Cascade seven-core optical fiber sensor applied to folic acid detection and preparation method thereof

Abstract

The invention discloses a cascade seven-core optical fiber sensor applied to folic acid detection and a preparation method of the cascade seven-core optical fiber sensor, and belongs to the technical field of optical fiber sensing and biological analysis detection. The sensor comprises a first single-mode optical fiber, a seven-core optical fiber section and a second single-mode optical fiber, a double-microsphere-cavity structure is arranged at the optical fiber connecting position to form a cascade interference optical path, and tapering is conducted on the middle portion of the seven-core optical fiber section to form a sensing area. According to the invention, mode matching is improved through the microsphere cavity, an evanescent field is enhanced through tapering, and high-sensitivity specific detection of folic acid is realized in cooperation with the folic acid binding protein biological functional layer fixed on the surface; the sensor obtains stable linear response to a folic acid standard solution in the range of 0-50 ng / mL, a clinical serum sample verifies that the detection result of the sensor is highly consistent with that of a chemiluminescence method, the sensor has good reusability, and the sensor has the advantages of being easy to prepare in structure, rapid in response, high in sensitivity, capable of being matched with a complex matrix and the like, and has good application prospects. The method has huge potential in clinical folic acid quantitative detection.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing and bioanalysis detection technology, specifically to a cascaded seven-core fiber optic sensor for folic acid detection and its preparation method. Background Technology

[0002] Folic acid is one of the micronutrient indicators that are routinely monitored in clinical practice, and it is in practical use in anemia, pregnancy-related risk assessment, and monitoring of some cancer treatment processes. While common methods in hospital testing systems, such as chemiluminescence, offer good accuracy, they are heavily reliant on instrument platforms, reagent systems, and testing scenarios. If the goal is to further develop more portable, faster, and lower-cost testing methods, traditional approaches often involve trade-offs between complex sample pretreatment, time-consuming testing, and expensive systems.

[0003] The advantages of fiber optic sensors lie in their compact structure, resistance to electromagnetic interference, ease of integration with spectral systems, and direct application in label-free detection. However, significant practical challenges remain: one issue stems from insufficient sensitivity to the light field, particularly in complex biological fluids where changes in effective refractive index caused by surface bonding are often masked by background factors such as bulk refractive index fluctuations and temperature drift. Another issue arises from insufficient surface stability, where physical adsorption or unstable coupling methods lead to significant drift in repeated measurements and difficulties in regeneration. A third challenge is the "structural fabrication barrier," where excessive reliance on micro / nano fabrication or complex coating processes rapidly amplifies conversion costs.

[0004] Therefore, developing a label-free folic acid detection scheme that is easy to prepare, highly sensitive, stable, and adaptable to complex matrices is of great practical significance. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a cascaded seven-core fiber optic sensor for folic acid detection and its preparation method, in order to solve the problems in the prior art.

[0006] To achieve the above objectives, this invention proposes a synergistic design approach of cascaded optical path interference enhancement, tapered evanescent field enhancement, and biorecognition molecule immobilization. The cascaded structure is used to amplify spectral line changes, the tapered region increases the proportion of light field leakage by reducing the outer diameter to enhance the coupling efficiency between the evanescent field and the interface refractive index perturbation, and the surface-immobilized folic acid binding protein provides selective binding sites, thereby achieving highly sensitive and specific detection of folic acid.

[0007] Specifically, the present invention provides a cascaded seven-core fiber optic sensor for folic acid detection, comprising: a first single-mode fiber, a seven-core fiber segment, and a second single-mode fiber. A first microsphere cavity structure is provided at the first connection between the first single-mode fiber and the seven-core fiber segment, and a second microsphere cavity structure is provided at the second connection between the seven-core fiber segment and the second single-mode fiber to form a cascaded interference optical path. The seven-core fiber segment has a tapered sensing region in the middle, which is formed by locally heating and stretching the seven-core fiber segment.

