Microfabricated Potentiometric Electrodes and Their In Vivo Applications.

Abstract

New technologies are yielding mass-produced, miniature, ion-selective electrodes that can be routinely used in biology E = (ε 1 + ε 2 + ε 3 + ε 4 + ε 5 ) + ε j + (ε´+ ε´´) = E o + ε j + ε M (1) in which E o is a constant potential term comprising the potential contributions ε 1 to ε 5 , which arise within the system from the two reference electrodes in the cell; ε j is the liquid junction potential; and ε M is the membrane potential. We usually seek electrolyte combinations for the reference electrode in which ε j is nearly zero. The essential part of an ISE is the ion-sensitive membrane that is commonly placed between two aqueous phases-for example, the sample and the inner electrolyte solution. Ion-selective membranes made from glass, single crystals (e.g., LaF 3 ), pressed pellets of insoluble precipitates (e.g., AgCl, AgBr, AgI), or solvent polymeric membranes (highly plasticized polymeric films, also known as liquid membranes) are widely used in analytical practice. In this article, we will focus our discussions on microfabricated ISEs based on solvent polymeric or liquid membranes.Typically, the membrane potential is divided into three separate potential contributions-the phase boundary potentials at both interfaces (ε´and ε´´) and the diffusion potential within the ion-selective membrane (3-5). However, the membrane's internal diffusion potential is zero if there are no concentration gradients within the membrane. The basic equation for the interfacial potentials εá nd ε´´is in which R is the gas constant, T is the absolute temperature, F is the Faraday constant, z i is the charge number of the primary ion i, k i is the single-ion partition coefficient, and a i and ā i are the primary ion solution and membrane activities, respectively. The phase boundary potential is a simple function of the sample ion activities only when ā i is not influenced significantly by the sample. Accordingly, for an ideally selective membrane, the membrane potential is directly related to the respective activities in the contacting solutions in which a i (1) and a i (2) are the ion activities in the sample and the inner filling solution, respectively. A Nernstian response is expected in a potentiometric cell when the liquid junction potential can be neglected and the inner filling solution of the electrode is kept constant:Most of the microfabricated ion sensors are based on solvent polymeric or liquid membranes. Overplasticized poly(vinyl chloride) (PVC) is most frequently used as the membrane matrix. The selectivity of these membranes is determined by the dielectric properties of the plasticizer and the selective, hydrophobic complexing agents, which are neutral or charged carriers (ionophores), within the membrane phase. A comprehensive review of the carrier-based ISEs and bulk optodes was published recently (6, 7).Calibrating ISEs by serial dilution gives a plot of the cell voltage as a function of the logarithm of the primary ion activity, as shown in Figure 2. It is linear over a wide concentration range (generally from...

Beyond potentiometry: Robust electrochemical ion sensor concepts in view of remote chemical sensing

“…Very similar problems are have also been major stumbling blocks in the realization of implantable sensing devices that can be left inside the body for prolonged periods of times, even after decades of active research in the field [16]. Perhaps the idea of using direct potentiometry as the readout principle for sensor devices that cannot be recalibrated has been flawed all along.…”

“…This design is most likely unacceptable for remote sensing applications, and direct potentiometric sensing will only be possible with pseudo-reference electrodes, such as a second ion-selective electrode selective for a background ion whose concentration remains sufficiently constant in the course of the measurement. Overall, current technology appears to be ill suited for prolonged use of miniature sensing systems that are supposed to give reliable measuring results over prolonged periods of time on the order of days or even months.Very similar problems are have also been major stumbling blocks in the realization of implantable sensing devices that can be left inside the body for prolonged periods of times, even after decades of active research in the field [16]. Perhaps the idea of using direct potentiometry as the readout principle for sensor devices that cannot be recalibrated has been flawed all along.…”

Electroanalytical Flow Measurements–Recent Advances

“…In case of miniaturized potentiometric detectors they have been mostly fabricated in recent years by the use of microelectronic technologies, characterized as thin-and thickfilm processes which allow their mass production [121]. Such electrodes, especially solid-contact devices, do not exhibit principally different properties than their respective microprobes, but they can be successfully employed in micro flow systems, specially in clinical applications.…”

Needle-Type Ultra Micro Silver/Silver Chloride Reference Electrode for Use in Micro-Electrochemistry

“…Therefore, many types of micro-indicator electrodes have been actively investigated. [1][2][3][4][5][6][7] In order to measure the accurate electrode potential of these micro indicator electrodes, we need the development of a reliable miniaturized reference electrode. However, no practical miniaturized reference electrode has yet been reported.…”