the tapered receiving optical fiber, the formic acid concentration can be selectively measured. The formic acid concentration sensor selectively detects the optical transmission path of the formic acid process. Figure 4 shown.

[0125] Combination Figure 4As shown, the output light intensity of the optical fiber sensor is calculated by the following steps:

[0126] S301: Define the light intensity of the incident optical fiber radiation as I in ;

[0127] S302: Light intensity is I in The light beam passes through the formic acid-containing air and is transmitted to the surface of the formic acid sensitive film, and then refracted into the formic acid sensitive film. The incident light intensity on the formic acid sensitive film surface is I 1 ;

[0128] When a light beam is transmitted in the air medium, according to the Lambert-Beer law, the input light intensity I in With light intensity I 1 The functional relationship between the concentration of formic acid C can be expressed as:

[0129] I 1 =I in exp(-σ 1 Cd / cosθ); (1)

[0130] Where: 1 is the intrinsic absorption coefficient of formic acid to a light beam under a certain wavelength condition, a constant that has nothing to do with concentration; d is the distance between the end face of the optical fiber and the mirror; θ is the incident angle of the light beam on the interface between the sensitive film and the air medium; C represents the formic acid concentration;

[0131] S303: Light intensity is I 1 The light beam is refracted on the surface of the formic acid sensitive film and enters the formic acid sensitive film. Then, the light beam is reflected at the interface between the formic acid sensitive film and the light reflector and enters the formic acid sensitive film again. Then, the light beam is refracted at the interface between the formic acid sensitive film and the air and enters the air medium containing formic acid again. The light intensity of the light beam after attenuation by the formic acid sensitive film is I 2 ;

[0132] When the light beam is transmitted in the formic acid sensitive film, the intensity I of the light beam when it enters the formic acid sensitive film 1 The intensity I of the light beam when it leaves the formic acid sensitive film 2 The functional relationship between them can be expressed as:

[0133] I 2 =I 1 exp(-σ 2 2l / cosθ 1 ); (2)

[0134] σ 2 =2πε / (λn 2 ); (3)

[0135] Where: 2is the attenuation coefficient of the formic acid sensitive film to light, which is related to the refractive index of the formic acid sensitive film; l is the film thickness of the formic acid sensitive film; ε is the formic acid sensitive film (MOF-802 / Ph-gC 3 N 4 / polyurethane composite material) imaginary part of the complex dielectric constant; λ is the wavelength of the sensor light source; n 2 is the refractive index of formic acid sensitive film, which increases with the MOF-802 / Ph-gC 3 N 4 / polyurethane composites selectively adsorbed formic acid with the increase of content;

[0136] S304: Light intensity is I 2 The light beam passes through the formic acid-containing air medium and reaches the first end of the tapered receiving optical fiber and is transmitted to the second end for output. The light intensity output by the tapered receiving optical fiber is I 3 ;

[0137] The outgoing light intensity I on the surface of the sensitive film 2 The light intensity I transmitted to the tapered receiving optical fiber end face after passing through the air medium 3 The functional relationship between the concentration of formic acid C can be expressed as:

[0138] I 3 =I 2 exp(-σ 1 Cd / cosθ); (4)

[0139] S305: Based on the light intensity I output by each tapered receiving optical fiber 3 Calculate the output light intensity I of the optical fiber sensor out ;

[0140] The output light intensity I received by the six tapered receiving fibers of the sensor out It can be described as:

[0141]

[0142] From the analysis of formula (5), it can be seen that when the waist radius, end radius and cone angle of the tapered receiving fiber are fixed, I in formula (5) 3 The previous coefficient can be simplified to k 1 :

[0143]

[0144] From equations (1), (2) and (5), we know that the functional relationship between the sensor input light intensity and output light intensity can be described as:

[0145] I out =k 1 I in exp(-σ 1Cd / cosθ)exp(-σ 2 2l / cosθ 1 )exp(-σ 1 Cd / cosθ); (7)

[0146] Where: r 1 r is the waist radius of the first end of the tapered receiving optical fiber; 2 is the end face radius of the first end of the tapered receiving optical fiber; β is the cone angle of the first end of the tapered receiving optical fiber; k 1 is a constant.

