Sensors and communication systems and computers *** with the composition of modern information processing systems. Sensors are equivalent to human senses, are the interface between computers and the natural world and society, and are a tool for providing information to computers.
Sensors are usually composed of sensitive (identification) elements, conversion elements, electronic circuits and corresponding structural accessories. Biosensors are sensors that use immobilized components of the organism (enzymes, antigens, antibodies, hormones, etc.) or the organism itself (cells, organelles, tissues, etc.) as sensing elements. Electrochemical biosensors, on the other hand, are sensors that use biological materials as sensitive elements, electrodes (solid electrodes, ion-selective electrodes, gas-sensitive electrodes, etc.) as conversion elements, and detect signals characterized by electric potential or current. Figure 1 is a schematic diagram of the basic composition of electrochemical biosensors. Due to the use of biological materials as the sensitive element of the sensor, the electrochemical biosensor has a high degree of selectivity, and is an ideal analytical tool for rapid and direct access to information on the composition of complex systems. Some research results have been applied in biotechnology, food industry, clinical testing, pharmaceutical industry, biomedicine, environmental analysis and other fields.
According to the different biomaterials used as sensitive elements, electrochemical biosensors are divided into enzyme electrode sensors, microbial electrode sensors, electrochemical immunosensors, tissue electrodes and organelle electrode sensors, electrochemical DNA sensors, and so on.
(1) Enzyme Electrode Sensor
Take the glucose oxidase (GOD) electrode as an example to briefly describe its working principle. Under the catalytic action of GOD, glucose (C6H12O6) is oxidized by oxygen to produce gluconic acid (C6H12O7) and hydrogen peroxide:
Based on the above reaction, it is obvious that the glucose content can be indirectly measured by the oxygen electrode (to measure the oxygen consumption), the hydrogen peroxide electrode (to measure the production of H2O2), and the pH electrode (to measure the change of acidity). Therefore, by fixing GOD on the surface of the above electrodes, a GOD sensor for glucose measurement can be formed. This is the so-called first-generation enzyme electrode sensor. Since this sensor is an indirect measurement method, there are many interfering factors. The second generation enzyme electrode sensor uses a redox electron mediator to transfer electrons between the redox activity center of the enzyme and the electrode. The second generation of enzyme electrode sensor is not limited by the measurement system, and the linear range of the measured concentration is wider with less interference. Now many researchers are trying to develop the third generation of enzyme electrode sensors, i.e., enzyme electrode sensors in which the redox active center of the enzyme directly exchanges electrons with the electrode surface. At present, the commercial enzyme electrode sensors include: GOD electrode sensor, L lactate monooxygenase electrode sensor, uric acid enzyme electrode sensor and so on. There are many more enzyme electrode sensors under research.
(2) Microbial Electrode Sensors
The expensive price and poor stability of dissociated enzymes have limited their application in electrochemical biosensors, which has led researchers to think of directly utilizing live microorganisms as sensitive materials for molecular recognition elements. This kind of electrochemical biosensors, which are composed of microorganisms (commonly used are bacteria and yeasts) immobilized on the electrode surface as sensitive materials, are called microbial electrode sensors. Its working principle can be roughly divided into three types: first, the use of microbial body contains the enzyme (single or complex enzyme) system to identify molecules, this type is similar to the enzyme electrode; second, the use of microorganisms on the assimilation of organic matter, through the detection of its respiratory activity (oxygen uptake) increase, that is, through the oxygen electrode to measure the reduction of oxygen in the system to indirectly determine the concentration of organic matter; third, through the determination of electrode-sensitive Third, by measuring the electrode-sensitive metabolites, we can indirectly determine some of the organic matter that can be assimilated by anaerobic microorganisms.
Microbial electrode sensor in the fermentation industry, food testing, health care and other fields have applications. For example: the determination of glucose in the food fermentation process of Pseudomonas fluorescens electrode; determination of methane flagellar methyl monocyte electrode; determination of antibiotics cephalosporins Citrobacterfreudii bacterial electrode and so on. Microbial electrode sensors have good application prospects due to low cost and long service life, but its selectivity and long-term stability need to be further improved.
(3) Electrochemical immunosensors
Antibodies have a unique recognition and binding function to the corresponding antigen. Electrochemical immunosensors are detection devices that utilize this recognition and binding function to combine antibodies or antigens with electrodes.
