CN106645026A - Quantum dot fiber gas sensor and preparation method thereof - Google Patents

Abstract

The invention discloses a quantum dot optical fiber gas sensor and a preparation method thereof. The quantum dot optical fiber gas sensor includes a gas sensitive layer composed of an optical fiber probe and colloidal quantum dots; the gas sensitive layer is evenly coated on the refractive index sensitive area of the optical fiber probe ; when the gas-sensitive layer absorbs gas, the carrier concentration changes, which causes the refractive index of the gas-sensitive layer to change, causing the optical field in the fiber probe to change, the extinction ratio of the spectrum to change, and the spectrum to drift at the same time; according to the extinction ratio It has the characteristics of high gas sensitivity and easy distributed networking; and the preparation method proposed in the present invention uses the layer-by-layer electrostatic self-assembly method to obtain uniform, adjustable thickness, and excellent adhesion. Sensitive layer; through doping or surface modification with short-chain ligand solution, the characteristics of different target gases can be adjusted to further improve the sensitivity and selectivity of target gases.

Description

A kind of quantum dot optical fiber gas sensor and preparation method thereof

technical field

The invention belongs to the technical field of gas detection and sensing, and more specifically relates to a quantum dot optical fiber gas sensor and a preparation method thereof.

Background technique

Gas sensors are widely used in the detection of flammable and explosive gases and toxic and harmful gases. The currently widely used semiconductor resistive gas sensor has the characteristics of simple use and good portability, but it has the disadvantages of high working temperature (usually needs to be heated to 200°C), and the detection accuracy is not high, making it difficult to accurately detect ppb-level low concentrations Gas; on the other hand, semiconductor resistive gas sensors rely heavily on carrier mobility to obtain a faster response, and their electrical response signals are susceptible to electromagnetic interference; and are not suitable for detecting gases dissolved in liquids.

Optical gas sensors based on gas non-dispersive infrared absorption (NDIR) spectroscopy can overcome the problems of poor selectivity and high operating temperature of electrical gas sensors, but in order to obtain a significant gas response signal, the optical path distance of the gas chamber is required to be long enough , thus limiting the miniaturization and portability of the sensor.

The optical fiber gas sensor based on the fluorescence characteristics of quantum dots deposits silicon quantum dots on the end face of the optical fiber, uses the characteristics that the fluorescence characteristics of quantum dots change with the target gas to realize gas detection, and uses optical fiber as the signal carrier to realize signal sensing; but In this sensor, only a small part of the quantum dots on the fiber end face can be excited by the light transmitted in the fiber core to generate fluorescence signals, and only a small part of these fluorescence can be collected by the fiber for spectrometer measurement and sensing, so The fluorescent properties of quantum dots have not been fully utilized, and the gas sensitivity of the fiber-optic gas sensor made is low.

Contents of the invention

Aiming at the above defects or improvement needs of the prior art, the present invention provides a quantum dot optical fiber gas sensor and its preparation method. The problem of low gas sensitivity.

In order to achieve the above object, according to one aspect of the present invention, a kind of quantum dot optical fiber gas sensor is provided, comprising an optical fiber probe and a gas-sensitive layer coated on the end of the optical fiber probe; the optical fiber probe is composed of a photonic crystal fiber and a single-mode The optical fiber interference structure formed by optical fiber fusion has a refractive index sensitive area at the end; the gas-sensitive layer uses colloidal quantum dots, and uses the room temperature gas-sensing effect of the colloidal quantum dots’ conductivity and refractive index to change with the gas to realize gas detection.

Preferably, the gas-sensing layer has a thickness of 50 nm to 500 nm.

Preferably, the colloidal quantum dots are lead sulfide (PbS) colloidal quantum dots or tin oxide (SnO ₂ ) colloidal quantum dots.

Preferably, the above-mentioned optical fiber probe is formed by fusion splicing and cutting of a photonic crystal fiber (PCF) and a single-mode optical fiber (SMF), and the fusion site has fusion bubbles; the inside of the retained PCF section has a pore structure; the end of the optical fiber probe is melted by the PCF The formed hemispherical fusion ball is the part sensitive to the refractive index on the fiber probe; the gas-sensitive layer is evenly coated on the end of the fiber probe.

