Screening and application of boron difluoride complexes of curcumin as colorimetric and ratiometric fluorescent probes for bisulfite

Abstract

Bearing a unique D–π–A–π–D structure, curcumin (Cur) and its difluoroboron complex can be used as fluorophores to construct fluorescent probes with β-diketone or phenolic hydroxyl groups as the recognition sites. With α,β-unsaturated ketone as the potential Michael acceptor, Cur and its four derivatives were screened for their response to cyanide, hydrogen sulphide, hypochlorite and bisulfite in aqueous solutions. It was found that only difluoroboron curcumin (Cur-BF₂) and difluoroboron dimethylated curcumin (Cur-Me-BF₂) responded to bisulfite selectively along with color changes from blue or pink to colorless. Good linear relationships were observed between the fluorescence intensity ratio I₄₈₅/I₇₅₀ or I₄₉₀/I₆₁₀ and the concentration of bisulfite; then, the detection limits were determined to be 5.22 ppb and 12.48 ppb for Cur-BF₂ and Cur-Me-BF₂, respectively. The sensing mechanism was attributed to the addition of bisulfite to the difluoroboron-complexed α,β-unsaturated ketone to form disulfonic acid, which was confirmed by ¹H NMR spectroscopy and HRMS. All the results suggested that Cur-BF₂ and Cur-Me-BF₂ could be used as colorimetric and ratiometric fluorescent probes for the sensitive, selective and quick-response detection of bisulfite. Moreover, the bioimaging of bisulfite in living H1975 cells with Cur-BF₂ was performed successfully.