A parallel double-cavity fiber-optic copper ion sensor and a detection method thereof

Abstract

The present application relates to the technical field of optical fiber sensing, and provides a parallel double-cavity type optical fiber copper ion sensor, which comprises an optical fiber coupler, a sensing Fabry-Perot interferometer and a reference Fabry-Perot interferometer; the sensing Fabry-Perot interferometer comprises a first single-mode optical fiber, a first capillary glass tube and a hydrogel; one end of the first single-mode optical fiber is connected with the optical fiber coupler, and the other end extends into the first capillary glass tube; the end faces of the first single-mode optical fiber and the first capillary glass tube are provided with the hydrogel; the reference Fabry-Perot interferometer comprises a second single-mode optical fiber, a second capillary glass tube and a third single-mode optical fiber; one end of the second single-mode optical fiber is connected with the optical fiber coupler, and the other end extends into the second capillary glass tube; the third single-mode optical fiber also extends into the second capillary glass tube; and a gap is left between the third single-mode optical fiber and the second single-mode optical fiber. The present application produces amplified frequency changes through the superposition of two reflected light spectra with a small frequency difference, so as to improve the sensitivity of copper ion measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber sensing, in particular to a parallel double-cavity optical fiber copper ion sensor and a detection method thereof. BACKGROUND

[0002] Copper ions are a common heavy metal ion, and industrial wastewater containing copper ions is often produced in chemical industry, dyeing, electroplating, non-ferrous metal mining, electronic material cleaning wastewater, and ignition production process. The discharge of a large amount of copper-containing wastewater has a huge impact on the soil resources and water resources on which human beings depend, and excessive intake of copper ions can also cause kidney damage, liver necrosis, hemolysis, and brain tissue lesions to the human body. Therefore, accurate detection of the concentration of copper ions in industrial wastewater and drinking water is a necessary means to ensure human health and avoid the impact of copper ion pollution on the ecological environment. Common methods for detecting copper ions include fluorescence labeling, atomic absorption spectrometry, and electrochemical methods, but these methods generally have problems such as high detection cost, complex sample pretreatment, and long detection time.

[0003] Compared with traditional sensors, optical fiber sensors have the advantages of small size, anti-electromagnetic interference, high sensitivity, simple process, low cost, high temperature resistance, corrosion resistance, and great potential in metal ion detection. By combining optical fiber sensors with specific recognition materials, accurate detection of metal ions by sensors can be achieved. However, existing optical fiber sensors still have certain deficiencies in sensitivity, repeatability, selectivity, etc. On the one hand, the detection capability for low-concentration copper ions is limited, which is difficult to meet the requirements of some high-precision detection applications; on the other hand, the sensor probe cannot be reused and is easily disturbed by other ions in complex environments, resulting in inaccurate detection results.

[0004] As a unique optical phenomenon, the optical vernier effect has attracted widespread attention in the field of optical measurement in recent years. By the interaction of two optical signals with a small frequency difference, it can produce amplified frequency changes, thereby improving the sensitivity and resolution of measurement. At present, although there have been some researches on sensors based on optical principles, the application of optical vernier effect to optical fiber copper ion sensors is still relatively rare. SUMMARY

[0005] The present application mainly solves the technical problems of low response sensitivity, long detection time, poor repeatability, and selectivity of existing optical fiber sensing technology, and proposes a parallel double-cavity optical fiber copper ion sensor and a detection method thereof. By superimposing two reflected spectra with a small frequency difference, amplified frequency changes are produced to improve the sensitivity of copper ion measurement.

[0006] The application provides a parallel double-cavity optical fiber copper ion sensor, comprising a fiber coupler, a sensing Fabry-Perot interferometer and a reference Fabry-Perot interferometer.

[0007] The sensing Fabry-Perot interferometer comprises a first single-mode optical fiber, a first capillary glass tube and a hydrogel.

[0008] One end of the first single-mode optical fiber is connected with the fiber coupler, and the other end is inserted into the first capillary glass tube; the end faces of the first single-mode optical fiber and the first capillary glass tube are aligned, and the end faces of the first single-mode optical fiber and the first capillary glass tube are provided with the hydrogel.

[0009] The reference Fabry-Perot interferometer comprises a second single-mode optical fiber, a second capillary glass tube and a third single-mode optical fiber.

[0010] One end of the second single-mode optical fiber is connected with the fiber coupler, and the other end is inserted into the second capillary glass tube; the third single-mode optical fiber is also inserted into the second capillary glass tube; the third single-mode optical fiber is oppositely arranged with the second single-mode optical fiber, and a gap is left between the third single-mode optical fiber and the second single-mode optical fiber.

