A chronoamperometric method to estimate changes in the membrane composition of ion-selective membranes.

A new chronoamperometric method is used to estimate changes in the membrane composition of mobile-site, ionophore-based membranes. The characteristic features of the chronoamperometric curves (initial current, slope, break time) of valinomycin-based, potassium-selective membranes loaded with potassium tetrakis(4-chlorophenyl)borate are correlated with the mobile-site and free ionophore concentration in the membrane. limiting cases for strong and negligible ion pair formation are distinguished. Replicate measurements indicate a relative standard deviation in the calculated values less than 10%. The practical applicability of the method was tested with membranes incorporated into conventional ion-selective electrode bodies or cast onto microfabricated planar sensor structures.

Analytical performance characteristics of thin and thick film amperometric microcells.

The analytical performance of amperometric microcells with different electrode geometries is compared for enzyme activity measurements. The microcells were fabricated with thin film photolithography or thick film screen-printing in four different designs. The cells made with the thin film process used flexible substrate with microelectrode array or a circular, disk-shaped working electrode. The screen-printed working electrodes had semicircle or disk shape on ceramic chips. Putrescine oxidase (PUO) activity measurement was used as a model. The determination of PUO activity is important in the clinical diagnosis of premature rupture of the amniotic membrane. An electropolymerized m-phenylenediamine size-exclusion layer was used to eliminate common interferences. The size exclusion layer revealed also to be advantageous in protecting the electrodes from fouling by putrescine (enzyme substrate). The electrode fouling of bare electrodes was insignificant for screen-printed electrodes, but very severe for electroplated platinum working electrodes. The microelectrode array electrodes demonstrated smaller RSD and higher normalized sensitivities for hydrogen peroxide and PUO activity. All the other electrodes were demonstrating comparable analytical performances.

Construction of a very high-density extracellular electrode array.

Cellular activation mapping (specifying in time and space the electrical activation sequence of cells) is a well-established basic research tool in cardiac, neural, and gastric physiology. Much recent research in cardiac mapping has focused on large arrays (>200 electrodes) with small electrodes (<500 microm). Construction of such arrays using standard techniques is tedious and yields irregular electrode spacing. We present a novel construction technique that rapidly produces large arrays with regularly spaced small electrodes. For methods, fine-pitch copper ribbon cables, insulated with either polyvinylchloride (PVC) or polyimide (flexible printed circuit; FPC), were assembled together such that the active surface was the cut end of the cable. The cut end was sanded and polished, then coated with silver and sometimes silver chloride. Once completed, the alternating current (AC) root-mean-square (rms) potential was measured between two adjacent, individual electrodes. Polarization testing was conducted according to a previously reported protocol (Witkowski FX and Penkoske PA. J Electrocardiol 21: 273-282, 1988). Activation mapping was conducted in the open-chest guinea pig with both pacing- and defibrillation- strength stimuli. In terms of results, four PVC and three FPC arrays were constructed, ranging from 4 to 400 electrodes. Two hours of labor were needed to create a complete electrode array, independent of the number of electrodes, including connectors and silver/silver chloride coating. As expected, the addition of a silver/silver chloride coating significantly reduced (0.76-0.42 mV, P < 0.001) the AC rms potential difference between two electrodes. A nearly immediate recovery of the potential difference between adjacent pairs of silver/silver chloride electrodes was observed after defibrillation stimuli.

A chronoamperometric method to estimate ionophore loss from ion-selective electrode membranes.

A novel chronoamperometric method was developed to estimate the concentration of a neutral ionophore in fixed-site, dioctyl sebacate plasticized, poly(vinyl chloride)-based, ion-selective electrode membranes. The membranes contained between 0.5 and 16 mmol/kg valinomycin. The chronoamperometric technique was used to estimate the valinomycin concentration in freshly prepared membranes and after extraction of some of the ionophore from the membranes using dioctyl sebacate. Replicate measurements indicated a relative standard deviation in the calculated valinomycin concentration of less than 10%, and these values accurately represented the true concentration of valinomycin within 10%. The method permitted an estimate of the valinomycin concentration after valinomycin was leached from a membrane. The results of preliminary experiments using heparinized dog blood suggest that blood protein adsorption does not interfere qualitatively or quantitatively with the analysis.