A simple and robust PET-based anthracene-appended O-N-O chelate for sequential recognition of Fe³⁺/CN^– ions in aqueous media and its multimodal applications

A very simple, robust, and pico/nanomolar-sensitive 9,10-diethanolamine-substituted fluorescent Fe³⁺/CN^– probe (PD) was synthesised, and its sensing abilities towards various ions were studied in mixed aqueous media. PD selectively recognised Fe³⁺ ions through a ‘turn on’ response with an excellent binding constant (K_a, 9.29 × 10⁶ M⁻¹) in 1:2 binding stoichiometry at pH 7.0 in phosphate-buffered saline (PBS). The in situ generated Fe³⁺·PD ensemble sequentially recognised CN^– ions with an excellent binding constant (K_a 1.72 × 10⁸ M⁻¹) via a ‘turn off’ mode by extruding Fe³⁺ ions from the ensemble. The highly selective sequential ‘on-off’ responses towards Fe³⁺ and CN^– ions were attributed to inhibition and restoration of photoinduced electron transfer (PET) and chelation-induced enhanced fluorescence (CHEF) effects from the chelating N and O heteroatoms. PD was able to detect Fe³⁺ and CN^– ions in real water samples satisfactorily at picomolar to sub-nanomolar levels. A colorimetric assay based on pyrocatechol violet (PCV) was also able to detect Fe³⁺/CN^– in a sequential manner (up to sub-micromolar level) by a change in colour from colourless to yellow/pale green without any interferences from other ions. Based on the complexation and decomplexation mechanism, bio-imaging photonic INHIBIT logic circuit strips were prepared for use under physiological conditions. In addition, solid-phase recognition of Fe³⁺/CN^– ions was demonstrated using cost-effective paper-based strips.