Voltammetric detection of heparin at polarized blood plasma/1,2-dichloroethane interfaces

Electroanalytical applications of ITIES - A review

Over the last decades, the interface between two immiscible electrolyte solutions (ITIES) attracted considerable attention of the scientific community due to their vast applications, such as extraction, catalysis, partition studies and sensing. The aim of this Review is to highlight the potential of electrochemistry at the ITIES for analytical purposes, focusing on ITIES-based sensors for detection and quantification of chemically and biologically relevant (bio)molecules. We start by addressing the evolution of ITIES in terms of number of publications over the years along with an overview of their main applications (Chapter 1). Then, we provide a general historical perspective about pioneer voltammetric studies at water/oil systems (Chapter 2). After that, we discuss the most impacting improvements on ITIES sensing systems from both perspectives, set-up design (interface stabilization and miniaturization, selection of the organic solvent, etc.) and optimization of experimental conditions to improve selectivity and sensitivity (Chapter 3). In Chapter 4, we discuss the analytical applications of ITIES for electrochemical sensing of several types of analytes, including drugs, pesticides, proteins, among others. Finally, we highlight the present achievements of ITIES as analytical tool and provide future challenges and perspectives for this technology (Chapter 5).

Plasticized PVC Membrane Modified Electrodes: Voltammetry of Highly Hydrophobic Compounds

In the last 50 years, plasticized polyvinyl chloride (PVC) membranes have gained unique importance in chemical sensor development. Originally, these membranes separated two solutions in conventional ion-selective electrodes. Later, the same membranes were applied over a variety of supporting electrodes and used in both potentiometric and voltammetric measurements of ions and electrically charged molecules. The focus of this paper is to demonstrate the utility of the plasticized PVC membrane modified working electrode for the voltammetric measurement of highly lipophilic molecules. The plasticized PVC membrane prevents electrode fouling, extends the detection limit of the voltammetric methods to sub-micromolar concentrations, and minimizes interference by electrochemically active hydrophilic analytes.

A “turn-off” red-emitting fluorophore for nanomolar detection of heparin

A simple fluorophore bearing a diethylaminocoumarin donor and a pyridinium acceptor was synthesized and utilized for the ultra-sensitive detection of heparin.

Electrochemical behaviour at a liquid-organogel microinterface array of fucoidan extracted from algae

The electrochemical behaviour of fucoidan, a sulfated polysaccharide, was investigated, leading to a detection strategy by adsorptive stripping voltammetry.

Detection of Metoprolol in Human Biofluids and Pharmaceuticals via Ion-Transfer Voltammetry at the Nanoscopic Liquid/Liquid Interface Array

Metoprolol (MTP) is one of the most widely used antihypertensive drugs yet banned to use in sport competition. Therefore, there has been an increasing demand for developing simple, rapid, and sensitive methods suited to the identification and quantification of MTP in human biofluids. In this work, ultrathin silica nanochannel membrane (SNM) with perforated channels was employed to support nanoscale liquid/liquid interface (nano-ITIES) array for investigation of the ion-transfer voltammetric behavior of MTP and for its detection in multiple human biofluids and pharmaceutical formulation. Several potential interfering substances, including small molecules, d-glucose, urea, ascorbic acid, glycine, magnesium chloride, sodium sulfate and large molecules, bovine serum albumin (BSA), were chosen as models of biological interferences to examine their influence on the ion-transfer current signal of MTP. The results confirmed that the steady-state current wave barely changed in the presence of small molecules. Although BSA displayed an apparent blockade on the transfer of MTP, the accurate determination of MTP in multiple human biofluids (i.e., urine, serum and whole blood) and pharmaceutical formulation were still feasible, thanks to the molecular sieving and antifouling abilities of SNM. A limit of detection (LOD) within the physiological level of MTP during therapy could be achieved for all cases, i.e., 0.5 and 1.1 μM for 100 times diluted urine and serum, respectively, and 2.2 μM for 1000 times diluted blood samples. These results demonstrated that the nano-ITIES array behaved as a simplified and integrated detection platform for ionizable drug analysis in complex media.

Voltammetric Ion Selectivity of Thin Ionophore-Based Polymeric Membranes: Kinetic Effect of Ion Hydrophilicity

The high ion selectivity of potentiometric and optical sensors based on ionophore-based polymeric membranes is thermodynamically limited. Here, we report that the voltammetric selectivity of thin ionophore-based polymeric membranes can be kinetically improved by several orders of magnitude in comparison with their thermodynamic selectivity. The kinetic improvement of voltammetric selectivity is evaluated quantitatively by newly introducing a voltammetric selectivity coefficient in addition to a thermodynamic selectivity coefficient. Experimentally, both voltammetric and thermodynamic selectivity coefficients are determined from cyclic voltammograms of excess amounts of analyte and interfering ions with respect to the amount of a Na(+)- or Li(+)-selective ionophore in thin polymeric membranes. We reveal the slower ionophore-facilitated transfer of a smaller alkaline earth metal cation with higher hydrophilicity across the membrane/water interface, thereby kinetically improving voltammetric Na(+) selectivity against calcium, strontium, and barium ions by 3, 2, and 1 order of magnitude, respectively, in separate solutions. Remarkably, voltammetric Na(+) and Li(+) selectivity against calcium and magnesium ions in mixed solutions is improved by 4 and >7 orders of magnitude, respectively, owing to both thermodynamic and kinetic effects in comparison with thermodynamic selectivity in separate solutions. Advantageously, the simultaneous detection of sodium and calcium ions is enabled voltammetrically in contrast to the potentiometric and optical counterparts. Mechanistically, we propose a new hypothetical model that the slower transfer of a more hydrophilic ion is controlled by its partial dehydration during the formation of the adduct with a "water finger" prior to complexation with an ionophore at the membrane/water interface.

