Electrode for measuring ocean pCO2 and preparation method thereof

Abstract

The invention discloses an electrode for measuring ocean pCO2 as well as a preparation method and application of the electrode. The electrode comprises an acrylic plate, an iridium oxide electrode and an Ag / AgCl reference electrode, the acrylic plate is provided with two grooves which are communicated up and down, the iridium oxide electrode and the Ag / AgCl reference electrode are transversely arranged in the upper groove and the lower groove respectively, the grooves are filled with bicarbonate buffer solutions, and the surface of the upper groove is covered with a PTFE breathable film in an adhesive mode; the iridium oxide electrode is prepared by adopting a carbonate melting oxidation technology, so that the iridium oxide electrode has excellent response capability, and shows extremely small drift and longer service life in long-term use. The electrode for measuring ocean pCO2 has the advantages of being simple in structure, small in potential drift, high in response speed, long in service life and the like, is suitable for measuring the pCO2 value of seawater, and has important significance and application value in the field of ocean environment monitoring.

Description

Technical Field

[0001] The invention relates to the technical field of marine environment monitoring and preparation of metal oxides, and in particular to an electrode for measuring marine pCO2 and a preparation method thereof. Background Art

[0002] The ocean's continuous absorption of excess atmospheric carbon dioxide (CO2) will lead to significant changes in the seawater carbonate system, posing a huge threat to a variety of marine organisms and even ecosystems that rely on the stability of the chemical environment. Therefore, high-quality seawater carbon dioxide partial pressure (pCO2) measurement products are urgently needed to accurately assess the ocean's carbon source and sink, and play an important role in predicting future changes in the ocean's ability to absorb anthropogenic CO2 emissions.

[0003] At present, the most widely used Severinghaus type pCO2 electrode is a typical membrane-covered sensor using the potentiometric method. Its structure consists of a pH electrode, a reference electrode, an electrolyte solution and a gas-permeable membrane. When external CO2 diffuses through the gas-permeable membrane into the electrolyte solution containing the pH electrode and the reference electrode, the pH value of the solution will change, and the potential change is proportional to the pH change of the electrolyte solution, and therefore is also proportional to the pCO2 value. Based on the structure and basic working principle of the Severinghaus type electrode, the response speed of the pCO2 electrode is mainly affected by two key processes: the diffusion of CO2 through the gas-permeable membrane and the chemical reaction in the internal bicarbonate solution. The response rate depends on the slowest step, and as the membrane thickness increases, the control step will change from hydration to diffusion. Therefore, in order to ensure the rapid response and high sensitivity of the electrode, the thickness of the gas-permeable membrane must be optimized, and the composition and concentration of the internal electrolyte solution must be precisely controlled to prevent the extension of the response time and the reduction of the detection limit caused by the diffusion of CO2.

[0004] In addition, the sensing performance of the internal pH electrode and the stability of the reference electrode will also have an important impact on the performance of the pCO2 electrode. The internal pH electrodes currently used to prepare pCO2 electrodes include ion-selective liquid membrane microelectrodes (LMMs), photoelectrodes, and ISFET sensors. However, these miniaturized electrodes have inherent problems in several important aspects, such as manufacturing, cost, stability, and reliability. For example, for LMMs, a smaller tip diameter is required to improve the response of the electrode under low pCO2, but the construction of a pCO2 microelectrode with an internal pH electrode and a small tip diameter is difficult and has a low success rate; due to the LIX membrane with both an external tip gas permeable membrane and an internal pH electrode tip, the response time may be prolonged; if the LIX membrane leaks, it may also cause potential drift and shorten the electrode life. Therefore, there is currently no completely successful micro pCO2 electrode, and most of the difficulties faced during use can be attributed to the internal pH electrode. Therefore, it is particularly important to simplify the structure and manufacturing process of the pCO2 electrode, and the cost needs to be further reduced. Summary of the invention

[0005] The object of the present invention is to provide an electrode for measuring ocean pCO2 and a preparation method thereof in view of the deficiencies of the prior art.

