Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics

Voltammetric and amperometric transduction for solvent polymeric membrane ion sensors

This paper describes basic response features of solvent polymeric membrane ion sensors with voltammetric and amperometric transduction. The model systems used here contain no ionophore for simplicity reasons. Reasonable simplifications of the theory are introduced that allow one to understand the response mechanism in view of a practical application of these sensors. It is shown that ion-sensing membranes preferentially contain no ion-exchanger properties in order to function optimally in a voltammetric mode. As with the systems studied by Kihara, both liquid-polymer interfaces of the membrane are preferably polarizable. Specifically, they contain the highly lipophilic electrolyte tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) in the membrane to improve lifetime, increase the magnitude of the potential window, and prohibit exchange reactions with sample ions. An ohmic behavior that is associated with an assisted electrolyte-transfer process is observed only above a threshold potential which can be quantitatively predicted by theory. The threshold potential depends on the nature and activity of sample anions and cations in the sample and inner filling solution of the membrane electrode. Within the experimental conditions discussed in this paper, these sensors seem to measure sample ion activities, not concentrations, since the rate-limiting step is the diffusion of extracted ions away from the interface into the membrane bulk. Similarly, no effect of sample stirring on the measured current is observed. This contrasts to work done on liquid-liquid electrolyte-transfer reactions, where large diffusion coefficients in the organic phase often lead to substantial sample depletion effects. The detection of anions and cations with the same membrane is demonstrated in a cyclic voltammetric mode. Direct continuous detection of one type of anion is accomplished by pulsed amperometry to ensure a rapid, repetitive renewal of the membrane composition between measurements.

Improving measurement stability and reproducibility of potentiometric sensors for polyions such as heparin

The growing importance of polymer membrane-based potentiometric polyion sensors in biomedical research and clinical measurements has brought up the question of how accurate and reproducible these sensors are. Indeed, recent research has revealed that these sensors behave quite differently than classical ion-selective electrodes. This paper explores ways to improve measurement reproducibility and long term potential stability by considering the unique pseudo steady-state response mechanism of the polyion sensors developed so far. Heparin may be stripped out of the phase boundary membrane surface with a high sample NaCl concentration and this characteristic is used to modify the calibration procedure in order to avoid memory effects. It is also attempted to reduce long term potential drifts by continuously stripping heparin out of the membrane at the membrane-inner filling solution side.

Carriers and channels: current progress and future prospects

Recent advances in the understanding of biological transport and in the design of artificial transport systems have resulted from the structural elucidation of the K+ ion channel and from synthesis of artificial receptors for cations and anions, as well as neutral and zwitterionic organic molecules. Sensors, gated carriers, and self-assembling capsules and nanotubes are all important offsprings of current efforts to mimic natural transport across biomembranes.

Microtiter plate-format optode

Microtiter plate-format optodes could be assembled by casting bulk-response membranes into the standard 96-well polypropylene-based plate or by screen printing them on an optically transparent substrate with 96-well pattern. The compositions of thick optode membranes, especially the ratios of poly(vinyl chloride) (PVC) to plasticizer [bis-(2-ethylhexyl) sebacate (DOS)], were carefully optimized to provide reproducible and rapid response. Adjusting the ratio of PVC to DOS by 1:6, bulk-response membranes containing neutral carrier (4-tert-butyl calix[4]arene tetraacetic acid tetraethyl ester for sodium-selective membrane or valinomycin for potassium-selective membrane) and lipophilic pH indicator (ETH 5294) could exhibit equilibrium response in 5 min. The practical utility of microtiter plate-format optodes has been examined by determining clinically relevant electrolytes in serum samples. It was demonstrated that microtiter plate-format optodes can provide high sample throughput (approximately 100 samples in less than 5 min), analytical performance comparable to that of a potentiometric clinical analyzer, and additional information on electrolytes using the same samples prepared for other colorimetric measurements.