Patch amplification folic acid optical fiber sensing system and folic acid concentration detection method thereof

Abstract

The invention discloses a complement amplification folic acid optical fiber sensing system and a folic acid concentration detection method thereof. The sensing system comprises an excitation light source, an optical fiber sensing probe, a micro-fluidic plate, a nanogold rod amplification solution and a spectrograph, the excitation light source, the optical fiber sensing probe and the spectrograph are sequentially communicated, the optical fiber sensing probe is introduced into the micro-fluidic plate, the nanogold rod amplification solution is introduced into the micro-fluidic plate, and the nanogold rod amplification solution is introduced into the spectrograph. The optical fiber sensing probe is a folic acid receptor protein modified gold-plated film optical fiber, and the nanogold rod amplification solution is a folic acid modified nanogold rod solution. Under the participation of the gold nanoparticles, the optical fiber sensing probe realizes enhanced plasma resonance wavelength shift, so that the sensitivity of folic acid molecule detection is remarkably improved, folic acid in a solution can be quickly and directly measured, and the application range is wide.

Description

Technical Field

[0001] This invention relates to folic acid content detection, specifically to a complementary amplified folic acid fiber optic sensing system and a method for detecting folic acid concentration. Background Technology

[0002] Folic acid, also known as vitamin B9, vitamin M, or pteroylglutamic acid, is a member of the vitamin B family. This important nutrient not only helps treat anemia caused by folic acid deficiency but also plays an indispensable role in maintaining human health. Folic acid insufficiency can lead to folic acid deficiency syndrome, which can result in a condition called megaloblastic anemia. Symptoms of this include fatigue, palpitations, difficulty breathing, ulcers on the tongue, and changes in the skin or hair. The benefits of folic acid extend far beyond this. It plays a crucial role in the development of the fetal nervous system, making sufficient folic acid especially important for pregnant women to ensure the health of the fetus. Furthermore, folic acid helps maintain normal DNA function and is essential for cell growth and repair. However, long-term intake of excessively high doses of folic acid can also have some health effects. Therefore, proper folic acid intake is very important.

[0003] Currently, the main methods for detecting folic acid in food include microbiological methods, chemiluminescence methods, voltammetry, enzyme-linked immunosorbent assay (ELISA), ultraviolet absorption spectroscopy, and high-performance liquid chromatography (HPLC). Microbiological methods are widely used for determining folic acid content in food due to their high sensitivity and broad applicability; however, they can only determine the total folic acid content in a sample and cannot individually determine the content of different folic acid derivatives. In addition, electrochemical and fluorescence methods are also used to determine folic acid content in food or multivitamin preparations, but they also have disadvantages such as high background and the need for pre-labeling.

[0004] Fiber optic sensors, with their compact size, lightweight design, strong anti-interference capabilities, and ability to perform remote detection and real-time monitoring, are widely used in the sensing field. The integration of fiber optic surface plasmon resonance (SPR) sensors with optical fibers provides scientists and researchers with a novel sensing approach. SPR fiber optic sensors fully utilize the light-guiding properties of optical fibers and the surface sensitivity of SPR technology, exhibiting highly sensitive detection performance and excellent real-time monitoring capabilities, enabling efficient, rapid, and accurate detection of biomolecules such as folic acid. Furthermore, fiber optic SPR sensors possess the unique advantage of real-time monitoring, allowing for the acquisition of reaction results in a short time without the need for labeled reagents or fluorescent probes. However, the sensitivity of fiber optic SPR sensors is limited due to the limited resonance peak shift caused by the dielectric constant of adsorbed biomolecules, necessitating further improvements in the sensitivity of fiber optic SPR sensing systems. Summary of the Invention

[0005] To address the current limitation in sensitivity of fiber optic SPR sensors for detecting folic acid concentration, this invention provides a complementary amplified fiber optic folic acid sensing system and a method for detecting folic acid concentration. By leveraging fiber optic technology, applying plasma resonance detection and complementary amplification techniques, and combining microfluidics, a novel high-sensitivity folic acid molecule detection system is constructed.

[0006] To achieve the above objectives, the present invention provides a folic acid amplification fiber optic sensing system, comprising an excitation source, a fiber optic sensing probe, a microfluidic plate, a gold nanorod amplification solution, and a spectrometer. The excitation source, the fiber optic sensing probe, and the spectrometer are connected in sequence. The fiber optic sensing probe is introduced into the microfluidic plate, and the gold nanorod amplification solution is introduced into the microfluidic plate. The fiber optic sensing probe is a gold-plated fiber modified with folic acid receptor protein, and the gold nanorod amplification solution is a folic acid-modified gold nanorod solution.

