Micro-nano fiber gas sensor based on in-situ grown polymer and preparation method thereof

Abstract

The present invention discloses a micro-nano fiber gas sensor based on in-situ grown polymer and a preparation method thereof, comprising: an input optical fiber and a fiber grating area probe connected in sequence; a part of the fiber grating area probe is subjected to etching treatment, and the surface of the fiber grating area probe is covered with a PANI film, and the PANI film is in-situ grown on the surface of the fiber grating area probe; the PANI film can adsorb a set gas in the environment to reduce its conductivity, thereby causing a drift in the center wavelength of the reflection spectrum in the fiber grating, and further realizing the detection of the concentration of the set gas. The present invention organically combines the refractive index sensitive characteristic of the micro-nano fiber and the gas sensitive characteristic of PANI, in-situ grows a PANI film on the surface of the fiber grating area, and the obtained sensor has high sensitivity, fast response / recovery time and good stability; it can be used for the test of the composition or concentration of gases and liquids.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and particularly relates to a micro-nano optical fiber gas sensor based on in-situ grown polymer and a preparation method thereof. Background Art

[0002] The statements in this part merely provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] With the rapid development of modern industry, the emissions of various industrial waste gases are increasing, which contain a large amount of toxic gases and flammable and explosive gases. It will not only cause environmental pollution but also pose a hazard to human health. After the power batteries of new energy vehicles and the energy storage batteries of energy storage power stations are damaged and before spontaneous combustion, various toxic gases and flammable and explosive gases are also generated. If detected in time and properly treated, the occurrence of battery spontaneous combustion or even explosion accidents can be avoided.

[0004] Among them, ammonia (NH 3 ) is a typical colorless, irritating toxic and harmful gas and a common air pollutant. At the same time, ammonia at a certain concentration will pose a hazard to human health. When people are in an environment of 25 ppm of NH 3 for 8 hours, or even in 15 minutes in 35 ppm of NH 3 , NH 3 will cause serious damage to people's eyes and respiratory tract. Therefore, timely and accurately monitoring and controlling the concentration of toxic and harmful gases such as NH 3 in the pollution source or the atmospheric environment is of great significance to environmental protection and human health.

[0005] Traditional electro-chemical gas sensors have high sensitivity and accuracy, but poor anti-interference ability and high operating temperature; spectral absorption type gas sensors are bulky, costly, and it is difficult to achieve in-situ online monitoring in high dust, high humidity, and vibration environments.

[0006] Compared with traditional sensors, optical fiber gas sensors have the advantages of strong anti-interference ability, high selectivity, multiplexing, low operating temperature, fast response speed, and can online in-situ real-time monitor gases. Fiber Bragg grating gas sensors belong to a widely studied type in optical fiber gas sensors, which utilize the fact that the contact between gas and the gas-sensitive material on the surface of the fiber grating region induces a certain change in the transmission spectrum or reflection spectrum of the fiber grating, thereby realizing the sensing of gas. Fiber gratings mainly include: short-period fiber gratings, namely Fiber Bragg Grating (FBG) and Long-Period Fiber Grating (LPFG).

[0007] Compared with LPFG sensors, FBG sensors have stronger anti-interference ability and more stable performance. However, the cores of commercial FBGs are wrapped by thick claddings and cannot be directly in contact with the external environment. Therefore, they cannot be directly used for gas detection based on the principle of optical refractive index change.

[0008] Therefore, at present, LPFG sensors are mostly used for biochemical sensing research. For the research on using FBG sensors to detect trace gases and their changes, many difficulties still need to be overcome.

