Fluorescent optical fiber sensor suitable for monitoring strongly alkaline pH and preparation method thereof

Abstract

The invention belongs to the field of fluorescence monitoring, and relates to a fluorescent optical fiber sensor suitable for monitoring strongly alkaline pH and a preparation method thereof. According to the method, resorcinol is used as a carbon source, ammonia water is used as a nitrogen source, and the nitrogen-doped carbon quantum dots are prepared through a one-step hydrothermal method. Attaching the nitrogen-doped carbon quantum dots to mesoporous silica nanoparticles, and coating the mesoporous silica nanoparticles with sodium alginate to prepare alginic acid / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles; when a cellulose acetate membrane solution is prepared, the alginic acid / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles are dispersed in the cellulose acetate membrane solution; and forming a thin film on the end part of the optical fiber with the protective layer removed by adopting a dip coating method to prepare the fluorescent optical fiber sensor suitable for monitoring the strongly alkaline pH. The fluorescence intensity and the pH value of the fluorescent optical fiber sensor provided by the invention are in a linear relationship in a range of 9-13, and accurate measurement of strongly alkaline pH can be realized.

Description

Technical Field

[0001] The invention belongs to the field of fluorescence monitoring and relates to a fluorescence optical fiber sensor suitable for monitoring strongly alkaline pH and a preparation method thereof. Background Art

[0002] Common methods for measuring pH include pH indicators, pH test paper, glass electrode pH meters, and pH fluorescence sensors. pH indicators and pH test paper utilize organic substances that display different colors in different acidic and alkaline solutions, allowing the color change to be used to determine the pH range. While these two methods are simple to use, their accuracy is low, affected by the naked eye's ability to discern color changes, and they are not reusable. A glass electrode pH meter uses a saturated calomel electrode as a reference electrode and a glass electrode as an indicator electrode. Together, they form a working cell with the water sample being measured. The pH meter then measures the working electromotive force, allowing the pH value to be read directly. Glass electrode pH meters offer advantages such as accurate and rapid measurement, and minimal interference from factors such as water color, turbidity, oxidants, reducing agents, and salinity. However, they also suffer from drawbacks such as susceptibility to electromagnetic interference, signal drift over time, and difficulty in miniaturization, making them difficult to implement for long-term online monitoring and analysis. In contrast, pH fluorescence sensors exploit the phenomenon of fluorescence quenching and enhancement in solutions of varying acidity and alkalinity, detecting pH based on the relationship between changes in fluorescence intensity and pH. pH fluorescence sensors offer advantages such as reusability, compact size, neutrality, strong resistance to electromagnetic interference, and a pollution-free design, making them ideal for long-term online monitoring and analysis.

[0003] Over the past decade, research on pH fluorescent probes has made significant progress, with their applications highly differentiated by their sensitivity range. Probes targeting the weakly acidic range (pH 4.00-6.00) are primarily used in cell biology research, particularly for real-time monitoring of lysosomal function (involving autophagy, disease mechanisms, and drug delivery) and endocytic pathway dynamics. They also play a role in plant vacuolar physiology (response to nutrient stress), acidity testing of specific foods (such as fermented products), and assessment of skin microenvironment health (linking the "acid coat" to disease). Probes focusing on the neutral physiological range (pH 6.80-7.40) are core tools in life sciences and medical diagnostics. They are primarily used for precise monitoring of intracellular pH homeostasis (regulating metabolism, proliferation, apoptosis, and signal transduction), analyzing the characteristic acidification of the tumor microenvironment (associated with development, metastasis, and treatment), assessing tissue ischemia / inflammatory states, and have applications in cerebrospinal fluid / brain tissue pH research (associated with neurological diseases), immediate pH monitoring of blood and body fluids, and optimizing the control of microbial fermentation processes. In contrast, research on fluorescent probes suitable for use in strongly alkaline environments (pH > 9.00) is relatively scarce. However, these probes are in urgent demand and hold great promise in areas such as demanding industrial processes (such as real-time monitoring and corrosion prevention in chlor-alkali production, papermaking, and metal processing), environmental monitoring (industrial alkali pollution early warning, landfill leachate and saline-alkali soil analysis), specific laboratory safety and quality control (strong alkali leak early warning, in vitro diagnostic reagent validation), and building materials science (concrete carbonation process and durability assessment). Therefore, while furthering the performance of weakly acidic and neutral probes, vigorously developing high-performance strongly alkaline pH fluorescent probes and their associated fiber optic sensors is of great scientific significance and application value, filling technological gaps and meeting the needs of cross-disciplinary precision monitoring. Summary of the Invention

[0004] In response to the above technical problems, the present invention provides a fluorescent optical fiber sensor suitable for monitoring strongly alkaline pH and a preparation method thereof.

