Effect of the activation strategy of nickel oxide-multi-walled carbon nanotubes on the immobilization of xylanase for synthesis of xylooligosaccharides.

Magnetic nickel oxide multi-walled carbon nanotubes (MWCNT-NiO) were employed in the immobilization of xylanase from Thermomyces lanuginosus, after modification with (3-glycidoxypropyl)trimethoxysilane or 3-aminopropyltriethoxysilane (APTES). The APTES-derivatized MWCNT-NiO particles were activated with glutaraldehyde to immobilize T. lanuginosus xylanase via covalent attachment. The (3-glycidoxypropyl)trimethoxysilane-derivatized MWCNT-NiO particles were directly used for the covalent immobilization of T. lanuginosus xylanase, or the formed epoxy groups were converted to aldehyde groups. The free xylanase had maximum activity at pH 7.5, whereas the immobilized samples showed an optimum pH of 7.0. The optimum temperature was 60°C for the xylanase samples. The thermal stability of xylanase increased at 7 and/or 12 folds after immobilization. The results of xylooligosaccharide synthesis showed that the main formed xylooligosaccharides were xylobiose, xylotriose, and xylotetraose for the immobilized xylanase samples. Furthermore, an effect of the enzyme loading could be found, an increase in this parameter promoted that xylobiose and xylotriose amounts slightly increased, whereas xylotetraose amount slightly decreased. The immobilized xylanase samples retained at least 80% of their initial activity after five reuses at pH 7.0 and 60°C. The results show that the new xylanase preparations were easily separable, thermally stable, and reusable in the synthesis of xylooligosaccharides.