A calixarene-based ion-selective electrode for thallium(I) detection.

Three new calixarene Tl(+) ionophores have been utilized in Tl(+) ion-selective electrodes (ISEs) yielding Nernstian response in the concentration range of 10(-2)-10(-6)M TlNO3 with a non-optimized filling solution in a conventional liquid contact ISE configuration. The complex formation constants (logβIL) for two of the calixarene derivatives with thallium(I) (i.e.

Evidence for a surface confined ion-to-electron transduction reaction in solid-contact ion-selective electrodes based on poly(3-octylthiophene).

The ion-to-electron transduction reaction mechanism at the buried interface of the electrosynthesized poly(3-octylthiophene) (POT) solid-contact (SC) ion-selective electrode (ISE) polymeric membrane has been studied using synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS), near edge X-ray absorption fine structure (NEXAFS), and electrochemical impedance spectroscopy (EIS)/neutron reflectometry (NR). The tetrakis[3,5-bis(triflouromethyl)phenyl]borate (TFPB(-)) membrane dopant in the polymer ISE was transferred from the polymeric membrane to the outer surface layer of the SC on oxidation of POT but did not migrate further into the oxidized POT SC. The TFPB(-) and oxidized POT species could only be detected at the outer surface layer (≤14 Ǻ) of the SC material, even after oxidation of the electropolymerized POT SC for an hour at high anodic potential demonstrating that the ion-to-electron transduction reaction is a surface confined process.

Water uptake in the hydrophilic poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) solid-contact of all-solid-state polymeric ion-selective electrodes.

Solid-contact (SC) ion-selective electrodes (ISEs) utilizing thin films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and plasticized poly(vinylchloride) (PVC) have been produced using a spin casting procedure. This study was carried out with a view of characterizing this popular and well known SC ISE using a series of complementary surface analysis techniques. This work revealed that PEDOT:PSS prevents the separation of an undesirable water layer at the buried interface of this SC ISE due to the high miscibility of water in the hydrophilic PEDOT:PSS layer.

Synchrotron radiation/Fourier transform-infrared microspectroscopy study of undesirable water inclusions in solid-contact polymeric ion-selective electrodes.

This paper reports on three-dimensional synchrotron radiation/Fourier transform-infrared microspectroscopy (SR/FT-IRM) imaging studies of water inclusions at the buried interface of solid-contact-ion-selective electrodes (SC-ISEs). It is our intention to describe a nondestructive method that may be used in surface studies of the buried interfaces of materials, especially multilayers of polymers. Herein, we demonstrate the power of SR/FT-IRM for studying water inclusions at the buried interfaces of SC-ISEs.

Potentiometric enzyme immunoassay using miniaturized anion-selective electrodes for detection.

An enzyme-linked immunosorbent assay (ELISA) for prostate specific antigen (PSA) detection in human serum was developed based on the potentiometric detection of 6,8-difluoro-4-methylumbelliferone (DiFMU). The assays were carried out in anti-human PSA capture antibody modified microtiter plates (150 microL volume). After incubation in the PSA-containing serum samples, beta-galactosidase-labeled PSA tracer antibody was added.

Memory Effects of Ion-Selective Electrodes: Theory and Computer Simulation of the Time-Dependent Potential Response to Multiple Sample Changes.

A straightforward theoretical description of the time-dependent response of ion-selective membrane electrodes to multiple sample changes is presented. The derivation makes use of an approximation for the ion fluxes in the membrane, and of the superposition of partial fluxes induced by the step-changes. The general theory allows for any number of samples and ions.

Theoretical Treatment and Numerical Simulation of Potential and Concentration Profiles in Extremely Thin Non-Electroneutral Membranes Used for Ion-Selective Electrodes.

The applicability of extremely thin non-electroneutral membranes for ion-selective electrodes (ISEs) is investigated. A theoretical treatment of potential and concentration profiles in space-charge membranes of << 1 μm thickness is presented. The theory is based on the Nernst-Planck equation for ion fluxes, which reduces to Boltzmann's formula at equilibrium, and on the Poisson relationship between space-charge density and electric field gradient.

Automatic compatibility tests of HSQC NMR spectra with proposed structures of chemical compounds.

A recently introduced similarity measure is extended here for comparing two-dimensional spectra. Its applicability is demonstrated with heteronuclear single-quantum correlation (HSQC) NMR spectra. For testing the compatibility of a spectrum with the proposed chemical structure, first, the spectrum is predicted on the basis of that structure and then, the proposed comparison algorithm is applied.

Limitations of current polarization for lowering the detection limit of potentiometric polymeric membrane sensors.

Ion fluxes across polymeric ion-selective membranes are a decisive parameter dictating the lower detection limit of potentiometric ion sensors. An applied current was earlier proposed to counteract such fluxes and reduce the detection limit to ultratrace levels. So far, however, the method has not been used in practical situations since the correct current amplitude requires prior knowledge of the sample composition.

Nanoscale potentiometry.

Potentiometric sensors share unique characteristics that set them apart from other electrochemical sensors. Potentiometric nanoelectrodes have been reported and successfully used for many decades, and we review these developments. Current research chiefly focuses on nanoscale films at the outer or the inner side of the membrane, with outer layers for increasing biocompatibility, expanding the sensor response, or improving the limit of detection (LOD).

Synthesis and Characterization of High Integrity Solid-Contact Polymeric Ion Sensors.

