## Results and Discussion
This is underpinned by the fact that the signals achieved at 70\({}^{\circ}\)C are much higher than for the other layers thus suggesting optimized uptake of the analyte. Imprinting strategies are not restricted to analytes of molecular scale, but also much larger particles can be used, such as enzymes or entire microorganisms. The synthetic procedures, however, have to be slightly modified, as imprinting into bulk materials is not possible in this case. First, for microorganisms with several \(\mu\)m in diameter the resulting layers would be much too thick to be useful for mass-sensitive detection. Second, as the analytes in this case are very bulky, they would diffuse through a three-dimensional polymer only very slowly (not speaking about the limitations imposed on template removal during synthesis) thus greatly enhancing the sensor response time and making it useless for sensing applications. For this purpose surface imprinting procedures have shown to be a suitable tool for sensor layer development. This principle has been successfully applied to yeast, that served as the microorganism used for the fundamental developments on the way to these sensors [11]. Figure 6: Enrichment factors of polyurethane layers imprinted with different template mixtures at 20\({}^{\circ}\)C and 70\({}^{\circ}\)C  
Figure 7 shows an AFM-image of one of these layers prepared by a surface stamping procedure as well as the respective sensor response obtained with a 10 MHz QCM towards yeast. The microorganisms on the stamp obtain a hexagonal packing and the pits left behind represent the typical size distribution of yeast cultures. These pits are optimally adapted to re-include cells from suspensions, which can also be seen in the sensor responses. The QCM frequency shift is perfectly reversible, the 1200 Hz obtained is the expected value for a monolayer of yeasts on the surface. Thus, the imprinted materials achieve antibody-like interaction properties towards the microorganisms used. The main difference to natural antibodies, however, is the ability to remove the cells from the material surface. This exactly meets the requirements of chemical sensors, as in this case high selectivity and reversibility is desired, which is often difficult to achieve. Viruses represent a group of microorganisms, for which it would be highly interesting to develop selective sensors systems, especially because up to now no easy on-line method for their detection exists. This is a result of their size (below 1 \(\upmu\)m) which makes them literally invisible for methods operating in the visible range of the electromagnetic spectrum (i.e. microscopy and scattering methods). The sensor layers are produced by a stamping method similarly to the yeast imprints, the procedure is shown in figure 8. Thus, the stamp and the polymer coating are prepared separately and the final material again is obtained by mechanically pressing the two substrates together. The polymerization takes place in the submicrometre-area around the templates that ensure an ideal adaptation of the cavities generated by forming an optimized interaction network between the reacting oligomeric chains and functional groups on the virus surface. An AFM-image of such a functionalized polymer is given in figure 9. The resulting pits once again can clearly be seen, however, the self organization of the yeasts is more pronounced. Nonetheless, the virus-temlated materials show large frequency shifts on mass-sensitive transducers, as can also be seen in figure 10, where the sensor characteristic for such a TMV-imprinted material is given. Figure 7: AFM-image of a yeast-imprinted polyurethane and sensor effect of that material towards a suspension of the template microorganism in buffer. Figure 8: Stamping technique for viruses  
Figure 9: TMV-imprinted polyurethane layer.