Preferably, the total length of the optical fiber probe is 300 μm to 600 μm; there are fusion bubbles with a diameter of 10 μm to 30 μm at the fusion joint between the photonic crystal fiber and the single-mode fiber; the distance between the PCF pore structure and the fusion bubble is 50 μm to 100 μm; the length of the PCF pore structure is 10 μm to 100 μm, and the radius of the welded ball is 60 μm to 100 μm.

In the quantum dot optical fiber gas sensor proposed by the present invention, when the gas-sensitive layer absorbs gas, the conductivity changes, the carrier concentration changes, and the real and imaginary parts of the dielectric constant all change, so that the real and imaginary parts of the refractive index of the gas-sensitive layer change. The change of the refractive index of the gas-sensitive layer makes the optical field in the fiber optic probe change, reflecting the change of the extinction ratio on the spectrum, and the shift of the spectral phase; the gas concentration is obtained according to the change of the extinction ratio and the spectral phase .

According to another aspect of the present invention, a kind of preparation method for the optical fiber probe of above-mentioned quantum dot optical fiber gas sensor is provided, comprising the steps:

(1) Welding the single-mode fiber (SMF) and the photonic crystal fiber (PCF) to obtain the fusion structure of SMF and PCF; the welding part has fusion bubbles;

(2) cutting the welded structure of SMF and PCF obtained in step (1), retaining a section of PCF;

(3) The retained PCF described in step (2) is melted, and the fusion part forms a hemispherical fusion ball, and the preparation of the optical fiber probe is completed.

According to another aspect of the present invention, a kind of preparation method of quantum dot fiber optic gas sensor is provided, comprising the following steps:

(1) Soak the above-mentioned optical fiber probe in an alkaline or acidic solution to ionize its surface; then soak in a colloidal quantum dot solution with opposite electrical properties, so that the colloidal quantum dots are combined on the surface of the optical fiber probe to form a quantum dot film;

(2) Use the short-chain ligand solution to soak the optical fiber probe covered with the quantum dot film obtained in step (1) to remove the long chain coated on the surface of the quantum dot and modify the film to facilitate the subsequent recombination of the quantum dot and film thickness growth;

(3) using anhydrous methanol solution to soak the product obtained in step (2), to remove residual short-chain ligands and by-products generated by the reaction between the short-chain ligand solution and the colloidal quantum dot solution;

(4) Steps (1) to (3) are repeated so that the thickness of the quantum dot film reaches 50 nm to 500 nm, and the preparation of the quantum dot optical fiber gas sensor is completed.

Preferably, the colloidal quantum dot solution used in the above step (1) is a PbS colloidal quantum dot solution or a SnO ₂ colloidal quantum dot solution.

According to another aspect of the present invention, another kind of preparation method of quantum dot fiber optic gas sensor is provided, comprising the steps:

(1) The fiber probe is placed in the colloidal quantum dot synthesis precursor to react, so that the quantum dots automatically grow and uniformly form a film on the surface of the fiber probe;

(2) annealing the optical fiber probe grown with quantum dot film obtained in step (1) to remove synthetic precursors and byproducts thereof;

(3) using short-chain ligand solution to soak the product obtained in step (2) and drying to modify the gas-sensitive layer to enhance the gas adsorption activity of the gas-sensitive layer;

(4) Step (3) is repeated to fully modify the gas-sensitive layer and complete the preparation of the quantum dot optical fiber gas sensor.

Preferably, in the above method for preparing a quantum dot optical fiber gas sensor, in step (1), the reaction temperature is 120°C-150°C, and the reaction time is 60s-300s.

Preferably, in the above method for preparing a quantum dot optical fiber gas sensor, in the step (2), the annealing temperature is 120°C-300°C, and the annealing time is 0.5h-3h.

