Immobilization of cellulase on TiO2 nanoparticles by physical and covalent methods: a comparative study.

Immobilization of cellulase from Aspergillus niger on TiO2 nanoparticles was studied by two different approaches--physical adsorption and covalent coupling. A. niger was selected, as it is generally non-pathogenic, is found in nature in the broad range of habitats and produces cellulase extracellulary. For covalent method, TiO2 nanoparticles were modified with aminopropyltriethoxysilane (APTS). The adsorbed and covalently immobilized enzymes showed 76% and 93% activity, respectively, as compared to the free enzyme. The catalytic efficiency V(max)/K(m) increased from 0.4 to 4.0 after covalent attachment, whereas in adsorption method, it increased slightly from 0.4 to 1.2. The covalently-immobilized and adsorbed cellulase lost only 25% and 50% of their activity, respectively after 60 min of incubation at 75 degrees C. The reusability and operational stability data also showed that covalent coupling increased the stability of the enzyme. The presence of enzyme on TiO2 nanoparticles was confirmed by Fourier-transform infrared spectroscopy. The high-resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM) studies indicated aggregation of enzyme when adsorbed on TiO2 surface and a monolayer of enzyme in covalent attachment. In conclusion, covalently attached cellulase retained good activity and thermal stability, as compared to physically adsorbed enzyme. The lower amount of enzyme activity and thermal stability in case of physically adsorbed immobilized enzyme was due to aggregation of the enzyme after adsorption on TiO2 nanoparticles, as revealed by HR-TEM and AFM. Thus, TiO2 nanoparticles could be suitable candidates for immobilization of cellulase for industrial applications like paper, textile, detergent and food industries.