[0147] Calculate the output light intensity I′ when there is no formic acid sensitive material in the formic acid sensitive unit of the optical fiber sensor out ; Based on the output light intensity I out and output light intensity I′ out Calculate the output signal K of the optical fiber sensor;

[0148] Formula (7) shows that when there is no formic acid sensitive film material in the sensor, after the air medium light beam is attenuated, the output light intensity of the sensor is I′ out :

[0149] I′ out =k 1 I in exp(-σ 1 C2d / cosθ). (8)

[0150] The output signal K of the optical fiber sensor is calculated by the following formula:

[0151] In order to eliminate the attenuation of light caused by other gases in the air medium, and thus interfere with the accuracy of formic acid measurement results, the relative measurement method is used to process the light intensity signal of the sensor output. The sensor output signal K can be described as:

[0152]

[0153] Substituting equation (2), equation (3) and equation (4) into equation (9), we can obtain:

[0154] K=exp{-4πεl / (λn 2 cosθ 1 )}; (10)

[0155] Since the refractive index of formic acid sensitive film n 2 is a function of the formic acid concentration C, so n 2 The functional relationship between and C can be described as:

[0156] n 2 =k 2 C+b; (11)

[0157] K=exp{-4πεl / [λcosθ 1 (k 2 C+b)]}; (12)

[0158] Where: n 2 represents the refractive index of formic acid sensitive film; k 2 , b are constants.

[0159] Formula (12) shows that when the sensor light source wavelength, the structural parameters of the incident optical fiber and the tapered receiving optical fiber, the number of tapered receiving optical fibers, the distance between the sensor end face and the formic acid selective sensitive membrane and other parameters are determined, the sensor output signal is only related to the formic acid concentration C. The larger the formic acid concentration C, the larger the sensor output signal K. Therefore, by measuring the change in the K value, the formic acid concentration in the air can be accurately measured.

[0160] It should be noted that the above calculation logic is an introduction to the internal working principle of the optical fiber sensor, which is mainly used to determine the calibration relationship between the output signal and the formic acid concentration. In the actual detection process, the optical fiber sensor can directly obtain the light intensity of the output light beam of the conical receiving optical fiber, and combine it with the pre-calculated I′ out , the output signal K is calculated using formula (9), and then the formic acid concentration is determined through the calibration relationship between the output signal and the formic acid concentration.

[0161] When the optical fiber sensor is working, the light intensity radiated into the coupler through the formic acid detection optical fiber. When the formic acid in the external environment is transmitted to the formic acid sensitive film (MOF-802 / Ph-g-C3N4 / polyurethane film) through the coupler, the formic acid sensitive film absorbs formic acid and its refractive index increases, and the absorption intensity of light with a wavelength of 292nm is enhanced and the reflectivity is reduced, causing the light intensity received by the tapered receiving optical fiber to decrease, so that the formic acid concentration in the environment can be accurately measured by measuring the relative variable of the output light intensity of the formic acid detection optical fiber; and the formic acid measurement result is not affected by other gas components and temperature changes. There is a linear relationship between the output signal of the optical fiber sensor and the formic acid concentration, which can be widely used in the in-situ online detection of formic acid concentration in various environments.

[0162] Sensor detection formic acid performance test

[0163] The prepared optical fiber sensor is placed in a formic acid environment. When the formic acid concentration is 0-500 ppm, the response spectrum of the sensor to formic acid and the relationship between the output signal K and the formic acid concentration C are as follows: Figure 5 As shown (sensor sampling time is 5 minutes).

[0164] like Figure 5As shown in (a), the optical fiber sensor produces a characteristic absorption peak at 292 nm, and the absorbance increases with the increase of formic acid. The reason is that with the increase of formic acid concentration, the number of formic acid molecules adsorbed by the sensitive material increases, resulting in an increase in the refractive index of the formic acid sensitive film, which causes the sensor light attenuation to increase. Therefore, the absorbance of the sensor output spectrum increases, which is consistent with the analysis results of the theoretical model of formula (12). Figure 5 (b) shows that when the wavelength of the light source is 292nm, there is a linear relationship between the sensor output signal (K) and the formic acid concentration (C), K = 0.000244C + 0.00611 (R 2 =0.992), the sensor sensitivity reached 2.4E-4AU / ppm. The reason is that MOF-802 and Ph-gC 3 N 4 It has strong adsorption ability and large adsorption capacity for formic acid. Therefore, when formic acid molecules are transmitted to the formic acid sensitive membrane, they will be adsorbed, thereby increasing its refractive index and causing light attenuation; the greater the formic acid concentration, the greater the refractive index increment, the greater the light attenuation, and the greater the sensor output signal K.

