Mass sensitive devices

Numerical simulation of the experiments [7] became increasingly available during the 1980s, and ultramicroelectrodes [8] opened the way not only to ever-faster timescales but also to finer lateral resolution when characterizing electrode processes. Finally, combinations with spectroscopic and mass-sensitive devices opened new ways to augment information available from molecular electrochemical experiments. 

Chemical sensors are by definition small, inexpensive and preferably hand-held devices, capable of continuously monitoring chemical constituents in liquids or gases. MIP sensors usually consist of an imprinted sensitive layer and a transducer to convert the chemical information, in real time, into an electrical or optical signal which is further evaluated electronically [12]. Figure 21.1 shows the set-up of chemosensors and two typical mass-sensitive devices. 

In particular, the detection of neutral or inert analytes, such as anaesthetics, odours or hazardous compounds via weak complex formation within the MIPs is the forte of mass-sensitive devices due to the ultra-low detection limits, such as 1 pg for a IGHz SAW. It is important that the sensitive layer is tightly bound to the metal electrode (QCM) or covalently linked to the piezo substrate (SAW) in order to achieve a stable coating in liquids. A stable link is sometimes crucial, for example when using polystyrenes (PSts). 

At this stage QCM applications in aqueous media are still a vivid research area. Further sensitivity improvements can be obtained by an STW resonator. However, the real time observation of the incorporation processes, either specific or nonspecific, in comparison to a non-imprinted device is performed much more easily utilising QCMs than with optical devices. In particular, non-specific adsorption of cross-sensitive substances can be excluded by the use of suitable materials, such as polyurethanes, for mass-sensitive devices. In the case of optical sensing, the responses might be influenced by quenching or self-absorption. 

Imprinted Polymers in Chemical Recognition for Mass-Sensitive Devices... 

When the QCM is used as a mass-sensitive device in electrochemical experiments, it is often important to control the electrical potential of the electrode that is facing the liquid. Thus, an additional (reference) electrode is introduced into the QCM chamber in order to provide well-defined electrochemical conditions and to allow for various kinds of electrochemical reactions at the crystal surface. A well-known example is the electrodeposition of metals on the electrode surface that is often used to calibrate the device and calculate its mass sensitivity. When these kind of electrochemical studies are combined with QCM readings, the acronym EQCM is used, abbreviating electrochemical quartz crystal microbalance. 

Kaspar M et al (2000) Thickness shear mode resonators ( mass-sensitive devices ) in bioanalysis. Presenius J Anal Chem 366(6-7) 602-610... 

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