[0005] The present invention is achieved through the following technical solutions: A method for preparing a fluorescent optical fiber sensor suitable for monitoring strongly alkaline pH, the method comprising: (1) Preparation of nitrogen-doped carbon quantum dots: Nitrogen-doped carbon quantum dots were prepared by a one-step hydrothermal method using m-diphenol as the carbon source and ammonia as the nitrogen source; (2) Preparation of fluorescent particles: attaching the nitrogen-doped carbon quantum dots to mesoporous silica nanoparticles and coating them with sodium alginate to prepare alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles; (3) Preparation of cellulose acetate membrane solution: When preparing the cellulose acetate membrane solution, the alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles are dispersed in the cellulose acetate membrane solution; (4) Fiber coating: The protective layer at one end of the bare quartz fiber is removed, and a uniform film is formed on the end of the fiber from which the protective layer has been removed using the pull-up coating method to prepare a fluorescent fiber optic sensor suitable for monitoring strong alkaline pH.

[0006] Furthermore, in step (1), resorcinol and ammonia water with a concentration of 25-30 wt.% are mixed in a mass ratio of (0.10-0.12): (0.06-0.08), the reaction temperature is 195-205°C, and the reaction time is 5.5-6.5h to prepare nitrogen-doped carbon quantum dots N-CQDs-OH.

[0007] Furthermore, step (2) is as follows: (2.1) Cetyltrimethylammonium bromide and triethanolamine were added to deionized water, condensed and refluxed with magnetic stirring, and ethyl orthosilicate was added dropwise to the reaction system with magnetic stirring. The reaction product was centrifuged and washed, and then added to a mixture of methanol and concentrated hydrochloric acid. The acid-alcohol reflux was performed to remove the cetyltrimethylammonium bromide. The product was centrifuged and washed to obtain mesoporous silica nanoparticles. (2.2) adding mesoporous silica nanoparticles and 3-aminopropyltrimethoxysilane to methanol, magnetically stirring the mixture at room temperature in a dark place, and centrifuging and washing the mixture to obtain amino-modified mesoporous silica nanoparticles; (2.3) Mixing the amino-modified mesoporous silica nanoparticles, nitrogen-doped carbon quantum dots solution, and phosphate buffer solution, stirring the mixture magnetically, and washing the mixture by centrifugation to obtain mesoporous silica / nitrogen-doped carbon quantum dots; (2.4) Mix mesoporous silica / nitrogen-doped carbon quantum dots, sodium alginate solution, and deionized water, stir magnetically at room temperature in a dark place, and centrifuge to obtain alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles.

[0008] Furthermore, step (2.1) is specifically as follows: (2-3) g of hexadecyltrimethylammonium bromide and (0.08-0.12) g of triethanolamine are added to 20 mL of deionized water, condensed and refluxed, and magnetically stirred at 900-1100 rpm for 1-2 h; (1-2) mL of ethyl orthosilicate is added dropwise to the reaction system, and magnetically stirred at 900-1100 rpm at 85-95 ° C for 1-2 h; the reaction product is centrifuged and washed 3-4 times, and then added to a mixture of 40 mL of methanol and 1-2 mL of 36-38 wt% concentrated hydrochloric acid, and the reaction product is uniformly dispersed in the mixture under ultrasound, and then connected to a condensation reflux device, and heated at 50-60 ° C for 200-500 The acid-alcohol reflux was performed with rpm magnetic stirring to remove hexadecyltrimethylammonium bromide. After the reaction for 10-12 hours, the acid-alcohol reflux solution was replaced and the hexadecyltrimethylammonium bromide was further removed. The acid-alcohol reflux was repeated 3-4 times and the product was centrifuged and washed 3-4 times to obtain mesoporous silica nanoparticles.

[0009] Furthermore, step (2.2) is specifically as follows: mesoporous silica nanoparticles and 3-aminopropyltrimethoxysilane are added to methanol in a mass ratio of (0.8-1.2):(2.4-3.6), magnetically stirred at room temperature in a dark place for 10-12 hours, and then centrifuged and washed 3-4 times to obtain amino-modified mesoporous silica nanoparticles.

