EchoTilt: An Acoustofluidic Method for the Capture and Enrichment of Nanoplastics Directed Toward Drinking Water Monitoring

Abstract

Micro- and nanoplastics have become increasingly relevant as contaminants to be monitored due to their potential health effects and environmental impact. Nanoplastics, in particular, have been shown to be difficult to detect in drinking water, requiring new capture technologies. In this work, we applied the acoustofluidic seed particle method to capture nanoplastics in an optimized, tilted grid of silica clusters even at the high flow rate of 5 mL/min. Moreover, we achieved, using this technique, the enrichment of nanoparticles ranging from 500 nm to 25 nm as a first in the field. We employed fluorescence to observe the enrichment profiles according to size, using a washing buffer flow at 0.5 mL/min, highlighting the size-dependent nature of the silica seed particle release of various sizes of nanoparticles. These results highlight the versatility of acoustic trapping for a wide range of nanoplastic particles and allow further study into the complex dynamics of the seed particle method at these size ranges. Moreover, with reproducible size-dependent washing curves, we provide a new window into the rate of nanoplastic escape in high-capacity acoustic traps, relevant to both environmental and biomedical applications.

Keywords: acoustic trap; acoustofluidics; microfluidic-based separation; microplastics; nanoplastics; seed particle method; silica-enhanced seed particle method.

Figure 1

(A) Bottom view of the EchoTilt microchannel, with a connected syringe pump and signal generator. (B) Side view schematic of the microfluidic assembly. (C) Schematic of the algorithm angle sweep (showing 0, 5 and 16 degrees as examples), with highlighted flow lines crossing the tilted grid. Green lines signify a high score of flow lines–cluster interaction, yellow lines a medium score and red lines a low score.

Figure 2

(A) Sixteen-degree-tilted acoustic field overlaid with a visual representation of the intersecting flow lines. (B) Acoustic pressure decay function extracted from COMSOL Multiphysics normalized for a 0–1 score. (C) Depiction of flow line score variation across the active capture area for different tilt angles in blue, orange and green (0, 5 and 16 degrees, respectively). The x-axis represents the normalized height, where 0.0 corresponds to the beginning and 1.0 to the end of the cluster array. The y-axis represents the score value associated with a flow line at each particular height. (D) Graph of the median grid score obtained by the algorithm, with the maximum median score at the 16° tilt angle highlighted.

Figure 3

(A) Picture of the EchoTilt device from the top, showing the milled slot in the back of the SDT. (B) Picture of the bottom (through the glass) of the EchoTilt device with the highlighted 1 cm wide silica cluster grid, visible with the naked eye. (C) Original EchoGrid (0-degree tilt) active trapping area with well-defined clusters. The surrounding silica outside of the square is also patterned with weaker forces and is washed off before the experiment. (D) EchoTilt optimization, showing the 16° tilted trapping area and the organized clusters within it. In (C,D), the flow direction is from left to right.

Figure 4

Silica-enhanced seed particle method nanoparticle enrichment at a flow rate of 5 mL/min using the EchoTilt device. The increase in fluorescence is attributed to the flowing nanoparticle solution interacting with the cluster. The particles used were 100 nm (green), 200 nm (blue) and 500 nm (magenta).

Figure 5

Silica-enhanced seed particle method nanoparticle enrichment at a flow rate of 5 mL/min using the EchoTilt device. The increase in fluorescence is attributed to the flowing nanoparticle solution interacting with the cluster. The particles used were 25 nm (red), and 50 nm (orange).

Figure 6

Nanoparticle washing from a silica cluster at a flow rate of 0.5 mL/min directly after enrichment at 5 mL/min. The decrease in fluorescence is attributed to the nanoparticles being washed away from the cluster. The particles used were 25 nm (red), 50 nm (orange), 100 nm (green), 200 nm (blue) and 500 nm (magenta).

Figure 7

(A) Enrichment performance at 5 mL/min, for 4 min, for a total of 20 mL of solution processed. (B) Enrichment performance at 2 mL/min, for 10 min, for a total of 20 mL of solution processed. (C) Washing at 0.5 mL/min after the nanoparticle enrichment experiments. The decrease in fluorescence is attributed the nanoparticles being displaced by the flowing MQ water from their clusters. The particles used were 50 nm (red), 100 nm (green), 200 (blue) and 500 (magenta). The experiments were performed in duplicate, and plotted with their average values.