Sodium ion concentration detection method based on optical fiber

Abstract

The invention discloses an optical fiber-based sodium ion concentration detection method. A used device comprises a broadband light source, a single-mode optical fiber, a polarization controller, a flow cell, a TFBG-SPR sensor and a spectrum analyzer. Firstly, the broadband light source emits light with the wavelength range of 1420-1620 nm, the polarization state of the light is adjusted to a P state through the polarization controller, then the light is input into the TFBG / PVC / NPOE / Sodium ion III / MB ion selective membrane, and finally a spectrum is displayed on the spectrum analyzer. The Sodium ion phore III on the surface of the TFBG can be combined with sodium ion molecules to cause the change of the refractive index, the change is shown as the drifting of a transmission spectrum on a spectrum analyzer, and the rapid, accurate and trace detection of the sodium ion concentration is realized by comparing the relationship between the concentrations of different sodium ion solutions and the drifting of the transmission spectrum. The invention provides a novel method which is simple in structure, high in accuracy and quick in response for sodium ion concentration measurement, and has great application potential.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and specifically to a method for detecting sodium ion concentration using an ion-selective membrane based on gold-plated TFBG combined with TFBG / PVC / NPOE / Sodium ionophore III / MB. Background Technology

[0002] Sodium ions are the most abundant cation in extracellular fluid, maintaining osmotic pressure balance inside and outside cells and regulating cell volume and morphology. In medical testing, the measurement of plasma sodium ion concentration is an important clinical diagnostic indicator used to detect electrolyte imbalances (such as hyponatremia and hypernatremia). Changes in sodium ion concentration are also associated with various diseases, such as kidney disease, heart disease, and endocrine disorders.

[0003] Sodium ion concentration is a crucial parameter in water quality analysis, used to assess the salinity and pollution levels of water bodies. Detection of sodium ions helps monitor the ion balance in groundwater, seawater, and industrial wastewater. The sodium ion content in soil affects its physical and chemical properties, such as water retention and aeration. Excessive accumulation of sodium ions can lead to soil salinization, negatively impacting crop growth.

[0004] Currently, various sodium ion detection technologies have been developed, including fluorescence sensors, absorption spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), electrochemical sensors, atomic absorption spectroscopy (AAS), and ion chromatography (IC). However, these methods require sophisticated and expensive instruments, involve complex sample pretreatment processes, and the equipment is primarily laboratory-grade. Their bulkiness, portability, and inability to be used for on-site testing significantly limit their widespread adoption.

[0005] In recent years, ultra-low lower limit fiber surface plasmon resonance sensors have shown great application potential in many fields such as biomedicine and chemical detection due to their advantages such as strong anti-electromagnetic interference capability, low cost, high sensitivity and corrosion resistance.

[0006] A tilted fiber Bragg grating (TFBG) is a special type of short-period fiber grating that is tilted at a certain angle relative to the fiber axis. By depositing gold on the surface of the TFBG, specific cladding modes can be converted into SPR modes. These modes are extremely sensitive to local refractive index changes around the gold-coated TFBG surface; any minute physicochemical reaction can lead to wavelength shifts and amplitude changes. Meanwhile, the core mode Bragg resonance of the TFBG is unaffected by changes in the surrounding refractive index, which helps to eliminate interference from ambient temperature variations and fluctuations in light source power. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this study developed an ultra-low lower limit fiber optic sodium ion detection sensor based on tilted fiber Bragg grating plasmon resonance (TFBG-SPR) technology. This sensor's detection capability is comparable to that of sodium ions... + The ion-selective film modification obtained by combining the elements onto the TFBG surface includes the following steps: S1, Preparation of gold-plated TFBG; S2. Preparation and modification of ion-selective membranes of TFBG / PVC / NPOE / Sodium ionophore III / MB that can bind to sodium ions; S3. Connection of broadband light source (1), polarization controller (2), TFBG-SPR sensor (5), and spectrometer (6); S4. Detection of sodium ion concentration and specificity.

