Manipulation of Amorphous‐to‐Crystalline Transformation: Towards the Construction of Covalent Organic Framework Hybrid Microspheres with NIR Photothermal Conversion Ability

Abstract

An approach to transforming amorphous organic networks into crystalline covalent organic frameworks (COFs) with retention of the colloidal nanosize and uniform morphology is presented. Specifically,F e 3 O 4 nanoclusters are encapsulated by ad isordering polyimine network via the Schiff-base reaction. The formed imine bonds could be reconstructed under thermodynamic control to reform the polyimine networks into imine-linked COFs in situ. Suchacore-shell microsphere exhibits the uniform sizea nd spherical shape, controllable COF shell thickness,a ccessible surface modification, and improved solution dispersibility as well as maintenance of high surface area, periodic micropores,and superior magnetic responsiveness.A dditionally,t he photothermal conversion effect is demonstrated for the first time on the nanoCOF layers upon exposure to near infrared light, providing convincing evidence for potential use in phototherapy.Covalent organic frameworks (COFs) are two-or threedimensional crystalline porous polymers that originate from the topological polymerization of building blocks with predesigned geometry and symmetry by virtue of dynamic covalent bonds (e.g.i mine, [1] enamine, [2] hydrazine, [3] azine, [4] b-ketoenamine, [5] and boronate ester [6] ). This emerging family presents high and regular porosity,tunable pore size and pore wall chemistry,a nd structural predictability and stability. [7] These characteristics endow COFs with outstanding performances for aw ide-range of applications,s uch as gas storage, [8] heterogeneous catalysis, [9] luminescence, [10] optical sensing, [11] proton conduction, [12] and photoconduction. [13] Against this backdrop,t here has been significant interest very recently in the miniaturization of COFs to the nanometer scale, [14] for the sake of extending the applicability and enhancing the proper-ties of COFs.I ndeed, the nanoCOFs can overcome,t os ome extent, the poor solution properties of the corresponding bulk materials,and they have been developed into nanocarriers for biomedical applications,such as drug delivery [15] and enzyme immobilization. [14b] However,most of the materials described were not very compatible with biomedical and pharmaceutical applications,a nd, with few exceptions,t hey were not engineered as dispersible nanoparticles to enable in vivo circulation by intravenous administration. As far as is known, the COF assemblies are prepared with no way of controlling size and morphology under solvothermal conditions.

Recent Trends in Covalent and Metal Organic Frameworks for Biomedical Applications

“…Photothermal therapy is a modern antitumor treatment whereby targeted radiation excites a photosensitizer molecule, which in turn generates heat and kills cancer cells by thermal ablation. This phenomenon has been demonstrated with some success in graphene [ 60 ]. Covalent photosensitizers are believed to be advantageous for this technique, as they tend to be biocompatible, efficient photoconverting agents.…”

“…One novel technique described by Tan [ 60 ] is an amorphous-to-crystalline transformation, whereby Fe 3 O 4 nanoclusters are embedded in an amorphous polyimine network whose bonds were reconstructed under thermodynamic control to yield a COF shell. This method allows for retention of certain properties (uniform spherical size and shape) and addition of new ones (controllable COF shell thickness, PEG-modifiable surface, crystallinity and surface area that increase with reaction time).…”

Water-Soluble Metalated Covalent Organic Nanobelts with Improved Bioavailability for Protein Transportation

“…Modification of exfoliated COFs with specific receptors also could implement the target delivery of loaded drugs 6 . In addition to shuttle cargoes, nanoscale COFs have been actively pursued as a brand-new platform for phototherapy 7 , enzyme immobilization 8 , and biomolecular detection 9 . However, the early reports have rarely investigated the fundamental interactions of COFs with living systems for improving the transportation ability of these materials.…”

Dynamic covalent chemistry steers synchronizing nanoparticle self-assembly with interfacial polymerization

“…One of the major challenges to integrating COFs with other materials is that COFs are typically formed as insoluble microcrystalline powders, which are difficult to process or not compatible with other materials. Recently, a two-step strategy that enables the encapsulation of nanoparticles into imine-based COF spheres has been reported 43,44 . This relies on the encapsulation of nanoparticles into amorphous imine-based spheres, followed by the crystallization of imine-based polymers under acidic conditions.…”

“…This relies on the encapsulation of nanoparticles into amorphous imine-based spheres, followed by the crystallization of imine-based polymers under acidic conditions. Another method for integrating COFs with other materials is to use amine functionalized materials that enables the condensation reaction between amine and aldehyde, which ultimately results in the overgrowth of COFs on the amine functionalized materials with a core-shell structure 44 .…”