Cellulose Nanofibers Prepared Using the TEMPO/Laccase/O₂ System

The 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/laccase/O₂ system was used to prepare cellulose nanofibers from wood cellulose without requiring any chlorine-containing oxidant. Laccase was degraded by oxidized TEMPO (TEMPO⁺) formed by laccase-mediated oxidation with O₂, which competed with the oxidation of wood cellulose. Thus, large amounts of laccase and TEMPO and a long reaction time were needed to introduce ∼0.6 mmol g^-1 of C6-carboxylate groups onto wood cellulose. The TEMPO/laccase/O₂ system underwent one-way reaction from TEMPO to reduced TEMPO through TEMPO⁺. When the oxidation was applied again to the oxidized wood cellulose following isolation and purification, the C6-carboxylate groups increased to ∼1.1 mmol g^-1, which was sufficient to convert the sample to cellulose nanofibers by sonication in water. However, the higher the carboxylate content of the oxidized celluloses, the lower their degree of polymerization.