Assisted ion transfer at micro-ITIES supported at the tip of micropipettes

Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase

Here, we report on the successful demonstration and application of carbonate (CO₃^2-) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO₃^2--selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO₃^2-, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO₃^2- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO₃^2-ionophore (CO₃^2-ionophore VII) by voltammetrically and amperometrically sensing CO₃^2- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO₃^2- ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, k⁰ = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO₃^2- ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO₃^2--selective amperometric nanoprobes is further demonstrated by measuring the CO₃^2- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H₂PO₄^-, Cl^-, and SO₄^2-.

Potential Difference-Modulated Synthesis of Self-Standing Covalent Organic Framework Membranes at Liquid/Liquid Interfaces

Covalent organic framework (COF) membranes with tailored functionalities hold great promise in diverse applications, but the key to realize their full advantages of highly ordered pore structures is the development of membrane fabrication approaches. In this work, we report a potential difference-modulated biphasic strategy to fabricate large-area, self-standing COF membranes under ambient conditions. The fabrication was conducted at the polarized water/1,2-dichloroethane (water/DCE) interface, where HCl was dissolved in water as a catalyst and monomers (both amine and aldehyde) were added to DCE. The external polarization of the water/DCE interface by cyclic voltammetry can continuously pump H⁺ from water to DCE to boost the Schiff base reaction of monomers and the growth of COF membranes. Moreover, the growth process can be real-time-monitored by interfacial double-layer capacitance measurement, and the permeability of COF membranes can be in situ-examined by heterogeneous ion transfer voltammetry. Given that the potential difference across the water/DCE interface can be also facilely modulated by dissolving proper electrolyte ions in two phases, the fabrication of large-area COF membranes is made possible in beakers. Using this strategy and different monomers, three types of centimeter-scale, free-standing COF membranes with tunable pore size and surface functionality were prepared, and their defect-free structure was proved by the molecular permeance and ultrafiltration test. We believe that this biphasic strategy offers a controllable and scalable way to fabricate COF membranes and sheds light on development of novel self-supporting membranes with unique functions.

Defined Ion-Transfer Voltammetry of a Single Microdroplet at a Polarized Liquid/Liquid Interface

A strategy for the fast analysis of ion transfer/facilitated ion transfer toward a tiny (femtoliter) water-in-oil droplet has been established. This scenario is embodied by the fusion of a w/o microdroplet at the micro liquid/liquid (L/L) interface, with the use of Fourier transform fast-scan cyclic voltammetry (FT-FSCV) to express the apparent half-wave potentials of anions or cations encapsulated inside the w/o microdroplet. First, the half-wave potential is in strict accordance with the transfer Gibbs free energy of either cations or anions. Second, the half-wave potential has been found to be positively proportional to the logarithmic concentration of ions, shedding thermodynamic insight into ion transfer. Third, as an instance of multivalent biopolymers, the transfer of protamine inside the single w/o microdroplet has been investigated. Obvious discrepancies in the behaviors of the fusion impacts at different pH, as well as in the absence and presence of the cationic surfactant DNNS^-, are revealed. The internal mechanism of protamine transfer has been thoroughly investigated. This work proposes a strategy to sensitively and quickly determine the transfer Gibbs energy and the concentration of ions encapsulated in a single microdroplet, and it provides the possibility of analyzing the interfacial transfer properties of trace biomacromolecules inside an aqueous micro- or nanoscale droplet.

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.

Faradaic Counter for Liposomes Loaded with Potassium, Sodium Ions, or Protonated Dopamine

Collisional electrochemistry between single particles and a biomimetic polarized micro-liquid/liquid interface has emerged as a novel and powerful analytical method for measurements of single particles. Using this platform, rapid detection of liposomes at the single particle level is reported herein. Individual potassium, sodium, or protonated dopamine-encapsulated (pristine or protein-decorated) liposomes collide and fuse with the polarized micro-liquid/liquid interface accompanying the release of ions, which are recorded as spike-like current transients of stochastic nature. The sizing and concentration of the liposomes can be readily estimated by quantifying the amount of encapsulated ions in individual liposomes via integrating each current spike versus time and the spike frequency, respectively. We call this type of nanosensing technology "Faradaic counter". The estimated liposome size distribution by this method is in line with the dynamic light scattering (DLS) measurements, implying that the quantized current spikes are indeed caused by the collisions of individual liposomes. The reported electrochemical sensing technology may become a viable alternative to DLS and other commercial nanoparticle analysis systems, for example, nanoparticle tracking analysis.

