Calcium ion concentration detection method based on hydrogel-coated tilted fiber bragg grating

Abstract

The invention provides a calcium ion concentration detection sensor based on a hydrogel-coated tilted fiber bragg grating and a preparation method of the calcium ion concentration detection sensor. The sensor is composed of a broadband light source, a polarization controller, a TFBG sensor and a spectrum analyzer. The broadband light source is used for providing a light source, the polarization controller is used for obtaining the P polarization state of input light, the hydrogel functionalized TFBG sensor is used for detecting the concentration of a calcium ion solution in the flow cell, the spectrometer is used for monitoring and recording the spectrum change condition, and the solution concentration is obtained according to the transmission spectrum intensity change condition. The device has the advantages of high sensitivity, wide detection range, low detection limit, reusability, electromagnetic interference resistance and small size.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology and relates to a method for detecting trace calcium ion concentration in solution based on surface plasmon resonance of tilted fiber grating. Background Technology

[0002] Calcium ions (Ca 2+ Calcium ions (CIE) are a key element in maintaining normal physiological functions of organisms, participating in important physiological processes such as cell signal transduction, muscle contraction, and nerve conduction. Their concentration changes are closely related to various physiological functions and pathological states. In the field of environmental monitoring, the concentration of calcium ions in water is one of the important indicators for assessing water hardness, and it is of great significance for water treatment, aquaculture, and environmental protection. Furthermore, monitoring calcium ion concentration in industrial processes is indispensable for protecting equipment from corrosion and ensuring product quality.

[0003] Although various methods for calcium ion detection have been developed, such as atomic absorption spectrometry, fluorescence methods, and colorimetric methods, these methods often have limitations such as expensive equipment, complex operation, and difficulty in achieving rapid on-site detection. Therefore, developing a new method for calcium ion detection that is rapid, highly sensitive, low-cost, and easy to apply in the field has significant theoretical and practical value.

[0004] Fiber optic sensing technology is considered an ultrasensitive trace detection technique that detects changes in physical, chemical, or environmental parameters by monitoring changes in characteristic parameters within the optical fiber. In recent years, fiber optic sensing technology has demonstrated enormous application potential in monitoring various parameters such as temperature, stress, and humidity due to its unique advantages, including high sensitivity, rapid response, strong resistance to electromagnetic interference, small size for easy integration, and remote monitoring. In particular, the use of fiber optic sensors for chemical and biological analysis has become a research hotspot in recent years. However, despite the significant advantages of fiber optic sensors, research in the field of calcium ion detection is quite limited. To date, most studies have focused on using fiber optic sensors to collect fluorescence spectra to detect calcium ions, failing to fully utilize the physical properties of the fiber optic sensors themselves. Instead, they are used as carriers for traditional fluorescent probes. While this method is effective, it is often affected by sample background light, making rapid on-site monitoring difficult and failing to fully realize the potential of fiber optic sensing technology.

[0005] Hydrogels, as materials with multiple advantages such as biocompatibility, tunability, and high specific surface area, are commonly used in biomedicine, drug delivery, and biochemical sensing, enabling high sensitivity, high selectivity, and real-time monitoring of calcium ions. This invention combines a three-dimensional network hydrogel with a functional fiber optic sensor, fully leveraging the advantages of both to provide a novel calcium ion detection method characterized by high sensitivity, low detection limit, and specific detection. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a calcium ion sensing and detection system based on surface plasmon resonance of a tilted fiber Bragg grating, which can achieve trace detection of calcium ions in aqueous solutions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a fiber surface plasmon resonance (SPR) sensor for calcium ion detection, comprising a broadband light source, a polarization controller, a tilted fiber Bragg grating sensor, and a spectrometer. The polarization controller is used to obtain the P polarization state of the input light. The TFBG-SPR sensor is fabricated from a photosensitive fiber using a phase mask method, and the fiber surface is functionalized with hydrogel after depositing a gold film. The spectrometer is used to monitor and record spectral changes.