High integrity solid-contact (SC) polymeric ion sensors have been produced by using spin casting and electropolymerization techniques in the preparation of the SC employing the conductive polymer, poly(3-octylthiophene) [POT]. The physical and chemical integrity of the POT SCs have been evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). Furthermore, the electrochemical stability of SC polymeric ion sensors has been investigated using electrochemical impedance spectroscopy (EIS).

Covalent binding of biorecognition groups to solids using poly(hydromethylsiloxane) as linkage.

By activating SiH bonds, poly(hydromethylsiloxane) can be covalently bound in a first step to various metal or polymer surfaces. In a second step, unreacted SiH bonds can be brought to react with organic compounds having adequate functional groups such as double or triple bonds, carbonyl or hydroxyl groups. This scheme is used to bind biorecognition groups to solids.

The phase-boundary potential model.

The response of ion-selective electrodes (ISEs) can be described on the basis of two different theoretical approaches. On one hand, the phase-boundary model is based on the assumption of local equilibria at the aqueous/organic interface. The phase-boundary model allows the description of all practically relevant cases of steady state and even transient responses with sufficient accuracy.

Backside calibration chronopotentiometry: using current to perform ion measurements by zeroing the transmembrane ion flux.

A recent new direction in ion-selective electrode (ISE) research utilizes a stir effect to indicate the disappearance of an ion concentration gradient across a thin ion-selective membrane. This zeroing experiment allows one to evaluate the equilibrium relationship between front and backside solutions contacting the membrane by varying the backside solution composition. This method is attractive since the absolute potential during the measurement is not required, thus avoiding standard recalibrations from the sample solution and a careful control of the reference electrode potential.

Elimination of undesirable water layers in solid-contact polymeric ion-selective electrodes.

This study aimed to develop a novel approach for the production of analytically robust and miniaturized polymeric ion sensors that are vitally important in modern analytical chemistry (e.g., clinical chemistry using single blood droplets, modern biosensors measuring clouds of ions released from nanoparticle-tagged biomolecules, laboratory-on-a-chip applications, etc.

Theory and Computer Simulation of the Time-Dependent Selectivity Behavior of Polymeric Membrane Ion-Selective Electrodes.

A theoretical treatment of the time-dependent potential response of ion-selective electrodes to sample solutions containing primary and interfering ions is presented. The theory accounts for the influence of ion fluxes in the electrode membrane and the contacting aqueous sample layer and describes the variations in the apparent selectivity behavior as a function of the measuring time. The applicability of the theory is demonstrated by comparing predicted response curves with results of virtual experiments based on computer simulation.

Aptamer-based potentiometric measurements of proteins using ion-selective microelectrodes.

We here report on the first example of an aptamer-based potentiometric sandwich assay of proteins. The measurements are based on CdS quantum dot labels of the secondary aptamer, which were determined with a novel solid-contact Cd2+-selective polymer membrane electrode after dissolution with hydrogen peroxide. The electrode exhibited cadmium ion detection limits of 100 pM in 100 mL samples and of 1 nM in 200 microL microwells, using a calcium-selective electrode as a pseudoreference electrode.

Evidence of a water layer in solid-contact polymeric ion sensors.

This paper presents the very first direct structural evidence for the formation of a 100 +/- 10 A water layer in coated-wire polymeric-membrane ion-selective electrodes (ISEs).

Sensitivity and working range of backside calibration potentiometry.

A new direction in potentiometric sensing, termed backside calibration potentiometry, was recently introduced. It makes use of the fact that the stir effect disappears in the absence of an ion-ionophore complex concentration gradient across supported liquid ion-selective membranes. This method is especially suitable for measurements in which recalibration in the sample is not feasible, such as in remote monitoring applications.

Novel potentiometric and optical silver ion-selective sensors with subnanomolar detection limits.

Ten Ag+-selective ionophores have been characterized in terms of their potentiometric selectivities and complex formation constants in solvent polymeric membranes. The compounds with pi-coordination show much weaker interactions than those with thioether or thiocarbamate groups as the coordinating sites. Long-term studies with the best ionophores show that the lower detection limit of the best Ag+ sensors can be maintained in the subnanomolar range for at least 1 month.

Potentiometric immunoassay with quantum dot labels.

Potentiometric sensors based on polymer membrane electrodes, if properly optimized, are useful for measurements at trace levels. The expected independence of the electrochemical signal of the sample size makes them extremely attractive for measurements in small volumes. Here, we report on electrodes for the potentiometric detection of cadmium ions that reach a detection limit of 6 nM and utilize a Na(+)-selective electrode as pseudoreference in order to facilitate measurements in 150-microL samples.

Hybridization-modulated ion fluxes through peptide-nucleic-acid- functionalized gold nanotubes. A new approach to quantitative label-free DNA analysis.

The inner walls of gold nanotubes, prepared by template synthesis in the nanopores of polycarbonate track etch membranes, have been chemically modified with peptide nucleic acid (PNA) and used for label-free quantification of complementary DNA sequences. Selective binding of DNA to the PNA-modified nanotubes is shown to decrease the flux of optically detected anionic markers through the nanotubes in a concentration-dependent manner. The strong dependence of the biorecognition-modulated ion transport through the nanopores on the ionic strength suggests a dominantly electrostatic exclusion mechanism of the ion flux decrease as a result of DNA binding to the PNA-modified nanopores.