[0011] Preferably, the hydrogel is sodium alginate / graphene oxide hydrogel.

[0012] Preferably, the preparation method of the sodium alginate / graphene oxide hydrogel is as follows:

[0013] 0.3-0.4g of graphene oxide is added into 37-50mL of deionized water, and the mixture is stirred for more than 1h by using a magnetic stirrer and is ultrasonically cleaned for more than 1h by using an ultrasonic cleaner to obtain a uniformly dispersed graphene oxide solution;

[0014] 0.8-1.0g of sodium alginate is added into 35-45mL of deionized water, and the mixture is stirred for more than 1h by using a magnetic stirrer to obtain a sodium alginate sol;

[0015] Then, 80-90mg of the graphene oxide suspension is added into the sodium alginate hydrogel, and the mixture is stirred for more than 2h to obtain a sodium alginate / graphene oxide sol;

[0016] 5mL of Cu(NO3)2 solution with a concentration of about 1g / L is added into the sodium alginate / graphene oxide sol, and the mixture is stirred for more than 20min to obtain a yellowish sol;

[0017] The common end faces of the first single-mode optical fiber and the first capillary glass tube are dipped in an appropriate amount of the sol, and the mixture is placed in a CaCl2 solution with a mass fraction of 5% for more than 3h; then, the sensing Fabry-Perot interferometer is washed in deionized water to elute the excess copper ions, and the sodium alginate / graphene oxide hydrogel with specific response to copper ions is obtained.

[0018] The reflected light intensity I of the sensing Fabry-Perot interferometer is preferably s is expressed as:

[0019]

[0020] wherein I s1 represents the light intensity reflected back to the first single-mode optical fiber from the interface between the hydrogel and the first single-mode optical fiber, I s2 represents the light intensity reflected back to the first single-mode optical fiber from the interface between the hydrogel and the solution to be measured, n s represents the refractive index of the hydrogel, L s represents the thickness of the hydrogel, is an initial phase;

[0021] When the reflected light intensity I of the sensing Fabry-Perot interferometer in the interference spectrum reaches the minimum value, the wavelength of the reflection spectrum valley is expressed as: s

[0022]

[0023] wherein λ dip represents the wavelength of the reflection spectrum valley, and m is the order of the interference peak;

[0024] The free spectral range of the sensing Fabry-Perot interferometer is:

[0025]

[0026] wherein FSR s represents the free spectral range of the sensing Fabry-Perot interferometer;

[0027] When the concentration of copper ions in the external environment changes, the change amount of the corresponding valley of the sensing Fabry-Perot interferometer is expressed as:

[0028]

[0029] wherein Δλ dip represents the change amount of the corresponding valley of the sensing Fabry-Perot interferometer;

[0030] The wavelength of the reflection spectrum valley of the reference Fabry-Perot interferometer is expressed as:

[0031]

[0032] wherein λ′ dip represents the wavelength of the reflection spectrum valley of the reference Fabry-Perot interferometer, n is the refractive index of air, and L r is the air cavity length;

[0033] The free spectral range of the reference Fabry-Perot interferometer is expressed as:

[0034] ​

[0035] FSR r FSR

[0036] FSR

[0037]

[0038] Preferably, the free spectral range of the sensing Fabry-Perot interferometer and the reference Fabry-Perot interferometer is 5.7-9.1 nm.

[0039] Correspondingly, the application also provides a detection method of the parallel double-cavity optical fiber copper ion sensor provided by any embodiment of the application, which comprises the following steps:

[0040] Step 1, placing the sensing Fabry-Perot interferometer in the solution to be measured and placing the reference Fabry-Perot interferometer outside the solution to be measured without contacting the solution to be measured; connecting the optical fiber coupler to the optical spectrometer;

[0041] Step 2, standing for a period of time, the sensing Fabry-Perot interferometer and the reference Fabry-Perot interferometer produce an optical vernier effect, the interference spectrum of the sensing Fabry-Perot interferometer and the reference Fabry-Perot interferometer is superimposed through the optical fiber coupler, and the interference spectrum of the sensor is displayed by the optical spectrometer;

[0042] Step 3, recording the wavelength drift of the corresponding trough in the interference spectrum of the sensor;

[0043] Step 4, bringing the wavelength drift of the trough into the fitting curve of the wavelength drift and the copper ion concentration to obtain the copper ion concentration in the solution to be measured.

