CN110133072B - Trace phosphate and pH combined detector and method thereof - Google Patents
Trace phosphate and pH combined detector and method thereof Download PDFInfo
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Abstract
The invention discloses a trace phosphate and pH combined detector and a method thereof, belonging to the field of electrochemistry. The detector comprises a double-channel screen printing electrode, wherein two working electrodes, a reference electrode and a counter electrode are arranged on a substrate of the double-channel screen printing electrode, and the four electrodes are respectively connected with a contact through wiring; the reference electrode and the counter electrode are arranged at intervals, two areas for placing the working electrodes are clamped between the reference electrode and the counter electrode, and the two working electrodes share one reference electrode and one counter electrode; in the two working electrodes, the surface of the first working electrode is modified with a pH sensitive nano material, and the surface of the second working electrode is modified with a phosphate sensitive nano material. The joint detector can simultaneously detect the pH value and the phosphate concentration of the solution, and can ensure the accuracy of the detection of the phosphate concentration by predetermining whether the pH value of the solution is strong acid or not. The invention has the advantages of realizing in-situ detection of phosphate and pH, having sensitivity, stability and reproducibility.
Description
Technical Field
The invention relates to the field of electrochemical detection, in particular to a trace phosphate and pH combined detector and a method thereof.
Background
The phosphate is one of the most important inorganic salt components in an ecological system, can provide necessary nutrients and nutrient elements for agricultural production and growth and development of animals and plants, and can also cause a series of adverse effects such as eutrophication pollution on a water environment due to excessive content. Therefore, it is very critical to develop a method for rapidly and sensitively detecting the phosphate concentration in different environments including water and soil.
The current standard method for detecting phosphate in laboratory is colorimetry, namely ammonium molybdate and ascorbic acid in a certain proportionThe acid and antimony (III) are added sequentially to the sample to be tested containing orthophosphate, followed by the formation of a blue phosphomolybdate complex. The method has a minimum detection limit of 0.01mg L-1. In addition, chromatography, optical fluorescence and spectrophotometry can also be used for detecting phosphate, but these techniques have high requirements on the operation of workers in the detection process, and the instrument cost is high, so that the in-situ detection of phosphate ions is not facilitated.
In recent years, with the development of electrochemical detection technology, more and more substances are rapidly detected in situ by an electrolytic method, a potential method and an electric conduction method. Therefore, the chemical analysis of phosphate ions by electrochemical voltammetry has a certain market potential. The principle of electrochemical detection of phosphate is mainly based on the complex reaction of phosphate ions and molybdate, and can generate a complex with electrochemical activity, so as to indirectly obtain the concentration of the phosphate ions, and the specific reaction equation (1) is as follows:
[ PMo ] in complexes12O40]3-The novel electrochemical detection scheme has good electrochemical activity, ions taking Mo as a center can generate oxidation-reduction reaction under the drive of potential change in cyclic voltammetry after being adsorbed on the surface of a working electrode, so that an obvious oxidation-reduction peak shape (equation 2) is presented in a scanned image, and compared with the currently commonly used inorganic phosphorus colorimetric detection method, the novel electrochemical detection scheme has lower detection limit and higher detection speed.
H3PMo(VI)i2O40+ reducing agent → [ H →4PMo(VI)sMo(V)4O40]3-(2)
The inorganic phosphate has different existing forms in the aqueous solution due to different pH values
Is present in an aqueous solution. In an acidic environment with H3PO4、H2PO4 -Exists in a structure of (A), and HPO4 2-It is its main ionic structure in alkaline environment, and as for neutral conditions, there are three forms. In order to allow the response peak current exhibited in the electrochemical reaction to fully reflect the presence of all soluble phosphate ions in the sample, it is necessary to adjust the pH of the electrolyte solution to conditions where the orthophosphate activity is more suitable for the complexation reaction to occur. Therefore, if the phosphate and the pH can be detected in a combined electrochemical manner, the method has a good effect on detecting trace phosphate ions by an electrochemical voltammetry method.
Disclosure of Invention
The invention aims to solve the problems in the prior art and provide a trace phosphate and pH combined detector which can simultaneously detect the pH and the phosphate concentration of a solution and ensure the accuracy of the detection of the phosphate concentration by predetermining whether the pH value of the solution is strong acid or not.
The technical scheme adopted by the invention is as follows:
a joint detector for trace phosphate and pH comprises a double-channel screen printing electrode, wherein two working electrodes, a reference electrode and a counter electrode are arranged on a substrate of the double-channel screen printing electrode, and the four electrodes are respectively connected with a contact through wiring; the reference electrode and the counter electrode are arranged at intervals, two symmetrical regions for placing the working electrodes are clamped between the reference electrode and the counter electrode, and the two working electrodes share one reference electrode and one counter electrode; in the two working electrodes, the surface of the first working electrode is modified with a pH sensitive nano material, and the surface of the second working electrode is modified with a phosphate sensitive nano material.
