Effective encapsulation of laccase in an aluminium silicate nanotube hydrogel

Aluminium silicate nanotubes (ASNT), with a length to width ratio of four, were synthesized from aluminium chloride and sodium silicate, and the ASNT hydrogel was easily prepared by adjusting the pH to 7. The hydrogel nanotube concentration was 1.5 wt%. Laccase, a type of oxidase, was encapsulated during ASNT hydrogel formation. This encapsulation method will have fewer negative effects on the relatively unstable enzyme because of the milder conditions used, which are different from 501 gel silica formation by acidic catalysis using strong acids such as hydrochloric acid. The obtained hydrogels were fully characterized by various methods such as field-emission scanning electron microscopy and Fourier transform infrared spectroscopy. The ASNT-hydrogel encapsulated enzyme worked well; notably, laccase ASNT hydrogels prepared from the shortest nanotubes exhibited a higher activity than the free laccase in solution because of an improvement in the substrate affinity of the encapsulated enzyme. Tryptophan fluorescence spectroscopy indicated that the highly ordered structure of laccase was not altered once bound to the nanotubes within the hydrogel. Notably, after nine repeated reactions, laccase encapsulated in the ASNT hydrogel retained its activity. The cycle performance of the encapsulated enzyme indicates that no enzyme was released from the ASNT hydrogel. In addition, the laccase ASNT hydrogel was easily used to prepare transparent thin films on glass cover slips, while still maintaining the enzyme activity. Another oxidase, myoglobin, was also encapsulated in the same type of ASNT hydrogel. Although free myoglobin in solution demonstrated oxidation activity, the activity of the bound protein was remarkably decreased due to changes in its tertiary structure when inside the ASNT hydrogel.