Yimin Fan

Preparation of Chitin Nanofibers from Squid Pen β-Chitin by Simple Mechanical Treatment under Acid Conditions

A procedure for preparing individualized chitin nanofibers 3-4 nm in cross-sectional width and at least a few microns in length was developed. The key factors to prepare the chitin nanofibers with such high aspect ratios are as follows: (1) squid pen beta-chitin is used as the starting material and (2) ultrasonication of the beta-chitin in water at pH 3-4 and 0.1-0.3% consistency for a few minutes. Transparent and highly viscous dispersions of squid pen beta-chitin nanofibers in water can be obtained by this method. No N-deacetylation occurs on the chitin molecules during the nanofiber conversion procedure. Moreover, the original crystal structure of beta-chitin is maintained, although crystallinity index decreases from 0.51 to 0.37 as a result of the nanofiber conversion. Cationization of the C2 amino groups present on the crystallite surfaces of the squid pen beta-chitin under acid conditions is necessary for preparing the nanofibers.

Multifunctional Coating Films by Layer-by-Layer Deposition of Cellulose and Chitin Nanofibrils

An environmentally benign surface modification process for plastic films was demonstrated by fabricating composite coatings through layer-by-layer assembly with green solid materials: aqueous dispersions of two kinds of crystalline polysaccharide nanofibrils. Anionic cellulose nanofibrils were obtained by the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation of native cellulose, while cationic β-chitin nanofibrils were prepared by the protonation of squid pen chitin. Uniform layer depositions, driven by electrostatic attraction and enhanced by hydrogen bonding, were observed on silicon wafers and then reproduced onto poly(ethylene terephthalate) films. Contact angle measurements and dyeing tests on the resulting films revealed their hydrophilic nature and the sorption of both charged and uncharged substances. Antireflection properties were also confirmed via the light transmittance measurements. As might be presumed from all these properties, this composite coating exhibited its unique behavior largely due to its structure, which was distinct from both those of nanofibril cast films and polymer films.

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.

Chemically Functionalized Silk for Human Bone Marrow-Derived Mesenchymal Stem Cells Proliferation and Differentiation

To produce biocompatible, mechanically robust, and conductive materials for bone tissue engineering, chemical oxidation using sodium hyprochlorite (NaClO) was utilized to introduce carboxyl groups onto silk fibroin (SF). A final carboxyl content of 1.09 mM/g SF was obtained, corresponding to ∼47% of the primary hydroxymethyl groups on the silk. Interestingly, both infrared (IR) spectroscopy and circular dichroism (CD) spectra demonstrated that the resulting oxidized silk (OxSF) self-assembled into β-sheet structures under aqueous conditions and this contributed to the mechanical properties of the as-prepared silk-based scaffolds and the mineralized OxSF scaffolds (M-OxSF). The OxSF scaffolds had a compressive modulus of 211 ± 75 KPa in the hydrated state, 10 times higher than that of the SF scaffolds, and the modulus of the M-OxSF scaffolds was increased to 758 ± 189 KPa. Human bone marrow-derived mesenchymal stem cells (hMSCs) grown on the scaffolds during osteogenesis showed that the OxSF scaffolds supported the proliferation and differentiation of hMSCs in vitro.

Reinforced chitosan beads by chitin nanofibers for the immobilization of β-glucosidase

A chitosan (CS) solution was filled with partially deacetylated α-chitin nanofibers (DEChN) and the composite (DEChN/CS) beads prepared thereof were used for the immobilization of β-glucosidase. The filling of chitin nanofibers resulted in an increase of the shear modulus (G′, G′′), which indicated an improvement to the mechanical properties of the DEChN/CS composite beads. Meanwhile, the specific surface area was improved from 46.47 m2 g−1 to 229.71 m2 g−1 when the chitosan beads were filled with the chitin nanofibers, and the prepared DEChN/CS composite beads had a broad pore size ranging from 20 nm to 60 nm. Moreover, a more porous and fibrous structure in the SEM image of the DEChN/CS beads was observed compared to that of the sole CS beads. The chitin nanofibers provided a fibrous network in the chitosan matrix that enhanced both the mechanical and porous properties of the DEChN/CS composite beads, which is normally considered contradictory. All of these enhanced features improved the efficiency of enzyme immobilization from 40% in the chitosan (CS) beads to 67% in the DEChN/CS composite beads.

Dissolution of Lignocelluloses with a High Lignin Content in a N-Methylmorpholine-N-oxide Monohydrate Solvent System via Simple Glycerol-Swelling and Mechanical Pretreatments

Lignocelluloses (LCs) with various amounts of lignin (even as high as 18.4%) were successfully dissolved in N-methylmorpholine-N-oxide monohydrate (NMMO/H2O) solution with stirring at 85 °C within 5 h. For the developmental dissolution methods of LCs with a high lignin content in NMMO/H2O solution, the following two pretreatment steps of LCs were necessary: (1) glycerol swelling and (2) mechanical extrusion. The mechanical extrusion pretreatment under glycerol swelling dissociated the fiber bundles of LCs to thinner fibers and, thus, enhanced the accessibility and solubility of the LCs in NMMO/H2O. The crystal structure of the pretreated LCs had no significant transformation during pretreatment, while the diameters of the fiber bundles were reduced from 50-60 to 10-12 μm, as investigated by X-ray diffraction and scanning electron microscopy. After the dissolution-regeneration process of LCs, the fiber bundles of the LCs disappeared and the crystal type of cellulose in the LCs was transformed from cellulose I to cellulose II, which indicated the complete dissolution of LCs.