Preferably, the pH sensitive nanomaterial is a nano metal oxide.
Further, the nano metal oxide comprises nano ruthenium dioxide, nickel oxide, manganese oxide, cobalt oxide, lead oxide and titanium oxide.
Preferably, the phosphate sensitive nano material is a ZrO 2-ZnO/multi-walled carbon nanotube/ammonium heptamolybdate tetrahydrate nano composite material, and the preparation method comprises the following steps:
1) reduction of Zn (Ac)2And ZrOCl2Powder is mixed according to the mass ratio of 1: 1 is dissolved in 2 wt.% acetic acid solution, then 2.0 wt.% chitosan is slowly added in the water bath environment at 80 ℃, the pH value of the solution is adjusted to 11.0-12.0, and the water bath at 80 ℃ is continuously maintained for 1 hour; the resulting precipitate was then washed, dried and the solid was placed in a muffle furnace at 25 ℃ for h-1Heating to 350 deg.C for 1 hr, and heating at 25 deg.C for another hour-1The temperature is raised to 450 ℃ and the high-temperature oxidation is carried out for 1 hour; after cooling the solid, ZrO is obtained2-a ZnO nanocomposite;
2) subjecting the ZrO to2Adding the-ZnO nano composite material into ethanol, and performing ultrasonic oscillation to form 300mg L-1The nano dispersion liquid of (4); then according to 100mg L-1Adding the pretreated multi-walled carbon nanotube, and heating for 24 hours in a water bath environment at 60 ℃ to obtain a suspension; after the suspension had cooled to room temperature, 0.064mol L of water was added to the suspension-1Ammonium heptamolybdate tetrahydrate and 0.1% Nafion, and uniformly mixing to obtain ZrO2-modification liquid of ZnO/multi-walled carbon nanotube/ammonium heptamolybdate tetrahydrate nanocomposite.
Preferably, the modifying liquid is modified on the surface of the second working electrode in a drying mode after being dripped.
Preferably, the pretreatment process of the multi-walled carbon nanotube is as follows: the multi-walled carbon nanotubes were refluxed in concentrated HNO3 for 5 hours.
Preferably, the wiring is a conductive silver layer, the working electrode is a carbon electrode, the reference electrode is an Ag/AgCl electrode or a carbon electrode, the counter electrode is a platinum wire electrode or a silver electrode, and the substrate is a PET substrate.
Another objective of the present invention is to provide a method for jointly detecting a trace amount of phosphate and pH by using the detector according to any of the above embodiments, which is used for simultaneously detecting the pH and the phosphate concentration of a solution to be detected, and comprises the following steps:
1) quantitatively dripping a sample solution to be detected on the surface of a three-electrode system containing a first working electrode, and detecting the pH value of the sample solution to be detected by an electrochemical method;
2) quantitatively mixing a sample solution to be tested with a strong acid electrolyte with the pH value less than or equal to 1 to obtain a test solution;
3) quantitatively dripping the test solution on the surface of a three-electrode system containing a second working electrode, and carrying out potential scanning to obtain the oxidation-reduction peak current of the phosphomolybdate complex; and obtaining the phosphate concentration in the sample solution to be detected according to the linear relation between the current and the phosphate concentration.
Another objective of the present invention is to provide a method for jointly detecting a trace amount of phosphate and pH by using the detector according to any of the above embodiments, for detecting the phosphate concentration in a solution to be detected, which comprises the following steps:
1) quantitatively mixing a sample solution to be tested with a strong acid electrolyte to obtain a test solution;
2) quantitatively dripping the test solution obtained in the step 1) onto the surface of a three-electrode system containing a first working electrode, detecting the pH value of the test solution by an electrochemical method, and determining that the pH value of the solution is less than or equal to 1;
3) quantitatively dripping the test solution with the pH value less than or equal to 1 on the surface of a three-electrode system containing a second working electrode, and carrying out potential scanning to obtain the oxidation-reduction peak current of the phosphomolybdate complex; and obtaining the phosphate concentration in the sample solution to be detected according to the linear relation between the current and the phosphate concentration.
Wherein the pretreatment of the sample solution to be tested is determined according to the practice. For example, for a water sample, it is necessary to filter the water sample in advance and use the filtrate for subsequent detection. If the sample to be detected is a soil sample, the phosphate in the soil sample needs to be extracted in advance, and the method comprises the following specific steps: drying and sieving a soil sample, quantitatively adding the soil sample into a centrifugal tube, and performing oscillation extraction by using acetic acid as an extracting agent; and (3) performing solid-liquid separation on the extracting solution by using an ultracentrifuge, and filtering the centrifuged supernatant to obtain a sample solution to be detected.