Preferably, in the preparation methods of the above two kinds of quantum dot optical fiber gas sensors, the short-chain ligand solutions used are ammonium chloride (NH ₄ Cl) solution, sodium nitrite (NaNO ₂ ) solution, copper chloride (CuCl ₂ ) solution, silver nitrate (AgNO ₃ ) solution or copper nitrate (Cu(NO ₃ ) ₂ ) solution.

Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

(1) The quantum dot optical fiber gas sensor provided by the present invention is different from the optical fiber gas sensor based on the fluorescence characteristics of quantum dots. The present invention utilizes the room temperature gas-sensing effect that the conductivity and refractive index of colloidal quantum dots change with the gas, and does not rely on Mobility; due to the huge specific surface area, colloidal quantum dots have high surface activity and are very sensitive to ambient gases. Once the target gas is adsorbed, the carrier concentration doubles and the charge transfer is faster. Combined with the design and preparation process of fiber optic probes The selection of medium and short-chain ligand solutions and the control of soaking time, quantum dot synthesis temperature and time can obtain a significant room temperature gas-sensitive response signal;

(2) The quantum dot optical fiber gas sensor provided by the present invention, compared with the semiconductor resistive gas sensor based on colloidal quantum dots, the present invention utilizes the optical fiber sensing technology with large amount of transmission information and own reference to realize a A new type of fiber optic gas sensor that is passive and anti-electromagnetic interference is suitable for the detection of dissolved gases in liquids. In addition, it is easy to array and suitable for remote sensing and networking. Combined with back-end detection technology and pattern recognition technology, it can Applied to the monitoring of multi-component complex gas environment;

(3) The preparation method of the quantum dot optical fiber gas sensor provided by the present invention, because the colloidal quantum dot has excellent room temperature film-forming performance, is easy to be deposited on the surface of the optical fiber, compared with existing methods such as magnetron sputtering, thermal evaporation, chemical Time-consuming, complicated and costly film deposition technologies such as gas phase deposition, the present invention uses a simple and efficient layer-by-layer electrostatic self-assembly method to obtain a gas-sensitive layer with uniform thickness, adjustable thickness and excellent adhesion;

On the other hand, the physical and chemical properties of colloidal quantum dots are highly controllable, and different short-chain ligand solutions can be used for doping or surface modification, and the characteristics of different target gases can be adjusted to improve the activity of the target gas and reduce the impact on the target gas. The activity of other interfering gases improves stability, combined with the optimization of quantum dot components and their grain size, further improves the sensitivity and selectivity to target gases, and reduces the detection limit.

Description of drawings

Fig. 1 is the quantum dot fiber optic gas sensor structure schematic diagram that embodiment 1 provides;

Fig. 2 is the system structural representation that adopts the quantum dot fiber optic gas sensor that embodiment 1 provides to carry out gas testing;

Fig. 3 is the structural representation of the optical fiber probe that embodiment 2 adopts;

Fig. 4 is the structural representation of the optical fiber probe that embodiment 3 adopts;

Fig. 5 is the flow chart of the preparation method of the quantum dot optical fiber gas sensor provided by the present embodiment 4;

Throughout the drawings, the same reference numerals are used to designate the same elements or structures, wherein:

1-SMF, 2-PCF, 3-fusion bubble, 4-PCF core air hole structure, 5-fusion ball, 6-gas sensitive layer, 7-broadband light source, 8-spectrometer, 9-circulator, 10-quantum dot Fiber optic gas sensor, 11-target detection gas, 12-air cavity, 13-Bragg grating.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

The quantum dot optical fiber gas sensor provided by the present invention includes an optical fiber probe and a gas-sensing layer; the gas-sensing layer is evenly coated on the end of the optical fiber probe; it will be specifically described in conjunction with the examples below.