[0044] Preferably, the fitting curve formula of the wavelength drift and the copper ion concentration is:

[0045] y = sC (Cu2+)

[0046] wherein y represents the wavelength drift, C (Cu2+) represents the copper ion concentration, and s represents the corresponding sensitivity of the sensor.

[0047] The application provides a parallel double-cavity optical fiber copper ion sensor and a detection method thereof, and is based on an optical vernier effect and a Fabry-Perot interference principle, and through superposition of two reflected light spectrums with a small frequency difference, amplified frequency change is generated, so that the sensitivity of copper ion measurement is improved. The application can effectively make up for the shortcomings of existing copper ion detection methods and optical fiber sensors, realize high-sensitivity detection of copper ions, and improve the performance of the optical fiber copper ion sensor; the application has excellent dynamic response characteristics and repeatability; the application has good selectivity, can accurately identify copper ions in a complex environment, avoids interference of other ions, and ensures the accuracy of the detection result. The application prepares sodium alginate / graphene oxide hydrogel with specific spatial structure and binding sites for copper ions through ion imprinting, has high selective recognition ability for copper ions, can specifically bind copper ions, improves the repeatability and stability of the sensor, and shortens the response time of the sensor.

[0048] The sensor is small in size, light in weight, convenient to install and use, and suitable for various complex detection environments. The application has good application prospect in the fields of environmental detection, water quality analysis and industrial production, can be used for detection of copper ions in water bodies, timely grasps the environmental quality, and provides strong support for environmental protection; and can be used for detection of copper ion concentration in waste liquid in the chemical and electronic industries. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a structural schematic diagram of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0050] Figure 2 is a reflected light spectrum diagram of the parallel double-cavity optical fiber copper ion sensor provided by the application when the copper ion concentration is 0 and 0.2 mg / L;

[0051] Figure 3 is a copper ion concentration and wavelength drift amount schematic diagram of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0052] Figure 4 is a dynamic response diagram of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0053] Figure 5 is a time and wavelength drift amount schematic diagram of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0054] Figure 6 is a result diagram of a repeatability experiment of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0055] Figure 7 is a result columnar diagram of a selectivity experiment of the parallel double-cavity optical fiber copper ion sensor provided by the application;

[0056] Figure 8 This is a graph showing the results of a selective experiment conducted on the parallel dual-cavity fiber optic copper ion sensor provided by this invention. Detailed Implementation

[0057] To make the technical problems solved by this invention, the technical solutions adopted, and the technical effects achieved clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings, not all of them.

[0058] Example 1

[0059] like Figure 1 As shown in the figure, an embodiment of the present invention provides a parallel dual-cavity fiber optic copper ion sensor, comprising: a fiber optic coupler 1, a sensing Fabry-Perot interferometer 2, and a reference Fabry-Perot interferometer 3.

[0060] The sensing Fabry-Perot interferometer 2 includes: a first single-mode optical fiber 201, a first capillary glass tube 202, and a hydrogel 203. One end of the first single-mode optical fiber 201 is connected to an optical fiber coupler 1, and the other end extends into the first capillary glass tube 202; the end faces of the first single-mode optical fiber 201 and the first capillary glass tube 202 are aligned, and the hydrogel 203 is disposed at the end faces of the first single-mode optical fiber 201 and the first capillary glass tube 202.

[0061] The reference Fabry-Perot interferometer 3 includes: a second single-mode fiber 301, a second capillary glass tube 302, and a third single-mode fiber 303. One end of the second single-mode fiber 301 is connected to the fiber coupler 1, and the other end extends into the second capillary glass tube 302; the third single-mode fiber 303 also extends into the second capillary glass tube 302; the third single-mode fiber 303 is arranged opposite to the second single-mode fiber 301, and a gap is left between the third single-mode fiber 303 and the second single-mode fiber 301, which serves as the interference cavity of the reference Fabry-Perot interferometer 3.

[0062] The hydrogel 203 is a sodium alginate / graphene oxide hydrogel.

