Focusing and weighing microanalytes in liquid enabled by higher-order piezoelectric resonant membranes.

Micro-electromechanical-system (MEMS) resonators have been recognized as powerful tools for precision mass detection. However, achieving both flexible on-chip capture of microanalytes and high-sensitivity liquid-phase sensing remains a technical challenge. We present a fully anchored multi-modal piezoelectric resonant membrane (PRM) that utilizes self-excited bulk acoustic waves to rapidly enrich sparsely dispersed microparticles within milliliter droplets and precisely locate them at the region of highest vibrational amplitude - the most sensitive area for resonance detection. The design of higher-order resonance modes preserves acoustofluidic enrichment performance while enabling reconfigurable particle distribution patterns through modulation of the acoustic field distribution. Experimental results demonstrate that the resonant frequency shift induced by the same mass load increases with the mode order, indicating that the sensing sensitivity is proportional to the resonance order. We systematically characterized the acoustofluidic parameters and electrical measurement performance of transverse modes from (0, 0) to (4, 0). This work shows that extending the integrated actuation-and-sense approach to consider multiple modes of resonances presents a toolbox for designing such miniaturized platforms to address nanogram-level mass analysis in liquid phase.