The structure of the quantum dot fiber optic gas sensor provided by embodiment 1 is as shown in Figure 1, including an optical fiber probe and a gas-sensitive layer; the gas-sensitive layer is coated on the hemispherical end of the fiber-optic probe;

In Example 1, the optical fiber probe is composed of a single-mode optical fiber and a photonic crystal optical fiber; the fusion site has a fusion bubble with a radius of 10 μm to 30 μm, and inside the PCF, there is a photonic crystal with a length of 100 μm to 300 μm at a distance of 200 μm from the fusion bubble. Optical fiber air hole structure, the end of the optical fiber probe is a hemispherical fusion ball with a radius of 60 μm to 100 μm obtained by melting PCF; the total length of the optical fiber probe (from the fusion bubble to the end) is 400 μm to 700 μm; the fiber probe’s The spectrum is multi-beam interference, which changes with the change of the refractive index at the end of the fiber probe; in Example 1, the gas-sensitive layer is a PbS quantum dot coating with a thickness of 50nm.

Adopt the quantum dot fiber optic gas sensor that embodiment 1 provides to carry out gas concentration test, test system as shown in Figure 2, comprises broadband light source, circulator, quantum dot fiber optic gas sensor and spectrometer; Quantum dot fiber optic gas sensor is placed in the gas to be measured , the refractive index of the gas-sensitive layer responds to changes in gas concentration.

The light emitted from the broadband light source reaches the fiber probe through the circulator; since the gas changes the refractive index of the quantum dots in the gas-sensitive layer, the change in the refractive index is reflected on the interference pattern of the fiber probe; the reflected light containing gas information passes through The circulator is received by the spectrometer, and the gas concentration information is obtained through the change of the interference pattern of the fiber optic probe on the spectrometer; compared with the application test system of the gas sensor through the fluorescence effect, the system is simple, does not require excitation light, and the optical signal intensity loss Small enough to be observed with ordinary spectrometers.

The fiber probe structure adopted in embodiment 1, its preparation process is as follows:

(1) The single-mode fiber (SMF) and the photonic crystal fiber (PCF) are welded, and fusion bubbles are formed at the fusion joint;

(2) cutting the welded structure of SMF and PCF obtained in step (1), and retaining a PCF of 500 μm to 1000 μm;

(3) The retained PCF is melted to obtain a fused sphere, and the interference spectrum of the fiber probe is sensitive to the refractive index of the fused sphere at the end.

The optical fiber probe that the present invention can adopt is not limited to the structure that embodiment 1 provides; The optical fiber probe that adopts in embodiment 2, its structure as shown in Figure 3, is to use the method for focusing ion beam to etch SMF, A rectangular area is obtained in the center of the SMF to form a microstructure fiber interference probe. Its preparation process is as follows:

(1) Cut the SMF to keep a smooth fiber end face;

(2) The SMF fiber obtained in step (1) is etched on the SMF by means of focused ion beam implantation (FIB), and a rectangular area with a length of 20 μm to 10 μm and a width greater than 10 μm is obtained by etching in the center of the SMF. The distance from the end of the SMF is equivalent to the length of the air hole etched by the FIB, forming a microstructure optical fiber interference probe;

The optical fiber probe interference structure prepared by the above method modulates the wavelength through multiple FP interferences between the air region and the terminal quartz region, and its interference spectrum is sensitive to the refractive index of the end and the air hole.

The fiber probe adopted in embodiment 3 has a structure as shown in Figure 4; the SMF is fused with the thin optical fiber, the length of the thin optical fiber is controlled to be 300 μm to 500 μm, and Bragg grating (FBG) is etched on the thin optical fiber; its preparation The process is as follows;

(1) Connect SMF and thin fiber (T-F);

(2) cutting the thin optical fiber part, and retaining the thin optical fiber with a length of 300 μm to 500 μm;

(3) Use a femtosecond laser to periodically etch the thin fiber according to the designed FBG parameters;

The optical fiber probe interference structure prepared by the above method can measure the refractive index change at the end of the FBG central wavelength.

The preparation method of the quantum dot fiber optic gas sensor provided by the present invention will be described in detail below in conjunction with specific examples.

