ELSEVIER Analytica Chimica Acta 299 (1994) 179-187 ANALmcA CHIMICA ACTA Flow-through type calcium ion selective optodes based on novel neutral ionophores and a lipophilic anionic dye Hideaki Hisamoto a, Kazuhiko Watanabe a, Eriko Nakagawa a, Dwi Siswanta a, Yushi Shichi b, Koji Suzuki a~* a Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan b Surface Analysis Section, Research Department, Nissan ARK, Ltd., Natsushima-cho, Yokosuka 237, Japan Received 17 May 1994; revised manuscript received 12 July 1994 zyxwvutsrqponmlkjihgfedcbaZYXW Abstract 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 Cazf 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 lo-’ M to 10-l M with 0.05 M Tris-HCl of pH 7.0 as the flowing buffer solution. This optode showed excellent Ca2+ selectivity of over 10P4 in selectivity coefficient values (k?‘; i = primary ion ( Ca’ l ) ; j = interfering ion) against Nat , K* and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ Mg’ + 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. Keywords: Ion-selective electrodes; Calcium; Neutral ionophores; Lipophilic anionic dye 1. Introduction Accurate determination of calcium ion without the interference of other ions (Na+, K+, Mg2+, etc.) is important in the biomedical and clinical fields, because calcium ion has physiological action useful for medical diagnostics [ 1,2]. At present, in order to determine calcium ion in human blood or serum, ion-selective electrodes (ISEs) have been most commonly used [3-141. * Corresponding author. 0003-2670/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO3-2670(94)00351-3 Recently, we reported a highly selective calcium ion electrode based on the neutral ionophore of a double- armed macrocyclic compound. This ionophore is a dia- zacrown ether derivative having two diglycolic amide side chains that form a 1:l complex with Ca*+ [ 151. On the other hand, we prepared a sensitive chromo- phore (color changeable anionic dyes of diphenyl amine type: L,AD and W-3, Fig. 1) and applied them to the Li+- and Kf -selective optodes [ 16,171. These dyes give a reversible absorbance change in the visible region by the deprotonation-protonation at the second- ary amine group of the dye molecule in the neutral pH region.