Microfabricated cantilevers attracted an enormous attention because of their capability to measure an extreme small mass, temperature variation, chemical reaction, or in biological applications single cells. However, operating these sensors in the dynamic mode makes detection in a fluidic environment quite intricate. In this case the quality factor drops down because of the dynamic viscosity of the fluid and possible effects of the surface energy and makes sensitive measurements almost impractical.

We present an adapted sensor design for the detection of single cells in a fluidic environment.  It relies on an oscillating column oriented perpendicular to the surface instead of the cantilever geometry with their parallel orientation to the surface. The column is immersed in a gas and only the top surface is used to interact with the fluid. In this way we propose to improve the quality factor and also the sensitivity of the set-up. For the detection of the resonance frequency of the vibrating column the beam deflection technique known from atomic force microscopy was used. To avoid severe problems with the diffraction of the laser beam at the air/liquid interface we mounted the column on a supporting membrane. With this optimized design the detection beam path and the volume of interaction was separated.

So far we investigated the damping of the oscillation by the viscous properties of pure gases. We observed that the damping is predominantly governed by the molecular mass and only to a minor extent by their viscosity. Furthermore, we were capable to detect a single latex bead with 2.5 µm diameter (12 pg weight). With a sensitivity of approximately 1fg/Hz and an uncertainty of the frequency detection of ca. 5 Hz we end up with a mass detection limit in the order of 10 fg for columnar resonators. First experiments on the operation of membrane sensors will be presented.

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