A graphene-based PCF-SPR methane sensor and its preparation method and application

Abstract

The present invention belongs to the field of sensor preparation and gas detection, and specifically relates to a graphene-based PCF-SPR methane sensor and its preparation method and application. The present invention provides a graphene-based PCF-SPR methane sensor, the graphene-based PCF-SPR methane sensor includes a photonic crystal fiber, the end face of the photonic crystal fiber is provided with a plurality of detection holes, and a metal coating, a methane sensitive layer coating and a graphene coating are sequentially arranged on the inner surface of the detection hole. The graphene-based PCF-SPR methane sensor of the present invention is based on the good adsorption characteristics and atomic thickness of graphene, so that methane molecules are uniformly and tightly adsorbed in the sensitive coating, and can clearly distinguish the methane concentrations in different intervals, and has high sensitivity and high stability for low-concentration methane detection, and realizes high-precision methane concentration measurement, and has good practical application value.

Description

Technical Field

[0001] The invention belongs to the field of sensor preparation and gas detection, and specifically relates to a graphene-based PCF-SPR methane sensor and a preparation method and application thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] Photonic crystal fiber sensors can not only inspect illegal drugs, monitor the concentration of various flammable, explosive, toxic and harmful gases in real time and alarm for leaks, but also monitor the air quality in real life. The huge market demand has increased the enthusiasm of scientific researchers to develop new gas sensors and detection technologies. PCF gas sensors are usually used to detect the concentration of a certain gas. In principle, changes in the physical or chemical properties of the detected gas can cause specific optical effects, and optical fiber gas sensors can be directly or indirectly applied to them. In the case of photonic crystal fiber gas sensors, the sensing information will change the wavelength, phase, intensity and polarization state of the transmitted light, and the gas concentration can be detected by observing the changes in the characteristics of the transmitted light.

[0004] With the discovery of the surface plasmon resonance phenomenon, photonic crystal fiber sensors based on surface plasmon resonance came into being and have attracted great attention from scientific researchers. Compared with traditional optical fiber sensors, photonic crystal fiber sensors based on surface plasmon resonance (PCF-SPR sensors) have made great improvements in major issues such as cross-sensitivity, coupling loss, and polarization-maintaining characteristics. They can manufacture multi-dimensional structures and have a series of outstanding advantages such as a wide operating wavelength range, a large mode field area, and the ability to achieve multi-parameter measurement.

[0005] The principle of the PCF-SPR sensor is that the core mode leaked to the surface of the metal film interacts with the free electrons on the surface of the metal film. The substance to be measured is filled in the air holes coated with the metal film. When the refractive index of the substance to be measured changes, the transmission characteristics of the transmitted light will also be affected accordingly. When the PCF-SPR sensor is used for extremely low concentration gas detection, the metal film cannot produce strong adsorption and binding force on the gas to be measured, resulting in a low detection limit and low sensitivity, which is not conducive to the application of PCF-SPR sensors. Summary of the invention

[0006] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a graphene-based PCF-SPR methane sensor and its preparation method and application. Based on the theoretical basis of the surface plasmon resonance type PCF gas sensor in design, as well as the finite element method, coupled mode theory and loss spectroscopy, the present invention designs and optimizes a highly sensitive SPR-PCF methane sensor, and further provides a graphene-based PCF-SPR methane sensor. At the same time, the graphene-based PCF-SPR methane sensor of the present invention is based on the good adsorption characteristics and atomic thickness of graphene, so that methane molecules are uniformly and tightly adsorbed in the sensitive coating, and can clearly distinguish the methane concentrations in different intervals, and has high sensitivity and stability for low-concentration methane detection, and realizes high-precision methane concentration measurement, which has good practical application value.

[0007] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0008] In a first aspect, the present invention provides a graphene-based PCF-SPR methane sensor, which includes a photonic crystal fiber, wherein the end face of the photonic crystal fiber is provided with a plurality of detection holes, and a metal coating, a methane sensitive layer coating and a graphene coating are sequentially arranged on the inner surface of the detection holes.

