A dipodal thiourea-ionic liquid conjugate system for selective ratiometric detection of HSO4 ion in purely aqueous medium: application to real sample analysis

Abstract

A benzimidazolium based thiourea conjugate (IL) receptor has been designed, synthesized and characterized spectroscopically. The prepared receptor (IL) shows the sensitive and selective ratiometric sensing for HSO4 over the other anions as evident by UV–visible absorption and fluorescence spectroscopy. With the addition of HSO4 ion, the parent absorption band of IL in UV–visible absorption at 311 nm was shifted to 261 nm having an isosbestic point at 294 nm, which clearly indicates an interaction between HSO4 ion and the IL. Further, upon excitation at 310 nm, the fluorescence emission of IL at 455 nm was observed. Furthermore, the gradual addition of HSO4 ion results in a drastic decrease in emission at 455 nm and simultaneous appearance of a new emission band at 379 nm with isosbestic point at 430 nm was observed. The binding mechanism of HSO4 with IL was also explored with 1H NMR titration, mass spectrometry and DFT calculations. These studies revealed the involvement of hydrogen bonding with –NH (thiourea) and –CH (benzimidazolium) functionalities towards the recognition of HSO4 ion. The association constant (Ka) and lowest detection limits for HSO4 were determined to be 5.437 104 M 1 and 5.0 nM, respectively. The real sample analysis by synthesized sensor probe for HSO4 was also performed which shows the practical applicability of the developed sensor system.