[0006] The present invention first provides an electrode for measuring ocean pCO2, the electrode comprising an acrylic plate, a first groove is arranged at the center of the upper surface of the acrylic plate, a second groove is connected directly below the first groove, the first groove and the second groove are each connected to a horizontally arranged cylindrical channel, and the two cylindrical channels are arranged vertically and parallelly;

[0007] The electrode for measuring ocean pCO2 also includes an iridium oxide electrode, an Ag / AgCl reference electrode, an internal electrolyte solution and a breathable membrane, wherein the iridium oxide portion of the iridium oxide electrode is arranged in a first groove, and its filamentous portion is fixed by an upper cylindrical channel; the silver chloride portion of the Ag / AgCl reference electrode is arranged in a second groove, and its silver filamentous portion is fixed by a lower cylindrical channel; the breathable membrane covers the first groove, and the first groove and the second groove are filled with an electrolyte solution.

[0008] According to a preferred embodiment of the present invention, the internal electrolyte solution is prepared by mixing 2-5 mM sodium bicarbonate solution and 1-2 M sodium chloride solution in a volume ratio of 1:1, adding 0.5-1 mg carbonic anhydrase II (from bovine erythrocytes, activity: ≥2000 W-A units / mg protein, Sigma C-3934) per mL of the mixed solution to prepare a buffer solution containing 0.5-1 mg / mL carbonic anhydrase II; the iridium oxide part of the iridium oxide electrode and the silver chloride part of the Ag / AgCl reference electrode are both immersed in the internal electrolyte solution. A sealing rubber ring is provided in the cylindrical channel, which fixes the iridium oxide electrode and the Ag / AgCl reference electrode on the one hand, and prevents the internal electrolyte solution from overflowing from the cylindrical channel on the other hand.

[0009] According to a preferred embodiment of the present invention, the iridium oxide electrode is prepared by the following steps:

[0010] 1) Solder the iridium sheet to the gold wire and polish and clean it with alumina powder;

[0011] 2) The iridium sheet was treated by immersion in piranha solution, then ultrasonically cleaned in ethanol and deionized water and dried;

[0012] 3) mixing lithium carbonate and sodium peroxide and heating and melting them, suspending an iridium sheet in the melt and keeping it for a certain period of time, and soaking the melt in dilute nitric acid (2.8-3.2M) after cooling to obtain a hydrated iridium oxide electrode;

[0013] 4) The iridium oxide electrode is cleaned and dried, and then the gold wire and welding point of the electrode are covered with a polytetrafluoroethylene tube and stored in a buffer solution of pH=4 to activate the iridium oxide electrode.

[0014] Furthermore, in the step 1), the particle size of the aluminum oxide powder is 50 nm to 1.5 μm.

[0015] Furthermore, in step 2), the piranha solution is prepared by mixing concentrated sulfuric acid and hydrogen peroxide in a volume ratio of 3:1.

[0016] Furthermore, in step 3), the molar ratio of lithium carbonate and sodium peroxide is 9:1, and the heating step is to first heat to 100°C, maintain for 60 minutes, and then heat the crucible to 750°C within 240 minutes, at which time iridium is immersed in the melt and maintained at 750°C for 300 minutes.

[0017] The present invention also provides the application of the pCO2 electrode in the quantitative detection of pCO2 values ​​in a marine environment. Specifically, when used, the filamentous part of the iridium oxide electrode and the Ag / AgCl reference electrode are externally connected to an electrochemical workstation, and then the entire pCO2 electrode is immersed in a water body with a known pCO2 concentration. The CO2 in the water body diffuses into the internal electrolyte solution through the gas permeable membrane, changing the pH value of the solution. The electrochemical workstation detects the electrode potential signal corresponding to the pH, and establishes a linear response model between the electrode potential and the pCO2 concentration based on the intrinsic coupling relationship between the parameters of the marine carbonate system; when detecting the water body to be tested (such as natural seawater), the electrochemical workstation detects the electrode potential signal and, in combination with the previously determined linear response model, infers the pCO2 concentration in the measured seawater.