[0009] In a second aspect, the present invention provides a method for preparing the graphene-based PCF-SPR methane sensor according to the first aspect, comprising the following steps:

[0010] A photonic crystal fiber is prepared, and a plurality of detection holes and air holes are formed by axially penetrating the photonic crystal fiber. A metal coating, a methane sensitive layer coating and a graphene coating are sequentially prepared in the detection holes.

[0011] In a third aspect, the present invention provides an application of the graphene-based PCF-SPR methane sensor described in the first aspect in methane detection.

[0012] In a fourth aspect, the present invention provides a methane detection method, comprising using the graphene-based PCF-SPR methane sensor described in the first aspect to detect methane gas in an environment.

[0013] The beneficial effects achieved by one or more technical solutions of the present invention are as follows:

[0014] The present invention proposes a novel methane photonic crystal fiber sensor configuration, analyzes the influence of key parameters in the configuration on the sensor performance, and selects the optimal parameters, so that the detection instrument can detect the response loss spectrum in the methane sensor within a wider frequency range, and make the loss spectrum smooth and complete, and adjust the resonance peak to the communication band.

[0015] In order to enhance the response of the loss spectrum to the methane concentration, the present invention combines the photonic crystal fiber and the SPR technology, and deliberately introduces four large-sized detection holes into the cladding, wherein the four large-sized detection holes are coated with a composite film and a graphene layer that are sensitive to methane, and the excellent optical and electrical control capabilities of graphene are utilized to realize the control of the optical properties of the optical fiber. Based on the good adsorption characteristics and atomic thickness of graphene, the methane molecules are uniformly and tightly adsorbed in the sensitive coating, which can clearly distinguish the methane concentrations in different intervals, has high sensitivity and high stability for low-concentration methane detection, and realizes high-precision methane concentration measurement.

[0016] The present invention utilizes the law that the refractive index of methane sensitive materials changes linearly with the methane concentration, and measures the methane concentration by measuring the displacement of the loss peak, which has high accuracy. Finally, a linear relationship between the displacement of the peak value of the loss spectrum peak and the linear change of the methane concentration is constructed, and the function of the displacement of the loss peak and the methane concentration is obtained by least squares fitting. Through fitting, the sensitivity of the graphene-based PCF-SPR methane sensor is -1.19451 / %, and the linearity is 99.97%.

[0017] The low-loss, optically and electrically adjustable graphene-based PCF-SPR methane sensor provided by the present invention plays an important role in the fields of optical fiber communication, sensing, and new optical devices. It solves the problem that traditional sensors cannot accurately measure gas concentrations, further improves the measurement sensitivity, and has strong durability, low production cost, and reduces the difficulty of the manufacturing process.

[0018] Experimental results show that the graphene-based PCF-SPR methane sensor of the present invention solves the problems of low sensitivity and complex manufacturing process of PCF-SPR gas sensors, and can accurately measure the concentration of methane gas in the environment, and the measurement results are relatively precise. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0020] Figure 1 Schematic diagram of the end face of the graphene-based four-hole PCF-SPR methane sensor provided in Example 1 of the present invention, wherein tn is the thickness of the graphene coating, tm is the thickness of the gold coating, tc is the thickness of the methane sensitive layer coating, a is the lattice period, r is the radius of the air hole, and d is the diameter of the detection hole;

[0021] Figure 2 A relationship diagram of the effect of the lattice period on the optical fiber loss spectrum provided in Example 1 of the present invention;

[0022] Figure 3 A relationship diagram showing the effect of the air hole radius on the optical fiber loss spectrum provided in Example 1 of the present invention;

[0023] Figure 4 A relationship diagram showing the effect of the thickness of the methane sensitive layer on the optical fiber loss spectrum provided in Example 1 of the present invention;

[0024] Figure 5 A relationship diagram showing the effect of the thickness of the metal film on the optical fiber loss spectrum provided in Example 1 of the present invention;