[0008] In the technical solution of the present invention, the microsphere cavity structure is used to improve the mode field mismatch between the single-mode fiber and the seven-core fiber, and to promote the power redistribution of the incident light between the seven-core supermode and the higher-order cladding modes, thereby enhancing the spectral response to external refractive index perturbations.

[0009] Furthermore, the tapered sensing region includes a transition region and a narrow waist region. The outer diameter of the transition region gradually changes along the axial direction to reduce reflection and additional insertion loss, while the outer diameter of the narrow waist region decreases to enhance the evanescent field.

[0010] Among them, the outer diameter of the tapered region is a key parameter affecting performance and usability. When the outer diameter of the tapered region is greater than 20 μm, the guided mode is still mainly limited to propagation within the fiber core and cladding, the evanescent field leakage ratio is limited, and the interaction with the surface functional layer and the analyte molecules is insufficient, resulting in a small effective refractive index change caused by refractive index perturbation, an insignificant spectral line drift, and overall sensitivity that is difficult to meet detection requirements. Conversely, when the outer diameter of the tapered region is less than 5 μm, although the evanescent field can be further enhanced, the cross-sectional area of ​​the fiber is significantly reduced, the defect sensitivity increases, and the bending and tensile strength of the device drops sharply. Under conditions such as encapsulation, liquid phase reaction, and repeated rinsing, it is extremely easy to break, making it difficult to achieve stable reuse and engineering applications. Therefore, the minimum outer diameter of the tapered sensing region is optimized to be 5~20 μm, preferably about 10 μm, and the length is 0.5~2.5 cm, preferably about 1.2 cm.

[0011] To achieve specific folic acid detection, a biofunctional layer for specific folic acid recognition is fixed on the surface of the tapered sensing area. This biofunctional layer is formed by covalently fixing folic acid binding protein (FBP) to the surface of an optical fiber, and after fixing, non-specific sites are blocked by bovine serum albumin (BSA). Folic acid binding protein (FBP) is covalently fixed through a "hydroxylation-silanization-glutaraldehyde crosslinking" method.

[0012] Furthermore, both the first and second microsphere cavity structures are integrated ellipsoidal transition zones formed by fusing two prefabricated ellipsoidal end faces.

[0013] The present invention also provides a method for fabricating the above-mentioned cascaded seven-core fiber optic sensor, including the steps of constructing a dual microsphere cascaded structure, fabricating a tapered sensing region, and performing biofunctional modification on the surface of the sensing region.

[0014] Specifically, the preparation method includes: Step 1: Form a first ellipsoidal end face structure at the end of the first single-mode fiber, form a second ellipsoidal end face structure at one end of the seven-core fiber segment, fuse the two together and form a first microsphere cavity structure at the connection point; Step 2: Form a third ellipsoidal end face structure at the other end of the seven-core fiber segment and a fourth ellipsoidal end face structure at the end of the second single-mode fiber. Fuse the two together and form a second microsphere cavity structure at the connection point to obtain a dual microsphere cavity cascaded structure. Since the diameters of the single-mode fiber (SMF) and the seven-core fiber (SCF) are mismatched and the core spacing of the seven-core fiber (SCF) is fixed, the microsphere cavity helps to guide the incident light more fully into the seven fiber cores and makes it easier to excite higher-order cladding modes, thus providing a stronger mode basis for refractive index sensitivity. Step 3: Apply hydrogen-oxygen flame heating to the middle part of the seven-core fiber segment and perform symmetrical stretching to form a tapered sensing area; this enhances the evanescent field and further amplifies the effective refractive index change brought about by the bonding of the fiber surface. Step 4: The cascaded seven-core fiber optic sensor is thoroughly cleaned, hydroxylated, and silanized in sequence to introduce amino functional groups on the surface, followed by cleaning and drying. Step 5: Place the sensor treated in Step 4 in a glutaraldehyde solution for cross-linking activation, clean and dry it again, and then immerse it in a folic acid-binding protein solution under light-protected conditions for covalent fixation. Step 6: The sensor immobilized with folic acid binding protein (FBP, a biomolecule that specifically recognizes folic acid) is blocked with bovine serum albumin (BSA, used to block non-specific adsorption sites) solution to reduce non-specific adsorption, thus obtaining a cascaded seven-core fiber optic sensor that can specifically detect folic acid.

