Hierarchical covalent organic framework-foam for multi-enzyme tandem catalysis

Abstract

Covalent organic frameworks (COFs) are ideal host matrices for biomolecule immobilization and biocatalysis due to their high porosity, various functionalities, and structural robustness. However, the porosity of COFs is limited to the micropore dimension, which restricts the immobilization of enzymes with large volumes and obstructs substrate flow during enzyme catalysis. A hierarchical 3D nanostructure possessing micro-, meso-, and macroporosity could be a beneficial host matrix for such enzyme catalysis. In this study, we employed an in situ CO₂ gas effervescence technique to induce disordered macropores in the ordered 2D COF nanostructure, synthesizing hierarchical TpAzo COF-foam. The resulting TpAzo foam matrix facilitates the immobilization of multiple enzymes with higher immobilization efficiency (approximately 1.5 to 4-fold) than the COF. The immobilized cellulolytic enzymes, namely β-glucosidase (BGL), cellobiohydrolase (CBH), and endoglucanase (EG), remain active inside the TpAzo foam. The immobilized BGL exhibited activity in organic solvents and stability at room temperature (25 °C). The enzyme-immobilized TpAzo foam exhibited significant activity towards the hydrolysis of p-nitrophenyl-β-D-glucopyranoside (BGL@TpAzo-foam: K_m and V_max = 23.5 ± 3.5 mM and 497.7 ± 28.0 μM min⁻¹) and carboxymethylcellulose (CBH@TpAzo-foam: K_m and V_max = 18.3 ± 4.0 mg mL⁻¹ and 85.2 ± 9.6 μM min⁻¹ and EG@TpAzo-foam: K_m and V_max = 13.2 ± 2.0 mg mL⁻¹ and 102.2 ± 7.1 μM min⁻¹). Subsequently, the multi-enzyme immobilized TpAzo foams were utilized to perform a one-pot tandem conversion from carboxymethylcellulose (CMC) to glucose with high recyclability (10 cycles). This work opens up the possibility of synthesizing enzymes immobilized in TpAzo foam for tandem catalysis.