[0010] Furthermore, step (2.3) is specifically as follows: amino-modified mesoporous silica nanoparticles, 0.9-1.1 mmol / L nitrogen-doped carbon quantum dot solution, and phosphate buffer are mixed in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and magnetically stirred for 20-24 h; centrifuged and washed 3-4 times to obtain mesoporous silica / nitrogen-doped carbon quantum dots; Furthermore, step (2.4) is specifically as follows: mesoporous silica / nitrogen-doped carbon quantum dots, 0.9-1.1 wt% sodium alginate solution, and deionized water are mixed in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and stirred at room temperature in a dark place for 10-12 hours, followed by centrifugation and washing 3-4 times to wash away excess sodium alginate solution to obtain alginate / mesoporous silica / nitrogen-doped carbon quantum dots.

[0011] Furthermore, step (3) is as follows: Cellulose acetate, acetone, alginate / mesoporous silica / alginate / mesoporous silica / nitrogen-doped carbon quantum dots were mixed in a mass ratio of (0.15-0.17):(2.30-2.50):(0.05-0.06), and stirred on a magnetic stirrer at 200-500 rpm for 18-24 h to prepare the cellulose acetate membrane solution.

[0012] Furthermore, in step (4), the coating is carried out by using a pulling coating method, and the parameters of the coating operation are set as: pulling speed 900-1100µm / s, descending speed 900-1100µm / s, immersion time 10-30s, and interval time 10-30s.

[0013] A fluorescence fiber optic sensor suitable for monitoring strongly alkaline pH. The fluorescence intensity of the fluorescence fiber optic sensor is linearly related to the pH value in the range of 9-13, and is used for accurately measuring strongly alkaline pH.

[0014] Beneficial technical effects of the present invention: The preparation method provided by the present invention has a green and simple preparation step for nitrogen-doped carbon quantum dots. The preparation is carried out in one step by a hydrothermal method using resorcinol. The reaction conditions are mild and no toxic byproducts are produced. The prepared nitrogen-doped carbon quantum dots have the characteristics of high fluorescence intensity, pH sensitivity and biocompatibility. The preparation method provided by the present invention avoids the reliance on toxic heavy metals in the production of traditional semiconductor quantum dots (such as quantum dots containing cadmium, lead, and selenium), eliminating the risk of heavy metal pollution from the source; the synthesis method is more environmentally friendly.

[0015] The fluorescent optical fiber sensor prepared by the preparation method provided by the present invention has the advantages of simple operation, no pollution, high accuracy, high stability, strong ion selectivity and resistance to photobleaching.

[0016] The fluorescence fiber optic sensor provided by the present invention has a high fluorescence response to high pH values. The fluorescence intensity of the fluorescence fiber optic sensor prepared using ammonia as the nitrogen source is linearly related to the pH value in the range of 9-13, which can achieve accurate measurement of strongly alkaline pH. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To further describe the present invention in detail, the accompanying drawings are provided below. Figure 1 is the fluorescence intensity of nitrogen-doped carbon quantum dots (N-CQDs-OH) synthesized at different temperatures; Figure 2 The fluorescence intensity of nitrogen-doped carbon quantum dots (N-CQDs-OH) synthesized at 200°C with different reaction times; Figure 3 FTIR images of nitrogen-doped carbon quantum dots and alginate / mesoporous silica / nitrogen-doped carbon quantum dots; Figure 4a is the TEM image of nitrogen-doped carbon quantum dots (N-CQDs-OH) (scale bar 100 nm); Figure 4b is the TEM image of nitrogen-doped carbon quantum dots (N-CQDs-OH) (scale bar 0.5 nm); Figure 5 This is the TEM image of alginate / mesoporous silica / nitrogen-doped carbon quantum dots; Figure 6 is the relationship between the fluorescence intensity of nitrogen-doped carbon quantum dots (N-CQDs-OH) and pH; Figure 7 Schematic diagram of the fluorescence sensor monitoring system built; Figure 8a The fluorescence spectra of the nitrogen-doped carbon quantum dots (N-CQDs-OH) fluorescence optical fiber sensor were placed in different alkaline pH solutions; Figure 8b Schematic diagram of linear fitting between the fluorescence intensity of nitrogen-doped carbon quantum dots (N-CQDs-OH) fluorescence optical fiber sensor at 520 nm and pH; Reference numerals: 1. 365 nm wavelength ultraviolet light emitting source; 2. Y-type optical fiber; 3. Connector; 4. 400 μm optical fiber holder; 5. 400 μm optical fiber coated with a fluorescent sensitive film; 6. Miniature marine optical spectrometer; 7. Computer. Specific implementation plan

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] On the contrary, the present invention covers any alternatives, modifications, equivalents, and solutions that fall within the spirit and scope of the present invention as defined by the claims. Furthermore, to facilitate a better understanding of the present invention, certain specific details are described in detail below in the detailed description of the present invention. Those skilled in the art will be able to fully understand the present invention without these details.

