Study of the morphological, structural and photophysical properties of dual emission europium-doped ZIF-8 particles

The development of functional materials with synergistic properties, including fluorescent probes based on a metal-organic framework (MOFs) and lanthanide ions with dual emission, has attracted interest from the scientific community. In this regard, a series of ZIF-8 materials containing europium ions (ZIF-8:x% Eu where, x = 1, 3 or 7%) was prepared by a simple and fast method at room temperature (25 °C) and the structural, morphological and photophysical properties were evaluated. The results showed that the insertion of europium ions decreases the degree of crystallinity from 71% for ZIF-8 to 67%, 54% and 32% for ZIF-8:1% Eu, ZIF-8:3% Eu and ZIF-8:7% Eu, respectively. Nevertheless, the crystalline structure of ZIF-8 is maintained and only a small displacement in the planes was observed, which can be attributed to the substitution of europium ions in the zinc sites of the MOF structure. In general, the presence of europium ions improved the thermal stability and decreased the surface area compared to undoped ZIF-8. The emission spectra for the Eu-doped ZIF-8 reveal a signal in the blue region related to ZIF-8 emission and line profiles of the Eu3+ ion in the coordination polymer, which are in accordance with the formation of a heterostructured material with dual emission. In addition, the europium f-f transitions increased in intensity with the amount Eu3+, indicating an increment in the efficiency of this energy transfer. This study provides an example of progress in developing dual-emission MOFs based on the synergy properties of ZIF-8 and europium ions, leading to a material which can act as an efficient and stable dual-emission sensor.