[0063] A method for fabricating a parallel dual-cavity fiber optic copper ion sensor according to the present invention: First, the fiber optic coupler and single-mode fiber are pretreated by stripping the coating layer from the fiber surface and cutting it flat with a fiber optic cleaver. One fiber of a 1×2 50:50 fiber optic coupler 1 is fitted onto a first capillary glass tube 202. Under the assistance of a microscope, the end face of the fiber is made flush with the end face of the first capillary glass tube 202. The first capillary glass tube 202 is fixed to the single-mode fiber with glue. Sodium alginate / graphene oxide sol is dipped into the sensor and cured in a 5% calcium chloride solution to form a hydrogel. An interference spectrum is generated on a spectrometer. The hydrogel is then immersed in deionized water to wash away excess copper ions, forming a Fabry-Perot interferometer 2 for sensing. Another fiber of the fiber coupler 1 is fitted with the second capillary glass tube 302, and then a section of neatly cut fiber is inserted into the second capillary glass tube 302. The distance between the two fiber sections (the second single-mode fiber 301 and the third single-mode fiber 303) is adjusted until a periodic envelope is generated on the spectrometer, which is used as the reference Fabry-Perot interferometer 3.

[0064] This invention utilizes ion imprinting to prepare sodium alginate / graphene oxide hydrogels that specifically respond to copper ions. The preparation method of the sodium alginate / graphene oxide hydrogels is as follows:

[0065] (1) Preparation of graphene oxide suspension: Add 0.3-0.4g of graphene oxide to 37-50mL of deionized water, stir with a magnetic stirrer for more than 1 hour, and sonicate with an ultrasonic cleaner for more than 1 hour to obtain a uniformly dispersed graphene oxide solution.

[0066] (2) Preparation of sodium alginate hydrogel: Add 0.8-1.0g of sodium alginate to 35-45mL of deionized water and stir magnetically for more than 1h to obtain sodium alginate sol.

[0067] (3) Preparation of sodium alginate / graphene oxide hydrogel: Add 80-90mg of graphene oxide suspension to the sodium alginate hydrogel and stir for more than 2 hours to obtain sodium alginate / graphene oxide sol.

[0068] (4) Add about 5 mL of Cu(NO3)2 solution with a concentration of 1 g / L to sodium alginate / graphene oxide sol, stir for more than 20 min to obtain a slightly yellow sol;

[0069] (5) Dip the common end face of the first single-mode optical fiber 201 and the first capillary glass tube 202 into an appropriate amount of sol, place it in a CaCl2 solution with a mass fraction of 5% for more than 3 hours, and then place the sensing Fabry-Perot interferometer 2 in deionized water to wash away excess copper ions, thereby obtaining a sodium alginate / graphene oxide hydrogel that specifically responds to copper ions.

[0070] As an anionic polysaccharide, sodium alginate contains a large number of carboxyl groups on the molecular chain, which are dissociated to carry negative electricity in aqueous solution, and are attracted to copper ions carrying positive electricity by electrostatic attraction. At the same time, the carboxyl groups can undergo complexation reaction with copper ions to form stable complexes. Graphene oxide is rich in oxygen-containing functional groups such as hydroxyl, carboxyl and epoxy groups on the surface, which are also ionized to carry negative electricity, and have electrostatic effect with copper ions, and the oxygen-containing functional groups can participate in complexation reaction to enhance the adsorption of copper ions. In addition, the hydrogel has a three-dimensional porous network structure, which provides adsorption space and channels for copper ions, and traps copper ions in the pore inner surface and the surface of graphene oxide through physical adsorption. Furthermore, the hydrogen bonding between sodium alginate and graphene oxide makes them closely combined, maintains the stability of the hydrogel structure, ensures the effective adsorption sites, and thus realizes the efficient adsorption of copper ions.

[0071] Ion imprinting method refers to that in the presence of copper ions, sodium alginate / graphene oxide hydrogel interacts with copper ions to form a sodium alginate / graphene oxide-copper ion complex with specific spatial structure and binding sites. Then, calcium ions are added as a crosslinking agent to initiate polymerization, so that the complex forms a highly crosslinked polymer network. After polymerization is completed, the excess copper ions are washed away by deionized water, so that the imprinting cavities matching the shape, size and binding sites of copper ions are left in the polymer. When a solution containing copper ions contacts the imprinted polymer, the imprinted cavities have a high selective recognition ability for copper ions and can specifically bind copper ions.

[0072] The sensor of the present application produces an optical vernier effect due to the parallel connection of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3, and the periodic envelope formed can amplify the change of the interference spectrum of the sensing Fabry-Perot interferometer 2. The principle is as follows:

[0073] The sensing Fabry-Perot interferometer 2 can be regarded as a double-beam interference, and the reflected light intensity I s can be expressed as:

[0074]

[0075] In the formula, I s1 represents the light intensity reflected back to the first single-mode optical fiber 201 at the interface between the first single-mode optical fiber 201 and the hydrogel 203, I s2 represents the light intensity reflected back to the first single-mode optical fiber 201 at the interface between the hydrogel 203 and the solution to be measured, n s represents the refractive index of the hydrogel 203, L s represents the thickness of the hydrogel 203, is the initial phase.