[0020] Example 1: A method for preparing a fluorescence optical fiber sensor suitable for monitoring strongly alkaline pH, the method comprising: (1) Preparation of nitrogen-doped carbon quantum dots: Nitrogen-doped carbon quantum dots were prepared by a one-step hydrothermal method using m-diphenol as the carbon source and ammonia as the nitrogen source; (2) Preparation of fluorescent particles: attaching the nitrogen-doped carbon quantum dots to mesoporous silica nanoparticles and coating them with sodium alginate to prepare alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles; (3) Preparation of cellulose acetate membrane solution: When preparing the cellulose acetate membrane solution, the alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles are dispersed in the cellulose acetate membrane solution; (4) Fiber coating: The protective layer at one end of the bare quartz fiber is removed, and a uniform film is formed on the end of the fiber from which the protective layer has been removed using the pull-up coating method to prepare a fluorescent fiber optic sensor suitable for monitoring strong alkaline pH.

[0021] In step (1), in order to obtain the optimal reaction temperature and reaction time, the present invention sets the muffle furnace temperature to (155-165, 195-205, 235-245, 275-285) ° C, respectively, and heats for 6 hours to obtain the optimal heating temperature. After obtaining the optimal temperature, the muffle furnace heating time is set to (4, 6, 8) hours to obtain the optimal heating time.

[0022] In this embodiment, in step (1), resorcinol and 25-30wt.% ammonia water are mixed in a mass ratio of (0.10-0.12): (0.06-0.08), added to deionized water, stirred evenly, and the solution is transferred to a reactor and heated in a muffle furnace. After the reactor is cooled to room temperature, the reaction solution is filtered with a 0.22 μm filter membrane, and then the filtrate is dialyzed to obtain nitrogen-doped carbon quantum dots N-CQDs-OH, which are then stored at 4°C for future use; according to Figure 1 、 Figure 2 , the optimal reaction temperature is 195-205°C, and the optimal reaction time is 5.5-6.5h.

[0023] In this embodiment, step (2) is specifically as follows: Step (2.1): add (2-3) g of hexadecyltrimethylammonium bromide (CTAB) and (0.08-0.12) g of triethanolamine to 20 mL of deionized water, condense and reflux, and magnetically stir at 900-1100 rpm for 1-2 h; then add (1-2) mL of tetraethyl orthosilicate (TEOS) dropwise to the reaction system, and magnetically stir at 900-1100 rpm at 85-95°C for 1-2 h; the reaction product is centrifuged and washed 3-4 times, and then added to a mixture of 40 mL of methanol and 1-2 mL of 36-38 wt% concentrated hydrochloric acid, first uniformly disperse the reaction product in the mixture under ultrasound, then connect a condensation reflux device, and at 50-60°C for 200-500 The CTAB was removed by acid-alcohol reflux with magnetic stirring at rpm. After reacting for 10-12 hours, the acid-alcohol reflux solution was replaced and the CTAB was further removed. This was repeated 3-4 times, and the product was centrifuged and washed 3-4 times to obtain mesoporous silica nanoparticles.

[0024] Step (2.2): adding mesoporous silica nanoparticles and 3-aminopropyltrimethoxysilane in a mass ratio of (0.8-1.2):(2.4-3.6) to methanol, magnetically stirring at 200-500 rpm for 10-12 hours at room temperature in a dark place, and then centrifuging and washing 3-4 times to obtain amino-modified mesoporous silica nanoparticles; Step (2.3): The amino-modified mesoporous silica nanoparticles, 0.9-1.1 mmol / L nitrogen-doped carbon quantum dot solution, and phosphate buffer were mixed in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and magnetically stirred at 200-500 rpm for 20-24 h; centrifuged and washed 3-4 times to obtain mesoporous silica / nitrogen-doped carbon quantum dots; the specific material of the phosphate buffer was 0.01 M PBS buffer, and the preparation method was as follows: 8.0 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4, and 0.24 g KH2PO4 were weighed and dissolved in 800 mL distilled water, the pH of the solution was adjusted to 7.4, and finally distilled water was added to make up the volume to 1 L.