Further, the time of shaking extraction is preferably 1 h.
Further, the method comprisesThe concentration of the acetic acid is preferably 0.50mol L-1。
The invention has the beneficial effect of providing the trace phosphate and pH combined detector which is provided with a double-channel screen printing electrode and can be used as portable equipment to independently detect two independent pH values and phosphate values. Moreover, tests prove that in a strong acid environment, phosphate can be converted into an orthophosphate form to the maximum extent, and a phosphomolybdate complex form is further formed to carry out subsequent oxidation-reduction reaction, so that whether the pH value of the solution to be detected is strong acid or not can be determined in advance by using the joint detector before detecting phosphate ions, and measurement errors are reduced.
In addition, a novel modified material ZrO is adopted on the working electrode of the detector aiming at the detection of phosphate ions2-ZnO/MWCNTs composite nano material. ZrO (ZrO)2The Zn composite nano-particles have large specific surface area and rich hydroxyl functional groups, the material shows good conductivity after being doped with the multi-wall carbon nano-tube, and the working electrode shows excellent sensitivity, selectivity and stability for phosphomolybdate complex in the potential scanning process. The formed characteristic oxidation reduction peak is complete and the current peak value thereof effectively proves the occurrence of the complex reaction of the phosphate and the molybdate and the content of reaction products, thereby indirectly obtaining the corresponding addition concentration of phosphate ions. The electrode modified by the material is 0.2mol L-1H2SO4In a KCl (pH 1.0) electrolyte solution, the lowest detection limit of phosphate ions reaches 2 x 10-8mol L-1。
The invention has the beneficial effect of providing a joint detection method of trace phosphate and pH. Aiming at the detection of phosphate ions, provides a novel modified material ZrO2-ZnO/MWCNTs/AMT composite nano material. ZrO (ZrO)2The Zn composite nano-particles have large specific surface area and rich hydroxyl functional groups, the material shows good conductivity after being doped with the multi-wall carbon nano-tube, and the working electrode shows excellent sensitivity, selectivity and stability for phosphomolybdate complex in the potential scanning process. The characteristic redox formed therebyThe occurrence of the complex reaction of the phosphate and the molybdate and the content of a reaction product are effectively proved by the complete peak value and the current peak value, so that the corresponding addition concentration of phosphate ions is indirectly obtained.
Drawings
FIG. 1 is a schematic structural diagram of a trace phosphate and pH combined detector according to the present invention;
FIG. 2 is a design drawing of a dual-channel silk screen electrode screen in the present invention;
FIG. 3 is a scanning electron microscope image of various materials in the present invention; wherein (A) to (C) are each ZrO2-ZnO nanocomposite (A), ZrO2-ZnO Individual modified Screen-printed electrode (B) and ZrO2-ZnO/MWCNTs/AMT composite modification screen printing electrode surface (C);
FIG. 4 is ZrO23.7 multiplied by 10 added to-ZnO/MWCNTs/AMT composite modified electrode-7μmol L-1H2PO4-(a) And no addition of H2PO4-The cyclic voltammetry electrochemical behavior under the condition (b) (-0.067v is the oxidation peak of the change of valence state of molybdenum).
Detailed Description
The advantages and features of the present invention will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention taken in conjunction with the accompanying drawings, the scope of which is defined more clearly.
Example 1
As shown in fig. 1, the core structure of the combined trace phosphate and pH detector in this embodiment is a dual-channel screen-printed electrode. The double-channel screen printing electrode is provided with two working electrodes, a reference electrode and a counter electrode on a substrate, and the four electrodes are respectively connected with contacts through wiring. The reference electrode is formed by two 1/4 circles in a tangent connection mode, the counter electrode is formed by two 1/2 circles in a tangent connection mode, the reference electrode and the counter electrode are arranged at a certain distance, two mirror symmetry areas for placing the working electrodes are formed by clamping the reference electrode and the counter electrode, the two working electrodes share one reference electrode and one counter electrode, and two three-electrode systems are formed. The two working electrodes can be divided into a first working electrode and a second working electrode, wherein the surface of the first working electrode is modified with a pH sensitive nano material, and the surface of the second working electrode is modified with a phosphate sensitive nano material. Wherein the first working electrode is useful for pH detection; the second working electrode can be used for detecting phosphate, and the two working electrodes are matched to carry out two independent pH and phosphate detection.
