What is a Microelectrode?

A microelectrode is an electrode with a small working area, and the limit of the electrode area is not very strict. Microelectrode includes two meanings: refers to the miniaturization of the electrode. Such as miniaturized ion-selective electrodes, used to directly observe the activity changes of some important ions in body fluids and even cells. Glass capillary (tip inner diameter below one millionth meter) electrode, under the control of a micromanipulator, is placed near the cell surface or inserted into the cell to observe the electrical activity of a single cell. Microelectrodes are a tool for studying cells in medicine. refers to a type of electrode in which the electrode area is small but the entire electrode is not miniaturized in electrochemical analysis. Such as the polarographic method and voltammetry used in the electrode, mercury drop electrode, suspended mercury electrode, indicator electrode in Coulomb titration, micro platinum electrode, etc. are also called microelectrodes. This type of electrode is prone to concentration polarization due to its small electrode surface and high current density. Microelectrodes have extremely high mass transfer rates.

A microelectrode is an electrode that has a size of at least one dimension in micrometers or nanometers (that is, <100 micrometers).

Microelectrode electrochemistry is a new electrochemical discipline developed in the 1970s. As a frontier of electrochemistry and electroanalytical chemistry, it has many new characteristics and provides people with an exploration of the microstructure of matter. A powerful tool. Because when the one-dimensional size of the electrode is reduced from the millimeter level to the micron and nanometer level, it exhibits many excellent electrochemical characteristics different from the conventional electrode.

Fabrication of combined disc electrodes: Ultra-fine metal wires of platinum, gold, and carbon are carefully arranged equidistantly on the surface of the insulator, and fixed and glued with an adhesive such as a redox resin. After curing, it is ground to expose the cross section of the electrode, and then further polished, and the other end is connected and led out with a silver conductive adhesive by a metal wire.

Fabrication of nano-scale disc-cylindrical electrodes: The radius of the prepared disc electrode can reach 100 nm or less. For the production method with a radius of 1-0.01um, nanometer-sized metal wire can be used by chemical etching or optical etching, or it can be used in a single cell to control a capillary with a diameter of 10-5cm. The inner wall of the capillary obtained by the controller is made of metal ring electrode with metal ink. ^[1]

(1) The inherent RC time constant of the ultramicroelectrode is small, which makes it useful for studying fast, transient electrochemical reactions;

(2) The small polarization current on the ultra-microelectrodes reduces the IR drop of the system, making it applicable to high-resistance systems, including low supported electrolyte concentrations or even unsupported electrolyte solutions, gas phase systems, semi-solid and all-solid systems ;

(3) The material on the ultramicroelectrode diffuses extremely fast, and the fast out-of-phase rate constant can be measured by steady-state voltammetry;

(4) The small size of the ultra-micro electrode ensures that the measured object will not be changed or destroyed during the experiment, so that the ultra-micro electrode can be applied to the low damage detection of living organisms and the space-resolved detection in the micro volume.

(1) Assembly: firstly perform activation / derivation treatment on the surface of the microelectrode to make the surface have functional groups, and then react / assemble the modified electrode by covalent / non-covalent methods.

(2) Electrodeposition / electropolymerization: firstly perform activation / derivation treatment on the surface of the microelectrode, and then perform electrodeposition / electropolymerization modification.

(3) Dip coating: immerse the microelectrode in the modified solution or suspension, and take it out to dry.

(4) Drop coating: Place the microelectrode on a glass plate, and apply the modified solution or suspension to the position of the glass plate where the microelectrode is located. After drying, a layer of modification material is naturally attached to the microelectrode.