[0008] This invention fabricates a calcium ion sensor based on a tilted fiber Bragg grating (TFBG). By depositing a gold film on the surface of the TFBG grating region, the sensor exhibits highly sensitive surface plasmon resonance (SPR) phenomena to changes in the refractive index of the microenvironment on the gold film surface. The sensor's detection capability is achieved by modifying the gold-plated TFBG with a hydrogel that specifically binds to calcium ions, including the following steps:

[0009] a. Preparation of TFBG;

[0010] b. Deposition of a gold film on the surface;

[0011] c. Preparation and modification of hydrogels;

[0012] d. Assembly and connection of broadband light source, polarization controller, TFBG-SPR sensor and spectrometer.

[0013] The steps for preparing TFBG in step a are as follows: Select a photosensitive fiber that has undergone hydrogen pretreatment, and use the phase mask method to write tilted fiber gratings on the fiber core. After writing, the TFBG is annealed in an oven at 125°C for 12 hours.

[0014] The surface gold film deposition process in step b is as follows: TFBG is immersed in a piranha solution to remove surface impurities, and then ultrasonically cleaned in deionized water. Subsequently, the cleaned TFBG is placed in a vacuum coating machine for gold film magnetron sputtering deposition.

[0015] The hydrogel preparation and modification steps in step c are as follows: An equal volume of polyvinyl alcohol aqueous solution is added to a cellulose nanocrystal suspension, and the mixture is mechanically stirred using a magnetic stirrer to obtain a mixed suspension. Gold-plated TFBG is sequentially washed with deionized water and anhydrous ethanol, and then dried under nitrogen. Subsequently, the dried gold-plated TFBG is contact-modified with the mixed suspension to allow the hydrogel to adhere to the fiber surface, and evaporation-induced self-assembly forms a photonic composite film (CNC / PVA). Afterwards, the fiber is placed in a 100°C oven for dehydration, and the fiber with the CNC / PVA film is immersed in a sodium alginate solution and then removed for static adsorption in a water bath. Finally, the film surface is rinsed with deionized water, and the above adsorption steps are repeated three times to obtain a surface-modified sodium alginate-doped hydrogel (CNC / PVA-SA) functional film, which is then dried under nitrogen.

[0016] The assembly and connection steps of the broadband light source, polarization controller, TFBG-SPR sensor, and spectrometer in step d are as follows: The broadband light source, polarization controller, TFBG-SPR sensor, and spectrometer are sequentially connected via single-mode fiber (SMF). The broadband light source provides the light source, the polarization controller is used to obtain the P polarization state of the input light, the hydrogel-functionalized TFBG-SPR sensor is used to detect the concentration of calcium ions in the flow cell, and the spectrometer is used to monitor and record spectral changes. When the hydrogel film on the TFBG surface comes into contact with calcium ions in the solution, the ionic crosslinking of sodium alginate causes the hydrogel network to shrink, resulting in changes in the hydrogel volume and refractive index, ultimately leading to a shift in wavelength and a change in amplitude in the SPR spectrum. The change in calcium ion concentration is determined by analyzing the changes in the transmission spectrum.

[0017] Preferably, the TFBG has a length of 1 cm and an inclination angle of 10°.

[0018] Preferably, the thickness of the gold film is 50 nm.

[0019] Preferably, the polyvinyl alcohol aqueous solution has a mass fraction of 2 wt%, the cellulose nanocrystal suspension has a mass fraction of 2 wt%, and the sodium alginate solution has a mass fraction of 1 wt%.

[0020] Preferably, the wavelength of the broadband light source is 1480-1610nm.

