title: Ionizable (Thia)calix[4]crowns as highly selective 226Ra2+ ionophores.
abstract: The 226Ra2+ selectivity of the ionizable (thia)calix[4]crowns 1-4 was determined in the presence of a large excess of the most common alkali and alkaline earth cations. Selective 226Ra2+ (2.9 x 10(-)(8) M) extraction occurs even at extremely high M(n+)/226Ra2+ ratios of 3.5 x 10(7) [M(n+) = Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Sr2+ (1M)] and an ionophore concentration of 10(-4) M. The selectivity coefficients log(K(Ra)(ex)/K(M)(ex)) are approximately 3.5 for Mg2+, Ca2+, and Sr2+. In the presence of Ba2+, which has very similar chemical properties, only the thiacalix[4]crown-6 derivative 4 showed a selectivity for 226Ra2+. In addition to the remarkable 226Ra2+ selectivities, the effective pH range (pH 8-13) of the thiacalix[4]crown dicarboxylic acids (3 and 4) allows for full regeneration of the ionophores at lower pH values (pH <6).
authors: Laboratory of Supramolecular Chemistry and Technology, MESA Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
authors: Fijs W B
authors: FW
authors: van Leeuwen
authors: Hans
authors: H
authors: Beijleveld
authors: Cornelis J H
authors: CJ
authors: Miermans
authors: Jurriaan
authors: J
authors: Huskens
authors: Willem
authors: W
authors: Verboom
authors: David N
authors: DN
authors: Reinhoudt