Extremely Large 3D Cages in Metal–Organic Frameworks for Nucleic Acid Extraction

Three-dimensional (3D) cages in the mesopore regime (2–50 nm) assembled from molecular building blocks are highly desirable in biological applications; however, their synthesis in crystalline form is quite challenging, as well as their structure characterization. Here, we report the synthesis of extremely large 3D cages in MOF crystals, with internal cage sizes of 6.9, and 8.5 nm in MOF-929; 9.3 and 11.4 nm in MOF-939, in cubic unit cells, a = 17.4 and 22.8 nm, respectively. These cages are constructed from relatively short organic linkers with the lengths of 0.85 and 1.3 nm, where the influence from molecular motion is minimized, thus favoring their crystallization. A 0.45 nm linker length elongation leads to a maximum 2.9 nm increase in cage size, giving a supreme efficiency in cage expansion. The spatial arrangements of these 3D cages were visualized by both X-ray diffraction and transmission electron microscopy. The efforts to obtain these cages in crystals pushed forward the size boundary for the construction of 3D cages from molecules and also exploited the limit of the area in space possibly supported per chemical bond, where the expansion efficiencies of the cages were found to play a critical role. These extremely large 3D cages in MOFs were useful in the complete extraction of long nucleic acid, such as total RNA and plasmid from aqueous solution.