The material used for the printed electrode can be adjusted as required, in this embodiment, the wiring is a conductive silver layer, the working electrode is a carbon electrode, the reference electrode is an Ag/AgCl electrode (a carbon electrode can also be used), the counter electrode is a platinum wire electrode (a silver electrode can also be used), and the substrate is a PET substrate. The printing electrode needs four screen printing plates in the printing process, wherein the four screen printing plates are respectively a conductive silver layer, a carbon electrode, an Ag/AgCl reference, a platinum wire electrode and an insulating layer, and the screen printing plates are designed as shown in figure 2. The modification material on both working electrodes may be selected as desired. For example, the pH sensitive nanomaterial may be a nano metal oxide, including nano ruthenium dioxide, nickel oxide, manganese oxide, cobalt oxide, lead oxide, titanium oxide, and the like, and the modification process may be performed in any feasible manner in the prior art, such as spraying, doping, and the like. The phosphate sensitive nano material can also adopt the existing phosphate sensitive film in the prior art as required, and ZrO in the subsequent embodiments2ZnO/MWCNTs/AMT nanocomposites are just one of the preferred implementations.
In the above detector, the first working electrode may be used to detect the pH of an original solution to be detected, or may be used to detect the pH of an acidified solution to be detected, so as to ensure that the solution to be detected added to the surface of the second working electrode is in a strongly acidic environment, so that the response peak current exhibited in the electrochemical reaction can completely reflect the presence of all soluble phosphate ions in the sample. Therefore, based on the detector, two using methods can be designed.
The first trace phosphate and pH combined detection method is used for simultaneously detecting the pH and the phosphate concentration in a solution to be detected, and comprises the following steps:
1) and dropwise adding the sample solution to be detected to the surface of a three-electrode system containing a first working electrode, and detecting the pH value of the sample solution to be detected according to the potential through an electrochemical workstation externally connected to the contact.
2) Then quantitatively mixing the sample solution to be tested with strong acid electrolyte (preferably pH is less than or equal to 1) to make the mixed solution also be strong acid. Then quantitatively dripping the test solution with the pH value less than or equal to 1 on the surface of a three-electrode system containing a second working electrode, and carrying out potential scanning by adopting a cyclic voltammetry or a square wave voltammetry (considering the peak appearance condition of trace phosphate, preferably the square wave voltammetry) to obtain the oxidation-reduction peak current (-0.067V) of the phosphomolybdate complex, wherein the same applies below; and converting according to the linear relation between the current and the phosphate concentration to obtain the phosphate concentration in the solution of the sample to be detected.
The second trace phosphate and pH combined detection method is used for accurately detecting the phosphate concentration in a solution to be detected, and comprises the following steps:
1) quantitatively mixing a sample solution to be tested with a strong acid electrolyte (preferably, pH is less than or equal to 1) to obtain a test solution;
2) quantitatively dripping the test solution obtained in the step 1) onto the surface of a three-electrode system containing a first working electrode, detecting the pH value of the test solution by an electrochemical method, and confirming that the pH value of the solution is less than or equal to 1. If the pH is less than or equal to 1, the next step is carried out, otherwise, acid is continuously added according to the pH to reduce the pH.
3) Quantitatively dripping the test solution with the pH value less than or equal to 1 on the surface of a three-electrode system containing a second working electrode, and performing potential scanning by adopting a cyclic voltammetry or a square wave voltammetry (considering the peak appearance condition of trace phosphate, preferably the square wave voltammetry) to obtain the oxidation-reduction peak-to-peak current of the phosphomolybdate complex; and converting according to the linear relation between the current and the phosphate concentration to obtain the phosphate concentration in the solution of the sample to be detected.
The first method is suitable where simultaneous determination of the pH and phosphate concentration of a sample is required.
The second mode is suitable for in-situ rapid quantitative detection of phosphate concentration in a field environment, the pH of a water sample to be detected cannot be accurately determined in a non-laboratory environment, and the pH of the water sample has volatility, so that the acid adding amount cannot be accurately controlled. In addition, the phosphate in the water sample is trace, and if the phosphate is lower than the detection limit, the mixing ratio of the water sample and the electrolyte may need to be increased, so that the acidity of the mixed solution is easy to be insufficient. However, the trace phosphate and pH combined detector can determine that the test solution is already in a strong acid environment when the second working electrode is added, and ensure the accuracy of the measurement result.
In both methods, the linear relationship existing between the current and the phosphate concentration allows determination of different dihydrogen phosphate ions H in the same manner2PO4-Standard solution was obtained.
