B. R. Putra, L. Tshwenya, M. A. Buckingham, J. Chen, K. J. Aoki, K. Mathwig, O. A. Arotiba, A. K. Thompson and F. Marken, Electroanalysis 33 (2021) 1398. [link, pdf]
Ionic rectifier membranes or devices generate uni‐directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nano‐ and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.
Membrane materials with semi-permeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed poly-acrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semi-permeability, also the ionic current rectification effect switches for phPAN materials deposited asymmetrically onto a 5, 10, 20, or 40 µm diameter microhole in a 6 µm thick polyethylene-terephthalate (PET) film substrate. Therefore, ionic rectifier behavior can be tuned and used to monitor and characterize semi-permeability. Effects of electrolyte type and concentration, and pH (relative to the zeta potential at approximately 3.1) are investigated by voltammetry, chronoamperometry, and impedance spectroscopy. A computational model provides good qualitative agreement with observed electrolyte concentration data. High rectification effects are observed for both cations (pH > 3.1) and anions (pH < 3.1), but only at relatively low ionic strengths.
In ion-annihilation electrochemiluminescence (ECL), luminophore ions are generated by oxidation as well as reduction at electrodes surfaces, and subsequently recombine into an electronically excited state, which emits light. The intensity of the emitted light is often limited by the kinetic rate of recombination of the luminophore ion species. Recombination or annihilation rates are high ranging up to approximately 1010 M−1 s−1 and can be difficult to determine using scanning electrochemical microscopy or high-frequency oscillations of an electrode potential. Here, we propose determining annihilation kinetics by measuring the relative change of the emitted light intensity as a function of luminophore concentration. Using finite element simulations of annihilation ECL in a geometry of two closely spaced electrodes biased at constant potentials, we show that, with increasing concentrations, luminescence intensity crosses over from a quadratic dependence on concentration to a linear regime—depending on the rate of annihilation. Our numerical results are applicable to scanning electrochemical microscopy as well as nanofluidic electrochemical devices to determine fast ion-annihilation kinetics.
In vitro digestions are essential for determining the bioavailability of compounds, such as nutrients. We have developed a cell-free, miniaturized enzymatic digestive system, employing three micromixers connected in series to mimic the digestive functions of the mouth, stomach and small intestine. This system continuously processes samples, e.g. containing nutrients, to provide a constant flow of digested materials which may be presented to a subsequent gut-on-a-chip absorption module, containing living human intestinal cells. Our system incorporates three-compartment enzymatic digestion, one of the key functions of the gastrointestinal tract. In each of these compartments, we modify the chemical environment, including pH, buffer, and mineral composition, to closely mimic the local physiological environment and create optimal conditions for digestive processes to take place. It will therefore provide an excellent addition to existing gut-on-a-chip systems, providing the next step in determining the bio-availability of orally administered compounds in a fast and continuous-flow ex vivo system. In this paper, we demonstrate enzymatic digestion in each separate compartment using compounds, starch and casein, as model nutrients. The use of transparent, microfluidic micromixers based on chaotic advection, which can be probed directly with a microscope, enabled enzyme kinetics to be monitored from the very start of a reaction. Furthermore, we have digested lactoferrin in our system, demonstrating complete digestion of this milk protein in much shorter times than achievable with standard in vitro digestions using batch reactors.
Single-molecule detection schemes offer powerful means to overcome static and dynamic heterogeneity inherent to complex samples. However, probing biomolecular interactions and reactions with high throughput and time resolution remains challenging, often requiring surface-immobilized entities. Here, we introduce glass-made nanofluidic devices for the high-throughput detection of freely-diffusing single biomolecules by camera-based fluorescence microscopy. Nanochannels of 200 nm height confine the movement of biomolecules. Using pressure-driven flow through an array of parallel nanochannels and by tracking the movement of fluorescently labelled DNA oligonucleotides, we observe conformational changes with high throughput. In a device geometry featuring a T-shaped junction of nanochannels, we drive steady-state non-equilibrium conditions by continuously mixing reactants and triggering chemical reactions. We use the device to probe the conformational equilibrium of a DNA hairpin as well as to continuously observing DNA synthesis in real time. Our platform offers a straightforward and robust method for studying reaction kinetics at the single-molecule level.
Microfabricated nanofluidic electrochemical devices offer a highly controlled nanochannel geometry; they confine the volume of chemical reactions to the nanoscale and enable greatly amplified electrochemical detection. Here, the generation of stable light emission by electrochemiluminescence (ECL) in transparent nanofluidic devices is demonstrated for the first time by exploiting nanogap amplification. Through continuous oxidation and reduction of [Ru(bpy)3]2+ luminophores at electrodes positioned at opposite walls of a 100-nm nanochannel, we compare classic redox cycling and ECL annihilation. Enhanced ECL light emission of attomole luminophore quantities is evidenced under ambient conditions due to the spatial confinement in a 10-femtoliter volume, resulting in a short diffusion timescale and highly efficient ECL reaction pathways at the nanoscale.
We suspended a single nanoskived gold nanowire in a microfluidic channel. In this preliminary report, a 200-nm-diameter nanowire was used as an electrode to perform hydrodynamic voltammetry in the center of solution flow. Suspended nanowires exhibit superior current response due to highly efficient mass transport in the area of fastest flow.
Nanoscale channels and electrodes for electrochemical measurements exhibit extreme surface-to-volume ratios and a correspondingly high sensitivity to even weak degrees of surface interactions. Here, we exploit the potential-dependent reversible adsorption of outer-sphere redox species to modulate in space and time their concentration in a nanochannel under advective flow conditions. Induced concentration variations propagate downstream at a species-dependent velocity. This allows one to amperometrically distinguish between attomole amounts of species based on their time-of-flight. On-demand concentration pulse generation, separation, and detection are all integrated in a miniaturized platform.
A cationic diode is fabricated by hot-pressing a commercial cation-conducting ionomer membrane (Fumasep FKS-30) onto a polyethylene terephthalate (PET) substrate with microhole of 5, 10, 20, or 40 μm diameter. Both, symmetric (ionomer on both sides) and asymmetric (ionomer only on the working electrode side) cases are investigated in a 4-electrode measurement cell. A 5-electrode measurement cell in generator-collector mode is employed to directly detect competing proton transport through the ionomer. Only the asymmetric device allows ion current rectification to be observed. With decreasing microhole diameter the rectification effect increases. With increasing electrolyte concentration (for aqueous HCl, NaCl, LiCl, NH4Cl, MgCl2, CaCl2) the rectification effect diminishes. Competition between cation transport and proton transport is observed in all cases. A qualitative impedance model is developed to diagnose the quality and performance of these cationic diodes.
Klaus Mathwig 