Richard J.M. Egberink

Silver selective electrodes based on thioether functionalized calix[4]arenes as ionophores

Silver selective electrodes based on thioether functionalized calix[4]arenes 1 and 2 as ionophores were investigated. For both ionophores the selectivity coefficients (log kAg,M) were lower than −2.2 for Hg(II) and lower than −4.6 for other cations tested. The best results were obtained with membranes containing dithioether functionalized calix[4]arene (ionophore 2), potassium tetrakis(4-chlorophenyl) borate (KTpCIPB) and bis(1-butylpentyl)adipate (BBPA) as a plasticizer. The Ag(I)-response functions exhibited almost theoretical Nernstian slopes in the activity range 10−6–10−1M of silver ions.

Lead selective electrodes based on thioamide functionalized calix[4]arenes as ionophores

Lead selective electrodes based on a di- and tetrathioamide functionalized calix [4] arene as ionophores were investigated. The Pb(II)-response functions exhibited almost theoretical Nernstian slopes in the activity range 10?6?10?2M of lead ions. For both ionophores a preference for lead over other cations was observed. The best results were obtained for membranes containing the tetrathioamide derivative (ionophore 1), potassium tetrakis(4-chlorophenyl) borate and o-nitrophenyl octyl ether as a plasticizer. This electrode showed greatly improved selectivity over copper, zinc and cadmium ions as compared with commercial solid-state lead selective electrodes.

Uranyl salophenes as ionophores for phosphate-selective electrodes

Anion selectivities of poly(vinylchloride) (PVC) plasticized membranes containing uranyl salophene derivatives were presented. The influence of the membrane components (i.e. ionophore structure, dielectric constant and structure of plasticizer, the amount of incorporated ammonium salt) on its phosphate selectivity was investigated. The highest selectivity for H2PO4− over other anions tested was obtained for lipophilic uranyl salophene III (without ortho-substituents) in PVC/o-nitrophenyl octylether (o-NPOE) membrane containing 20 mol% of tetradecylammonium bromide (TDAB). Ion-selective electrodes (ISEs) based on these membranes exhibited linear response in the range 1–4 of pH2PO4− with a slope of 59 mV/decade. The introduction of ortho-methoxy substituents in ionophore structure decreased the phosphate selectivity of potentiometric sensors.

Substantial rate enhancements of the esterification reaction of phthalic anhydride with methanol at high pressure and using supercritical CO2 as a co-solvent in a glass microreactor

The esterification reaction of phthalic anhydride with methanol was performed at different temperatures in a continuous flow glass microreactor at pressures up to 110 bar and using supercritical CO(2) as a co-solvent. The design is such that supercritical CO(2) can be generated inside the microreactor. Substantial rate enhancements were obtained, viz. a 53-fold increase was obtained at 110 bar and 60 degrees C. Supercritical CO(2) as a co-solvent gave rise to a 5400-fold increase (both with respect to batch experiments at 1 bar at the same temperature).

Durable phosphate-selective electrodes based on uranyl salophenes

Lipophilic uranyl salophenes derivatives were used as ionophores in durable phosphate-selective electrodes. The influence of the ionophore structure and membrane composition (polarity of plasticizer, the amount of incorporated ionic sites) on the electrode selectivity and long-term stability were studied. The highest selectivity for H2PO4− over other anions tested was obtained for lipophilic uranyl salophene III (with t-butyl substituents) in poly(vinylchloride)/o-nitrophenyl octyl ether (PVC/o-NPOE) membrane containing 20 mol% of tetradecylammonium bromide (TDAB). Moreover, phosphate-selective electrodes based on this derivative exhibited the best long-term stability (2 months). The electrode durability can be improved decreasing the amount of the ammonium salt in membrane to 5 mol%.