In addition, in the two methods, the sample solution to be measured may be directly dropped on the surface of the second working electrode for cyclic voltammetry measurement, or the sample solution to be measured may be dropped on the surface of the second working electrode first, and after drying, the strongly acidic electrolyte solution may be dropped for measurement. If the concentration of phosphate in the sample is too low, a certain amount of the solution to be tested containing phosphate is firstly dripped on the working electrode, dried and then placed in the electrolyte solution for potential scanning, which is a preferred recommended method. This is because the detected phosphate is a trace amount and is not easy to peak when directly mixed in the electrolyte and subjected to potential scanning by cyclic voltammetry. And after the membrane is dried in advance, potential scanning is carried out in the electrolyte solution, so that peaks can be better generated, and the measurement accuracy is improved. However, pre-drying can increase the time required for testing, and is not recommended for use in large-scale sampling. In addition, for trace phosphate, the solution to be detected can be quantitatively mixed with the electrolyte solution, then the mixed solution is directly dripped onto the three-electrode system or the three-electrode system is placed in the mixed solution, and the potential scanning by adopting square wave voltammetry is easier to generate peaks compared with cyclic voltammetry.
Example 2
In this example, a combined detector of trace phosphate and pH designed based on example 1 further provides a phosphate sensitive nanomaterial, i.e., ZrO 2-ZnO/multiwall carbon nanotube/ammonium heptamolybdate tetrahydrate (ZrO)2-ZnO/MWCNTs/AMT) nanocomposite.
In this example, ZrO was prepared2The process of the-ZnO/MWCNTs/AMT nanocomposite material is as follows:
1) preparation of ZrO2-ZnO nanocomposite
1.5g of Zn (Ac)2And 1.5g of ZrOCl2Dissolving the powder (mass ratio is 1: 1) in 30ml of 2 wt.% acetic acid solution, slowly adding chitosan under the environment of 80 ℃ water bath to enable the chitosan concentration to be 2.0 wt.%, dropwise adding 30% NaOH solution under the condition of continuous stirring, adjusting the pH of the solution to 11.0-12.0, and continuously maintaining the 80 ℃ water bath for 1 hour to generate white precipitate in the solution.
The white precipitate produced by filtration was then washed repeatedly with deionized water and ethanol and placed in an oven and dried at 100 ℃. After the precipitate is completely dried, putting the solid precipitate into a muffle furnace, and heating for 25 ℃ for h-1The temperature is raised to 350 ℃ and heated for 1 hour at constant temperature, and then the temperature is continuously raised to 450 ℃ at the same temperature raising rate for 1 hour of high-temperature oxidation. After the solid is completely cooled, ZrO is obtained2-ZnO nanocomposites.
2) Preparation of ZrO2- -ZnO/MWCNTs/AMT nano composite modified material
A certain amount of the ZrO prepared as described above2Adding the-ZnO nano composite material into ethanol, and performing ultrasonic oscillation for more than 1h to form 300mg L-1ZrO of2-ZnO nanodispersion. Pretreating the multi-wall carbon nano-tube: refluxing multi-wall carbon nanotubes (avastin, diameter 3-5nm, length 50 μm) in concentrated HNO3 for 5 hr to activate its molecular activity, and adding pretreated multi-wall carbon nanotubes (MWCNTs) into dispersion with MWCNTs concentration of 100mg L-1And continuously heating for 24 hours under the water bath environment at the temperature of 60 ℃ to obtain a suspension. After the suspension was cooled to room temperature, 0.064mol L of the suspension was added to increase the stability of the post-modification film-1Ammonium heptamolybdate tetrahydrate (AMT) and 0.1% Nafion (AMT and Nafion are dissolved in ethanol in advance and then added), and ZrO is obtained after uniform mixing2-modification liquid of ZnO/multi-walled carbon nanotube/ammonium heptamolybdate tetrahydrate nanocomposite.
When the finishing liquid is used, the finishing liquid can be dripped on the surface of a working electrode, and ZrO is prepared after drying2-ZnO/MWCNTs/AMT composite modified screen printing electrode (ZrO 2-ZnO/M)WCNTs/AMT/SPE)。
ZnO-ZrO easy to observe2The composite nano-particles and the morphology characteristics of the surface of the modified screen-printed electrode are observed by using a Japanese JSM-5600LV scanning electron microscope to form and diameter. In this example, for ZrO2The morphological appearance of the ZnO nanocomposite and the structures on the modified electrode before and after doping the multi-walled carbon nanotube thereof are characterized, and FIG. 3(A) is ZrO2-ZnO nanocomposite scanning electron microscopy image, image show when Zn (Ac)2And ZrOCl2Mixing the raw materials in a ratio of 1: 1 in the acetic acid solution containing 2.0 wt% of chitosan, a uniform nanorod structure is formed by high-temperature calcination, the length of the nanorod is in the range of 300-500nm, and the width of the nanorod from the tip to the bottom is kept at about 100 nm. While FIG. 3(B) shows when ZrO is incorporated alone2The ZnO nano composite material is modified on the surface of the screen printing electrode, compared with the unmodified material, the structure of the ZnO nano composite material has the advantages that the form of nano particles is changed to a certain extent after ultrasonic oscillation and other treatment, the dispersion density of nano rods is more uniform, the specific surface area is also improved to a certain extent, and the change effectively promotes the increase of diffusible sites of adsorbates. Meanwhile, as shown in the scanning image of fig. 3(C), on the modification film doped with the multi-walled carbon nanotubes, ZrO is coated on the surface of each carbon nanotube2-ZnO composite nanoparticles, the structure not only making use of ZrO2The high affinity of the ZnO nano composite material to phosphate ions can improve the reaction rate and the reaction degree of the phosphomolybdate ion redox reaction by means of the high conductivity of the multi-walled carbon nano tube.
