Ion-Selective Microchemical Sensors with Reduced Preconditioning Time. Membrane Biostability Studies and Applications in Blood Analysis

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Spray-coated all-solid-state potentiometric sensors

A novel fully spray coating-based method for the preparation of all-solid-state ion-selective electrodes for simplified construction is proposed.

Performance evaluation criteria for preparation and measurement of macro- and microfabricated ion-selective electrodes (IUPAC Technical Report)

Over the last 30 years, IUPAC published several documents with the goal of achieving standardized nomenclature and methodology for potentiometric ion-selective electrodes (ISEs). The ISE vocabulary was formulated, measurement protocols were suggested, and the selectivity coefficients were compiled. However, in light of new discoveries and experimental possibilities in the field of ISEs, some of the IUPAC recommendations have become outdated. The goal of this technical report is to direct attention to ISE practices and the striking need for updated or refined IUPAC recommendations which are consistent with the state of the art of using macro- and microfabricated planar microelectrodes. Some of these ISE practices have never been addressed by IUPAC but have gained importance with the technological and theoretical developments of recent years. In spite of its recognized importance, a generally acceptable revision of the current IUPAC recommendations is far beyond the scope of this work.

A planar pCO2 sensor with enhanced electrochemical properties

To develop planar microchemical pCO2 sensing devices with improved electrochemical properties, we combined two advanced technologies. One is a differential sensor arrangement to simplify the microfabrication procedure by employing pH-sensitive gas-permeable membranes, and the other is the use of an enzyme (carbonic anhydrase) to shorten total measurement time by accelerating the rate of CO2 hydration. The adhesion of the polyurethane-matrix gas-permeable membrane is enhanced significantly by incorporating a silanizing reagent (silicon tetrachloride), improving the stability and extending sensor lifetime. The proposed differential pCO2 microelectrodes exhibited significantly improved performance in their preconditioning period, response and recovery times, stability, response slope, and lifetime.

Swelling and delamination of multi-electrode sensor arrays studied by variable-pressure scanning electron microscopy

Multi-electrode sensor arrays are made of soft and wet materials not easily examined by most microscopic techniques. In this paper, we have demonstrated that low-vacuum scanning electron microscopy (LVSEM) and energy-dispersive X-ray analysis (EDX) are adequate for studying the hydration, swelling, and possible delamination of multi-electrode sensor arrays. We found that the LVSEM environment had no detectable effect on the morphology of Na(+), K(+), and Ca(++) sensors, and EDX analysis indicated that all three membranes have similar compositions. However, once hydrated, the sensors exhibited different behaviors. The K(+) and Ca(++) sensors swelled more than the Na(+) sensor did. This swelling is due principally to water sorption in the membrane. We believe that the larger thickness of the K(+) and Ca(++) membrane is partly responsible for the observed swelling effect. A simple Griffith analysis of the interface rupture confirms the experimental evidence that these thicker membranes also are more prone to delamination failure.

Photosensitive polyurethanes applied to the development of CHEMFET and ENFET devices for biomedical sensing

Chemical microsensors based on ion-selective field effect transistor (ISFET) transducers with ion-selective and enzymatic membranes have been fabricated. In this case, photolithographically patterned membranes based on acrylated urethanes have been developed and applied onto the gate area of ISFET chips. Aliphatic urethane diacrylate has been used for K+ and NH+4 membranes, while a photocurable hydrogel formulation based on other type of acrylated urethane has been optimized for urea-FET sensors. Resulting potassium and ammonium sensors show similar performances to those found when PVC membranes are employed. An integrated packaging process for ISFET-based sensors has been developed giving the possibility of carrying out most of the encapsulation on wafer level. For this purpose, a photocurable polyurethane encapsulant formulation has been optimized to be microstructured by photolithography. Finally, a preliminary study of biocompatibility of photosensitive formulations containing urethane oligomers has been performed in order to examine future applications in biomedical and clinical analysis.

Effect of surface-attached heparin on the response of potassium-selective electrodes

Heparin (or hydrolyzed heparin) was covalently attached on the surface of derivatized cellulose triacetate membranes, which were subsequently impregnated with the potassium-selective ionophore valinomycin. The resulting ion-selective electrodes presented near-Nernstian response to potassium and had selectivity coefficients of the same order of magnitude as those of conventional poly-(vinyl chloride)-based electrodes. It was found that the heparin layer does not alter significantly the response characteristics of the electrodes. The biological activity of the immobilized heparin (or hydrolyzed heparin) was measured in terms of its inactivation of blood coagulation factor Xa. It was found that the covalently anchored hydrolyzed heparin was not biologically active, but the immobilized heparin was able to inactivate factor Xa. Therefore, by covalently attaching heparin on the surface of ion-selective electrodes, electrodes with improved blood compatibility characteristics may be prepared.