A coumarin–hemicyanine hybrid as a ratiometric fluorescent sensor of microenvironment proticity

Abstract

The coumarin–hemicyanine hybrid is an internal charge transfer (ICT) dye that features a donor-π-acceptor motif and displays two well-resolved absorption bands at ∼350 nm for the donor coumarin and ∼550 nm for the fully-conjugated dye. Herein, we demonstrate that preferential excitation of the dye at 350 nm, ascribed to a twisted conformation, gives rise to two well-resolved emission bands at ∼450 nm for the coumarin and ∼650 nm for the planar dye that are highly quenched in water. Unexpectedly, the coumarin blue emission was found to ‘turn-on’ to aprotic and viscous solvents and respond to binding bovine serum albumin (BSA) in aqueous media. In contrast, the red ICT emission was sensitive to protic solvent polarity and responded to binding the G-quadruplex (GQ) DNA structure produced by the heme binding aptamer (PS2.M). Our studies also demonstrate the influence of the alkyl group attached to the dye quaternary nitrogen atom in determining the microenvironment sensitivity and ability to differentiate biopolymer types.