QCM and SAW transducers allow analyte detection from nanometer- to micrometer-dimensions using imprinting techniques

Mass-sensitive devices like the quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices show a major advantage as a transducer principle as every analyte can be detected due to its mass. In order to transfer QCMs and SAWS into chemical sensors a layer has to be applied in which the desired analyte is preferentially incorporated. Such coatings can vary from molecular hollows like calix[n]arenes to monolayers and molecular imprinted polymers (MIP). MIPs are produced by polymerization of carefully selected monomers around a template, the desired analyte. Such monomers can carry functional groups which interact with the analyte. When the template is removed by evaporation or washed out, it leaves behind specially adapted hollows in respect to size and interactions in which the analyte can be re-included. With these sensitive layers it was possible to achieve selectivities for poly aromatic hydrocarbons which are comparable to that of natural antibodies. Bulk imprinted MIPs allow the synthesis of,highly packed artificial, receptor sites for small organic molecules. The high amount of sites within the coating of a QCM/SAW allows detection limits down to the ppb range. Due to diffusion limitations the imprinting technique has to be adapted to the size of the analyte. The technique is not limited to single compounds, complex mixtures can also be used as templates. In this way it was possible to determine motor oil degradation. Even whole cells can act as imprinting media.