Kinetics of the transfer of acetylcholine across the water + sucrose/ 1,2-dichloroethane interface

An electrochemical perspective on the interfacial width between two immiscible liquid phases

Molecular dynamics simulations and vibrational sum-frequency spectroscopy are historically the main techniques applied to the description of the molecular structure and dynamics of the immiscible liquid/liquid interface. A molecular sharpness is estimated for oil/water interfaces, with an interfacial width that extends from hundreds of Å to 1 nm. However, electrochemical studies have elucidated a deeper liquid/liquid interface on the order of several micrometers. The breaking down of single-entity electrochemistry to simpler systems and the combination of high-resolution microscopies is confirming a larger extension of the interface. What can be the role of the electrochemist in clarifying this fundamental question? We try to give a suggestion at the end of a brief historical overview of the liquid/liquid interface studies.

Disposable-micropipette tip supported electrified liquid-organogel interface as a platform for sensing acetylcholine

Sensing acetylcholine has been predominantly based on enzymatic strategies using acetylcholine esterase and choline oxidase because of its electrochemical inertness. Electrified liquid-liquid interfaces are not limited to oxidation/reduction processes, and can be utilized to detect non-redox molecules which cannot be detected using conventional solid electrodes. In this study, a disposable micropipette tip based liquid-organogel interface, in the presence/absence of calixarene has been developed as a platform for sensing acetylcholine. We also explored a liquid-liquid interface approach for sensing acetylcholine using a pre-pulled glass micropipette. In both approaches, the configuration, i.e., liquid-organogel and liquid-liquid interface-current linearly increases during the backward transfer of acetylcholine. The simple and facilitated ion transfer of acetylcholine across the liquid-organogel exhibited a linear range of 10-50 μM and 1-30 μM with a detection limit of 0.18 μM and 0.23 μM and a sensitivity of 9.52 nA μM^-1 and 9.20 nA μM^-1, respectively. Whereas, the detection limit of simple and facilitated ion transfer of liquid-liquid interface using pre-pulled glass micropipette was found to be 0.42 μM and 0.13 μM with a sensitivity of 5 × 10^-3 nA μM^-1 and 3.39 × 10^-2 nA μM^-1. The results indicate that the liquid-organogel configuration supported on a disposable micropipette tip without any pre-fabrication is highly suitable for electrified soft interface sensing applications.

Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes

The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E_1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k⁰), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i-t amperometry. The E_1/2 of ACh transfer at the NPOE/water nanoITIES was -0.342 ± 0.009 V versus the E_1/2 of tetrabutylammonium (TBA⁺). The limit of detection of ACh at the NPOE/water nanoITIES was 37.1 ± 1.5 μM for an electrode with a radius of ∼127 nm. We also determined the ion-transfer kinetics parameters, α and k⁰, of TBA⁺ at the NPOE/water nanoITIES by fitting theoretical cyclic voltammograms to experimental voltammograms. This work lays the basis for future cellular studies using ACh detection at the nanoscale and for studies to detect other analytes. The NPOE/water ITIES offers a potential window distinct from that of the 1,2-dichloroethane (DCE)/water ITIES. This unique potential window would offer the ability to detect analytes that are not easily detected at the DCE/water ITIES.

Quantitative Analysis of Redox-Inactive Ions by AC Voltammetry at a Polarized Interface between Two Immiscible Electrolyte Solutions

The interface between two immiscible electrolyte solutions (ITIES) is ideally suited to detect redox-inactive ions by their ion transfer. Such electroanalysis, based on the Nernst-Donnan equation, has been predominantly performed using amperometry, cyclic voltammetry, or differential pulse voltammetry. Here, we introduce a new electroanalytical method based on alternating-current (AC) voltammetry with inherent advantages over traditional approaches such as avoidance of positive feedback iR compensation, a major issue for liquid|liquid electrochemical cells containing resistive organic media and interfacial areas in the cm² and mm² range. A theoretical background outlining the generation of the analytical signal is provided and based on extracting the component that depends on the Warburg impedance from the total impedance. The quantitative detection of a series of model redox-inactive tetraalkylammonium cations is demonstrated, with evidence provided of the transient adsorption of these cations at the interface during the course of ion transfer. Since ion transfer is diffusion-limited, by changing the voltage excitation frequency during AC voltammetry, the intensity of the Faradaic response can be enhanced at low frequencies (1 Hz) or made to disappear completely at higher frequencies (99 Hz). The latter produces an AC voltammogram equivalent to a "blank" measurement in the absence of analyte and is ideal for background subtraction. Therefore, major opportunities exist for the sensitive detection of ionic analyte when a "blank" measurement in the absence of analyte is impossible. This approach is particularly useful to deconvolute signals related to reversible electrochemical reactions from those due to irreversible processes, which do not give AC signals.