With CHI660E electrochemical workstation, for the ZrO produced2And (4) performing cyclic voltammetry electrochemical analysis detection on the ZnO/MWCNTs/AMT composite modified electrode. In the embodiment, an integrated three-electrode system is adopted in electrochemical analysis and detection: ZrO (ZrO)2And (4) taking ZnO/MWCNTs/AMT/SPE as a working electrode, and simultaneously taking a carbon electrode and a silver electrode as a reference electrode and a counter electrode respectively. 10 mu L of 3.7 multiplied by 10 is dripped on the surface of the modified film of the working electrode-7μmol L-1Dihydrogen phosphate radical ion H2PO4 -Standard solution, dried for 1 hour until itCompletely coated on the electrode surface. With 0.2mol L of oxygen removed-1H2SO4Taking a/KCl solution (pH is 1) as an electrolyte solution, placing a three-electrode system in the electrolyte solution at room temperature, selecting potentials of-1.0V, +1.0V and-1.0V as a starting point potential, a highest point potential and an end point potential of potential scanning respectively, and keeping a scanning speed at 50mVs-1. At the same time, 3.7 multiplied by 10 without adding is also arranged-7μmol L-1Dihydrogen phosphate radical ion H2PO4 -Control group of standard solution, the same electrochemical analysis detection is carried out.
The results showed that the concentration was 3.7X 10-7μmol L-1ZrO with or without addition of dihydrogen phosphate ions2Cyclic voltammetric response of ZnO/MWCNTs/AMT/SPE in pH 1 electrolyte solution as shown in fig. 4, the electrode always shows an anode response peak at 0.101V during potential scan, which is attributed to oxidation reaction of hydrogen ions on the surface of the working electrode (curve a); after adding dihydrogen phosphate ions, the curve b shows a corresponding oxidation peak at a potential of-0.067V, which reflects the complex change process of the molybdenum valence state in the phosphomolybdate complex, and the electrochemical reaction occurring during the formation of the two oxidation peaks is shown in equations 3 and 4:
it is worth noting that the oxidation peak response current generated at the potential of-0.067V will increase with the increase of the phosphate ion concentration, so this reaction indirectly provides a feasible solution for detecting inorganic phosphate ions by electrochemical technology, i.e. the phosphate concentration in the sample solution to be detected is obtained by conversion according to the linear relationship existing between the current and the phosphate concentration. Since the oxidation peak occurs only when the surface of the electrode is loaded with phosphate ions, the electrode has good detection selectivity.
Phosphoric acidThe electrochemical detection and analysis of the salt are carried out under the condition that the surface potential of the electrode changes from negative → positive → negative, the salt compound taking molybdenum element as the center is pre-complexed with phosphate to generate phosphomolybdate complex with electrochemical activity, and the substance is forced to carry out redox reaction to generate a response peak signal. Mo/H in this reaction+The content ratio is controllable, and the testing environment must be ensured to be strong acid.
Example 3
The sample solutions to be detected in the embodiments 1 and 2 are all aqueous solutions, and can be directly used for detecting the phosphate content in a water sample. However, if the phosphate content in the soil sample needs to be determined, the inorganic phosphate ions in the soil sample need to be extracted in advance. The following optimization of extraction parameters based on the integrated three-electrode system of example 2 was performed to ensure that the content of inorganic phosphate in the soil sample is maximally reflected on the cyclic voltammetry curve. Therefore, the optimum extractant is needed not only to completely dissolve the phosphate, but also to contain ions which do not interfere with the detection result, i.e., do not affect the potential current value of the oxidation-reduction peak. In this example, four extraction reagents, acetic acid, sodium bicarbonate, potassium sulfate and MES buffer, were used as extraction reagent candidate solutions.
