Ratiometric detection of hydroxy radicals based on functionalized europium(III) coordination polymers

A ratiometric fluorescent probe is described that is based on a functionalized europium(III) coordination polymer (Eu/DPA-TA). It can be fabricated by using dipicolinic acid (DPA) as a bridging ligand (to sensitize the Eu³⁺ fluorescence) and terephthalic acid (TA) acting as a functional ligand to recognize the hydroxy radical (^•OH). The quenching of Eu³⁺ fluorescence (measured at excitation/emission wavelengths of 288/615 nm) by water is strongly reduced in the presence of TA. This leads to a significant enhancement in the emission lifetime and intensity of Eu³⁺. Upon the addition of ^•OH, the fluorescence of Eu/DPA-TA showed ratiometric changes in that the TA sensitized fluorescence (peaking at 445 nm) is switched on, while the plain Eu³⁺ fluorescence (peaking at 615 nm) decreases. This finding led to a method for the ratiometric (2-wavelength) detection of OH˙ with a 0.5 μM detection limit. The method also allows OH˙ to be detected with bare eyes at OH˙ concentrations as low as 10 μM. In our perception, this study paves the way towards the design of new functionalized lanthanide coordination polymers for fluorometric assays and biomedical imaging. Graphical abstract Schematic of a ratiometric fluorescent assay for the detection of hydroxy radical (^•OH) based on the use of a europium(III)/dipicolinic acid coordination polymer (Eu/DPA CP) functionalized with terephthalic acid (TA). A gradual color change from red to blue can be observed and correlated to ^•OH concentrations.