Quartz crystal microbalance 
Quartz crystal microbalance 
Quartz Crystal Microbalance (QCM) development began in the early 60s of last century, it is a very sensitive quality testing instruments, the measurement accuracy up to nanogram level, than the sensitivity of the microgram level of electronic micro-balance 100 times higher, the theoretically measurable mass change corresponds to a fraction of the monolayer or atomic layer. Quartz crystal microbalance uses the piezoelectric characteristics of the quartz crystal resonator, quartz crystal surface quality of the electrode into quartz crystal oscillation circuit output signal frequency changes, and then through the computer and other auxiliary equipment to obtain high-precision data.


The basic principle of quartz crystal microbalance is to take advantage of the piezoelectric effect of quartz crystal: each crystal lattice inside the quartz crystal is a hexagonal shape without external force. If mechanical pressure is applied on both sides of the crystal lattice, Of the charge center shift and polarization, then in the corresponding direction of the wafer will produce an electric field; the other hand, if the quartz crystal in the two electrodes plus an electric field, the wafer will have mechanical deformation, the physical phenomenon is called pressure Electric effect. If an alternating voltage is applied to both poles of the wafer, the wafer will have mechanical vibrations, and the mechanical vibration of the wafer will 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. However, when the frequency of the applied alternating voltage is a certain value, the amplitude is significantly increased. This phenomenon is called piezoelectric resonance. In fact, it is very similar to the resonance of the LC loop: when the crystal does not vibrate, it can be regarded as a flat-plate capacitor called the electrostatic capacitance C, generally about several PF to tens of PF; when the crystal oscillates, the inertia of the mechanical vibration Available inductance L to be equivalent, the general L value of tens of mH to several hundred mH. This constitutes a quartz crystal microbalance oscillator, the circuit oscillation frequency equal to the resonant frequency of the quartz crystal oscillator, and then measured by the host resonant frequency into electrical signals output. As the resonant frequency of the wafer itself is basically only related to the cutting mode, geometry and size of the wafer, and can be accurately done, the oscillation circuit using the quartz resonator can obtain high frequency stability.
In 1959, Sauerbrey obtained the conclusion that the variation of resonant frequency of QCM is proportional to the applied mass under the assumption that the applied energy is uniformly and rigidly attached to the surface of the gold electrode of QCM. For rigid deposits, the change in crystal oscillation frequency, ΔF, is proportional to the change in mass, △ M, of the deposit on the working electrode. Through this relationship can be QCM electrode surface quality changes.

The main structure

QCM mainly consists of quartz crystal sensors, signal detection and data processing and other components. Quartz crystal sensor is basically the basic structure: from a quartz crystal along the main optical axis of the quartz crystal 35 ° 15 'cut (AT-CUT) to obtain quartz crystal oscillating piece, in its two corresponding surfaces coated with silver Layer as an electrode, quartz crystal sandwiched between two electrodes to form a sandwich structure. Each electrode soldered a lead to the pin, coupled with the package shell to form a quartz crystal resonator, the product is generally packaged with a metal case, but also useful glass, ceramic or plastic package.
The other components of quartz crystal microbalance structure in different models and specifications of the instruments are not the same, according to the measurement needs of selected or combined. General ancillary structures also include oscillation lines, frequency counters, computer systems, etc .; electrochemical quartz crystal microbalance on the basis of this include potentiostat, electrochemical cell, auxiliary electrode, reference electrode, etc .; often additional auxiliary output Equipment, such as monitors, printers, etc.

Application and Outlook

QCM system
QCM (Quartz Crystal Microbalance) has the advantages of simple structure, low cost, large vibration Q value, high sensitivity and measurement accuracy up to nanogram level, and is widely used in chemistry, physics, biology, medicine and surface science And other fields, for gas, liquid composition analysis and micro-mass measurement, film thickness detection. Depending on your needs, you can also selectively apply a coating on metal electrodes to further broaden your application. For example, if a selective adsorption film is added to the electrode surface, it can be used to detect the chemical composition of the gas or to monitor the progress of the chemical reaction. With the rapid development of biological science, QCM has broad prospects as a bio-sensor in the field of biology.
QCM has the advantages of on-line tracking to detect the microscopic process and the advantages of rich online information, which is unmatched by other methods. With its advantages of simplicity, quickness, high sensitivity and on-line tracking, this technology will inevitably become a very effective means to study the microscopic process and mechanism of action, detection of trace and trace substances, and be widely used in combination with other technologies From the simple determination of concentration to the kinetic mechanism of the study. The future direction of development focused on the following aspects:
1. The theoretical treatment of the viscoelastic layer
2. Other surface technologies, such as surface-infrared spectroscopy, are used to investigate the interfacial properties of the crystal surface.
3. Research on the detection and function of trace substances. Such as biological tissue as a molecular recognition element, such as the role of trace elements.
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