Reversible Surface Functionalization of Cellulose Nanocrystals Mediated by Metal-Phenolic Networks

The incorporation of small, rigid fillers can effectively reduce stress concentrations in the materials. Nanocellulose has attracted significant attention as a biobased nanofiller due to increasing environmental concerns. To improve its compatibility with matrix materials, surface hydrophobization of nanocellulose is essential; however, achieving this without complex processes remains a challenge. In this study, we demonstrate a facile and reversible surface modification of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanocrystals (CNCs) via metal-phenolic networks (MPNs). The CNCs derived from tunicates were coated with MPNs formed from tannic acid (TA) and Fe3+ in water. Following the MPN coating, octadecylamine (ODA) was covalently bonded to TA, increasing the CNC surface hydrophobicity. ODA was attached at the 3-position of the galloyl moiety of TA. After modification with ODA, the CNCs became nondispersible in water but dispersible in organic solvents including ethanol. The surface coverage of CNCs, estimated using a quartz crystal microbalance (QCM), was 86%. Treatment with HCl solution effectively disassembled the coating, and inductively coupled plasma mass spectrometry (ICP-MS) analysis confirmed that more than 95% of Fe3+ was removed from the CNC surfaces. These results highlight the potential of MPNs as primer layers for the reversible surface modification of nanocellulose.