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Nov 20 2019

Principles and Types of Biosensors

Ⅰ. What is Biosensor?

Biosensor is an instrument which is sensitive to biological substances and converts its concentration into electrical signal for detection. Biosensor has the function of receiver and converter. Because enzyme membrane, mitochondrial electron transport system particle membrane, microbial membrane, antigen membrane and antibody membrane have the selective recognition function to the molecular structure of biomaterials and only have the catalytic activation function to specific reactions, so biosensors have very high selectivity. The disadvantage is that the biofilm is not stable.

Biosensors are mainly used in clinical diagnosis, treatment monitoring, fermentation industry, food industry, environment and robotics.

Biosensor is an interdisciplinary subject combining bioactive materials(Enzyme, protein, DNA, antibody, antigen, biofilm, etc)with physical and chemical transducers. It is an advanced detection method and monitoring method necessary for the development of biotechnology, and it is also a rapid and microanalysis method at the molecular level. In the 21st century, in the development of knowledge economy, biosensor technology will be a new growth point between information and biotechnology. It will have a wide application prospect in clinical diagnosis, industrial control, food and drug analysis(including biopharmaceutical research and development), environmental protection, biotechnology, biochip and other research in the national economy. All kinds of biosensors have the following common structures: including one or several related bioactive materials(Biofilm) and physical or chemical transducers(sensors) that can convert the signals expressed by bioactivity into electrical signals. The two are combined to reprocess the biological signals with modern microelectronics and automatic instrument technology to form a variety of usable biosensor analysis devices, instruments and systems.

Biosensor

Ⅱ. Principle of biosensors 

The substance to be measured enters into the bioactive material through diffusion, and after molecular recognition, biological reaction occurs. The information generated is then transformed into a quantitative and treatable electrical signal by the corresponding physical or chemical transducer, and then amplified and output by the secondary instrument, the concentration of the substance to be measured can be known.

Ⅲ. Characteristics of biosensors 

    (1)The biosensor uses the immobilized bioactive substance as the catalyst, and the expensive reagent can be reused many times, which overcomes the shortcomings of high cost of enzyme analysis reagent and complicated chemical analysis in the past.
    (2)Strong specificity, only react to specific substrate, and not affected by color and turbidity.
    (3)The analysis speed is fast, and the results can be obtained in one minute.
    (4)High accuracy; general relative error can reach 1%.
    (5)The operating system is simple and easy to realize automatic analysis.
    (6)Low cost; only a few cents per measurement in continuous use. 
    (7)Some biosensors can reliably indicate the oxygen supply and by-products in the microbial culture system. In the process of production control, many complex information can be obtained only by the comprehensive action of physical and chemical sensors. At the same time, they also pointed out the direction of increasing the yield of products.

Ⅳ. Types of biosensors

According to the classification of life substances used in biosensors, biosensors can be divided into microbial sensors, immune sensors, tissue sensors, cell sensors, enzyme sensors, DNA sensors, etc.

According to the principle of sensor detection, it can be divided into thermosensitive biosensor, FET biosensor, piezoelectric biosensor, optical biosensor, acoustic channel biosensor, enzyme electrode biosensor, mediator biosensor, etc.

According to the type of interaction between sensitive substances, it can be divided into two types: affinity type and metabolism type.

