One-component room temperature vulcanizing-type silicone rubber-based calcium-selective electrodes.

New calcium-selective membranes for all-solid-state ion sensors are developed with a highly adhesive one-component room temperature vulcanizing-type silicone rubber (RTV-1-type SR) matrix. The membranes are formulated with 21.6 wt % bis(2-ethylhexyl) adipate, 0.3 wt % tetradodecylammonium tetrakis(p-chlorophenyl)borate (ETH 500), 0.1 wt % potassium tetrakis(p-chlorophenyl)borate, and 0.8 wt % calcium-selective neutral carrier ETH 129 or ETH 1001. Plasticizer added to the RTV-1-type SR matrix not only decreases the bulk membrane resistance but also increases the solubility of electroactive components incorporated in the membrane without significantly deteriorating its adhesive strength. It is found that the lipophilic salt ETH 500 remarkably enhances the calcium selectivity of ETH 129 or ETH 1001 ligands in the SR matrix; the selectivity coefficients ([Formula: see text] by separate solution method, where j = Li(+), Na(+), K(+), or Mg(2+)) for the optimized membranes were below 10(-5). Potentiometric characteristics of planar-type Ag electrodes coated with optimized RTV-1-type SR membranes, e.g., response slope 29.0 ± 0.5 mV/decade, detection limit below 5.0 × 10(-7) M a(Ca)((2+)), and 2-3 mV of potential drift per day, were virtually the same as those of the corresponding poly(vinyl chloride) membrane-based conventional electrodes, but with greatly enhanced sensor-to-sensor reproducibility and lifetime (3-9 weeks).