Real-Time Operation of Microcantilever-Based in-Plane Resonators Partially Immersed in a Microfluidic Sampler

This paper reports a real-time liquid-phase quantitative analysis system, where in-plane flexural piezoresistive Si cantilever resonators are utilized as microsensors. The new-developed system comprises a 3D-printed sampler, which enables a controllable setting of the immersion depth of the thermally self-actuated cantilevers of rectangular and triangular shapes into a microfluidic volume. By electromechanical amplitude modulation (EAM) of the supply voltage of the piezoresistive strain gauge, parasitic actuator-detector coupling effects were removed from the read-out signal enabling real-time tracking of resonant-frequency (rf) shifts using a phase-locked loop (PLL) circuit. High quality factors (Q: ~100-680) and low rf reduction (max. ~2.5%) with respect to operation in air were observed with cantilevers triangular-tip immersed into DI water. A real-time limit of detection (LOD) of ~1.0 ng in liquid and quasi-real-time LOD of ~150 pg have been obtained.