[0009] The prior art discloses a preparation method of an FBG gas sensor, which is a preparation method of an FBG sensor with a gas-sensitive polymer film coated by thermal spin coating. A gas-sensitive polymer film is coated on the surface of the fiber grating region, which can ensure the coating quality of the polymer film on the surface of the fiber grating region and make the film thickness uniform. However, this fiber optic sensor utilizes the volume expansion or contraction effect induced by the adsorption of gas by the polymer film coated on the surface of the fiber grating region, so as to change the grating pitch of the FBG and achieve the purpose of gas detection. This type of FBG gas sensor is only applicable to environments with high-concentration gases and cannot detect ppm-level gases. Summary of the Invention

[0010] To solve the above problems, the present invention proposes a micro-nano fiber gas sensor based on in-situ growth of polymers and a preparation method thereof, which utilizes the characteristic that micro-nano fibers are sensitive to the optical refractive index of the environment to detect gas concentration. At the same time, to solve the problems of low sensitivity and poor selectivity, polyaniline (hereinafter referred to as PANI) is introduced as a gas-sensitive material. PANI is in-situ grown on the surface of the grating region of the fiber grating after etching and necessary modification treatments are performed. At the same time, by real-time monitoring the reflection spectrum intensity during the polymer growth process, a suitable PANI film thickness and coating uniformity are obtained to ensure good and stable fiber optic signal intensity and effectively ensure the bonding firmness between PANI and the surface of the grating region of the fiber grating.

[0011] In some embodiments, the following technical solutions are adopted:

[0012] A micro-nano fiber gas sensor based on in-situ growth of polymers, comprising: an input optical fiber and a fiber grating region probe connected in sequence; a part of the fiber grating region probe is etched, and the surface of the fiber grating region probe is covered with a PANI film, and the PANI film is in-situ grown on the surface of the fiber grating region probe.

[0013] The PANI film can adsorb the set gas in the environment and reduce its conductivity, thereby causing the drift of the center wavelength of the reflection spectrum in the fiber grating, and further realizing the detection of the concentration of the set gas.

[0014] As a specific example, the above fiber grating region probe can be an FBG fiber grating region probe. The present invention organically combines the optical refractive index sensitivity of the micro-nano FBG fiber gas sensor and the gas sensitivity of PANI, and in-situ grows a PANI film on the surface of the fiber grating region, solving the problems of low gas sensitivity and poor selectivity of the FBG sensor itself; at the same time, through the in-situ growth method of the PANI film, the technical problems of thickness control, uniform coating of the polymer on the surface of the FBG fiber grating, and stable and firm combination with the FBG fiber grating are effectively solved.

[0015] In some other embodiments, the following technical solutions are adopted:

[0016] A preparation method of a micro-nano fiber gas sensor based on in-situ growth of a polymer, comprising:

[0017] Etch the fiber grating region, and at the same time monitor the central wavelength of the fiber grating reflection spectrum. After the central wavelength deviates from the set value, soak the grating region with deionized water to remove the residual impurities on the surface of the grating region;

[0018] Take out the fiber from the deionized water, perform surface treatment on the grating region; then perform drying treatment on the fiber;

[0019] Perform in-situ growth of PANI on the surface of the fiber grating region.

[0020] As a further solution, the process of performing in-situ growth of PANI on the surface of the fiber grating region is specifically as follows:

[0021] Disperse aniline monomers in an acid solution with a set pH value to form an aniline-acid solution;

[0022] Disperse ammonium persulfate as an initiator uniformly in an acid solution with the same pH value as above to form an ammonium persulfate-acid solution;

[0023] Vertically immerse the surface-treated fiber grating region into the aniline-acid solution and cool it; add the ammonium persulfate-acid solution to the aniline-acid solution to form a reaction solution, and make aniline start to polymerize and grow on the surface of the grating region;

[0024] At the same time, monitor the intensity of the central wavelength of the fiber grating reflection spectrum. After meeting the set requirements, take out the fiber.

[0025] As a further solution, after the in-situ growth is completed, it further includes:

[0026] Immerse the fiber in deionized water to wash the residual impurities on the surface of the grating region;

[0027] Completely immerse the grating region in an ammonia water solution for de-doping treatment of the PANI film;

[0028] Take the optical fiber out of the ammonia aqueous solution, then immerse it in deionized water to remove the residual ammonia molecules on the surface, and then immerse it in the corresponding acid solution to adjust its pH value for redoping the PANI film; finally, take out the optical fiber and dry it thoroughly.

