Electrokinetic separation techniques for studying nano- and microplastics

Abstract

In recent years, microplastics have been found in seawater, soil, food, and even human blood and tissues. The ubiquity of microplastics is alarming, but the health and environmental impacts of microplastics are just beginning to be understood. Accordingly, sampling, separating, and quantifying exposure to microplastics to devise a total risk assessment is the focus of ongoing research. Unfortunately, traditional separation methods (i.e., size- and density-based methods) unintentionally exclude the smallest microplastics (<10 μm). Limited data about the smallest microplastics is problematic because they are likely the most pervasive and have distinct properties from their larger plastic counterparts. To that end, in this Perspective, we discuss using electrokinetic methods for separating the smallest microplastics. Specifically, we describe three methods for forming electric field gradients, discuss key results within the field for continuously separating microplastics, and lastly discuss research avenues which we deem critical for advancing electrokinetic separation platforms for targeting the smallest microplastics.

Scheme 1. (a) Representation of a permselective nanochannel connecting two microfluidic channels. (b) Schematic illustration of an electric field gradient in a microchannel and its effect on a single charged ion. (c) Same as (b), but for two ions having different mobilities. Adapted and reprinted from Chem. Sci., 2020, 11, 5547–5558 (Copyright © 2020 Royal Society of Chemistry).

Fig. 1. (a) Schematic illustration and image of a microfluidic device designed to utilize ICP for continuous separations. (b) Micrograph showing the channel bifurcation of the device in (a) during operation. (a and b) Adapted from: Anal. Chem., 2011, 83, 7348–7355 (copyright © 2011 American Chemical Society). (c) Schematic illustration of the microfluidic device designed to continuously separate microplastics. (d and e) Micrographs of the device shown in (c) when separating different sizes of microplastics. (c–e) Adapted from Sci. Rep., 2013, 3, article number: 3483 (copyright © 2013 Nature Publishing Group).

Fig. 2. (a) Schematic illustration of a microfluidic device designed to use ICP for the continuous separation and enrichment of anionic dyes. (b) Fluorescence micrograph of the device shown in (a) when continuously separating and enriching three anionic dyes. Adapted from: Anal. Chem., 2020, 92, 4866–4874 (copyright © 2011 American Chemical Society).

Scheme 2. Schematic representation of the operation of a bipolar electrode. Adapted from: Angew. Chem., Int. Ed., 2013, 52, 10438–10456 (copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Fig. 3. (a) Schematic illustration and image of a microfluidic device designed to utilize fICP for continuous separations. (b) Fluorescence micrograph showing the channel bifurcation of the device in (a) during operation. (a and b) Adapted with permission from ChemElectroChem, 2018, 5, 877–884 (copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim). (c) Schematic illustration of the microfluidic device designed to continuously separate microplastics. (d) Micrograph of the device shown in (c) when separating different sizes of microplastics. (c and d) Adapted from Chem. Sci., 2020, 11, 5547–5558 (copyright © 2020 Royal Society of Chemistry).

Fig. 4. (a) Schematic illustration of ionic current variations caused by BPEs. Adapted with permission from ChemElectroChem, 2022, 9, e202200251 (Copyright © 2022 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim). (b) Optical micrograph of a bifurcated microchannel during an electrochemical microplastic separation experiment in buffer-free solution. Adapted from Chem. Sci., 2021, 12, 13744–13755 (copyright © 2022 Royal Society of Chemistry).

Jonathan R. Thompson

Richard M. Crooks