CN112345799B - PH measurement method based on single-molecule electrical detection - Google Patents
PH measurement method based on single-molecule electrical detection Download PDFInfo
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- KKEYFWRCBNTPAC-UHFFFAOYSA-N Terephthalic acid Chemical compound OC(=O)C1=CC=C(C(O)=O)C=C1 KKEYFWRCBNTPAC-UHFFFAOYSA-N 0.000 description 2
- 239000002585 base Substances 0.000 description 2
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- 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 single-molecule electrical detection. The invention aims to overcome the defects of the prior solution pH detection, and provides a pH measurement method based on single-molecule electrical detection, which can continuously respond within the pH value range from 0 to 5 and realize real-time monitoring of an interfacial acid-base chemical reaction on a single-molecule scale. The technical scheme adopted by the invention is as follows: the quantitative analysis of interface acidity and alkalinity is realized by utilizing the change of the conductivity intensity of the molecular junction under different pH environments measured by STM-BJ, wherein 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 of being capable of quickly and largely constructing the molecular junctions, measuring the electrical properties of the molecular junctions and obtaining a large amount of reliable data for statistical analysis in a short time.
Description
Technical Field
The invention relates to the technical field of solution pH detection, in particular to a pH measurement method based on single-molecule 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 common sense. The pH is one of the most basic chemical properties of an aqueous solution, and measuring pH is not only critical to determining the chemical nature of a substance, but also the initial step of controlling the chemical reaction. At present, the pH value measurement is widely applied to various fields, including almost all industries which are intersected with water, not only the chemical industry, but also public organization, agriculture and fishery related industries, so that it is important to explore a convenient and quick pH detection method. The conventional solution pH detection methods mainly include an indicator method, a potentiometric method (mainly depending on the use of pH electrodes), an optical fiber sensing technique, and a spectroscopic method (ultraviolet spectrophotometry, fluorescent probe technique, etc.). With the development of science and technology, the conventional pH detection method cannot meet the research requirement in various fields, or is difficult to be suitable for related detection of other substances or industries. In addition, the traditional pH detection technology often has the defects of easy breakage, low precision, multiple times of calibration, high impedance, poor universality and the like of a detection instrument. With the rapid development of single-molecule detection technology, for example, detection technology represented by a scanning tunneling microscope, measurement of certain relevant properties of molecules can be realized on a single-molecule scale, and the advantages are expected to be used as implementation means for combining single-molecule junction conductivity measurement with pH detection. Compared with the traditional research method, the single-molecule technology has the advantages of rapid detection, real-time monitoring and the like, reaches the limit of single-molecule detection sensitivity, can explore the interaction between acid and alkali on a solid-liquid interface, and provides a new opportunity for chemical and physical property research beyond electric measurement. Based on this, it is significant to develop a single-molecule pH detection technique that has a sustained response to single-molecule junction conductance over a range of pH, and that is capable of revealing the relationship between molecular reactions and pH changes at the single-molecule level, and identifying and quantitatively describing them.
Disclosure of Invention
The invention aims to overcome the defects of the prior solution pH detection, and provides a pH measurement method based on single-molecule electrical detection, which can continuously respond within the pH value range from 0 to 5 and realize real-time monitoring of an interfacial acid-base chemical reaction on a single-molecule scale.
The invention adopts the following technical scheme: a pH measuring method based on single-molecule electrical detection and based on scanning tunneling microscope (STM-BJ) technology comprises the following steps:
the method comprises the steps of controlling the movement of a needle point to continuously approach a metal substrate by using piezoelectric ceramics, controlling the needle point to extend forwards in the direction of the substrate for a certain distance after reaching a preset current value so as to ensure the contact between the needle point and the substrate, then controlling the needle point to be far away from the substrate at a certain speed, stretching the needle point and the substrate in the process, separating the needle point from the substrate after undergoing a single-atom contact process, recording a curve of change of conductance with time or distance in the process, correspondingly observing the occurrence of a conductance step, and finally carrying out statistical analysis on data;
secondly, utilizing the fact that molecules in a liquid phase contain anchoring groups capable of forming bonds with metal to form needle point-molecule-substrate connection, namely metal-molecule-metal junction, wherein a large number of molecules can be connected to two ends of a metal electrode at first, but as two electrodes continue to be far away, the middle molecules gradually transition from a plurality of molecules to three molecules, two molecules and one molecules until the molecules are completely broken, recording a curve of conductivity change with time or distance in the process, and accordingly observing the appearance of a conductivity step so as to measure the conductivity of a single-molecule junction; 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 is counted, and when molecules are not contained in a liquid phase, steps and statistical peaks cannot be observed;
according to the method, the steps are repeated under different pH environments, a large amount of molecular junction conductivity information data can be obtained, and under a single molecular level, in different pH environments, the dissociation degree of molecules is different, so that in the process of changing the environmental pH, a process of conducting statistical peak shape change with Gaussian distribution can be monitored, the dependence of the molecular junction conductivity intensity and the environmental pH is obtained, and quantitative detection and analysis of interface acidity and alkalinity on a single molecular scale are realized.
