Synthesis of novel water-soluble chitosan-based “off–on” fluorescent probes for successive recognitions of Fe3+ and F− ions

Water-soluble fluorescent probes have been synthesised by chemical labelling of 9-anthraldehyde onto chitosan via Schiff base reaction in the presence of a biocompatible polymer, i.e., poly(ethylene glycol) (PEG). Interestingly, PEG controlled the much needed water-solubility of the probe through hydrogen bonding whereas its proportion was optimized to achieve the required water solubility of the product. The synthesised compound was characterised by various techniques such as Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, fluorescence studies, nuclear magnetic resonance spectroscopy, scanning electron microscopy, thermogravimetric analysis-differential thermogravimetry and time-correlated single-photon counting. The fluorescence signal of the probe was quenched (turn off) upon binding to Fe 3+ ions and was regenerated (turn on) quantitatively due to the addition of F − ions. Further, the photo physical process (off and on) was understood in the light of photo-induced electron transfer and complex formation. Again, the static quenching mechanism was understood by life-time measurement. The detection limits for Fe 3+ and F − ions were found to be (2.98 × 10 –7 M) and (3.12 × 10 –7 M) in the “turn off” and “turn-on” modes, respectively. The synthesized materials were exposed to the simultaneous detection of Fe 3+ and F − ions for the first time. The developed material and detection technique were robust, selective and environmentally benign. Such “off–on” fluorescent probe being water soluble and highly photo-stable showed potential suitability for simultaneous recognition of Fe 3+ and F − ions in the real systems. Graphical abstract