Electrified liquid-liquid interface as an electrochemical tool for the sensing of putrescine and cadaverine

Putrescine and cadaverine are biogenic amines that serve as potential biomarkers for several types of cancers and monitoring food quality. Electrochemical sensing of putrescine and cadaverine by non-enzymatic routes remains a challenge because of their inertness at unmodified electrode surfaces and hence a liquid-liquid interface strategy has been employed for their detection. In the present study, electrochemical sensing of cadaverine and putrescine has been demonstrated by simple and facilitated ion-transfer processes using a liquid-liquid microinterface supported by a microcapillary. A microinterface was constructed in different configurations by varying the aqueous phase composition in the absence and presence of dibenzo-18-crown-6, and the ion-transfer ability of putrescine and cadaverine was studied in these configurations. A peak shaped voltammogram was observed in the backward scan, due to the linear diffusion of putrescine and cadaverine from the organic to the aqueous phase. The detection ability in the presence of dibenzo-18-crown-6 was observed in the concentration ranges of 0.25-25 μM and 0.25-40 μM for putrescine and cadaverine with detection limits of 0.11 and 0.17 μM respectively. In the presence of dibenzo-18-crown-6, the electrochemical sensing of putrescine and cadaverine was more pronounced compared to the simple ion-transfer process.

Recent progress in simulating microscopic ion transport mechanisms at liquid-liquid interfaces

Transport of ions through liquid-liquid interfaces is of fundamental importance to a wide variety of applications. However, since it is quite challenging for experimentalists to directly and selectively observe molecules at the interfaces, microscopic mechanisms of ion transport have been largely presumed from kinetic information. This Perspective illustrates recent examples that molecular dynamics simulations with proper free energy surfaces clarified mechanistic pictures of ion transport. The key is a proper choice of coordinates and defining/calculating free energy surfaces in multidimensional space. Once the free energy surfaces for realistic systems are available, they naturally provide new insight into the ion transport in unprecedented details, including water finger, transient ion pairing, and electron transfer.

Investigation of Dendrimer Transfer Behaviors at the Micro-Water/1,2-Dichloroethane Interface Facilitated by Dibenzo-18-Crown-6

Trans-interfacial behaviors of multiple ionic species at the interface between two immiscible electrolyte solutions (ITIES) are of importance to biomembrane mimicking, chemical and biosensing, and interfacial molecular catalysis. Utilizing host-guest interaction to facilitate ion transfer is an effective and commonly used method to decrease the Gibbs energy of transfer of a target molecule. Herein, we investigated a facilitated ion transfer (FIT) process of poly(amidoamine)dendrimer (PAMAM, G0-G2) by dibenzo-18-crown-6 (DB18C6) at the microinterfaces between water and 1,2-dichloroethane (μ-W/DCE). Because of the host-guest interaction between a dendrimer and a ligand, negative shifts of the transfer potentials were observed using cyclic voltammetry or Osteryoung square wave voltammetry. From the FIT behavior of the dendrimer, we revealed that each DB18C6 could selectively coordinate with one amino group. We first evaluated the protonated status of the intermediate state (1:2) exactly under the conditions the dendrimer (G1) transfers across the interface using the electrochemical mass spectrometry (EC-MS)-hyphenated technique, which is much smaller than the protonated status in the water phase (1:8 to 14). Using the same methodology, we also studied the facilitated transfer behaviors of G0 and G2. Based on these results, we put forward the mechanism of the FIT process, which might involve a deprotonating process at the interface for higher-generation dendrimers.

