CN112345799A - pH measuring method based on monomolecular electrical detection - Google Patents
pH measuring method based on monomolecular electrical detection Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
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- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/10—STM [Scanning Tunnelling Microscopy] or apparatus therefor, e.g. STM probes
Abstract
The invention relates to a pH measuring method based on monomolecular electrical detection. The invention aims to overcome the defects of the existing solution pH detection, and provides a method for measuring pH based on monomolecular electrical detection, which can continuously respond within the range of pH value from 0 to 5 and realize the real-time monitoring of interface acid-base chemical reaction on monomolecular scale. The technical scheme adopted by the invention is as follows: the STM-BJ technology is a method for realizing repeated construction of a large number of single molecular junctions based on mutual impact and lifting between metal electrodes, and has the advantages that a large number of molecular junctions can be quickly constructed, the electrical properties of the molecular junctions can be measured, and a large amount of reliable data can be obtained in a short time for statistical analysis.
Description
Technical Field
The invention relates to the technical field of solution pH detection, in particular to a pH measuring method based on monomolecular electrical detection.
Background
The pH is also called hydrogen ion concentration index and pH value, and is a measure of the acid-base degree of the solution in a general sense. pH is one of the most basic chemical properties of aqueous solutions, and measuring pH is not only critical for determining the chemical properties of a substance, but also for controlling the initiation of a chemical reaction. At present, pH value measurement is widely applied to various fields, including almost all industries which deal with water, not only chemical industry, but also public organization, agriculture and fishery related industries, so that the exploration of a convenient and rapid pH detection method is very important. The traditional solution pH detection methods mainly comprise an indicator method, a potentiometric method (mainly relying on the use of a pH electrode), an optical fiber sensing technology and a spectroscopic method (ultraviolet spectrophotometry, fluorescent probe technology and the like). With the development of scientific technology, the traditional pH detection method can not meet the research requirements in various fields, or is difficult to be applied to the relevant detection of other substances or industries. In addition, the traditional pH detection technology has the defects of easy damage of a detection instrument, low precision, multiple times of calibration, high impedance, poor universality and the like. With the rapid development of single molecule detection technology, for example, detection technology represented by scanning tunneling microscope, the method can realize the determination of some related properties of molecules on single molecule scale, and is expected to be an implementation means for combining single molecule junction conductivity determination and pH detection based on the advantages. Compared with the traditional research method, the unimolecular technology has the advantages of rapid detection, real-time monitoring and the like, reaches the limit of the unimolecular detection sensitivity, can explore the interaction between acid and alkali on a solid-liquid interface, and provides a new opportunity for the research of chemical and physical properties except for electrical measurement. Based on this, it is meaningful to develop a single molecule pH detection technology that has a sustained response of single molecule junction conductance in a certain pH range, and can reveal the relationship between molecular reaction and pH change at a single molecule level, and identify and quantitatively describe them.
Disclosure of Invention
The invention aims to overcome the defects of the existing solution pH detection, and provides a method for measuring pH based on monomolecular electrical detection, which can continuously respond within the range of pH value from 0 to 5 and realize the real-time monitoring of interface acid-base chemical reaction on monomolecular scale.
