Method for preparing fluorescent probe and optical fiber sensing system for monitoring concentration of chlorine ion

Abstract

The application belongs to the technical field of non-destructive monitoring of chloride ion concentration in concrete, and relates to a preparation method of a fluorescent probe and a fiber-optic sensing system for monitoring chloride ion concentration. The fluorescent probe is a quartz bare fiber as a carrier, one end of which is plated with a sol-gel film wrapping chloride ion sensitive fluorescent nanoparticles after surface modification, and the other end is connected to a bifurcated optical fiber through a fluorescent probe connector. The other end of the bifurcated optical fiber after being connected to the fluorescent probe is divided into two paths, one of which is connected to a fluorescence spectrometer and then to a computer, and the other of which is connected to a light source. The light emitted by the light source converges into the fluorescent probe through the Y-shaped bifurcated optical fiber. When the fluorescent nanoparticles at the film-plated end of the fluorescent probe contact chloride ions and fluorescence quenching occurs, the degree of fluorescence quenching is received by the fiber-optic spectrometer through the bifurcated optical fiber, and the fluorescence intensity is recorded. The chloride ion concentration is obtained through the Stern-Volmer equation, and the chloride ion concentration monitoring is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of non-destructive monitoring of chloride ion concentration in concrete, and relates to a preparation method of a fluorescent probe and a fiber-optic sensing system for monitoring chloride ion concentration. BACKGROUND

[0002] Reinforced concrete is a composite material that adds steel bars and other materials to concrete to improve the mechanical properties of concrete. It has become one of the most widely used building and structural materials in the world due to its easy pouring, excellent mechanical properties and wide range of raw materials. However, to date, the structure of reinforced concrete often appears premature cracking during service, one of the main reasons being the influence of chloride ions. Chloride ions gradually penetrate the concrete cover to reach the surface of the steel bars, and when the chloride ion concentration reaches a certain threshold, it will destroy the passivation film on the surface of the steel bars, causing corrosion of the steel bars. After that, the expansion of the corrosion products will cause the concrete to crack, ultimately reducing the load-bearing capacity of the reinforced concrete structure and leading to a shorter life cycle of the concrete structure and an increase in maintenance costs. Therefore, it is necessary to monitor the chloride ions in concrete in real time at an early stage, to obtain the chloride ion concentration at the position of the steel bars in a timely manner, to detect steel corrosion problems as soon as possible, and to take necessary measures to reduce the safety problems of the concrete structure caused by steel corrosion.

[0003] At present, the methods for testing chloride ion concentration in concrete are mainly divided into destructive monitoring and non-destructive monitoring: destructive monitoring aims to drill and sample the concrete by physical methods, and then use traditional laboratory chemical concentration titration monitoring methods to obtain the chloride ion concentration. This method will damage the integrity of the concrete structure and cannot achieve continuous monitoring, and cannot obtain the chloride ion concentration distribution over time. Non-destructive monitoring does not damage the integrity of the concrete structure, and usually uses external non-contact measurement or embedded sensors in the concrete to obtain the chloride ion concentration in the concrete, which can achieve continuous monitoring of the chloride ion concentration. The main methods include potential measurement, laser-induced breakdown spectroscopy, terahertz frequency electromagnetic wave spectroscopy (near-infrared, microwave, millimeter wave), optical fiber sensors, etc.

[0004] Fiber optic sensors, characterized by their resistance to electromagnetic interference and small size, are an excellent choice for chloride ion monitoring because they can be embedded in any part of concrete structures for real-time, multi-point monitoring. Fiber optic sensors used for chloride ion monitoring include long-period grating type sensors and fluorescent type sensors. Long-period grating type sensors operate based on sensitivity to the environmental refractive index; however, chloride ions are not the only factor affecting changes in the environmental refractive index. Therefore, long-period grating type sensors have poor ion specificity for chloride ion monitoring and are susceptible to interference from other ions in the concrete, leading to deviations in the measured chloride ion concentration. Furthermore, long-period grating type sensors are sensitive to temperature and external forces; changes in temperature and external forces can cause a shift in the center wavelength of the transmission spectrum. Therefore, in practical chloride ion monitoring, additional work is required to improve specificity and compensate for temperature and external force effects, which increases monitoring error. Fluorescent fiber optic sensors, based on the principle of fluorescence quenching, can achieve rapid and accurate monitoring of chloride ion concentration by selecting specific chloride ion fluorescent indicators and fixing them to the fiber surface. However, the measurement principle of fluorescent fiber optic sensors dictates that their assembly and manufacturing costs are relatively high, and their sensitivity is affected by the choice of materials and fiber parameters. Currently, in many fluorescent fiber optic sensors embedded in concrete, the connection method between the fluorescent probe and the sensing system is not well applied in practical measurements. For example, one existing fluorescent probe involves pressing a fluorescent dye into a pellet, fixing it to the end face of the fiber, encapsulating it, and then connecting it to the sensing system. In this method, the fluorescent dye is physically pressed into a pellet and then fixed to the fiber end face through splicing, but the pellet is not securely fixed at the fiber end. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing a fluorescent probe and an optical fiber sensing system for monitoring chloride ion concentration. This fluorescent optical fiber sensing system has high sensitivity, good accuracy, and can monitor the chloride ion concentration in freshly mixed concrete in real time.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a fluorescent probe, the method comprising:

[0008] (1) Treat both ends of the bare quartz fiber to expose the quartz core at both ends of the bare quartz fiber;

[0009] (2) Surface modification of the quartz fiber core at one end of the bare quartz fiber to hydroxylate the surface of the fiber core at the corresponding end.

[0010] (3) Prepare a sol-gel casting solution doped with fluorescent nanoparticles, wherein the sol-gel casting solution doped with fluorescent nanoparticles is prepared by using tetraethyl orthosilicate, ethanol, HCl, SiO2 fluorescent nanoparticles and water.

