Water-stable Ni(ii)-MOFs as ratiometric fluorescent probes for the detection of perfluorocarboxylic acids

Abstract

Ratiometric fluorescent probes can be used as efficient and stable means of detection, but they usually require rare earth metals. Thus, it is necessary to develop proportional fluorescent probes with earth-abundant metals rather than rare earth ones. In this work, we utilise an abundant metal, Ni(II), as metal nodes linked by 1,1,2,2-tetrakis(4-(imidazole-1-yl)phenyl)ethene (Tipe) and dicarboxylic acid of 4-[(E)-2-(4-carboxyphenyl)ethenyl]benzoic acid (H₂BA), 4-[4-(4-carboxyphenyl)phenyl]benzoic acid (H₂TDA) to construct metal–organic frameworks (MOFs) with the formulae [Ni₂(BA)₂(Tipe)]·4EtOH·8H₂O (MOF1) and [Ni(TDA)(Tipe)]·6H₂O (MOF2). Structural analyses show that MOF1 and MOF2 belong to 4-fold interpenetrated mog and 2-fold interpenetrated fsc topologies, respectively. Ratiometric fluorescence responses of Ni-MOFs toward various per-/polyfluoroalkyl substances (PFASs) in aqueous media were investigated by fluorescence sensing assays. The sensing performance of MOF1 and MOF2 toward perfluorooctanoic acid (PFOA) and trifluoroacetic acid (TFA) revealed that Ni-MOFs have low detection limits and high immunity to interference as well as excellent recycling stability. Mechanism investigations show that MOF1's response is mainly due to PFOA affecting the coordination environment of Ni(II), whereas that of MOF2 is caused by interactions between the TFA and the framework resulting in a proportional fluorescence response to the analytes.