According to the structure of electrochemical immunosensors, they can be divided into two categories: direct and indirect. The direct type is characterized by the recognition and binding of the antibody to its corresponding antigen, while the information of its immune response is directly converted into an electrical signal. These sensors can be further categorized structurally into binding and separating types. The former is to fix the antibody or antigen directly on the electrode surface, the sensor and the corresponding antibody or antigen binding at the same time to produce a potential change; the latter is to use the antibody or antigen to make antibody or antigen membrane, when it reacts with the corresponding ligand, the membrane potential changes, the electrode to determine the membrane potential and the membrane is separate. The indirect type is characterized by the conversion of information about the binding of antigen and antibody into another kind of intermediate information, which is then converted into an electrical signal. These sensors can also be further categorized structurally into two types: bound and separated. In the former, the antibody or antigen is immobilized on the electrode; in the latter, the antibody or antigen and the electrode are completely separated. Indirect electrochemical immunosensors are usually labeled with enzymes or other electroactive compounds to chemically amplify the concentration information of the antibody or antigen being measured, thus achieving very high sensitivity.
Examples of electrochemical immunosensors are: hCG immunosensors for diagnosis of early pregnancy; alpha-fetoprotein (AFP or αFP) immunosensors for diagnosis of primary liver cancer; immunosensors for the determination of human serum albumin (HSA); IgG immunosensors, insulin immunosensors, and so on.
(4) Tissue electrode and organelle electrode sensors
Directly using plant and animal tissue as sensitive elements of electrochemical sensors called tissue electrode sensors, the principle is to use plant and animal tissues of the enzyme, the advantages of the enzyme activity and its stability than the dissociated enzyme is high, easy to obtain the material, easy to prepare, long service life and so on. However, there are still deficiencies in selectivity, sensitivity and response time.
Animal tissue electrodes include: kidney electrode, liver electrode, intestinal electrode, muscle electrode, thymus electrode and so on. Determination of the main objects: glutamine, glucosamine 6 phosphate, D amino acids, H2O2, digoxin, insulin, adenosine, AMP and so on. Plant tissue electrodes have a wide range of sensitive components, including roots, stems, leaves, flowers and fruits of different plants. Plant tissue electrodes are simpler to prepare than animal tissue electrodes, less costly and easy to store.
Organelle electrode sensors are sensors that utilize plant and animal organelles as sensitive elements. Organelle refers to the tiny "organs" that exist in the cell surrounded by the membrane, such as mitochondria, microsomes, lysosomes, peroxisomes, chloroplasts, hydrogenase particles, magnetosomes and so on. The principle is to utilize the enzymes contained in the organelle (often a multi-enzyme system).
(5) Electrochemical DNA sensors
Electrochemical DNA sensors are a new idea of biosensors that have developed rapidly in recent years. Its use is to detect genes and some substances that can have special interactions with DNA. Electrochemical DNA sensor is the use of single-stranded DNA (ssDNA) or gene probe as a sensitive element fixed on the surface of the solid electrode, coupled with the recognition of hybridization information of the electroactive indicator (known as hybridization indicator)*** with the composition of the detection of specific genes of the device. The working principle is to use a specific sequence of ssDNA fixed on the electrode surface and the homologous sequence in the solution of the specific recognition of the role (molecular hybridization) to form double-stranded DNA (dsDNA) (electrode surface properties change), at the same time, with the help of a hybridization indicator that can recognize ssDNA and dsDNA signal change in the current response to achieve the detection of the gene purpose.
Electrochemical DNA sensors with detection sensitivity as high as 10-13g/mL have been reported, and Hashimoto et al[8] used a 20-aggregate nucleotide probe modified on a gold electrode to detect the oncogene v myc on the PatⅠ fragment of PVM623. Electrochemical DNA sensors are still quite far from practical use, mainly because of the stability, reproducibility, and sensitivity of the sensor. The electrochemical DNA sensor is still far away from practical use, mainly because the stability, reproducibility and sensitivity of the sensor need to be improved. The research on DNA-modified electrodes is of great significance for gene detection, but it can also be used for other biosensor research, for the interaction between DNA and exogenous molecules [9], such as anticancer drug screening, anticancer drug mechanism research; and for the detection of DNA binding molecules. Undoubtedly, it will become a very vital frontier field of bioelectrochemistry.
Bioelectrochemistry involves a wide range of aspects and is very rich in content. The above is only an overview of some areas of this interdisciplinary field. It is believed that bioelectrochemistry will further flourish with the development of related disciplines.