Thin layer coulometry based on ion-exchanger membranes for heparin detection in undiluted human blood

We explore here for the first time a potentially calibration-free methodology for the detection of protamine (and, by titration, heparin) in undiluted human blood in the therapeutic concentration range from 20 to 120 mg L(-1). The use of a thin layer sample (5.8 μL) confined between a tubular protamine selective membrane (inner diameter, 600 μm) and a Ag/AgCl wire (diameter 400 μm) achieves an exhaustive depletion from the sample. Coulometry detection was chosen for the interrogation of the thin layer, employing a double pulse technique with 120 s for each pulse. Protamine calibration curves were recorded at physiological concentrations and in undiluted human blood. A linear relationship was obtained in both cases, but a diminished sensitivity was observed in contact with blood, which is explained with a partial passivation of the inner Ag/AgCl element. Heparin-protamine titrations were performed in undiluted human blood samples, mimicking the final application with patients undergoing critical care. The observed values correlate satisfactorily with those of an alternative technique, so-called flash-chronopotentiometry on planar membranes.

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.

Stripping voltammetry of nanomolar potassium and ammonium ions using a valinomycin-doped double-polymer electrode

Here, we report on the first application of an ionophore-doped double-polymer electrode for ion-transfer stripping voltammetry (ITSV) to explore the nanomolar limit of detection (LOD) and multiple-ion detectability. We developed a theoretical model for ITSV at a thin ionophore-doped membrane on the solid supporting electrode to demonstrate that its LOD is controlled by the equilibrium preconcentration of an aqueous analyte ion as an ionophore complex into the thin polymer membrane and is lowered by the formation of a more stable ion-ionophore complex. The theoretical predictions were confirmed using valinomycin as a K(+)-selective ionophore, which forms a ∼60 times more stable complex with K(+) than with NH(4)(+), as confirmed by cyclic voltammetry. A LOD of 0.6 nM K(+) was achieved by ITSV using commercial ultrapure water as a K(+)-free media, where NH(4)(+) contamination at a higher concentration was also detected by ITSV. The dependence of the ITSV response on the preconcentration time was monitored under the rotating-electrode configuration and analyzed theoretically to directly determine ∼100 nM NH(4)(+) and ∼5 nM K(+) contaminations in commercial ultrapure water and laboratory-purified water, respectively, without the background ITSV measurement of an analyte-free blank solution.

Revisiting the Response Mechanism of Polymeric Membrane Based Heparin Electrodes

Potentiometric membrane electrodes that respond to heparin and other polyanions were introduced in the early 1990s. Herein, the mechanism of polymer membrane electrode type heparin sensors is revisited. The extraction/diffusion of heparin is studied via both potentiometric and impedance spectroscopic techniques using a pre-fractionated heparin preparation that contains polyanionic species > 10000 Daltons. The reversal in EMF response using this heparin preparation indicates diffusion of higher MW heparin fragments to the backside of the membrane. Diffusion coefficients are calculated using a novel formula derived from the phase boundary potential model and Fick's second law of diffusion. Impedance spectroscopy is also employed to show that high MW heparin species are extracted and diffuse across the PVC membranes.

Polyion-sensitive membrane-based electrodes for heparin-binding foldamer analysis

Polymer membrane-based electrodes sensitive to low molecular weight heparin (LMWH) have been used to examine the binding between several preparations of LMWH and heparin-binding foldamers, which have recently been developed as potential inhibitors of the anticoagulant activity of LMWHs. It was found that the structure of the heparin-binding foldamer affects the equilibrium binding constant, K(eq), determined by analysis of the titration curves of the foldamers with LMWHs monitored with these electrodes, and further, the strength of binding depends on the specific LMWH preparation. Additionally, polymer membrane-based electrodes utilizing dinonylnaphthalene sulfonate as the ion-exchanger were developed to measure the heparin-binding foldamers directly in whole blood, and the response was found to depend on the lipophilicity and charge density of the foldamer.

Voltammetric extraction of heparin and low-molecular-weight heparin across 1,2-dichloroethane/water interfaces

Heparin and low-molecular-weight heparin are voltammetrically extracted across 1,2-dichloroethane/water interfaces for the detection of these highly sulfated polysaccharides widely used as anticoagulants/antithrombotics in many medical procedures. A new heparin ionophore, 1-[4-(dioctadecylcarbamoyl)butyl]guanidinium, is the first to enable the voltammetric extraction of various polyanionic heparins with average molecular weights of up to approximately 20 kDa including those in commercial preparations (i.e., Arixtra (1.5 kDa), Lovenox (4.5 kDa), and unfractionated heparin (15 kDa), as well as chromatographically fractionated heparins (7, 9, 15, and 20 kDa)). Facilitated Arixtra extraction is fully and quantitatively characterized by micropipet voltammetry to propose that cooperative effects from strong heparin-binding capability and high lipophilicity of this ionophore are required for the formation of an electrically neutral and highly lipophilic complex of a heparin molecule with multiple ionophore molecules to be extracted into the nonpolar organic phase. At the same time, the participation of multiple ionophore molecules in interfacial complexation with a heparin molecule slows down its extraction across the interface. This kinetic limitation is enhanced by fast mass transfer at a micropipet-supported interface to compromise thermodynamically favorable selectivity for heparin and an important contaminant, oversulfated chondroitin sulfate, thereby requiring a macroscopic interface for sensing applications. Another highly lipophilic guanidinium ionophore, N,N-dioctadecylguanidinium, cannot completely extract even Arixtra, which indicates the importance of elaborate ionophore design for heparin extraction.