[0029] In some other embodiments, the following technical solutions are adopted:

[0030] A sensing system for detecting the concentration of NH 3 comprises: an optical fiber grating demodulator, the above-mentioned micro-nano optical fiber gas sensor and a computer; the input optical fiber of the micro-nano optical fiber gas sensor is connected to the optical fiber grating demodulator, and the optical fiber grating demodulator is connected to the computer.

[0031] When detecting the concentration of NH 3 , the optical signal output by the built-in light source of the optical fiber grating demodulator enters the grating region through the input optical fiber of the micro-nano optical fiber gas sensor. The PANI film on the surface of the grating region adsorbs NH 3 in the environment. NH 3 molecules capture the protons on the imine nitrogen in the PANI molecular chain, causing the conductivity of the PANI film to decrease, resulting in the drift of the center wavelength of the reflection spectrum in the optical fiber core, and then calculating the concentration of NH 3 .

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] (1) The present invention organically combines the refractive index sensitivity characteristic of the micro-nano optical fiber and the gas sensitivity characteristic of PANI, in-situ grows the PANI film on the surface of the optical fiber grating region. The obtained sensor can detect NH 3 in the concentration range of 1-100 ppm, and has the advantages of high sensitivity, fast response / recovery time and good stability; it can be used for the test of gas, liquid components or concentration.

[0034] (2) During the in-situ growth of PANI on the surface of the optical fiber grating region of the present invention, by monitoring the signal change of the micro-nano optical fiber grating in real time through the demodulator, and at the same time controlling the growth rate and morphology of PANI by adjusting the pH value of the reaction solution, a PANI film with uniform particle size, uniform thickness and porous can be prepared, and the PANI film is firmly combined with the surface of the optical fiber grating region and is not easy to fall off; a gas sensor with stable signal, moderate film thickness and excellent gas sensitivity can be obtained.

[0035] (3) In the present invention, the dedoping and redoping processes of the PANI film are, on the one hand, to remove the unreacted small molecules, oligomers and initiator molecules during the reaction process, eliminate their interference with the gas sensitivity performance of the sensor, and on the other hand, to realize the reversible regulation of the conductivity of PANI and obtain the rapid response / recovery characteristic to NH 3 .

[0036] Other features and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the micro-nano FBG gas sensor structure in an embodiment of the present invention;

[0038] Figure 2 For detecting NH 3 Concentration sensing system structure diagram;

[0039] Figure 3 Electron microscope photograph of the micro-nano optical fiber with surface in-situ growth of PANI in an embodiment of the present invention;

[0040] Figure 4 Based on the surface in-situ growth of PANI in the micro-nano FBGNH 3 Relationship curve between the response value of the sensor and the NH 3 Concentration;

[0041] Figure 5 Stability test curve of the sensor within 60 days in an embodiment of the present invention;

[0042] Wherein, 1. Input optical fiber, 2. Fiber grating area probe, 3. Fiber grating demodulator, 4. Computer; 101. Protective layer, 102. Cladding, 103. Core, 201. Grating, 202. PANI film. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0044] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0045] Embodiment 1

[0046] As described in the background, due to the micro-nano optical fiber itself being sensitive to NH 3The detection sensitivity is low and it does not have excellent gas selectivity. Therefore, in this embodiment, this problem is solved by coating a special ammonia-sensitive material. And how to realize the optimization and processing of the fiber grating diameter, how to realize the optimization and modification processing of the sensitive coating material on this basis, how to realize the thickness control and uniform coating of the sensitive coating material on the surface of the fiber grating, and how to stably and firmly bond with the fiber grating are the main challenges faced at present.

