Artificial organic–inorganic hybrid materials produced through mineralization in/on biomolecular assemblies under aqueous-based mild conditions have attracted much attention due to the sustainability derived from environmentally friendly and low-energy production processes and excellent mechanical properties resulting from their highly organized structures. In this study, organic–inorganic hybrid hydrogels composed of crystalline nanoribbon assemblies of terminally carboxylated cellulose oligomers and hydroxyapatite (HAp) were produced via cellodextrin phosphorylase-catalyzed syntheses of the oligomers and in situ HAp mineralization achieved by combining phosphate ions kinetically fed by the enzyme reaction with coexisting calcium ions. Chemical structure characterizations revealed successful syntheses of the oligomers from the appropriate substrates (namely, monomers and primers). Crystallographic characterizations revealed that the cellulose moieties crystallized as the cellulose II allomorph, thereby leading to an antiparallel molecular arrangement in the assemblies, and that the calcium phosphate produced was assignable to HAp. Microscopic observations revealed the production of surface-carboxylated nanoribbon assemblies of the oligomers onto which HAp granules were hybridized, while the hybrid structure was not observed for nanoribbon assemblies of plain cellulose oligomers even after HAp mineralization. Mechanical property characterizations revealed that the stiffness (namely, Young’s modulus) of the hybrid hydrogel was significantly greater than it was without surface carboxylation of nanoribbon assemblies or HAp hybridization, suggesting that HAp hybridization to surface-carboxylated nanoribbon assemblies is essential for improving the mechanical properties of cellulose oligomer hydrogels. Our findings open a new avenue for production of synthetic nanocellulose–inorganic hybrid materials with advanced functions.

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