“Chitosan-enhanced zirconium-based metal-organic framework: Enabling cellulase immobilization for improved cellulose breakdown”

In this study, cellulase derived from Aspergillus niger was effectively immobilized on an innovative epoxy-linked chitosan-zirconium metal-organic framework (CS@ZMOF-66) matrix. Detailed characterization was achieved using field emission scanning electron microscopy-energy dispersive X-ray (FESEM-EDX), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDX). The immobilized cellulase showed a high conjugation yield of 0.68 ± 0.01 mg/cm², maintaining 85.00 ± 0.04 % of its specific enzymatic activity and demonstrating improved catalytic efficiency. The immobilized enzyme exhibited peak activity at pH 5.0, a temperature of 65 °C and a saturating substrate concentration of 0.8 × 10⁻² mg/ml. The reduction in activation energy, enthalpy change and Gibbs free energy change, along with an increase in entropy change upon immobilization, suggested that the enzyme efficiency, stability and spontaneity in catalyzing the reaction were augmented by immobilization. The epoxy-linked CS@ZMOF-66/cellulase assembly was successfully applied for the hydrolysis of rice husk, achieving a remarkable conversion efficiency of 95 %. The method demonstrated a linear range from 0.1 to 0.9 % (0.1 × 10⁻² to 0.9 × 10⁻² mg/ml) and showed a strong correlation (R² = 0.998) with the widely used 3,5-dinitrosalicylic acid (DNS) method. The epoxy/CS@ZMOF-66 bound cellulase exhibited remarkable thermal stability, retaining 50 % of its activity at 80 °C, in contrast to just 33 % for the free enzyme and displayed a half-life of 24 days after storage at 4 °C compared to 9 days for the free enzyme. Furthermore, it retained over 90 % activity after 12 h at pH levels of 4.5 and 5.0 and showcased excellent reusability, maintaining activity over 25 cycles. Overall, this method offers high conversion efficiency and selectivity under benign conditions, with no undesirable by-products, making it a cost-effective solution for the routine hydrolysis of lignocellulosic biomass feedstock.