     4.1 Acoustic biosensor

Acoustic biosensor is a kind of sensor to detect the change of acoustic frequency caused by the substance to be detected. Among them, quartz crystal microbalance(QCM) biosensor has been studied most. In the piezoelectric crystal of quartz crystal microbalance biosensor, AT mode is often used to form two parallel metal((Au,Ag,Pt,Ni,Pd etc.)) membrane electrodes on both sides of the crystal by ion beam deposition. (At cutting means that the cutting surface is 25.15 ° to the main optical axis of quartz crystal. At this moment, the temperature coefficient of crystal resonance is close to zero at room temperature)The recognition molecules are fixed on the surface of the membrane electrode. Because of their specificity, the recognition molecules combine with the molecules to be detected, causing the quality change of the electrode surface, thus changing the oscillation frequency of the quartz crystal. If the nanoparticles are modified on the molecules to be detected, the quality of the molecules to be detected will be significantly improved, and the detection signal will also be enhanced. Ward et al. Labeled the antibody with nano colloidal particles, and combined it to the surface of quartz crystal by antibody antigen immunoassay. Because the modified colloidal particles(The diameter of sol particles is 5 "100nm) improved the quality of the labeled molecules, according to Sauerbrey equation, the oscillation frequency of quartz crystal was correspondingly increased, so the detection signal was amplified, the detection sensitivity was improved, and the detection lower limit was also reduced.

Acoustic biosensor

     4.2 optical biosensor

Nano metal particles can be used for optical resonance detection. Bauer et al. Fixed nano metal particles on the surface of conductive materials by antigen antibody or protein receptor binding methods. Due to the interaction of reflection dipoles of nano particles, the resonance of reflected light is enhanced. The materials to be detected can be detected by detecting resonance signals. Nanoparticles can also be used to locate tumors. Fluorescein labeled recognition factors bind to tumor receptors, and then the size and location of tumors can be displayed in vitro.

Nano metal particles can also be used as a general fluorescent annihilation group. Maxwell and other scientists labeled gold nanoparticles and fluorescence excitation groups at both ends of oligonucleotide probe molecules respectively. The probe formed a "hairpin" structure due to complementary bases, and the proximity of fluorescence excitation group and gold nanoparticles resulted in excitation fluorescence annihilation. When the probe combined with specific target DNA, its conformation changed, and the gold nanoparticles and fluorescence excitation groups were separated, so as to excite Fluorescence. The principle can be used for real-time fluorescence detection of nucleic acids and single base mutation polymorphism detection.

Optical biosensor

     4.3 Magnetic biosensor

Magnetic nanoparticles have important application value in biological detection and drug analysis. By using magnetic materials to label biomolecules and molecular recognition technology, complex operations such as sample mixing, separation and detection can be realized. Scientists label molecules with magnetic materials, and realize the separation and detection of samples under the magnetic field gradient. Richardson et al. Used magnetic counter to detect magnetic labeled molecules by magnetic immunoassay. In addition, the distribution and position of magnetic particles in vivo can be measured in vitro after the identification factor is labeled with nano magnetic particles and combined with the target recognition device on the tumor surface, so as to locate the tumor.

Chemla and other scientists used paramagnetic nanoparticles and a microscope based on high temperature transient DC superconducting quantum interface device (SQUID) to propose a novel rapid detection technology for biological samples. Firstly, the magnetic particles of the fixed antibody are suspended in the solution, and then the magnetized nanoparticles are generated under the instantaneous magnetic field pulse. When the magnetic field disappears, the particles tend to be free distribution, because the particles without the antibody are Brownian motion, so there is no detection signal; while the nanoparticles with the target molecule move in the way of Neel relaxation, resulting in a slowly attenuated magnetic signal, The substance to be detected can be analyzed by the signal collected by the squid. This technology can directly detect the labeled molecules without separating the nanoparticles which are not combined with the molecules to be detected, which shortens the detection time and improves the detection efficiency.

Magnetic biosensor

     4.4 Electrochemical biosensor

Colloidal gold is the most common metal nanoparticles, which can be used to mark biomolecules, thus realizing signal detection and amplification; in addition, it can also be widely used in TEM, SEM characterization and paper strip color. Many literatures also reported the signal amplification of colloidal gold in various biosensors. Gonzalez Garcia and other scientists used colloidal gold labeling and electrochemical methods to study the interaction between biotin and avidin. By modifying biotinylated albumin on the electrode surface and then reacting with avidin labeled by colloidal gold with a diameter of 10 nm, scientists found that the current response caused by colloidal gold was linearly related to the concentration of avidin(2.5×10-9mol/L "2.5×10–5mol/L).

