Synthesis, characterization and sol-gel entrapment of a crown ether-styryl fluoroionophore

The synthesis and initial evaluation of a new dye-functionalized crown-ether, 2-[2-(2,3,5,6,8,9,11,12,14,15-decahydro-1,4,7,10,13,16-benzohexaoxacyclooctadecin)ethenyl]-3-methyl benzothiazolium iodide (denoted BSD), are reported. This molecule contains a benzyl 18-crown-6 moiety as the ionophore and a benzothiazolium to spectrally transduce ion binding. Binding of K⁺ to BSD in methanol causes shifts in the both absorbance and fluorescence emission maxima, as well as changes in the molar absorptivity and the emission intensity. Apparent dissociation constants (K_d) in the range 30-65 mM were measured. In water and neutral buffer, K_d values were approximately 1 mM. BSD was entrapped in sol-gel films composed of methyltri-ethoxysilane (MTES) and tetraethylorthosilicate (TEOS) with retention of its spectral properties and minimal leaching. K⁺ binding to BSD in sol-gel films immersed in pH 7.4 buffer causes significant fluorescence quenching, with an apparent response time of approximately 2 min and an apparent K_d of 1.5 mM.