III-V Nitride based microcantilevers, with AlGaN/GaN heterostructure field effect transistor as the piezoresistive deflection transducer, have been investigated under steady state, transient, ac, and UV illuminated conditions and compared to theoretical calculations. The steady state transverse gauge factor (GFt) was found to be much larger than theoretical estimates and increased regularly with more negative gate bias. Transient GFt demonstrated opposite sign but similar gate bias dependence and was measured as high as ∼860. Measurements under ac biasing conditions and UV illumination resulted in a lower GFt of ∼13, which agrees with theoretical calculations owing to elimination of charge trapping effects.
Cell-based screening assays are now widely used for identifying compounds that serve as ion channel modulators. However, instrumentation for the automated, real-time analysis of ion flux from clonal and primary cells is lacking. This study describes the initial development of an ion-sensitive field effect transistor (ISFET)-based screening assay for the acquisition of K+ efflux data from cells cultured in multi-well plates. Silicon-based K+-sensitive ISFETs were tested for their electrical response to varying concentrations of KCl and found to display a linear response relationship to KCl in the range of 10 µM to 1 mM. The ISFETs, along with reference electrodes, were inserted into fast-flow chambers containing either human colonic T84 epithelial cells or U251-MG glioma cells. Application of the Ca2+ ionophore A23187 (1 µM), to activate Ca2+-activated non-selective cation (NSC) channels (T84 cells) and large conductance Ca2+-activated K+ (BK) channels (U251 cells), resulted in time-dependent increases in the extracellular K+ concentration ([K+]o) as measured with the ISFETs. Treatment of the cells with blockers of either the NSC or BK channels, caused a strong inhibition of the A23187-induced increase in [K+]o. These results were consistent with ion current measurements obtained using the whole-cell arrangement of the patch clamp procedure. In addition, K+ efflux data could be acquired in parallel from multiple cell chambers using the ISFET sensors. Given the non-invasive properties of the probes, the ISFET-based assay should be adaptable for screening ion channels in various cell types.
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