A Zn-MOF functionalized with alkyne groups: ultrasensitive detection of Cu+ and Pd2+ ions in aqueous medium

Abstract

An alkyne-functionalized Zn(II)-based luminescent metal–organic framework (MOF) 1 has been synthesized and structurally characterized. MOF 1 functions as an ultrasensitive and ultrafast chemosensor for the ‘turn-off’ detection of Cu⁺ and Pd²⁺ ions in an aqueous medium. MOF 1 exhibits not only an extremely low detection limit of 53 and 71 nM for Cu⁺ and Pd²⁺ ions, respectively, but also an ultrafast response time of ca. 10 s. The emission quenching in 1 is attributed to FRET due to the interaction of alkyne–π bonds with the d-orbitals of soft Cu⁺ and Pd²⁺ ions. Such a π–d interaction was confirmed by the solid-state NMR and FTIR spectral studies, XPS studies, zeta potential measurements and EDX analyses. To further validate the role of the alkyne group in controlling the said interaction in MOF 1, its structural analogue MOF 2 but carrying a saturated alkyl group was synthesized and structurally characterized. MOF 2 failed to recognize both Cu⁺ and Pd²⁺ ions, thus confirming a critical role of the alkyne group in participating in the interaction with the Cu⁺ or Pd²⁺ ions via the propargyl group of MOF 1 which resulted in fluorescence quenching via a dynamic quenching pathway. MOF 1 selectively detects both Cu⁺ and Pd²⁺ ions in different water samples as well as in the presence of other interfering metal ions, while MOF-based paper strips are used for real-life monitoring.