[0025] Figure 6 A relationship diagram of the effects of different graphene film thicknesses on optical fiber loss spectrum provided in Example 1 of the present invention;

[0026] Figure 7 A relationship diagram showing the influence of different detection hole diameters on the optical fiber loss spectrum provided in Example 1 of the present invention

[0027] Figure 8 A curve showing a change in the refractive index of the methane sensitive film as a function of the methane concentration provided in Example 1 of the present invention;

[0028] Fig. 9 A mode field distribution diagram provided in Example 1 of the present invention;

[0029] Fig.10 A diagram showing the regularity of the loss spectrum shifting with different methane concentrations provided in Example 1 of the present invention;

[0030] Fig.11 The loss spectrum peak provided in Example 1 of the present invention moves as the methane concentration changes. DETAILED DESCRIPTION

[0031] The first typical embodiment of the present invention is a graphene-based PCF-SPR methane sensor, which includes a photonic crystal fiber. The end face of the photonic crystal fiber is provided with a plurality of detection holes, and a metal coating, a methane sensitive layer coating and a graphene coating are sequentially arranged on the inner surface of the detection holes.

[0032] Graphene coating can actively respond to surface ion concentration and charge, which can improve the sensitivity and flexibility of traditional sensors and shorten the reaction time. Using graphene materials as sensor materials has the advantages of low cost, small size and simple preparation process.

[0033] In one or more embodiments of this implementation, the number of the detection holes is even, and the detection holes are symmetrically distributed along the axis.

[0034] In one or more embodiments of this implementation, the number of detection holes is 4, and the detection holes are arranged in a cross shape on the end face; specifically, the central angle between two adjacent columns is 90°, then detection holes arranged in horizontal and vertical directions with the axis as the origin are formed on the end face and distributed on the x-axis and y-axis, and a metal coating is provided on the inner surface of the detection holes, and the surface of the metal coating is sequentially coated with a methane sensitive layer coating and a graphene layer.

[0035] In order to enhance the response of the loss spectrum to the methane concentration, four large-sized side holes are introduced into the cladding and coated with a composite film. By adjusting the loss spectra in the x-polarization direction and the y-polarization direction, combining the losses in the two directions and averaging the two sets of data, the concentration of methane in the mixed gas can be accurately measured, and the methane concentration can be effectively detected.

[0036] In one or more embodiments of this implementation, the end face of the photonic crystal fiber is modified with a SiO2 inverse opal film; further, the SiO2 inverse opal film is self-assembled on the end face of the optical fiber bundle using a sol-gel collaborative self-assembly method to form a composite colloidal crystal film, and the composite film is calcined to remove the PS microspheres to obtain the SiO2 inverse opal film.

[0037] The inverse opal structure uses the opal structure as a template and is produced by removing the opal template to create a highly ordered detection pore structure. It has the characteristics of regular and orderly pore structure, uniform pore size, large specific surface area, and easy mass transfer. It also has the characteristics of easy control of pore size. When the ratio of the refractive index of the inverse opal structure material to that of the surrounding medium is greater than 2.8, the inverse opal photonic crystal can produce a complete photonic band gap.

