Torsional and lateral resonant modes of cantilevers as biosensors: alternatives to bending modes.

We show here novel cantilever designs that express torsional and lateral modes exhibit excellent mass-change sensitivity to molecular self-assembly on gold (75-135 fg/Hz) which is superior to that of widely investigated bending modes. Lead zirconate titanate (PZT) millimeter-sized cantilevers were designed with two types of anchor asymmetry that induced expression of either torsional or lateral modes in the 0-80 kHz frequency range. Experiments and supporting calculations show that anchor asymmetry enables resonant mode impedance-coupling. The sensitive torsional and lateral modes enabled measurement of self-assembled monolayer formation rate at picomolar levels. The anchor design principle was extended to microcantilevers via finite element simulations, which caused both 97% sensitivity improvement relative to conventional designs, as well as new nonclassical resonant mode shapes.