Effect of MWCNTs surface properties on lipase immobilization and its catalytic activity

This study investigates the surface properties effect of high aspect ratio synthesized MWCNTs on the immobilization efficiency, and the catalytic activity of Candidia rugosa lipase enzyme for olive oil hydrolysis reactions. High aspect ratio MWCNTs (H-MWCNTs) and low aspect ratio MWCNTs (L-MWCNTs) were synthesized in a vertical injection CVD reactor. These two customized MWCNTs types were compared to commercial MWCNTs (C-MWCNTs) with a lower aspect ratio, but higher surface area as confirmed by SEM and BET characterizations. All MWCNTs were functionalized by nitric acid treatment followed by the immobilization of lipase. FTIR and TGA characterization confirmed the functionalization of all MWCNTs types with carboxyl groups and the covalent attachment of lipase. In addition, TGA showed that the variation of the aspect ratio and surface area did not significantly affect the percent lipase loading, although H-MWCNTs had the highest loading of 43%. Immobilization of lipase on all three types of MWCNTs increased the amount of fatty acids produced from the hydrolysis of olive oil. C-MWCNTs showed the highest lipase activity (35 μmol·min⁻¹·mg⁻¹) due to their higher surface area, which enhanced the dispersion of lipase in the oil emulsion interface region. H-MWCNTs demonstrated the lowest lipase activity (10.4 μmol·min⁻¹·mg⁻¹) due to their long bundled structure that restricts the movement of lipase molecules lowering their contact with the substrate.