## 3 Small Molecules  
Molecular imprinting can lead to almost antibody-like selectivity (see e.g. for nandrolone imprints [4]). One example for this is shown in Figure 4: the left part - (A) shows the frequency responses of a  
Figure 4: (A) **Fregquency response of a 10 MHz QCM coated with a p-xylene imprint.** (B) Selectivity pattern of the oo- and the  
Figure 3: QCM damping spectra after different electrode pre-treatment strategies10-MHz-QCM coated with 8 kHz (i.e. 320 nm) of a p-xylene imprinted polyurethane layer towards p-xylene and o-xylene, respectively. Although the molar mass of the analytes as well as the vapor pressure are the same, the two compounds lead to very different sensor responses with a very appreciable selectivity factor. On the right-hand side - (B) - the selectivity of both the p- and the o-xylene imprint towards all xylene isomers is given. All sensor signals are normalized towards a layer thickness of 10 kHz (=400 nm) and an analyte concentration of 100 ppm in air. Both imprints clearly favor their respective template compound with very high selectivity over the other two components. In both cases the ortho/para-selectivity is somewhat superior to the one towards the m-isomer. A remarkable phenomenon, however, is the fact that both sensors lead almost to the same signal for m-xylene. Evidently, the geometrical difference between template and analyte - which is similar for both imprints - is correctly reproduced by the sensor responses.  
One major advantage of imprinting techniques is the fact that they rely on self-organization processes to generate selectivity in a material. In contrast to the synthesis of supramolecular hollows (e.g. calixarenes [5]) the geometrical shape and the chemical properties of the sensitive material are therefore determined directly by the analyte-to-be, which makes it possible to design chemical sensors for systems that would otherwise not be accessible. One example for this is oxidative degradation processes that e.g. occur in engine oils [6]. Both organic and inorganic polymers have already proven their ability as coating materials for QCM. Further improvement of the sensor quality, however, can be achieved by using higher fundamental frequencies, as it is known that the frequency effects increase with the square of the device frequency [7], whereas noise increases only linearly. We therefore coated shear-wave SAW devices based on Li tantalate as piezoelectric medium with titanate sol-gel-layers imprinted with capric acid that acts as model compound for oxidized base oil components. The table below shows the resulting frequency shifts both for the imprinted channel and a reference channel as a function of engine oil degradation time:  
\begin{tabular}{|l|r|r|r|} \hline
**Oil age:** & **109 h** & **136 h** & **266 h** \\ \hline (1) Coated & -4.9 kHz & -4.3 kHz & -12.3 kHz \\ \hline (2) Uncoated & 1.2 kHz & 4.6 kHz & 7.8 kHz \\ \hline (3) Difference (1)-(2) & -6.1 kHz & -9.9 kHz & - 20.9 kHz \\ \hline \end{tabular}  
The frequency of the uncoated sensor increases, whereas on the imprinted one it decreases. Of course, the reference signal in this case also depends mainly on the viscosity change during the degradation process in the test stand. However, the frequency shift is in contrast to a QCM, where an increase in sample viscosity always leads to a decrease in resonance frequency. On the other hand the imprint channel exhibits negative frequency shifts that are of course a consequence of mass incorporation into the sensitive layer. Once again, the sensor signal clearly depends on the degradation time of the oil. Compared with QCM measurements the sensor signals in this case are indeed much higher: titanate layers on QCM lead to a mass effect in the range of some kilohertz compared with some ten kHz for the STW.