Bacterial Cellulose Nanofibers Modified with Quaternary Ammonium Salts for Antimicrobial Applications

Nanofibrillated bacterial cellulose (NFBC) is a recyclable, biodegradable, and nontoxic biopolymer whose fibers are much longer than other cellulose nanofibers prepared from wood pulp. NFBC is a promising material for use as an additive in food and pharmaceutical industries and as a nanofiller in fiber-reinforced nanocomposite resins; however, it is easily microbially degraded and, consequently, cannot be used in long-term applications. In this study, NFBC was endowed with antimicrobial properties through modification by an aqueous alkaline solution of 2,3-epoxypropyltrimethylammonium chloride (EPTMAC). The structure of the modified NFBC was analyzed in detail using solid-state NMR and infrared spectroscopy, as well as X-ray diffractometry, which revealed that only the surface layer of each fiber had been conjugated with quaternary ammonium EPTMAC groups, with the amount of EPTMAC added during preparation determining the degree of molar substitution (MS) of the cationic groups. In addition, increasing the MS of the modified NFBC resulted in a positive surface charge that led to an increase in the transparency of the nanofiber dispersion owing to electrostatic repulsion between nanofibers. Despite these differences, the fiber morphologies and crystal structures of the modified fibers were almost identical to those of pristine NFBC, suggesting that EPTMAC selectively reacts with the surface hydroxyl groups of NFBC. In addition, bacterial cell culture experiments revealed that the modified nanofibers are antimicrobial against Klebsiella pneumoniae, a Gram-negative bacterium, most likely because the surface of the modified NFBC is positively charged. Therefore, the quaternary-ammonium-salt-modified NFBC is a functional nanofiber endowed with both nanofiber features and antimicrobial properties and can possibly be used in fiber-reinforced nanofillers and as a flocculant for water-based paints and polar resins.