[0021] The working principle of the trace calcium ion concentration detection sensor based on hydrogel-coated TFBG in this invention is as follows: TFBG is a fiber Bragg grating with a certain angle between its grating plane and the plane perpendicular to the fiber axis. This tilted grating plane breaks the circular symmetry of the fiber cross-section, allowing for strong coupling between the core mode and the cladding mode. Due to the large penetration depth of the evanescent field of the excited higher-order cladding modes, TFBG exhibits high sensitivity to changes in the surrounding refractive index, making it well-suited for detecting calcium ions. 2+ Concentration sensing. Based on the phase-matching condition, the resonant wavelengths of each mode in TFBG are expressed as: λ core =2n eff，co ∧ / cosθ#(1) λ clad，i =(n eff，cl +n eff，co )∧ / cosθ#(2) In the formula λ core and λ clad，i Let n be the wavelength of the core mode and the i-th cladding mode, respectively. eff，cl and n eff，co ∧ represents the effective refractive index of the i-th cladding mode and the core mode, respectively. ∧ is the interference period, and θ is the angle between the grating and the fiber axis. The effective refractive index n of the cladding mode. eff，cl Influenced by the refractive index of the surrounding medium, the resonance intensity of the cladding mode during TFBG propagation is expressed as: T = 1 - tanh 2 (κL)#(3) T represents the transmission amplitude of the cladding mode, κ is the coupling coefficient between the cladding mode and the core mode, and L is the length of the TFBG. Typically, a metal film is deposited on the TFBG surface to excite surface plasmon resonance (SPR) effects. Under the influence of light of a certain frequency, free electrons on the metal surface will generate collective oscillations; these collective oscillations of electrons bound to the metal surface are called surface plasmon waves (SPWs). According to the principle of total internal reflection, when the incident light wave undergoes total internal reflection at the interface, an evanescent wave is generated. The evanescent wave excites surface plasmon waves (SPWs) on the metal surface. When the two satisfy certain phase matching conditions, resonance occurs, and the incident light energy at the resonance angle or wavelength is absorbed by the surface plasmon. The energy of the reflected beam generated by total internal reflection drops sharply, forming a resonance peak in the reflection spectrum. In this invention, when calcium ions are present in the flow cell solution, the sodium ions in the gulonic acid (G unit) of sodium alginate in the TFBG surface hydrogel are replaced by calcium ions. Under the action of calcium ions, the G units undergo ionic cross-linking, making the bonds between different sodium alginate chains tighter and increasing the cross-linking density. This causes the entire hydrogel network to shrink, resulting in an increase in its effective refractive index. Due to the evanescent field effect of the cladding mode, as shown in equations (1), (2), and (3), when the refractive index of the surrounding medium changes, the core mode is unaffected, while the wavelengths and transmission intensities corresponding to each cladding mode change. Calcium ion detection can be achieved by analyzing the spectral drift in solutions with different concentrations of calcium ions.

[0022] Compared with the prior art, the present invention has the following substantial features and significant advantages:

[0023] 1. This invention uses an insoluble hydrogel as the sensitive layer of the sensor, cellulose nanocrystals as the natural framework for the experimental hydrogel, and polyvinyl alcohol as a soft filler to maintain the swelling capacity of the hydrogel. The combination of polyvinyl alcohol and cellulose nanocrystals via hydrogen bonding has been shown to form a stable hydrogel in an aqueous environment, exhibiting dynamic switching capabilities between dry and wet states, and achieving specific functionalization through swelling absorption. Sodium alginate is added to this hydrogel network as an adsorption probe for calcium ions, thereby specifically recognizing calcium ions in aqueous solution. Compared with other sensing materials, the hydrogel is reusable, effectively reducing detection costs.

[0024] 2. This invention utilizes tilted fiber grating surface plasmon resonance (TFBG-SPR) technology, combining the high-sensitivity optical properties of TFBG with the surface sensitivity of SPR, providing a high-performance platform for calcium ion sensing. This platform features a wide concentration detection range, high sensitivity, low detection limit, and rapid response. Furthermore, the TFBG structure itself is sensitive to temperature changes, allowing for compensation to mitigate these variations, thus ensuring that the accuracy and reliability of calcium ion concentration measurement are unaffected by temperature fluctuations. Attached Figure Description

[0025] Figure 1 A schematic diagram of a calcium ion concentration detection sensing system based on a hydrogel-coated tilted fiber grating.

[0026] Figure 2 Transmission spectra of the sensor in calcium ion solutions of different concentrations and fitting curves of resonance peak changes in different SPR modes.

[0027] Figure 3 Comparison of sensor-specific response intensity.

[0028] Figure 4 Sensor response time diagram.

