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

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 H-1 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 (K-a) and lowest detection limits for HSOz were determined to be 5.437 x 10(4) 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. (C) 2019 Elsevier Ltd. All rights reserved.