Voltammetric Analysis of Redox Reactions and Ion Transfer in Water Microdroplets

We report a set of voltammetric experiments for studying redox reactions and ion transfer in water microdroplets emulsified in 1,2-dichloroethane (DCE). The electrochemistry of microdroplets (r_drop ∼ 700 nm) loaded with either ferrocyanide ([Fe(CN)₆]^4-) or ferricyanide ([Fe(CN)₆]^3-), chosen due to their hydrophilic nature, was tracked using cyclic voltammetry. These heterogeneous reactions necessitated ion transfer at the droplet interface to maintain charge balance in the two liquid phases during oxidation or reduction, which was facilitated by the tetrabutylammonium perchlorate ([TBA][ClO₄]) salt in the DCE phase. Experiments were performed with (1) a single macrodroplet (10^-7 L) on a macroelectrode (r ∼ 1.5 mm), (2) millions of microdroplets (10^-15 L) adsorbed on to a macroelectrode (r ∼ 1.5 mm), and (3) at the single microdroplet level via observing individual microdroplet collisions at an ultramicroelectrode (r ∼ 5 μm). We demonstrate that when millions of microdroplets are adsorbed onto a macroelectrode, there are two surprising observations: (1) the half-wave potential (E_1/2) for the [Fe(CN)₆]^3-/4- redox couple shifts by +100 mV, which is shown to depend on the number of droplets on the electrode surface. (2) The reduction of [Fe(CN)₆]^3-, which is assisted by the transfer of TBA⁺ into the water droplet, displays two waves in the voltammogram. This dual-wave behavior can be explained by the formation of TBA_xK_3-xFe(CN)₆, which is soluble in DCE. Additionally, we demonstrate that the adsorption of microdroplets onto an electrode surface offers significant amplification (×10³) of the water/oil/electrode three-phase boundary when compared to the adsorption of larger macrodroplets, permitting a rigorous evaluation of heterogeneous chemistry at this distinct interface. In combination, these experiments provide new energetic and mechanistic insights for droplet systems, as well as reactivity differences between microscale and bulk multiphase systems.

Revealing Transient Shuttling Mechanism of Catalytic Ion Transport through Liquid-Liquid Interface

Hard, hydrophilic ions that hardly transport over the water-oil interface by imposing external electric potential could undergo facile transport with a trace of ligand. Such phenomena, called "shuttling", are elucidated by microscopic investigation with molecular dynamics simulations. The catalytic role manifests itself in a 2-D free-energy surface within the nanometer range of the interface. The free-energy landscape clearly distinguishes the condition that the catalytic shuttling plays a vital role in the ion transport. The mechanism associated with transient complex formation at the interface is shown to be widely relevant to the ion kinetics and extends the conventional concept of facilitated ion transport.

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Flow-Pattern Characterization of Biphasic Electrochemical Cells by Magnetic Resonance Imaging under Forced Hydrodynamic Conditions

The fluid dynamics of a liquid|liquid system inside a four-electrode electrochemical cell were studied by velocimetry magnetic resonance imaging (MRI) and flow propagator measurements. To characterize this system fully, three different cell configurations operating at two rotational frequencies were analyzed. Quantitative information about the stability of the liquid|liquid interface and the dynamics of the organic phase were determined. The NMR spectroscopy results were in agreement with the electrochemical measurements performed by using the same experimental setup.

Investigation of proton-driven amine functionalized tube array as ion responsive biomimetic nanochannels

A simple amine embellished tube array was assembled at the liquid–liquid interface to study ion transfer behavior.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Coextraction of water into nitrobenzene with organic ions

Various organic anions (sulfonates (RSO3(-)), carboxylates (RCO2(-)), and phenolates (RO(-))) and ammonium cations (RNH3(+), R2NH2(+), and R3NH(+)) were distributed in the nitrobenzene (NB)-water system by using Crystal Violet and dipicrylaminate, respectively. The number of water molecules (n) being coextracted into NB with an ion was then determined by the Karl Fischer method. The n values determined and those reported previously showed the variation from 0.51 to 3.4, depending on not only the charged groups but also the noncharged R-groups. In this study, we focused our attention to the strong electric field on the charged group and its facilitation effect for binding water molecules in NB. The local electric field (Ei) on the surface of an organic ion was evaluated by using Gaussian09 program with a subprogram developed in our recent study. It was found that the n values showed a clear dependence on the average value of Ei on oxygen or hydrogen atoms, respectively, of an anionic or cationic group.

