## 2 Measuring Technique  
Mass-sensitive devices rely on a piezoelectric substrate that transforms a mechanical oscillation into an electrical one, where the frequency is determined by material parameters and also prominently by the mass loading of the device. One aspect for successful sensor characterization is the design of suitable measuring cell setups. Figure 1 in the upper right corner shows a QCM cell design based on casted silicone for micro organism detection. In this case the device is optimized towards low sample volumes, as especially in the case of virus sensing the sample amounts are generally limited. One aspect, however, has to be born in mind: especially in liquid phase, QCM shows substantial responses to temperature changes, mainly because the sample viscosity is usually highly temperature dependent. One strategy to overcome this problem, e.g. if only insufficient thermostating can be provided, is to use devices with more electrodes deposited on the same substrate. One can see a main reason for this also in Figure 1: here, the frequency responses of a dual-channel QCM are given when changing thewater temperature from 15\({}^{\circ}\)C to 25\({}^{\circ}\)C in steps on 1\({}^{\circ}\)C. As can bee seen, one degree of temperature change results in a frequency shift of about 40 Hz. This is not negligible, as the sensor responses attributed to imprinting effects usually reach values between 100 and 1000 Hz. However, the difference between the two channels in Fig. 1 remains absolutely constant. So the combination of the small cell size with a single transducer substrate carrying all sensors leads to ideal compensation of the occurring physical effects, as demonstrated here by temperature/viscosity changes.  
Screen-printed QCM devices can be improved by optimizing a further parameter: the electronic quality turned out to depend distinctly on the surface roughness of the electrodes. Figure 2 shows three AFM scan lines over different electrodes, respectively, where each scan covers 50 \(\upmu\)m. The uppermost graphic shows the roughness of a gold layer after screen printing and burning at 400\({}^{\circ}\)C. Evidently, the highest vertical distances reach up to about 250 nanometers with an average roughness of about 100 nm. Spinning the quartz for a minute at 6000 rpm after printing reduces this roughness to values around 40 nm with maximum distances being below 100 nm (as to be seen in the middle). Even further improvement can be achieved by placing the printed QCM into a chamber with an atmosphere saturated with dichloromethane (lower part of Fig. 2), a dilutor for the gold paste. After just one minute the roughness has dropped to 5 nanometres on an average. This reduction in surface unevenness has substantial impact on the electronic properties of the resulting devices, as can be seen in Figure 3, where the impedance spectra obtained with a network analyzer for the three respective preparation methods are given: the as-prepared quartz has a damping of -3.2 dB, after spinning -1.9 are reached and exposing it to methylene chloride vapors leads to -1.1 dB. As the dB-scale logarithmic,  
Figure 1: Low-volume measuring cell for bioanalyte detection and temperature curves in water.  
these are substantial improvements. Lower electronic losses usually lead to a better electronic quality and frequency sharpness and therefore to reduced noise in the oscillator circuit.  
Figure 2: **Influence of electrode pre-treatment on the electronic properties.**