[0076] When the reflected light intensity I sThe wavelength of the reflection spectrum valley is expressed as:

[0077]

[0078] where λ dip represents the wavelength of the reflection spectrum valley, and m is the interference peak order.

[0079] According to the above formula, the free spectral range of the sensing Fabry-Perot interferometer 2 is:

[0080]

[0081] where FSR s represents the free spectral range of the sensing Fabry-Perot interferometer 2.

[0082] When the external copper ion concentration changes, the corresponding valley change amount of the sensing Fabry-Perot interferometer 2 is expressed as:

[0083]

[0084] where Δλ dip represents the corresponding valley change amount of the sensing Fabry-Perot interferometer 2.

[0085] Therefore, by observing the wavelength drift of the valley in the interference spectrum, the change amount of the copper ion concentration can be obtained.

[0086] Similarly, the wavelength of the reflection spectrum valley of the reference Fabry-Perot interferometer 3 is expressed as:

[0087]

[0088] where λ' dip represents the wavelength of the reflection spectrum valley of the reference Fabry-Perot interferometer 3, n is the air refractive index, and L r is the air cavity length. According to the above formula, the free spectral range of the reference Fabry-Perot interferometer 3 is expressed as:

[0089]

[0090] where FSR r represents the free spectral range of the reference Fabry-Perot interferometer 3.

[0091] When the free spectral range (FSR) of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3 is about 5.7-9.1 nm, the external copper ion concentration changes, the interference spectrum of the sensing Fabry-Perot interferometer changes, and the interference cavity of the reference Fabry-Perot interferometer 3 only has air and is not in contact with the to-be-measured solution, so the interference spectrum of the reference Fabry-Perot interferometer 3 does not change. After the interference spectra of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3 are superimposed through the fiber coupler 1, an envelope is formed, which is analogous to a vernier caliper, and the optical vernier effect is generated in the parallel sensing Fabry-Perot interferometer 2, and the periodic envelope amplifies the change of the original interference spectrum of the sensing Fabry-Perot interferometer 2. The interference spectrum can be modulated by the envelope, and the free spectral range (FSR) of the simplified envelope can be described by the relationship between the free spectral ranges of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3 as follows:

[0092]

[0093] As can be seen from the above formula, the closer the free spectral ranges of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3, the greater the FSR of the envelope, and the higher the response sensitivity of the sensor, thereby realizing the improvement of the response sensitivity of the sensor.

[0094] The effects of the parallel double-cavity optical fiber copper ion sensor of the present application are experimentally described as follows:

[0095] Experiment 1: The sensing Fabry-Perot interferometer 2 is placed in the to-be-measured solution, and the reference Fabry-Perot interferometer 3 is placed outside the solution and does not contact the to-be-measured solution. The to-be-measured solution has a copper ion concentration of 0 (deionized water), 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 mg / L. First, the sensing Fabry-Perot interferometer 2 is placed in deionized water, and the trough movement of the interference spectrum is observed. After a period of standing, when the interference spectrum is stable, the data is recorded and saved. Then a certain amount of 100 mg / L copper ion solution is added to the deionized water to change the copper ion concentration in the beaker to 0.2 mg / L, and the experiment is repeated until the copper ion concentration of the to-be-measured solution changes to 1.4 mg / L. As shown in FIG. 2, when the copper ion concentration of the to-be-measured solution is 0.2 mg / L, the trough of the interference spectrum of the sensing Fabry-Perot interferometer 2 moves to 0.2 nm, and when the copper ion concentration of the to-be-measured solution is 1.4 mg / L, the trough of the interference spectrum of the sensing Fabry-Perot interferometer 2 moves to 1.4 nm. Figure 2 、 Figure 3, with the copper ion concentration from 0 to 0.2mg / L, the envelope moves to the long wavelength (right) direction, select a trough, record its corresponding wavelength in deionized water (marked as the initial wavelength), and then record the wavelength corresponding to the trough when the copper ion concentration changes to 0.2mg / L, and so on, record the wavelength corresponding to the trough when the copper ion concentration is 1.4mg / L, respectively, subtract the wavelength corresponding to the trough in different concentrations of copper ion solution from the initial wavelength, and obtain the wavelength drift of the sensor in different concentrations of copper ion solution. After linear fitting of these data, the response sensitivity of the sensor is about 19.58nm / (mg / L), the correlation coefficient R 2 =0.98739, and the response sensitivity of the parallel sensor is 9.9 times higher than that of the Fabry-Perot interferometer. The parallel double-cavity optical fiber copper ion sensor has high sensitivity.

