Urease covalently immobilized on cotton-derived nanocellulose-dialdehyde for urea detection and urea-based multicomponent synthesis of tetrahydro-pyrazolopyridines in water

The urease Schiff-base covalently bonded to the designed high-content nanocellulosedialdehyde (HANCD) prepared from cotton-derived nanocellulose (NC) via tandem acid-hydrolysis and periodate-oxidation reactions was termed HANCD@urease. No change in the aldehyde content of HANCD after Schiff-base bonding to urease and similarity in the relative enzyme activities for HANCD@urease and free enzyme supported that the preparation conditions for HANCD-loaded urease are mild enough to prevent denaturation of the enzyme. As the immobilized urease showed higher stability and reusability versus free enzyme, the HANCD@urease was efficiently used to determine the urea concentration in aqueous solutions and blood serum samples. Alternatively, the catalytic efficiency of the HANCD@urease was demonstrated for the production of ammonia from urea in the multicomponent synthesis of 3,5-dimethyl-4-aryl-1,4,7,8-tetrahydrodipyrazolo[3,4-b:4′,3′-e]pyridines (THPPs) in water. This new environment-friendly urea sensor showed 90% preservation of the enzyme activity after the six cycles of reuse in enzymatic reactions, while its catalytic activity in the reaction of benzaldehyde, hydrazine hydrate, and alkyl acetoacetate with urea instead of hygroscopic ammonium salts did not change significantly after the sixth run. Detection and production of ammonia by a bio-compatible sensor and catalyst under mild conditions are features of this new green protocol.