Label-Free Electrochemical Monitoring of Concentration Enrichment during Bipolar Electrode Focusing

Focusing, sorting, and separating microplastics by serial faradaic ion concentration polarization

In this article, we report continuous sorting of two microplastics in a trifurcated microfluidic channel using a new method called serial faradaic ion concentration polarization (fICP). fICP is an electrochemical method for forming ion depletion zones and their corresponding locally elevated electric fields in microchannels. By tuning the interplay between the forces of electromigration and convection during a fICP experiment, it is possible to control the flow of charged objects in microfluidic channels. The key findings of this report are threefold. First, fICP at two bipolar electrodes, configured in series and operated with a single power supply, yields two electric field gradients within a single microfluidic channel (i.e., serial fICP). Second, complex flow variations that adversely impact separations during fICP can be mitigated by minimizing convection by electroosmotic flow in favor of pressure-driven flow. Finally, serial fICP within a trifurcated microchannel is able to continuously and quantitatively focus, sort, and separate microplastics. These findings demonstrate that multiple local electric field gradients can be generated within a single microfluidic channel by simply placing metal wires at strategic locations. This approach opens a vast range of new possibilities for implementing membrane-free separations.

Bipolar Electrodes with 100% Current Efficiency for Sensors

A bipolar electrode (BPE) is an electron conductor that is embedded in the electrolyte solution without the direct connection with the external power source (driving electrode). When the sufficient voltage was provided, the two poles of BPE promote different oxidation and reduction reactions. During the past few years, BPEs with wireless feature and easy integration showed great promise in the various fields including asymmetric modification/synthesis, motion control, targets enrichment/separation, and chemical sensing/biosensing combined with the quantitative relationship between two poles of BPE. In this perspective paper, we first describe the concept and history of the BPE for analytical chemistry and then review the recent developments in the application of BPEs for sensing with ultrahigh current efficiency (η_c = i_BPE/i_channel) including the open and closed bipolar system. Finally, we offer the guide for possible challenge faced and solution in the future.

Unconventional Electrochemistry in Micro-/Nanofluidic Systems

Electrochemistry is ideally suited to serve as a detection mechanism in miniaturized analysis systems. A significant hurdle can, however, be the implementation of reliable micrometer-scale reference electrodes. In this tutorial review, we introduce the principal challenges and discuss the approaches that have been employed to build suitable references. We then discuss several alternative strategies aimed at eliminating the reference electrode altogether, in particular two-electrode electrochemical cells, bipolar electrodes and chronopotentiometry.

Two-Step Bipolar Electrochemistry: Generation of Composition Gradient and Visual Screening of Electrocatalytic Activity

Bipolar electrochemistry (BE) is employed for both creating electrocatalysts composition gradient and visual screening of the prepared composition on a single substrate in just two experiment runs. In a series of proof-of-principle experiments, we demonstrate gradient electrodeposition of Ni-Cu using BE; then the electrocatalytic activity of the prepared composition gradient toward the hydrogen evolution reaction (HER) is visually screened in the BE system using array of BPEs. Moreover, the morphology and the chemical composition of the Ni-Cu gradient are screened along the length of the bipolar electrode (BPE). By measuring the potential gradient over the BPE, it is also demonstrated that by controlling the concentration of the metals precursor and the supporting electrolyte, the length of the bipolar electrodeposited gradient can be controlled.

Concurrent DNA Preconcentration and Separation in Bipolar Electrode-Based Microfluidic Device

This paper presents a bipolar electrode (BPE) device in a microfluidic dual-channel design for concurrent preconcentration and separation of composite DNA containing samples. The novelty of the present effort relies on the combination of BPE-induced ion concentration polarization (ICP) and end-labeled free-solution electrophoresis (ELFSE). The ion concentration polarization effect arising from the faradaic reaction on the BPE is utilized to exert opposing electrophoretic and electroosmotic forces on the DNA samples. Meanwhile, end-labeled free-solution electrophoresis alters the mass-charge ratio to enable simultaneous DNA separation in free solution. The microfluidic device was fabricated using standard and soft lithography techniques to form gold-on-glass electrode capped with a PDMS microfluidic channel. Experimental testing with various DNA samples was carried out over a range of applied electric field. Concentration ratios up to 285× within 5 minutes for a 102-mer DNA, and concurrent preconcentration and free-solution separation of binary mixture of free and bound 102-mer DNA within 6 minutes was demonstrated. The effect of applied electric field was also interrogated with respect to pertinent performance metrics of preconcentration and separation.