The invention adopts the following technical scheme: a pH measuring method based on single-molecule electrical detection is based on a scanning tunneling microscope (STM-BJ) splitting technology and comprises the following steps:
the method includes the steps that the piezoelectric ceramic is utilized to control a needle point to move and approach a metal substrate continuously, the needle point is controlled to extend forwards for a certain distance towards the substrate after a preset current value is reached so as to ensure the contact between the needle point and the substrate, then the needle point is controlled to be far away from the substrate at a certain speed, the needle point and the substrate can stretch in the process, the needle point and the substrate are separated after a single-atom contact process is carried out, a curve of the electric conduction changing along with time or distance is recorded in the process, the appearance of a conducting step can be observed correspondingly, and finally statistical analysis and processing are carried out on data;
secondly, by utilizing that molecules in a liquid phase contain anchoring groups capable of forming bonds with metals, a needle point-molecule-substrate connection, namely a metal-molecule-metal junction is formed, a large number of molecules can be connected to two ends of a metal electrode at the beginning, but with the continuous separation of two electrodes, the molecules in the middle are gradually transited from a plurality of molecules to three, two or one molecules until the two electrodes are completely broken, in the process, a curve of the change of the conductance along with time or distance is recorded, the appearance of a conductance step can be correspondingly observed, and the conductance of a single-molecule junction is further measured; specific molecular junction conductivity information data can be obtained by counting a large number of conductivity curves, a conductivity statistical analysis result with Gaussian distribution can be obtained after the data are counted, and steps and statistical peaks cannot be observed when a liquid phase does not contain molecules;
according to the method, a large amount of molecular junction conductivity information data can be obtained by repeating the steps in different pH environments, and the dissociation degree of molecules is different in different pH environments at a single molecular level, so that in the process of changing the pH of the environment, the process of conductivity statistical peak shape change with Gaussian distribution can be monitored, the dependency relationship between the conductivity intensity of the molecular junction and the pH of the environment is obtained, and the quantitative detection and analysis of the pH of the interface on a single molecular scale are realized.
In the method for measuring the pH value based on single-molecule electrical detection, the preset current value in the step is between 8nA and 80 nA.
In the pH measuring method based on single-molecule electrical detection, gold is adopted as the metal.
In the pH measuring method based on monomolecular electrical detection, the liquid phase of the second step contains a carboxylic acid anchoring group bonded with metal.
Based on the characteristic that the monomolecular junction conductivity intensity is dependent on the pH value of the environment, the invention adopts the technical scheme that: the STM-BJ technology is a method for realizing repeated construction of a large number of single molecular junctions based on mutual impact and lifting between metal electrodes, and has the advantages that a large number of molecular junctions can be quickly constructed, the electrical properties of the molecular junctions can be measured, and a large amount of reliable data can be obtained in a short time for statistical analysis. Meanwhile, the change of the surface appearance of the sample can be observed in the measuring process, and the molecular conductance of the solution system can be conveniently measured and regulated at room temperature. The invention can be used for carrying out a series of electrical tests under different conditions to obtain the correlation information of the ionization degree of molecules and the electric conductivity of molecular junctions, and further reflect the dependency relationship between the pH value of the solution and the electric conductivity of single molecules.
Compared with the traditional pH sensor, the invention has the advantages that: by taking the STM-BJ method as a core and combining a data statistical analysis processing method, the ionization degree of molecules in a solution can be monitored in real time, the resolution can reach a single molecule level, and the measurement method has good response in a certain pH range. This result provides an important idea for the realization of single molecule sensitive chemical/biological sensors or detectors.
Drawings
FIG. 1 is a schematic top view of an experimental apparatus according to the present invention.
FIG. 2 is a schematic side view of the experimental set-up of the present invention.
FIG. 3 is a graph showing the one-dimensional conductance statistics of 4-methylthiobenzoic acid under different acidic pH conditions in the examples of the present invention.
FIG. 4 is a graph showing the relationship between the intensity of one-dimensional conductance peak of 4-methylthiobenzoic acid and pH under different acidic pH conditions in the example of the present invention.
FIG. 5 is a linear graph of one-dimensional conductance peak intensity of 4-methylthiobenzoic acid versus pH at different acidic pH conditions in examples of the present invention.
Detailed Description
The present invention will be further described below by way of examples with reference to the accompanying drawings.
A pH measuring method based on single-molecule electrical detection is based on a scanning tunnel microscope junction cracking technology and comprises the following steps:
the method includes the steps that the piezoelectric ceramic is utilized to control a needle point to move and approach a metal substrate continuously, the needle point is controlled to extend forwards for a certain distance towards the substrate after a preset current value is reached so as to ensure the contact between the needle point and the substrate, then the needle point is controlled to be far away from the substrate at a certain speed, the needle point and the substrate can stretch in the process, the needle point and the substrate are separated after a single-atom contact process is carried out, a curve of the electric conduction changing along with time or distance is recorded in the process, the appearance of a conducting step can be observed correspondingly, and finally statistical analysis and processing are carried out on data; the preset current value is 8 nA.