[0011] (4) Using the dip-coating method, one end of the modified quartz fiber core in step (2) is coated with a chloride ion sensitive film to prepare a fluorescent probe sensitive to chloride ions.

[0012] Further, in step (3), the preparation method of the sol-gel casting solution doped with fluorescent nanoparticles is as follows: tetraethyl orthosilicate, ethanol, HCl, SiO2 fluorescent nanoparticles and water are mixed in a volume ratio of tetraethyl orthosilicate: ethanol: HCl: H2O: SiO2 fluorescent nanoparticles = (20-30): (32-48): (0.064-0.096): (8-12): (0.0015-0.0021). The mixture is then stirred continuously on a magnetic stirrer for 16-32 hours to obtain a sol-gel solution. The solution is stored in the dark for 1-2 days until it becomes a semi-gel state to obtain the sol-gel casting solution doped with fluorescent nanoparticles.

[0013] Furthermore, the method for synthesizing the SiO2 fluorescent nanoparticles is as follows:

[0014] H2O, cyclohexane, Tritium X-100, and n-hexanol were mixed in a volume ratio of H2O:cyclohexane:Tritium X-100:n-hexanol = (9-11):(3.8-4.6):(0.9-1.1):(0.9-1.1) to prepare a microemulsion. An aqueous solution of chloride ion-sensitive fluorescent dye and an aqueous solution of chitosan were then added to the microemulsion to obtain a mixed solution. The pH of the mixed solution was adjusted to neutral, and tetraethyl orthosilicate and ammonia were added. The mixture was stirred continuously on a magnetic stirrer for 24-36 hours until an emulsion was formed. After centrifugation, washing, and drying, SiO2 fluorescent nanoparticles were obtained.

[0015] The concentration of the chloride ion-sensitive fluorescent dye in the aqueous solution is 0.005-0.015 mol / L, and the concentration of chitosan in the aqueous solution is 0.25-0.75 g / L.

[0016] When preparing the mixed solution, the volume ratio of the microemulsion, the aqueous solution of the chloride ion-sensitive fluorescent dye, and the aqueous solution of chitosan is (14.6-17.8):(4.5-5.5):(9-11);

[0017] When preparing the emulsion, the volume ratio of the mixed solution, tetraethyl orthosilicate, and ammonia is (28.1-34.3):(5.4-6.6):(3.6-4.4).

[0018] Further, step (1) specifically involves: placing both ends of the bare quartz fiber into concentrated sulfuric acid for acid etching to remove the skin layer and sheath layer, exposing the quartz fiber core at both ends of the bare quartz fiber, and then rinsing the surface of the quartz fiber core with deionized water.

[0019] After acid etching, the exposed length of the quartz fiber core at one end of the bare quartz fiber is 1-2cm, and the exposed length of the quartz fiber core at the other end is 4-6cm; the end with the exposed length of 1-2cm of quartz fiber core is then moved to step (2) for surface modification.

[0020] Furthermore, step (2) specifically involves:

[0021] The bare quartz fiber core at one end was immersed in a potassium hydroxide isopropanol solution, then rinsed with a large amount of distilled water and dried with compressed nitrogen.

[0022] The dried quartz fiber core was immersed in Piranha solution to remove surface organic matter and hydroxylate the fiber core surface; it was then rinsed in distilled water and dried to complete the surface modification.

[0023] Further, in step (4), the coating process specifically includes:

[0024] The coating was performed using the dip coating method, with 50 coating operations performed in each coating cycle. The parameters for each coating operation were set as follows: dip speed 2083 μm / s, descent speed 2083 μm / s, immersion time 10s, and interval time 10s.

[0025] After each coating cycle is completed, the sample is placed in an oven at 40-60℃ for 20-30 minutes to dry before proceeding to the next cycle. The entire coating process consists of 6 coating cycles. After completing the entire coating process, a fluorescent probe sensitive to chloride ions is prepared.

[0026] This invention employs a dip-coating method to coat fluorescent probes in a sol-gel casting solution containing fluorescent nanoparticles. The fluorescent nanoparticles can be better fixed to the end of the optical fiber due to the adhesion of the sol-gel film, further improving the durability of the fluorescent probe and enabling it to be better applied to the detection of chloride ion concentration in concrete.

[0027] A fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete includes: a light source, a fiber optic spectrometer, a Y-shaped bifurcated fiber, a chloride ion-sensitive fluorescent probe, and a fluorescent probe connector.

[0028] The Y-shaped branched optical fiber includes a first branch, a second branch, and a combining end; the first branch and the second branch are respectively connected to the light source and the optical fiber spectrometer;

[0029] One end of the fluorescent probe is a coated end for detecting chloride ions, and the other end is connected to the combining end of the Y-shaped bifurcated optical fiber through the fluorescent probe connector.

[0030] When the coated end of the fluorescent probe comes into contact with chloride ions, the chloride ion-sensitive fluorescent nanoparticles in the coating undergo fluorescence quenching. The degree of fluorescence quenching is focused by a Y-shaped bifurcated optical fiber and displayed on a spectrometer. The chloride ion concentration is obtained through the Stern-Wolmer equation, enabling real-time monitoring of the chloride ion concentration in concrete.

[0031] Furthermore, the fluorescent probe connector is used to connect the Y-type fiber combiner end and the fluorescent probe, and includes a fluorescent probe fixing unit and a cylindrical connecting unit;

[0032] The fluorescent probe fixing unit is used to fix the fluorescent probe;

[0033] The cylindrical connecting unit is used to connect the fluorescent probe and the combining end of the Y-shaped branched optical fiber; the cylindrical connecting unit is internally provided with three plano-convex lenses: a first plano-convex lens, a second plano-convex lens, and a third plano-convex lens.