In the method for measuring the pH based on single-molecule electrical detection, the preset current value in the step is 8nA to 80 nA.
In the pH measuring method based on single-molecule electrical detection, gold is adopted as the metal.
In the pH measurement method based on single-molecule electrical detection, the liquid phase of the step (ii) contains a carboxylic acid anchoring group bonded to a metal.
Based on the characteristic that the single-molecule junction conductivity intensity and the environmental pH value have dependence, the invention adopts the following technical scheme: the quantitative analysis of interface acidity and alkalinity is realized by utilizing the change of the conductivity intensity of the molecular junction under different pH environments measured by STM-BJ, wherein 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 of being capable of quickly and largely constructing the molecular junctions, measuring the electrical properties of the molecular junctions and obtaining a large amount of reliable data for statistical analysis in a short time. Meanwhile, the change of the surface morphology of the sample can be observed in the measuring process, and the measurement and regulation of the molecular conductivity of the solution system can be conveniently carried out 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 conduction intensity of the molecular junction, and further reflect the dependency relationship between the pH value of the solution and the electric conduction intensity of single molecules.
The invention has the advantages compared with the traditional pH sensor that: the STM-BJ method is used as a core, and the method of data statistics analysis processing is combined, so that the ionization degree of molecules in the solution can be monitored in real time, the resolution can reach a single molecular level, and the measuring method has good response in a certain pH range. This result provides an important idea for chemical/biological sensors or detectors that achieve single molecule sensitivity.
Drawings
Fig. 1 is a schematic top view of the experimental apparatus of 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 one-dimensional conductance statistics of 4-methylthiobenzoic acid under different acidic pH conditions in an example of the present invention.
FIG. 4 is a graph showing the relationship between the intensity of one-dimensional conductivity peak of 4-methylthiobenzoic acid and pH under different acidic pH conditions in the examples of the present invention.
FIG. 5 is a linear plot of one-dimensional conductivity peak intensity versus pH for 4-methylthiobenzoic acid under different acidic pH conditions in an example of the present invention.
Detailed Description
The invention will be further illustrated by the following examples in the figures.
A pH measuring method based on single-molecule electrical detection and based on scanning tunnel microscope cleavage technology comprises the following steps:
the method comprises the steps of controlling the movement of a needle point to continuously approach a metal substrate by using piezoelectric ceramics, controlling the needle point to extend forwards in the direction of the substrate for a certain distance after reaching a preset current value so as to ensure the contact between the needle point and the substrate, then controlling the needle point to be far away from the substrate at a certain speed, stretching the needle point and the substrate in the process, separating the needle point from the substrate after undergoing a single-atom contact process, recording a curve of change of conductance with time or distance in the process, correspondingly observing the occurrence of a conductance step, and finally carrying out statistical analysis on data; the preset current value is 8nA.
The method comprises the steps that (1) anchoring groups capable of forming bonds with metals, namely carboxylic acid, are contained in molecules in a liquid phase to form needle point-molecule-substrate connection, namely metal-molecule-metal junction, a large number of molecules are connected to two ends of a metal electrode at first, but as two electrodes continue to be far away, the middle molecules gradually transition from a plurality of molecules to three molecules, two molecules and one molecules until the molecules are completely broken, a curve of conductivity change with time or distance is recorded in the process, the occurrence of a conductivity step can be correspondingly observed, and then the conductivity of a single-molecule junction is 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 is counted, and when molecules are not contained in a liquid phase, steps and statistical peaks cannot be observed;
according to the method, the steps are repeated under different pH environments, a large amount of molecular junction conductivity information data can be obtained, and under a single molecular level, in different pH environments, the dissociation degree of molecules is different, so that in the process of changing the environmental pH, a process of conducting statistical peak shape change with Gaussian distribution can be monitored, the dependence of the molecular junction conductivity intensity and the environmental pH is obtained, and quantitative detection and analysis of interface acidity and alkalinity on a single molecular scale are realized.
Referring to fig. 1 to 5, the present invention specifically operates as follows:
1. preparation step
Cleaning a plurality of solvent bottles, volumetric flasks, electrolytic cells and O-rings, and putting the solvent bottles, the volumetric flasks, the electrolytic cells and the O-rings which are ready for use into a solution containing concentrated sulfuric acid in a volume ratio of: hydrogen peroxide = 3:1, soaking in a beaker of the piranha solution for at least 1h, pouring the waste liquid into a waste liquid tank for centralized treatment after soaking, removing possible residual organic substances, other impurities and the like, repeatedly flushing the spare parts with a large amount of ultrapure water for multiple times, boiling with the ultrapure water for 3 times, putting the cleaned material into a baking oven at 105 ℃ and drying for standby.