A high spatiotemporal study of somatic exocytosis with scanning electrochemical microscopy and nanoITIES electrodes

Extra-synaptic exocytosis is an essential component of cellular communication. A knowledge gap exists in the exocytosis of the non-redox active transmitter acetylcholine. Using the nano-interface between two immiscible electrolyte solutions and scanning electrochemical microscopy (SECM), a high resolution spatiotemporal study of acetylcholine exocytosis is shown from an individual neuronal soma. The nanoelectrode was positioned ∼140 nm away from the release sites on the soma using an SECM. The quantitative study enables the obtaining of key information related to cellular communication, including extracellular concentration of the neurotransmitter, cellular permeability, Ca2+ dependence on somatic release, vesicle density, number of molecules released and the release dynamics. Measurements were achieved with a high signal to noise ratio of 6-19. The released neurotransmitter with a concentration of 2.7 (±1.0) μM was detected at the nanoelectrodes with radii of 750 nm to 860 nm.

GABA Detection with Nano-ITIES Pipet Electrode: A New Mechanism, Water/DCE-Octanoic Acid Interface

Interface between two immiscible electrolyte solutions (ITIES) supported on the orifice of a pipet have become a powerful platform to detect a broad range of analytes. We present here the detection of γ-aminobutyric acid (GABA) with the nanoITIES pipet electrodes for the first time. GABA has a net charge of zero in an aqueous solution at pH ≈ 7, and it has not previously been detected at ITIES. In this work, we demonstrated GABA detection at ITIES in an aqueous solution at pH ≈ 7, where we introduced a novel detection strategy based on "pH modulation from the oil phase". To the best of our knowledge, this is the first report of such. Current increases linearly with increasing concentrations of GABA, ranging from 0.25 mM to 1.0 mM. The measured half-wave transfer potential of GABA is -0.401 ± 0.010 V ( n = 22) vs E1/2,TBA. The measured diffusion coefficient for GABA detection at nanoITIES pipet electrode is 6.09 (±0.58) × 10-10 m2/s ( n = 5). Experimental results indicate that protons generated from octanoic acid dissociation in the oil phase do not come out from the oil phase into the aqueous phase; neither were protons produced in the aqueous phase. NanoITIES pipet electrodes with radii of 320-340 nm were used in the current study. This new strategy and knowledge presented here lays the groundwork for the future development of ITIES pipet electrodes, especially for the detection of electrochemically nonredox active analytes.

Void-Assisted Ion-Paired Proton Transfer at Water-Ionic Liquid Interfaces

At the water-trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) ionic liquid interface, the unusual electrochemical transfer behavior of protons (H(+)) and deuterium ions (D(+)) was identified. Alkali metal cations (such as Li(+), Na(+), K(+)) did not undergo this transfer. H(+)/D(+) transfers were assisted by the hydrophobic counter anion of the ionic liquid, [FAP](-), resulting in the formation of a mixed capacitive layer from the filling of the latent voids within the anisotropic ionic liquid structure. This phenomenon could impact areas such as proton-coupled electron transfers, fuel cells, and hydrogen storage where ionic liquids are used as aprotic solvents.

Toxic effects of heavy metal terbium ion on the composition and functions of cell membrane in horseradish roots

The environmental safety of rare earth elements (REEs), especially the toxic effect of REEs on plants, has attracted increasing attention. However, the cellular mechanism of this toxic effect remains largely unknown. Here, the toxic effects of heavy REE terbium ion [Tb(III)] on the cell membrane of horseradish roots were investigated by using electron microscope autoradiography (EMARG) and histochemical methods. The results indicated that Tb(III) was distributed in the extracellular and intracellular spaces of the roots after horseradish was treated with Tb(III). Moreover, the percentage contents of the unsaturated fatty acids in the membrane lipids, the current of the outward K(+) channel and the average diameter of membrane proteins in the roots of horseradish treated with Tb(III) were decreased; on the contrary, the percentage contents of the saturated fatty acids and malondialdehyde in the roots of horseradish treated with Tb(III) were increased. Furthermore, the contents of intracellular N, P, Mg and Fe in the roots of horseradish treated with Tb(III) were decreased, while the contents of intracellular K and Ca in the roots of horseradish treated with Tb(III) were increased. Finally, the effects of Tb(III) on horseradish roots were increased with increasing concentration or duration of Tb(III) treatment. In conclusion, after horseradish was treated with Tb(III), Tb(III) could enter the cells of horseradish roots and lead to the toxic effects on horseradish, which caused the oxidation of the unsaturated fatty acids in the membrane lipids, the changes in the membrane proteins (including the outward K(+) channel), the decrease in the membrane fluidity, and then the inhibition of the intracellular/extracellular-ion exchange in horseradish roots.

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.

Kinetic study of rapid transfer of tetraethylammonium at the 1,2-dichloroethane/water interface by nanopipet voltammetry of common ions

Steady-state voltammetry at the pipet-supported liquid/liquid interface has previously been used to measure kinetics of simple and facilitated ion transfer (IT) processes. Recently, we showed that the conventional experimental protocol and data analysis produce large uncertainties in kinetic parameters of rapid IT processes extracted from pipet voltammograms. Here, we used a new mode of nanopipet voltammetry, in which a transferable ion is initially present as a common ion in both liquid phases, and improved methodology for silanization of the outer pipet wall to investigate the kinetics of the rapid transfer of tetraethylammonium (TEA(+)) at the 1,2-dichloroethane/water interface. This reaction was often employed as a model system to check the IT theory. The determined standard rate constant and transfer coefficient of the TEA(+) transfer are compared with previously reported values to demonstrate limitations of conventional nanopipet voltammetry with a transferrable ion present only in one liquid phase.

