A Ca2+ controlled thioether linked bichromophoric squaraine foldamer for “turn on” fluorescent sensing of oxalate

•A bichromophoric squaraine-based probe for oxalate is reported.•The detection limit for oxalate is 5.2 nM (3σ/k).•The signaling is based on oxalate induced unfolding of Ca2+ controlled foldamer.•The binding mechanism is discussed.

Several thioether linked bichromophoric squaraines 1 were prepared and characterized. Among these, the bichromophore 1c, containing three sulfur atoms, was found to selectively bind with Ca2+ and lead to a folded conformation. The selective binding of Ca2+ resulted in an absorption blue shift and fluorescence quenching due to the cation-induced folding of the bichromophore and the resultant exciton coupling between the squaraine chromophores. Job's plot confirmed a 1:1 binding between 1c and Ca2+. Anions such as oxalate can extract Ca2+ from squaraine foldamers via the formation of insoluble salts and restore the fluorescent signal. This sensing approach can be applied to the “turn on” fluorescent detection of oxalate with a low detection limit of 5.2 nM (3σ/k).

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