J.A.J. Brunink

Chemically modified field-effect transistors; a sodium ion selective sensor based on calix[4]arene receptor molecules

The development of an ion-sensitive field-effect transistor for sodium ions is described. Cahx[4]arene derivatives incorporated in a poly(vinyl chloride)-based membrane provide the selectivity. A poly(2-hydroxyethyl methacrylate) interlayer between the silicon dioxide gate and the sensing membrane is necessary to obtain a Na+-sensitive ISFET with Nernstian behaviour. The potentiometric selectivity coefficients (log Kij pot) for Na+ over K+ and Li+ are ?1.9 and ?2.5,

The design of durable Na+-selective CHEMFETs based on polysiloxane membranes

The design of durable sodium-selective CHEMFETs based on the covalent attachment of a sodium-selective ionophore and tetraphenylborate anions to a polysiloxane membrane matrix is described. Simulations of the membrane potential of such CHEMFETs using an extended version of the model developed previously in our group, revealed that a membrane with a reduced mobile ionophore and completely immobilized anionic sites should give a sub-nernstian response owing to a counteracting diffusion potential. CHEMFETs with all possible combinations of free and covalently bound ionophore and borate anions were prepared and the effect of covalent binding on the sensing behaviour was studied. The attachment of both electroactive components to a polysiloxane membrane matrix results in CHEMFETs that respond to Na+ activities in aqueous solution with good selectivity, and an almost nernstian slope (56.7 mV decade?1). The polarity of the membrane plays a crucial role. The durability is improved by the covalent attachment of the electroactive components (more than 90 days).

Effects of anionic sites on the selectivity of sodium-sensitive CHEMFETs

Calix[4]arene amides incorporated in poly(vinyl chloride) membranes provide good Na+ selectivity for CHEMFETs. A remarkable difference in the Na+/Ca2+ selectivity was observed depending on the presence or absence of tetraphenylborate ions in the membrane matrix. In the latter case the potentiometric selectivity coefficient (log KNa,CaPot) increased from −2.1 to −3.6. Calix[4]arene amides proved to be excellent ionophores to obtain Na+ selectivity in the presence of K+, i.e. log KNa,KPot = −2.7.

A Conformational Study of the Calixspherand and Its Complexes with Alkali-Metal Cations

The calixpherand 2 forms kinetically very stable complexes with alkalimetal cations. This molecule is not completely preorganized for binding of a cation, as is evidenced from the results of NOESY spectroscopy and X-ray diffraction measurements. Both in CDCl3 solution and in the solid state the free ligand adopts a cone conformation, whereas the Na+ complex adopts a flattened partial cone conformation. Molecular-mechanics calculations with different programs give rather biased results. Calculations with QUANTA(the all atom CHARMM-force field) correctly predict the conformation of the free ligand but not of the complexes, whereas with MACROMODEL(the united atom AMBER-force field) the experimentally observed conformation had the lowest energy only for the Na+ complex. The calculated geometries of the experimentally found conformations of the free ligand and the Na+ complex agree well with the X-ray structures, especially for the structures that were obtained with QUANTA. A comparison of the calculated structures of the Na+, K+ and Rb+ complexes showed that larger cations force the terphenyl bridge to bend away, thereby opening up the cage of the ligand and making the cation more accessible to solvent molecules. This might explain the considerably lower kinetic and thermodynamic stability of the Rb+ complex compared with those of the Na+ and K+ complexes.