[0008] In step S1, after loading hydrogen for 24 days, TFBG was etched into the fiber core using a phase mask method with ultraviolet light, with a length set to 1 cm. The tilt angle between the phase mask (period of 541 nm) and the fiber was set to 8 degrees, and then the fiber was annealed in a 125°C temperature chamber for 12 hours. Finally, a 50 nm thick layer of gold was deposited on the TFBG in a magnetron sputtering deposition chamber.

[0009] In step S2, an ion-selective membrane is spin-coated onto the gold-plated TFBG surface for modification, and hydrochloric acid and Tris-HCl buffer are used to activate and stabilize the ion-selective membrane.

[0010] In step S2, five reagents—polyvinyl chloride (PVC), 2-nitrophenyl octyl ether (NPOE), methylene blue (MB), sodium ionophore III, and tetrakis(3,5-di(trifluoromethyl)phenyl)boronic acid (TFPB)—are added to 99% tetrahydrofuran (THF) and stirred until completely dissolved to obtain a homogeneous solution (THF is a volatile organic compound, so this step requires dissolution under sealed conditions). Then, the gold film surface is rinsed with deionized water to remove impurities from the gold-plated sensor surface; this step needs to be repeated 2-3 times. The solution is then evenly spin-coated onto the gold film surface, and the organic solvent THF is allowed to evaporate and solidify at room temperature, leaving a complete ion-selective membrane. Next, TFBG is immersed in a hydrochloric acid solution at pH 2 for 10 minutes to activate the ion carriers in the ion-selective membrane. Finally, the fiber optic sensor is immersed in a Tris-HCl buffer solution at pH 7 for 5 hours to stabilize the optical and chemical properties of the hydrochloric acid-activated ion-selective membrane.

[0011] In step S3, the broadband light source (1), polarization controller (3), TFBG-SPR sensor (5), and spectrometer (6) are connected as follows: the broadband light source (1) is connected to the left end of the polarization controller (3) via a single-mode fiber (2); the polarization controller (3) is connected to the TFBG-SPR sensor (5); the TFBG-SPR sensor (5) is fixed in the flow cell (4); and its right end is connected to the spectrometer (6). The broadband light source (1) is used to provide the light source, the polarization controller (3) is used to obtain a greater stripe contrast, and the flow cell (4) is used to add the Na to be measured. + The solution is used to monitor and record the spectral changes. When the ion-selective membrane modified on the surface of TFBG-SPR (5) binds sodium ions, the refractive index changes, which is reflected in the drift of the transmission spectrum on the spectrometer (6). By comparing the relationship between different sodium ion concentrations and the transmission spectrum drift, the concentration of sodium ions can be detected.

[0012] Preferably, the wavelength of the broadband light source (1) is 1420-1620 nm.

[0013] The sodium ion concentration detected in step S4 is modulated to: 10 -16 M, 10 -15 M, 10 -14 M, 10 -13 M, 10 -12 M, 10 - 11 M, 10 -10 M, 10 -9 M, 10 -8 M, 10 -7 M, 10 -6 M, 10 -5 M, 10 -4 M.

[0014] The cation included in the specific detection in step S4 is: Na. + Cu 2+ K + Li + Ca 2+ .

[0015] The working principle of the sodium ion concentration detection sensor based on gold-plated TFBG prepared in this invention is as follows: When light waves enter the optical fiber and are completely reflected by the fiber sidewalls, the TFBG can also excite evanescent surface plasmon resonance waves on the gold surface. Due to the presence of a gold film on the sensor surface, when the p-polarized component of light enters the gold film, the free electrons of the gold film interact with the p-polarized component of the light, generating plasma. At the interface between the optical fiber and the gold film, plasma vibration forms surface plasmon waves. When the horizontal component of the evanescent wave vector matches the wave vector of the surface plasmon wave and energy transfer occurs, the evanescent wave and the surface plasmon wave resonate, generating surface plasmon resonance. Light energy near the resonance wavelength is absorbed, resulting in a resonance valley in the output spectrum. When the sodium ion concentration changes, the local refractive index of the sensitive film on the TFBG surface changes, and the resonance valley and resonance wavelength also change. Therefore, the sodium ion concentration can be inferred by monitoring the shift in the resonance wavelength.