[0038] Through polishing, a plane is formed at one end of the optical fiber bundle, which is perpendicular to the optical fiber axis. The other end of the optical fiber is free. The free end is used to access the detection light, couple the light into the end face of the optical fiber bundle, and couple the signal light reflected from the end face for detection. The composite colloidal crystal film is self-assembled on the end face of the optical fiber bundle by the sol-gel collaborative self-assembly method, and then the composite film is calcined to remove the PS microspheres to obtain the SiO2 inverse opal film. After ultrasonic dispersion of the PS microsphere solution, it is mixed with a TEOS solution composed of alcohol, hydrochloric acid and ethyl orthosilicate in a ratio of 2:1:1. The TEOS solution spontaneously fills the gaps between the microspheres as a precursor solution of SiO2. After the liquid evaporates, a gel is formed between the microspheres. After the gel is sintered, it can be converted into SiO2. In the solution, the surface of the colloidal particle spheres carries a charge. Under appropriate charge density and particle concentration, the spheres self-organize and grow into a periodic structure through electrostatic interaction to form colloidal crystals. In the capillary container, the electrostatic interaction between the colloidal particles and the charged glass wall is used. When the volume fraction of the colloid particles is high, the colloidal suspended particles are stacked in a face-centered cubic (FCC) lattice; when the volume fraction is low, they tend to be stacked in a body-centered cubic (BCC) lattice, and the close-packed planes of the crystals are parallel to the surface of the wall. The colloidal crystals are carefully dehydrated to obtain an opal film on the end face of the optical fiber bundle. The experimental device is placed in a constant temperature drying oven with an evaporation temperature of 45°C ± 1°C, and the indoor humidity is properly controlled. After about 30 hours, the solution evaporates completely, and a layer of composite colloidal crystal film is formed on the end face of the optical fiber bundle. The prepared composite colloidal film is sintered at 500-700°C for about 5 minutes to remove the PS microspheres and obtain an ordered SiO2 inverse opal structure.

[0039] In one or more embodiments of this implementation, the thickness of the graphene coating is 1-3nm; the preparation method of the graphene coating includes: obtaining graphene by high-pressure chemical vapor deposition (CVD) technology, achieving the growth of graphene by carburization or surface growth, converting the graphene grown on the optical fiber into graphene oxide by Hummers oxidation method, preparing a plating solution and then sulfonating and sensitizing and activating it to finally form a graphene coating.

[0040] Using methane as a carbon source, it is decomposed in a high-temperature environment outside the substrate to obtain graphene. The graphene provided by the CVD method has good quality assurance and is not complicated to operate. More importantly, it can be transferred to other substrates. Therefore, this method is very popular in graphene transistors and transparent conductive films, and has obvious advantages in the quality of producing graphene films.

[0041] Specifically, the method of growing graphene using chemical vapor deposition generally includes the following steps:

[0042] Step 1: Place the bare optical fiber in a chemical vapor deposition reactor, introduce inert gas protection and heat to 1000-1200°C;

[0043] Step 2: Keep the temperature constant, introduce methane and hydrogen to react for 2-5 hours, and after the reaction is completed, turn off the carbon source and take out the sample to obtain an optical fiber with a surface completely covered with graphene.

[0044] The growth of graphene coating can be achieved through two paths, carburization or surface growth, depending on the substrate:

[0045] First, carburization and carbon precipitation: If the matrix material is made of metals such as nickel with high carbon solubility, since its carbon solubility is not high, the carbon atoms formed in a high temperature environment penetrate into the matrix, and then when the temperature drops, the carbon atoms will precipitate and form nuclei, and finally form a graphene film;

[0046] Second, surface growth: If the base material is a metal such as copper with a low carbon solubility, the carbon atoms released under high temperature will be adsorbed on its outside, thus producing a "graphene island", which will continue to expand in two dimensions.

[0047] The graphene grown on the optical fiber is made into graphene oxide by the Hummers oxidation method, and the graphene coating is finally formed by preparing a plating solution and then subjecting it to sulfonation treatment and sensitization activation treatment.

[0048] In one or more examples of this implementation, the metal used in the metal coating is gold, the thickness of the metal coating is 32-38 nm, and the metal coating is prepared by high pressure chemical vapor deposition technology.

[0049] When the thickness of the metal layer increases from 32nm to 38nm at 20℃, the peak of the loss spectrum decreases continuously when the wavelength is between 570 and 600nm, and a red shift occurs. When the thickness of the gold layer is 35nm, the half-wave band is the widest, the effect is obviously the best, and the loss limitation is more appropriate.

