Flow-through type calcium ion selective optodes based on novel neutral ionophores and a lipophilic anionic dye

Flow-through type calcium ion selective optodes based on novel neutral ionophores and a lipophilic anionic dye were developed. The novel calcium ion selective neutral ionophores designed and synthesized were double-armed diazacrown ether compounds possessing glycolic amides as the binding arms. Cation sensing material was prepared by coating pellicular-type ODS (octadecylsilica) beads with a lipophilic organic liquid which incorporates a novel neutral calcium ionophore and a lipophilic anionic dye of the diphenyl amine type. These beads were packed in a flow-through optical cell, and the Ca2+ measurement was performed to measure the absorbance change of the sensing beads at 516 nm. The best optode tested exhibited a good response to calcium ions in the concentration range from 10−7 M to 10−1 M with 0.05 M Tris-HCl of pH 7.0 as the flowing buffer solution. This optode showed excellent Ca2+ selectivity of over 10−4 in selectivity coefficient values (ki,jopt.; i = primary ion (Ca2+); j = interfering ion) against Na+, K+ and Mg2+ which are primary interferents for human serum analysis. Sensor response theory, response time, selectivity, and application to the determination of calcium ion concentration in human serum were also discussed together with experimental results.