Continuous micellar electrokinetic focusing of neutral species driven by ion concentration polarization

Modulating selective ionic enrichment and depletion zones in straight nanochannels via the interplay of surface charge modulation and electric field mediated fluid-thickening

We report the possibilities of achieving highly controlled segregation of ion-enriched and ion-depleted regions in straight nanochannels. This is achieved via harnessing the interplay of an axial gradient of the induced transverse electric field on account of electrical double layer phenomenon and the localized thickening of the fluid because of intensified electric fields due to the large spatial gradients of the electrical potential in extreme confinements. By considering alternate surface patches of different charge densities over pre-designed axial spans, we illustrate how these effects can be exploited to realize selectively ion-enriched and ion-depleted zones. Physically, this is attributed to setting up of an axial concentration gradient that delves on the ionic advection due to the combined effect of an externally applied electric field and induced back-pressure gradient along the channel axis and electro-migration due to the combinatorial influences of the applied and the induced electrostatic fields. With an explicit handle on the pertinent parameters, our results offer insights on the possible means of imposing delicate controls on the solute-enrichment and depletion phenomena, a paradigm that remained unexplored thus far.

Molecularly Selective Polymer Interfaces for Electrochemical Separations

The molecular design of polymer interfaces has been key for advancing electrochemical separation processes. Precise control of molecular interactions at electrochemical interfaces has enabled the removal or recovery of charged species with enhanced selectivity, capacity, and stability. In this Perspective, we provide an overview of recent developments in polymer interfaces applied to liquid-phase electrochemical separations, with a focus on their role as electrosorbents as well as membranes in electrodialysis systems. In particular, we delve into both the single-site and macromolecular design of redox polymers and their use in heterogeneous electrochemical separation platforms. We highlight the significance of incorporating both redox-active and non-redox-active moieties to tune binding toward ever more challenging separations, including structurally similar species and even isomers. Furthermore, we discuss recent advances in the development of selective ion-exchange membranes for electrodialysis and the critical need to control the physicochemical properties of the polymer. Finally, we share perspectives on the challenges and opportunities in electrochemical separations, ranging from the need for a comprehensive understanding of binding mechanisms to the continued innovation of electrochemical architectures for polymer electrodes.

Ion-Depleting Action of Perm-Selective Membranes for Enhancing Electrical Communication and Gated Ion Channel Activity in Cell Cultures

Ion-depletion action of an ion-selective membrane produces a moat channel that electrically insulates a cell colony and elevates the cell medium potential uniformly to synchronously activate and deactivate the voltage-gated ion channels of all cells. The result is robust synchronization with strong intercellular electrical communication and the discovery of ion channel deactivation that is only possible when the cells are in communication. The study suggests that the collective response of a cell colony to external stimuli is distinct from that of a single cell. Cell proliferation must hence be guided with strong intercellular communication and proper exogenous stimuli.

Recent advances in paper-based preconcentrators by utilizing ion concentration polarization

One of the most cited limitations of biochemical detection is its poor sensitivity, owing to the relatively high complexity of micro-samples. Moreover, some samples cannot be easily self-replicated and their abundance cannot be increased through traditional technologies. Therefore, the preconcentration of low-abundance samples is a key requirement for microfluidic biological analysis. In recent years, the ion-concentration polarization phenomenon has aroused widespread interest in the application of microfluidic technology. In addition, paper-based materials are readily available, easy to modify, and exhibit good hydrophilicity. The study of the ion-concentration polarization preconcentration of micro-samples in paper-based microfluidic chips is of considerable significance. In this review, we discuss the development and applications of ion-concentration polarization paper-based preconcentrator in the past 5 years, with emphasis on key progresses in chip fabrication and performance optimization under different conditions. The current needs and development prospects in this field have also been discussed.

Liquid biopsy technologies based on membrane microfluidics: High-yield purification and selective quantification of biomarkers in nanocarriers

Liquid biopsy, screening cancer non-invasively and frequently by detecting and quantifying molecular markers in physiological fluids, would significantly improve cancer survival rate but it remains a distant goal. The key obstacles presented by the highly heterogeneous samples are rapid/high-yield purification and precise/selective marker capture by their antibody and oligo probes. As irregular expressions of these molecular biomarkers are the key signals, quantifying only those from the cancer cells would greatly enhance the performance of the screening tests. The recent discovery that the biomarkers are carried by nanocarriers, such as exosomes, with cell-specific membrane proteins suggests that such selection may be possible, although a new suite of fractionation and quantification technologies would need to be developed. Although under-appreciated, membrane microfluidics has made considerable contributions to resolving these issues. We review the progress made so far, based on ion-selective, track-etched, and gel membranes and advanced electrophoretic and nano-filtration designs, in this perspective and suggest future directions.

Tutorial review: Enrichment and separation of neutral and charged species by ion concentration polarization focusing

Ion concentration polarization focusing (ICPF) is an electrokinetic technique, in which analytes are enriched and separated along a localized electric field gradient in the presence of a counter flow. This field gradient is generated by depletion of ions of the background electrolyte at an ion permselective junction. In this tutorial review, we summarize the fundamental principles and experimental parameters that govern selective ion transport and the stability of the enriched analyte plug. We also examine faradaic ICP (fICP), in which local ion concentration is modulated via electrochemical reactions as an attractive alternative to ICP that achieves similar performance with a decrease in both power consumption and Joule heating. The tutorial covers important challenges to the broad application of ICPF including undesired pH gradients, low volumetric throughput, samples that induce biofouling or are highly conductive, and limited approaches to on- or off-chip analysis. Recent developments in the field that seek to address these challenges are reviewed along with new approaches to maximize enrichment, focus uncharged analytes, and achieve enrichment and separation in water-in-oil droplets. For new practitioners, we discuss practical aspects of ICPF, such as strategies for device design and fabrication and the relative advantages of several types of ion selective junctions and electrodes. Lastly, we summarize tips and tricks for tackling common experimental challenges in ICPF.