[0016] The core mechanism by which the ion-selective membrane responds to changes in sodium ion concentration in this invention is the ion-pair / ion exchange principle. This principle is based on the interaction between the ionophore and the target ion (such as sodium ions), and the influence of this interaction on the degree of dye deprotonation in the ion-selective membrane.

[0017] An ionophore is an organic molecule that can selectively bind to specific ions.

[0018] In this invention, the ion support used is Sodium ionophore III, which exhibits high selectivity for sodium ions. Sodium ions can combine with Sodium ionophore III to form ion pairs, at which point hydrogen ions (H+) in the ion-selective membrane... + The sodium ions will be exchanged. If a pH-sensitive cationic dye, such as methylene blue, is added to the ion-selective membrane, its optical properties (such as absorption spectra) will change with the degree of deprotonation. When Na... + When bound to Sodium ionophore III, H in the ion-selective membrane + Decreasing concentration leads to increased deprotonation of the dye. This not only alters the absorption spectrum of the cationic dye but also changes the refractive index of the entire ion-selective membrane, resulting in a change in the detection spectrum.

[0019] The ion-pair / ion-exchange principle of this invention can be described by the following chemical equation:

[0020]

[0021]

[0022]

[0023] S O Indicates the form of the ionophore in the organic phase, i W + Indicates the form of the target ion (e.g., sodium ion) in the aqueous phase, Si O + H represents the form of the ion pair formed between the ion support and the target ion in the organic phase. W + D represents hydrogen ions in the aqueous phase. O DH represents the deprotonated form of a cationic dye in an organic phase. O + R represents the protonated form of a cationic dye. O - HR indicates the form of anionic additives in the organic phase. O - This indicates the protonated form of the anionic additive.

[0024] Ion pair / ion exchange ion exchange equilibrium constant K exch for:

[0025]

[0026] Equilibrium constant K exch It plays a crucial role in ion-selective optical sensors. It not only describes the equilibrium state of ion exchange reactions but also calculates the concentration of target ions, directly affecting the sensor's sensitivity and selectivity. Through the rational design and selection of ion carriers, Kconcentration can be optimized. exch The value of is increased to increase the refractive index of the material, thereby improving the performance of the sensor.

[0027] The target ion i of this invention + Yes + The neutral ion carrier S is Sodium ionophore III, the deprotonated cationic dye D is methylene blue (MB), and the lipophilic anionic additive R is tetrakis(3,5-bis(trifluoromethyl)phenyl)boronic acid (TFPB).

[0028] In summary, this invention provides a TFBG-SPR sensor capable of detecting ultra-low lower limit sodium ion concentration. This sensor is based on the fact that Sodium ionophore III can bind with sodium ions and change the effective refractive index, causing a shift in the transmission spectrum, thereby enabling the detection of sodium ions. It has the advantages of simple structure, low detection limit, and fast response time.

[0029] The beneficial effects of this invention are as follows: First, a thin gold film is deposited on the surface of TFBG to excite the SPR effect. Then, an ion-selective membrane composed of TFBG / PVC / NPOE / Sodium ionophore III / MB is used as the sensing material for sodium ions. The ion-selective membrane significantly enhances the SPR signal of TFBG, while the binding of sodium ions with Sodium ionophore III causes a change in the refractive index of the sensitive membrane on the surface of the TFBG-SPR sensor. The detection limit (LOD) of this sensor reaches 2.63 × 10⁻⁶. -14 M, dynamic range coverage 10 -16 Up to 10 -4 M represents a performance improvement of at least seven orders of magnitude compared to existing sodium ion concentration sensors. Attached Figure Description

[0030] Figure 1 A schematic diagram of an experimental setup for sodium ion sensing TFBG-SPR spectroscopy measurement.

[0031] Figure 2 Transmission spectra of the TFBG-SPR sensor in sodium ion solutions of different concentrations and fitting curves of transmission peak intensity changes.

[0032] Figure 3 Specific response diagram of the TFBG-SPR sensor for sodium ion detection. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to specific embodiments:

[0034] Example 1: Preparation of gold-plated TFBG.