[0096] Experiment 2: Place the sensing Fabry-Perot interferometer 2 in the solution to be measured with a copper ion concentration of 0, collect the interference spectrum data of the sensor at a frequency of 30Hz (i.e. record the interference spectrum data every 15s), add a certain amount of 100mg / L copper ion solution to the deionized water to change the copper ion concentration in the beaker to 0.2mg / L, and observe the movement of the interference spectrum. After 10 minutes of standing, ensure that the spectrum image is stable, then change the copper ion concentration in the solution to be measured to 0.4mg / L, and repeat the above operation until the copper ion concentration in the solution to be measured is 1.4mg / L. Extract the spectrum data, select a trough, find the wavelength corresponding to the trough in each data, calculate the wavelength drift of the trough, and draw a time-wavelength drift graph, i.e. the copper ion concentration dynamic response experiment curve of the sensor. As shown in Figure 4 、 Figure 5 In order to further observe the response time, the part from 0.2mg / L to 0.4mg / L of the copper ion concentration is enlarged, and under the condition of a change of 0.2mg / L, the response time of the sensor is 27 seconds. The parallel double-cavity optical fiber copper ion sensor can quickly respond.

[0097] Experiment 3: Select the copper ion concentration of 0 (deionized water), 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 mg / L of the test solution. First, place the sensing Fabry-Perot interferometer 2 in deionized water, observe the trough movement of the interference spectrum, stand for a period of time, and record and save the data after the interference spectrum is stable, then add a certain amount of 100 mg / L copper ion solution to change the copper ion concentration in the beaker to 0.2 mg / L, repeat the experiment until the copper ion concentration of the test solution becomes 1.4 mg / L. Record and save the data, calculate the wavelength shift of the selected trough, and linearly fit it, denoted as one experiment. After one experiment, place the sensing Fabry-Perot interferometer 2 in deionized water for desorption, observe the movement of the interference spectrum, and wait for the interference spectrum to be stable, then move the sensing Fabry-Perot interferometer 2 to deionized water, repeat the above experiment operation until three experiments are completed. For example Figure 6 , the response sensitivity of the sensing Fabry-Perot interferometer 2 in three experiments is 2.00768 nm / (mg / L), 2.06946 nm / (mg / L), and 2.03536 nm / (mg / L), with a maximum difference of 0.062 nm / (mg / L). Taking the solution with a copper ion concentration of 1 mg / L as an example, the standard deviation of the sensor is about 0.0425 mg / L. After repeated experiments, the fitting curve formula of the wavelength shift of the sensor and the copper ion concentration is y=sC (Cu2+) , wherein y represents the wavelength shift, C (Cu2+) represents the copper ion concentration, and s represents the corresponding sensitivity of the sensor. The sensor has good repeatability. The parallel double-cavity optical fiber copper ion sensor of the application has good repeatability.

[0098] Experiment 4: Select equal concentrations of copper ion solution, copper ion and iron ion mixed solution, copper ion and lead ion mixed solution and copper ion and magnesium ion mixed solution as the test solution respectively. The concentration thereof is 0 (deionized water), 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 mg / L respectively. First, the sensing Fabry-Perot interferometer 2 is placed in deionized water, the trough movement of the interference spectrum is observed, and after a period of standing, the data is recorded and saved after the interference spectrum is stable, then a certain amount of 100 mg / L copper ion solution is added to the deionized water to change the copper ion concentration in the beaker to 0.2 mg / L, and the experiment is repeated until the copper ion concentration in the test solution is 0.8 mg / L. Record and save the data, calculate the wavelength shift of the selected trough, make a column chart, and linearly fit it. Then the sensing Fabry-Perot interferometer 2 is placed in deionized water, and after the spectrum is stable, the concentrations of copper ion and iron ion in the test solution are both 0.2 mg / L, the spectrum is waited to be stable, the data is recorded, and the experiment is repeated until the concentrations of copper ion and iron ion in the test solution are both 0.8 mg / L, the data is recorded and saved, the wavelength shift of the selected trough is calculated, a column chart is made, and linear fitting is performed. Repeat the above experimental operation, measure the wavelength shift of the sensing Fabry-Perot interferometer 2 in the copper ion and lead ion mixed solution and the copper ion and magnesium ion mixed solution respectively, make a column chart, and linearly fit it. Explore the response characteristics of the sensor in the presence of other metal ion interference. For example Figure 7 , Figure 8 In the concentration range of 0-0.8 mg / L, the wavelength shift of the sensing Fabry-Perot interferometer 2 in the copper ion and iron ion mixed solution, the copper ion and lead ion mixed solution and the copper ion and magnesium ion mixed solution is approximately equal to the wavelength shift of the sensing Fabry-Perot interferometer 2 in the copper ion solution. After linear fitting of these data, the response sensitivity of the sensor in the presence of other ion interference is also approximately equal to the response sensitivity of the sensor only to copper ions, indicating that even if there are other metal ions in the test solution, the performance of the sensor, i.e. the wavelength shift and the response sensitivity, is similar to that when there is only copper ion in the test solution, indicating that the sensor can accurately identify copper ions and the detection result is accurate and reliable. The parallel double-cavity optical fiber copper ion sensor of the present application has high selectivity.

