Study on a highly selective fluorescent chemosensor for Fe3+ based on 1,3,4-oxadiazole and phosphonic acid

• New fluorescent sensor 2 based on oxadiazole and phosphonic acid was designed.
• The novel sensor 2 was shown to selectively recognize Fe3+ cations.
• Competition experiments indicate the sensor 2 can discriminate between mixtures of metal ions.
• The recognition mechanism of the sensor 2 toward Fe3+ was evaluated by FTIR.

A high selectivity and novel phosphonic acid-functionalized fluorescent sensor 2,5-bis[2-phosphonic acid methyl)phenyl]-1,3,4-oxadiazole (2) for Fe3+ ions has been designed and synthesized. It can display on–off type fluorescence change with high selectivity and sensitivity toward Fe3+ in DMF–H2O solution. The recognition mechanism of the sensor toward Fe3+ was evaluated by FTIR through the disappearance of the characteristic sorption peak of PO, POH and the form of new broad band (POFe stretching bands) in FTIR spectra. It was verified that the Fe3+ ion caused the quenching of sensor 2 fluorescence emission, due to the Fe3+ ion binding to sensor 2 via the O-atoms of the phosphonic group. A linear relationship that flowed with the Perrin model of static quenching was observed between the relative fluorescence intensity (lnF0/F) of sensor 2. The detection limit of the sensor 2 toward Fe3+ was 1 × 10−8 M, while other ions including Hg2+, Ni+, Pb2+, Ca2+, Mg2+, Cd2+, Cu2+, Co2+, Zn2+, Na+, K+, Mn2+, Ag+, Cr3+ and Fe2+ had no influence on the probing behavior. Stern–Volmer analysis showed the binding stoichiometry to be 1:1 (host–guest) and a binding constant of 1.6 × 104 M−1, calculated using the Benesi–Hilderbrand equation.

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