[0015] By sequentially performing NaOH hydroxylation (generating hydroxyl groups on the fiber surface to provide reaction sites for subsequent silanization), APTES (3-aminopropyltriethoxysilane, a silanizing agent used to introduce amino groups) silanization, and glutaraldehyde (GA, a cross-linking agent used to connect amino groups and biomolecules) cross-linking activation on the surface of the tapered sensitive region, and then immobilizing folic acid-binding protein (FBP) to provide folic acid-specific binding sites, non-specific adsorption is reduced using blocking agents such as bovine serum albumin (BSA), ensuring a more stable baseline and response during multiple detections.

[0016] In the preparation method of the present invention, the process parameters are optimized as follows: the length of the seven-core optical fiber segment is 0.5~2.5cm, preferably about 1.2cm; the minimum outer diameter of the tapered region is 5~20μm, preferably about 10μm; the effective heating width of the hydrogen-oxygen flame is 2~10mm, preferably about 5mm; and the stretching amount of the tapered region is 0.3~1.5cm, preferably about 0.9cm.

[0017] Furthermore, the specific processes for forming the ellipsoidal end face and fusion splicing in steps 1 and 2 are as follows: Using a commercial fusion splicer, in manual mode, an arc discharge is performed on the end face of the single-mode fiber with parameters of 160~200 mA discharge current and 10~30 μm advance distance to form an ellipsoidal end face; an arc discharge is performed on the end face of the seven-core fiber with parameters of 200~260 mA discharge current to form an ellipsoidal end face; the two fibers with pre-formed ellipsoidal end faces are aligned and fused with a discharge current of 80~120 mA and a center advance mode, forming a fusion transition zone in the middle of the two ellipsoids, thereby forming the microsphere cavity structure of the sensor at the connection point.

[0018] Further, the specific process of tapering in step 3 is as follows: the optical fiber is fixed on an electric displacement stage, and the middle part of the seven-core optical fiber segment is uniformly heated by an oxyhydrogen flame with a heating width of 2~10 mm. After the optical fiber softens, the two displacement stages on both sides are stretched in opposite directions at the same speed with a stretching amount of 0.3~1.5 cm to form a tapered sensing area with an outer diameter of 5~20 μm in the narrow waist region.

[0019] When the above-mentioned cascaded seven-core fiber optic sensor is used for folic acid detection, the following steps are included: connecting the sensor to the spectral system, reading the peak position shift Δλ of the characteristic peak of the spectral line within a fixed time (5 min), and obtaining the folic acid concentration according to the pre-established Δλ-concentration calibration curve; after each detection, washing with alkaline phosphate buffer solution (PBS) and deionized water to support continuous detection.

[0020] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. By introducing microsphere cavity structures at two fusion connection points of the SMF–SCF–SMF cascade structure and performing hydrogen-oxygen flame tapering treatment on the SCF sensing area, the optical field is more fully coupled between the seven-core and cladding correlation modes, and an enhanced evanescent field is formed in the waist region. This significantly improves the device's response sensitivity to external refractive index and interface perturbations, and obtains clearer and more stable interference spectrum characteristics and peak position drift signals.

[0021] 2. By immobilizing folic acid-binding protein (FBP) on the surface of the optical fiber region to form a specific recognition interface, and using a covalent immobilization method of "hydroxylation-silanization-glutaraldehyde crosslinking", combined with bovine serum albumin (BSA) blocking and washing regeneration process, non-specific adsorption and baseline drift are effectively suppressed, so that the detection signal mainly comes from the change in the equivalent refractive index of the interface caused by the binding of target molecules, thereby improving the anti-interference ability and result reliability in complex samples such as serum.