[0165] In order to characterize the selectivity of the sensor for formic acid measurement, the sensor was placed in a 500 ppm SO 2 , formic acid, acetone, acetic acid and CO 2 In gas, the sensor output signal is as follows Figure 6 As shown. Figure 6 As shown in the figure, the optical fiber sensor is sensitive to 500ppm SO 2 , formic acid, acetone, acetic acid and CO 2 The absorption characteristic wavelengths of gases are different, and the sensor's response sensitivity to formic acid of the same concentration is higher than that to other gases; the reason is that Ph-gC 3 N 4 Highly selective for formic acid.

[0166] comprehensive Figure 5 and Figure 6 It can be seen that the optical fiber sensor of the present invention can achieve selective and accurate measurement of formic acid gas at a light wavelength of 292 nm.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. Formic acid concentration detection optical fiber sensor, characterized in that: include: A formic acid detection optical fiber, a coupler coupled to the formic acid detection optical fiber, and a formic acid sensitive unit disposed inside the coupler; The formic acid sensitive unit includes a light reflector facing the detection end of the formic acid detection optical fiber when in use and a formic acid sensitive film arranged on the reflection surface of the light reflector; the formic acid sensitive film is a MOF-802 / Ph-g-C3N4 / polyurethane film composed of MOF-802 compound, Ph-g-C3N4 and polyurethane, wherein Ph-g-C3N4 is a graphite carbon nitride material modified by phenyl functionalization.

2. The formic acid concentration detection optical fiber sensor according to claim 1, characterized in that: The formic acid sensitive membrane was prepared by the following steps: 1) dissolving zirconium oxychloride octahydrate and 1h-pyrazole-3,5-dicarboxylic acid in a mixed solvent of DMF and formic acid, stirring to completely dissolve, and transferring the mixed solution into a polytetrafluoroethylene-lined autoclave; 2) heating the polytetrafluoroethylene-lined autoclave, cooling it to room temperature, and then taking it out; washing it with ethanol and drying it, and grinding the obtained MOF-802 into powder for later use; 3) Mixing cyanuric acid and 2,4-diamino-6-phenyl-1,3,5-triazine in the presence of ethanol and grinding, transferring the obtained powder to a crucible, and heating it in a tube furnace; cooling to room temperature after heating, and ultrasonically treating the obtained material in deionized water; centrifuging the colloidal suspension solution obtained by ultrasonic treatment to remove large particles, discharging the aqueous solution, placing it in a vacuum drying oven, and drying it to obtain dry Ph-g-C3N4, and grinding the obtained Ph-g-C3N4 into powder for later use; 4) adding MOF-802 powder and Ph-g-C3N4 powder to polyurethane and stirring to obtain MOF-802 / Ph-g-C3N4 / polyurethane sol, i.e., formic acid selective sensitive sol; 5) The formic acid selective sensitive sol is coated on the surface of the light reflecting lens by a spin coating method and dried to obtain a formic acid sensitive film.

3. The formic acid concentration detection optical fiber sensor according to claim 2, characterized in that: Before coating the formic acid selective sensitive sol, the light reflecting lens is first cleaned with alcohol and deionized water, and then the cleaned light reflecting lens is blown dry with nitrogen.

4. The formic acid concentration detection optical fiber sensor according to claim 1, characterized in that: The formic acid detection optical fiber includes an incident optical fiber, a plurality of tapered receiving optical fibers and an optical fiber holder; the detection ends of the incident optical fiber and the plurality of tapered receiving optical fibers are fixed together by the optical fiber holder and the plurality of tapered receiving optical fibers are distributed around the incident optical fiber; The detection ends of the incident optical fiber and a plurality of tapered receiving optical fibers are the detection ends of the formic acid detection optical fibers.

5. The formic acid concentration detection optical fiber sensor according to claim 4, characterized in that: The detection end of the tapered receiving optical fiber is tapered by a fusion taper method to form a truncated cone structure with a taper angle.

6. The formic acid concentration detection optical fiber sensor according to claim 4, characterized in that: The coupler is a hollow Teflon tube with an open hole at one end and a sealed end at the other end; The formic acid detection optical fiber is inserted into the opening of the Teflon tube through an optical fiber fixture to achieve coupling with the coupler, and the incident optical fiber of the formic acid detection optical fiber and the detection ends of several tapered receiving optical fibers face the inside of the Teflon tube.

