Characterization of cellulose nanofibrils prepared by direct TEMPO-mediated oxidation of hemp bast

Hemp bast (α-cellulose 79.4%, Klason lignin 4.9%) was directly oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation in water at pH 10 and room temperature for 2 h. The level of added NaClO in oxidation varied from 5 to 30 mmol/g (based on dry weight of hemp bast). Weight recovery ratios of the TEMPO-oxidized hemp bast celluloses were in the range of 81–91%, and their carboxylate contents increased up to 1.2 mmol/g with the increased NaClO addition level. The lignin contents decreased to 0.5–0.9% after oxidation, and the viscosity-average degrees of polymerization decreased from 1100 to 560 because of depolymerization during oxidation. Thus, direct TEMPO-mediated oxidation of hemp bast introduced a significant number of carboxylate groups and simultaneously achieved sufficient delignification. Small amounts of xylose, mannose, and rhamnose originating from hemicelluloses remained in the TEMPO-oxidized hemp bast samples prepared by oxidation with 5–20 mmol/g NaClO. However, oxidation with 30 mmol/g NaClO completely removed these hemicellulose-originating sugars, and produced almost pure TEMPO-oxidized cellulose. When TEMPO-oxidized hemp bast samples were mechanically disintegrated in water, their nanofibrillation yields were 58–65%. After removal of unfibrillated fractions by centrifugation, transparent dispersions showed birefringence when observed between cross-polarizers, while atomic force microscopy images showed near-individually dispersed nanofibril elements with widths of ~2 nm.     

Silver-nanoparticle-containing handsheets for antimicrobial applications

Development of self-sanitizing cellulose and cellulose paper-based products will increase human safety and hygiene. In the present work, a softwood bleached kraft pulp (SBKP) was oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation in water at pH 10 at two NaClO addition levels (3 and 5 mmol g^?1 based on the dry weight of SBKP). The fibrous TEMPO-oxidized SBKPs (TO-SBKPs) were subsequently incorporated with silver nanoparticles (AgNPs) by soaking in aqueous silver nitrate (AgNO₃) solution and subsequent thermal reduction. The C=O absorption band in FTIR spectra of AgNP-containing TO-SBKPs increased with increasing Ag content, showing that the C2/C3 hydroxy groups in TO-SBKPs were oxidized to ketones by reduction of Ag⁺ ions to AgNPs during heating at 100 °C for 1 h. Scanning electron microscopy images showed that the AgNPs were almost homogenously distributed on the surface of each TO-SBKP fiber with an average diameter of 32–40 nm regardless of different Ag contents. Handsheets were prepared from SBKP and the AgNP-containing TO-SBKP at various weight ratios. The handsheets showed sufficient antimicrobial activities against a Gram-negative Escherichia coli strain and a Gram-positive Staphylococcus aureus strain. The tensile strength of the handsheets was significantly improved by mixing the AgNP-containing TO-SBKP with SBKP. The 20% TO-SBKP/Ag-containing SBKP sheets were optimal in terms of efficient antimicrobial activities and good mechanical properties. Thus, the AgNP-containing TO-SBKP sheets have potential for use as antimicrobial paper and related packaging materials produced using the conventional papermaking process.

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