[0050] In one or more examples of this implementation mode, the thickness of the methane sensitive layer coating is 260-300 nm; the methane sensitive layer coating is made of cage-shaped supramolecular material Cryptophane A and cured fluorosilicone.

[0051] When the thickness of the methane sensitive layer is within the range of 260-300 nm, the loss spectrum undergoes a red shift, the size of the limiting loss decreases, and the half-peak width increases.

[0052] A second typical embodiment of the present invention is a method for preparing a graphene-based PCF-SPR methane sensor according to the first typical embodiment, comprising the following steps:

[0053] A photonic crystal fiber is prepared, and a plurality of detection holes and air holes are formed by axially penetrating the photonic crystal fiber. A metal coating, a methane sensitive layer coating and a graphene coating are sequentially prepared in the detection holes.

[0054] In one or more embodiments of this implementation, the preparation method of the metal coating is a high-pressure chemical vapor deposition method, specifically: through high-pressure heating, the gaseous or steam-state metal plasma undergoes a chemical reaction and is deposited in an atomic state on the inside of the air holes, thereby forming a metal coating film.

[0055] In one or more embodiments of this implementation, the method for preparing the methane sensitive layer coating is to adopt capillary immersion coating technology to apply a dilute solution of cage-shaped supramolecular material Cryptophane A and fluorosilicone on the surface of the metal coating, and then polish the inner surface of the film with a nitrogen flow of 0.4MPa, and dry the film under ultraviolet irradiation.

[0056] In one or more embodiments of this implementation, the method for preparing the graphene coating includes: obtaining graphene using high-pressure chemical vapor deposition technology, achieving graphene growth through carburization and carbon precipitation or surface growth, converting the graphene grown on the optical fiber into graphene oxide through Hummers oxidation method, preparing a plating solution and then subjecting it to sulfonation treatment and sensitization and activation treatment to finally form a graphene coating.

[0057] A third typical embodiment of the present invention is the application of the graphene-based PCF-SPR methane sensor described in the first typical embodiment in methane detection.

[0058] A fourth typical embodiment of the present invention is a methane detection method, which comprises using the graphene-based PCF-SPR methane sensor described in the first typical embodiment to detect methane gas in the environment.

[0059] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in combination with specific embodiments and comparative examples.

[0060] Example 1

[0061] A graphene-based four-hole PCF-SPR methane sensor comprises: a photonic crystal fiber, an air hole, a detection hole, a gold coating, a methane sensitive layer coating, and a graphene coating; a gold coating is applied in four adjacent detection holes of the photonic crystal fiber by using a chemical deposition technique, and the methane sensitive layer coating and the graphene coating are applied in sequence on the gold coating.

[0062] Photonic crystal fiber is a fiber made of glass or plastic that can be used as a light transmission tool. It is generally surrounded by a glass envelope with a lower refractive index than the core.

[0063] The end face of the photonic crystal fiber is modified by an inverse opal film, and four cross-shaped detection holes are arranged along the axial direction. The inner surface of the detection hole is successively provided with a gold coating, a methane sensitive layer coating, and a graphene coating. Figure 1 As shown, tn is the thickness of the graphene coating, tm is the thickness of the gold coating, and tc is the thickness of the methane sensitive layer coating.

[0064] Design Theory

[0065] The lattice period in the sensor is a, the radius of the air hole is r, and the diameter of the detection hole is d. The four detection holes are plated with a gold coating with a thickness of tm, a methane sensitive layer with a thickness of tc, and a graphene coating with a thickness of tn, so that they are coupled with the fiber core fundamental mode to stimulate the surface plasmon resonance effect.

[0066] like Figure 2 As shown in Figure 2, the change of lattice period a in the range of 2.6 to 3 μm has a slight effect on the central wavelength and intensity of the loss spectrum. Figure 3 As shown, when the air hole radius r varies from 0.52 to 0.6 um, the intensity of the loss spectrum decreases as the radius increases.