7. The formic acid concentration detection optical fiber sensor according to claim 6, characterized in that: An opening area is provided on the side wall of the Teflon round tube to connect the inside of the Teflon round tube with the outside, so that the formic acid-containing air can enter the inside of the Teflon round tube through the opening area.

8. A method for detecting formic acid concentration based on an optical fiber sensor, characterized in that: The formic acid concentration detection optical fiber sensor according to claim 7 is implemented, comprising: S1: The incident optical fiber of the optical fiber sensor transmits the light beam emitted by the light source; S2: The detection end of the incident optical fiber of the optical fiber sensor radiates the light beam into the interior of the Teflon tube containing formic acid air, and the light beam is transmitted to the formic acid sensitive unit after passing through the formic acid air; S3: The detection end of each tapered receiving optical fiber of the optical fiber sensor receives the light beam reflected by the formic acid sensitive unit; the output light intensity of the optical fiber sensor is calculated based on the light intensity output by each tapered receiving optical fiber; S4: Calculate the corresponding output signal based on the output light intensity of the optical fiber sensor; S5: Calculating the formic acid concentration of the formic acid-containing air by using the output signal of the optical fiber sensor and a predetermined calibration relationship between the output signal and the formic acid concentration.

9. The method for detecting formic acid concentration based on an optical fiber sensor according to claim 8, characterized in that: In step S3, the output light intensity of the optical fiber sensor is calculated by the following steps: S301: Define the light intensity of the incident optical fiber radiation as I in ; S302: Light intensity is I in The light beam passes through the formic acid-containing air and is transmitted to the surface of the formic acid sensitive film, and then refracted into the formic acid sensitive film. The incident light intensity on the formic acid sensitive film surface is I1; The formula is: I1=I in exp(-σ1Cd / cosθ); Where: σ1 is the intrinsic absorption coefficient of formic acid to a light beam under a certain wavelength; d is the distance between the end face of the incident optical fiber and the surface of the formic acid sensitive film; θ is the incident angle of the light beam on the interface between the formic acid sensitive film and the air containing formic acid; C represents the formic acid concentration; S303: The light beam with the light intensity of I1 is refracted on the surface of the formic acid sensitive film and enters the formic acid sensitive film. Then, the light beam is reflected at the interface between the formic acid sensitive film and the light reflector and enters the formic acid sensitive film again. Then, the light beam is refracted at the interface between the formic acid sensitive film and the air and enters the air medium containing formic acid again. The light intensity of the light beam after attenuation by the formic acid sensitive film is I2. The formula is: I2=I1exp(-σ22l / cosθ1); σ2=2πε / (λn2); Where: σ2 is the attenuation coefficient of the formic acid sensitive film to the light beam; l is the film thickness of the formic acid sensitive film; ε is the imaginary part of the complex dielectric constant of the formic acid sensitive film; λ is the wavelength of the light beam; n2 is the refractive index of the formic acid sensitive film; θ1 is the refraction angle of the light beam; S304: the light beam with a light intensity of I2 reaches the detection end of the tapered receiving optical fiber after passing through the air medium containing formic acid, and the light intensity output by the tapered receiving optical fiber is I3; The formula is: I3=I2exp(-σ1Cd / cosθ); S305: Calculate the output light intensity I of the optical fiber sensor based on the light intensity I3 output by each tapered receiving optical fiber out ; The formula is: in: I out =k1I in exp(-σ1Cd / cosθ)exp(-σ22l / cosθ1)exp(-σ1Cd / cosθ); Wherein: r1 is the cone waist radius of the cone receiving optical fiber detection end; r2 is the end face radius of the cone receiving optical fiber detection end; β is the cone angle of the cone receiving optical fiber detection end; k1 is a constant; n represents the number of receiving optical fibers.

10. The method for detecting formic acid concentration based on an optical fiber sensor according to claim 8, characterized in that: In step S4, the output light intensity I′ is calculated when the formic acid sensitive unit of the optical fiber sensor does not contain formic acid sensitive material. out ; Based on the output light intensity I out And the output light intensity I′ out Calculate the output signal K of the optical fiber sensor; The formula is: I′ out =k1I in exp(-σ1C2d / cosθ);

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