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生物感測器原理與應用 Quartz Crystal Microbalance
What does QCM stand for? QCM stands for Quartz Crystal Microbalance.
What other abbreviations are used for QCMs? A Quartz Crystal Microbalance is sometimes called a QMB, for "Quartz MicroBalance". When used by electrochemists, a QCM is often called an EQCM for "Electrochemical Quartz Crystal Microbalance".
What is a QCM, physically? Physically, a QCM consists of a thin, usually round, slice of crystalline quartz with an electrode on each side. The slice is cut in a particular orientation called an "AT-cut". The electrode can be made of any metal, but gold is the most common choice because it does not oxidize in air. If gold is used, a thin "undercoat" of chromium is usually put onto the quartz first. This is because gold by itself does not stick all that strongly to quartz. However, chromium sticks well to both gold and quartz. So using a chromium undercoat improves adhesion.
How does a QCM work? If the two electrodes are put at different potentials an electric field results across the QCM, i.e. in the "Y direction". Because of the piezoelectric properties of quartz, such an electric field in the "y direction" couples to shear motion "around" the z-axis, and vice versa. The end result is that shear waves in the quartz, in which the mechanical displacement is in the "x" direction, also called the electric axis, are coupled to voltage between the electrodes.
What are QCMs used for? Almost all QCMs are used as sensitive detectors of mass deposited on them. This added mass decreases the resonant frequency of the QCM. By measuring the decrease in the resonant frequency of the QCM, and knowing something about the physics of the QCM you can calculate the added mass per unit area on the QCM. Because frequency changes can be measured to very high precision, QCMs are very sensitive. They can measure amounts of deposited material with an average thickness of less than a single atomic layer. Hence the "microbalance" part of their name.
What is a simple useful approximate equation for the frequency shift of a QCM as a function of added mass? 1957, Sauerbrey
Application areas • Biomolecular adsorption • Antibody-antigen interactions • Cell adhesion • Polymer science • Marine biofouling • Surfactants • Surface Coatings
ANTIGEN-ANTIBODY REACTION First HSA (a 67 kD protein, present in blood) is introduced to the surface (a). Upon adsorption, f decreases in proportion to the increased mass on the crystal while D increases only slightly, indicating a fairly rigid film. The frequency change corresponds to a mass change of 180 ng/cm2. In the next step (b) the sensor surface is rinsed from excess HSA by buffer. An antibody against HSA (150 kD) is added (c). The decrease in f shows that the antibody binds to the HSA on the sensor surface. A significant increase in D indicates the formation of a non-rigid layer of antibodies. Finally the surface is rinsed again with buffer, which removes loosely bound antibodies (d).
LIPID VESICLE ADSORPTION Monodisperse phospholipid vesicles (~25 nm diameter) were deposited on sensor crystals with alkane terminated thiols, SiO2, and oxidized gold surfaces respectively.