The preparation method of the quantum dot fiber optic gas sensor provided by embodiment 4 comprises the steps:

(1) Using lead oxide (PbO) as the lead source and bistrimethylsilylsulfane (TMS) as the sulfur source, Preparation of PbS colloidal quantum dot solution by colloidal chemical reaction;

Specifically, 0.9g (4mmol) PbO was dissolved in 3ml oleic acid (OA) and 17ml octadecene (ODE) under a nitrogen atmosphere and heated to 90°C to prepare the precursor of lead oleate as a lead source; After reaching 8 hours, the temperature of the precursor was raised to 120°C; 180ul (1mmol) TMS was dissolved in 10ml ODE as a sulfur source;

Inject the sulfur source into the lead source at 120°C. After the color of the reaction system turns black completely (about 15s), put the solution into cold water to quickly lower the temperature to room temperature; add acetone to the cooled solution, and remove it after centrifugal stirring. The supernatant is then dispersed in toluene and centrifuged in acetone for multiple cycles until the supernatant is pure white; the final product is dried into powder and dispersed in n-octane to obtain a 50mg/ml lead sulfide quantum dot solution;

(2) Splice the single-mode fiber and photonic crystal fiber (PCF) to prepare the fiber probe; adjust the parameters to form fusion bubbles in the fusion zone; keep the PCF of 200 μm to 500 μm, cut it, and melt the cut end to form a hemisphere shaped end to obtain a fiber optic probe;

The fiber probe is a hybrid interference structure of intermode interference and FP interference; first, the fundamental mode of the single-mode fiber is reflected by the bubble to obtain reflected light 1, and after passing through the PCF, it is reflected again by the spherical end face to obtain reflected light 2, The bubble excites its high-order mode in the fusion area. During the reflection process, the high-order mode passes through the PCF twice and is filtered by the PCF. When passing through the bubble, it is coupled to a near-single-mode fiber to form a beam 3; these three beams are interference main source of

(3) Preparation of quantum dot fiber optic gas sensor:

(3.1) Immerse the fiber optic probe in a 1% potassium hydroxide solution for 10 minutes, take it out and let it stand for 60 seconds;

(3.2) Immerse the product obtained in step (3.1) in the PbS quantum dot solution for 5 minutes, take it out and let it stand for 60 seconds;

(3.3) Immerse the product obtained in step ( _3.2 ) in NaNO solution for 45s, take it out and let it stand for 15s; repeat the process of immersion and standing twice;

(3.4) Immerse the product obtained in step (3.3) in anhydrous methanol solution for 45s, take it out and let it stand for 15s;

(3.5) Repeat steps (3.2) to (3.4) 5 times; make the thickness of the quantum dot film deposited on the surface of the fiber optic probe reach 50nm to 60nm, and prepare a quantum dot fiber optic gas sensor with [PbS/NaNO ₂ ] structure.

The preparation method of the quantum dot optical fiber gas sensor provided by embodiments 5 to 8 has the same steps as in embodiment 4, the difference being the number of repetitions of steps (3.2) to (3.4); the specific parameters are shown in Table 1 below;

Table 1 Embodiment 5～8 Coating times and film thickness parameter list

Example number Step 3.2～3.4 Repeat times Quantum dot film thickness (nm) 5 5 50～60 6 8 60～80 7 12 100～150 8 20 300～500

The preparation method of the quantum dot fiber optic gas sensor provided by embodiment 9 comprises the following steps:

(1) 0.6g of tin chloride pentahydrate (SnCl ₄ 5H ₂ O) powder, 20ml of oleic acid and 2.5ml of oleylamine were mixed and stirred under vacuum conditions and heated to 80°C to prepare the precursor of tin oleate;

After evacuating for 6 hours, the solution became clear, and the heating and stirring were turned off, and nitrogen gas was fed into it to allow it to cool naturally;

Inject 10ml of absolute ethanol into the obtained solution, and after mixing evenly, the mixed solution is transferred to a stainless steel hydrothermal kettle; meanwhile, the optical fiber probe adopted in Example 1 is removed from the cladding, and 8 ～15mm part, put it into the mixture of the above hydrothermal kettle; react at 180℃ for 3h in the hydrothermal kettle; cool to room temperature, take out the fiber optic probe;

