2.2. TEMPO Oxidation of BC

The synthesis of TOCNs via the conventional process (C-TOCNs): 20 g of a bacterial cellulose sheet that had been cut into small pieces was suspended in 500 mL of distilled (DI) water containing dissolved sodium hypochlorite and 20 mg of TEMPO catalyst. The oxidation reaction was maintained at a pH of 10 with 0.5 M NaOH. The mixture was vigorously agitated overnight using a magnetic stirrer at 25 °C. The oxidation reaction was then quenched by adding ethanol to the suspension. The products were washed with DI by centrifugation at 10,000× g several times, until all of the reactants were completely removed. The final product was stored at room temperature for later use.

The synthesis of TOCNs via the proposed one-pot process (O-TOCNs): the same amounts of bacterial cellulose and sodium hypochlorite were used for the chemical treatment of the process-cellulose. Before SiliaCat-TEMPO was used in the oxidation reaction, it was pretreated to activate the catalyst (Figure 1) by stirring a mixture containing SiliaCat-TEMPO (1 eq.), 4 M HCl (6 eq.) in dioxane, and a 0.5 M solution of N-chlorosuccinimide (5 eq.) in dichloromethane (DCM) was stirred for 15 min. The activated SiliaCat TEMPO was then washed with ethanol and dried completely under vacuum to ensure that all of the reactant had evaporated [15]. An oxidation reaction consisting of bacterial cellulose, sodium hypochlorite, and the activated SiliaCat-TEMPO was facilitated by stirring while the pH was maintained at 10.

Schematic showing the structure of the immobilized 2,2,6,6–tetramethyl–1–piperidine–N–oxy radical (TEMPO) on silica beads.

A pure bacterial cellulose aqueous solution was obtained by preparing the aqueous dispersion of cellulose as follows. Ascorbic acid was added to the reacted cellulose solution to neutralize the hypochlorite acid. Then, the SiliaCat-TEMPO was removed by filtering the solution through a vacuum decompression device using a nylon mesh through which only the cellulose nanofiber solution could pass. As a result, the SiliaCat-TEMPO beads remained on the nylon mesh, while the cellulose nanofiber solution passed through to produce a pure nanofiber solution.