Firstly, adding 1.0g of soil powder which is dried in the sun and screened by a screen with a screen hole of 0.5mm diameter into a plurality of 50mL centrifuge tubes, setting a plurality of groups of tests of different extracting agents, and respectively adding 20mL of 0.50mol L of the extracting agent into different test groups-1HAc、NaHCO3、K2SO4And MES solution as an extractant. Then 1mL of 0.025mol L is added into each centrifuge tube-1KHP solution is used as the ionic strength regulator. The tube was then placed in a constant temperature shaker (170rpm, 25 ℃) for shaking extraction, which was optimized for 10 minutes, 20 minutes, 30 minutes, 1 hour, and 2 hours, respectively. After the mixture was shaken, it was subjected to solid-liquid separation using an ultracentrifuge (5000rpm, 20 minutes, 25 ℃). The centrifuged liquid was re-filtered and transferred to a 25mL centrifuge tube and stored at 4 ℃ until the samples were examined.
The test results show that the preferred order of the extractants is HAc>NaHCO3>K2SO4>MES. The acetic acid solvent is used as an optimal extractant, the cyclic voltammetry curve formed by phosphate ions extracted by acetic acid in a potential scanning stage is complete in line type, a clear redox peak appears, and a response signal is very strong. And as for the extraction time, under the condition that the shaking extraction time is 1 hour, the final detection effect is shown to be optimal. Therefore, the oscillation extraction optimization conditions of the soil are as follows: at 0.50mol L-1Acetic acid is used as extractant, and the shaking extraction time is set as 1 h.
Under the optimized condition, the sample containing phosphate ions after extraction is subjected to electrochemical concentration analysis, 10 mu L of sample solution is removed by a micro-injector and is dripped and coated on the surface of the modified screen printing electrode, and after the sample solution is dried at room temperature, the sample solution is placed in 0.2mol L-1H2SO4In a/KCl (pH 1) electrolyte solution, potential scanning is carried out by adopting a cyclic voltammetry method, so that the redox peak-peak current of a phosphomolybdate complex is obtained, and the concentration of soluble phosphate ions in a sample is indirectly determined according to the linear relation between the current and the concentration of phosphate. Meanwhile, in order to evaluate the feasibility of measuring inorganic phosphate ions in soil by using an electrochemical voltammetry method, the detection result of the cyclic voltammetry method is compared with the result obtained by using a standard colorimetric method, and the detection accuracy of the working electrode is evaluated. Soil samples were taken from three fields, Guangdong, Hunan and Jiangsu, respectively, two samples per field.
The results of the standard colorimetry and cyclic voltammetry measurements of the concentration of inorganic phosphate ions in the same sample are shown in Table 1, wherein cyclic voltammetry measures the concentration of inorganic phosphate ions in the sample by adjusting the pH value so that the inorganic phosphate is mainly H2PO4 -The form of (a) participates in an electrochemical reaction, and standard colorimetric rules test according to the complexation reaction that occurs between conventional phosphates and molybdates. The results of the tests showed that the colorimetry gave slightly lower phosphate concentrations than the cyclic voltammetry technique, which is suspected to be due to the addition of ascorbic acid to the colorimetry, a substance that disrupts the electron transfer during the redox reaction, making 10% -20% of the complex phosphoric acidThe salt compound cannot be identified in the detection process.
TABLE 1 comparison of the results of the standard colorimetric method and cyclic voltammetry method for the detection of the content of phosphate ions in soil
The above-described embodiments are merely preferred embodiments of the present invention, which should not be construed as limiting the invention. Various changes and modifications may be made by one of ordinary skill in the pertinent art without departing from the spirit and scope of the present invention. Therefore, the technical scheme obtained by adopting the mode of equivalent replacement or equivalent transformation is within the protection scope of the invention.
