Effects of biologically compatible buffers on the electrical response of gravimetric sensors operating at GHz frequencies

Gravimetric biosensors based on piezoelectric resonators have their active area just atop an electrical contact. Sometimes both contacts are located on the same surface (coplanar electrodes). The use of bio-buffers with high ionic content makes often electrical isolation of the electrodes from the liquid mandatory. This restricts the use of surfaces for functionalization and worsens the response of the resonator. In this paper we demonstrate that, if operating frequencies are high enough, there is little effect of the liquid ionic conductivity on the performance of the resonators with both electrodes in contact with the latter. We test the response of 1.32 GHz AlN shear mode resonators in NaCl solutions of different concentrations to show the effects of the ionic conductivity on their performance. The conclusion is that, even the presence of a parallel conductance due to the ionic conductivity into the liquid, the shear mode resonance is not significantly disturbed. To prove the viability of non-isolated AlN resonators as biosensors, we present measurements of thrombin detection using devices with aptamer-functionalized iridium electrode surfaces.