Size-controlled synthesis of unusual hydrogen-bonded imine-linked covalent organic framework for trypsin immobilization and drug delivery

The interlayer hydrogen-bonded, uniform, and spherical covalent organic frameworks (USCOFs) and non-hydrogen bonded, and spherical covalent organic polymers (USCOPs) were synthesized in a series of aromatic and nonaromatic solvents at room temperature. The presence of interlayer hydrogen bonds between USCOFs were investigated via electron density difference (EDD) and stacking energy calculation in various solvents by DFT methods. The in-situ mid-angle X-ray scattering measurements indicate that the (100)-plane at 0.33 Å continuously grows in USCOFs, which was absent in USCOPs. The crystalline USCOFs exhibit a high surface area of 658 m²g⁻¹, with a pore size of 3 nm, whereas USCOPs exhibit a surface area of 16 m² g⁻¹ with a pore size of 19 nm. Therefore, USCOFs formation in aromatic solvents, involving solvation and reduced π–π stacking, which allow the defective intermediates to dissociate and react reversibly, leading to well-ordered molecular arrangements. These solvent characteristics are absent in nonaromatic solvents; therefore, they fail to produce well-ordered molecular arrangement, forming amorphous USCOPs. Finally, the as-synthesized USCOFs were used for a protein(trypsin)-loading and drug delivery application. In addition, the loaded protein retains its original activity. Notably, the conducted MTT assay and cellular uptake proved COF-DOX selectively killed the cancer cells rather than normal cells.