Synthesis and characterisation of new fluorescent Na+ and K+ indicators

The synthesis of new fluorescent indicators for Na+, Benz-15-5 or Diaz-15-5, and for K+, Bent-18-6 or Diaz-18-6, respectively, is reported. They consist of a benzocrown or a diazacrown ether linked to a pyrazoline fluorophore substituted with carboxylate groups to ensure water solubility. They were obtained via reaction of an alpha,beta-unsaturated ketone with an aryl hydrazine. The fluorescence intensity of Bent-15-5 is nearly insensitive to Na+. Bent-18-6 displays a four-fold selectivity for K+ over Na+, with a dissociation constant of 3.6 mM for the K+:indicator complex. Diaz-15-5 and Diaz-18-6 have only a weak affinity for monovalent cations, so that they only can be used for the determination of high Na+ and K+ concentrations. (C) 1999 Elsevier Science Ltd. All rights reserved.