Electrochemical Sensing and Imaging Based on Ion Transfer at Liquid/Liquid Interfaces

Here we review the recent applications of ion transfer (IT) at the interface between two immiscible electrolyte solutions (ITIES) for electrochemical sensing and imaging. In particular, we focus on the development and recent applications of the nanopipet-supported ITIES and double-polymer-modified electrode, which enable the dynamic electrochemical measurements of IT at nanoscopic and macroscopic ITIES, respectively. High-quality IT voltammograms are obtainable using either technique to quantitatively assess the kinetics and dynamic mechanism of IT at the ITIES. Nanopipet-supported ITIES serves as an amperometric tip for scanning electrochemical microscopy to allow for unprecedentedly high-resolution electrochemical imaging. Voltammetric ion sensing at double-polymer-modified electrodes offers high sensitivity and unique multiple-ion selectivity. The promising future applications of these dynamic approaches for bioanalysis and electrochemical imaging are also discussed.

Chiral interactions of the drug propranolol and α1-acid-glycoprotein at a micro liquid-liquid interface

The investigation of chiral interactions of drugs with plasma proteins is of fundamental importance for drug efficacy and toxicity studies. In this paper, we demonstrate a simple liquid-liquid interface procedure for investigating chiral interactions. Chiral discrimination of the enantiomers of a basic drug, propranolol, was achieved at a micro liquid-liquid interface, using α(1)-acid-glycoprotein (AGP) as a chiral acute phase plasma protein. When the protein is added to an aqueous phase containing the enantiomers of propranalol hydrochloride, the binding of (S)- and (R)-propranolol hydrochloride to the protein results in a decrease in the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) current responses corresponding to the decrease in transfer of propranolol at an aqueous-1,2-dichloroethane interface. This decrease is a consequence of the complexation of the drug and the protein. The complex drug-protein does not transfer across the interface nor changes the transfer potential of the uncomplexed form of propranolol enantiomers. The bound concentration of propranolol enantiomers in the presence of AGP was found to be greater for (S)-propranolol than (R)-propranolol for solutions containing constant concentrations of AGP (50 μM). Scatchard analysis yielded association constants of 2.7 and 1.3 × 10(5) M(-1) for (S)- and (R)-propranolol, respectively.

Electrochemical mechanism of ion-ionophore recognition at plasticized polymer membrane/water interfaces

Here, we report on the first electrochemical study that reveals the kinetics and molecular level mechanism of heterogeneous ion-ionophore recognition at plasticized polymer membrane/water interfaces. The new kinetic data provide greater understanding of this important ion-transfer (IT) process, which determines various dynamic characteristics of the current technologies that enable highly selective ion sensing and separation. The theoretical assessment of the reliable voltammetric data confirms that the dynamics of the ionophore-facilitated IT follows the one-step electrochemical (E) mechanism controlled by ion-ionophore complexation at the very interface in contrast to the thermodynamically equivalent two-step electrochemical-chemical (EC) mechanism based on the simple transfer of an aqueous ion followed by its complexation in the bulk membrane. Specifically, cyclic voltammograms of Ag(+), K(+), Ca(2+), Ba(2+), and Pb(2+) transfers facilitated by highly selective ionophores are measured and analyzed numerically using the E mechanism to obtain standard IT rate constants in the range of 10(-2) to 10(-3) cm/s at both plasticized poly(vinyl chloride) membrane/water and 1,2-dichloroethane/water interfaces. We demonstrate that these strongly facilitated IT processes are too fast to be ascribed to the EC mechanism. Moreover, the little effect of the viscosity of nonaqueous media on the IT kinetics excludes the EC mechanism, where the kinetics of simple IT is viscosity-dependent. Finally, we employ molecular level models for the E mechanism to propose three-dimensional ion-ionophore complexation at the two-dimensional interface as the unique kinetic requirement for the thermodynamically facilitated IT.