What is Quartz Crystal Microbalance
Pierre and Marie Curie showed in 1880 that crystals of Rochelle salt could produce electricity when pressure was applied in certain crystallographic directions.Later they also showed the converse effect i.e. production of strain by application of electricity. these findings were the discovery of the piezoelectric effect.piezoelectricity did not receive lot of interest in the beginning and a more detailed study of piezoelectricity was not started until 1917 when it was showed that quartz crystals could be used as transducers and receivers of ultrasound in water. in 1919 several devices of everyday interest based on the piezoelectricity of Rochelle salt was described i.e. loudspeakers, microphones and sound pick-ups. in 1921 the first quartz crystal controlled oscillator was described. these first quartz crystal controlled oscillators were based on X-cut crystals, which have the drawback of being very temperature sensitive. therefore, the X-cut crystals are nowadays used in applications where the large temperature coefficient is of little importance, such as transducers in apace sonars.
The dominance of the quartz crystal for all kind of frequency control applications started in 1934 when the AT-cut quartz crystal was introduced. The advantage with the AT-cut quartz crystal is that it has nearly zero frequency drift with temperature around room temperature. From the very beginning of using quartz crystal resonators as frequency control elements it was common to increase the frequency of the resonator by drawing pencil marks on the electrodes, or decreasing the frequency by rubbing of some electrode material with an eraser. The understanding of this mass induced frequency shift was only known on the qualitative basis, However, in 1959 Sauerbrey published a paper what showed that the frequency shift of a quartz crystal resonator is directly proportional to the added mass. Saeuerbreys work is generally taken as the breakthrough and the first step towards a new quantitative tool to measure very small masses i.e. the quartz crystal microbalance.
Hence, one can describe the QCM to be an ultra-sensitive mass sensor. The heart of the QCM is the piezoelectric AT-cut quartz crystal sandwiched between a pair of the electrodes. When the electrodes are connected to an oscillator and an AC voltage is applied over the electrodes the quartz crystal starts to oscillate at its resonance frequency due to the piezoelectric effect. This oscillation is generally very stable due to the high quality of the oscillation(high Q factor).
If a rigid layer is evenly deposited on one or both of the electrodes the resonant frequency will decrease proportionally to the mass of the adsorbed layer according to the Sauerbrey equation:
There are situations where the Sauerbrey equation does not hold, for example, (a)when the added mass is not rigidly deposited on the electrode surfaces, (b)slips on the surface, (c)not deposited evenly on the electrodes. Therefore, the Sauerbrey equation is only strictly applicable to uniform, rigid, thin film deposits. Due to this the QCM was for many years just regard as a gas-phase mass detector. Not until 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. Much of the pioneering work in liquid phase QCM measurements have been done by Kanazawa and co-workers, who showed that the change in resonant frequency of a QCM taken from air into a liquid is proportional to the square root of the liquid's density-viscosity product:
After it was found out that an excessive viscous loading would not prohibit use of the QCM in liquids and that the response of the QCM is still extremely sensitive to mass changes at the solid-liquid QCMs have been used in the direct contect with liquids and/or visco-elastic films to assess changes in mass and visco-elasitc 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.
Applications of quartz crystal microbalance
QCM is basically a mass sensing device with the ability to measure very small changes on a quartz crystal resonator in real-time, The sensitivity of the QCM is approximately 100 times higher than an electronic fine balance with a sensitivity of 0.1ug. This means that QCM capable of measuring mass changes as small as a fraction of a monolayer or single layer or atoms. 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 have 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.
Thin film thickness monitoring in thermal, ebeam, sputtering, magnetron, ion and laser deposition.
Electrochemistry of interfacial processes at electrode surfaces
interactions of DNA and RNA with complementary strands
Specific recognition of protein ligands by immobilized receptors, immunological reactions
Detections of virus capsids, bacteria, mammalian cells
Adhesion of cells, liposomes and proteins
Biocompatibility of surfaces
Formation and prevention of formation of biofilms
Creation of selective surfaces
Thin film formation
Langmuir and langmuir-Blodgett film
Surfactant interactions with surfaces
Effectiveness of surfactants
Dissolution of polymer coatings
Molecular interaction of drugs
Cell response to pharmacological substances
Liquid Plating & Etching
In situ monitoring of lubricant and petroleum properties