Single-Crystal 2D Covalent Organic Frameworks for Plant Biotechnology

Molecules chemically synthesized as periodic two-dimensional(2D)frameworks via covalent bonds can form some of the highest-surfacearea and -charge density particles possible. There is significantpotential for applications such as nanocarriers in life sciences ifbiocompatibility can be achieved; however, significant synthetic challengesremain in avoiding kinetic traps from disordered linking during 2Dpolymerization of compatible monomers, resulting in isotropic polycrystalswithout a long-range order. Here, we establish thermodynamic controlover dynamic control on the 2D polymerization process of biocompatibleimine monomers by minimizing the surface energy of nuclei. As a result,polycrystal, mesocrystal, and single-crystal 2D covalent organic frameworks(COFs) are obtained. We achieve COF single crystals by exfoliationand minification methods, forming high-surface area nanoflakes thatcan be dispersed in aqueous medium with biocompatible cationic polymers.We find that these 2D COF nanoflakes with high surface area are excellentplant cell nanocarriers that can load bioactive cargos, such as theplant hormone abscisic acid (ABA) via electrostatic attraction, anddeliver them into the cytoplasm of intact living plants, traversingthrough the cell wall and cell membrane due to their 2D geometry.This synthetic route to high-surface area COF nanoflakes has promisefor life science applications including plant biotechnology.