First, the basic principle of quartz crystal microbalance:
The most basic principle of quartz crystal microbalance is to use the piezoelectric effect of quartz crystal: each crystal lattice inside the quartz crystal is a regular hexagon when it is not subjected to external force. If mechanical pressure is applied on both sides of the wafer, the crystal lattice will be formed. If the charge center is shifted and polarized, an electric field will be generated in the corresponding direction of the wafer; conversely, if an electric field is applied to the two electrodes of the quartz crystal, the wafer will be mechanically deformed. This physical phenomenon is called pressure. Electrical effect. If an alternating voltage is applied across the two poles of the wafer, the wafer will mechanically vibrate and the mechanical vibration of the wafer will again produce an alternating electric field. In general, the amplitude of the mechanical vibration of the wafer and the amplitude of the alternating electric field are very small, but when the frequency of the applied alternating voltage is a certain value, the amplitude is significantly increased. This phenomenon is called piezoelectric resonance. It is actually very similar to the resonance phenomenon of the LC loop: when the crystal does not vibrate, it can be regarded as a plate capacitor called electrostatic capacitance C, generally about several PF to several tens of PF; when the crystal oscillates, the inertia of mechanical vibration It can be equivalent by the inductance L. Generally, the value of L is several tens of mH to several hundred mH. Thus, the oscillator of the quartz crystal microbalance is formed. The oscillation frequency of the circuit is equal to the resonance frequency of the quartz crystal oscillation plate, and the measured resonance frequency is converted into an electrical signal output by the host. Since the resonant frequency of the wafer itself is basically only related to the cutting mode, geometry, and size of the wafer, and can be made precise, an oscillation circuit composed of a quartz resonator can achieve high frequency stability.
Second, the main structure of quartz crystal microbalance:
QCM is mainly composed of quartz crystal sensor, signal detection and data processing. The basic structure of the quartz crystal sensor is roughly as follows: a quartz crystal oscillation plate is obtained from a quartz crystal along a 35015' cut (AT-CUT) with the main optical axis of the quartz crystal, and a silver layer is applied on the two corresponding faces thereof. The electrode, the quartz crystal is sandwiched between the two electrodes to form a sandwich structure. A lead wire is attached to each of the electrodes to the pins, and a package is formed to form a quartz crystal resonator. The products are generally packaged in a metal case and are also packaged in a glass case, ceramic or plastic. The other components of the quartz crystal microbalance are also different in different models and specifications, and can be selected or used according to the measurement needs. The general auxiliary structure also includes an oscillating circuit, a frequency counter, a computer system, etc.; the electrochemical quartz crystal microbalance further includes a potentiostat, an electrochemical cell, an auxiliary electrode, a reference electrode, etc.;
3. Analytical chemistry application of quartz crystal microbalance
QCM was first applied to the detection of gas phase components and toxic and explosive gases. The detection of toxic and explosive gases such as SO2, H2 S, HCI, NH3, NO2, Hg, CO, and other hydrocarbons and cyanides has been conducted. For the first time, Dr. S. Iijima of Japan discovered carbon nanotubes (CNTs) whose structure is hollow carbon formed by single-layer (single-walled carbon nanotubes) or two layers (MWCNTs) and very fine cylindrical graphite sheets. Cage tube. Using MWCNTs as gas sensing material, it is uniformly coated on the surface of QCM to form a sensitive film. Using MWCNTs sensitive film to adsorb 16 mg/m3 formaldehyde and 9.64 mg/m3 water vapor, formaldehyde and 19. The concentration signal of 64 mg/m3 water vapor is converted into a frequency signal to detect 16 mg/m3 formaldehyde and 9.64 mg/m3 water vapor.
Fourth, the application of quartz crystal microbalance in biomedicine
In biomedical science, a piezoelectric crystal biosensor is fabricated by modifying a bioactive specific selective functional membrane on the QCM probe electrode. Because of its high sensitivity to mass change, the sensor has good specificity, high sensitivity, low cost and operation. Simple and so on. It has been widely used in molecular biology, pathology, medical diagnostics, bacteriology and other research fields. This year, it has played an important role in the research and detection of proteins, microorganisms, nucleic acids, enzymes, cells, etc., and has broad development. prospect.