[0029] Figure 5 Sensor repeatability response diagram. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to specific embodiments:

[0031] Example 1: Construction of the Sensing Platform

[0032] In this embodiment, see Figure 1 A TFBG sensor for calcium ion detection includes a broadband light source 1, a polarization controller 2, a TFBG sensor 3, and a spectrometer 4. The broadband light source 1 has a wavelength range of 1480-1610 nm. The TFBG sensor 3 is fixed above the flow cell using an optical fiber clamp. The experimental instruments also include a cleaver and a fusion splicer. A single-mode fiber (SMF) is used to sequentially connect the broadband light source 1, the polarization controller 2, the TFBG sensor 3, and the spectrometer 4. The broadband light source 1 provides the light source, the polarization controller 2 obtains the p-polarization state of the input light, and the hydrogel-functionalized TFBG sensor 3 detects the concentration of calcium ions in the flow cell, specifically by changes in spectral wavelength and amplitude. The spectrometer 3 monitors and records these spectral changes.

[0033] Example 2: The specific steps for calcium ion detection based on hydrogel-coated tilted fiber optic gratings are as follows:

[0034] (a) The coating layer of the photosensitive fiber was stripped off, and the fiber was cleaned sequentially with deionized water and anhydrous ethanol. It was then subjected to a hydrogen loading process for 14 days at 15.2 MPa pressure and 20°C. A 10 mm long phase mask with a period of 541 nm was used to write tilted fiber gratings (TFBGs) onto the fiber core. The TFBGs were 1 cm long, tilted at 10°, and had a Bragg wavelength of approximately 1601 nm. After writing, the TFBGs were annealed in a 125°C oven for 12 hours.

[0035] (b) The TFBG was immersed in a piranha solution (deionized water, ammonium hydroxide, and hydrogen peroxide in a ratio of 8:1:1) for 30 minutes to remove surface impurities, and then ultrasonically cleaned in deionized water. The cleaned TFBG was then placed in a magnetron sputtering deposition chamber, the sputtering current was set to 0.2A and sputtered for 100s to obtain a gold film with a thickness of approximately 50nm.

[0036] (c) 5 ml of polyvinyl alcohol aqueous solution (2 wt%) was added to 5 ml of cellulose nanocrystal suspension (2 wt%). The mixed suspension was mechanically stirred at room temperature for 1 h and then allowed to stand. The gold-plated TFBG was washed sequentially with deionized water and anhydrous ethanol and dried under nitrogen. Subsequently, the gold-plated TFBG was contacted with the mixed suspension for 1 h to allow the hydrogel to adhere to the fiber surface, and evaporation-induced self-assembly was carried out to form a photonic composite film (CNC / PVA). After the fiber was placed in a 100°C oven for 1 h to dehydrate, the fiber with the CNC / PVA film was immersed in 10 ml of sodium alginate solution (1 wt%) and then placed in a 30°C water bath for static adsorption for 30 min. Finally, the film surface was rinsed with deionized water, and the above adsorption steps were repeated 3 times to obtain a surface-modified sodium alginate-doped hydrogel (CNC / PVA-SA) functional film, which was then dried under nitrogen.

[0037] (d) Connect the modified TFBG to the sensing platform and record the raw spectrum of the TFBG-SPR sensor in deionized water. Add the calcium ion solution to be tested to the flow cell and let it stand for 10 min, then acquire the spectral signal. Subsequently, the sensor probe is rinsed with EDTA elution buffer and then repeatedly rinsed with deionized water for reuse. The concentration gradient is set to 0.1 fM, 1 fM, 0.01 pM, 0.1 pM, 1 pM, 0.01 nM, 0.1 nM, 1 nM, 0.01 uM, 0.1 uM, 1 uM. Different concentrations of calcium ion solution are added sequentially from low to high, and the above steps are repeated.

[0038] (e) Using a 0.01 pM calcium ion solution, spectra were acquired from 0 to 60 min to evaluate the time response of the TFBG-SPR sensor.

[0039] (f) Using a 0.01 pM calcium ion solution, after acquiring spectral signals from 0 to 60 min, the functional membrane was rinsed with EDTA and repeatedly rinsed with deionized water. This process was repeated three times. The standard deviation of the spectral response was calculated to evaluate the repeatability of the hydrogel-coated TFBG-SPR sensor.

