Fabrication of agar-gel microelectrodes and their application in the study of ion transfer across the agar–water∣1,2-dichloroethane interface

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.

Electrochemical Behavior and Detection of Diclofenac at a Microporous Si3N4 Membrane Modified Water–1,6-dichlorohexane Interface System

The electrochemical behavior when the liquid–liquid interface was modified by commercially available, microporous silicon nitride membrane, was achieved using cyclic voltammetry with tetramethyl ammonium. The transfer characteristics of the ionizable drug diclofenac ( DCF − ), as an anti-inflammatory, anti-rheumatic, antipyretic, and analgesic treatment in common use in biomedical applications, were also investigated across microporous silicon nitride-modified liquid interface. Thus, some thermodynamic variables for DCF − , such as the standard Gibbs energy of transfer, the standard transfer potential and lipophilicity were estimated. Furthermore, the influence of possible interfering substances (ascorbic acid, sugar, amino acid, urea, and metal ions) on the detection of DCF − was investigated. An electrochemical DCF sensor is investigated using differential pulse voltammetry (DPV) as the quantification technique, a linear range of 8–56 µM and a limit of detection of 1.5 µM was possible due to the miniaturized interfaces formed within silicon nitride.

Mechanistic Study of Oxygen Reduction at Liquid/Liquid Interfaces by Hybrid Ultramicroelectrodes and Mass Spectrometry

Proton-coupled electron transfer (PCET) reactions at various interfaces (liquid/membrane, solid/electrolyte, liquid/liquid) lie at the heart of many processes in biology and chemistry. Mechanistic study can provide profound understanding of PCET and rational design of new systems. However, most mechanisms of PCET reactions at a liquid/liquid interface have been proposed based on electrochemical and spectroscopic data, which lack direct evidence for possible intermediates. Moreover, a liquid/liquid interface as one type of soft interface is dynamic, making the investigation of interfacial reactions very challenging. Herein a novel electrochemistry method coupled to mass spectrometry (EC-MS) was introduced for in situ study of the oxygen reduction reaction (ORR) by ferrocene (Fc) under catalysis from cobalt tetraphenylporphine (CoTPP) at liquid/liquid interfaces. The key units are two types of gel hybrid ultramicroelectrodes (agar-gel/organic hybrid ultramicroelectrodes and water/PVC-gel hybrid ultramicroelectrodes), which were made based on dual micro- or nanopipettes. A solidified liquid/liquid interface can be formed at the tip of these pipettes, and it serves as both an electrochemical cell and a nanospray emitter for mass spectrometry. We demonstrated that the solidified L/L interfaces were very similar to typical L/L interfaces. Key CoTPP intermediates of the ORR at the liquid/liquid interfaces were identified for the first time, and the four-electron oxygen reduction pathway predominated, which provides valuable insights into the mechanism of the ORR. Theoretical simulation has further supported the possibility of formation of intermediates. This type of platform is promising for in situ tracking and identifying intermediates to study complicated reactions at liquid/liquid interfaces or other soft interfaces.

Transport of neutral and ionic solutes: the gel/electrode and gel/electrolyte interfaces

Transport through a polysaccharide gel phase has been investigated voltammetrically using both redox voltammetry at a gel covered electrode and ion transfer voltammetry across the gel/liquid interface (Gel/L). The apparent diffusion coefficients, D(app), of a range of neutral and ionic electroactive species have been determined in both uncharged and anionic polysaccharide media, agar, and kappa-carrageenan, respectively. It is shown that the diffusion of electroactive species in agar gel occurs at a rate similar to that of diffusion in aqueous solution for a range of redox couples. In the kappa-carrageenan medium, by contrast, the diffusion coefficient obtained for cationic solutes was found to be approximately an order of magnitude lower than the value in aqueous solution. The difference in D(app) is attributed to two independent processes: electrostatic interactions between the charge of the sulfonate groups of the kappa-carrageenan gel and the charge of the solute, as well a change in hydration of the solute molecules.

Ion-transfer voltammetry at silicon membrane-based arrays of micro-liquid-liquid interfaces

Microporous silicon membranes, fabricated by lithographic patterning and wet and dry silicon etching processes, were used to create arrays of micro-scale interfaces between two immiscible electrolyte solutions (muITIES) for ion-transfer voltammetry. These membranes served the dual functions of interface stabilization and enhancement of the rate of mass-transport to the interface. The pore radii were 6.5 microm, 12.8 microm and 26.6 microm; the pore-pore separations were ca. 20- to 40-times the pore radii and the membrane thickness was 100 microm. Deep reactive ion etching (DRIE) was used for pore drilling through the silicon, which had been previously selectively thinned by potassium hydroxide etching. DRIE produces hydrophobic fluorocarbon-coated internal pore walls. The small pore sizes and large pore-pore separations used resulted in steady-state voltammograms for the transfer of tetramethylammonium cation (TMA(+)) from the aqueous to the organic phase, whereas the reverse voltammetric sweeps were peak-shaped. These asymmetric voltammograms are consistent with the location of the ITIES at the aqueous side of the silicon membrane such that the organic phase fills the micropores. Comparison of the experimental currents to calculated currents for an inlaid disc micro-interface revealed that the interfaces were slightly recessed, up to 10 microm (or 10% of the pore length) in one case. Facilitated ion transfer, with an organic-phase ionophore, confirmed the location of the organic phase within the pores. These microporous silicon membranes offer opportunities for various analytical operations, including enhancing the rate of mass transport to ITIES-based sensing devices and stabilization of the ITIES for hydrodynamic applications.

Cyclic and pulse voltammetric study of dopamine at the interface between two immiscible electrolyte solutions

The detection of dopamine by differential pulse voltammetry (DPV) and square wave voltammetry (SWV) at the interface between two immiscible electrolyte solutions (ITIES) has been studied. Voltammetry at the liquid/liquid (water/1,2-dichloroethane) interface provides a simple method for overcoming the major problem associated with dopamine detection by voltammetry at solid electrodes: the co-existence of ascorbate at higher concentrations. Selectivity for dopamine was achieved by the use of dibenzo-18-crown-6 as an ionophore for the facilitated transfer voltammetry of protonated dopamine across the ITIES. Under these conditions, ascorbate is not transferred and hence does not interfere in the ion transfer current for dopamine. By use of DPV and SWV, the lowest concentration detectable can be lowered from ca. 0.1 mM (obtained with cyclic voltammetry) to 2 microM. Evaluation of the effect of some other physiologically important species (acetylcholine, sodium, potassium and ammonium ions) on the dopamine transfer voltammetry has been studied, indicating the need for improved ionophore designs in order to achieve practically useful selectivity.

Influence of the presence of a gel in the water phase on the electrochemical transfer of ionic forms of beta-blockers across a large water 1,2-dichloroethane interface

The transfer of ionic species of three beta-blockers (propranolol, sotalol and timolol) has been studied by cyclic voltammetry at a macroscopic water 1,2-dichloroethane (1,2-DCE) interface. The aqueous solution has been gellified in order to study the effect of the gel on the transport properties of the drugs. The gelling agent also stabilizes the interface overcoming mechanical instability. The standard potential and standard Gibbs energy of transfer across the interface, the partition coefficient and the diffusion coefficient of each drug were determined in the presence of a gelled interface. The diffusion coefficients were shifted relative to those obtained at normal water 1,2-DCE interfaces (free of gel).