## 4 Bioanalyte detection  
Therefore, molecular imprinting leads to excellent sensitivities for the detection of small molecules, as shown with the examples above. Similar recognition abilities can be achieved when detecting biological species such as micro organisms [8] or viruses [9]. One example for this is the selective detection of different human rhinovirus serotypes: the HRV has an icosahedral structure with 30 nm diameter. Different serotypes have the same geometrical features; however, the amino acid composition of their outer protein shell differs from each other. These chemical differences can be detected with QCM. Our measurements with an HRV-1a imprint inducated a frequency effect of 370 Hz for HRV-1a, whereas HRV-2 leads to a responseof 110 Hz. Therefore, the imprint evidently prefers the template species with a very appreciable selectivity factor. This selectivity is even more impressive given the fact, that both analytes belong to the so-called minor group of HRV serotypes, whose members are chemically very closely related to each other.  
A further example for detecting geometrically similar species by imprinted layers via differences in surface chemistry is the selective sensing detection erythrocytes. Erythrocytes - the red blood cells (RBC) - are responsible for transporting both oxygen and carbon dioxide between lung and cells in human and animal organisms. They have a diameter of approximately 7 um (see also Figure 5) but are very flexible and thus fit through the smallest veins. RBC's are classified in four blood groups (O, A, B, AB), with each of them being defined by a characteristic antigen on the cell surface. This is usually an oligosaccharide with six sugars, except for BG O that has one sugar less and is the precursor for all other groups (and thus present on any RBC). Groups A and B differ by the sixth sugar molecule, which is N-acetyl-D-galactosamine for A and D-galactoside for B.  
Figure 6: (A) Erythrocyte ghost in a polyurethane surface and cross-section.  
(B) Erythrocyte-ghost imprinted surface after washingRBCs are very elastic, so the quality of the imprint sites can be further improved by preparing erythrocyte ghosts [10] as template, as they have higher form stability are easier to handle. Such ghosts consist exclusively of the outer membrane and are prepared by treating the cells with a hypertonic NaCl solution for a few minutes which makes cells bursting. Figure 6 shows an AFM image of such a ghost in a polyurethane surface (A) and a respective imprint after washing with SDS (B). Due to the reduced elasticity, the pit in the middle of the cell is much more pronounced. Similarly, the structure is more accurately reproduced in the polymer surfaces. Ghost-imprinted polymers are excellently suited for erythrocyte detection, as can be seen in Figure 7. Here, the QCM sensor curves are given for a quartz, where the electrodes have been coated with BG A and B erythrocyte ghost imprints, respectively, and exposed to suspensions containing RBCs of the four blood groups. Obviously each imprint clearly prefers the \"own\" template. The selectivity factor for blood group A is higher, because the antigen oligosaccharidesacharide concentration is higher for A than for B. However, both imprints yield similar signals for AB, which further supports chemical recognition, as AB erythrocytes contain both saccharides. Finally, the selectivity picture is completed for both imprints by the BG O signals, as each imprint clearly prefers the own template.