Switching off FRET by analyte-induced decomposition of squaraine energy acceptor: A concept to transform 'turn off' chemodosimeter into ratiometric sensors

FRET cassette-type sensors, SN-1 and SN-2, which contain two naphthalimide donors and a squaraine acceptor, have been developed on the concept of switching off FRET through removing the spectral overlap by the analyte-induced decompositions of acceptor chromophore. SN-2 shows sensitive responses to fluoride anion and cyanide through both naked-eye detectable colour changes and ratiometric fluorescence changes. As the first example, SN-2 confirms that transformation of the common 'turn-off' chemodosimeter into ratiometric sensors can be realized conveniently based on the novel 'switching FRET off' approach.