[0099] Example Two

[0100] The present application provides a detection method of a parallel double-cavity optical fiber copper ion sensor according to any embodiment of the present application, comprising the following processes:

[0101] Step 1, place the sensing Fabry-Perot interferometer 2 in the test solution, and place the reference Fabry-Perot interferometer 3 outside the test solution without contacting the test solution. The optical fiber coupler 1 is connected to the spectrometer.

[0102] Step 2, after a period of time, the optical vernier effect is generated between the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3, and the interference spectrum of the sensing Fabry-Perot interferometer 2 and the reference Fabry-Perot interferometer 3 is superimposed through the fiber coupler 1, and then the interference spectrum of the sensor is displayed by the spectrometer. Wait until the interference spectrum is stable and no longer moves.

[0103] Step 3, record the wavelength shift of the corresponding trough in the interference spectrum of the sensor.

[0104] Step 4, the wavelength shift of the trough is brought into the fitting curve of the relationship between the wavelength shift and the copper ion concentration, and the copper ion concentration in the solution to be measured can be obtained.

[0105] The fitting curve formula of the relationship between the wavelength shift and the copper ion concentration is:

[0106] y = sC (Cu2+)

[0107] Wherein, y represents the wavelength shift, C (Cu2+) represents the copper ion concentration, and s represents the corresponding sensitivity of the sensor.

[0108] Step 5, the sensing Fabry-Perot interferometer 2 is taken out of the solution to be measured and placed in deionized water for desorption, and can be reused after the interference spectrum is stable.

[0109] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: the technical solutions recorded in the foregoing embodiments are modified, or some or all of the technical features are replaced, without making the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A parallel dual-cavity fiber-optic copper-ion sensor, characterized in that, The application relates to a fiber coupler (1), a sensing Fabry-Perot interferometer (2) and a reference Fabry-Perot interferometer (3). The sensing Fabry-Perot interferometer (2) comprises a first single-mode optical fiber (201), a first capillary glass tube (202) and a hydrogel (203). One end of the first single-mode optical fiber (201) is connected with the fiber coupler (1), and the other end is inserted into the first capillary glass tube (202); the end faces of the first single-mode optical fiber (201) and the first capillary glass tube (202) are aligned, and the end faces of the first single-mode optical fiber (201) and the first capillary glass tube (202) are provided with the hydrogel (203); the hydrogel (203) is a sodium alginate / graphene oxide hydrogel. The reference Fabry-Perot interferometer (3) comprises a second single-mode optical fiber (301), a second capillary glass tube (302) and a third single-mode optical fiber (303). One end of the second single-mode optical fiber (301) is connected with the fiber coupler (1), and the other end is inserted into the second capillary glass tube (302); the third single-mode optical fiber (303) is also inserted into the second capillary glass tube (302); the third single-mode optical fiber (303) is arranged opposite to the second single-mode optical fiber (301), and a gap is left between the third single-mode optical fiber (303) and the second single-mode optical fiber (301). The preparation method of the sodium alginate / graphene oxide hydrogel is as follows:

