Quarz Crystal Microbalance, QCM(QCM Z-500 for impedance-based QCM) Definition: The Quartz Crystal Microbalance, QCM is an ultra-sensitive mass sensor utilizing the piezoelectric properties of quartz crystals. The QCM can detect adsorbed masses in the nanogram range. Summary/Description: The heart of the QCM is the piezoelectric AT-cut quartz crystal sandwiched between a pair of electrodes. When the electrodes are connected to an oscillator and an AC voltage is applied over the electrodes the quart crystal starts to oscillate at its resonance frequency due to the piezoelectric effect. Any mass addition or adsorption to the quartz crystal surface will change the resonance frequency. In ideal conditions, i.e. rigid layer deposited evenly on the quartz surface, this change in resonance frequency is directly proportional to the added mass. Hence, the QCM is basically a mass sensing device with the ability to measure mass changes in the nanogram range on a quartz crystal resonator in real-time. This means that QCM´s are capable of measuring mass changes as small as a fraction of a monolayer or single layer of atoms. The QCM was in the beginning only used for monitoring adsorption of materials in air or vacuum, but in the beginning of 1980´s scientists realized that a quartz crystal can be excited to a stable oscillation when it was completely immersed in a liquid without loosing its sensitivity to mass changes at the solid-liquid interface. After this finding QCMs have been used in direct contact with liquids and/or visco-elastic films to assess changes in mass and visco-elastic properties. Even in air or vacuum, where the damping of layers has been considered to be negligible or small the QCM has been used to probe dissipative processes on the quartz crystal. This is especially true for soft condensed matters such as thick polymer layers deposited on the quartz surface. Due to the influence of dissipative processes in many systems it is nowadays generally accepted that measuring only the fundamental frequency changes are not enough, and it is recommended to measure frequency and dissipation changes at multiple overtones. This is enabled by using an impedance analyzer for determining the quartz crystal properties at multiple overtones. The high sensitivity and the real-time monitoring of mass changes on the sensor crystal make QCM a very attractive technique for a large range of applications. Especially, the development of QCM systems for use in fluids or with visco-elastic deposits has dramatically increased the interest towards this technique. Major advantages of the QCM technique used for liquid systems are that it allows a label-free detection of molecules. A partial list of the application areas of the QCM is shown below, and it seems that the application areas are only limited by your imagination.
|
Back
