MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas

Abstract

The present invention relates to an optical fiber sensor, and in particular to a MOF-SPR sensor for online detection of changes in the refractive index of hydrogen sulfide gas. The MOF-SPR sensor is an optical fiber sensor, wherein a central air hole is provided in the cladding of the optical fiber, and two symmetrical open-loop gaps are provided on the outer wall of the cladding. Ten symmetrically distributed outer-layer air holes are provided on both sides of the open-loop gap, and the 10 outer-layer air holes are symmetrically arranged on the sides of a regular hexagon. Two symmetrically distributed air holes are provided between the central air hole and the open-loop gap, and two inner-layer air holes are provided on both sides of the central air hole. The inner-layer air holes are symmetrically distributed on both sides of the open-loop gap; a gold film is plated on the outer wall of the cladding and the inner wall of the open-loop gap. The refractive index change of hydrogen sulfide gas is detected online using SPR technology and the principle of energy coupling. Ultrasensitive expression of hydrogen sulfide gas in a wide detection range is achieved in an operable infrared working band. The structural design is reasonable and the application is wide.

Description

Technical Field

[0001] The present invention relates to an optical fiber sensor, in particular to a MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas. Background Art

[0002] Hydrogen sulfide (HS) is a highly corrosive, toxic, and dangerous gas. It affects multiple production processes, including oil and gas extraction, mineral exploration, chemical processing, and steel production. Leakage accidents are particularly prone to occur during the exploration and development of high-sulfur oil and gas fields, polluting the environment, threatening personal safety, and corroding and damaging oil and gas equipment. Therefore, effective, real-time, online monitoring of HS is crucial for addressing these issues.

[0003] Currently, hydrogen sulfide sensors based on electrochemical, gas chromatography, oxide resistivity, and fiber optic methods have been proposed. Among them, electrochemical methods suffer from the problem of impurities easily reacting with the electrolyte, causing sensor failure; gas chromatography methods have disadvantages such as poor qualitative analysis of the measured medium and complex equipment systems; hydrogen sulfide sensors based on oxide resistivity changes often require high temperatures to function and have poor gas selectivity and stability.

[0004] Fiber-optic sensors have attracted significant attention due to their outstanding advantages, including small size, excellent compatibility, long-distance transmission, and adaptability to harsh environments. They have been gradually optimized and upgraded as they are adopted for real-time monitoring of hydrogen sulfide gas. Microstructured fiber, a new type of specialty optical fiber with a quartz substrate and a series of air holes introduced within the cladding, boasts unique and outstanding properties, such as flexible structural design, endless single-mode transmission, adjustable chromatic dispersion, and excellent nonlinearity, injecting immense vitality into the field of fiber-optic sensing.

[0005] Surface plasmon resonance (SPR) is an optical phenomenon involving the stimulated oscillation of free electron clusters between a dielectric and a metal surface. Due to its extreme sensitivity to changes in surrounding physical parameters (strain, temperature, refractive index, displacement, and magnetic field), and its particularly high sensitivity and resolution in detecting the refractive index of analytes (gaseous or liquid) in the environment, SPR is often introduced into the field of fiber optic sensing. Due to its high sensitivity, low detection cost, fast response speed, and real-time label-free monitoring, it is widely used in the field of environmental safety for the detection, compositional analysis, identification, and authentication of samples.

[0006] The combination of MOF sensors and SPR technology has fostered the rapid development of nanosensing technology. Compared to conventional fiber-optic SPR sensors, MOF-SPR sensors effectively confine incident light to the fiber core and optimize the light's outward transmission path by manipulating the arrangement of air holes within the cladding, further enhancing mode coupling efficiency and stimulating SPR to produce a strong resonance effect. In recent years, MOF-SPR sensors with diverse structural designs have emerged for refractive index detection of samples, ranging from high-refractive-index liquids (n>1.33) to low-refractive-index gases (n<1.33). Even for low-refractive-index materials, the accessible refractive index range is generally between 1.10 and 1.33, making materials with refractive indices below 1.10 virtually undetectable. Hydrogen sulfide gas has a refractive index between 1.00 and 1.10, necessitating the design of high-performance MOF-SPR sensors capable of detecting gases with ultra-low refractive indices. Summary of the Invention

[0007] To address the above technical problems, the present invention provides an ultrasensitive MOF-SPR sensor device for online detection of refractive index changes in hydrogen sulfide gas. This device utilizes SPR technology and the principle of energy coupling to perform real-time online detection of refractive index changes in the hydrogen sulfide gas sample. Slight changes in the sample's refractive index can affect the resonant wavelength of the loss curve. This device achieves ultrasensitive detection of hydrogen sulfide gas over a wide detection range (1.00-1.38) within the operational infrared operating band (900-2750 nm). The upper and lower open-loop structures enable bidirectional SPR excitation, enabling ultrasensitive detection of the sample in the ultra-low refractive index range of 1.00-1.10. The device boasts a rational structural design and broad application potential.

