Hydration energy or hydration force? Origin of ion-specificity in ion selective electrodes

The origin of ion-specificity (also known as Hofmeister effect) in potentiometric ion selective electrodes (ISE) with polymeric membranes has been traditionally assigned to the differences in lipophilicities of ions, with hydration energies described in the framework of Born theory as being of purely electrostatic nature. This is in clear contrast to the current understanding of the Hofmeister effect in colloid and interface science, where it is viewed as resulting from an interplay between the electrostatic and non-electrostatic interactions, the latter often referred to as “hydration” forces. The two approaches to ion-specificity in ISE, simplistically termed “hydration energy” (ion partitioning between an aqueous phase and the ISE membrane) and “hydration force” (ion adsorption from an aqueous phase to the electrically charged ISE membrane) are described and compared. Two major conclusions are drawn: i) ion-specificity can be included in both approaches, although it is more natural within the “hydration force” approach with ion-surface interactions; ii) both ion partitioning into, and ion adsorption onto the ISE membrane should be considered in order to fully understand the origin of ion-specificity in ISE.     

Horseradish peroxidase-screen printed biosensors for determination of Ochratoxin A

This work summarizes the manufacturing procedure of Horseradish peroxidase (HRP) based biosensors for the determination of the mycotoxin Ochratoxin A (OTA). The biosensors have been fabricated using the single technology of screen-printing. That is to say, an HRP containing ink has been directly screen-printed onto carbon electrodes, which offers a higher rapidity and simplicity in the manufacturing process of biosensors for OTA determination. The formal redox potential of the Fe(III/II) moiety of HRP has been used to demonstrate the effective loading of enzyme into the ink. The chronoamperometric oxidation current registered has been successfully related to the concentration of OTA in solution from different samples, including beer ones. Under the optimum conditions of the experimental variables, precision in terms of reproducibility and repeatability has been calculated in the concentration range from 23.85 to 203.28 nM. A relative standard deviation for the slopes of 10% (n = 4) was obtained for reproducibility. In the case of repeatability, the biosensor retained a 30% of the initial sensitivity after the third calibration. The average capability of detection for 0.05% probabilities of false positive and negative was 26.77 ± 3.61 nM (α = 0.05 and β = 0.05, n = 3).     

Determination of mercury in ambient water samples by anodic stripping voltammetry on screen-printed gold electrodes

The applicability of commercial screen-printed gold electrodes (SPGEs) for the determination of Hg(II) in ambient water samples by square wave anodic stripping voltammetry has been demonstrated. Electrode conditioning procedures, chemical and instrumental variables have been optimized to develop a reliable method capable of measuring dissolved mercury in the low ng mL⁻¹ range (detection limit 1.1 ng mL⁻¹), useful for pollution monitoring or screening purposes. The proposed method was tested with the NIST 1641d Mercury in Water Standard Reference Material (recoveries 90.0–110%) and the NCS ZC 76303 Mercury in Water Certified Reference Material (recoveries 82.5–90.6%). Waste water samples from industrial origin and fortified rain water samples were assayed for mercury by the proposed method and by a reference ICP-MS method, with good agreement. Screen printing technology thus opens a useful way for the construction of reliable electrochemical sensors for decentralized or even field Hg(II) testing.     

A solid-contact Pb-selective electrode using poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) as ion-to-electron transducer

In this work, a novel all-solid-state polymeric membrane Pb²⁺-selective electrode was developed by using for the first time poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) as solid contact. To demonstrate the ion-to-electron transducing ability of MEH-PPV, chronopotentiometry and electrochemical impedance spectroscopy measurements were carried out. The proposed electrodes showed a Nernstian response of 29.1 mV decade⁻¹ and a lower detection limit of subnanomolar level. No water film was observed with the conventional plasticized PVC membrane. This work demonstrates a new strategy for the fabrication of robust potentiometric ion sensors.     

Gold nanoparticles-based catalysis for detection of S-nitrosothiols in blood serum

Accumulating evidence suggests that S-nitrosothiols (RSNOs) play key roles in human health and disease. To clarify their physiological functions and roles in diseases, it is necessary to promote some new techniques for quantifying RSNOs in blood and other biological fluids. Here, a new method using gold nanoparticle catalysts has been introduced for quantitative evaluation of RSNOs in blood serum. The assay involves degrading RSNOs using gold nanoparticles and detecting nitric oxide (NO) released with NO-selective electrodes. The approach displays very high sensitivity for RSNOs with a low detection limit in the picomolar concentration range (5.08 × 10⁻¹¹ mol L⁻¹, S/N = 3) and is free from interference of some endogenous substances such as NO₂⁻ and NO₃⁻ co-existing in blood serum. A linear function of concentration in the range of (5.0-1000.0) × 10⁻⁹ mol L⁻¹ has been observed with a correlation coefficient of 0.9976. The level of RSNOs in blood serum was successfully determined using the described method above. In addition, a dose-dependent effect of gold nanoparticles on the sensitivity for RSNOs detection is revealed, and thereby the approach is potentially useful to evaluate RSNOs levels in various biological fluids via varying gold nanoparticles concentration.     