[0034] Furthermore, the first plano-convex lens is disposed at one end close to the fluorescent probe, and the plane of the first plano-convex lens faces the direction of the fluorescent probe; the planes of the second plano-convex lens and the third plano-convex lens are disposed on the same plane, the convex surface of the second plano-convex lens faces the direction of the fluorescent probe, and the plane of the third plano-convex lens faces the direction of the fluorescent probe.

[0035] By controlling the positions of the three convex lenses, light from the second light source enters the second plano-convex lens through the first optical path of the Y-shaped branched optical fiber and then is incident on the first plano-convex lens, and light from the first plano-convex lens is converged to the third plano-convex lens.

[0036] Furthermore, the fluorescent fiber optic sensing system is used to monitor the concentration of chloride ions in concrete pore fluid and fresh concrete.

[0037] The working principle of the fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete provided by this invention is as follows: Fluorescent nanoparticles are prepared using a fluorescent dye sensitive to chloride ions. A sol-gel casting solution containing the fluorescent nanoparticles is then prepared and deposited onto the end of a surface-modified quartz fiber core using a dip-coating method to obtain a fluorescent probe. This fluorescent probe is then used to construct the fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete. In this system, light emitted from a light source is focused through a Y-shaped optical fiber and incident on the sensitive film on the surface of the fluorescent probe, exciting a fluorescence signal. The light is then refocused into the Y-shaped optical fiber through a pre-set convex lens, and the initial fluorescence intensity is recorded in a fiber optic spectrometer. When chloride ions in the concrete come into contact with the fluorescent nanoparticles through the sensitive film, fluorescence quenching occurs, weakening the reflected fluorescence signal and creating a fluorescence intensity difference with the initial fluorescence intensity. The fiber optic spectrometer records the fluorescence intensity change during this process. The relationship between fluorescence intensity and chloride ion concentration is obtained using the Stern-Wolmer equation, calibrating the standard equation of the fluorescent probe, and then monitoring the chloride ion concentration.

[0038] The beneficial technical effects of this invention are as follows: The fluorescent probe produced by this invention uses glossy fluorescent dyes with high fluorescence intensity and good chloride ion sensitivity as raw materials; the sol-gel casting solution doped with fluorescent nanoparticles perfectly preserves the performance of the fluorescent dye; the bare quartz fiber used in the manufacturing process is low-cost and has minimal loss during optical transmission; after modifying the optical fiber, the casting solution can better adhere to the quartz fiber core, ensuring the fluorescence intensity and stability of the probe; when connecting the fabricated fluorescent probe to the sensing system, a fluorescent probe connector with a convex lens is designed, which not only ensures the connection between the fluorescent probe and the sensing system but also changes the optical path, allowing more light emitted from the light source to converge into the probe, enabling the spectrometer to receive more fluorescence signals, reducing losses during light transmission, and improving test accuracy.

[0039] The chloride ion-sensitive fluorescent fiber optic sensing and monitoring system provided by this invention has a short measurement time, high accuracy, and good stability, and can continuously monitor the chloride ion concentration in freshly mixed concrete. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete, as described in this embodiment of the invention.

[0041] Figure 2 This is an optical path diagram of the fluorescent fiber optic sensing system in an embodiment of the present invention;

[0042] Figure 3a This is a schematic diagram of the fluorescent probe connector structure in an embodiment of the present invention;

[0043] Figure 3bThis is a schematic diagram of the columnar body of the fluorescent probe connector in an embodiment of the present invention;

[0044] Figure 3c This is a schematic diagram of the cylindrical connection unit structure of the fluorescent probe connector in an embodiment of the present invention;

[0045] Figure 4 This is a fluorescence spectrum of a sol-gel film doped with fluorescent nanoparticles in an embodiment of the present invention;

[0046] Figure 5 is a fluorescence spectrum recorded by the chloride ion-sensitive fluorescent fiber optic sensor in an embodiment of the present invention; Figure 5a To record fluorescence spectra under different coating parameters, Figure 5b and Figure 5c Fluorescence spectra were recorded under different coating cycles;

[0047] Figure 6 is a fitted curve showing the relationship between the fluorescence intensity of the calibrated fluorescent probe and the chloride ion concentration during the chloride ion concentration monitoring process. Figure 6a The fluorescence intensity-concentration curves of the fluorescent probes for six coating cycles are shown. Figure 6b Fluorescence intensity-concentration curves of fluorescent probes coated for eight periods.

[0048] Reference numerals: 1. Fluorescent probe; 2. Fluorescent probe connector; 3. Y-shaped branched optical fiber; 4. Fiber optic spectrometer; 5. Light source; 6. Computer; 21. Fluorescent probe fixing unit; 22. Cylindrical connecting unit; 21-1. Columnar body; 21-2. Receiving cavity; 21-3. Snap-fit; 21-4. Spring; 22-1. First plano-convex lens; 22-2. Second plano-convex lens; 22-3. Third plano-convex lens. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0050] Conversely, this invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the invention as defined in the claims. Furthermore, to provide a better understanding of the invention, certain specific details are described in detail below. However, those skilled in the art will fully understand the invention even without these detailed descriptions.

[0051] Example 1: A method for preparing a fluorescent probe, the method comprising:

[0052] (1) Treat both ends of the bare quartz fiber to expose the quartz core at both ends of the bare quartz fiber;

[0053] (2) Surface modification of the quartz fiber core at one end of the bare quartz fiber is performed to make the surface of the fiber core at the corresponding end hydroxylated; the fiber core after hydroxylation treatment can better combine with the coating solution during coating due to the presence of surface hydroxyl groups.