1, protein detection:
The QCM method detects protein based on the principle of immune reaction. The antigen/antibody is immobilized on the surface of the quartz electrode and immunologically bound to the specific antibody/antigen in the solution to be tested. The formed complex is deposited on the surface of the electrode, causing the oscillation frequency of the quartz crystal to decrease. A quartz crystal microbalance prepared by using an antigen-antibody immunological binding reaction to cause a change in mass. At present, QCM technology has been applied to the detection of immunoglobulin, albumin, fibrin (original) and degradation products, complement, enzyme protein, thyroxine, human chorionic gonadotropin and cortisol. Based on the realization of liquid crystal oscillation of piezoelectric quartz crystal, the reaction system was sampled and continuously monitored. In 1996, researchers in China studied the adsorption process of IgM antigen on the surface of solid antibody in IgM immunoscreening, and deduced the antigen excess. The reaction kinetics equation below calculates the Arrhenius activation energy of the immune response for the first time, and the theoretical derivation is consistent with the experimental results. In recent years, Q-sense AB of Sweden has developed QCM-D technology, the core of which is a quartz crystal sensor. An alternating voltage is applied across the electrodes to induce a small shearing vibration at the resonant frequency of the quartz crystal sensor. When the alternating voltage is turned off, the vibration is exponentially attenuated, and this attenuation is recorded to obtain the resonant frequency (f) and consumption. The scattering factor (D) is two parameters. Multiple frequency and dissipation factor data are obtained per second, which is convenient for calculation of reaction kinetics. Compared with radioimmunoassay and enzyme-linked immunosorbent assay, it has better selectivity, wider linear range, and does not require biomolecular labeling.
2. Microorganisms:
The bacteria are detected using the principle of combining bacterial antigen-target bacterial antibodies. In 1992, Plomer et al. detected Enterococcus by this method for the first time. It has been developed on a large scale and has mastered the detection methods of various bacteria such as Candida albicans, Escherichia coli, Salmonella, Vibrio cholerae. Fung et al. improved the adsorption process of the antibody on the surface of the foundation to coagulate, the working range reached 102 ~ 105 cells / L, the detection limit was 1. 7 × 102 cells / L. The most widely used virus detection method is to detect viral antibodies in a solution to be tested by a QCM technique in which a viral antigen is immobilized on an electrode surface. Some scholars have fixed the monoclonal antibody of the virus on the QCM electrode by staphylococcal protein A, and directly detected the virus particles (5*104~1*109/quartz crystal surface). Compared with the enzyme-labeled immunoreactivity assay, in addition to the absence of molecular markers, false positive reactions can be reduced a lot. Currently used to detect infectious diseases such as Coxsackie virus, hepatitis virus, herpes virus, dengue virus and HIV. The response of the quartz crystal microbalance to viscosity and density was determined indirectly by measuring the solidification process of the gelatin medium film after the addition of the bacterial solution using QCM coated with a solid medium film.
3. Nucleic acid:
In 1988, Fawcett et al first used the QCM technique to determine the hybridization of RNA/DNA molecules, thus setting a precedent for the study of enzymes in DNA piezoelectric sensors. The DNA piezoelectric sensor works by immobilizing the ssDNA probe on the surface of the electrode and then immersing it in a solution containing the target ssDNA molecule. When the nucleic acid probe on the electrode hybridizes with the target ssDNA molecule in the solution, the surface quality of the crystal increases to cause a decrease in the oscillation frequency. The biggest advantage of QCM detection nucleic acid technology is that it is easy and fast to operate, especially in the experimental diagnosis of pathogenic microorganisms with difficult artificial cultivation and complex identification process. In addition, quartz crystal microbalances are also used for the detection of genetic mutations. Applying MutS to the surface of the QCM electrode, a protein that binds to a mismatched base pair in the dsDNA molecule. If there is a mismatch in the target dsDNA molecule, a binding reaction occurs, resulting in complex deposition, which can detect single base mutations and 1~ 4 base pair insertional mutations.
4. Coagulation factor:
The conventional method is based on the plasma coagulation reaction, and the end point of the reaction is judged by observing the change of viscosity and optical density in the system. Due to the inconsistency of the naked eye observation and manual operation by the experimenter, there are many disadvantages. Using QCM to detect clotting factor activity, using a 10MHz silver-plated quartz crystal, the starting point and the end point are judged by real-time changes. This method is fast, accurate, and low-cost.
5. Cell detection:
At present, there are two main applications. One is to establish a QCM system to detect the content of cells in various blood cells based on the receptor-ligand affinity reaction. The other is the detection of tumor cells, which immobilizes the extracellular matrix protein on the piezoelectric quartz electrode. Under the integrin-mediated, the human ovarian cancer cells to be tested adhere to the extracellular matrix protein. Formation of extracellular matrix protein-integrin-cancer-like sandwich-like structure, the increase in electrode mass and the decrease in electrode oscillation frequency are in accordance with the Sauerbrey equation.
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