[0022] 3. This invention uses wavelength shift of characteristic peaks in transmission spectroscopy to achieve label-free detection, and quantitative analysis can be completed within a fixed reaction time. The sensitivity can reach 118 pm / (ng / mL) within the linear range, and the detection limit can be as low as 750 pg / mL. At the same time, it shows high consistency with the results of chemiluminescence assay in the validation of delabeled clinical serum samples, and has the potential to be further applied to clinical quantitative detection. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the manufacturing process of the cascaded seven-core fiber optic sensor of the present invention; Figure 2 The simulated optical field distribution and corresponding spectrum of the seven-core fiber tapered structure of this invention are shown below. Figure 3 This is a schematic diagram of the sensor surface functionalization process of the present invention; Figure 4 This is a schematic diagram of the refractive index calibration curve of the sensor of the present invention; Figure 5 This is a schematic diagram of the sensor's detection response and linear fitting to folic acid standard solution according to the present invention; Figure 6 This is a schematic diagram of the sensor of the present invention detecting serum-spiked folic acid samples; Figure 7 This is a schematic diagram illustrating the consistency between the sensor of this invention detecting clinical samples and the chemiluminescence method control.

[0024] In the figure: 1. First single-mode fiber; 2. Second single-mode fiber; 3. Seven-core fiber segment; 4. First microsphere cavity structure; 5. Second microsphere cavity structure; 6. Arc discharge device; 7. Hydrogen-oxygen flame ignition device; 8. Tapering device. Detailed Implementation

[0025] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0026] Example 1 refer to Figure 1This embodiment details the sensor fabrication process.

[0027] The first single-mode fiber 1 is fixed in the fixture of a commercial fusion splicer. The manual fusion splicing mode is selected, and the initial parameters are set as follows: the discharge current of the arc discharge device 6 is 180 mA and the advance distance is 20 μm. Arc discharge is performed on the end face of the first single-mode fiber 1. Since the arc power here is higher than that under conventional fusion splicing conditions, the material at the end of the fiber softens and gradually bulges under the action of surface tension, eventually forming a stable ellipsoidal end face structure.

[0028] The same approach was used to prepare an ellipsoidal structure for the end face of the seven-core fiber segment 3. Considering that the outer diameter of the seven-core fiber segment 3 is larger than that of the single-mode fiber, its discharge parameters were set to 220mA.

[0029] After the ellipsoids at both ends are prefabricated, the prepared seven-core fiber segment 3 and the first single-mode fiber 1 are placed on both sides of the fusion splicer, and the end faces of the ellipsoids are aligned and axially adjusted. At this time, the connection and fusion are still performed in manual mode. The parameters are adjusted as follows: discharge current 100 mA, and the propulsion mode is center propulsion. After discharge, the two ellipsoids form a fusion transition zone in the middle, thereby obtaining the first microsphere cavity structure 4 of the sensor.

[0030] To obtain the required sensor region length, the other end of the seven-core optical fiber 3 is cut and the end face is trimmed using a cutter. By controlling the length, interference structures with different effective lengths can be prepared. The preferred sensor region length is 1.2 cm.

[0031] The cut seven-core fiber segment 3 is put back into the fusion splicer, and the end of the seven-core fiber segment 3 is discharged multiple times using the same discharge procedure as above, so that its end face also forms an ellipsoidal structure. At the same time, the above steps are repeated for the second single-mode fiber 2 to prepare an ellipsoidal end face. The second single-mode fiber 2 and the seven-core fiber segment 3 are clamped and aligned respectively, and the discharge fusion is completed using the same fusion parameters as above to prepare the second microsphere cavity structure 5.