Chemically modified field effect transistors: the effect of ion-pair association on the membrane potentials

A theoretical model has been developed which relates physically accessible parameters to the formation of a membrane potential. The description is an extension of a theoretical description presented previously by our group, now including divalent cations and ion-pair association. Simulations of the overall membrane potential reveal several factors that may lead to non-Nernstian response curves. For monovalent and divalent cations a reduction in the slope of the response curve (sub-Nernstian response) should virtually always be expected when ion-pair association takes place in the membrane. Ion-pair association of divalent cations and sample anions can lead to a super-Nernstian response. A diffusion potential generally reduces the Nernstian slope of the response curve. In addition, several experimental results are described which illustrate and confirm our theoretical model.

The effects of covalent binding of the electroactive components in durable CHEMFET membranes—impedance spectroscopy and ion sensitivity studies

The effects of covalent attachment of the electroactive components (i.e. the cation receptor molecules and the tetraphenylborate anions) in durable CHEMFET membranes is described. CHEMFETs for monovalent cations having either one or both electroactive components covalently bound in the membrane matrix exhibit Nernstian responses and good selectivities for Na+, or K+ ions, however, covalent attachment of borate in the membrane results in non-functioning sensors. Durability studies with CHEMFETs modified with polysiloxane membranes which are selective for Na+, K+ and Pb2+ ions show long lifetimes upon continuous exposure to a water stream. Na+ selective CHEMFETs with covalently attached Na+ ionophores and free borate in the membrane are still Na+ selective after 82 weeks of continuous exposure to water. Similar K+ and Pb2+ selective polysiloxane CHEMFETs showed good selectivities for at least 26 and 19 weeks, respectively. Impedance measurements showed that in all cases the lowest membrane resistance and the most stable CHEMFETs were obtained with polysiloxane membranes containing covalently attached ion-selective calix[4]arene-based receptor molecules and free anionic sites.

Durable nitrate-selective chemically modified field effect transistors based on new polysiloxane membranes

Polysiloxane copolymers with different amounts and types of substituents have been synthesized and characterized. Polar substituents determine the polarity and methacrylate groups allow cross-linking and covalent binding of electroactive species. These chemically well-defined homogeneous polymers have been applied as ion-selective membrane material in nitrate-selective field effect transistors (CHEMFETs). The selectivity of the CHEMFETs is influenced by the nature of the polar substituents in the polymer. The incorporation of methacrylate moieties in the polysiloxane allows covalent attachment of quaternary ammonium sites to the membrane matrix and this enlarges the durability of the sensor.

Gallium-containing polymer brush film as efficient supported Lewis acid catalyst in a glass microreactor.

Summary Polystyrene sulfonate polymer brushes, grown on the interior of the microchannels in a microreactor, have been used for the anchoring of gallium as a Lewis acid catalyst. Initially, gallium-containing polymer brushes were grown on a flat silicon oxide surface and were characterized by FTIR, ellipsometry, and X-ray photoelectron spectroscopy (XPS). XPS revealed the presence of one gallium per 2–3 styrene sulfonate groups of the polymer brushes. The catalytic activity of the Lewis acid-functionalized brushes in a microreactor was demonstrated for the dehydration of oximes, using cinnamaldehyde oxime as a model substrate, and for the formation of oxazoles by ring closure of ortho-hydroxy oximes. The catalytic activity of the microreactor could be maintained by periodic reactivation by treatment with GaCl3.

Chemically modified ion-sensitive field-effect transistors application in flow-injection analysis cells without polymeric encapsulation and wire bonding

New wall jet and flow-through cells have been developed which can be used for flow-injection analysis using chemically modified ion-sensitive field-effect transistors (CHEMFETs). In these designs traditional bonding and encapsulation with epoxy resin have become superfluous. This ISFET chip has been designed to meet the requirements of these two cell types. Measurements with plasticized PVC-valinomycin-modified CHEMFETs in these flow cells are presented. The effects of flow-rate and sample size on the recorded peak were investigated. The operational performance of the wall jet and flow-through cells were compared; the recorded peaks with the wall jet cell are less symmetrical than those with the flow-through cell.