[0007] Preferably, the probe is D-shaped, with a sensing length of 1.5-3 cm and a gold film thickness of 40-60 nm.

[0008] Preferably, the concentration of the nano-gold rod amplification solution is 5-8 µmol / L.

[0009] Preferably, the excitation light source is a 5W halogen lamp.

[0010] A second aspect of the present invention provides a method for detecting folic acid concentration using the above-described sensing system, comprising the following steps: S1. Inject the fiber optic sensing probe into a buffer solution (usually PBS buffer solution with pH 7.4) through a microfluidic plate, and introduce the excitation light source into the fiber optic sensing probe to generate surface plasmon resonance. Receive the light with a spectrometer, detect the plasmon resonance spectrum, and obtain the first plasmon resonance peak. S2. Inject the test solution into the fiber optic sensing probe through the microfluidic plate and incubate for 15-20 min, then inject buffer solution to wash. S3. Inject the gold nanorod amplification solution into the fiber optic sensing probe through a microfluidic plate and incubate for 10-15 min to achieve the replacement of the fiber optic folic acid receptor by the gold nanorod. Wash with buffer solution and detect the plasmon resonance spectrum to obtain the second plasmon resonance peak. S4. Analyze the surface plasmon resonance peaks, compare the measured shifts of the two peaks with the standard curve, and obtain the folic acid concentration of the test solution.

[0011] Specifically, in step S4, the method for drawing the standard intensity curve includes: S4.1. Inject the fiber optic sensing probe into the buffer solution through the microfluidic plate, and introduce the excitation light source into the fiber optic sensing probe to generate surface plasmon resonance. Receive the signal with a spectrometer, detect the plasmon resonance spectrum, and obtain the third plasmon resonance peak. S4.2 Then, folic acid concentrations of 25, 12.5, 6.25, 3.12, 1.56 and 0.78 ng / mL were injected into the fiber optic sensing probe through the microfluidic plate and incubated for 15-20 min, followed by washing with buffer solution. S4.3. Inject the gold nanorod amplification solution into the fiber optic sensing probe through a microfluidic plate and incubate for 10-15 min. Wash with buffer solution and detect the plasmon resonance spectrum to obtain multiple fourth plasmon resonance peaks. S4.4 Analyze the surface plasmon resonance peaks and plot a standard curve with folic acid concentration as the abscissa and the wavelength shift between the fourth and third plasmon resonance peaks as the ordinate.

[0012] A third aspect of the present invention provides a method for constructing the above-described sensing system, comprising: Step 1: Constructing the fiber optic sensing probe (1.1) Deposit a gold film on the surface of a D-type optical fiber; (1.2) The optical fiber obtained in step (1.1) is incubated in a carboxylated polyethylene glycol mercapto solution for 5-6 h to obtain a carboxylated optical fiber; (1.3) The carboxylated optical fiber was activated by immersing it in a mixture of EDC (1-ethyl-(3-dimethylaminopropyl)carbodiimide) and NHS (N-hydroxysuccinimide) for 4-5 hours; (1.4) The activated optical fiber was incubated in a folic acid receptor protein solution for 10-12 h to construct an optical fiber sensing probe, which was then connected to a microfluidic spectroscopy detection plate for later use. Step 2: Constructing a scale-up solution for gold nanorods (2.1) Mix carboxyl polyethylene glycol thiol solution with gold nanorods and incubate at room temperature for 5-6 h to obtain carboxyl-functionalized gold nanorods; (2.2) The carboxyl-functionalized gold nanorods were activated by soaking in a mixture of EDC and NHS for 4-5 hours; (2.3) The activated gold nanorods were incubated in folic acid solution for 10-12 h to complete the folic acid modification.

[0013] Preferably, in step (1.2), the concentration of the carboxylated polyethylene glycol thiol solution is 0.3-0.4 mg / mL; in step (1.3), the concentration of EDC in the mixture is 3-4 mg / mL and the concentration of NHS is 1-2 mg / mL; in step (1.4), the concentration of the folic acid receptor protein solution is 1-2 mg / L.

[0014] Preferably, in step (2.1), the concentration of the carboxyl polyethylene glycol mercapto solution is 0.3-0.4 mg / mL; in step (2.2), the mass ratio of carboxyl-functionalized gold nanorods, EDC and NHS is 1:(3-4):(2-3).