Secondly, by utilizing that molecules in a liquid phase contain anchoring groups capable of forming bonds with metals, namely carboxylic acid, needle tip-molecule-substrate connection, namely metal-molecule-metal junctions are formed, a large number of molecules can be connected to two ends of a metal electrode at the beginning, but with the continuous separation of the two electrodes, the molecules in the middle are gradually transited from a plurality of molecules to three, two or one molecules until the two electrodes are completely broken, in the process, a curve of the electrical conductivity along with the change of time or distance is recorded, the appearance of an electrical conductivity step can be correspondingly observed, and the electrical conductivity of the single-molecule junctions can be further measured; specific molecular junction conductivity information data can be obtained by counting a large number of conductivity curves, a conductivity statistical analysis result with Gaussian distribution can be obtained after the data are counted, and steps and statistical peaks cannot be observed when a liquid phase does not contain molecules;
according to the method, a large amount of molecular junction conductivity information data can be obtained by repeating the steps in different pH environments, and the dissociation degree of molecules is different in different pH environments at a single molecular level, so that in the process of changing the pH of the environment, the process of conductivity statistical peak shape change with Gaussian distribution can be monitored, the dependency relationship between the conductivity intensity of the molecular junction and the pH of the environment is obtained, and the quantitative detection and analysis of the pH of the interface on a single molecular scale are realized.
Referring to the attached drawings 1-5, the invention specifically operates as follows:
1. preparation step
Cleaning a plurality of solvent bottles, volumetric flasks, electrolytic cells and O rings, and putting the standby solvent bottles, volumetric flasks, electrolytic cells and O rings into a container containing concentrated sulfuric acid in a volume ratio: hydrogen peroxide is 3: 1 for at least 1 hour, pouring the waste liquor into a waste liquor tank for centralized treatment after soaking, wherein the soaking aims to remove possible residual organic substances, other impurities and the like, repeatedly washing the spare parts for many times by using a large amount of ultrapure water, boiling the spare parts for 3 times by using the ultrapure water, putting the washed spare parts into a drying oven at 105 ℃, and drying the spare parts for later use.
2. Preparing molecular solution
1.68mg of 4-methionine benzoic acid molecule (C)8H8O2S) and 612.30mg of sodium perchlorate (NaClO) as supporting electrolyte4) Placing the solution in a 100mL volumetric flask, fixing the volume with ultrapure water and dissolving the solution until the powder is completely dissolved to obtain 0.1mM mother solution, wherein the concentration of sodium perchlorate is 50mM, and equivalently transferring 10mL mother solution by a liquid transfer gun and respectively placing the 10mL mother solution in 6 10mL clean reagent bottles for later use.
3. Standard solution preparation
Adding perchloric acid with different amounts into the 6 reagent bottles to prepare standard solutions with different pH values, wherein
pH 0 (1136. mu.L concentrated perchloric acid pipetted with pipette into 10mL of stock solution)
pH 1.3 (114. mu.L of concentrated perchloric acid was pipetted into 10mL of stock solution)
pH 2.0 (100. mu.L of 1M perchloric acid solution was pipetted into 10mL of molecular mother liquor)
pH 3.0 (10. mu.L of 1M perchloric acid solution was pipetted into 10mL of molecular mother liquor)
pH 4.1(0.1mM stock solution)
pH 5.0 (60. mu.L of 10mM sodium hydroxide solution was pipetted into 10mL of molecular mother liquor)
The pH values of the solutions used in the experiments were determined by means of a pH meter, manufactured by Sidersles scientific instruments Germany.
