Detection of NaCN in aqueous media using a calixarene-based fluoroionophore containing ruthenium(ii)-bipyridine as the fluorogenic unit

Abstract

A new molecular sensor containing calixarene and ruthenium(II)-bipyridine as the fluorophore bridged by an amide moiety has been synthesised and characterized, and its anion binding properties have been investigated. It selectively detected cyanide in 95 : 5 water : acetonitrile when sodium salts of various anions such as F⁻, Cl⁻, Br⁻, I⁻, PO₄²⁻, IO₄⁻, BO₃⁻, CH₃COO⁻, CN⁻ and SO₄²⁻ were investigated. The recognition event was monitored by fluorescence spectroscopy and the lower detection limit found was 70 ppb. However, when tetrabutylammonium salts of the same anions were used then, in addition to CN⁻, CH₃COO⁻ was also detected under similar experimental conditions. Interestingly, CN⁻ exhibited substantial quenching, whereas CH₃COO⁻ showed an enhancement in emission intensity. The interaction of anions with the fluoroionophore was also monitored by electrochemical technique and the result obtained is consistent to that found by fluorogenic method. Binding constants were determined from emission titration, composition of the anion-complexes formed were determined from mass and emission titration data, mechanistic aspects of the interaction have been discussed with the aid of NMR data and the role of cations in the contrast fluorescent off and on behaviour has also been discussed. This sensor has also been used to estimate cyanide in real samples and the result obtained is satisfactory.