[0032] To further improve the refractive index sensitivity, a controllable hydrogen-oxygen flame heating device 7 is used to taper three segments of a seven-core optical fiber. The optical fiber is fixed on an electric displacement stage, and after the middle part is uniformly heated to soften, the two tapering devices 8 on both sides stretch it in opposite directions at the same speed, so that the optical fiber is extended by about 0.9 cm to form a tapered seven-core optical fiber region. The minimum diameter of the waist is controlled at about 10 μm, and a semi-finished cascaded seven-core optical fiber sensor is obtained.

[0033] Example 2 This invention uses RSoft software to perform numerical simulation of the tapered structure of a seven-core fiber (SCF). It is mainly used to illustrate from three perspectives—structural change, optical field evolution, and spectral response—that the tapered morphology directly changes the energy distribution and further affects the interference fringe characteristics and refractive index response of the device.

[0034] Figure 2 The field distribution and monitoring results along the fiber axis are presented. The simulation results show that after the light enters the tapered section, the energy is no longer mainly limited to propagation within a single fiber core, but is redistributed between the supermode formed by multi-core coupling and the cladding-related mode, exhibiting energy exchange characteristics that vary with propagation distance. The monitoring curve near the waist fluctuates more significantly, indicating that the mode coupling is stronger and the power exchange is more frequent in this region, which is the main region affecting the device's interference behavior and sensing response.

[0035] The corresponding spectral results are as follows Figure 2 As shown, quasi-periodic interference fringes can be observed in the 1200-2400 nm band, accompanied by a certain amplitude modulation, indicating that there is a stable multimode interference process inside the device. The free spectral range and contrast of the fringes are mainly affected by the effective refractive index difference and the equivalent coupling length. At the same time, the evanescent field enhancement in the narrow waist region makes the interaction between the light field and the external medium more complete, so that the external refractive index change is more easily converted into spectral line drift, thereby improving the device's sensitivity to interface disturbances.

[0036] Example 3 To achieve folic acid-specific detection, a biofunctional layer needs to be formed by immobilizing folic acid-binding protein (FBP) in the region of a tapered seven-core fiber (TSCF). The process is as follows: Figure 3 As shown.

[0037] The specific steps are as follows: Step 1: Thoroughly clean the sensor prepared in Example 1 with ethanol and deionized water (3 cycles) to remove contaminants.

[0038] Step 2: Immerse the sensor in 1.0 M NaOH solution for 1 hour to perform hydroxylation treatment, thereby enriching the surface with hydroxyl groups.

[0039] Step 3: Immerse the fiber optic sensor in a 5% (v / v) APTES (3-aminopropyltriethoxysilane) ethanol solution for 3 hours to complete silanization and introduce amino functional groups, then clean and dry.

[0040] Step 4: Treat the sensor with a 5% (v / v) glutaraldehyde (GA) aqueous solution for 2.5 hours for crosslinking activation, then clean and dry again. This step uses the APTES+GA scheme: the silyl group at one end of APTES can form a stable bond with the hydroxyl group on the surface of the optical fiber, and the amino group at the other end facilitates subsequent reactions; the aldehyde groups (-CHO) at both ends of GA can crosslink with the amino group exposed by APTES, thereby achieving stable fixation without additional activation pretreatment of biomolecules.

[0041] Step 5: Then, under light-protected conditions, the sensor was incubated in a 1 μg / mL folic acid binding protein (FBP) solution for 2 hours to complete the fixation.

[0042] Step 6: Block with 5% (w / v) BSA (bovine serum albumin) / PBS (phosphate buffer solution) for 1 hour to reduce non-specific adsorption, and finally complete the preparation of a cascaded seven-core fiber optic sensor for folic acid detection.