[0067] Figure 4 This is a relationship diagram of the effect of methane sensitive layer film thickness on optical fiber loss spectrum. In the thickness range of 260 to 300 nm, the loss spectrum red-shifts, the limiting loss decreases, and the half-peak width increases. In order to adjust the position of the central wavelength and select the appropriate limiting loss for easy detection, the methane sensitive layer film thickness tc = 280 nm is finally selected.

[0068] Figure 5 The relationship between the gold coating thickness and the optical fiber loss spectrum. When the thickness of the gold coating at 20°C increases from 32nm to 38nm, the peak value of the loss spectrum decreases continuously when the wavelength is between 570 and 600nm. A red shift phenomenon occurs. When the gold coating thickness is 35nm, the half-wave band is the widest, the effect is obviously the best, and the loss control is more appropriate. Therefore, the gold coating thickness of 35nm is selected for easy detection.

[0069] Figure 6 The figure shows the relationship between the influence of different graphene film thicknesses on the optical fiber loss spectrum. After simulation, it was found that when the thickness of the graphene layer plated in the methane sensor increased from 1nm to 3nm, the loss spectrum blue-shifted, and the size of the limiting loss decreased, and the half-peak width decreased. In order to adjust the position of the central wavelength and select the appropriate limiting loss for easy detection, the graphene thickness m = 2nm was finally selected.

[0070] Through experimental simulation, the present invention finds that the diameter of the air hole can also affect the performance of the sensor. Figure 7This is the effect of the diameter of the ultra-large air hole on the optical fiber loss spectrum. When the detection hole diameter d increases, the loss resonance wavelength moves toward the long wavelength direction, and the loss peak begins to increase, and a red shift phenomenon occurs. In order to limit the loss size within an acceptable range, it is necessary to select a suitable loss spectrum and then select the parameters: d = 1.2μm.

[0071] The effective refractive index of the methane sensitive material changes linearly with the change of methane concentration. The methane sensitive film can fully react with the methane gas in the surrounding environment, and the methane sensitive film has good repeatability. The relationship between the refractive index of the methane sensitive film and the methane concentration is as follows Figure 8 As shown, according to Figure 8 The specific formula for the change of the refractive index of the methane sensitive film with the gas concentration is:

[0072] n eff1 =1.4478-0.0038C CH4 .

[0073] For the gold coating, the relationship between the dielectric constant and the refractive index can be described by the Drude model as follows:

[0074]

[0075] n m (λ)+jIm(n eff )

[0076] where εm, λc and λp are the dielectric constant, plasma wavelength and collision wavelength of gold respectively, λc and λp are constants and λp=1.4541×10 -7 m,λc=1.7614×10 -5 m; the real and imaginary parts of the refractive index of gold are represented by nm and km respectively.

[0077] Light intensity transmittance (T) can be expressed as follows:

[0078]

[0079] The response spectrum is analyzed by loss spectrum analysis. The characteristics of the loss spectrum are analyzed by plotting a graph of the limiting loss with respect to the wavelength. When the phase matching condition is met, a surface plasmon resonance (SPR) will occur in the photonic crystal fiber. At this time, the limiting loss of the core mode can be calculated based on the imaginary part of the effective refractive index of the light in this propagation mode, as shown in the following formula:

[0080]

[0081] The fundamental mode effective refractive index that can be obtained by numerically calculating the photonic crystal fiber in Comsol simulation software is expressed in complex form, as shown in the following formula:

[0082] n eff =Re(n eff )+jIm(n eff ).

[0083] The simulated mode field distribution diagram obtained by Comsol simulation software is as follows Fig. 9 shown.

