Water soluble and efficient amino acid Schiff base receptor for reversible fluorescence turn-on detection of Zn2+ ions: Quantum chemical calculations and detection of bacteria

• An amino acid Schiff base receptor has been synthesized and characterized.
• The receptor can sense Zn2+ in aqueous media selectively at a 1.04 × 10− 8 M level.
• The receptor exhibits 40 fold fluorescence enhancement with Zn2+ ions.
• The binding mechanism of the receptor and Zn2+ was studied by DFT calculations.
• The receptor shows excellent microbial sensing for Escherichia coli and Staphylococcus aureus.

An amino acid Schiff base (R) capable of recognizing Zn2+ ions selectively and sensitively in an aqueous medium was prepared and characterized. Upon addition of Zn2+ ions, the receptor exhibits fluorescence intensity enhancements (~ 40 fold) at 460 nm (quantum yield, Φ = 0.05 for R and Φ = 0.18 for R–Zn2+) and can be detected by naked eye under UV light. The receptor can recognize the Zn2+ (1.04 × 10− 8 M) selectively for other metal ions in the pH range of 7.5–11. The Zn2+ chelation with R decreases the loss of energy through non-radiative transition and leads to fluorescence enhancement. The binding mode of the receptor with Zn2+ was investigated by 1H NMR titration and further validated by ESI-MS. The elemental color mapping and SEM/EDS analysis were also used to study the binding of R with Zn2+. Density functional theory calculations were carried out to understand the binding mechanism. The receptor was applied as a microbial sensor for Escherichia coli and Staphylococcus aureus.

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