Claims (11)
1. The joint detector for trace phosphate and pH is characterized by comprising a double-channel screen printing electrode, wherein the double-channel screen printing electrode is provided with two working electrodes, a reference electrode and a counter electrode on a substrate, and the four electrodes are respectively connected with a contact through wiring; the reference electrode and the counter electrode are arranged at intervals, two symmetrical regions for placing the working electrodes are clamped between the reference electrode and the counter electrode, and the two working electrodes share one reference electrode and one counter electrode; in the two working electrodes, the surface of the first working electrode is modified with a pH sensitive nano material, and the surface of the second working electrode is modified with a phosphate sensitive nano material;
the phosphate sensitive nano material is ZrO2-ZnO/multi-walled carbon nanotube/ammonium heptamolybdate tetrahydrate nanocomposite, and the preparation method comprises the following steps:
1) reduction of Zn (Ac)2And ZrOCl2Powder is mixed according to the mass ratio of 1: 1 is dissolved in 2 wt% acetic acid solution, then 2.0 wt% chitosan is slowly added in 80 ℃ water bath environment, the pH value of the solution is adjusted to 11.0-12.0, and the 80 ℃ water bath is continuously kept for 1 hour; then washing and drying the generated precipitate to obtain a solid; the solid was placed in a muffle furnace at 25 ℃ for h-1Heating to 350 deg.C for 1 hr, and heating at 25 deg.C for another hour-1The temperature is raised to 450 ℃ and the high-temperature oxidation is carried out for 1 hour; after cooling the solid, ZrO is obtained2-a ZnO nanocomposite;
2) subjecting the ZrO to2Adding the-ZnO nano composite material into ethanol, and performing ultrasonic oscillation to form 300mg L-1The nano dispersion liquid of (4); then according to 100mg L-1Adding the pretreated multi-walled carbon nanotube, and heating for 24 hours in a water bath environment at 60 ℃ to obtain a suspension; after the suspension had cooled to room temperature, 0.064mol L of water was added to the suspension-1Ammonium heptamolybdate tetrahydrate and 0.1% Nafion, and uniformly mixing to obtain ZrO2-modification liquid of ZnO/multi-walled carbon nanotube/ammonium heptamolybdate tetrahydrate nanocomposite.
2. The combined detector for trace phosphate and pH according to claim 1, wherein the pH sensitive nanomaterial is a nano metal oxide.
3. The trace phosphate and pH combination detector according to claim 2, wherein the nano-metal oxide comprises nano-sized ruthenium dioxide, nickel oxide, manganese oxide, cobalt oxide, lead oxide and titanium oxide.
4. The combined detector for trace phosphate and pH according to claim 1, wherein the modifying solution is modified on the surface of the second working electrode by means of drying after dropping.
5. The combined detector for trace phosphate and pH according to claim 1, wherein the pretreatment process of the multi-walled carbon nanotubes comprises: in concentrated HNO3The multi-walled carbon nanotubes were refluxed for 5 hours.
6. The joint detector for trace phosphate and pH according to claim 1, wherein the wiring is a conductive silver layer, the working electrode is a carbon electrode, the reference electrode is an Ag/AgCl electrode or a carbon electrode, the counter electrode is a platinum wire electrode or a silver electrode, and the substrate is a PET substrate.
7. A trace phosphate and pH combined detection method using the detector as claimed in any one of claims 1 to 6, which is used for simultaneously detecting the pH and the phosphate concentration of a solution to be detected, and is characterized by comprising the following steps:
1) quantitatively dripping a sample solution to be detected on the surface of a three-electrode system containing a first working electrode, and detecting the pH value of the sample solution to be detected by an electrochemical method;
2) quantitatively mixing a sample solution to be tested with a strong acid electrolyte with the pH value less than or equal to 1 to obtain a test solution;
3) quantitatively dripping the test solution on the surface of a three-electrode system containing a second working electrode, and carrying out potential scanning to obtain the oxidation-reduction peak current of the phosphomolybdate complex; and obtaining the phosphate concentration in the sample solution to be detected according to the linear relation between the current and the phosphate concentration.
8. A method for jointly detecting trace phosphate and pH by using the detector as claimed in any one of claims 1 to 6, which is used for detecting the phosphate concentration in a solution to be detected, and is characterized by comprising the following steps:
1) quantitatively mixing a sample solution to be tested with a strong acid electrolyte to obtain a test solution;
2) quantitatively dripping the test solution obtained in the step 1) onto the surface of a three-electrode system containing a first working electrode, detecting the pH value of the test solution by an electrochemical method, and determining that the pH value of the solution is less than or equal to 1;
3) quantitatively dripping the test solution with the pH value less than or equal to 1 on the surface of a three-electrode system containing a second working electrode, and carrying out potential scanning to obtain the oxidation-reduction peak current of the phosphomolybdate complex; and obtaining the phosphate concentration in the sample solution to be detected according to the linear relation between the current and the phosphate concentration.
9. The method for jointly detecting trace phosphate and pH according to claim 7 or 8, wherein the sample to be detected is a soil sample, and the phosphate in the soil sample needs to be extracted in advance, and the method comprises the following specific steps: drying and sieving a soil sample, quantitatively adding the soil sample into a centrifugal tube, and performing oscillation extraction by using acetic acid as an extracting agent; and (3) performing solid-liquid separation on the extracting solution by using an ultracentrifuge, and filtering the centrifuged supernatant to obtain a sample solution to be detected.
10. The method for jointly detecting trace phosphate and pH according to claim 9, wherein the shaking extraction time is 1 hour.
11. The method for jointly detecting trace phosphate and pH according to claim 9, wherein the concentration of acetic acid is 0.50mol L-1。
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