[0047] Based on this, in one or more embodiments, a micro-nano fiber gas sensor based on in-situ growth of polymers on the surface is disclosed. In this embodiment, an FBG gas sensor is taken as an example (of course, other micro-nano fibers based on the refractive index sensitive mechanism are also within the protection scope of the present invention, and the same applies to the following embodiments), combined with Figure 1 , specifically including: an input optical fiber 1 and a fiber grating area probe 2 connected in sequence; wherein, the input optical fiber 1 is a single-mode optical fiber, including a core 103, a cladding 102, and a protective layer 101 arranged from the inside to the outside in sequence; a part of the fiber grating area probe 2 is etched, and a grating 201 is arranged inside the part of the fiber grating area probe, and a PANI film 202 is covered on the surface of the fiber grating area probe 2, and the PANI film is grown in-situ on the surface of the fiber grating area probe.

[0048] In this embodiment, an optical fiber Bragg grating is used. The optical fiber diameter is 250 μm, the grating area diameter (after removing the polyimide or polyamide protective layer on the surface) is 125 μm, the core diameter is about 8 - 9 μm, and the length of each grating area is 3 - 10 mm.

[0049] The diameter of the fiber grating area probe is 4 - 12 μm, and a layer of PANI film is covered on its surface; the thickness of the PANI film is 0.1 - 2 μm, and the PANI film is grown in-situ on the surface of the fiber grating area probe.

[0050] The PANI film can adsorb a set gas (such as NH 3 ) in the environment and reduce its conductivity, thereby causing the drift of the center wavelength of the reflection spectrum in the fiber grating, and further realizing the detection of the concentration of the set gas.

[0051] The thickness of the PANI film is related to the in-situ growth time, and the in-situ growth time is determined by the intensity of the reflection spectrum in the fiber grating. When the intensity of the reflection spectrum decays to a set range, the in-situ growth is controlled to end.

[0052] In some embodiments, it further includes: a substrate for fixing the fiber grating area probe, and both ends of the fiber grating area probe are fixed on the substrate. The substrate for fixing the grating area can be used to fix the etched grating area to avoid its adverse effects on the in-situ growth of PANI and the gas-sensing test process due to unstable fluttering during subsequent operations.

[0053] Due to its good chemical and environmental stability, low cost, simple synthesis, controllable conductivity, and unique doping method, etc., PANI has been widely used in the field of gas sensing. In this embodiment, PANI is introduced as the sensitive material, combining the refractive index sensitive characteristics of micro-nano optical fiber and the gas sensitive characteristics of PANI organically. A PANI film is in-situ grown on the surface of the fiber grating region, solving the problem that the FBG sensor itself is insensitive to gas detection; at the same time, through the method of in-situ growth of the PANI film, the technical problems of thickness control and uniform coating of the polymer on the surface of the fiber grating and its stable and firm combination with the fiber grating are effectively solved.

[0054] The micro-nano FBG gas sensor based on in-situ growth of polymer on the surface in this embodiment has high detection accuracy, high sensitivity and excellent stability, and can be used for testing the composition or concentration of gases and liquids. For example, it can realize the detection of the concentration of gases such as NH 3 、H 2 S、N 2 O, etc.

[0055] Example Two

[0056] In one or more embodiments, a preparation method of a micro-nano optical fiber gas sensor based on in-situ growth of polymer on the surface is disclosed. Taking the FBG gas sensor as an example, the specific process is as follows:

[0057] Step (1): Connect an optical fiber to a demodulator, and etch the fiber grating region with hydrofluoric acid (HF) solution. When the central wavelength of the fiber grating reflection spectrum is monitored to blue-shift by 1.2 - 1.8 nm, take out the optical fiber and wash the surface of the grating region with deionized water multiple times to remove the residual HF molecules on the surface of the grating region;

[0058] Step (2): Take out the optical fiber from deionized water and perform surface treatment on the grating region; The surface treatment method can be soaking treatment with an aqueous solution of ammonia - hydrogen peroxide (NH 3 ·H 2 O-H 2 O 2 ), or it can also be soaking treatment with a silane coupling agent solution;

[0059] Among them, the NH 3 ·H 2 O-H 2 O 2 aqueous solution is a solution prepared by mixing NH 3 ·H 2 O, H 2 O 2 and deionized water in a certain volume ratio; The silane coupling agent can be selected from vinyl silane, amino silane or methacryloxy silane type coupling agent.