[0084] The optimal settings of each parameter are shown in Table 1. The thickness of the perfect matching layer PML = 2.4μm, and the total radius of the fiber cross section R = 10a. The perfect matching layer is the annular material wrapped around the outermost layer of the optical fiber during simulation. Its implementation principle is to set a boundary condition with the same material and matching wave impedance as the cladding material on the outside of the optical fiber cladding. Therefore, the perfect matching layer can be regarded as a non-reflective absorption layer. When the light beam is incident on the perfect matching layer, it will not be reflected back immediately, but will gradually attenuate until it is finally absorbed. When adding a perfect matching layer, it is necessary to manually set a suitable thickness for it. Within a certain thickness range, the thicker the thickness of the perfect matching layer, the higher the calculation accuracy. When the thickness exceeds this range, that is, the thickness of the perfect matching layer reaches a certain level, the calculation accuracy will no longer change significantly with the increase of the thickness of the perfect matching layer, but will gradually remain constant. In general, 3-5 times the wavelength of the incident light beam is used as the standard measurement for the thickness setting of the perfect matching layer.

[0085] Table 1 Optimal setting parameters of graphene-based four-hole PCF-SPR methane sensor

[0086] tn tm tc a r d 1nm 35nm 280nm 2.8um 0.56um 1.2um

[0087] In the actual environment, the concentration of methane is about 0-3.5%, so the methane concentration is set to 0-3.5% and scanned every 0.5%. Since the refractive index of the methane sensitive layer changes linearly with the methane concentration, the methane concentration can be measured by measuring the shift of the loss spectrum peak under different methane concentrations.

[0088] like Fig.10 As shown in the curve of the loss spectrum changing with the methane concentration, the peak of the loss spectrum moves toward the short-wave direction as the methane concentration increases, and this law is linear. Fig.11 It is the law that the loss spectrum moves with the difference of methane concentration. Through the computer least square method, it can be calculated that the sensitivity of the photonic crystal fiber methane sensor can reach k=-1.18nm / % when sensing methane concentration, and its linearity is as high as 99.8%.

[0089] Example 2

[0090] Preparation method of graphene-based four-hole PCF-SPR methane sensor

[0091] Prepare photonic crystal fiber, polish to form a plane at one end of the fiber bundle, the plane is perpendicular to the fiber axis, and the other end of the fiber is free. The free end is used to access the detection light, couple the light into the end face of the fiber bundle, and couple the signal light reflected from the end face for detection. After ultrasonic dispersion of PS microsphere solution, mix it with TEOS solution composed of alcohol, hydrochloric acid and ethyl orthosilicate in a ratio of 2:1:1. Immerse the photonic crystal fiber in TEOS solution and put it in a constant temperature drying oven with an evaporation temperature of 45℃±1℃, and properly control the indoor humidity. After 30 hours, the solution evaporates completely, and a layer of composite colloidal crystal film will be formed on the end face of the fiber bundle. Sinter the prepared composite colloidal film at 600℃ for 5min, remove the PS microspheres, and obtain an ordered SiO2 inverse opal structure.

[0092] Four detection holes and a plurality of air holes are formed by axially penetrating the photonic crystal optical fiber, and the four detection holes are arranged in a cross shape in the end face.

[0093] By means of high-pressure heating, the gold plasma in gaseous or vapor state undergoes a chemical reaction and is deposited in an atomic state on the inside of the air holes, thereby forming a gold coating film.

[0094] The capillary dip coating technique is adopted to coat the cage-shaped supramolecular material Cryptophane A and a dilute solution of fluorosilicone on the surface of the metal coating, and then the inner surface of the film is polished with a nitrogen flow of 0.4 MPa, and the film is dried under ultraviolet irradiation to obtain a methane sensitive layer coating.

[0095] The bare optical fiber is placed in a chemical vapor deposition reactor, inert gas is introduced for protection and the temperature is raised to 1100°C; the temperature is kept constant, methane and hydrogen are introduced for reaction for 3 hours, and after the reaction is completed, the carbon source is turned off and the sample is taken out to obtain an optical fiber with the surface of the methane sensitive layer completely covered with graphene.

[0096] The graphene grown on the optical fiber is made into graphene oxide by the Hummers oxidation method, and the graphene coating is finally formed by preparing a plating solution and then subjecting it to sulfonation treatment and sensitization activation treatment.

