Development of a cell permeable ratiometric chemosensor and biomarker for hydrogen sulphate ions in aqueous solution

Abstract

A newly designed organic moiety, 5H-5,7a,12-triaza-dibenzo[a,e]azulen-6-one (L) containing a seven membered ring behaves as a hydrogen sulphate ion selective ratiometric chemosensor. The formulation and detailed structural characterisation of L have been established using physico-chemical, spectroscopic tools and single crystal X-ray diffraction study. On additions of hydrogen sulphate ions to the solution of L in HEPES buffer (1 mM; water : ethanol (v/v), 98 : 2) at 25 °C at biological pH, a new fluorescence peak generated at 483 nm was increased with concomitant decrease of the weak fluorescence of L at 430 nm through an isoemissive point at 449 nm due to the selective binding of HSO₄⁻ ions with L in a 1 : 1 ratio with a binding constant (K) of 4.13 × 10⁶ M⁻¹, and detects HSO₄⁻ ions as low as 5.5 × 10⁻⁷ M. The ratiometric enhancement of the fluorescence is based on intermolecular hydrogen bonding assisted chelation enhanced fluorescence (CHEF) process which has been evidenced by ¹HNMR titration and supported by theoretical (DFT) calculations. The probe (L) having no cytotoxic effect is also useful for the detection of intracellular HSO₄⁻ ion concentrations under a fluorescence microscope.