Partially deacetylated α-chitin nanofibers/nanowhiskers mixtures (DEChNs) and TEMPO-oxidized α-chitin nanowhiskers (TOChNs) that had positive and negative charges, respectively, were transformed into hydrogels with mass concentrations of 0.2, 0.4, 0.6, 0.8, and 1.0% under ammonium hydroxide or hydrochloric acid "gas phase coagulation". To the best of our knowledge, 0.2% is the lowest mass content reported for the successful preparation of physical self-standing hydrogels based on chitin nanofibers/nanowhiskers. The even and uniform coagulation under "gas phase" is one of the key aspects of preparing hydrogels with quite low mass content. The storage modulus achieved the highest value of 8.35 and 3.73 KPa for DEChN and TOChN hydrogels, respectively, at the mass concentration of 1.0%, and these are known to be the highest values reported in the literature for hydrogels at the same mass concentration of chitin nanofibers/nanowhiskers. The equilibrium swelling ratio (ESR) of both DEChN and TOChN hydrogels decreased with increasing mass content at neutral pH. As the pH increased from 2 to 10, the swelling degree of DEChN hydrogels decreased from 268 to 130, whereas the swelling degree of TOChN hydrogels increased from 128 to 242. Additionally, due to the electrostatic attraction between the hydrogels and dyes, DEChN hydrogels had significant adsorption of Reactive Blue 19, while TOChN hydrogels had effective adsorption of Basic Green 4. The different pH-dependent swelling behavior and adsorption affinity of the DEChN and TOChN hydrogels were related to their designed opposite surface charges corresponding to the surface amino groups on the DEChNs and carboxyl groups on the TOChNs.

The 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/laccase/O 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 O, 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 of C6-carboxylate groups onto wood cellulose. The TEMPO/laccase/O 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, 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.

Different from single charged or uncharged nanocrystals, amphoteric chitin nanocrystals (A-ChNCs) with both amine and carboxylate groups prepared with 2,2,6,6-tetramethylpiperidine-1-oxy-radical (TEMPO)-mediated oxidation and partial deacetylation were individually nanodispersed by sonicating in water at pH 3 and pH 11. The effects of the amount of NaClO2 added in TEMPO-oxidation, deacetylation time, and sequence of the two treatments on the weight recovery ratios of the A-ChNCs were investigated. The A-ChNCs prepared under optimum conditions had an average length of ∼544 nm and an average width of ∼10 nm. The A-ChNCs nanodispersed in water at pH 3 and pH 11 had absolute ζ-potentials of >30 mV; however, in neutral water, they formed aggregations, which were nanodispersed again when pH was adjusted to 3 or 11, showing pH sensitivity. Hydrogels of A-ChNC were prepared by adding saturated NaCl solution and adsorbed both anionic and cationic dyes. Freeze-dried A-ChNC aerogels had three-dimensional network structures containing abundant pores.

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Aryl-aryl bond formation is one of the most important tools in modern organic synthesis. Therefore, there is a high level of interest to develop green, effective reaction system to obtain biaryls. This review summarized the recent advances in the metal-catalyzed Ullmann reaction in which the aryl-aryl bond was formed directly. Furthermore, different types of catalytic mechanisms, especially the surface reaction, have been summarized to help the design of the catalyst.