[0015] Through the above technical solution, the present invention achieves the following beneficial effects: 1. This invention utilizes optical fiber-modified folic acid receptors to collect and identify folic acid, which has the advantages of not requiring special detection markers, high specificity, and high accuracy.

[0016] 2. This invention utilizes optical fiber evanescent wave excitation of plasmonic resonance and employs folic acid-modified gold rods and optical fibers for a replacement reaction to amplify the surface plasmonic resonance signal.

[0017] 3. This invention utilizes optical fiber evanescent wave excitation of plasmonic resonance, and takes advantage of the dual plasmonic resonance peak changes caused by the adsorption reaction of folic acid and gold rods. Furthermore, it utilizes an optical fiber + microfluidic structure to achieve folic acid detection, thus requiring a small sample amount and providing more sensitive detection.

[0018] 4. This invention utilizes fiber optic technology and applies plasma resonance detection and amplification techniques, combined with microfluidics, to construct a novel high-sensitivity folic acid molecule detection system. This achieves an organic combination of multiple existing technologies, improves sensitivity, simplifies the operation process, and provides a new approach for folic acid detection. Attached Figure Description

[0019] Figure 1 This is the sensing and detection system and schematic diagram of the present invention; Figure 2 Yes, the optical magnification of the D-type region of the fiber optic sensing probe (1), the composition of the fiber optic folic acid sensing part (2), the transmission electron microscope image and absorption spectrum of the gold nanorod (3), and the scanning electron microscope image of the surface of the D-type region of the fiber optic (4). Figure 3 These are the plasmon resonance spectra before and after folic acid binding, and before and after the gold rod sensitization. Figure 4 These are plasmon resonance images of gold rods at different folic acid incubation times in a fiber optic folic acid sensing system. Figure 5 These are the plasmon resonance diagrams and peak point diagrams under different folic acid concentrations in Example 3 of the present invention; Figure 6 This is a concentration detection graph of the vegetable folic acid extract in Example 4 of the present invention. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in detail below with reference to examples. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0021] In the following embodiments, such as Figure 1 As shown, the microfluidic plate used is a transparent plate (such as an acrylic plate, 10cm×3cm), with an optical fiber groove 1 (500µm wide) for the optical fiber sensing probe to pass through and a sample groove 2 (2-3cm long and 2mm wide). Several guide grooves 3 (1.5mm in diameter) are set around the sample groove 2 to improve the rinsing efficiency. One guide groove on one side of the sample groove 3 has a liquid injection port 4, and one guide groove on the other side has a liquid drainage port 5.

[0022] Example 1: Construction of a novel complementary amplified folic acid fiber optic sensing system Step 1: After removing the protective layer of the 10cm long, 400µm core diameter quartz fiber, grind the middle section for 1.5cm in length and 50µm in depth to obtain a D-shaped quartz fiber. Clean the D-shaped end of the fiber repeatedly with alcohol and deionized water three times, and then dry it with nitrogen. Step 2: Place the D-type optical fiber in a vacuum coating machine for sputtering gold deposition, with a gold film thickness of approximately 50 nm. Step 3: After gold coating, the optical fiber is rinsed three times with deionized water, incubated with a solution of 0.33 mg / mL carboxylated polyethylene glycol mercapto (SH-PEG-COOH) for 5 hours, and then dried with nitrogen to obtain carboxylated optical fiber. Step 4: Clean the optical fiber three times with 15mM MES (pH=6.0) solution, then immerse it in 3ml of coupling agent (EDC is 12mg and NHS concentration is 6mg) for 4h for activation. Finally, clean the activated optical fiber with MES 6.0. Step 5: Place the above optical fiber in 3 mL of folate receptor protein (FOLR1) solution (concentration of 1 mg / L) and incubate for 12 h to allow the folate receptor to be fully modified, constructing a plasmonic resonance nanoprobe. After washing with PBS (pH 7.4), connect it to a microfluidic spectroscopy detection plate for later use. Step Six: Mix 0.33 mg / mL of carboxylated polyethylene glycol thiol with gold nanorods (AuNP) to allow self-assembly via Au-S bonds, and incubate at 25°C for 5 hours. Next, extract the obtained carboxylated functionalized gold nanorods by centrifugation (12000 rpm, 5 minutes). Step 7: Dissolve 1 mg AuNP in 5 mL MES (pH 6.0), add 20 mg EDC and 10 mg NHS, stir and incubate at 25°C for 4 hours, centrifuge the solution (12000 rpm, 10 minutes), discard the supernatant, and wash the precipitate 3 times with PBS (pH 7.4). Step 8: Dissolve the activated gold nanorods in 5 mL of folic acid solution for 12 hours with stirring. After completing the folic acid modification, centrifuge the solution to extract the gold nanorods and resuspend them in 1 mL of PBS (pH 7.4) for later use.