(2) annealing the optical fiber probe obtained in step (1) at 70° C. for 2 hours; the quantum dot film grown on the optical fiber probe is heated at high temperature, the organic solvent is volatilized, the by-product is removed, and the effect of aging is played. Changes in grain properties;

(3) Soak the optical fiber probe obtained in step (2) with AgNO ₃ solution for 60s, take it out and let it stand for 15s, repeat 3 times; then soak it in anhydrous methanol for 45s, take it out and stand it for 15s, repeat 2 times, and complete the preparation of the device . Repeated immersion makes the replacement of ligands on the surface of quantum dots more complete and improves the gas-sensing performance.

The preparation method of the quantum dot optical fiber gas sensor provided in Examples 10-12 has the same steps as in Example 9, the difference being the annealing temperature and detectable target gas in step (2); the specific parameters are shown in Table 2 below;

Table 2 Example 10～12 annealing temperature and target gas list

Example number quantum dot Annealing temperature target gas 10 PbS 25°C NO ₂ 11 _SnO2 70°C H ₂ S 12 PbS 135°C H ₂ S

The preparation method of the quantum dot optical fiber gas sensor provided in embodiment 13, its steps (1)～(2) are the same as embodiment 5, the difference is in step (3), and its step (3) is specifically as follows:

(3) Preparation of quantum dot fiber optic gas sensor device:

(3.1) Soak the optical fiber probe in 1% potassium hydroxide solution for 10 minutes, take it out and let it stand for 15 seconds;

(3.2) Dip the soaked optical fiber probe obtained in step (3.1) into the PbS quantum dot solution for 5mins through light, form a film using the photothermal effect, and turn off the light source after taking it out;

(3.3) Put the optical fiber probe obtained in step ( _3.2 ) into the NaNO solution for 45 seconds, take it out and let it stand for 15 seconds, and repeat the soaking and standing process twice;

(3.4) Soak the optical fiber probe obtained in step (3.3) in anhydrous methanol solution for 15s, take it out and let it stand for 15s;

(3.5) Steps (3.2)-(3.4) were repeated 5 times until the thickness of the quantum dot film deposited on the surface of the optical fiber reached 50nm-100nm, and a quantum dot optical fiber gas sensor with [PbS/NaNO ₂ ] structure was prepared.

The quantum dot synthesis method adopted in the present invention, the proportion of the precursor used, the synthesis temperature and the reaction time are not limited to the specific parameters proposed in the embodiments of the present invention, and the selection of different parameters has an impact on the size and gas-sensing activity of the synthesized quantum dots. great impact.

The annealing temperature in the present invention is not limited to the specific parameter that the example of the present invention provides, and the annealing temperature has determined the specific property (p type or n type, or p-n junction type) that quantum dot shows in air, and then influences gas selection sex.

The short-chain ligand solution in the present invention is not limited to NaNO ₂ solution and AgNO ₃ solution, and NH ₄ Cl solution and CuCl ₂ solution can be used; the ligand solution can effectively remove the long chain of oleylamine wrapped on the surface of quantum dots, Improve the gas adsorption activity of quantum dots, thereby improving the gas selectivity and sensitivity of the sensor.

The quantum dot optical fiber gas sensor proposed in the present invention is designed and prepared from the optical fiber probe and the optimization of the gas-sensitive layer; different from the optical fiber SPR gas measurement scheme, the use of quantum dot measurement has better resolution of gas, and is not easily affected by electric and magnetic fields Interference; Compared with the existing film-forming scheme, the preparation method provided by the present invention takes less time in the film-forming process, does not need additional equipment to facilitate operation, and has a better controllable film thickness; compared with the prior art using fluorescence effect Compared with the spectroscopic measurement scheme, it does not require a complex gas chamber structure, and the structure is simpler, which is convenient for forming a distributed network.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (10)