Cholesteric film of Cu(II)-doped cellulose nanocrystals for colorimetric sensing of ammonia gas

With the increasing demand of environmental monitoring for toxic and odorous ammonia gas it is desired to develop specific green, cost-effective and in situ passive colorimetric alternatives to current complex instrumentations. In this work, we designed an ammonia gas sensor based on cholesteric liquid crystal films of copper(II)-doped cellulose nanocrystals (CNCCu(II)) whose structure, optical and sensing properties were investigated. The hybrid films using the low doping Cu(II) as a color-tuning agent inherited the chiral nematic signature and optical activity of CNCs, suggesting a strong chelation between copper ions and negatively charged CNCs. The sensing performance illustrates that the CNCCu(II)125 film was sensitive to ammonia gas which could merge into nematic layers of CNCs and trigger-sensed to copper ions chelated on CNCs, consequently arousing a red-shift of reflective wavelength as well as an effective colorimetric transition. Such a hybrid film is anticipated to boost a new gas sensing regime for fast and effective on-site qualitative investigations.

Adsorption of Reactive Blue 19 from aqueous solution by chitin nanofiber-/nanowhisker-based hydrogels

Physical hydrogels prepared from partially deacetylated chitin nanofibers/nanowhiskers (DEChNs) were prepared and evaluated as a new adsorbent for Reactive Blue 19 (RB19) solutions. The effects of pH, initial dye concentration, contact time and temperature were investigated. The optimum pH value for the adsorption experiments was found to be 1.0; as pH increases, the dye adsorption capacity decreases gradually. The adsorption of RB19 onto partially deacetylated chitin nanofiber-/nanowhisker-based hydrogels (DEChNs-Gels) was relatively fast, as the equilibrium could be reached in almost 20 min. The maximum adsorption capacity was found to be 1331 mg g−1 at pH = 1 (degree of deacetylation (DDA) = 23%, dye concentration = 1000 mg L−1), considering the practical applications, the adsorption capacity in pH = 5 (838 mg g−1) was believed to have more practical significance. A pseudo-second-order kinetics model agreed very well with the experimental results. Equilibrium data also fitted well to the Freundlich adsorption isotherm model in this study. The DEChNs-Gels exhibited a high efficiency for removing RB19 from aqueous solutions as a result of their nanofibrillar network and excellent pore structure accompanied by the presence of amino groups. Even when the DDA was lowered to 15%, the adsorption capacity reached 940 mg g−1 due to its nanostructural assembly of nanofibers/nanowhiskers, which showed great advantages compared to highly deacetylated chitosan-based adsorbents (DDA > 70%). Considering the issue of environmental protection and adsorption efficiency, DEChNs-Gels have become a potential substitute for chitosan-based adsorbents due to the milder deacetylation process and superior performance, making this material an attractive adsorbent for textile dyes.

Chitin nanocrystals prepared by oxidation of α-chitin using the O 2 /laccase/TEMPO system

Laccase mediator oxidation was applied to chitin at pH 6.8 and 30 °C to prepare chitin nanocrystals with a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). When 40 mM TEMPO and a total of 500 U laccase were added to 1 g chitin, the yield of water-insoluble oxidized chitin was more than 95%, and the carboxylate content was 0.43 mmol/g. Adsorption of laccase molecules on chitin particles occurred in a buffer at pH 6.8, which may have been caused by electrostatic interactions between positively charged C2-ammonium groups of chitin and anionically charged groups of laccase. Rod-like chitin nanocrystals (ChNCs) were obtained with average lengths and widths of 480 ± 200 nm and 24 ± 17 nm, respectively, by sonication of the oxidized chitin/water suspensions. The O2/laccase/TEMPO oxidation caused no decrease in the degree of N-acetylation or the crystallinity of the original chitin based on FTIR and X-ray diffraction data.

Self-assembling oxidized silk fibroin nanofibrils with controllable fractal dimensions

Negatively charged carboxyl groups were introduced onto silk fibroin by using sodium hypochlorite (NaClO) oxidation. The nanofibrillar structure of oxidized silk fibroin (OxSF) was investigated by thioflavin T fluorescence and TEM over time. The fractal dimension (Df) value of 1.21 was obtained in 0.5 wt% OxSF with 2 mM NaClO oxidation (1.09 mM per g protein of carboxyl content), suggesting that the formation of individual nanofibrils was key during aggregation. Moreover, the fractal structure of the resulting nanofibrils was regulated by the charge distribution of OxSF. Df values of 1.68, 1.48 and 1.24 were obtained in 0.2 wt% OxSF with 0.5, 1 and 2 mM NaClO in which the corresponding carboxyl content was 0.58, 0.75 and 1.09 mM per g protein, respectively. Nanoindentation measurements demonstrated reinforced hardness and modulus in OxSF-chitosan composite materials obtained by layer-by-layer deposition, with the highest average values of 1.23 ± 0.06 and 20.01 ± 1.64 GPa, respectively.