*3.3. Characterization*

The ATR-FTIR technique was applied to study the structural variation of pBC before and after polymer coating. As seen in Figure 2b and c, functional groups including –OH (3343 cm−1), C–H (2902 cm<sup>−</sup>1), and –C–O–C– (1109 and 1164 cm−1) can be found in the bare pBC [33–35]. After the polymer coating, two absorption bands at 1609 and 1493 cm−<sup>1</sup> emerged. These corresponded to the stretching vibration of C=N and N–H shearing vibration in benzenoid amine structure [36,37]. Moreover, these two signals became obvious with the increase of polymer amount coating on the nanofibers.

The morphology of the final product was examined by the SEM technique, taking pBC-Polym-0.04 as an example. The morphology of the surface section was shown in Figure 3a that the gap between the nanofibers was filled with polymer. The result was identical for the cross-section of the product (Figure 3b). That is to say, the poly Schiff base uniformly coated on the nanofibers within the body of bacteria cellulose. The TEM images are given in Figure 3c (agglomeration) and 3d (single fiber). As shown, the incorporated thick polymer evolved to sheath-like structure to encapsulate the cellulose nanofibers, which was consistent with the SEM results. For pBC-Polym-0.02, only nanoparticles can be found on the nanofibers (Supplementary Figure S2). On the other hand, pBC-Polym-0.08 showed a similar morphology but a higher density of coating (Supplementary Figure S3). The N2 adsorption-desorption isotherm and porosity are shown in Supplementary Section S4, which indicated that the products were mainly composed of mesopores and macropores. In addition, the product exhibited good thermal stability (Supplementary Section S5), and this was beneficial for its practical applications.

**Figure 3.** The SEM images of pBC-Polym-0.04: surface (**a**) and cross-section (**b**), and the TEM images of pBC-Polym-0.04: agglomerated fibers (**c**) and single fiber (**d**).