2. The parallel dual-cavity fiber-optic copper-ion sensor according to claim 1, wherein, 0.3-0.4g of graphene oxide is added into 37-50mL of deionized water, and the mixture is stirred for more than 1h by using a magnetic stirrer and is ultrasonically cleaned for more than 1h by using an ultrasonic cleaning machine to obtain a uniformly dispersed graphene oxide solution; 0.8-1.0g of sodium alginate is added into 35-45mL of deionized water, and the mixture is stirred for more than 1h by using a magnetic stirrer to obtain a sodium alginate sol; 80-90mg of graphene oxide suspension is further added into the sodium alginate hydrogel, and the mixture is stirred for more than 2h to obtain a sodium alginate / graphene oxide sol; 5mL of Cu(NO3)2 solution with a concentration of 1g / L is added into the sodium alginate / graphene oxide sol, and the mixture is stirred for more than 20min to obtain a yellowish sol; the common end faces of the first single-mode optical fiber (201) and the first capillary glass tube (202) are dipped in the sol, and the mixture is placed in a CaCl2 solution with a mass fraction of 5% for more than 3h; after the sensing Fabry-Perot interferometer (2) is washed with deionized water to remove the excess copper ions, the sodium alginate / graphene oxide hydrogel which is specific to copper ions is obtained. The free spectral range of the sensing Fabry-Perot interferometer (2) is:

3. The parallel dual-cavity fiber-optic copper-ion sensor according to claim 1, wherein, The reflected light intensity of the sensing Fabry-Perot interferometer (2) is represented as: ； wherein, represents the light intensity of the light reflected back to the first single-mode optical fiber (201) at the interface between the first single-mode optical fiber (201) and the hydrogel (203), represents the light intensity of the light reflected back to the first single-mode optical fiber (201) at the interface between the hydrogel (203) and the solution to be measured, represents the refractive index of the hydrogel (203), represents the thickness of the hydrogel (203), is the initial phase; When the intensity of the reflected light in the interference spectrum of the sensing Fabry-Perot interferometer (2) reaches a minimum , the wavelength of the reflection spectrum valley is expressed as: ; wherein λm represents the wavelength of the reflection spectrum valley, and m is the interference peak number. When the concentration of copper ions in the outside environment changes, the change amount of the corresponding wave trough of the sensing Fabry-Perot interferometer (2) is represented as: ； wherein denotes the free spectral range of the known sensing Fabry-Perot interferometer (2); The wavelength of the reflection spectrum wave trough of the reference Fabry-Perot interferometer (3) is represented as: wherein, represents the amount of change in the corresponding trough of the sensor Fabry-Perot interferometer (2); The free spectral range of the reference Fabry-Perot interferometer (3) is represented as: ； wherein denotes the wavelength of the reflection spectrum minimum of the reference Fabry-Perot interferometer (3), n is the air refractive index, is the air cavity length; The relationship between the free spectral ranges of the sensing Fabry-Perot interferometer (2) and the reference Fabry-Perot interferometer (3) is: ； wherein, denotes the free spectral range of the reference Fabry-Perot interferometer (3); The free spectral ranges of the sensing Fabry-Perot interferometer (2) and the reference Fabry-Perot interferometer (3) are different by 5.7-9.1nm. 。 4. The parallel dual-cavity fiber-optic copper-ion sensor according to claim 3, wherein, The application further discloses a preparation method of the sodium alginate / graphene oxide hydrogel.

5. A method of detection of a parallel dual-cavity fiber-optic copper ion sensor according to any one of claims 1 to 4, characterized in that, ​ Step 1, the sensing Fabry-Perot interferometer (2) is placed in the solution to be measured, and the reference Fabry-Perot interferometer (3) is placed outside the solution to be measured and does not contact the solution to be measured; the fiber coupler (1) is connected to the optical spectrometer; Step 2, after standing for a period of time, the sensing Fabry-Perot interferometer (2) and the reference Fabry-Perot interferometer (3) produce an optical vernier effect, and the interference spectra of the sensing Fabry-Perot interferometer (2) and the reference Fabry-Perot interferometer (3) are superimposed through the fiber coupler (1), and then the interference spectrum of the sensor is displayed by the optical spectrometer; Step 3, the wavelength drift amount corresponding to the wave trough in the interference spectrum of the sensor is recorded; Step 4, the wavelength drift amount of the wave trough is brought into the fitting curve of the relationship between the wavelength drift amount and the copper ion concentration to obtain the copper ion concentration in the solution to be measured.

6. The detection method according to claim 5, characterized in that, The fitting curve formula of the relationship between the wavelength drift amount and the copper ion concentration is: y = sC (Cu2+) where y represents the wavelength shift amount, C (Cu2+) represents the copper ion concentration, and s represents the corresponding sensitivity of the sensor.

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