[0008] The technical solution adopted by the present invention is: a MOF-SPR sensor (MOF-SPR is the abbreviation of microstructured fiber surface plasmon resonance) for online detection of changes in the refractive index of hydrogen sulfide gas. The MOF-SPR sensor for online detection of changes in the refractive index of hydrogen sulfide gas is an optical fiber sensor. The optical fiber has a central air hole in its cladding, two symmetrically distributed open-loop gaps are opened on the outer wall of the cladding, 10 symmetrically distributed outer-layer air holes are provided in the cladding on both sides of the open-loop gap, and the 10 outer-layer air holes are symmetrically arranged on the sides of a regular hexagon. Two symmetrically distributed air holes are provided between the central air hole and the open-loop gap, and two inner-layer air holes are provided on both sides of the central air hole. The two inner-layer air holes are symmetrically distributed on both sides of the open-loop gap. The outer wall of the cladding and the inner wall of the open-loop gap are plated with a gold film, and the background material of the cladding is silicon dioxide.

[0009] Furthermore, the refractive index-based MOF-SPR gas sensor is a microstructured optical fiber sensor. The cladding of the microstructured optical fiber is equipped with air holes of three different sizes. The 10 outer air holes are arranged in a hexagonal pattern. The combination of the two inner air holes and the central air hole on the horizontal central axis divides the equivalent fiber core into two upper and lower regions, forming a dual-core structure. The paired air holes are distributed in two groups between the fiber core and the metal film, effectively confining most of the energy to the fiber core and optimizing the light transmission path. The alternating arrangement of the three air hole sizes in the cladding forms a hexagonal dual-core sensing structure. The outer wall of the cladding is micro-polished, and the cladding corresponding to the core region is double-opened on both sides. The outer wall of the cladding and the double-opened gap are coated with gold film, constructing a sensing device that excites SPR on both sides simultaneously.

[0010] Furthermore, the radius of the cladding is 6 mm-6.4 mm; the radius of the air hole is 0.4 mm-0.5 mm; the radius of the central air hole is 0.45-0.55 mm; the radius of the outer air hole and the inner air hole is 0.75 mm-0.85 mm; and the thickness of the gold film is 65 nm-75 nm.

[0011] Furthermore, the radius of the cladding is 6.2 mm; the radius of the air hole is 0.45 mm; the radius of the central air hole is 0.5 mm; the radius of the outer air hole and the inner air hole is 0.8 mm; and the thickness of the gold film is 70 nm.

[0012] Furthermore, the open-loop gap is a circular gap with blocked ends.

[0013] Furthermore, the microstructured optical fiber of the optical fiber sensor is of the TIR (Turning Index Refractive Index) type.

[0014] The beneficial effects of the present invention are as follows: A supersensitive MOF-SPR sensor for online detection of refractive index changes in hydrogen sulfide gas is provided. SPR technology and the principle of energy coupling are used to perform real-time online detection of refractive index changes in the hydrogen sulfide gas sample to be tested. Slight changes in the sample's refractive index can affect the resonant wavelength of the loss curve. Supersensitive expression of hydrogen sulfide gas over a wide detection range is achieved in the operable infrared operating band. The upper and lower open-loop structures enable bidirectional SPR excitation; achieving supersensitive detection of the sample to be tested in the ultra-low refractive index range. The advantages of the refractive index-based MOF-SPR hydrogen sulfide gas sensor are as follows:

[0015] (1) It can detect substances in both gas and liquid states, especially gases in the ultra-low refractive index range (1.00-1.10);

[0016] (2) The refractive index range of detectable samples is wide: 1.00-1.38;

[0017] (3) The operable working wavelength can be extended to the infrared spectrum region: 900-2750 nm;

[0018] (4) Bidirectional excitation SPR greatly improves the mode coupling efficiency and presents excellent optical properties. The best sensitivity and resolution are 20000 nm / RIU and 5×10 -6 RIU;

[0019] (5) The structural design is reasonable and easy to manufacture and apply in multiple fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the main structure of the optical fiber in Example 1;