Potentiometric responses of ion-selective electrodes after galvanostatically controlled incorporation of primary ions

A new method of quantitative incorporation of primary cations into ion-selective membrane by means of galvanostatic cathodic polarization/conditioning, before measurement step, was proposed and tested on the example of potassium-selective electrode with ionophore - valinomycin in poly(vinyl chloride) based membrane and with polypyrrole solid contact. Open circuit potential values recorded after polarization can be quantitatively explained by changes of primary cations and ionophore concentration in the surface part of the membrane. The influence of potassium ions concentration in the membrane (in relation to ion exchange sites amount) on the shape of potentiometric calibration plots was also observed. Improved characteristics, with extended linear range, can be obtained for membrane of minor loading with primary cations (around 25%), the responses are relatively stable in course of following calibrations.     

Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell

The present work aims to explore the activity of Cu/CeO₂ composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, P_i-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO₂ with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO₂ were investigated by voltage-current density-power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H₂ production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and P_i-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu-CeO₂ electrodes remained essentially unaffected.     

Optical and electrochemical properties of ordered macroporous gold array on the ITO surface

The electrochemical and optical properties of transparent, two-dimensional macroporous gold film were investigated. Colloidal crystal templates were assembled onto indium-doped tin oxide (ITO) glass surface through vertical depositing method. Following gold electrodeposition, they were removed by dissolution with tetrahydrofuran (THF). The highly ordered macroporous gold array was achieved. It was characterized by scanning electron microscope (SEM) and ultraviolet visible (UV-vis) spectrophotometry. The optical transparency of the gold film was near 25% and fairly constant between 300 and 900 nm. The macroporous gold film electrode was mounted into a thin-layer transmission cell. The electrochemical response was evaluated by thin-layer cyclic voltammograms (CV) of the Fe (CN)₆³⁻/Fe (CN)₆⁴⁻ couple. Thin-layer cell exhibits good shape of waves and nearly symmetric cathodic and anodic waves. E⁰′ value and n of TMPD⁺/TMPD (TMPD is acronyms for N,N,N′,N′-tetramethyl-p-phenylenediamine, and TMPD⁺ is its mono-cation radical) couple were determined. Furthermore, results demonstrated electrolytic equilibrium was faster reached in macroporous gold film than ITO electrode.     

Performances of RETGEM with resistive electrodes made of kapton foils

THGEM with resistive Kapton electrodes (RETGEM) has been developed to make the THGEM more tolerant to discharges. At higher gains with resistive electrodes, serious discharges may travel to the streamer mode, in contrast to violent sparks in conventional GEMs. These streamers are mild and less dangerous to the detector and the front-end electronics. RETGEM looks very promising, and its basic properties are being studied. Recently we developed and tested the THGEM with electrodes using 20 um thick resistive kapton foils. The new RETGEM performs at a lower discharge current, has a lower discharge probability, and has a good energy resolution of 27% and a high effective gas gain and long-term stability.     

Particle‐in‐Cell Simulation for the Effect of Segmented Electrodes Near the Exit of an Aton‐Type Hall Thruster on Ion Focusing Acceleration

The effect of floating conductive electrodes near the channel exit of an Aton‐type Hall thruster on ion focusing acceleration is studied by simulating the two‐dimensional plasma flow with a fully kinetic Particle‐in‐Cell method for the gas flow rate j_a ranged in 1～3 mg/s. Numerical results show that low‐emissive electrodes can reduce plume divergence if the electrode length is less than 2 mm due to the low secondary electron emissive characteristic, but widen plume in all the gas flow rate range if the electrode length is greater than 2mm since the conductive property of segmented electrodes trends to make equipotential lines convex toward channel exit and is even parallel to the wall surface in the near‐wall region. Further investigation predicts that the combination of high emissive dielectric wall and segmented low‐emissive dielectric wall is a promising way to reduce plume divergence (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)