[0025] Step (2.4): Mix mesoporous silica / nitrogen-doped carbon quantum dots, 0.9-1.1 wt% sodium alginate solution, and deionized water in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and stir at room temperature at 200-500 rpm for 10-12 h in a dark place, then centrifuge and wash 3-4 times to wash away excess sodium alginate solution to obtain alginate / mesoporous silica / nitrogen-doped carbon quantum dots.

[0026] Step (3) of this embodiment is as follows: Cellulose acetate, acetone, alginate / mesoporous silica / alginate / mesoporous silica / nitrogen-doped carbon quantum dots were mixed in a mass ratio of (0.15-0.17):(2.30-2.50):(0.05-0.06), and stirred on a magnetic stirrer at 200-500 rpm for 18-24 h to prepare the cellulose acetate membrane solution.

[0027] In this embodiment, in step (4), the prepared cellulose acetate membrane solution containing alginate / mesoporous silica / alginate / mesoporous silica / nitrogen-doped carbon quantum dots is plated on a quartz bare fiber by a pulling coating method to prepare a fluorescent optical fiber sensor. The parameters of the coating operation are set as follows: pulling speed 900-1100µm / s, descending speed 900-1100µm / s, immersion time 10-30s, and interval time 10-30s.

[0028] FTIR and TEM analysis of nitrogen-doped carbon quantum dots and alginate / mesoporous silica / nitrogen-doped carbon quantum dots Figure 3 The FTIR spectra of nitrogen-doped carbon quantum dots (N-CQDs-OH) and alginate / mesoporous silica / nitrogen-doped carbon quantum dots (Alginate / mesoporous / N-CQDs-OH) are shown in Figure 3000-3750 cm -1is the stretching vibration region of NH and HO, 2970 cm -1 、2880 cm -1 is the stretching vibration peak of CH, 1380 cm -1 It is the internal bending vibration peak of -CH3, 1090cm -1 is the stretching vibration peak of CN, 1044 cm -1 is the stretching vibration peak of CO, 880 cm -1 is the external bending vibration of CH on the benzene ring, 1650 cm -1 The peaks are the C=O double bond stretching vibration peaks. The presence of these chemical bonds indicates the successful doping of nitrogen atoms. Nitrogen-doped carbon quantum dots contain abundant hydroxyl and amine groups, which make them exhibit excellent water solubility. 1000-1200 cm -1 It is the stretching vibration peak range of Si-O, OH, CN, and CO, 800 cm -1 is the external bending vibration of Si-O. The existence of Si-O proves the synthesis of mesoporous silica particles. Figure 3 In the middle, the fluorescent nanoparticles are at 1410 cm -1 、1640 cm -1 The peak shift is attributed to the hydrogen bonding between the hydrogen atoms on the CH groups and the oxygen atoms on the C=O groups, which results in a shift in the characteristic spectrum. The presence of chemical bonds between carbon quantum dots on silica indicates the successful integration of silica nanoparticles and nitrogen-doped carbon quantum dots.

[0029] To investigate the microscopic morphology of nitrogen-doped carbon quantum dots and alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent nanoparticles, transmission electron microscopy (TEM) was used to characterize them. The nitrogen-doped carbon quantum dots and alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent nanoparticles were added to anhydrous ethanol and thoroughly dispersed by ultrasonication for 5 minutes. A small amount was then pipetted onto a copper grid and allowed to dry naturally at room temperature before being scanned using a TEM to observe their morphology.

[0030] Figure 4 is a TEM image of nitrogen-doped carbon quantum dots (N-CQDs-OH). As can be seen from the figure, the nitrogen-doped carbon quantum dots are approximately spherical in shape and are evenly distributed with a particle size distribution of 2-2.5 nm.

[0031] Figure 5 This is a TEM image of alginate / mesoporous silica / nitrogen-doped carbon quantum dots (Alginate / mesoporous / N-CQDs-OH). As shown, the mesoporous silica is spherical with a particle size distribution of 150-250 nm. Observing the surface of the mesoporous silica reveals a dense distribution of nitrogen-doped carbon quantum dots, similar to the appearance shown in Figure 4.