2. Preparing a molecular solution
1.68mg of 4-methylsulfuric acid benzoic acid molecule (C 8 H 8 O 2 S) and 612.30mg of supporting electrolyte sodium perchlorate (NaClO) 4 ) Placing the mixture into a 100mL volumetric flask, using ultrapure water to fix the volume and dissolving, obtaining 0.1mM mother liquor after the powder is completely dissolved, wherein the concentration of sodium perchlorate is 50mM, and respectively placing 10mL mother liquor into 6 10mL clean reagent bottles for standby by using a pipette.
3. Standard solution configuration
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 of concentrated perchloric acid was removed with a pipette and added to 10mL of the molecular mother liquor)
ph=1.3 (114 μl of concentrated perchloric acid was removed with a pipette and added to 10mL of the molecular mother liquor)
ph=2.0 (100 μl of 1M perchloric acid solution was removed with a pipette and added to 10mL of the molecular mother liquor)
ph=3.0 (10 μl of 1M perchloric acid solution was removed with a pipette and added to 10mL of the master-molecule solution)
ph=4.1 (0.1 mM molecular mother liquor)
ph=5.0 (60 μl of 10mM sodium hydroxide solution was added to 10mL of the molecular mother liquor by pipetting with a pipette)
The pH values of the solutions used in the experiments were all determined by a pH meter, which was manufactured by Sidoriko instruments, germany.
Preparation and treatment of Au (111) substrates
Taking a gold wire (the purity is more than or equal to 99.999 percent, phi=0.5 mm), and preparing single-crystal electrode Au (111) by adopting a Clavilier method; fixing the prepared Au (111) on a gold substrate, observing by using a microscope, finding out bright and flat surfaces in at least three directions, and finally fixing the Au with oxyhydrogen flame again firmly; in order to ensure the cleanliness of the experiment, au (111) is put in 0.5M dilute sulfuric acid solution for electrolysis for 10s by applying 5V direct voltage before each experiment, then the power supply is turned off, the experiment is flushed with ultrapure water, finally the experiment is soaked in 0.5M dilute hydrochloric acid solution for 10s, and then a large amount of ultrapure water is used for flushing; repeating the steps for two times, keeping for at least 3min when the solution is soaked in dilute hydrochloric acid for the last time, and then washing the solution with a large amount of ultrapure water; and finally, annealing the polished gold balls, and then cooling the gold balls in nitrogen for standby.
5. Needle tip preparation and installation
Taking a section of gold wire (the purity is more than or equal to 99.999 percent and phi=0.25 mm) with about 1.5cm, preparing the gold wire by adopting a mechanical shearing method, cleaning scissors and tweezers by using cotton balls dipped with absolute ethyl alcohol before shearing, straightening the gold wire, clamping a needle point by using the tweezers, and shearing the tweezers and the scissors cleanly at about 45 degrees; placing the sheared needle tip under a microscope to observe whether the needle tip is suitable for experiments; a proper needle point is generally smooth in section, sharp in top end and not bent; after the needle tip is prepared, placing the needle tip on an electric soldering iron at 150 ℃ to perform insulation encapsulation treatment by using polymethyl styrene so as to reduce the interference of Faraday current on experiments; 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 by self according to the requirement of STM experimental environment, the manufacturing material is mainly polytrichlorofluoroethylene, the structure of the overlook and side view is 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 tightly pressed, and screws are fixed in threaded holes at two ends of the electrolytic cell; meanwhile, a counter electrode Pt wire is clamped in the central ring of the electrolytic cell and is connected into an external measuring loop; when the experiment is carried out, the gold substrate is placed on the steel sheet, then covered by the electrolytic cell, finally fixed by the screw, and the installation is completed.
7. Testing and data acquisition
And (3) slowly dripping about 200 mu L of solution into the electrolytic cell by using a pipetting gun, 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 a tunnel current area is reached, starting the piezoelectric ceramic of a scanning head, controlling the needle point to move by changing the voltage of the piezoelectric ceramic until the whole device is in a stable state, namely mechanical drift and thermal drift are negligible, maintaining the bias voltage of the needle point and the substrate 100mV, driving the needle point to continuously approach the substrate and impacting each other, controlling the needle point to be far away from the substrate at a speed of 20nm/s, and simultaneously recording a current-lifting distance curve or a current-time curve at a sampling frequency of 20kHz, and repeating the test until thousands of lifting curves are obtained.