[0054] (3) Prepare a sol-gel casting solution doped with fluorescent nanoparticles, wherein the sol-gel casting solution doped with fluorescent nanoparticles is prepared by using tetraethyl orthosilicate (TEOS), ethanol (EtOH), HCl, SiO2 fluorescent nanoparticles and water.

[0055] (4) A coating method is used to coat one end of the modified quartz fiber core in step (2) to coat one end of the modified quartz fiber core with a chloride ion-sensitive membrane, thereby preparing a fluorescent probe sensitive to chloride ions. The coating process uses the sol-gel casting solution doped with fluorescent nanoparticles prepared in step (3).

[0056] In step (3) of this embodiment, the preparation method of the sol-gel casting solution doped with fluorescent nanoparticles is as follows: Tetraethyl orthosilicate, ethanol, HCl, SiO2 fluorescent nanoparticles, and water are mixed in a volume ratio of tetraethyl orthosilicate:ethanol:HCl:H2O:SiO2 fluorescent nanoparticles = (20-30):(32-48):(0.064-0.096):(8-12):(0.0015-0.0021). The mixture is then continuously stirred on a magnetic stirrer for 16-32 hours to obtain the sol-gel solution. This preparation method ensures that the SiO2 fluorescent nanoparticles are uniformly dispersed in the sol-gel solution. After being stored in the dark for 1-2 days until it becomes a semi-gel state, it is used as the casting solution.

[0057] In this embodiment, the method for synthesizing the SiO2 fluorescent nanoparticles is as follows:

[0058] H2O, cyclohexane, Tritium X-100, and n-hexanol were mixed in a volume ratio of H2O:cyclohexane:Tritium X-100:n-hexanol = (9-11):(3.8-4.6):(0.9-1.1):(0.9-1.1) to prepare a microemulsion. An aqueous solution of chloride ion-sensitive fluorescent dye and an aqueous solution of chitosan were then added to the microemulsion to obtain a mixed solution. The pH of the mixed solution was adjusted to neutral, and tetraethyl orthosilicate and ammonia were added. The mixture was stirred continuously on a magnetic stirrer for 24-36 hours until an emulsion was formed. After centrifugation, washing, and drying, SiO2 fluorescent nanoparticles were obtained.

[0059] Specifically, the chloride ion-sensitive fluorescent dye used is the chloride ion-sensitive fluorescent dye Gloss (purchased from Aladdin Biochemical Technology Co., Ltd.);

[0060] The concentration of the chloride ion-sensitive fluorescent dye in the aqueous solution is 0.005-0.015 mol / L, and the concentration of chitosan in the aqueous solution is 0.25-0.75 g / L.

[0061] When preparing the mixed solution, the volume ratio of the microemulsion, the aqueous solution of the chloride ion-sensitive fluorescent dye, and the aqueous solution of chitosan is (14.6-17.8):(4.5-5.5):(9-11);

[0062] When preparing the emulsion, the volume ratio of the mixed solution, tetraethyl orthosilicate, and ammonia is (28.1-34.3):(5.4-6.6):(3.6-4.4).

[0063] In this embodiment, step (1) specifically involves: placing both ends of the bare quartz fiber into concentrated sulfuric acid for acid etching to remove the skin layer and sheath layer, exposing the quartz fiber core at both ends of the bare quartz fiber, and then rinsing the surface of the quartz fiber core with deionized water.

[0064] After acid etching, the exposed length of the quartz fiber core at one end of the bare quartz fiber is 1-2cm, and the exposed length of the quartz fiber core at the other end is 4-6cm; the end with the exposed length of 1-2cm of quartz fiber core is then moved to step (2) for surface modification.

[0065] In this invention, the fluorescent probe is prepared using bare quartz fiber, which includes a core, a sheath, and a protective layer. The core has a diameter of 400 μm, the sheath has a thickness of 30 μm, and the protective layer has a thickness of 200 μm.

[0066] In this embodiment, step (2) specifically includes:

[0067] The bare quartz fiber core at one end was immersed in a potassium hydroxide isopropanol solution, then rinsed with a large amount of distilled water and dried with compressed nitrogen.

[0068] The dried quartz fiber core was immersed in Piranha solution to remove surface organic matter and hydroxylate the fiber core surface; it was then rinsed in distilled water and dried to complete the surface modification.

[0069] Specifically, the exposed quartz fiber core at one end is immersed in an isopropanol solution of potassium hydroxide (1.50-1.60 mol / L) for 25-35 minutes, followed by rinsing with plenty of distilled water and drying with compressed nitrogen. Then, it is immersed in a Piranha solution (H₂O₂:H₂SO₄ = 30:70 (v / v)) for 25-35 minutes (this removes all organic matter and simultaneously hydroxylates the material surface), followed by rinsing in distilled water for 10-20 minutes. Finally, it is dried at 90-110℃ for 25-35 minutes to complete surface modification, thereby hydroxylating the fiber core surface. After this modification, the surface of the bare quartz fiber is hydroxylated, allowing the casting solution to better bind with the hydroxyl groups and adhere to the optical fiber during coating.

[0070] In step (4) of this embodiment, the coating process is specifically as follows:

[0071] The coating was performed using the dip coating method, with 50 coating operations performed in each coating cycle. The parameters for each coating operation were set as follows: dip speed 2083 μm / s, descent speed 2083 μm / s, immersion time 10s, and interval time 10s.

[0072] After each coating cycle, the probe is dried in an oven at 40-60℃ for 20-30 minutes before proceeding to the next cycle. The entire coating process consists of 6 coating cycles. After completing the entire coating process, a fluorescent probe sensitive to chloride ions is prepared. The fluorescent probe is dried and stored for subsequent use. The coating method provided by this invention is based on the optimal coating parameters obtained by comparing different coating parameters in examples. The fluorescent probe prepared using these coating parameters can retain the fluorescence emission wavelength of the fluorescent dye, and the fluorescence intensity is also relatively ideal.