[0023] The constructed folic acid fiber optic sensing system is as follows Figure 1 As shown, it includes an excitation source, an optical fiber sensing probe, a microfluidic plate, a gold nanorod amplification solution, and a spectrometer. The excitation source, optical fiber sensing probe, and spectrometer are connected in sequence. The optical fiber sensing probe is introduced into the microfluidic plate, and the gold nanorod amplification solution is introduced into the microfluidic plate.

[0024] The testing process of this invention is as follows: First, PBS (pH 7.4) is injected into the microfluidic plate embedded with optical fiber for cleaning, and then light is passed through for excitation. Next, a folic acid sample solution is injected, incubated for 15 min, and then cleaned with PBS (pH 7.4). Then, a gold nanorod solution is injected to induce a replacement reaction of the folic acid receptor on the optical fiber, followed by PBS (pH 7.4) cleaning. Finally, the surface plasmon resonance spectrum is detected again, and the peak shift is calculated to determine the folic acid concentration in the solution.

[0025] The theoretical basis for this invention, which utilizes a folic acid sensor to detect folic acid molecules, can be represented by the following process: In this invention, D-type optical fiber is an optical transmission device that has been theoretically and experimentally proven. Because of the addition of a high-dielectric mode for optical transmission, D-type optical fiber can generate stronger evanescent waves. By utilizing these evanescent waves to excite plasmonic resonance, it exhibits higher sensitivity and stronger anti-interference capabilities. It has been widely studied and applied.

[0026] When a thick gold film is deposited on the surface of a D-type optical fiber and light is introduced, surface plasmon resonance occurs, consuming light energy in a certain wavelength band and forming a resonance peak. This resonance peak is related to the refractive index of the surface medium. When the analyte molecule comes into contact with the recognition molecules on the fiber sensing surface, an adsorption and recognition reaction occurs, causing a change in the refractive index and resulting in plasmon resonance transfer.

[0027] Plasmon resonance absorption on the surface of gold nanorods exhibits resonance absorption in two directions: short-wavelength transverse localized plasmon resonance absorption and long-wavelength longitudinal localized plasmon resonance absorption along the long axis. This absorption phenomenon can be used to tune the plasmon resonance on the fiber surface, causing a greater shift in the plasmon resonance peak than that of folic acid alone.

[0028] During detection, only some folic acid receptors bind to the folic acid molecules being detected, while the remaining vacant portions are further filled by folic acid-modified gold nanorods, thereby inducing a greater shift in the surface plasmon resonance peak.

[0029] The above fiber optic probes were subjected to shape inspection: 1. Use a micro-CCD camera to photograph the D-shaped polished area of ​​the prepared D-shaped fiber probe to obtain an image of the D-shaped fiber. It can be seen that the cladding on one side of the fiber has been polished away, resulting in a D-shaped concavity. Figure 2 (1)).

[0030] 2. Use a CCD camera to photograph the D-shaped fiber optic probe mounted on the microfluidic plate to obtain an outline diagram of the fiber optic folic acid sensing section. The diagram shows that the fiber optic cable is embedded in a pre-fabricated groove, and the sensing and guiding parts are covered and sealed by a thin plastic sheet. Light enters from the left end and is received at the right end. Figure 2 (2)).

[0031] 3. Characterization of gold nanoparticles: A drop of gold nanoparticle solution was placed on a copper grid, and its morphological characteristics were observed using an electron microscope (TEM), with photographs taken and recorded. Additionally, the gold nanoparticle solution was placed in a glass container, and its absorption spectrum was detected using a spectrometer, with a spectral range of 400-1200 nm. The results showed that the nanoparticles were rod-shaped, approximately 50 nm long and 20 nm wide, exhibiting broad absorption spectra with two peaks at 520 nm and 820 nm. Figure 2 (3)).

[0032] 4. Place a fiber optic probe incorporating gold nanoparticles on the sample stage and observe the D-type region using a scanning electron microscope (SEM), taking photographs as needed. The image shows that the gold nanoparticles are adsorbed onto the surface of the gold-coated and molecularly modified fiber. This adhesion will affect surface plasmon resonance. Figure 2 (4)).