8. Standard working curve drawing
According to the above steps, the conductance of the 4-methylthiobenzoic acid molecular junction under different pH environments is measured respectively, the log statistics is carried out after the data collected by each experiment are combined, as shown in FIG. 3, a one-dimensional conductance statistical chart of the 4-methylthiobenzoic acid molecular junction using Au as an electrode can be obtained, because the intensity of the conductance peak is in direct proportion to the probability of forming the molecular junction, and for carboxylic acid as an anchoring group molecule, the molecular junction formation is in direct proportion to the coverage rate of carboxylic acid molecules with deprotonated surfaces in a certain range, and therefore, the relationship between the intensity (I) of the conductance peak and the molecular coverage (theta) can be expressed as follows:
wherein I is min And I max The minimum and maximum values of the intensity of the conductance peak are measured. Under different pH conditions, the degree of dissociation of the interface pH value and the COOH groups of the surface adsorption molecules can be expressed by the Henderson-Hasselbalch equation:
wherein the method comprises the steps ofIs the ionization equilibrium constant at the interface. From the above equation, lg [ (I-I) can be obtained min )/(I max -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 conductivity peak intensity variation showed very good linear response with a standard deviation value of 0.99 over the range of ph=0 to ph=5. In addition, the terephthalic acid and 3-methylthiobenzoic acid molecules are also selected to be subjected to the same test in different pH solutions, and the linear relation results are obtained, so that the detection technology has certain universality in pH measurement application.
Actual solution pH measurement.
A solution containing 4-methylthiobenzoic acid molecules (unknown solution pH) was selected and subjected to the experimental procedures [0020] to [0027 ]. The result was a molecular junction conductivity peak intensity i=0.973. The standard operating curve of FIG. 3 can be used, and the computer directly gives a pH of 0.91. The pH value of the unknown solution measured by a commercial pH meter is 0.9, and the result is quite consistent with the result measured by the technology of the invention, thereby proving the reliability and the practicability of the technology of the invention.
Claims (3)
1. The pH measurement method based on single-molecule electrical detection is characterized by comprising the following steps of:
the method comprises the steps of controlling the movement of a needle point to continuously approach a metal substrate by using piezoelectric ceramics, controlling the needle point to extend forwards in the direction of the substrate for a certain distance after reaching a preset current value so as to ensure the contact between the needle point and the substrate, then controlling the needle point to be far away from the substrate at a certain speed, stretching the needle point and the substrate in the process, separating the needle point from the substrate after undergoing a single-atom contact process, recording a curve of change of conductance with time or distance in the process, correspondingly observing the occurrence of a conductance step, and finally carrying out statistical analysis on data;
the method comprises the steps that (1) anchoring groups capable of forming bonds with metal are contained in molecules in a liquid phase to form needle point-molecule-substrate connection, the needle point-molecule-substrate connection is a metal-molecule-metal junction, a large number of molecules are connected to two ends of a metal electrode at the beginning, the middle molecules gradually transition from a plurality of molecules to three molecules, two molecules and one molecules along with the continuous separation of the two electrodes until the molecules are completely broken, a curve of conductivity change with time or distance is recorded in the process, the appearance of a conductivity step can be correspondingly observed, and the conductivity of a single-molecule junction is 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 is counted, and when molecules are not contained in a liquid phase, steps and statistical peaks cannot be observed; the liquid phase contains carboxylic acid anchoring groups bonded with metal;
according to the method, the steps are repeated under different pH environments, a large amount of molecular junction conductive information data can be obtained, and under a single molecular level, in different pH environments, the dissociation degree of molecules is different, so that in the process of changing the environmental pH, a process of conducting statistical peak shape change with Gaussian distribution can be monitored, the dependence of the molecular junction conductive strength and the environmental pH is obtained, and quantitative detection and analysis of interface acidity and alkalinity on a single molecular scale are realized;
the dependency relationship is specifically expressed as follows: because the intensity of the conductivity peak is in direct proportion to the formation probability of the molecular junction, for molecules with carboxylic acid as an anchoring group, the formation of the molecular junction is in direct proportion to the coverage rate of carboxylic acid molecules with the surface deprotonated, so that the relation between the intensity I of the conductivity peak and the molecular coverage rate theta is expressed as follows;
wherein I is min And I max Minimum and maximum values of the intensity of the measured conductivity peak; under different pH conditions, the dissociation degree of the interface pH value and the COOH groups of the surface adsorption molecules is expressed by using a Henderson-Hasselbalch equation:
wherein the method comprises the steps ofIs the ionization equilibrium constant at the interface; according to the above equation, lg [ (I-I) min )/(I max -I)]Linear function of solution pH.
2. The pH measuring method based on single-molecule electrical detection as claimed in claim 1, wherein the preset current value in the step is 8nA to 80 nA.
3. A pH measurement method based on single molecule electrical detection according to claim 1 or 2, characterized in that the metal is gold.
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