[0032] Fluorescence analysis of nitrogen-doped carbon quantum dots at different pH values: Figure 6 As shown in the figure, within the pH range of 5.23-13.01, the fluorescence intensity of N-CQDs-OH first increases and then decreases with increasing OH- concentration. From pH 5.23 to pH 9.47, the fluorescence intensity of N-CQDs-OH increases significantly, accompanied by a blue shift in the fluorescence peak of N-CQDs-OH. From pH 9.47 to pH 13.01, the fluorescence intensity of N-CQDs-OH gradually decreases, and the fluorescence peak of N-CQDs-OH no longer exhibits a blue shift. This is due to the fluorescence response of N-CQDs-OH to different pH values.

[0033] Example 2: Fluorescence fiber optic sensor suitable for monitoring alkaline pH.

[0034] Fluorescence analysis was performed on the N-CQDs-OH based fluorescence optical fiber sensor prepared using ammonia as the nitrogen source in Example 1: The measurement platform built for fiber optic sensor testing involves equipment including a light source emitting at a wavelength of 365nm, a Y-type optical fiber, an Ocean Optics spectrometer, an SMA adapter, a 400nm optical fiber holder, and a computer that receives data. The light emitted by the light source is collected by the Y-type optical fiber and enters the fiber optic sensor. The SMA adapter and the 400nm optical fiber holder are used to connect the Y-type optical fiber to the fiber optic sensor. After the light enters the fiber optic sensor, it excites the sol-gel film to produce fluorescence. The fluorescence enters the other optical fiber of the Y-type optical fiber from the fiber optic sensor and is eventually captured by the Ocean Optics spectrometer. Figure 7 shown.

[0035] Prepare a solution with a pH of 9-13 using sodium hydroxide and measure its pH using a PHS-3E pH meter. Immerse the fiber optic sensor in the solution and record its fluorescence intensity after the fluorescence stabilizes. Remove the fiber optic sensor and rinse it with deionized water before measuring the next pH solution.

[0036] The fluorescence emission spectra of the fluorescence optical fiber sensor were measured in solutions with different pH values. The results showed that the fluorescence of the fluorescence optical fiber sensor gradually decreased with the increase of the alkalinity of the solution. Figure 8a This is the fluorescence spectrum of the fluorescence fiber optic sensor in solution pH = 9-13. Figure 8b This is a linear fitting diagram of the fluorescence intensity at the peak 500nm and pH. The linear equation is F 500nm / F0=-0.04127x+1.36774(R 2 =0.99668).

[0037] It can be seen that in this embodiment, the fluorescence intensity of the fluorescence optical fiber sensor based on N-CQDs-OH is linearly related to the pH value in the range of 9-13, and can be used to accurately measure the pH value of strong alkalinity.

[0038] The above is only one embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH, characterized in that: The method comprises: (1) Preparation of nitrogen-doped carbon quantum dots: Nitrogen-doped carbon quantum dots were prepared by a one-step hydrothermal method using m-diphenol as the carbon source and ammonia as the nitrogen source; (2) Preparation of fluorescent particles: attaching the nitrogen-doped carbon quantum dots to mesoporous silica nanoparticles and coating them with sodium alginate to prepare alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles; (3) Preparation of cellulose acetate membrane solution: When preparing the cellulose acetate membrane solution, the alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles are dispersed in the cellulose acetate membrane solution; (4) Fiber coating: The protective layer at one end of the bare quartz fiber is removed, and a thin film is formed on the end of the fiber from which the protective layer has been removed using the pull-up coating method to prepare a fluorescent fiber optic sensor suitable for monitoring strong alkaline pH.

2. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 1, characterized in that: In step (1), resorcinol and ammonia water with a concentration of 25-30 wt.% are mixed in a mass ratio of (0.10-0.12): (0.06-0.08), the reaction temperature is 195-205°C, and the reaction time is 5.5-6.5h to prepare nitrogen-doped carbon quantum dots N-CQDs-OH.

3. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 1, characterized in that: Step (2) is as follows: (2.1) Cetyltrimethylammonium bromide and triethanolamine were added to deionized water, condensed and refluxed with magnetic stirring, and ethyl orthosilicate was added dropwise to the reaction system with magnetic stirring. The reaction product was centrifuged and washed, and then added to a mixture of methanol and concentrated hydrochloric acid. The acid-alcohol reflux was performed to remove the cetyltrimethylammonium bromide. The product was centrifuged and washed to obtain mesoporous silica nanoparticles. (2.2) adding mesoporous silica nanoparticles and 3-aminopropyltrimethoxysilane to methanol, magnetically stirring the mixture at room temperature in a dark place, and centrifuging and washing the mixture to obtain amino-modified mesoporous silica nanoparticles; (2.3) Mixing the amino-modified mesoporous silica nanoparticles, nitrogen-doped carbon quantum dots solution, and phosphate buffer solution, stirring the mixture magnetically, and washing the mixture by centrifugation to obtain mesoporous silica / nitrogen-doped carbon quantum dots; (2.4) Mix mesoporous silica / nitrogen-doped carbon quantum dots, sodium alginate solution, and deionized water, stir magnetically at room temperature in a dark place, and centrifuge to obtain alginate / mesoporous silica / nitrogen-doped carbon quantum dot fluorescent particles.

4. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 3, characterized in that: Step (2.1) is specifically as follows: (2-3) g of hexadecyltrimethylammonium bromide and (0.08-0.12) g of triethanolamine are added to 20 mL of deionized water, condensed and refluxed, and magnetically stirred at 900-1100 rpm for 1-2 h; (1-2) mL of ethyl orthosilicate is added dropwise to the reaction system, and magnetically stirred at 900-1100 rpm at 85-95 ° C for 1-2 h; the reaction product is centrifuged and washed 3-4 times, and then added to a mixture of 40 mL of methanol and 1-2 mL of 36-38 wt% concentrated hydrochloric acid, and the reaction product is evenly dispersed in the mixture under ultrasound, and then connected to a condensation reflux device, and heated at 50-60 ° C for 200-500 The acid-alcohol reflux was performed with rpm magnetic stirring to remove hexadecyltrimethylammonium bromide. After the reaction for 10-12 hours, the acid-alcohol reflux solution was replaced and the hexadecyltrimethylammonium bromide was further removed. The acid-alcohol reflux was repeated 3-4 times and the product was centrifuged and washed 3-4 times to obtain mesoporous silica nanoparticles.

5. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 3, characterized in that: Step (2.2) is specifically as follows: mesoporous silica nanoparticles and 3-aminopropyltrimethoxysilane are added to methanol in a mass ratio of (0.8-1.2):(2.4-3.6), magnetically stirred at room temperature in a dark place for 10-12 hours, and then centrifuged and washed 3-4 times to obtain amino-modified mesoporous silica nanoparticles.

6. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 3, characterized in that: Step (2.3) is specifically as follows: amino-modified mesoporous silica nanoparticles, 0.9-1.1 mmol / L nitrogen-doped carbon quantum dot solution, and phosphate buffer solution are mixed in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and magnetically stirred for 20-24 h; centrifuged and washed 3-4 times to obtain mesoporous silica / nitrogen-doped carbon quantum dots.

7. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 3, characterized in that: Step (2.4) is specifically as follows: mesoporous silica / nitrogen-doped carbon quantum dots, 0.9-1.1 wt% sodium alginate solution, and deionized water are mixed in a mass ratio of (0.8-1.2):(2.4-3.6):(240-360) and stirred at room temperature in a dark place for 10-12 hours, followed by centrifugation and washing 3-4 times to wash away excess sodium alginate solution to obtain alginate / mesoporous silica / nitrogen-doped carbon quantum dots.

8. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 1, characterized in that: Step (3) is as follows: Cellulose acetate, acetone, alginate / mesoporous silica / alginate / mesoporous silica / nitrogen-doped carbon quantum dots were mixed in a mass ratio of (0.15-0.17):(2.30-2.50):(0.05-0.06), and stirred on a magnetic stirrer at 200-500 rpm for 18-24 h to prepare the cellulose acetate membrane solution.

9. The method for preparing a fluorescent optical fiber sensor suitable for monitoring strong alkaline pH according to claim 1, characterized in that: In step (4), the coating is carried out by the pulling coating method, and the parameters of the coating operation are set as: pulling speed 900-1100µm / s, descending speed 900-1100µm / s, immersion time 10-30s, and interval time 10-30s.

10. A fluorescent optical fiber sensor suitable for monitoring strongly alkaline pH, prepared by the method according to any one of claims 1 to 9, characterized in that: The fluorescence intensity of the fluorescence optical fiber sensor is linearly related to the pH value in the range of 9-13, and is used to accurately measure the pH of strongly alkaline solutions.

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