[0073] Example 2: A fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete, such as... Figure 1 As shown, the fluorescent fiber optic sensing system includes: a light source 5, a fiber optic spectrometer 4, a Y-shaped bifurcated fiber 3, a chloride ion-sensitive fluorescent probe 1, and a fluorescent probe connector 2; it also includes a computer 6 for analyzing and processing data; the chloride ion-sensitive fluorescent probe is the fluorescent probe prepared in Example 1; the Y-shaped bifurcated fiber includes a first branch, a second branch, and a combining end; the first branch and the second branch are respectively connected to the light source and the fiber optic spectrometer;

[0074] In this embodiment, the Y-shaped branched optical fiber is used to connect the optical path. The two branches connect the light source and the spectrometer, and the combined end is connected to the fluorescent probe through the fluorescent probe connector. This connection method allows light to enter the fluorescent probe after leaving the light source to excite fluorescence. Then, the fluorescence intensity fluctuation caused by the change in chloride ion concentration is recorded and fed back to the spectrometer to realize real-time monitoring of chloride ion concentration in concrete.

[0075] One end of the fluorescent probe is a coated end for detecting chloride ions, and the other end is connected to the combining end of the Y-shaped bifurcated optical fiber through the fluorescent probe connector.

[0076] During measurement, a fluorescent probe is embedded in freshly mixed concrete. Light emitted from a light source is focused by a Y-shaped bifurcated optical fiber and a convex lens and directed into the fluorescent probe to excite a fluorescence signal. The reflected signal is then reflected back through the convex lens into the bifurcated optical fiber and enters the fluorescence spectrometer. When the fluorescent nanoparticles in the sol-gel film on the fluorescent probe come into contact with chloride ions, the chloride ion-sensitive fluorescent nanoparticles in the coating undergo fluorescence quenching, weakening the reflected fluorescence signal and creating a fluorescence intensity difference from the initial fluorescence intensity. This change in fluorescence intensity is recorded in the fiber optic spectrometer, and the chloride ion concentration in the freshly mixed concrete is calculated using the Stern-Walmer equation, achieving real-time, sensitive, and accurate monitoring of the chloride ion concentration in the concrete. The Y-shaped bifurcated optical fiber meets the optical path requirements, allowing the light source to illuminate the fluorescent probe and the fiber optic spectrometer to collect the fluorescence signal emitted by the probe. The length of the bare quartz fiber used to prepare the fluorescent probe can be determined according to the actual application, i.e., it can be arbitrarily extended to facilitate insertion into the freshly mixed concrete at different depths for chloride ion concentration monitoring.

[0077] In this embodiment, the fluorescent probe connector is used to connect the Y-type fiber combiner end and the fluorescent probe, and includes a fluorescent probe fixing unit 21 and a cylindrical connecting unit 22.

[0078] like Figures 3a-3c As shown, the fluorescent probe fixing unit is used to fix the fluorescent probe; specifically, the fluorescent probe fixing unit 21 includes a columnar body 21-1 (specifically a cylindrical or square columnar body) and a cylindrical receiving cavity 21-2 penetrating the columnar body. The diameter of the receiving cavity is equal to or slightly about the diameter of the quartz fiber core exposed at one end of the fluorescent probe (in this embodiment, the diameter of the receiving cavity is 400-430 μm); a snap-fit ​​receiving hole communicating with the receiving cavity is provided on the columnar body, and a snap-fit ​​21-3 is provided in the snap-fit ​​receiving hole; one end of the snap-fit ​​located in the receiving cavity is connected to a spring 21-4 fixed in the receiving cavity; the snap-fit ​​includes a cylindrical body and a snap-fit ​​hole penetrating the cross-section of the cylindrical body (the diameter of the snap-fit ​​hole is greater than or equal to the diameter of the receiving cavity, and in this embodiment, the diameter of the snap-fit ​​hole ranges from 400-450 μm);

[0079] The specific operation for fixing the fluorescent probe is as follows: Press the buckle in the buckle receiving hole to align the buckle hole on the cross-section of the buckle with the cylindrical receiving cavity inside the cylindrical body. At this time, the receiving cavity inside the cylindrical body is connected. Insert the exposed quartz fiber core (unmodified end) of the fluorescent probe into the receiving cavity, with a portion protruding. When the end of the quartz fiber core reaches the designated position in the cylindrical connecting unit, release the buckle. Under the action of the spring, the buckle clamps the quartz fiber core in the buckle hole, thereby fixing the fluorescent probe. In this embodiment, the position of the fluorescent probe can be easily adjusted through the buckle, which is beneficial for the connection operation of the fluorescent probe and the Y-shaped branched optical fiber.

[0080] The bottom surface of the columnar body in the fluorescent probe fixing unit is provided with threads, and the surface of the combining end of the Y-shaped branched optical fiber is also provided with threads; both ends of the cylindrical connecting unit are provided with threads; the bottom of the cylindrical connecting unit, the columnar body, and the combining end of the Y-shaped branched optical fiber are all connected by threaded nuts.

[0081] Inside the cylindrical connecting unit 22, a first plano-convex lens 22-1, a second plano-convex lens 22-2, and a third plano-convex lens 22-3 are provided; the diameters of the three plano-convex lenses are all the same as the diameter of the quartz fiber core in the fluorescent probe (in this embodiment of the invention, all are 400 μm); the other parts inside the cylindrical connecting unit (i.e., the parts other than the parts where the three convex lenses are provided) are hollow structures.

