Fluorescence analysis for characterizing the alkali stability of metal-organic frameworks: an informative complement to X-ray diffraction

The stability of metal-organic frameworks (MOFs) in solutions is of essential importance from the perspectives of scientific research and practical applications. Powder X-ray diffraction (PXRD) with isolated solids is a common protocol to judge the stability. However, solid-state characterization may lead to ignorance of possible chemical leaching and even partial collapse, and the ignorance may lead to confusing or even false conclusions about stability and relevant properties. Herein, based on the pH-responsive fluorescence associated with alkaline hydrolysis of MOFs, a fluorometric method is proposed for characterizing the stability of luminescent or luminogenic MOFs in basic solutions. Exemplified by two Zr(iv) MOFs, Zr-TCPBP (tetracarboxylate linker) and UiO-66-NH₂ (dicarboxylate), the method has been validated by a comparative study with solid-state characterization using PXRD, scanning electron microscopy (SEM) and thermogravimetry. The method has the advantages of facile operation, high sensitivity and good complementarity to PXRD and SEM. It can be used to continuously monitor the whole hydrolytic process starting with nondestructive local hydrolysis and ending with abrupt and destructive hydrolysis, giving the pH values for different hydrolytic stages. The method has great potential to be extended for characterizing the stability of MOFs towards acids or other substrates in solution.