[0043] Example 4 To evaluate the sensor's response to changes in refractive index, refractive index calibration was first performed. A sucrose solution with a refractive index in the range of 1.3333–1.3394 was selected, and the precise refractive index was measured using an Abbe refractometer (error ±0.0002). The sensor was then immersed in the solution, and the shift Δλ of its characteristic peak with changes in refractive index was recorded. The refractive index sensitivity was then fitted to obtain the result. Figure 4 As shown, the refractive index sensitivity in this embodiment can reach approximately 1352.1 nm / RIU, providing a high-sensitivity structural basis for subsequent biological detection.

[0044] Example 5 The sensor prepared above enabled label-free rapid detection of folic acid. The experiment used 0.01 M PBS as the buffer system to prepare folic acid solutions with concentrations of 0, 10, 20, 30, 40, and 50 ng / mL. For each test, 1 mL of sample was added to a custom-designed Ω-shaped groove (maximum diameter 3 mm) to completely immerse the sensing area. Transmission spectra were recorded after 5 min of immersion. To avoid residual analytes affecting subsequent tests, the sensor and groove were rigorously cleaned after each measurement: first rinsed with alkaline PBS (phosphate buffer solution), then rinsed with deionized water (DI water) to remove residual folic acid. To improve the consistency and efficiency of rinsing, a mechanically pump-driven flow rinsing system was used. Multiple characteristic peaks were tracked in the 1730-1780 nm band to obtain Δλ, and a Δλ-concentration calibration curve was established. Figure 5 As shown.

[0045] In this embodiment, a sensitivity of 118 pm / (ng / mL) was obtained. The limit of detection (LOD) was calculated according to the 3σ / S rule, where σ is the standard deviation of 10 blank sample measurements and S is the sensitivity of the standard curve. The calculated LOD can be as low as 750 pg / mL. After testing, the baseline drift of the sensor was less than ±0.2 nm after 20 "detection-cleaning" cycles, and the response sensitivity remained above 95% of the initial value, indicating that it has good reusability.

[0046] Example 6 Aliquots of the same human serum sample were prepared, and different amounts of folic acid standard were added to each sample to create a 0-50 ng / mL spiking gradient. The samples were then analyzed using the same procedure as in Example 4. The results are as follows: Figure 6 As shown, this result indicates that the spiked concentration gradient can still be distinguished in the serum matrix, and the response can be repeated after multiple washing and regeneration, demonstrating the sensor's good specificity and reusability in complex matrices.

[0047] Example 7: Clinical Sample Comparison and Validation Several delabeled patient serum samples were collected (10 in this example). Each sample was divided into two parts: one part was sent to the hospital for folic acid concentration determination using chemiluminescence immunoassay, and the other part was measured using the sensor of this invention. Clinical sample testing could be performed after appropriate dilution (e.g., to one-third of the original volume concentration) without compromising accuracy. Each sample was collected after a fixed incubation time (5 min), and peak shift was calculated. Concentration was converted using a calibration curve. Comparison results showed that the platform of this invention and the chemiluminescence immunoassay yielded highly consistent results with minimal deviation (e.g., ...). Figure 7 As shown in the figure, it has the feasibility for clinical application.

[0048] In summary, the present invention provides a cascaded seven-core fiber optic sensor for folic acid detection and its preparation method. Through a unique dual-microsphere cascaded structure and a tapered evanescent field enhancement design, combined with stable surface biofunctionalization technology, it achieves highly sensitive, specific, rapid, and repeatable detection of folic acid, and has broad application prospects in clinical testing and portable diagnostics.

[0049] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A cascaded seven-core fiber optic sensor for folic acid detection, characterized in that, include: A first single-mode fiber (1), a seven-core fiber segment (3), and a second single-mode fiber (2); a first microsphere cavity structure (4) is provided at the first connection point between the first single-mode fiber (1) and the seven-core fiber segment (3), and a second microsphere cavity structure (5) is provided at the second connection point between the seven-core fiber segment (3) and the second single-mode fiber (2) to form a cascaded interference optical path. The seven-core fiber segment (3) has a tapered sensing area in the middle, which is formed by local heating and stretching of the seven-core fiber segment; the surface of the tapered sensing area is fixed with a biological functional layer for specific recognition of folic acid.