[0082] In this embodiment, the first plano-convex lens is disposed at one end close to the fluorescent probe, and the plane of the first plano-convex lens faces the direction of the fluorescent probe; the planes of the second plano-convex lens and the third plano-convex lens are disposed on the same plane, the convex surface of the second plano-convex lens faces the direction of the fluorescent probe, and the plane of the third plano-convex lens faces the direction of the fluorescent probe.

[0083] By controlling the positions of the three convex lenses, light from the second light source enters the second plano-convex lens through the first optical path of the Y-shaped branched fiber, then is incident on the first plano-convex lens, and the light from the first plano-convex lens is converged to the third plano-convex lens. The adjustment sequence for the positions of the three convex lenses is as follows: first, determine the distance between the plane containing the second and third plano-convex lenses and the third plano-convex lens; then, move the second and third plano-convex lenses vertically on the same plane to control the optical path; finally, fix the three convex lenses in place after determining their positions.

[0084] like Figure 2As shown, after the fluorescent fiber optic sensing and monitoring system is installed, the light from the light source enters the second plano-convex lens and the first plano-convex lens (one side of the plano-convex lens is in contact with one end of the fluorescent probe) through the first optical path of the Y-shaped branched fiber, and then enters the fluorescent probe; the light after fluorescence quenching passes through the first plano-convex lens and the third plano-convex lens and enters the second optical path of the Y-shaped branched fiber, and the fluorescence intensity is recorded by the spectrometer.

[0085] The cylindrical connecting unit and the Y-shaped branched fiber are connected by a threaded nut, which allows for slight adjustment of the position of the three plano-convex lenses inside the cylindrical connecting unit. This allows light from the light source to enter the second plano-convex lens through the first optical path of the Y-shaped branched fiber. By using the three plano-convex lenses, the optical path can be changed, reducing the loss when the light source enters the fluorescent probe and receives the fluorescent signal.

[0086] Specifically, in this invention, the diameters of the first and second branches of the Y-shaped branched optical fiber are the same as the core diameter of the bare quartz fiber (400 μm in this embodiment); the inner diameter of the cylindrical connecting unit is 1000 μm, the surface of the Y-shaped branched optical fiber combining end is covered with a rubber layer, and the outer diameter of the Y-shaped branched optical fiber combining end is 1000 μm; the Y-shaped branched optical fiber combining end can be more tightly connected with the cylindrical connecting unit in the fluorescent probe connector, reducing light loss.

[0087] The fluorescent fiber optic sensing system is used to monitor the pore fluid in concrete and the chloride ion concentration in freshly mixed concrete.

[0088] Example 3: A method for preparing a fluorescent probe, comprising:

[0089] 1) Preparation of SiO2 fluorescent nanoparticles: First, a microemulsion with a volume ratio of H2O:cyclohexane:TritionX-100:n-hexanol = 10:4.2:1:1 was prepared. Then, a chloride ion-sensitive fluorescent dye solution was added to the microemulsion. After adjusting the pH of the mixed solution to neutral, a certain amount of tetraethyl orthosilicate and ammonia water were added, and the mixture was continuously stirred on a magnetic stirrer for 24 hours. The stirred emulsion was then centrifuged, washed, and dried to obtain SiO2 fluorescent nanoparticles. Due to the presence of the three-dimensional framework of SiO2 nanoparticles, the fluorescent dye can be effectively encapsulated and fixed, preventing photobleaching and leakage of the fluorescent dye, and ensuring the long-term photostability and lifespan of the fluorescent dye.

[0090] 2) The fluorescent nanoparticles prepared in step 1) are embedded in a sol-gel membrane and mixed in a ratio of TEOS:EtOH:HCl:H2O:fluorescent nanoparticles = 25:40:0.08:10:0.0018 (V / V). The mixture is then stirred continuously on a magnetic stirrer for 24 hours. After stirring, the mixture is poured into a brown bottle and stored in the dark. It is then used as a casting solution for probes after 1-2 days of storage.

[0091] 3) Preparation of fluorescent probes: Bare quartz fibers were selected as the material, with core, sheath, and cladding diameters of 400 μm, 430 μm, and 730 μm, respectively. A 1-2 cm length of the sheath and sheath was removed from one end of the bare quartz fiber to expose the core, which was used for surface modification and deposition of a chloride ion-sensitive film. A 4-6 cm length of the sheath and sheath was removed from the other end for connection to the fluorescent probe connector. The end of the quartz fiber with the 1-2 cm sheath and sheath removed was immersed in a 1.56 mol / L potassium hydroxide solution in isopropanol for 30 min, followed by rinsing with copious amounts of distilled water and drying with compressed nitrogen. Then, it was immersed in a Piranha solution (H₂O₂:H₂SO₄ = 30:70 (V / V)) for 30 min, rinsed with distilled water for 15 min, and finally dried at 100 °C for 30 min to complete surface modification. After surface modification, a chloride ion-sensitive sol-gel film was coated onto its surface.

[0092] Bare quartz fibers were fixed on a Czochralski coating machine. The surface-modified end was immersed in the casting solution prepared in step 2) for Czochralski coating. The coating parameters were: lifting speed 2083 μm / s, descent speed 2083 μm / s, immersion time 10 s, interval time 10 s, and 50 coating cycles. After each 50 coating cycles, the bare fiber was dried in a 50℃ oven for 20-30 min. This process was considered one cycle, and each fluorescent probe required 6 cycles to complete. After completion, the fluorescent probe was stored in a dark environment for 3 days until the sol-gel film deposited on the end face completely solidified. Then, the chloride ion concentration was tested.

[0093] 4) Monitoring process: The fluorescence intensity of the prepared sol-gel film doped with fluorescent nanoparticles was measured, and its fluorescence spectrum was obtained as follows: Figure 4 As shown, the fluorescent nanoparticles exhibit good fluorescence intensity after being embedded in the sol-gel membrane.