[0018] Compared with the prior art, the electrode for measuring ocean pCO2 (referred to as pCO2 electrode) prepared by the present invention has the following advantages:

[0019] (1) In terms of the overall structural design of the electrode, the present invention combines the iridium oxide electrode and the Ag / AgCl electrode in a more compact design. Grooves and channels are precisely formed on the acrylic plate to fix the two electrodes, and the interior is filled with a bicarbonate solution containing carbonic anhydrase II, and finally a film layer is covered on the groove. This innovative structural design not only simplifies the manufacturing process, but also significantly improves the sensing performance of the pCO2 electrode, providing a more efficient and reliable pCO2 sensing solution. Compared with the prior art, the present invention does not need to construct a pCO2 microelectrode with a small tip diameter glass sleeve, and the structure is simpler, which greatly simplifies the preparation process, saves preparation costs, and has a higher success rate in manufacturing new electrodes.

[0020] (2) The present invention significantly improves the overall performance of the pCO2 electrode by optimizing the design of the internal pH sensitive electrode. Compared with other electrodes that have been put into practice in pCO2 electrodes, the present invention innovatively uses iridium oxide electrodes prepared by carbonate molten oxidation technology, which exhibits more excellent response capabilities, while showing extremely small drift and longer service life in long-term use. This innovative internal electrode design not only speeds up the sensor's response to changes in seawater pCO2, but also ensures the stability and reliability of the sensor in various complex marine environments.

[0021] (3) The present invention innovatively optimizes the internal electrolyte solution formula of the Severinghaus-type pCO2 electrode to meet the detection needs of low-concentration pCO2 in the marine environment. By precisely controlling the concentration of sodium bicarbonate at 2mM, the present invention effectively avoids the risk of CO2 diffusing from the internal solution to the breathable membrane, thereby ensuring the rapid response time and low detection limit of the electrode. In addition, the present invention introduces carbonic anhydrase into the bicarbonate solution, which significantly accelerates the hydration reaction process of CO2, greatly shortens the response time and hysteresis of the electrode, and significantly improves the response slope and overall sensitivity of the electrode. These optimization measures not only improve the detection accuracy of the electrode, but also expand its application range under low pCO2 concentration conditions in the ocean, providing a more accurate and reliable tool for marine environmental monitoring.

[0022] (4) In order to increase the CO2 conductivity while reducing potential interference factors, the present invention accurately controls the thickness of the gas permeable membrane between 35 and 80 μm. This optimization not only ensures the high flux of the membrane, but also maintains the mechanical strength of the membrane, thereby significantly improving the response efficiency and stability of the electrode while avoiding the conversion of the hydration process to diffusion control. This precise thickness control provides a solid guarantee for the rapid response and long-term reliability of the electrode.

[0023] The present invention can rapidly measure pCO2 in seawater and has important significance and application value in the field of marine environment monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the overall structure of the rectangular acrylic plate used for the pCO2 electrode.

[0025] Figure 2 This is the upper surface structure diagram of the rectangular acrylic plate.

[0026] Figure 3 AA cross-sectional view of the upper surface of a rectangular acrylic plate.

[0027] Figure 4 BB cross-section of the upper surface of a rectangular acrylic plate.

[0028] Figure 5 It is the CC cross-section of the upper surface of the rectangular acrylic plate.

[0029] Figure 6 This is a diagram of the completed assembly of the pCO2 electrode obtained in Example 1.

[0030] Figure 7 This is a potential response diagram of the iridium oxide electrode obtained in Example 1 to changes in solution pH.

[0031] Figure 8 This is the potential-pH calibration curve of the iridium oxide electrode obtained in Example 1.

[0032] Fig. 9 This is the potential response diagram of the pCO2 electrode obtained in Example 2 to the change of pCO2 in artificial seawater.

[0033] Fig.10 This is the potential-pCO2 calibration curve of the pCO2 electrode obtained in Example 2.

[0034] Fig.11 This is the potential-logpCO2 calibration curve of the pCO2 electrode obtained in Example 2. DETAILED DESCRIPTION

[0035] The present invention is further described and illustrated below in conjunction with specific embodiments. The embodiments are merely exemplary of the present disclosure and do not define the scope of limitation. The technical features of each embodiment of the present invention may be combined accordingly without conflicting with each other.