Shuttling Mechanism of Ion Transfer at the Interface between Two Immiscible Liquids

The transfers of hydrophilic ions between aqueous and organic phases are ubiquitous in biological and technological systems. These energetically unfavorable processes can be facilitated either by small molecules (ionophores) or by ion-transport proteins. In absence of a facilitating agent, ion-transfer reactions are assumed to be "simple", one-step processes. Our experiments at the nanometer-sized interfaces between water and neat organic solvents showed that the generally accepted one-step mechanism cannot explain important features of transfer processes for a wide class of ions including metal cations, protons, and hydrophilic anions. The proposed new mechanism of ion transfer involves transient interfacial ion paring and shuttling of a hydrophilic ion across the mixed-solvent layer.

Amperometric glucose biosensor based on assisted ion transfer through gel-supported microinterfaces

A novel amperometric glucose sensor was developed based on the facilitated proton transfer across microinterfaces between two immiscible electrolyte solutions. The combination of a 1,3:2,4-dibenzylidene sorbitol/2-nitrophenyl octyl ether gel membrane and 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine as the ionophore allows the transfer of protons from water to the gellified organic phase; the gel membrane is supported on arrays of microholes drilled on a polyester film. The protons are generated as the result of the dissociation of gluconic acid produced during the enzymatic degradation of glucose by glucose oxidase. The characteristics of the glucose sensor were investigated using several experimental conditions, namely, the concentration of ligand and enzyme. The electrochemical response is typical of an enzymatic electrode and displays a linear behavior in the range 0.2-3 mM glucose. The effect of the experimental parameters of the voltammetric technique was also optimized with the aim of improving sensor sensitivity.

Electron Transfer Kinetics at Polarized Nanoscopic Liquid/Liquid Interfaces

Rapid kinetics of electron transfer (ET) reactions across the interface between water and 1,2-dichloroethane were measured by steady-state voltammetry at nanopipet electrodes (50- to 400-nm orifice radius). The origins of previously reported imperfect voltammetric responses of ET reactions at micropipets were investigated. Several new experimental systems were explored, and two of them yielded high-quality voltammograms suitable for kinetic experiments. The determined standard rate constants were compared to those measured previously at polarized and nonpolarized liquid/liquid interfaces. The effect of the interfacial dimensions on the magnitude of the apparent ET rate constant is discussed. A new approach to ET kinetic measurements based on the use of the scanning electrochemical microscope with a nanopipet tip and a metallic substrate has been developed and employed to check the validity of determined kinetic parameters.

Electrochemical Study of Ion Transfer of Acetylcholine Across the Interface of Water and a Lipid-Modified 1,2-Dichloroethane

The ion transfer of acetylcholine (AcH(+)) ions across the unmodified and phospholipid-modified water|1,2-dichloroethane (DCE) interface has been studied by means of square-wave and cyclic voltammetry, as well as by electrochemical impedance spectroscopy. After being transferred in the organic phase, the AcH(+) ions undergo chemical reactions with the phospholipids. The overall behavior of the experimental system studied in the presence of phospholipids has been compared with the theoretical results of an ECrev reaction. The kinetic parameters of the chemical interactions between AcH(+) and the phospholipids have been determined from the voltammetric and impedance measurements. Additional characterization of those interactions has been made by using the surface tension measurements.

Dual-pipet techniques for probing ionic reactions

Novel dual-pipet electrodes prepared by pulling borosilicate theta-tubing are described. Three types of electrochemical experiments employing such devices include the following: (1) generation/collection experiments in which ions are ejected from one of two micropipets ("generator") into the external solution and collected at the second pipet ("collector"), (2) measurements of ohmic current-voltage curves, and (3) ion-transfer voltammetry "in the air". The first setup is used for probing ion transfers at the interface between two immiscible liquids and homogeneous reactions in solution involving ionic species. Such experiments are reported for two model processes, i.e., simple and facilitated transfers of potassium between aqueous and organic phases and complexation of potassium with dibenzo-18-crown-6 in organic solution. The second arrangement is used for characterization of theta-pipets. The last arrangement can be useful for preparation of gas sensors. The possibility of measuring the concentration of volatile substances (e.g., ammonia and nitric acid) in the gaseous phase has been demonstrated.

Molecular structure and dynamics at liquid-liquid interfaces

The structural, dynamical, and electrical properties of the interface between two immiscible liquids are described. The adsorption of solute molecules and the processes of ion transfer across the interface and of electron transfer at the interface are discussed. The microscopic perspective is emphasized by focusing on selected recent experimental results and on results obtained from molecular dynamics and Monte Carlo computer simulations. The validity of some of the existing models of the interface is examined. A proper account of the molecular structure of the interface is important for understanding solvation and charge transfer processes at the interface.