[0097] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A graphene-based PCF-SPR methane sensor, characterized in that: The graphene-based PCF-SPR methane sensor comprises a photonic crystal fiber, the end face of the photonic crystal fiber is provided with a plurality of detection holes, and a metal coating, a methane sensitive layer coating and a graphene coating are sequentially provided on the inner surface of the detection holes; The number of the detection holes is 4, and the detection holes are arranged in a cross shape in the end face; specifically, the central angle between two adjacent columns is 90°, so that the detection holes arranged in the horizontal and vertical directions with the axis as the origin are formed on the end face and distributed on the x-axis and y-axis, and a metal coating is provided on the inner surface of the detection hole, and the surface of the metal coating is sequentially coated with a methane sensitive layer plating layer and a graphene layer; The end face of the photonic crystal fiber is modified by SiO2 inverse opal film; The thickness of the graphene coating is 2 nm; and the preparation method of the graphene coating comprises: Graphene is obtained by high pressure chemical vapor deposition technology, and the growth of graphene is achieved by carburization and carbon precipitation or surface growth. The graphene grown on the optical fiber is made into graphene oxide by Hummers oxidation method, and the graphene coating is finally formed by preparing a plating solution and then subjecting it to sulfonation treatment and sensitization and activation treatment; The metal used in the metal coating is gold, the thickness of the metal coating is 35nm, and the metal coating is prepared by high pressure chemical vapor deposition technology; The thickness of the methane sensitive layer coating is 280nm; the methane sensitive layer coating is made of cage-shaped supramolecular material Cryptophane A and cured fluorosilicone; The design theory of the sensor is: the lattice period in the sensor is a, the radius of the air hole is r, the diameter of the detection hole is d, the gold coating thickness is tm, the methane sensitive layer coating thickness is tc, and the graphene coating thickness is tn on the above four detection holes, so that they are coupled with the fiber core fundamental mode to stimulate the surface plasma resonance effect.

2. The graphene-based PCF-SPR methane sensor according to claim 1, characterized in that: The SiO2 inverse opal film adopts a sol-gel coordinated self-assembly method to self-assemble a composite colloidal crystal film on the end face of an optical fiber bundle, and then the composite film is calcined to remove PS microspheres to obtain the SiO2 inverse opal film.

3. The method for preparing the graphene-based PCF-SPR methane sensor according to any one of claims 1 to 2, characterized in that: The following steps are involved: A photonic crystal optical fiber is prepared, and a plurality of detection holes are formed by axially penetrating the photonic crystal optical fiber. A metal coating, a methane sensitive layer coating and a graphene coating are sequentially prepared in the detection holes.

4. The preparation method according to claim 3, characterized in that: The preparation method of the metal coating is a high-pressure chemical vapor deposition method, specifically: by means of high-pressure heating, a gaseous or steam-state metal plasma undergoes a chemical reaction and is deposited in an atomic state on the inner side of the air pores, thereby forming a metal coating film; Alternatively, the methane sensitive layer coating is prepared by applying a dilute solution of the cage-shaped supramolecular material Cryptophane A and fluorosilicone on the surface of the metal coating using a capillary dip coating technique, then polishing the inner surface of the film with a nitrogen flow of 0.4 MPa, and drying the film under ultraviolet irradiation; Alternatively, the preparation method of the graphene coating includes: obtaining graphene by high-pressure chemical vapor deposition technology, achieving the growth of graphene by carburization and carbon precipitation or surface growth, converting the graphene grown on the optical fiber into graphene oxide by Hummers oxidation method, preparing a plating solution and then subjecting it to sulfonation treatment and sensitization and activation treatment to finally form a graphene coating.

5. Application of the graphene-based PCF-SPR methane sensor according to any one of claims 1 to 2 in methane detection.

6. A methane detection method, characterized in that: The method comprises using the graphene-based PCF-SPR methane sensor according to any one of claims 1 to 2 to detect methane gas in the environment.

Images