[0040] (g) Deionized water and 0.01 nM Na were added to the flow cell respectively. + ,Tb 3+ Sn 2+ Co 2+ ,K+ The solution was allowed to stand for 10 minutes before collecting spectra. After testing with different metal ion analyte solutions, the hydrogel functional membrane was rinsed with EDTA solution and repeatedly rinsed with deionized water. The specificity of the hydrogel-coated TFBG-SPR sensor for different ions was evaluated.

[0041] like Figure 2 As shown, for calcium ion concentrations of 0.1 fM, 1 fM, 0.01 pM, 0.1 pM, 1 pM, 0.01 nM, 0.1 nM, 1 nM, 0.01 μM, 0.1 μM, and 1 μM, the intensities of SPR modes 1 and 2 decreased with increasing concentration gradient, while the intensities of SPR modes 3, 4, and 5 increased, exhibiting a good linear relationship. The sensitivity of SPR mode 1 reached 0.39 dB / log(M).

[0042] like Figure 3 As shown, the TFBG-SPR sensor exhibits the strongest selectivity for calcium ions.

[0043] like Figure 4 As shown, the TFBG-SPR sensor responds rapidly within 1 minute, then shows a gradual upward trend, and reaches stability after 10 minutes.

[0044] like Figure 5 As shown, the TFBG-SPR sensor exhibits good repeatability in three repeated uses.

[0045] The above embodiments have provided a detailed description of the technical solution of the present invention. However, 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 scope of protection of the present invention.

Claims

1. A method for detecting calcium ion concentration based on hydrogel-coated TFBG, comprising a broadband light source (1), a single-mode optical fiber (2), a polarization controller (3), a TFBG sensor (4), a flow cell (5), and a spectrometer (6), characterized in that: A broadband light source (1) is connected to the left end of a polarization controller (3) via a single-mode optical fiber (2). The polarization controller (3) is connected to a TFBG sensor (4). The right end of the TFBG sensor (4) is connected to a spectrometer (6). The TFBG sensor (4) is fixed in the center of a flow cell (5). A calcium ion solution is introduced from the flow cell (5) to contact the TFBG sensor (4). The TFBG sensor (4) is made of a photosensitive optical fiber. A tilted fiber grating with a tilt angle of 10° is etched in the fiber core, and the etched length is 1 cm. The coating layer on the surface of the TFBG sensor (4) has been stripped off, and the exposed part is the sensing area, including the fiber core (41), cladding (42), gold film (43), and hydrogel coating layer (44). The specific steps of this method are as follows: Step 1: First, the photosensitive fiber is pretreated with hydrogen for 14 days. Then, the core (41) of the photosensitive fiber is etched with TFBG using the phase mask method and annealed. Subsequently, the surface of the photosensitive fiber is ultrasonically cleaned. Finally, a gold film (43) is deposited by magnetron sputtering. The thickness of the gold film ranges from 50 nm to 70 nm. Step 2: Immerse the gold-plated TFBG from Step 1 in a mixed suspension of polyvinyl alcohol and cellulose nanocrystals for 1 hour to allow the suspension to adhere to the surface of the optical fiber and form a photonic composite film through evaporation-induced self-assembly; after dehydrating the TFBG surface film, immerse it in a sodium alginate solution and remove it, perform static adsorption in a water bath, repeat Step 2 three times to obtain a sodium alginate-doped hydrogel coating layer (44). The polarization controller (3) is used to obtain the P polarization state of the input light, and the spectrometer analyzer (6) is used to monitor and record the spectral changes. Deionized water and the calcium ion solution to be tested are added to the flow cell (5), and the solution is left to stand for 10 minutes. Spectral signals are collected respectively. After the TFBG sensor (4) is immersed in the solution, the hydrogel coating layer (44) interacts with the calcium ion solution, the refractive index of the surrounding medium changes, the effective refractive index of the cladding mode changes, and the wavelength and transmission intensity corresponding to each cladding mode change. The calcium ion concentration is detected by analyzing the linear change of the transmitted light intensity in calcium ion solutions of different concentrations.

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