[0060] Step (3): Place the fiber optic that has been surface-treated in a drying oven for drying treatment, and control the temperature of the drying oven at 60 - 120°C;

[0061] Step (4): Conduct in-situ growth of PANI on the surface of the fiber optic grating region.

[0062] In this step, the specific process of conducting in-situ growth of PANI is as follows:

[0063] Step (4-1): Disperse aniline monomers in an acid solution with a certain pH value to form an aniline-acid solution; among them, the acid solution can be one of inorganic acids or organic acids, such as hydrochloric acid, sulfuric acid, or camphorsulfonic acid.

[0064] Step (4-2): Disperse ammonium persulfate, the initiator, in an acid solution with the same pH value as above, and stir evenly to form an ammonium persulfate-acid solution.

[0065] Step (4-3): Vertically immerse the surface-treated fiber optic grating region into the above-mentioned aniline-acid solution, and place it in an ice-water bath (0 - 5°C) for cooling. Then, gradually and slowly add the ammonium persulfate-acid solution to the aniline-acid solution to enable aniline to start polymerizing and growing on the surface of the grating region;

[0066] Step (4-4): Observe the intensity of the center wavelength of the reflection spectrum of the fiber optic grating region output by the demodulator on the computer. When its intensity drops to a certain range, take out the fiber optic.

[0067] In this embodiment, the thickness of the PANI film grown on the surface of the fiber optic grating region is related to the growth time. The longer the in-situ growth time, the thicker the film. However, in this embodiment, the PANI film should not be too thick, as being too thick easily causes a large attenuation of the signal intensity of the fiber optic, to the extent that it cannot be detected. Therefore, it is necessary to monitor the change in signal intensity during the growth process in real time, and control the end time of the growth of the polymer film by the attenuation of the signal intensity to a certain range, that is, the film thickness that meets the fiber optic signal intensity range is more appropriate.

[0068] After the above in-situ growth process is completed, conduct acid doping and re-doping treatment of PANI. The specific process is as follows:

[0069] Take out the fiber optic on which the surface growth of PANI has been completed, immerse it in deionized water to wash away impurities such as unreacted aniline monomers, oligomers, and initiators remaining on the surface of the grating region; then completely immerse the grating region in an ammonia water solution for a period of time to conduct de-doping treatment of the PANI thin film; finally, take out the fiber optic from the ammonia water solution, immerse it in deionized water again to remove the ammonia molecules remaining on the surface, and then immerse it in the corresponding acid solution for a period of time to adjust its pH value to conduct re-doping of the PANI thin film; after completion, take out the fiber optic and transfer it to a drying oven for thorough drying.

[0070] In this embodiment, the dedoping and re-doping processes of the PANI film are carried out for two purposes. On the one hand, it is to remove unreacted aniline monomers, oligomers and residual initiator molecules during the reaction process, eliminate their interference with the gas-sensing performance of the sensor, and obtain a sensor with stable performance; on the other hand, it is to realize the reversible regulation of the conductivity of PANI and obtain a rapid response / recovery characteristic to NH 3 .

[0071] The preparation method of this embodiment will be specifically described below in different implementation manners.

[0072] 1. Taking the preparation process of the micro-nano fiber NH 3 sensor doped with hydrochloric acid PANI as an example, the preparation method of this embodiment will be specifically described.