[0036] like Figures 1 to 6 As shown, the pCO2 electrode includes an acrylic plate, a first groove is arranged at the center of the upper surface of the acrylic plate, a second groove is connected directly below the first groove, the first groove and the second groove are each connected to a horizontally arranged cylindrical channel, and the two cylindrical channels are arranged vertically and parallelly;

[0037] The electrode for measuring ocean pCO2 also includes an iridium oxide electrode, an Ag / AgCl reference electrode, an internal electrolyte solution and a breathable membrane, wherein the iridium oxide portion of the iridium oxide electrode is arranged in the first groove, and its filamentous portion is fixed by the upper cylindrical channel; the silver chloride portion of the Ag / AgCl reference electrode is arranged in the second groove, and its silver wire portion is fixed by the lower cylindrical channel; the breathable membrane covers the first groove, and the first groove and the second groove are filled with an electrolyte solution. The breathable membrane is made of a PTFE hydrophobic breathable membrane with a membrane thickness of 35 to 80 μm, which completely covers the entire first groove. The first groove is a square groove, and the second groove is a rectangular groove; the depth of the first groove is 1.4 to 1.6 mm; the depth of the second groove is 1.5 to 1.6 mm. The diameters of the two cylindrical channels are both 1.2 to 1.3 mm, and the upper channel is 0.3 to 0.4 mm away from the lower channel.

[0038] In a specific embodiment of the present invention, the acrylic plate is rectangular, 30 mm long, 20 mm wide and 5 mm high. The first groove in the center of the upper surface of the rectangular acrylic plate is a square groove, 5 mm long and 1.4 mm thick. The second groove is a rectangular groove, 5 mm long, 2 mm wide and 1.6 mm thick. The two cylindrical channels on the right side of the rectangular acrylic plate have a diameter of 1.2 mm and a length of 12.5 mm, and the upper channel is 0.3 mm away from the lower channel. The breathable membrane is made of PTFE breathable membrane with a membrane thickness of 75 μm.

[0039] Example 1

[0040] The first step is to make a sheet of iridium oxide electrode

[0041] 1) An iridium sheet with a length of 5 mm and a width of 2.5 mm was heat-welded on a gold wire with a length of 60 mm, and the iridium sheet was polished with 1.5 μm, 0.5 μm and 50 nm alumina powders in sequence, and then washed with deionized water;

[0042] 2) Mix concentrated sulfuric acid and hydrogen peroxide in a volume ratio of 3:1 to obtain a piranha solution; soak the iridium sheet in the piranha solution for 15 minutes;

[0043] 3) The iridium sheet obtained in step 2) was ultrasonically cleaned in anhydrous ethanol and deionized water for 15 min respectively, and dried at 100° C. for 12 h;

[0044] 4) Lithium carbonate and sodium peroxide were mixed in a molar ratio of 9:1 and placed in a gold crucible. The crucible was placed in a muffle furnace and slowly heated from room temperature to 100°C. After maintaining for 60 minutes, the crucible was heated to 750°C within 240 minutes.

[0045] 5) vertically suspending the gold wire of the iridium sheet obtained in step 3) on the crucible described in step 2), immersing the entire iridium sheet in the melt, covering the crucible with a gold crucible cover, and maintaining the crucible at 750° C. for 300 min;

[0046] 6) After cooling to room temperature, the crucible was immersed in a 3M dilute nitric acid solution for 48 hours;

[0047] 7) Rinse the reacted iridium oxide with deionized water several times and dry at 120° C. for 12 h;

[0048] 8) Soaking the iridium oxide electrode dried in step 7) in a buffer solution of pH = 4 for 24 hours;

[0049] 9) using a heat-shrinkable polytetrafluoroethylene tube to cover the gold wire and the gold-iridium welding point of the iridium oxide electrode obtained in step 8) to obtain a sheet-shaped iridium oxide electrode;

[0050] Step 2: Make Ag / AgCl reference electrode

[0051] 1) A 60 mm long silver wire was polished with 1.5 μm, 0.5 μm and 50 nm alumina powders in sequence, and then ultrasonically cleaned in anhydrous ethanol and deionized water for 15 min respectively;

[0052] 2) immersing one end of the silver wire obtained in step 1) into molten silver chloride for 2 seconds, then standing to solidify, and covering the silver wire and the contact point with the silver chloride with a heat shrinkable polytetrafluoroethylene tube to obtain an Ag / AgCl reference electrode;

