Dip-in Nanomechanical Probes for Mass Characterization of Nanoparticles in Ultra-Low Volume Liquid Suspensions

The mass of nanoparticles has a critical influence on their phenomenology and functional performance, driving the need for measurement tools that combine high-throughput with high-resolution single-particle analysis. Mass characterization of large nanoparticle populations under such demands remains a persistent challenge, particularly for nanoparticles of large mass, where consolidated techniques encounter limitations. Here, a dip-in nanomechanical probe sensing approach for weighing nanoparticles in liquid suspensions which simultaneously provides high mass resolution and sensing rate is demonstrated. The method relies on nanoparticle transfer to nanowire resonator probes via iterative immersion into the suspension, requiring minimal volume and resulting in negligible losses. It also introduces ionic liquids as suspension media, enabling vacuum operation for optimizing mass resolution through single-mode resonance frequency tracking of the probes. Additionally, implementing acoustofluidic actuation on the suspension regulates nanoparticle adsorption/desorption during immersion, significantly enhancing the sensing rate. Measurements of 30 nm gold nanoparticles demonstrate accurate characterization of their mass distribution with a probe resolution of 0.1 ag, a sensing rate of 38 NPs min^-1, and a sample volume of a few nanoliters. The concurrent advances in all these performance characteristics reveal the potential of dip-in nanomechanical probes for precise and fast characterization of individual nanoparticles over a wide mass range.