Application in capillary electrophoresis-electrochemical detection system

Modification of microelectrodes is an ideal system that combines separation, enrichment, and measurement into one. Modified electrodes currently used in capillary electrophoresis (CE) -electrochemical detection (EC) systems include Hg modified microelectrodes, chemically modified microelectrodes, micrometal particle modified electrodes, and surface film modified microelectrodes. ^[1]

Application in scanning technology

Microelectrodes are used in scanning technology to study various forms of local corrosion, such as the occurrence and development of pitting corrosion; the growth and decline of crevice corrosion; the precursor potential effect of stress corrosion cracking; the corrosion behavior of welds; the corrosion inhibition mechanism and the material resistance In the research of the local measurement of local corrosion, etc., technologies that are difficult to obtain by other technologies can be obtained. Scanning probe microscopy (SPM) mainly includes scanning tunneling microscope (STM) and atomic force microscope (AFM). Its principle is completely based on the tunneling effect of quantum mechanics. By measuring the surface tunneling current distribution, it can be used in vacuum, air, and solutions. Under a variety of environmental conditions, it characterizes the morphology of the atomic structure of the electrode surface in real space. SPM has ultra-high-resolution surface testing technology, which has been widely used in the study of surface and interface processes, involving the fields of surface physical chemistry, materials science, life sciences, etc., and microelectrodes are an important part of it.

Microelectrode scanning technology can be divided electrochemically due to the different electrodes used: (1) using a single electrode system: using high-speed scanning technology on the test object to detect the intermediate in the future and before reaction; (2) Two-electrode system: For example, using SECM technology to study the mechanism of acrylonitrile polymerization to produce dinitrile. ^[1]

Application in sensors

The development direction of potential sensors is to study microelectrodes and nanoelectrodes. The purpose is to determine the activity or concentration of a single cell and various ionic molecules in a cell.

Biosensors use biochemical reactions between solid biocatalyst identification devices and chemical substances, and rely on electrochemical devices to select and measure the chemical substances generated or consumed. BLMS biosensor is also used to identify and detect odorous substances, and can also identify and measure many biologically active substances, such as glucose. The detection limit of glutamic acid by glutamate ion channel coulomb sensor is 3 × 10 ^-8 mol / L. Biosensor can quickly measure antigen, acetylcholine, urea and penicillin in less than 2 minutes, and the fastest response time is about 10 seconds. ^[1]

Application in Energy Electrochemistry and Disproportionation Catalysis

In the battery industry, scholars have drawn much attention to lithium batteries. Some scholars have applied microelectrodes to the research of lithium batteries and obtained important data. For example, Gendevs et al. Used copper microdisk electrodes (r _d = 40um) to study the fuse behavior of Li / Li ⁺ in THF (tetrahydrofuran) media. The results of the study show the application prospects of microelectrodes in energy electrochemistry and their superior electrode characteristics. The microelectrode has a small geometric size, a small IR drop, a short charging time, a thin effective diffusion layer, and a steady state is easily reached. A fast chemical reaction rate constant can be determined under steady state conditions. The disproportionation catalysis reaction on the disk electrode can be studied. The differential current equation of the disproportionation catalysis reaction is used to obtain the steady-state current expression of the microdisc electrode. Using the derived expression, the kinetics of the disproportionation catalysis reaction can be obtained. constant. ^[1]

Application in Spectroelectrochemistry

Micro-infrared spectroscopy electrochemical method can study specific micro-regions on the surface of the electrode and its vicinity, and obtain micro-information of the low-conductivity system due to the use of tiny working electrodes. Therefore, the electrochemical studies of microelectrode infrared spectroscopy and space-resolved infrared spectroscopy can be used.

Applications in Analytical Chemistry

The various characteristics of the microelectrode can reflect its advantages in analytical chemistry and are widely used in the field of analytical chemistry, such as: can be used as a variety of ion-selective electrodes; can be used as a biosensor; as a gas sensor to detect nitric oxide And nitrogen dioxide; can be used in clinical analysis to determine the oxygen content in blood; can be used to test the freshness of food; can be used in environmental analysis to detect heavy metal ions in water, etc. The acceleration of the material transfer rate and the decrease of the charging current on the microelectrode all help to increase the ratio of the Faraday current to the charging current, increase the signal-to-noise ratio, and significantly improve the sensitivity of the analysis.