Enzyme immobilization technology has become an effective way to promote the industrialization of biological macromolecules. Despite huge efforts in developing new strategies and materials to achieve this process, the structure of biological macromolecules such as enzymes is fragile, so maintaining enzyme activity remains a huge challenge. In this paper, a sacrificial template method was used to construct a hollow COF capsule with unstable MOF as a sacrificial template for encapsulation of enzymes. This method successfully maintains enzyme activity and is successfully applied to enzyme-catalyzed reactions! What‘s more important is the versatility of methods and concepts, which is of great inspiration for the future synthesis of COF coating materials (not only biomolecules, but also other unstable materials)! I think this strategy is very important and worth reading.

[COF Capsule-Origin]

The importance of enzymes as a natural catalyst is self-evident. However, the fragility of enzymes has greatly hindered their industrial application. And COF has porosity, high stability and adjustability, which makes it an excellent host for enzyme encapsulation. However, the harsh synthetic conditions required in the COF preparation process make it impractical to directly prepare bio-encapsulated COF capsulesby the "one-pot method" . The sister material of COFs, MOFs, has been reported to encapsulate and stabilize biomacromolecules in situ , however, resulting in a decrease in biomolecule activity. However, in MOFs capsules, unstable MOFs can be digested to release biomolecules without losing their activity . In addition, MOF @ COF materials have also been reported extensively! !! !! So far, it is conceivable to combine the above characteristics of COF and MOF, first encapsulate the enzyme into MOF, then cover the MOF with COF, and finally digest the MOF and release the enzyme , you can get the enzyme @COF capsule perfectly! !! !! excellent! !! As shown below:

Figure 1 Synthesis strategy of COF capsules

[COF Capsule-Synthetic]

ZIF-90 has been previously proven to effectively encapsulate various biologically active macromolecules (such as enzymes) and protect them from harsh conditions. At the same time , ZIF-90 is easily digested under mild acidic conditions . And, it is important that there are unreacted aldehyde groups in the ZIF-90 structure! Now, as long as you find a stable COF for the aldehyde-amine condensation reactionunder acidic conditions , everything is ready. After screening, the author chose COF-42 with the structure shown below. After grasping the above ideas, I think its synthesis method is as simple as how to put an elephant in the refrigerator. Synthetic steps: In the first step, the enzyme @MOF is enveloped in situ during the MOF (ZIF-90) synthesis; in the second step, the enzyme @MOF is thrown into the reaction for the synthesis of COF, so that the outer surface of the MOF is coated COF (choose COF-42B) to get the enzyme @ MOF @ COF; the third step is to digest the MOF in the enzyme @ MOF @ COF in a weak acid, and finally get the enzyme @COF. For some details that need attention during the synthesis process, please move to the original. Notes: The biological macromolecule selected by the author in the example is BSA (Bovine Serum Albumin, which is a protein, not an enzyme). For the convenience of description, the above are collectively called enzymes.

Figure 2 Synthesis of COF capsules

The results of each step were tested by the authors using PXRD and TEM. PXRD found that MOF and COF crystals coexisted during the synthesis. In addition, the thickness of the COF shell can be adjusted by the ratio of the enzyme @MOF to COF in the second step. The final result is shown in Figure 3:

Figure 3 PXRD and TEM characterization of COF capsules

In addition, the author explored the versatility of this strategy , using ZPF-2 (ZPF = Zeolitic Pyrimidine Framework), ZIF-8, etc. as sacrificial templates, and using COF-43-B as the COF shell, which can successfully synthesize COF capsule.

[COF Capsule-Application]

CAT is a known multimeric protein with four subunits, which can catalyze the decomposition of hydrogen peroxide (H 2 O 2 ) to produce O 2and H 2 O. Therefore, the author chose CAT enzyme, and MOF chose ZPF-2 and COF-42-B to synthesize CAT @ COF-42-B, a COF capsule to demonstrate its application in catalysis. As shown in Figure 4, TEM and mapping show that the characteristic element Fe in CAT is evenly dispersed in all samples, which indicates the successful encapsulation of CAT .

Figure 4 Electron micrograph of CAT @ COF-42-B

After that, the author tested the H 2 O 2 catalytic decomposition performance of the COF capsule ( CAT @ COF-42-B ) , mainly examining the performance comparison with the MOF complex CAT @ ZPF-2 and pure CAT two precursors. The results are shown in Figures a and b below. In short, the performance is super good. And the author also examined its stability chart c under different chemical conditions , and finally tested its cyclic stability as shown in Figure d, and found that the performance was still maintained in 10 cycles of testing, proving its stability.

Figure 5 Catalytic performance of COF enzyme

[Application Development]

Here the author encapsulated two or more enzymes in a COF capsule, and successfully achieved a cascade reaction between each enzyme (Figure 6). The realization of this result means that it has potential applications in the manufacture of bioreactors or advanced micro-devices.

Figure 6 Catalytic performance of various systems

【to sum up】

In summary, this article reports a simple three-step method for preparing new COF capsules to encapsulate various biological macromolecules and maintain the activity of biological macromolecules for further applications. I think this strategy can be applied to the COF encapsulation of other small molecule compounds , especially in various catalytic fields!

Original link: https://doi.org/10.1021/jacs.0c00285

Information source: COF person