Robert W. Hower

Enhanced electrochemical performance of solid-state ion sensors based on silicone rubber membranes

Results of optimization studies regarding the composition of silicone rubber films suitable for preparing miniaturized solid-state ion-selective sensors are presented. Using conventional macro ion-selective electrodes (ISEs) it was shown that the addition of more lipophilic ionic additives and small amounts of an appropriate plasticizer to the silicone matrix achieved optimal electroanalytical performance. The incorporation of these components into the membrane along with the required ionophore can, however, influence membrane curing time and adhesion of the film to silicon nitride surfaces. Solid-state sensors with optimized silicone membrane compositions exhibit theoretical and reproducible electrochemical performance.

Asymmetric membrane-based potentiometric solid-state ion sensors

Asymmetric ion-selective membranes are shown to be advantageous for the fabrication of solid-state ion sensors. These membranes have a hydrophilic, porous layer on top of the sensing surface of the ion-selective layer. Compared to conventional polymeric membranes, the asymmetric membranes show decreased interference from large, hydrophobic anions because of kinetic and exclusion mechanisms. Cellulose acetate-matrix asymmetric membranes are formed by hydrolyzing directly the surface of homogeneous, plasticized cellulose triacetate ion-selective membranes. The polyurethane-based asymmetric membrane system consists of a very thin hydrophilic polyurethane membrane coated on an underlying more hydrophobic plasticized polyurethane membrane containing the appropriate ion carrier. Asymmetric membrane-based carbonate and chloride ion sensors exhibit a remarkably reduced response to salicylate when compared to the PVC, unmodified cellulose triacetate, or polyurethane membrane-based ion sensors. The polyurethane-based asymmetric membrane system is especially promising for the fabrication of solid-state ion sensors.

Microfabricated sampling probes for in vivo monitoring of neurotransmitters.

Microfabricated fluidic systems have emerged as a powerful approach for chemical analysis. Relatively unexplored is the use of microfabrication to create sampling probes. We have developed a sampling probe microfabricated in Si by bulk micromachining and lithography. The probe is 70 μm wide by 85 μm thick by 11 mm long and incorporates two buried channels that are 20 μm in diameter. The tip of the probe has two 20 μm holes where fluid is ejected or collected for sampling. Utility of the probe was demonstrated by sampling from the brain of live rats. For sampling, artificial cerebral spinal fluid was infused in through one channel at 50 nL/min while sample was withdrawn at the same flow rate from the other channel. Analysis of resulting fractions collected every 20 min from the striatum of rats by liquid chromatography with mass spectrometry demonstrated reliable detection of 17 neurotransmitters and metabolites. The small probe dimensions suggest it is less perturbing to tissue and can be used to sample smaller brain nuclei than larger sampling devices, such as microdialysis probes. This sampling probe may have other applications such as sampling from cells in culture. The use of microfabrication may also enable incorporation of electrodes for electrochemical or electrophysiological recording and other channels that enable more complex sample preparation on the device.

New solvent system for the improved electrochemical performance of screen-printed polyurethane membrane-based solid-state sensors

In our previous work aimed at mass producing chemical sensors, we have used the fabrication techniques of semiconductor processing and screen printing. In that work, we showed that high boiling-point solvents (i.e., low evaporation rates) were needed to increase the membrane paste viscosity and thixotropy [1]. While this approach achieved a paste with good mechanical properties for screen printing, the resulting sensors for some ions exhibited poor electrochemical performance in terms of detection limits and slopes. This paper reports a new technique using a low boiling-point plasticizer, dimethyl phthalate (DMP), to control viscosity in the membrane paste. Use of the plasticizer facilitates screen printing, while maintaining good electrochemical characteristics.

