Self-assembly of CNCs and Bacteria
Basing on the interactions between bacteria and the CNCs, we achieve self-assembly in aqueous solution driven by multivalent interactions especially the multiple hydrogen bonds.
Theory and Method
There are abundant binding sites on the surface of bacteria for CNCs. The lipopolysaccharides, form the outer cell membrane of gram-negative bacteria, have plentiful hydroxyl groups to bind with the same functional groups of CNCs by multiple hydrogen bond interactions, while the Van der Waals participate a significant part in polymers as well. As for gram-positive bacteria, the peptidoglycans play an analogous role in the interactions with CNCs. Basing on the properties of the surface of bacteria, we achieve self-assembly between the bacteria and the CNCs driven by multivalent interactions especially the multiple hydrogen bonds.
Bacteria Interaction Assays
E.coli and Streptomycete were separately cultured until an OD600 of 0.5 was attained. The grown bacteria were centrifuged at 12000 rpm for 2 min, and the precipitation was washed in PBS buffer for three times. To E.coli(OD600 = 0.5 A, 1 mL) in PBS, CNC(0.25 mg/mL, 1 mL) was added in a 6-well plate and incubated in an incubator shaker (250 rpm) at 37 °C for 2 h.[]
Generation of the CNCs
Cellulose is the staple food of termites and it has many specific advantages, so we prepare CNC according to the feature of cellulose’s structure. The most used way of producing CNC is TEMPO-mediated oxidation[]. However, we decided to use the way of acidolysis because of its low cost and convenience. In this way, we finally obtained our product of CNC.
Structure of cellulose
Cellulose, is a linear chain of (1-4)-β-D-glucopyranana. Besides the β-(1-4)-glucosidic bond, the intrachain hydrogen bonding between hydroxyl groups and oxygens of the adjoining ring molecules also strengthen the linkage and stabilize the linear structure of the cellulose chain.
In the parallel stacking of multiple cellulose chains, the Van der Waals and intermolecular hydrogen bonds play a significant role, which promote the forming of elementary fibrils that further aggregate into larger microfibrils (5-50 nm in diameter and several microns in length)[].
The multivalent interactions stabilize the cellulose fibrils. Because of the existence of side chains, within these cellulose fibrils there are divided into two kinds of regions where one kind is arranged in a highly ordered (crystalline) structure, and the other kind is disordered (amorphous-like). After acid hydrolysis dissolved the amorphous-like regions, we will get cellulose nanocrystals (CNCs) from the crystalline regions[].
Acid hydrolysis of cellulose
α-cellulose(25 μm，Aladdin) was hydrolyzed at 40 °C with 8.75 mL of 50 wt % sulfuric acid/g of cellulose. To find the most proper time of acidolysis, We carry on a gradient experiment, set up four groups of experiments whose hydrolyzed time are 1h, 2h, 3h and 4h, respectively. The hydrolysis was quenched by diluting 10-fold with cold DI water. []The crystals were collected and washed once by centrifugation for 10 min at 9000 rpm and final solution was collected by centrifugation for 10 min at 5000 rpm. The solution was dialyzed in dialysis bag against ultrapure water until the pH was neutral.
Freeze-drying to get final product
Freeze-drying to get the solid CNC and solve it at 0.25 mg/mL. Crystal aggregates were disrupted by sonicating the suspension for 24 min under ice-bath cooling with a Vibracell ultrasonic processor.