[0073] In this embodiment, the grating region of the FBG is first etched with a 40 wt% HF solution. When the central wavelength of the reflection spectrum of the grating region is blue-shifted by 1.2 nm after etching, the optical fiber is taken out and the surface of the grating region is washed with deionized water multiple times. The SEM photograph of the etched bare optical fiber is as shown in Figure 3 (a). It can be seen that the thickness of the optical fiber grating region after etching is uniform, and the surface is relatively smooth, without obvious mottles and defects. Then the optical fiber grating region is immersed in NH 3 ·H 2 O-H 2 O 2 aqueous solution (the ratio is NH 3 ·H 2 O:H 2 O 2 :H 2 O = 1:1:5) for 3 h, and then the optical fiber is taken out and dried in an oven at 40 °C for 2 h. The dried optical fiber grating region is placed in an aniline-hydrochloric acid solution with a pH value of 0.3 and an aniline concentration of 0.1 mol / L. After cooling in an ice-water bath for 30 min, an ammonium persulfate-hydrochloric acid solution (0.025 mol / L) is slowly added dropwise to the aniline-hydrochloric acid solution to start the in-situ growth of PANI on the surface of the optical fiber grating region. At the same time, the optical fiber is connected to a demodulator, and the intensity change of the reflection spectrum is monitored in real time on the software user interface of the computer. When the intensity of the reflection spectrum drops to -25 dB, the optical fiber is taken out and immersed in deionized water to wash the residual impurity molecules on the surface. Subsequently, the optical fiber grating region is immersed in an ammonia water solution with a mass fraction of 1% for 30 min, then the optical fiber grating region is taken out, washed with deionized water, and then immersed in a hydrochloric acid solution with the same pH value for 30 min for re-doping. After that, the optical fiber is taken out and dried in an oven at 40 °C for 2 h. The SEM photograph of the optical fiber grating region with PANI grown on the surface prepared by this embodiment is as shown in Figure 3As shown in (b), it can be seen that a layer of PANI film grows uniformly on the surface of the fiber grating region. Moreover, the PANI film has uniform particle size, uniform thickness, is porous, and is firmly combined with the fiber grating region without falling-off defects.

[0074] 2. Taking the preparation process of the micro-nano FBG sensor doped with hydrochloric acid PANI as an example, the preparation method of this embodiment will be specifically described. 3 For example, taking the preparation process of the micro-nano FBG sensor doped with hydrochloric acid PANI as an example, the preparation method of this embodiment will be specifically described.

[0075] In this embodiment, the grating region of the FBG is first etched with a 24wt% HF solution. When the center wavelength of the reflection spectrum of the grating region is blue-shifted by 1.4 nm after etching, the optical fiber is taken out and the surface of the fiber grating region is washed with deionized water multiple times. Then, the fiber grating region is immersed in an aqueous solution of KH550 (γ-aminopropyltriethoxysilane) for 2 h, and then the optical fiber is taken out and placed in a drying oven at 80 °C for drying for 2 h. The dried fiber grating region is placed in an aniline-hydrochloric acid solution with a pH value of 0 and an aniline concentration of 0.1 mol / L, and the whole is placed in an ice-water bath for cooling for 30 min. Then, an ammonium persulfate-hydrochloric acid solution (0.05 mol / L) is slowly added dropwise to the aniline-hydrochloric acid solution to initiate the in-situ growth of PANI on the surface of the fiber grating region. At the same time, the optical fiber is connected to a demodulator, and the intensity change of the reflection spectrum is monitored in real time on the software user interface of the computer until the optical fiber is taken out when the intensity of the reflection spectrum drops to -30 dB, and it is immersed in deionized water to wash away the residual impurity molecules on the surface. Subsequently, the fiber grating region is immersed in an ammonia water solution with a mass fraction of 1% for 30 min, then the fiber grating region is taken out and washed with deionized water, and then immersed in a hydrochloric acid solution with the same pH value for 60 min for re-doping. After that, the optical fiber is taken out and placed in a drying oven at 40 °C for drying for 3 h. The SEM photo of the fiber grating region with PANI grown on the surface prepared by this embodiment is as Figure 3 As shown in (c), it can be seen that a layer of PANI film grows uniformly on the surface of the fiber grating region. Moreover, the PANI film has uniform particle size, a thickness thicker than that of the film in Example 1, and is tightly combined with the fiber grating region without falling-off defects.