2. The cascaded seven-core fiber optic sensor as described in claim 1, characterized in that, The tapered sensing region includes a transition region and a narrow waist region. The outer diameter of the transition region gradually changes along the axial direction to reduce reflection, and the outer diameter of the narrow waist region decreases to enhance the evanescent field. The minimum outer diameter of the tapered sensing region is 5~20 μm, and the length of the tapered sensing region is 0.5~2.5 cm.

3. The cascaded seven-core fiber optic sensor as described in claim 1, characterized in that, The biofunctional layer is formed by covalently immobilizing folic acid-binding protein onto the surface of an optical fiber, and after immobilization, non-specific sites are blocked by bovine serum albumin.

4. The cascaded seven-core fiber optic sensor as described in claim 1, characterized in that, Both the first microsphere cavity structure (4) and the second microsphere cavity structure (5) are integrated ellipsoidal transition zones formed by welding two prefabricated ellipsoidal end faces.

5. A method for preparing a cascaded seven-core fiber optic sensor as described in claim 1, characterized in that, Includes the following steps: Step 1: Form a first ellipsoidal end face structure at the end of the first single-mode fiber (1), form a second ellipsoidal end face structure at one end of the seven-core fiber segment (3), fuse the two together and form a first microsphere cavity structure (4) at the connection. Step 2: Form a third ellipsoidal end face structure at the other end of the seven-core fiber segment (3), and form a fourth ellipsoidal end face structure at the end of the second single-mode fiber (2). Weld the two together and form a second microsphere cavity structure (5) at the connection point to obtain a double microsphere cavity cascade structure. Step 3: Apply hydrogen-oxygen flame heating to the middle part of the seven-core fiber segment (3) and perform symmetrical stretching to form a tapered sensing area.

6. The method as described in claim 5, characterized in that, The specific processes for forming the ellipsoidal end face and fusion splicing in steps 1 and 2 are as follows: Using a commercial fusion splicer, in manual mode, the end face of the single-mode fiber is subjected to arc discharge with parameters of 160~200 mA discharge current and 10~30 μm advance distance to form an ellipsoidal end face; the end face of the seven-core fiber is subjected to arc discharge with parameters of 200~260 mA discharge current to form an ellipsoidal end face; the two fibers with pre-formed ellipsoidal end faces are aligned and fused with a discharge current of 80~120 mA and a center advance mode, forming a fusion transition zone in the middle of the two ellipsoids, thereby forming the microsphere cavity structure of the sensor at the connection point.

7. The method as described in claim 5, characterized in that, The specific process of tapering described in step 3 is as follows: the optical fiber is fixed on the electric displacement stage, and the middle part of the seven-core optical fiber segment (3) is uniformly heated by an oxyhydrogen flame. The heating width is 2~10 mm. After the optical fiber softens, the two displacement stages are stretched in opposite directions at the same speed. The stretching amount is 0.3~1.5 cm, forming a tapered sensing area with an outer diameter of 5~20 μm in the narrow waist region.

8. The method as described in claim 5, characterized in that, It also includes the step of constructing a biofunctional layer on the surface of the tapered sensing region: Step 4: The cascaded seven-core fiber optic sensor is thoroughly cleaned, hydroxylated, and silanized in sequence to introduce amino functional groups on the surface, followed by cleaning and drying. Step 5: Place the sensor treated in Step 4 in a glutaraldehyde solution for cross-linking activation, clean and dry it again, and then immerse it in a folic acid-binding protein solution under light-protected conditions for covalent fixation. Step 6: The sensor immobilized with folic acid binding protein is blocked with bovine serum albumin solution to reduce non-specific adsorption, thus obtaining a cascaded seven-core fiber optic sensor that can specifically detect folic acid.

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