[0033] Performance analysis of fiber optic probe for folic acid molecular detection: (1) Spectral data 1 was obtained by measuring the detection pure water of the uncoated fiber optic probe as the background; (2) Plasma resonance spectral data 2 was obtained by detecting the prepared fiber optic probe (modified with folic acid receptor) in pure water; (3) The prepared fiber optic probe (modified with folic acid receptor) was placed in 12.5 ng / mL folic acid solution for 15 min, rinsed with PBS, and the plasma resonance spectral data 3 was obtained; (4) Then the fiber optic probe was incubated with 5 mL (concentration of 0.4 mg / mL) of gold nanoparticles for 10 min, rinsed with PBS, and the plasma resonance spectrum of the fiber optic probe was detected. The results showed that the plasma resonance peak of the fiber optic probe modified with biomolecules shifted to the right, but the shift was small. However, the resonance peak shifted significantly after combining with gold rods, showing the characteristic of amplifying plasma resonance. Figure 3This property lays the technical foundation for using gold nanoparticles to enhance plasmon resonance detection of folic acid.

[0034] Example 2 Detection of surface plasmon resonance at different time points after folic acid binding and gold rod replacement: An optical fiber probe was placed in a 12.5 ng / mL folic acid solution for 1 min, then rinsed with PBS and incubated in a folic acid-gold nanoparticle solution for 10 min. Plasmon resonance (SPR) spectral data were then detected in PBS. The same procedure was repeated for folic acid incubation times of 2 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, and 40 min to obtain spectra. This test determined the optimal folic acid reaction time by observing the shift in SPR spectral wavelengths over different time periods. Results showed that too short an incubation time resulted in insufficient folic acid binding and the amplification effect of the gold nanoparticles, potentially masking the folic acid signal. Conversely, too long an incubation time resulted in excessive folic acid binding and insufficient gold nanoparticles, reducing the amplification effect. Therefore, to ensure signal significance and linearity, an intermediate time of approximately 15-20 min was deemed appropriate. Figure 4 ).

[0035] Example 3: Constructing a Standard Curve The detection performance of the novel folic acid-complementary amplified fiber optic sensing system prepared in Example 1 was calibrated using different concentrations of folic acid: A fabricated optical fiber probe was selected and incubated with folic acid at concentrations of 25, 12.5, 6.25, 3.12, 1.56, and 0.78 ng / mL for 15 min. Afterward, the probe was washed with PBS and gold nanoparticle solution was added (incubation time 10 min). Simultaneously, the plasmon resonance (SPR) spectra were obtained using optical transillumination. Based on the wavelength shift, a corresponding standard curve was obtained, with the equation Y = -50.60479lg(X) + 80.9923, where X is the concentration in ng / mL and Y is the relative peak shift (…). Figure 5 ).

[0036] The results showed that the shift of the plasmonic resonance peak was inversely proportional to the folic acid concentration; the lower the folic acid concentration, the greater the peak shift.

[0037] Example 4: Detection of folic acid concentration S1. Inject the fiber optic sensing probe into the PBS solution through the microfluidic plate, and introduce the excitation light source into the fiber optic sensing probe to generate surface plasmon resonance. Receive the spectrum with a spectrometer and detect the plasmon resonance spectrum to obtain the first plasmon resonance peak. S2. Inject a vegetable folic acid extract (20 ng / mL) of known folic acid concentration into the fiber optic sensing probe through a microfluidic plate and incubate for 15 min, then wash with PBS. S3. Inject the gold nanorod amplification solution into the fiber optic sensing probe through a microfluidic plate and incubate for 10 min to achieve the replacement of the fiber optic folic acid receptor by the gold nanorod. Wash with PBS and detect the plasmon resonance spectrum to obtain the second plasmon resonance peak. S4. Analyze the surface plasmon resonance peaks and compare the measured peak offset of 15 nm with the standard curve. Figure 6 The folic acid concentration of the test solution was calculated to be 20.1 ng / mL, which is within the allowable error range.

[0038] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0039] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0040] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A complementary amplified folic acid fiber optic sensing system, characterized in that, The device includes an excitation source, an optical fiber sensing probe, a microfluidic plate, a gold nanorod amplification solution, and a spectrometer. The excitation source, optical fiber sensing probe, and spectrometer are connected in sequence. The optical fiber sensing probe is embedded in the microfluidic plate. The gold nanorod amplification solution is introduced into the microfluidic plate. The optical fiber sensing probe is a gold-plated optical fiber modified with folic acid receptor protein, and the gold nanorod amplification solution is a folic acid-modified gold nanorod solution.