[0021] Figure 2 yes Figure 1 Schematic diagram of the cross section of AA;

[0022] Figure 3 is a confinement loss diagram of the plasma mode in Example 1;

[0023] Figure 4 1 is a dispersion curve of the core guided mode and plasma mode and a loss curve of the core guided mode in Example 1;

[0024] Figure 5 This is a diagram of the light energy transfer process during mode coupling between the core guided mode and the plasma mode in Example 1;

[0025] Figure 6 1. This is a graph showing the relationship between the limiting loss and the resonance wavelength when the refractive index is in the range of 1.00-1.09 when the MOF-SPR sensor in Example 1 detects the sample to be tested;

[0026] Figure 7 1. This is a graph showing the relationship between the limiting loss and the resonance wavelength when the refractive index is in the range of 1.10-1.19 when the MOF-SPR sensor in Example 1 detects the sample to be tested;

[0027] Figure 8 1. This is a graph showing the relationship between the limiting loss and the resonance wavelength when the MOF-SPR sensor in Example 1 detects a sample to be tested and has a refractive index in the range of 1.20-1.29;

[0028] Figure 9 1. This is a graph showing the relationship between the limiting loss and the resonance wavelength when the refractive index is in the range of 1.30-1.38 when the MOF-SPR sensor in Example 1 detects the sample to be tested;

[0029] Figure 10 3 is a polynomial fitting curve diagram of the resonant wavelength of the MOF-SPR sensor at different refractive indices and the refractive index of the sample to be measured in Example 1. DETAILED DESCRIPTION

[0030] Referring to the figures, a MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas is provided. The MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas is an optical fiber sensor. A central air hole 6 is provided in the cladding 1 of the optical fiber. Two symmetrically distributed open-loop gaps 2 are provided on the outer wall of the cladding 1. Ten symmetrically distributed outer air holes 4 are provided in the cladding 1 on both sides of the open-loop gap 2. The 10 outer air holes 4 are symmetrically arranged on the sides of a regular hexagon. Two symmetrically distributed air holes 3 are provided between the central air hole 6 and the open-loop gap 2. Two inner air holes 5 are provided on both sides of the central air hole 6. The two inner air holes 5 are symmetrically distributed on both sides of the open-loop gap 2. A gold film 7 is plated on the outer wall of the cladding 1 and the inner wall of the open-loop gap 2. The background material of the cladding 1 is silicon dioxide. The radius of the cladding 1 is 6.2 mm. The radius of the air hole 3 is 0.45 mm. The radius of the central air hole 6 is 0.5 mm; the radius of the outer air hole 4 and the inner air hole 5 is 0.8 mm; the thickness of the gold film 7 is 70 nm; the open-loop gap 2 is a circular gap with no connection at both ends.