1. A quantum dot optical fiber gas sensor, characterized in that, comprises an optical fiber probe and a gas-sensitive layer coated at the end of the optical fiber probe; structure, the end of which is a refractive index sensitive area; the gas-sensing layer adopts colloidal quantum dots, and realizes gas detection by utilizing the room temperature gas-sensing effect that the conductivity and refractive index of the colloidal quantum dots change with gas. 2. The quantum dot optical fiber gas sensor according to claim 1, wherein the colloidal quantum dots are lead sulfide colloidal quantum dots or tin oxide colloidal quantum dots. 3. quantum dot fiber optic gas sensor as claimed in claim 1 or 2, is characterized in that, described fiber optic probe is fused and cut to form by photonic crystal fiber and single-mode fiber, and welding position has fusion splicing bubble; Retained photonic crystal fiber The inside of the segment has an air hole structure; the end of the fiber probe is a hemispherical fusion ball formed by melting the photonic crystal fiber. 4. a preparation method for the optical fiber probe of the quantum dot optical fiber gas sensor described in any one of claims 1 to 3, is characterized in that, comprises the steps: (1) welding the single-mode optical fiber and the photonic crystal optical fiber to obtain a fusion structure of the single-mode optical fiber and the photonic crystal optical fiber; the welding part has fusion splicing bubbles; (2) cutting the fusion splice structure of the single-mode fiber and photonic crystal fiber obtained in step (1), and retaining a section of photonic crystal fiber; (3) Melting the retained photonic crystal optical fiber described in step (2), the melting part forms a hemispherical fusion ball, and the preparation of the optical fiber probe is completed. 5. A preparation method of the quantum dot fiber optic gas sensor as claimed in any one of claims 1 to 3, comprising the following steps: (1) Soak the fiber optic probe in an alkaline or acidic solution to ionize its surface; then soak it in a colloidal quantum dot solution with opposite electrical properties, so that the colloidal quantum dots are combined on the surface of the fiber optic probe to form a quantum dot film; (2) Soak the optical fiber covered with the quantum dot film obtained in step (1) with the short-chain ligand solution to remove the long chain coated on the surface of the quantum dot, so as to facilitate the recombination of the subsequent quantum dot and the growth of the film; (3) using anhydrous methanol solution to soak the product obtained in step (2), to remove residual short-chain ligands and by-products generated by the reaction between the short-chain ligand solution and the colloidal quantum dot solution; (4) Steps (1) to (3) are repeated so that the thickness of the quantum dot film reaches 50 nm to 500 nm, and the preparation of the quantum dot optical fiber gas sensor is completed. 6. The preparation method according to claim 5, wherein the colloidal quantum dot solution adopted in the step (1) is a lead sulfide colloidal quantum dot solution or a tin oxide colloidal quantum dot solution. 7. A preparation method of the quantum dot fiber optic gas sensor as claimed in any one of claims 1 to 3, comprising the following steps: (1) The fiber probe is placed in the colloidal quantum dot synthesis precursor to react, so that the quantum dots automatically grow and uniformly form a film on the surface of the fiber probe; (2) annealing the optical fiber probe grown with quantum dot film obtained in step (1) to remove synthetic precursors and byproducts thereof; (3) Soaking the product obtained in step (2) with a short-chain ligand solution and drying to modify the gas-sensing layer and enhance the gas adsorption activity of the gas-sensing layer; (4) Step (3) is repeated to fully modify the gas-sensitive layer and complete the preparation of the quantum dot optical fiber gas sensor. 8. The preparation method according to claim 7, characterized in that, in the step (1), the reaction temperature is 120°C-150°C, and the reaction time is 60s-300s. 9. The preparation method according to claim 7, characterized in that, in the step (2), the annealing temperature is 120°C-300°C, and the annealing time is 0.5h-3h. 10. the preparation method as claimed in claim 5 or 7, is characterized in that, the short-chain ligand solution that adopts in described step (2) is ammonium chloride solution, sodium nitrite solution, cupric chloride solution, silver nitrate solution or copper nitrate solution.