A bioinspired dual-crosslinked tough silk protein hydrogel as a protective biocatalytic matrix for carbon sequestration

The development of carbonic anhydrase (CA)-based materials for the environment-friendly sequestration of carbon dioxide (CO2) under mild conditions would be highly valuable for controlling emissions to the environment and for producing value-added chemicals. Here, a highly tough and stable CA-encapsulating silk protein hydrogel was developed as a robust biocatalyst for CO2 sequestration through a bioinspired dual-crosslinking strategy that employed photoinduced dityrosine chemical crosslinking followed by dehydration-mediated physical crosslinking. The target enzyme was efficiently encapsulated in the silk hydrogel with ~60% retention of the activity of free CA, and the encapsulated CA exhibited excellent overall multi-use, storage and thermal stabilities. The dual-crosslinked CA-encapsulating silk hydrogel exhibited a significant compressive modulus, which surpassed the moduli of most traditional and double-network hydrogels as well as those of enzyme-encapsulated hydrogels. This hydrogel also showed high resiliency and elasticity and outstanding structural stability. Importantly, the dual-crosslinked CA-encapsulating silk hydrogel facilitated the sequestration of CO2 into calcium carbonate with high CO2 hydration activity. Thus, the unique combination of bioinspired dual-crosslinking with silk fibroin protein and CA enzyme demonstrates the successful application of this protein hydrogel as a promising biocatalyst for CO2 sequestration by showing high activity, strong mechanical properties and outstanding structural stability. Natural enzymes that solidify carbon dioxide (CO2) gases into minerals can gain critical stability from a tough, silk-based hydrogel. In the body, carbonic anhydrase enzymes catalyze the reversible hydration of CO2 to maintain the acid–base balance. Hyung Joon Cha from Korea's Pohang University of Science and Technology and colleagues have found a way to improve the reliability of these compounds for carbon sequestration using a bio-inspired cross-linking strategy. After purifying the enzyme, the team stirred it into a suspension of silk fibroin – protein molecules used by silkworms to construct cocoons. Exposing the mixture briefly to light caused the silk to chemically link into an encapsulating gel with a strong internal framework of physically stacked sheets. The gel protected carbonic anhydrase against both thermal and mechanical shock, enabling up to six reuses with no loss in catalytic activity. The dc-ngCA-silk hydrogel is a highly protective biocatalytic matrix fabricated through bioinspired photoinduced chemical and dehydration-mediated physical dual-crosslinking of silk fibroin and carbonic anhydrase. The fabricated silk hydrogel exhibited significantly high toughness, resiliency and stability with good catalytic activity, which enabled its successful use as a robust biocatalyst for CO2 sequestration.