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 (Ka, 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 (Ka 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.
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