Multiionophore-based solid-state potentiometric ion sensor as a cation detector for ion chromatography

A new solvent/polymeric multiion-selective membrane electrode is described for use as a flow-through detector in ion chromatography. Unlike conventional ion-selective membranes that utilize only one type of ionophore, the multiion-selective membranes incorporate several different ionophores. In this study, a K+iNH,+/Na+/Ca*+ selective membrane has been prepared by incorporating four different neutral-carrier ionophores in a strongly adhesive polyurethane matrix. This membrane is cast on solid electrode surfaces, with no internal electrolyte solution, to form solid-state multiion sensors. The detection characteristics of the muhiionophore membranes, which are controlled by the amounts and ratios of the ionophores they contain, are examined in a flow-injection arrangement. The sensors have been demonstrated in the target application, a flow-through ion chromatography system.

Screen printing: a technology for partitioning integrated microsensor processing

Potentiometric chemical sensors are presented as a case study in the use of screen printing for process partitioning. After CMOS processing, the authors print silver epoxy over the openings in the overglass layer to the aluminum input pads; the silver forms a stable chemical interface to the membranes, and the epoxy forms a strong physical bond to them. The screen-printed silver epoxy allows the use of conventional aluminum metallization in the microelectronic circuits. The polymeric membranes are applied and patterned with screen printing. Membranes of different compositions can be deposited on the various sites of a multisensor chip by simply repositioning the mask and repeating the screen print/cure cycle. There is no cross-contamination of membranes.<<ETX>>

Improved stability of solid-state ion-selective sensors by incorporation of lipophilic silver-calix[4] arene complexers within polymeric films

The EMF stability of potentiometric solid-state ion-selective sensors is found to be enhanced by the incorporation of lipophilic silver-ligand complex of thioether derivatized calix[4]arene [Ag(TDC)+]. The membranes are made by adding silver-ligand complex with excess of free ligand (TDC) to polymer membrane doped with an appropriate ion-selective ionophore and ionic additives. The silver-ligand complex functions as a reversible electron transfer pair at the membrane-solid (Ag/sup 0/-epoxy) interface. The resulting solid-state sensors exhibit improved EMF stability during the first 20 hours of soaking in a solution of analyte ion and also continued to show improved stability over a 30-day period.

Enhanced Electrochemical Performance Of Solid-state Ion Sensors Based On Silicone Rubber Membranes

Results of optimization studies regarding the composition of silicone rubber films suitable for preparing miniaturized solid-state ion-selective sensors are presented. Using conventional macro ion-selective electrodes (ISE) it was shown that more lipophilic ionic additive species and the addition of small amounts of appropriate plasticizer to the silicone matrix is required to achieve optimal electroanalytical performance. The incorporation of these components into the membrane along with the required ionophore can, however, influence membrane curing time and adhesion of the film to silicone nitride surfaces. Solid-state sensors with optimized silicone membrane compositions exhibit theoretical and reproducible electrochemical performance.

New Solvent System For The Improved Electrochemical Performance Of Screen-printed Polyurethane Membrane-based Solid-state Sensors

In previous work to mass produce chemical sensors using screen printing, we showed the need for higher boiling solvents (i.e. lower evaporation rates) to increase the membrane paste viscosity and thixotrophy. While this approach achieved a paste with good mechanical properties for screen printing, the resulting sensors for some ions exhibited poor electrochemical performance in terms of detection limits and slopes. This paper reports a new technique using Dimethyl phthalate, a low boiling placticizer, for membrane formulation that facilitates screen printing achieving appropriate paste viscosity, while maintaining good electrochemical characteristics.

Development of a solid-state potentiometric heparin sensing cartridge based on photocrosslinked decyl methacrylate

A novel solid-state potentiometric sensing cartridge for detecting the polyanionic anticoagulant drug heparin is introduced. The device is based on photocrosslinked decyl methacrylate (DIMA) polyanion-sensitive films covalently immobilized over screen-printed silver epoxy electrodes. The fabrication and optimization of such a sensing arrangement are discussed. In accordance with potentiometric polyion response theory, a sensitive response to heparin can be achieved by decreasing the amounts of ion-exchanger, plasticizer, and crosslinker in the membrane. A novel hydrophilic copolymer can be used as an internal layer to improve EMF reproducibility. Clinical levels of heparin can be detected in as little as 50 /spl mu/L of whole blood.