Dual-emitting fluorescence of Eu/Zr-MOF for ratiometric sensing formaldehyde

A dual-emitting luminescent lanthanide/transition heterometal-organic frameworks of Eu/Zr-MOF was synthesized by incorporation of Eu3+ ions into NH2-UiO-66 under microwave irradiation condition. The multiband fluorescence derived from the characteristic emission of Eu3+ and linker-to-cluster (Eu- oxo or Zr- oxo) charge transfer (LCCT) transition was fabricated. By combination of the luminescent property with the intrinsic porosity and open sites of amine group to bind target analytes, the Eu/Zr-MOF exhibited small molecules-dependent luminescence enhancement and quench effects. Notably, a drastic enhancement of fluorescent at 465 nm induced by formaldehyde was observed. Thus, a ratiometric fluorescent sensing for formaldehyde was performed based on the intensity ratio of two emission bands at 465 and 615 nm for Eu/Zr-MOF. Under the excitation of 365 nm, the increase in intensity ratio of the two emission bands was nearly linearly proportional to the amount of formaldehyde. By this Eu/Zr-MOF sensor, the detection limit of formaldehyde was 0.2 mg/L. This sensing mechanism was ascribed to the binding interaction of free amino groups in Eu/Zr-MOF with the guest. An added electron transfer from amino group containing lone pair electrons to the positively charged formaldehyde leads to a drastic enhancement of luminescence at about 465 nm, while the characteristic emission of Eu3+ at 615 nm enhances slightly. These studies demonstrate that the strategy of multiband emissive heterometal-MOFs can be served as a facile method to fabricate sensitive and specific fluorescent probes of polluting organic small molecules.