[0031] The refractive index-based MOF-SPR hydrogen sulfide gas sensor is a microstructured optical fiber sensor. The cladding of the microstructured optical fiber is equipped with air holes of three different sizes. The 10 outer air holes are arranged in a hexagonal pattern. The combination of the two inner air holes and the central air hole on the horizontal central axis divides the equivalent fiber core into two upper and lower regions, forming a dual-core structure. The paired air holes are distributed in two groups between the fiber core and the metal film, effectively confining most of the energy to the fiber core while optimizing the light transmission path. The alternating arrangement of the three air hole sizes within the cladding forms a hexagonal dual-core sensing structure. The outer wall of the cladding is micro-polished, and the cladding opposite the core region is double-opened on both sides. The outer wall of the cladding and the inner gap of the double-opened ring are coated with gold film, creating a sensing device that simultaneously excites SPR on both sides. The refractive index type MOF-SPR hydrogen sulfide gas sensor uses a gold film coated on the surface of the optical fiber as a plasma sensing layer. In a sealed environment, the gas pressure and flow rate are adjusted to allow the hydrogen sulfide gas of the sample to be tested to fill the entire detection device, facilitating full contact with the plasma sensing material. When light is emitted from a supercontinuum broadband light source and enters the interior of the microstructured optical fiber through a single-mode optical fiber, the incident light of different wavelengths propagates in a specific mode along the axial direction in the optical fiber, while the surface plasma wave propagates in a fixed mode in the gold film. When the incident light of a certain wavelength in the optical fiber has the same wave vector as the surface plasma wave in the gold film, mode coupling will occur at the junction of the optical fiber medium and the gold film. At this time, the light energy in the optical fiber is reduced, and the light energy is coupled into the gold film. Figure 5 The process of light energy transfer during mode coupling is presented. Figure 3 and Figure 4 The dispersion and loss curves of the core guided mode at different wavelengths are depicted, along with the loss curves of the plasmon mode at the corresponding wavelengths. This figure intuitively demonstrates that when the core guided mode loss is maximum, the corresponding plasmon mode loss is minimum, indicating that more light energy is transferred from the core to the gold film surface at the resonant wavelength. At this point, phase matching is optimal, mode coupling is strongest, and the resonance effect is more pronounced. The wavelength corresponding to the maximum energy loss is the resonant wavelength. The energy loss spectrum is plotted based on the relationship between energy loss and resonant wavelength. Figure 6 - 9 describes the relationship between the limiting loss and the resonance wavelength of the refractive index type MOF-SPR hydrogen sulfide gas sensor when the refractive index of the sample to be tested is 1.00-1.38. It can be observed from the figure that when the refractive index of the sample to be tested outside the gold film 7 increases by 0.01, the resonance wavelength increases accordingly, the resonance peak becomes higher, the half-peak width decreases, and the coupling strength of the two modes increases. The sensitivity and resolution of the refractive index type MOF-SPR hydrogen sulfide gas sensor also increase with the increase of the refractive index of the sample to be tested. In particular, when the refractive index of the sample to be tested changes from 1.37 to 1.38, the sensitivity and resolution of the probe are 20000 nm / RIU and 5×10-6 RIU. Figure 10 A polynomial fitting curve diagram of the resonance wavelength under different refractive indices and the refractive index of the sample to be measured is presented. When the designed sensing device is placed in an environment filled with hydrogen sulfide gas, the external conditions of mode coupling are further changed by changing the gas pressure and flow rate. At this time, by observing the offset of the resonance wavelength under different conditions in the spectrometer, the spectral sensitivity and other optical characteristics of the sensor can be calculated. Ultimately, effective detection of hydrogen sulfide gas under different refractive indices is achieved, further achieving the practical purpose and significance of micro-nano fiber optic sensing.

[0032] An ultrasensitive MOF-SPR sensor capable of online detection of refractive index changes in hydrogen sulfide gas utilizes SPR technology and the principle of energy coupling to detect refractive index changes in the sample in real time. Small changes in the sample's refractive index can affect the resonant wavelength of the loss curve. Results demonstrate ultrasensitive detection of hydrogen sulfide gas over a wide detection range (1.00-1.38) within the operational infrared operating band (900-2750 nm) (optimal sensitivity and resolution of 20,000 nm / RIU and 5×10 -6 The upper and lower open-ring gap structures enable bidirectional SPR excitation and ultrasensitive detection of samples in the ultra-low refractive index range of 1.00-1.10. The rational structural design makes it easy to manufacture and apply in multiple fields.

Claims

1. A MOF-SPR sensor for online detection of refractive index changes in hydrogen sulfide gas, characterized by: The MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas is an optical fiber sensor. A central air hole (6) is provided in the cladding (1) of the optical fiber. Two symmetrically distributed open-loop gaps (2) are provided on the outer wall of the cladding (1). Ten symmetrically distributed outer-layer air holes (4) are provided in the cladding (1) on both sides of the open-loop gap (2). The ten outer-layer air holes (4) are symmetrically arranged on the sides of a regular hexagon. Two symmetrically distributed air holes (3) are provided between the central air hole (6) and the open-loop gap (2). Two inner-layer air holes (5) are provided on both sides of the central air hole (6). The two inner-layer air holes (5) are symmetrically distributed on both sides of the open-loop gap (2). A gold film (7) is plated on the outer wall of the cladding (1) and the inner wall of the open-loop gap (2). The background material of the cladding (1) is silicon dioxide. The open-loop gap (2) is a circular gap with two blocked ends.

2. A MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas according to claim 1, characterized in that: The radius of the cladding (1) is 6 mm-6.4 mm; the radius of the air hole (3) is 0.4 mm-0.5 mm; the radius of the central air hole (6) is 0.45-0.55 mm; the radii of the outer air hole (4) and the inner air hole (5) are 0.75 mm-0.85 mm; and the thickness of the gold film (7) is 65 nm-75 nm.

3. A MOF-SPR sensor for online detection of refractive index changes of hydrogen sulfide gas according to claim 2, characterized in that: The radius of the cladding (1) is 6.2 mm; the radius of the air hole (3) is 0.45 mm; the radius of the central air hole (6) is 0.5 mm; the radius of the outer air hole (4) and the inner air hole (5) is 0.8 mm; and the thickness of the gold film (7) is 70 nm.

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