Quartz crystal microbalance electronically conducting polymers

A variety of chemical gas sensors are or could be used in electronic nose instruments. So far, successful results have been reached with conductive polymer (CP) sensors, metal oxide semiconductor (MOS) sensors, metal oxide semiconductor field effect transistor (MOSFET) sensors, quartz crystal microbalance (QCM) sensors, and infrared sensors. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

Electronic noses The so-called electronic noses consist of chemical gas sensors that are able to monitor changes in the offgas composition of fermentation processes. The different sensors of electronic noses are based on conductive polymers (CP), metal oxide semiconductors (MOS), metal oxide semiconductor field effect transistors (MOSFET), or quartz crystal microbalance (QCM). CP-based sensors use the electrochemical properties of polymers like polypyrrole or polyindole. The absorbance of selected molecules of the off-gas into the polymer film causes changes in the sensors conductivity. MOS sensors possess an electrochemically active surface of metal oxides like tin oxide or copper oxide. The sensitivity... 

The conductivity of the polymer layer may also depend on the physical state of the polymer. For instance, the sorption of organic vapors (e.g., alcohol) [130, 144,151,156] or acetone [154] causes a swelling of the polymer that alters the rate of interchain electron hopping. The mass change caused by the sorption can be followed by a piezoelectric quartz-crystal microbalance (QCM) or by sitrface acoustic wave (SAW) sensors. Optical changes can also be detected, although this effect is less frequently utihzed in gas sensors. 

Sensorics, also known as the electronic nose, represent another interesting technology where polymers are used because synthetic chemosensors can only detect a molecule when it is dissolved in a polymeric layer present in the sensor. The technology involves the use of a quartz crystal microbalance device to detect by volume and analyse surface waves on piezoelectric materials. Swelling detection on the basis of variations in conductivity also forms part of the technology. The role of polymers in this technology was discussed in an article by Banhegyi [3]. 

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