[0094] The fluorescent probe was calibrated using the standard equation of fluorescence intensity versus chloride ion concentration. In this invention, sodium chloride solution was used as the standard solution for calibration. Sodium chloride solutions with concentrations of 0M, 0.01M, 0.03M, 0.06M, 0.08M, and 0.10M were prepared. The fluorescent probe was connected to the constructed fiber optic sensing system platform, and the fluorescence intensity of the fluorescent probe in air and in the prepared solutions was recorded respectively. (Note: When changing the solution, the fluorescent probe needs to be cleaned in deionized water before immersing in the next concentration solution.)

[0095] Before determining the final coating specifications, this example tested the performance of the fluorescent probe under various coating parameters. Several sets of coating parameters and fluorescence spectra used as examples are shown in Table 1 and... Figure 5a As shown in (①-④).

[0096] Table 1 Coating parameters

[0097]

[0098] Depend on Figure 5a It can be seen that the peak positions of curves ① and ② are not consistent. Therefore, the immersion time and interval time affect the wavelength of the curve and cause the peak position to shift. Compared with curves ① and ②, the peak position of curve ③ appears at the fluorescence emission wavelength of the fluorescent dye used, indicating that the correct immersion time and interval time have been found. Based on curve ③, with other parameters unchanged, increasing the number of coatings yields probe ④, whose fluorescence intensity curve is shown as line ④ in the figure. It can be seen that the peak appears at the same position, but the fluorescence intensity has increased. However, the fluorescence probe intensity is not ideal under the above coating parameters. In order to improve the fluorescence intensity, this invention starts from the emission wavelength of the fluorescent nanoparticles and modifies the film thickness by adjusting the pulling speed and the number of coatings, thereby increasing the thickness of the sol-gel film on the fluorescent probe, so that more sol-gel film containing fluorescent nanoparticles can be coated on the fluorescent probe. Based on literature review, this invention determined the lifting and lowering speed to be 2083 μm / s and proposed the concept of a coating cycle, which specifies 50 coating cycles. After 50 coating cycles, the bare fiber is placed in a 50°C oven for drying for 20-30 minutes, and the above process is considered as one cycle.

[0099] Subsequently, this example tested the fluorescence spectra and chloride ion solution under six and eight coating cycles, and the coating parameters are shown in Table 2.

[0100] Table 2 Coating parameters

[0101]

[0102] The fluorescence spectra of the fluorescent probes were tested over six coating cycles, as shown in the figure. Figure 5b As shown, the fluorescence spectra of the fluorescent probes during the eight coating cycles are as follows: Figure 5c As shown in Figure 4), when testing the solution using the monitoring method, the fluorescent probe only needs 10-20 seconds to record the fluorescence intensity upon entering the solution. After rinsing with deionized water, the next concentration test can be performed, demonstrating its short measurement time and facilitating continuous monitoring of chloride ion concentration in concrete. Then, based on the fluorescence spectrum, the recorded fluorescence intensity and its corresponding concentration are calibrated using the Stern-Wolmer equation to obtain the standard curve equation. The Stern-Wolmer equation is as follows:

[0103]

[0104] Where I0 and I represent the original fluorescence intensity and the fluorescence intensity after quenching of the fluorescent indicator, respectively, [Q] represents the concentration of the quencher (chloride ion solution), and K...SV is the quenching constant, and its value indicates the sensitivity of the fluorescent indicator to the quencher.

[0105] The standard equation for the fluorescence intensity-chloride ion concentration of the fluorescent probe can be derived from the data measured during the monitoring process in step 4), where I0 is the fluorescence intensity of the fluorescent probe in air and I is the fluorescence intensity of the fluorescent probe in the prepared solutions of various concentrations.

[0106] The fluorescence intensity-concentration curves of the fluorescent probe for six coating cycles were obtained by fitting multiple data points, as shown in the figure below. Figure 6a As shown, the standard curve equation is I0 / I = 1.2652 + 1.2548[Q](R 2 =0.999); The fluorescence intensity-concentration curves of the fluorescent probes coated for eight periods are shown in the figure. Figure 6b As shown, the standard curve equation is I0 / I = 1.5868 + 1.7127[Q](R 2 =0.954), and from the above two equations, it can be seen that the correlation coefficient R of the equation for the sixth coating is 0.954. 2 The accuracy of a fluorescent probe with six coatings is higher than that with eight coatings; and after measurement, the sensitivity of this fluorescent probe is 0.001M, that is, the minimum chloride ion concentration that can be measured is 0.001M.

[0107] Based on this fitted equation, when testing the chloride ion concentration in concrete, the fluorescence intensity I of the fluorescent probe in the concrete is recorded, and I0 is a known data, which can be substituted into the equation to calculate the chloride ion concentration [Q].

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a fluorescent probe, characterized in that, The preparation method includes: (1) Treat both ends of the bare quartz fiber to expose the quartz core at both ends; (2) Surface modification of the quartz fiber core at one end of the bare quartz fiber to hydroxylate the surface of the fiber core at the corresponding end; (3) Preparation of sol-gel casting solution doped with fluorescent nanoparticles, wherein the sol-gel casting solution doped with fluorescent nanoparticles is prepared by tetraethyl orthosilicate, ethanol, HCl, SiO2 fluorescent nanoparticles and water; the specific method is as follows: Tetraethyl orthosilicate, ethanol, HCl, SiO2 fluorescent nanoparticles, and water were mixed in a volume ratio of tetraethyl orthosilicate:ethanol:HCl:H2O:SiO2 fluorescent nanoparticles = (20-30):(32-48):(0.064-0.096):(8-12):(0.0015-0.0021). The mixture was then stirred continuously on a magnetic stirrer for 16-32 hours to obtain a sol-gel solution. The solution was stored in the dark for 1-2 days until it became a semi-gel state, which yielded a sol-gel casting solution doped with fluorescent nanoparticles. (4) Using the dip-coating method, one end of the modified quartz fiber core in step (2) is coated with a chloride ion sensitive film to prepare a fluorescent probe sensitive to chloride ions. The coating process is specifically as follows: The coating was performed using the dip coating method, with 50 coating operations performed in each coating cycle. The parameters for each coating operation were set as follows: dip speed 2083µm / s, descent speed 2083µm / s, immersion time 10 s, and interval time 10 s. After each coating cycle is completed, the sample is placed in an oven at 40-60℃ and dried for 20-30 minutes before proceeding to the next cycle. The entire coating process consists of 6 coating cycles. After completing the entire coating process, a fluorescent probe sensitive to chloride ions is prepared.

