Electrokinetic ion transport in nanofluidics and membranes with applications in bioanalysis and beyond

Abstract

Electrokinetic transport of ions between electrolyte solutions and ion permselective solid media governs a variety of applications, such as molecular separation, biological detection, and bioelectronics. These applications rely on a unique class of materials and devices to interface the ionic and electronic systems. The devices built on ion permselective materials or micro-/nanofluidic channels are arranged to work with aqueous environments capable of either manipulating charged species through applied electric fields or transducing biological responses into electronic signals. In this review, we focus on recent advances in the application of electrokinetic ion transport using nanofluidic and membrane technologies. We start with an introduction into the theoretical basis of ion transport kinetics and their analogy to the charge transport in electronic systems. We continue with discussions of the materials and nanofabrication technologies developed to create ion permselective membranes and nanofluidic devices. Accomplishments from various applications are highlighted, including biosensing, molecular separation, energy conversion, and bio-electronic interfaces. We also briefly outline potential applications and challenges in this field.

FIG. 1.

(a) Microscopy image of a reverse-biased bipolar membrane in the microfluidic channel that produces local pH changes through field-enhanced water splitting. (b) I-V characteristic of the bipolar membrane showing ionic rectification and water dissociation. (c) Schematics of the ion distribution and electric-field profile in a reverse-biased bipolar membrane. Reproduced with permission from Biomicrofluidics 5, 046502 (2011). Copyright 2011 AIP Publishing LLC.

FIG. 2.

(a) Integrated bipolar membrane generates enhanced water dissociation for electrical pH control in microfluidic devices. Reproduced with permission from Biomicrofluidics 5, 046502 (2011). Copyright 2011 AIP Publishing LLC. (b) On-chip free-flow isoelectric focusing (FF-IEF) utilizes the integrated bipolar membrane to produce the pH gradient in the downstream separation chamber for protein separation. Reproduced with permission from Lab Chip 7, 979 (2014). Copyright 2014 The Royal Society of Chemistry.

FIG. 3.

(a) Nonlinear I-V characteristics of an ion exchange membrane. (b) Ion concentration polarization (ICP) fulfilled by membrane-integrated microfluidics for preconcentration of molecules. Reproduced with permission from Top. Curr. Chem. 304, 153 (2011). Copyright 2011 Springer Nature. (c) Micro-ICP desalination platform formed by bounding two CEMs and two electrodes to a PDMS channel that bifurcates into desalted and brine downstream flows. Reproduced with permission from Sci. Rep. 6, 25349 (2016). Copyright 2016 Springer Nature (d) Nucleic acid sensing platform utilizes the integrated membrane for DNA sensing which relies on the change in its nonlinear I-V characteristics in response to the binding of target DNA on the membrane surface. Reproduced with permission from Talanta 145, 35 (2015), and Biosens. Bioelectron. 60, 92 (2014). Copyright 2014 and 2015 Elsevier.

FIG. 4.

(a) Fluidic-based ion memristor utilizes oxidization of the silicon electrode in the electrolyte to generate the memristive effect. (b) I-V characteristics and bistable resistive-state switching of an ion memristor. Reproduced with permission from Gongchen et al., Small 11, 5206 (2015). Copyright 2015 John Wiley and Sons.

FIG. 5.

(a) Ionic circuit powered by a reverse electrodialysis (RED) cell. (b) Ionic OR circuit with two ionic diodes formed by the connection of a pAMPSA cation exchange hydrogel and a pDADMAC anion exchange hydrogel. The input voltage was maintained at 3.1 V from RED. The digital output voltage corresponding to the bias across the ionic diode can be visualized by the fluorescence intensity of the pDADMAC stained by anionic fluorescein. The dark fluorescence indicates a forward-biased ionic diode. Reproduced with permission from Han et al., Sci. Rep. 7, 14068 (2017). Copyright 2017 Springer Nature.

FIG. 6.

(a) Bioelectronic neural pixel for both drug delivery and neuronal signal recording. (b) A neural pixel has an organic electronic ion pump (OEIP) composed of the cation-exchange membrane (CEM) and conductive polymer PEDOT:PSS that conducts and delivers positively charged inhibitory neurotransmitter, γ-aminobutyric acid (GABA), to the local neuron of a mouse hippocampal. The same PEDOT:PSS electrode measures the local neuronal response in terms of ion fluxes. (c) Epileptiform activity of a mouse hippocampal preparation recorded from a single pixel before and during GABA delivery to the same pixel. Reproduced with permission from Jonsson et al., Proc. Natl. Acad. Sci. U.S.A. 113, 9440 (2016). Copyright 2016 United States National Academy of Sciences.