Rate-Limited Reaction in TEMPO/Laccase/O2 Oxidation of Cellulose.

The environment-friendly oxidation of cellulose by the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)/laccase/O2 system is an alternative route with huge potential to prepare cellulose nanofibers. It is found that the concentration of TEMPO significantly affects the oxidation efficiency. An effective method for improving the oxidation effect is to increase the TEMPO concentration and prolong the oxidation time. To clarify the rate-limited step of TEMPO/laccase/O2 oxidation of cellulose, the academically accepted oxidation process is divided into individual pathways. A series of experiments is conducted with laccase and the three forms of organocatalyst (TEMPO, oxoammonium (TEMPO+), and hydroxylamine (TEMPOH)) to simulate individual reactions and calculate the reaction rates. The concentrations of TEMPO and oxoammonium are monitored by EPR spectroscopy. The oxidation rate of TEMPO by laccase varies at different pH conditions, and laccase activity is much higher at pH 4.5. Other reactions without laccase involved express a higher reaction rate when the pH value increased. TEMPO is mainly regenerated through a comproportionation reaction between oxoammonium and hydroxylamine. The acceleration of TEMPO regeneration by laccase is not obvious. The results indicate that the rate-limited reaction in TEMPO/laccase/O2 oxidation is cellulose oxidation by TEMPO+.

High internal phase Pickering emulsions stabilized by ε-poly-l-lysine grafted cellulose nanofiber for extrusion 3D printing.

An effective method for preparing food-grade three-dimensional (3D) printing materials was the use of highly concentrated oil-in-water emulsions. This research reported 3D printable materials constructed from food-grade high internal phase Pickering emulsions (HIPPEs) that were stabilized by ε-poly-l-lysine grafted cellulose nanofiber (ε-PL-TOCNs). The ε-PL-TOCNs were prepared via ε-poly-l-lysine grafting of 2, 2, 6, 6-tetramethylpiperidine-N-oxyl (TEMPO)-oxidized cellulose (TOC) and the successive mechanical treatment. Subsequently, the chemical structure, microstructure and surface properties of ε-PL-TOCNs were characterized. The results showed that the prepared ε-PL-TOCNs had excellent dispersion performances, cationic properties brought by amino groups, and hydrophilic/hydrophobic functions of chain structure, which confirmed the feasibility of preparing HIPPEs. The HIPPEs with an internal phase volume fraction of 82 % were obtained at 0.8 wt% ε-PL-TOCNs concentration and pre-emulsification followed by continuous oil feeding. The HIPPEs' storage stability, morphology, and rheological behavior were further discussed. The ultra stable HIPPEs with apparent shear-thinning behavior and high solid viscoelasticity were successful produced, which was suitable for 3D printing. This work expanded the application of nanocellulose in emulsions field and provided a new thinking to prepare food-grade 3D printable materials and porous foam.

Re-dispersible chitin nanofibrils with improved stability in green solvents for fabricating hydrophobic aerogels.

A facile formic acid (15 M) hydrolysis method was established to prepare chitin nanofibrils that exhibited much longer fibril size (538-1000 nm) and higher yield (87-95%) than traditional hydrochloric acid hydrolyzed ones. Interestingly, these chitin nanofibrils can well be dispersed in water/TBA or water/ethanol binary solvents, which can contribute to the functionality of chitin nanofibrils in different way. On one hand, chitin nanofibril aerogels with tunable inter-structure were prepared via direct freeze-drying of chitin nanofibril/water/TBA dispersion with specific surface area could be promoted 69%. Meantime, re-dispersibility was realized, and the higher concentration of TBA used, the better re-disperse performance. On the other hand, due to the well disperse nature of chitin nanofibrils in water/ethanol (ethanol can reach 90%), they can be well mixed with zein. Whereby, the hydrophobic chitin nanofibril/zein composite aerogels were prepared and showed great potential in absorbing oil or organic spillage off water.

A MoS2QDs/chitin nanofiber composite for improved antibacterial and food packaging.

Chitin nanofiber has potential application as antibacterial nanocomposite material because of its inherent biocompatibility, biological activity, amino containing macromolecular structure and nano-size effect. Molybdenum disulfide quantum dots (MoS2QDs) were uniformly bonded on partially deacetylated chitin nanofibers (DEChNs) by hydrothermal reactions. The antibacterial properties of MoS2QDs/DEChN against Escherichia coli were detected under different conditions. When the antibacterial agent was fixed at 200 µg/mL, the survival rates of bacteria were 2.77% (pH = 4), 5.58% (pH = 5) and 7.83% (pH = 6), which were lower than those in the DEChN groups. Unlike DEChN, which only had excellent antibacterial activity under acidic conditions (pH < 5), the combination of DEChN and MoS2QDs had antibacterial activity close to neutral conditions, with a bacteriostatic rate > 90%. When TEMPO-oxidized cellulose nanofibers (TOCN) were applied for the preparation of MoS2QDs/TOCN, they did not show obvious antibacterial ability, which proved the positive role of DEChN and its amino groups. The MoS2QDs/DEChN assembled film could be applied to preserve meat by delaying spoilage. The current study might inspire new ideas for designing food packaging based on the prepared MoS2QDs/DEChN films.

Zwitterionic chitin nanocrystals mediated composite and self-assembly with cellulose nanofibrils.

Zwitterionic dispersed chitin nanocrystals and TEMPO oxidized cellulose nanofibrils can be well mixed and self-assembled to be hydrogels/membranes. Active carboxyl groups ensure the well mixing of zwitterionic chitin nanocrystals and cellulose nanofibrils under neutral and alkaline condition. Electrostatic attraction between amino groups in chitin nanocrystals and carboxyl groups in chitin nanocrystals and cellulose nanofibrils further endows self-assemble property of composite suspensions. Simple standing for 12 h at room temperature is found enough for preparing self-assembled composite hydrogels. By 1-(3-dimethy-laminopropyl)-3-ethylcarbodiimide hydrochloride/N-hydroxy succinimide (EDC/NHS) mediated chemical crosslinking, the storage modulus of composite hydrogel can achieve almost 8 times higher than self-assembled hydrogel. Well dispersed composite suspensions also can be transformed to be membranes via filtration treatment. The strain increases almost 2.3 times higher with similar tensile strength for cellulose nanofibril rich samples, and chitin nanocrystals mainly contributes to the enhancement in strain of composite membranes.