Nanoparticles have excellent specific surface area, which can be used to immobilize biomolecules, increase the number of fixed molecules, and achieve signal amplification. Singh and other scientists used sol-gel method to synthesize silicon nanoparticles with a diameter of 20 nm or 200 nm. Acetylcholinesterase immobilized on the surface of nanoparticles can be used to make organophosphorus pesticide biosensor. Because of its high specific surface activity, combined with the detection of ion sensitive field effect tube, the metal nanoparticles with rapid response can be used as the carrier of catalyst, which can greatly improve the performance of catalyst. Enzyme colloidal gold is fixed on the surface of electrode and can be used for the electrochemical detection of H2O2, glucose, xanthine and hypoxanthine. Xu et al. Modified the surface of the screen printed carbon electrode with colloidal gold, combined with immunity and horseradish peroxidase (HRP) to make H2O2 biosensor. The results showed that the electrocatalytic performance and current response of HRP were significantly improved, the linear range of signal was greatly improved(0.8μM"1.0mM), and the detection limit was also reduced to 0.4 μ M.

Electrochemical biosensor

     4.5 Optical fiber nano biosensor

Compared with other types of biosensors, fiber-optic nano biosensors are not only small in size and high in sensitivity, but also free from electromagnetic interference and do not need reference devices. It can enter the interior of cells and measure the changes of structure and cytoplasm in vivo.

     (1)Optical fiber nano fluorescence biosensor

Kopelman was the first to use a fluorescent fiber-optic nano sensor to detect the pH value in the micro environment. Its working principle is to fix the fluorescent agent at the head of the optical fiber. When the fluorescent agent reacts reversibly with the proton, the optical property of the liquid changes. According to the change of the fluorescence intensity, the pH value can be determined. The optical fiber processing method is as follows: the optical fiber is drawn into a fiber probe with a head diameter of 100nm "1000nm by a fiber drawing instrument, and aluminum is plated on the surface of the optical fiber by a vacuum evaporator to prevent light from leaking during transmission. Then, the exposed optical fiber head is silanized, and the surface is modified into an active surface containing hydroxyl or amino group, and the antigen or antibody of the molecule to be detected is fixed and identified. Finally, the light The fiber head is combined with a pH selective fluorescent dye polymer. The response time of the nano sensor is 250ms, and it can detect the ion concentration of μ M. These characteristics are suitable for the detection of single cell and subcellular structure, such as the detection of pH value of mouse embryonic cell fluid.

(2)Optical fiber nano immune biosensor

Optical fiber nano immunosensor is a kind of sensor which applies optics and photonics technology to immunoassay. It can convert the amount of antigen or antibody to optical signal by using the characteristic that antigen and antibody can combine specifically. This kind of sensor combines the advantages of traditional immunoassay, optics and biosensor technology, and has high specificity, sensitivity and stability.At the same time, the fiber-optic nano immune sensor only uses nano products on sensitive components, so it not only retains many advantages of the original, but also makes it suitable for the measurement of single cells. Dinh et al. Have successfully developed an optical fiber nano immunosensor for the detection of BPT (benzo pyrene tetrol, a biomarker of DNA damage related to exposure to carcinogenic benzo [α] pyrene). They first made quartz fiber with a diameter of 10nm "100nm with a fiber drawing instrument, then silanized the fiber head, modified the fiber head with BPT antibody, and then plated the whole length of the fiber (except the modified fiber head) with silver to prevent light from leaking out. Finally, cell puncture and detection experiments were carried out on a single cell operated micromanipulator / microinjector, they used photomultiplier PMT to record the fluorescence produced by the binding of BPT and antibody, and detected the content of BPT in cells by measuring the change of fluorescence intensity. The minimum detection limit of the sensor can reach 10 – 21mol.


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