Preparation and treatment of Au (111) substrates
Taking a gold wire (the purity is more than or equal to 99.999 percent, and phi is 0.5mm), and preparing a single crystal electrode Au (111) by a Clavilier method; fixing the prepared Au (111) on a gold substrate, observing by using a microscope, finding out surfaces which are bright and flat in at least three directions, and finally fixing the surfaces firmly by using oxyhydrogen flame again; in order to ensure the cleanness of the experiment, before each experiment, Au (111) is put in 0.5M dilute sulfuric acid solution, 5V direct current voltage is applied for electrolysis for 10s, then the power supply is closed, the solution is washed by ultrapure water, finally the solution is soaked in 0.5M dilute hydrochloric acid solution for 10s, and then a large amount of ultrapure water is used for washing; repeating the above steps for two times, soaking with dilute hydrochloric acid solution for at least 3min, and washing with large amount of ultrapure water; finally, annealing the polished gold ball, and then placing the gold ball in nitrogen for cooling for later use.
5. Needle tip preparation and installation
Taking a section of gold wire of about 1.5cm (the purity is more than or equal to 99.999 percent, and the diameter is 0.25mm), preparing by adopting a mechanical shearing method, cleaning scissors and tweezers by using a cotton ball dipped with absolute ethyl alcohol before shearing, clamping a needle tip by using the tweezers after straightening the gold wire, and cleanly dropping the gold wire by using the tweezers and the scissors at an angle of about 45 degrees; placing the cut needle tip under a microscope to observe whether the needle tip is suitable for an experiment; generally, a proper needle point has a smooth section and a sharp top end without bending; after the needle point is prepared, placing the needle point on a 150 ℃ electric soldering iron, and carrying out insulation encapsulation treatment by using polymethyl styrene so as to reduce the interference of Faraday current to the experiment; and finally inserting the prepared needle tip into the STM scanning head.
6. Electrolytic cell installation
The electrolytic cell used in the experiment is manufactured according to the requirements of STM experimental environment, the manufacturing material is mainly polytrichlorofluoroethylene, the overlooking and side view structures of the electrolytic cell are shown in figures 1 and 2, an O-shaped sealing ring is arranged in a hole at the bottom of the electrolytic cell to be compressed, and a screw is fixed in threaded holes at two ends of the electrolytic cell; meanwhile, a counter electrode Pt wire is clamped in the center ring of the electrolytic cell and is connected to an external measuring loop; during the experiment, the gold substrate is placed on a steel sheet, then covered by an electrolytic cell, and finally fixed by screws, and the installation is finished.
7. Test and data acquisition
Using a pipette to transfer about 200 mu L of solution and slowly dripping the solution into an electrolytic cell, covering a shielding cover, starting a test, driving a motor to enable a needle point to approach an Au (111) substrate, stopping the motor after the solution reaches a tunnel current region, enabling piezoelectric ceramics of a scanning head to start working at the moment, controlling the needle point to move by changing the voltage of the piezoelectric ceramics, keeping the bias voltage of the needle point and the substrate at 100mV until the whole device is in a stable state, namely mechanical drift and thermal drift are negligible, driving the needle point to continuously approach the substrate and impacting mutually, controlling the needle point to be away from the substrate at the speed of 20nm/s, simultaneously recording a current-pulling distance curve or a current-time curve at the sampling frequency of 20kHz, and repeating the test until thousands of pulling curves are obtained.
