Double-armed crown ethers for calcium optical sensors

This paper presents the characterization of optical sensing membranes for calcium based on ionophore–chromoionophore chemistry. Six different ionophores, 18-membered crown ether derivatives, were studied, coming from 18-crown-6 ether and 4,13-diaza-18-crown-6 ether to a series of double-armed crown ethers with different type of terminal groups. The study of optical membranes containing the same transducer and plasticizer allow drawing some conclusions on the influence of lipophilicity and size of the terminal group of the side chain on calcium selectivity. We have calculated the exchange constant KexchILp for each equilibrium with alkaline and alkaline-earth ions and the selectivity coefficient kI,JOsel for each ion against calcium as a way for a full characterization of sensing membranes. In all cases the ion:ionophore stability constants for calcium were the highest and the ionophore V containing an (N-adamantylcarbamoyl) acetyl moiety originated the most selective membrane for calcium. Analytical parameters for calcium determination using prepared membranes were calculated.