[0035] In this embodiment, see Figure 1 First, the photosensitive optical fiber (PS1250 / 1500, FIBERCORE) underwent hydrogen loading treatment, a process that lasted for 14 days at a pressure of 15.2 MPa and a temperature of 20 degrees Celsius, to enhance the fiber's photosensitivity. Subsequently, a TFBG (transient fiber optic membrane) was fabricated in the fiber core using UV etching via a phase mask method, with a length set to 1 cm. The tilt angle between the phase mask (period of 541 nm) and the fiber was set to 8 degrees. After etching, the TFBG was annealed at 125 degrees Celsius for 12 hours. Then, the TFBG was immersed in a piranha solution (a mixture of deionized water, hydrogen peroxide, and ammonium hydroxide in an 8:1:1 ratio) for 30 minutes to remove surface contaminants. Finally, it was placed in a magnetron sputtering deposition chamber (PCXT350, PENGCHENG) for gold film deposition.

[0036] Example 2: Preparation and modification of an ion-selective membrane of TFBG / PVC / NPOE / Sodium ionophore III / MB that can bind to sodium ions.

[0037] In this embodiment, the preparation and modification of the ion-selective membrane of TFBG / PVC / NPOE / Sodium ionophore III / MB, which can bind to sodium ions, are carried out through the following steps: a) Add five reagents—polyvinyl chloride (PVC), 2-nitrophenyl octyl ether (NPOE), methylene blue (MB), sodium ionophore III, and tetrakis(3,5-bis(trifluoromethyl)phenyl)boronic acid (TFPB)—to 99% tetrahydrofuran (THF), stir and shake until completely dissolved to obtain a homogeneous solution (THF is an organic compound that is volatile, so this step must be performed under sealed conditions); b) Rinse the gold film surface with deionized water to remove impurities from the surface of the sensor after gold plating. This step needs to be repeated 2-3 times. c) Spin-coat the solution evenly onto the surface of the gold film, and wait for the organic solvent THF to evaporate and solidify at room temperature, leaving a complete ion-selective film; d) Immerse TFBG in hydrochloric acid solution with pH=2 for 10 minutes to activate the ion carrier in the ion-selective membrane; e) Immerse the fiber optic sensor in a Tris-HCl buffer solution at pH 7 for 5 hours to stabilize the optical and chemical properties of the ion-selective membrane activated by hydrochloric acid.

[0038] Example 3: Construction of the detection platform.

[0039] In this embodiment, see Figure 1 The detection platform is composed of a single-mode optical fiber (2) connecting a broadband light source (1), a polarization controller (3), a TFBG-SPR sensor (5) placed in a flow cell (4), and a spectrometer (6). The wavelength range of the broadband light source (1) is 1420-1620 nm, which is used to provide light source. The polarization controller (3) is used to obtain greater stripe contrast. The flow cell (4) is used to add the sodium ion solution to be tested. The spectrometer (6) is used to monitor and record the spectral changes. When the TFBG / PVC / NPOE / Sodium ionophore III / MB modified on the gold-plated TFBG surface binds sodium ions, the refractive index changes, which is reflected in the drift of the transmission spectrum on the spectrometer (6). By comparing the relationship between different sodium ion concentrations and the transmission spectrum drift, the concentration of sodium ions can be detected.

[0040] Example 4: Detection of sodium ion concentration and specificity.

[0041] Sodium ion solutions were used for specific detection, and all sodium ion solutions were prepared with Tris-HCl buffer. The specific steps were as follows:

[0042] Sodium ion concentration detection: The TFBG-SPR sensor modified with TFBG / PVC / NPOE / Sodium ionophore III / MB was rinsed multiple times with deionized water and Tris-HCl buffer, and placed in a flow cell (4). Sodium ion solutions of different concentrations were prepared using Tris-HCl buffer, and the concentration was adjusted to 10. -16 M, 10 -15 M, 10 -14 M, 10 -13 M, 10 -12 M, 10 -11 M, 10 -10 M, 10 -9 M, 10 -8 M, 10 -7 M, 10 -6 M, 10 -5 M, 10 -4 M. Tris-HCl buffer was added to the flow cell (4), and the initial spectrum was recorded using a spectrometer (6). The solution in the flow cell (4) was rinsed with deionized water, and then sodium ion solution was added to the flow cell (4) and allowed to stand. The transmission spectrum was then recorded using a spectrometer (6). Sodium ion solutions of different concentrations were added sequentially, and the above steps were repeated to obtain the transmission spectra of sodium ion solutions of different concentrations. Fitting curves were then plotted, as shown in the figure. Figure 2 As shown.

