Peter D. van der Wal

Modification of ISFESTs by covalent anchoring of poly(hydroxyethyl methacrylate) hydrogel. Introduction of a thermodynamically defined semiconductor-sensing membrane interface

Silicon dioxide ion-sensitive field effect transistors were modified by silylation with methacryloxypropyltrimethoxysilane (MPTS) and with in situ photopolymerized poly(hydroxyethyl methacrylate). Subsequently, the covalently linked methacrylate was swollen with a buffered potassium chloride solution, prior to the introduction of a hydrophobic sensing membrane. The introduced hydrogel layer effects a significant reduction in the peak-to-peak noise levels and eliminates completely interference from carbon dioxide. The method is compatible with integrated circuit photolithographic techniques and improves the development of potentiometric biosensors and chemical sensors.

New membrane materials for potassium-selective ion-sensitive field-effect transistors

Several polymeric materials were studied as membrane materials for potassium-selective ion-sensitive field-effect transistors (ISFETs) to overcome the problems related with the use of conventional plasticized poly(vinyl chloride) membranes casted on ISFET gate surfaces. Several acrylate materials, such as ACE, Epocryl and derivatives, showed no reproducible results. Three room-temperature vulcanizing (RTV)-type silicone rubbers were tested. The addition-type RTV-2 silicone rubber was not suitable as a membrane material, but the condensation-type RTV-1 and especially the RTV-2 silicone rubber showed good results. ISFETs with a Silopren membrane showed a durability of at least 2 months.

Reference field effect transistors based on chemically modified ISFETs

Different hydrophobic polymers were used for chemical modification of ion-sensitive field effect transistors (ISFETs) in order to prepare a reference FET (REFET). Chemical attachment of the polymer to the ISFET gate results in a long lifetime of the device. Properties of polyacrylate (polyACE) REFETs are described in detail. The polyACE-REFET is superior to other polymer modified REFETs, showing an excellent pH insensitivity (1 mV pH−1), a long lifetime and an electrically identical behaviour as an unmodified pH ISFET or a cation-selective PVC-MEMFET (membrane FET). The cation permeselectivity of the polymer can be significantly reduced by addition of immobile cations. The applicability of a polyACE-REFET in differential measurements with a pH ISFET and a K+ MEMFET is demonstrated.

Impedance spectroscopy and surface study of potassium-selective silicone rubber membranes

Impedance spectroscopy measurements of silicone rubber membranes containing potassium-selective neutral carriers are reported. Two types of silicone rubbers are studied viz. the commercially available Siloprene and a novel copolymer, that was synthesized for application on Ion-Sensitive Field Effect Transistors (ISFETs). Three different potassium-selective ionophores have been studied, the natural ionophore, valinomycin, and two hemispherand type ionophores. One of the hemispherands can be covalently bonded to the polysiloxane copolymer matrix. The bulk resistance of the valinomycin containing membranes was found to be dependent on the contacting electrolyte solution. The K+/Na+ selectivity of the membrane is reflected in the behavior of the bulk resistance. The presence of a surface film on Siloprene membranes reported in the literature is confirmed. A surface study revealed the presence of small droplets exuded by the Siloprene membrane. The copolymer seems not to suffer from the presence of a surface film.

Design and properties of a flow-injection analysis cell using potassium-selective ion-sensitive field-effect transistors as detection elements

The combination of flow-injection analysis (FIA) and chemically modified ion-sensitive field-effect transistors (CHEMFETs) is described. In a wall-jet cell, two identical potassium-selective CHEMFETs were used for a differential measurement using a platinum (pseudo-)reference electrode. Silicone-rubber membrane materials, chemically bound to the SiO2 gate oxide, were used with valinomycin as the ionophore. The optimized FIA system showed a linear response of 56 mV per decade for potassium concentrations above 5 × 10−5 M. Preliminary results of potassium determinations in human serum and urine samples are presented.

Molecular materials for the transduction of chemical information by CHEMFETs

This paper describes a strategy to overcome the problems associated with the use of MEMFETs as selective chemical sensors. The problems arising from the thermodynamically ill-defined insulator-membrane interface have been eliminated by a new sensor design, viz., the introduction of an intermediate hydrogel layer. Experiments to assess the beneficial effect of such a hydrogel are described. Furthermore, new membrane materials have been designed, synthesized, and characterized. Hence, photopolymerizable polysiloxanes are used which allow both the covalent attachment of electroactive components and the covalent bonding of the sensing membrane to the hydrogel. It is shown that polysiloxanes are versatile materials for the sensing membrane of an ISFET-based chemical sensor. The described technology results in a robust and stable sensor.

Thermodynamic stabilities of complexes of synthetic macrocyclic valinomycin mimics with alkali, barium, and guanidinium cations

Association constants of the 1:1 complexes of a series of pyrido crown ethers 1 (n=0–6) and a number of large ring (27–33 ring atoms) benzo and dibenzo crown ethers 2,3 with alkali, barium and guanidinium chlorides have been determined potentiometrically in MeOH. For most of the ions besides a global maximum for 18-membered rings, a local maximum for the complexation with 30-membered rings is observed. The appearance of local maxima is attributed to the tennisball-seam like conformations that large macrocycles can adopt. The larger crown ethers show a remarkbly high K+ /Na+ selectivity similar to the antibiotic valinomycin, a structurally related 36-membered macrocycle. Therefore, these large ring crown ethers can be regarded as synthetically easy to modify valinomycin mimics. The effects of the solvation of the hosts, guests and complexes on the overall (co-)complexation is discussed. Comparison of data from extraction experiments with association constants clearly shows that differences in the distribution of the complexes between the aqueous and organic phases may be a dominating factor for the selective extraction of cations.