[0076] The gas-sensing performance of the sensor prepared according to the above technical solution is tested. The sensor is respectively placed in a sealed container containing 10 ppm, 20 ppm, 50 ppm, and 100 ppm of NH 3 After the center wavelength of the reflection spectrum of the fiber grating region displayed on the software user interface of the computer is stable, its offset value is calculated, and the relationship curve between the wavelength offset and the concentration of NH Figure 4 as shown is obtained. It can be seen that the sensor prepared in this embodiment has a good response to NH 3 in the range of 10 - 100 ppm. 3 has a good response.

[0077] 3. Taking the preparation process of the micro-nano FBG doped with camphorsulfonic acid PANI NH 3 sensor as an example, the preparation method of this embodiment will be specifically described.

[0078] In this embodiment, the grating region of the FBG is first etched with a 24 wt% HF solution. When the central wavelength of the reflection spectrum of the grating region is blue-shifted by 1.4 nm after etching, the optical fiber is taken out and the surface of the fiber grating region is washed with deionized water multiple times. Then the fiber grating region is immersed in an aqueous solution of NH 3 ·H 2 O-H 2 O 2 (the ratio is NH 3 ·H 2 O:H 2 O 2 :H 2 O = 1:1:5) for 3 h, and then the optical fiber is taken out and placed in a drying oven at 40 °C for 2 h. The dried fiber grating region is placed in an aniline-camphorsulfonic acid solution with a pH value of 0.3 and an aniline concentration of 0.1 mol / L. The whole is placed in an ice-water bath and cooled for 30 min. Then an ammonium persulfate-camphorsulfonic acid solution (0.05 mol / L) is slowly added dropwise to the aniline-camphorsulfonic acid solution to start the in-situ growth of PANI on the surface of the fiber grating region. At the same time, the optical fiber is connected to a demodulator, and the intensity change of the optical reflection spectrum is monitored in real time on the software user interface of the computer. When the intensity of the reflection spectrum drops to -30 dB, the optical fiber is taken out and immersed in deionized water to wash away the residual impurity molecules on the surface. Subsequently, the fiber grating region is immersed in a 1% ammonia water solution by mass for 30 min, then the fiber grating region is taken out, washed with deionized water, and then immersed in a camphorsulfonic acid solution with the same pH value for 60 min for re-doping. After that, the optical fiber is taken out and placed in a drying oven at 40 °C for drying for 3 h. The SEM photograph of the fiber grating region with PANI grown on the surface prepared by this embodiment is as shown in Figure 3 (d) in, it can be seen that a layer of PANI film grows uniformly on the surface of the fiber grating region, and the particle size of the PANI film is uniform, the film thickness is thicker than that in Example 1, and it is tightly combined with the fiber grating region without shedding defects.

[0079] The gas-sensing stability of the sensor prepared according to the above technical solution is tested. Every 5 days, the sensor is placed in a sealed container of 100 ppm NH 3 , and a response test is carried out and the wavelength shift value is recorded. A total of 12 tests are carried out for 60 days, and the stability curve of the sensor as shown in Figure 5 is obtained. It can be seen that the sensor prepared in this embodiment has excellent stability.

[0080] Example Three

[0081] In one or more embodiments, a method for detecting NH 3 The concentration sensing system is based on the micro-nano FBG NH 3 The sensor is used as an example to illustrate the Figure 2 , specifically comprising: a fiber Bragg grating demodulator 3, the NH of the micro-nano FBG described in the first embodiment 3 Sensors and computers 4; NH of micro-nano FBG 3 The input optical fiber 1 of the sensor is connected to the fiber Bragg grating demodulator 3 , and the fiber Bragg grating demodulator 3 is connected to the computer 4 .

[0082] The NH of the micro-nano FBG in this embodiment 3 The sensor can be prepared by the method described in Example 2.

[0083] When detecting gas, place the sensor in a certain concentration of NH 3 In the environment; the PANI film on the surface of the fiber grating area absorbs NH 3 , NH 3 The molecules capture the protons on the imine nitrogen in the PANI molecular chain, which reduces the conductivity of the PANI film, thereby causing the change of the effective refractive index of the PANI / micro-nano fiber grating composite waveguide, and finally causing the drift of the central wavelength of the core reflection spectrum. The NH 3 concentration.

