Nanoparticle enrichment in a sessile droplet using secondary acoustic radiation force

Nanoparticles are widely utilzed in many applications including nanomedicine and drug delivery. Although the nanoparticle enrichment is fundemantal in nanoscience and nanotechnology, the conventional centrifugation and filtration methods are difficult to apply for the nanascale objects. Here, we propose an acoustic method to enrich the nanoscale objects using secondary acoustic radiation force. When MHz-frequency acoustic waves are applied to a mixture of micro- and nanoparticles are suspended in a sessile droplet, the incident waves are scattered off the microparticles; the nanoparticles are captured and enriched in between the microparticles by secondary acoustic radiation force. We experimentally demonstrated that the 700 nm polystyrene (PS) nanoparticles can be captured using the 7 μm PS microparticles suspended in a 3 μl water sessile droplet subject to a 72.5 MHz acoustic field within a few tens of seconds. We expect that the proposed method can be applied to droplet-based point-of-care testing using nanoscale biomolecules.