2. The sensing system according to claim 1, characterized in that, The probe is D-shaped, with a sensing length of 1.5-3 cm and a gold film thickness of 40-60 nm.

3. The sensing system according to claim 1, characterized in that, The concentration of the nano-gold rod amplification solution is 5-8 µmol / L.

4. The sensing system according to claim 1, characterized in that, The excitation light source is a 5W halogen lamp.

5. A method for detecting folic acid concentration using the sensing system according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Inject the fiber optic sensing probe into the buffer solution through the microfluidic plate, and introduce the excitation light source into the fiber optic sensing probe to generate surface plasmon resonance. Receive the spectrum with a spectrometer and detect the plasmon resonance spectrum to obtain the first plasmon resonance peak. S2. Inject the test solution into the fiber optic sensing probe through the microfluidic plate and incubate for 15-20 min, then inject buffer solution to wash. S3. Inject the nano-gold rod amplification solution into the fiber optic sensing probe through a microfluidic plate and incubate for 10-15 min to achieve the replacement of the fiber optic folic acid receptor by the nano-gold rod. Wash with buffer solution and detect the plasmon resonance spectrum to obtain the second plasmon resonance peak. S4. Analyze the surface plasmon resonance peaks, compare the offset of the two measured peaks with the standard curve, and obtain the folic acid concentration of the test solution.

6. The method according to claim 5, characterized in that, In step S4, the method for plotting the standard intensity curve includes: S4.

1. Inject the fiber optic sensing probe into the buffer solution through the microfluidic plate, and introduce the excitation light source into the fiber optic sensing probe to generate surface plasmon resonance. Receive the signal with a spectrometer, detect the plasmon resonance spectrum, and obtain the third plasmon resonance peak. S4.2 Then, folic acid concentrations of 25, 12.5, 6.25, 3.12, 1.56 and 0.78 ng / mL were injected into the fiber optic sensing probe through the microfluidic plate and incubated for 15-20 min, followed by washing with buffer solution. S4.

3. Inject the gold nanorod amplification solution into the fiber optic sensing probe through a microfluidic plate and incubate for 10-15 min. Wash with buffer solution and detect the plasmon resonance spectrum to obtain multiple fourth plasmon resonance peaks. S4.4 Analyze the surface plasmon resonance peaks and plot a standard curve with folic acid concentration as the abscissa and the wavelength shift between the fourth and third plasmon resonance peaks as the ordinate.

7. A method for constructing a sensing system according to any one of claims 1 to 4, characterized in that, include: Step 1: Constructing the fiber optic sensing probe (1.1) Deposit a gold film on the surface of a D-type optical fiber; (1.2) The optical fiber obtained in step (1.1) is incubated in a carboxylated polyethylene glycol mercapto solution for 5-6 hours to obtain a carboxylated optical fiber; (1.3) Immerse the carboxylated optical fiber in a mixture of EDC and NHS for 4-5 hours to activate it; (1.4) The activated optical fiber was incubated in a folic acid receptor protein solution for 10-12 h to construct an optical fiber sensing probe, which was then connected to a microfluidic spectroscopy detection plate for later use. Step 2: Constructing a scale-up solution for gold nanorods (2.1) Mix carboxyl polyethylene glycol thiol solution with gold nanorods and incubate at room temperature for 5-6 h to obtain carboxyl-functionalized gold nanorods; (2.2) The carboxyl-functionalized gold nanorods were activated by soaking in a mixture of EDC and NHS for 4-5 hours; (2.3) The activated gold nanorods were incubated in folic acid solution for 10-12 h to complete the folic acid modification.

8. The construction method according to claim 7, characterized in that, In step (1.2), the concentration of the carboxylated polyethylene glycol mercapto solution is 0.3-0.4 mg / mL; in step (1.3), the concentration of EDC in the mixture is 3-4 mg / mL and the concentration of NHS is 1-2 mg / mL; in step (1.4), the concentration of the folic acid receptor protein solution is 1-2 mg / L.

9. The construction method according to claim 7, characterized in that, In step (2.1), the concentration of the carboxyl polyethylene glycol mercapto solution is 0.3-0.4 mg / mL; in step (2.2), the mass ratio of carboxyl-functionalized gold nanorods, EDC and NHS is 1:(3-4):(2-3).

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