[0084] This embodiment is based on the surface in-situ growth of PANI micro-nano optical fiber NH 3 Sensor, acid-doped PANI to NH 3 It has reversible adsorption / desorption characteristics, which can significantly improve the sensor's detection of NH 3 The sensitivity of the molecule and the effective reduction of the response / recovery time; adsorption / desorption of NH 3 , changing the conductivity and dielectric constant of the PANI film, thereby changing the refractive index of the PANI film, and further changing the effective refractive index of the PANI / micro-nano optical fiber composite waveguide, thereby achieving different concentrations of NH 3 Detection.

[0085] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A micro-nano fiber gas sensor based on in-situ grown polymer, characterized in that, it includes: An input optical fiber and a fiber grating area probe connected in sequence; The fiber grating is a fiber Bragg grating; The preparation method of the micro-nano fiber gas sensor based on in-situ grown polymer includes: Etching the fiber grating area while monitoring the central wavelength of the fiber grating reflection spectrum. When the central wavelength blueshifts by 1.2 - 1.8 nm, soak the grating area with deionized water to remove the residual impurities on the surface of the grating area; Take the optical fiber out of the deionized water; soak it with an aqueous solution of ammonia - hydrogen peroxide, or soak it with a silane coupling agent solution; then dry the optical fiber; Perform in-situ growth of PANI on the surface of the fiber grating area; After the in-situ growth is completed, it further includes: Immerse the optical fiber in deionized water to clean the residual impurities on the surface of the grating area; Completely immerse the grating area in an ammonia water solution for dedoping treatment of the PANI film; Take the optical fiber out of the ammonia water solution, then immerse it in deionized water to remove the residual ammonia molecules on the surface, and then immerse it in the corresponding acid solution to adjust its pH value for re-doping of the PANI film; finally take out the optical fiber and dry it thoroughly; The process of performing in-situ growth of PANI on the surface of the fiber grating area is specifically: Disperse aniline monomer in an acid solution with a set pH value to form an aniline - acid solution; Disperse ammonium persulfate initiator evenly in an acid solution with the same pH value as above to form an ammonium persulfate - acid solution; Vertically immerse the surface-treated fiber grating area into the aniline - acid solution and cool it; add the ammonium persulfate - acid solution into the aniline - acid solution to form a reaction solution, and make aniline start to polymerize and grow on the surface of the grating area; At the same time, monitor the intensity of the central wavelength of the fiber grating reflection spectrum. After meeting the set requirements, take out the optical fiber.

2. A micro-nano fiber gas sensor based on in-situ grown polymer according to claim 1, characterized in that, it further includes: A substrate for fixing the fiber grating area probe, and both ends of the fiber grating area probe are fixed on the substrate.

3. A micro-nano fiber gas sensor based on in-situ grown polymer according to claim 1, characterized in that, The silane coupling agent is selected from vinyl silane, amino silane or methacryloxy silane type coupling agent.

4. A sensing system for detecting the concentration of NH 3 ​ characterized in that, it includes: A fiber grating demodulator, a micro-nano fiber gas sensor based on in-situ grown polymer according to any one of claims 1 - 3 and a computer; the input optical fiber of the micro-nano fiber gas sensor is connected to the fiber grating demodulator, and the fiber grating demodulator is connected to the computer.

5. A sensing system for detecting the concentration of NH 3 as described in claim 4 characterized in that, Conduct NH 3 During concentration detection, the optical signal output by the built-in light source of the fiber Bragg grating demodulator enters the grating area through the input optical fiber of the micro-nano optical fiber gas sensor, and the PANI film on the surface of the grating area absorbs NH 3 , NH 3 The molecule captures the proton on the imine nitrogen in the PANI molecular chain, which reduces the conductivity of the PANI film and causes the drift of the central wavelength of the reflection spectrum in the fiber grating. 3 concentration.

Images