[0043] Specific detection: 10 were prepared respectively. -10 The test solutions were NaCl, CuCl2, KCl, LiCl, and CaCl2. First, the transmission spectrum of the TFBG-SPR sensor in Tris-HCl buffer was measured. The optical fiber was rinsed multiple times with deionized water and Tris-HCl buffer. The test solutions were added to the flow cell and allowed to stand. The transmission spectra were then recorded using a spectrometer (6). The sensor was then rinsed multiple times with Tris-HCl buffer, and different test solutions were added sequentially. The above steps were repeated to evaluate the specificity of the TFBG-SPR sensor for different ions, such as… Figure 3 As shown.

[0044] The embodiments described above provide a detailed explanation of the technical solution of the present invention, but the scope of protection of this disclosure is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of the present invention.

Claims

1. A method for detecting sodium ion concentration based on optical fiber, characterized in that: The detection device consists of a broadband light source (1), a single-mode fiber (2), a polarization controller (3), a flow cell (4), a TFBG-SPR sensor (5), and a spectrometer (6); the broadband light source (1) is connected to the input end of the polarization controller (3) through the single-mode fiber (2); the output end of the polarization controller (3) is connected to the left end of the TFBG-SPR sensor (5), and the right end of the TFBG-SPR sensor (5) is connected to the spectrometer (6); The fabrication steps of the TFBG-SPR sensor (5) are as follows: First, hydrogen is loaded onto the photosensitive fiber (41) and then a tilted fiber grating (42) with a tilt angle of 8 degrees is formed by ultraviolet etching and annealing. Then, the tilted fiber grating (42) is placed in a magnetron sputtering deposition chamber and a gold film with a thickness range of 50 nm to 150 nm is deposited on its surface. Finally, TFBG / PVC / NPOE / Sodium ionophore III / MB is assembled on the gold film by surface self-assembly.

2. The detection steps of the fiber optic sodium ion concentration detection method are as follows: First, fix the TFBG-SPR sensor (5) in the flow cell (4), then add the sodium ion test solution to the flow cell (4). The sodium ions combine with the sodium ion selective membrane on the surface of the TFBG-SPR sensor (5) to achieve ion pair exchange. The light emitted by the broadband light source (1) passes through the TFBG-SPR sensor (5) and is recorded on the spectrometer (6) as a transmission spectrum. The data recorded on the spectrometer (6) for adding sodium ion solutions of different concentrations to the flow cell (4) are fitted to obtain the relationship between different sodium ion concentrations and transmission spectrum drift, thereby realizing the detection of sodium ion concentration. The steps for preparing and modifying the ion selective membrane are as follows: a) Add five reagents—polyvinyl chloride (PVC), 2-nitrophenyl octyl ether (NPOE), methylene blue (MB), sodium ionophore III, and tetrakis(3,5-bis(trifluoromethyl)phenyl)boronic acid (TFPB)—to 99% tetrahydrofuran (THF), stir and shake until completely dissolved to obtain a homogeneous solution (THF is an organic compound that is volatile, so this step must be performed under sealed conditions); b) Rinse the gold film surface with deionized water to remove impurities from the surface of the sensor after gold plating. This step needs to be repeated 2-3 times. c) Spin-coat the solution evenly onto the surface of the gold film, and wait for the organic solvent THF to evaporate and solidify at room temperature, leaving a complete ion-selective film; d) Immerse TFBG in hydrochloric acid solution with pH=2 for 10 minutes to activate the ion carrier in the ion-selective membrane; The optical fiber sensor was immersed in Tris-HCl buffer solution at pH 7 for 5 hours to stabilize the optical and chemical properties of the ion-selective membrane activated by hydrochloric acid.

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