Electrochemically-gated delivery of analyte bands in microfluidic devices using bipolar electrodes

A method for controlling enrichment, separation, and delivery of analytes into different secondary microchannels using simple microfluidic architecture is described. The approach, which is based on bipolar electrochemistry, requires only easily fabricated electrodes and a low-voltage DC power supply: no pumps or valves are necessary. Upon application of a voltage between two driving electrodes, passive bipolar electrodes (BPEs) are activated that result in formation of a local electric field gradient. This gradient leads to separation and enrichment of a pair of fluorescent analytes within a primary microfluidic channel. Subsequently, other passive BPEs can be activated to deliver the enriched tracers to separate secondary microchannels. The principles and performance underpinning the method are described.

Design of a wireless electrochemical valve

The control of motion of small objects is an emerging research area. Their design strongly depends on the strategy chosen to trigger the locomotion, which is typically obtained by either physical or chemical fueling. An ongoing challenge in this field is the remote control of the motion with space and time resolution. In this context, we describe in the present contribution a wireless electrochemical valve based on a chemo-mechanical feedback loop. This valve is driven by bipolar electrochemistry and works in aqueous solutions by converting reversibly an electrical input signal into periodic motion via electrogenerated hydrogen bubbles. We report the design of this very first bipolar valve, together with an analysis of the mechanism leading to the energy conversion, based on controlled production, storage and release of gas bubbles.

Dual-channel bipolar electrode focusing: simultaneous separation and enrichment of both anions and cations

In this paper we show that a microelectrochemical cell comprising two parallel microchannels spanned by a single bipolar electrode can be used to simultaneously enrich and separate both anions and cations within a single microchannel. This is possible because reduction and oxidation of water at the cathodic and anodic poles of the bipolar electrode, respectively, lead to ion depletion zones. Specifically, TrisH(+) is neutralized by OH(-) at the cathodic pole, while acetate buffer is neutralized by H(+) at the anodic pole. This action creates a local electric field gradient having both positive and negative components, and hence positive and negative ions follow their respective field gradients leading to separation. In the presence of an opposing counter-flow (pressure driven flow in this case), enrichment also occurs. In addition to separation and enrichment in a single channel, it is also possible to simultaneously enrich cations in one microchannel and anions in the other. Enrichment is achieved by controlling experimental parameters, including the type of buffer and the direction and magnitude of the opposing counter-flow.

Enhanced mass transport of electroactive species to annular nanoband electrodes embedded in nanocapillary array membranes

Electroosmotic flow (EOF) is used to enhance the delivery of Fe(CN)(6)(4-)/Fe(CN)(6)(3-) to an annular nanoband electrode embedded in a nanocapillary array membrane, as a route to high efficiency electrochemical conversions. Multilayer Au/polymer/Au/polymer membranes are perforated with 10(2)-10(3) cylindrical nanochannels by focused ion beam (FIB) milling and subsequently sandwiched between two axially separated microchannels, producing a structure in which transport and electron transfer reactions are tightly coupled. The middle Au layer, which contacts the fluid only at the center of each nanochannel, serves as a working electrode to form an array of embedded annular nanoband electrodes (EANEs), at which sufficient overpotential drives highly efficient electrochemical processes. Simultaneously, the electric field established between the EANE and the QRE (>10(3) V cm(-1)) drives electro-osmotic flow (EOF) in the nanochannels, improving reagent delivery rate. EOF is found to enhance the steady-state current by >10× over a comparable structure without convective transport. Similarly, the conversion efficiency is improved by approximately 10-fold compared to a comparable microfluidic structure. Experimental data agree with finite element simulations, further illustrating the unique electrochemical and transport behavior of these nanoscale embedded electrode arrays. Optimizing the present structure may be useful for combinatorial processing of on-chip sample delivery with electrochemical conversion; a proof of concept experiment, involving the generation of dissolved hydrogen in situ via electrolysis, is described.