[0053] Step 3: Assemble the pCO2 electrode

[0054] 1) Insert the silver wire part of the Ag / AgCl reference electrode into the rectangular groove of the acrylic plate, so that the silver wire covered with the heat shrink tube fits tightly into the hole and passes through, and the silver wire part covered with silver chloride remains in the groove;

[0055] 2) Insert the filamentous part of the sheet-like iridium oxide electrode into the square groove on the upper surface of the acrylic plate, so that the gold wire wrapped with the heat shrink tube fits tightly into the hole and passes through, and the sheet-like iridium oxide remains in the groove;

[0056] 3) 4 mM sodium bicarbonate solution and 1 M sodium chloride solution were mixed in a volume ratio of 1:1, and 1 mL of the mixed solution was added with 5 mg CA to prepare a buffer solution containing 5 mg / mL CA;

[0057] 4) The grooves described in steps 1) and 2) are filled with the buffer solution described in step 3), glue is applied around the edges of the square grooves on the upper surface of the acrylic plate, a breathable film is attached, and the mixture is cured under ultraviolet light for 2 minutes to complete the production of the pCO2 electrode.

[0058] from Figure 7 It can be seen that the iridium oxide electrode prepared in this example shows a clear step-like change in the process of pH value from 2 to 10 (and then from 10 back to 2). This shows that the electrode has good responsiveness to changes in pH. The potential shows good stability at each pH value, without obvious drift, and when the pH value returns from 10 to 2, the potential almost returns to the original value. This shows that the response of the electrode is reversible and has good stability and repeatability.

[0059] from Figure 8 It can be seen that the E-pH relationship of the iridium oxide electrode prepared in this embodiment when calibrated from pH 2 to 10 is E = -58.6pH + 842.8mV (R 2 =0.9998), the E-pH relationship when the pH value is calibrated from 10-2 is E=-58.8pH+846.2mV(R 2 =0.9997), the slope in the fitting equation is close to the theoretical value of -59 mV / pH, which is consistent with the prediction of the Nernst equation, indicating that the electrode response to pH is close to ideal. 2 The value is close to 1, indicating that the electrode is very sensitive to changes in pH with a high degree of precision. The potential recovers well when returning from high pH to low pH, indicating good reversibility of the electrode without significant hysteresis. The electrode exhibits high sensitivity, good linear response, stability and reversibility, making it very suitable for use as an internal electrode in a pCO2 sensor.

[0060] Example 2

[0061] 1) 250 mL of artificial seawater was placed in a four-necked flask, each bottle mouth was sealed with a rubber stopper, one rubber stopper was loaded with a pH glass electrode for seawater pH data collection, one was loaded with the pCO2 electrode obtained in Example 1, one was connected to the air inlet duct, and one was connected to the air outlet duct for balancing the internal and external gas pressures. The acrylic plate coating of the pCO2 electrode was partially immersed below the liquid surface, and the gold and silver wires wrapped in the heat shrink tube were respectively connected to the wires, and passed through the rubber stopper to connect to the electrochemical workstation. The air inlet duct was connected below the liquid surface, and the air outlet duct was above the liquid surface.

[0062] 2) Calibration using N2 and CO2: First, introduce N2 until the seawater carbonate system reaches equilibrium, then place the pCO2 electrode, which has been sealed with a rubber stopper, and test the changes in the pH value of seawater and the electrode potential; then introduce N2 / CO2 mixed gas for different lengths, and after the seawater system reaches equilibrium, place the pCO2 electrode, and test the changes in the pH value of seawater and the electrode potential.

[0063] 3) Use the pH and TA values ​​of artificial seawater to calculate the pCO2 value for calibration.

[0064] from Fig. 9 It can be seen that the pCO2 electrode prepared by the present invention can respond quickly and stably in low pCO2 seawater. As the pCO2 value of seawater increases, the response time is prolonged. In about 20 minutes, the lowest pCO2 value that can be detected is 120 μatm, indicating that the electrode is suitable for low pCO2 seawater measurement.

[0065] from Fig.10 The logistic model fitted to the calibration data captures the full range response of the pCO2 electrode. This wide measurement range (120 to 400,000 μatm) can effectively cover the changes from low to high pCO2 concentrations, indicating that the prepared pCO2 electrode can be applied to pCO2 measurement needs in various marine environments. 2 =0.99488, indicating a good fit.