8. Drawing of standard working curve
According to the steps, the conductance of the 4-methylthiobenzoic acid molecule junction under different pH environments is measured respectively, the data collected in each experiment are combined and subjected to log statistics, as shown in figure 3, a 4-methylthiobenzoic acid molecule junction one-dimensional conductance statistical graph taking Au as an electrode can be obtained, as the conductance peak intensity is in direct proportion to the formation probability of the molecule junction, and as carboxylic acid is an anchoring group molecule, the formation of the molecule junction is in direct proportion to the coverage rate of the carboxylic acid molecule subjected to surface deprotonation in a certain range, the relation between the conductance peak intensity (I) change and the molecular coverage rate (theta) can be expressed as follows:
wherein, IminAnd ImaxThe minimum value and the maximum value of the intensity of the measured conductance peak are shown. The degree of dissociation of the interfacial basicity of pH from the COOH groups of the surface adsorbed molecules at different pH's can be expressed by the Henderson-Hasselbalch equation:
wherein
Is the ionic equilibrium constant on the interface. From the above equation, lg [ (I-I)min)/(Imax-I)]Is a linear function of the pH of the solution. We will therefore plot the experimentally obtained relationship, see e.g. fig. 3. The conductance peak intensity change showed very good linear response with a standard variance value of 0.99 with pH 0 to pH 5. In addition, the terephthalic acid and the 3-methylthiobenzoic acid molecules are selected to be tested in the solutions with different pH values in the same way, and the linear relation results are obtained, so that the detection technology has certain universality in pH measurement application.
Actual solution pH measurements.
A solution containing 4-methylthiobenzoic acid molecules (solution pH unknown) was selected for the above [0020] to [0027] experimental procedures. As a result, the peak intensity of the molecular junction conductance was measured to be 0.973. The standard working curve of FIG. 3 can be used, with the computer directly giving a pH of 0.91. The pH value of the unknown solution is 0.9 by utilizing a commercial pH meter, the result is quite consistent with the result measured by the technology of the invention, and the reliability and the practicability of the technology of the invention are proved.
Claims (4)
1. A pH measuring method based on single-molecule electrical detection is characterized in that the method is based on a scanning tunneling microscope (STM-BJ) split junction technology and comprises the following steps:
the method includes the steps that the piezoelectric ceramic is utilized to control a needle point to move and approach a metal substrate continuously, the needle point is controlled to extend forwards for a certain distance towards the substrate after a preset current value is reached so as to ensure the contact between the needle point and the substrate, then the needle point is controlled to be far away from the substrate at a certain speed, the needle point and the substrate can stretch in the process, the needle point and the substrate are separated after a single-atom contact process is carried out, a curve of the electric conduction changing along with time or distance is recorded in the process, the appearance of a conducting step can be observed correspondingly, and finally statistical analysis and processing are carried out on data;
secondly, by utilizing that molecules in a liquid phase contain anchoring groups capable of forming bonds with metals, a needle point-molecule-substrate connection, namely a metal-molecule-metal junction is formed, a large number of molecules can be connected to two ends of a metal electrode at the beginning, but with the continuous separation of two electrodes, the molecules in the middle are gradually transited from a plurality of molecules to three, two or one molecules until the two electrodes are completely broken, in the process, a curve of the change of the conductance along with time or distance is recorded, the appearance of a conductance step can be correspondingly observed, and the conductance of a single-molecule junction is further measured; specific molecular junction conductivity information data can be obtained by counting a large number of conductivity curves, a conductivity statistical analysis result with Gaussian distribution can be obtained after the data are counted, and steps and statistical peaks cannot be observed when a liquid phase does not contain molecules;
according to the method, a large amount of molecular junction conductivity information data can be obtained by repeating the steps in different pH environments, and the dissociation degree of molecules is different in different pH environments at a single molecular level, so that in the process of changing the pH of the environment, the process of conductivity statistical peak shape change with Gaussian distribution can be monitored, the dependency relationship between the conductivity intensity of the molecular junction and the pH of the environment is obtained, and the quantitative detection and analysis of the pH of the interface on a single molecular scale are realized.
2. The method for measuring pH based on single-molecule electrical detection according to claim 1, characterized in that the preset current value in the step is between 8nA and 80 nA.
3. The method for measuring pH based on single-molecule electrical detection according to claim 1 or 2, wherein the metal is gold.
4. The method for measuring pH based on monomolecular electrical detection according to claim 1 or 2, characterized in that the liquid phase of the two phases contains a carboxylic acid anchoring group bonded to the metal.
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| CN202011213412.5A CN112345799B (en) | 2020-11-04 | 2020-11-04 | PH measurement method based on single-molecule electrical detection |
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