Echoseed: Modeling Nanoparticle Release in Silica Clusters and Pioneering New Seed Particle Materials for Acoustofluidics

This abstract reports on the investigation of controlled capture and release of nanoparticles from 500 to 50 nm using the silica enhanced seed particle (SE-SP) method.In addition, we explore how different materials behave as seed particle clusters in an acoustic field by taking advantage of the recently developed EchoGrid 1 acoustofluidic device (Figure 1A).The successful formation of levitating clusters using materials other than silica (Figure 1C-D) or polymers is a novel result (Figure 1B) in the field of acoustic actuation in microfluidics and shows great promise in potentially expanding the realm of applications of the seed particle method.The EchoGrid device was developed to address micro and nanoplastic contamination in drinking water, a complex issue due to the various types, shapes and sizes of this pollutants 2 .In this work, we aimed to model the capture and release of various nanoparticles (50, 100, 200 and 500 nm) using the SE-SP method (Figure 2), as these have the potential to be damaging to biological systems and human health 3 .Furthermore, we theorised that different pre-seeding particle materials could have significant implications in other fields.The polymer seed particles used in acoustofluidics interfere with the spectroscopic analysis of micro and nanoplastics of interest in a sample and cannot be trapped at high flow rates.Silica particles have emerged as a solution to circumvent this and we have achieved 50 mL/min sustained capture using our SE-SP method in a work just accepted by Analytical Chemistry, while other authors have achieved 5 mL/min, which is still considerable for microfluidics 4 .Silica's high density (2 g/cm 3 ) compared to polystyrene (1 g/cm 3 ) contributes significantly to trapping behaviour, but other materials can present even densities.For this reason, we investigated the clustering behaviour of other materials, namely: barium titanate glass (4.4 g/cm 3 -Figure 3A-B), titanium (4.5 g/cm 3 -Figure 3C-D), stainless steel (7.8 g/cm 3 -Figure 3E-F) and conductive silver-coated silica (4.1 g/cm 3 -Figure G-I).In a first, we've created seed particles of all these materials, opening doors for a myriad of applications.Potential applications include the study of biocompatibility and immune response of human cells in titanium seed particle clusters 5 , antibacterial efficacy trials for clinical drugs using silver-coated microparticles for conjugation 6 , nanomedical and nonlinear optics studies in barium titanate seed particle clusters 7 , and corrosion-resistant stainless steel coat testing for electrochemical purposes 8 .We observed unexpected complex acoustic phenomena when clustering these materials simultaneously in a high-power field (Figure 3J), namely a highly compact 3-D cluster being kept together with the primary and secondary acoustic radiation forces while acoustic streaming continuously recirculates a composite core of the cluster made of every aforementioned material.In conclusion, this work pushes the boundary in acoustofluidics by trapping several sizes of nanoplastic using the SE-SP method and then characterizing their release during a washing step.Furthermore, we investigate a new avenue of the field by successfully capturing different materials as levitated seed particles, significantly expanding the potential application of the seed particle method not only for micro and nanoplastic detection but also for other purposes.