[0066] from Fig.11 It can be seen that the calibration curve of the prepared electrode for seawater with pCO2 varying from 120 to 400000 μatm is E = 51.36 log pCO2 -10.684 mV, R 2 =0.995 indicates that the electrode has good linear correlation. The electrode sensitivity is 51.36mV / decade, indicating that the electrode can respond significantly to changes in pCO2 concentration.

[0067] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. An electrode for measuring ocean pCO2, characterized in that: The electrode comprises an acrylic plate, wherein a first groove is arranged at the center of the upper surface of the acrylic plate, a second groove is connected directly below the first groove, the first groove and the second groove are each connected to a horizontally arranged cylindrical channel, and the two cylindrical channels are arranged vertically and parallelly; The electrode for measuring ocean pCO2 also includes an iridium oxide electrode, an Ag / AgCl reference electrode, an internal electrolyte solution and a breathable membrane, wherein the iridium oxide portion of the iridium oxide electrode is arranged in a first groove, and its filamentous portion is fixed by an upper cylindrical channel; the silver chloride portion of the Ag / AgCl reference electrode is arranged in a second groove, and its silver filamentous portion is fixed by a lower cylindrical channel; the breathable membrane covers the first groove, and the first groove and the second groove are filled with an electrolyte solution.

2. The electrode for measuring ocean pCO2 according to claim 1, characterized in that The internal electrolyte solution is prepared by mixing 2-5 mM sodium bicarbonate solution and 1-2 M sodium chloride solution in a volume ratio of 1:1, and adding 0.5-1 mg carbonic anhydrase II per mL of the mixed solution to prepare a buffer solution containing 0.5-1 mg / mL carbonic anhydrase II; The iridium oxide portion of the iridium oxide electrode and the silver chloride portion of the Ag / AgCl reference electrode are both immersed in the internal electrolyte solution.

3. The electrode for measuring ocean pCO2 according to claim 1, characterized in that The breathable membrane is made of a PTFE hydrophobic breathable membrane with a thickness of 35 to 80 μm, which completely covers the entire first groove.

4. The electrode for measuring ocean pCO2 according to claim 1, characterized in that The first groove is a square groove, and the second groove is a rectangular groove; the depth of the first groove is 1.4-1.6 mm; the depth of the second groove is 1.5-1.6 mm.

5. The electrode for measuring ocean pCO2 according to claim 1, characterized in that The diameters of the two cylindrical channels are both 1.2-1.3 mm, and the distance between the upper channel and the lower channel is 0.3-0.4 mm.

6. The electrode for measuring ocean pCO2 according to claim 1, characterized in that The iridium oxide electrode is prepared by the following steps: 1) Solder the iridium sheet to the gold wire and polish and clean it with alumina powder; 2) The iridium sheet was treated by immersion in piranha solution, then ultrasonically cleaned in ethanol and deionized water and dried; 3) mixing lithium carbonate and sodium peroxide and heating and melting them, suspending an iridium sheet in the melt and keeping it for a certain period of time, and soaking the melt in dilute nitric acid after cooling to obtain a hydrated iridium oxide electrode; 4) The iridium oxide electrode is cleaned and dried, and then the gold wire and welding point of the electrode are covered with a polytetrafluoroethylene tube and stored in a buffer solution of pH=4 to activate the iridium oxide electrode.

7. The method for preparing an iridium oxide electrode according to claim 6, wherein: In the step 1), the particle size of the aluminum oxide powder is 50 nm to 1.5 μm.

8. The method for preparing an iridium oxide electrode according to claim 6, wherein: In the step 2), the piranha solution is prepared by mixing concentrated sulfuric acid and hydrogen peroxide in a volume ratio of 3:

1.

9. The method for preparing an iridium oxide electrode according to claim 6, wherein: In the step 3), the molar ratio of lithium carbonate and sodium peroxide is 9:1, and the heating step is to first heat the temperature to 90-110°C, maintain it for 60-90 minutes, and then heat the crucible to 720-780°C within 220-260 minutes. At this time, iridium is immersed in the melt and maintained at 720-780°C for 300-360 minutes.

10. Use of the pCO2 electrode as claimed in claims 1 to 9 for quantitative detection of pCO2 values ​​in a marine environment.

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