2. The method for preparing a fluorescent probe according to claim 1, characterized in that, The synthesis method of the SiO2 fluorescent nanoparticles is as follows: H2O, cyclohexane, Tritium X-100, and n-hexanol were mixed in a volume ratio of H2O:cyclohexane:Tritium X-100:n-hexanol = (9-11):(3.8-4.6):(0.9-1.1):(0.9-1.1) to prepare a microemulsion. An aqueous solution of chloride ion-sensitive fluorescent dye and an aqueous solution of chitosan were then added to the microemulsion to obtain a mixed solution. The pH of the mixed solution was adjusted to neutral, and then tetraethyl orthosilicate and ammonia were added. The mixture was stirred continuously on a magnetic stirrer for 24-36 hours until an emulsion was formed. After centrifugation, washing, and drying, SiO2 fluorescent nanoparticles were obtained. The concentration of the chloride ion-sensitive fluorescent dye in the aqueous solution is 0.005-0.015 mol / L, and the concentration of chitosan in the aqueous solution is 0.25-0.75 g / L. When preparing the mixed solution, the volume ratio of the microemulsion, the aqueous solution of the chloride ion-sensitive fluorescent dye, and the aqueous solution of chitosan is (14.6-17.8):(4.5-5.5):(9-11); when preparing the emulsion, the volume ratio of the mixed solution, the tetraethyl orthosilicate, and the ammonia is (28.1-34.3):(5.4-6.6):(3.6-4.4).

3. The method for preparing a fluorescent probe according to claim 1, characterized in that, Step (1) specifically involves: placing both ends of the bare quartz fiber into concentrated sulfuric acid for acid etching to remove the skin and sheath layers, exposing the quartz fiber core at both ends of the bare quartz fiber, and then rinsing the surface of the quartz fiber core with deionized water. After acid etching, the exposed length of the quartz fiber core at one end of the bare quartz fiber is 1-2cm, and the exposed length of the quartz fiber core at the other end is 4-6cm; the end with the exposed length of 1-2cm of quartz fiber core is then moved to step (2) for surface modification.

4. The method for preparing a fluorescent probe according to claim 1, characterized in that, Step (2) specifically involves: The bare quartz fiber core at one end was immersed in a potassium hydroxide isopropanol solution, then rinsed with a large amount of distilled water and dried with compressed nitrogen. The dried quartz fiber core was immersed in Piranha solution to remove surface organic matter and hydroxylate the fiber core surface; it was then rinsed in distilled water and dried to complete the surface modification.

5. A fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete, characterized in that, The fluorescent fiber optic sensing system includes: a light source, a fiber optic spectrometer, a Y-shaped bifurcated fiber, a chloride ion-sensitive fluorescent probe, and a fluorescent probe connector; the chloride ion-sensitive fluorescent probe is a fluorescent probe obtained according to any one of claims 1-4. The Y-shaped branched optical fiber includes a first branch, a second branch, and a combining end; the first branch and the second branch are respectively connected to the light source and the optical fiber spectrometer; One end of the fluorescent probe is a coated end for detecting chloride ions, and the other end is the combining end of the fluorescent probe connector that connects to the Y-shaped bifurcated optical fiber. When the coated end of the fluorescent probe comes into contact with chloride ions, the chloride ion-sensitive fluorescent nanoparticles in the sol-gel film undergo fluorescence quenching. The degree of fluorescence quenching is focused by a Y-shaped bifurcated optical fiber and displayed on a spectrometer. The chloride ion concentration is obtained through the Stern-Wolmer equation, enabling real-time monitoring of the chloride ion concentration in concrete. The fluorescent probe connector is used to connect the Y-type fiber combiner end and the fluorescent probe. The fluorescent probe connector includes a fluorescent probe fixing unit and a cylindrical connecting unit. The fluorescent probe fixing unit is used to fix the fluorescent probe; The cylindrical connecting unit is used to connect the fluorescent probe and the combining end of the Y-shaped branched optical fiber; the cylindrical connecting unit is internally provided with three plano-convex lenses: a first plano-convex lens, a second plano-convex lens, and a third plano-convex lens.

6. The fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete according to claim 5, characterized in that, The first plano-convex lens is disposed at one end close to the fluorescent probe, and the plane of the first plano-convex lens faces the direction of the fluorescent probe; the planes of the second plano-convex lens and the third plano-convex lens are disposed on the same plane, the convex surface of the second plano-convex lens faces the direction of the fluorescent probe, and the plane of the third plano-convex lens faces the direction of the fluorescent probe. By controlling the positions of the three plano-convex lenses, light from the second light source enters the second plano-convex lens through the first optical path of the Y-shaped branched fiber and then is incident on the first plano-convex lens, and light from the first plano-convex lens is converged to the third plano-convex lens.

7. The fluorescent fiber optic sensing system for monitoring chloride ion concentration in concrete according to claim 6, characterized in that, The fluorescent fiber optic sensing system is used to monitor the pore fluid in concrete and the chloride ion concentration in freshly mixed concrete.

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