*3.2. Schi*ff *Base Loading in Bacteria Cellulose*

To allow the precursor coat onto the nanofibers, the bacteria cellulose firstly interacted with acidic aqueous mixture of *m*-phenylenediamine and glutaraldehyde. The pretreated bacteria cellulose immersed into precursor solution as soon as their contact. In contrast, the raw bacteria cellulose took ~30 min to be absolutely wetted by the solution. That should be caused by the pretreatment of bacteria cellulose, which alters its surface properties. After the saturation of precursor, the cellulose was put into the alkaline solution to speed up the polymerization process. Due to the strong interaction between –NH2 and –OH, the poly Schiff base would stably coat onto the nanofibers of bacteria cellulose. The white cellulose became brown after the polymer coating (Figure 1g), which had ultra-low density. The final product was named as pBC-Polym-*x*, where pBC is the pretreated bacteria cellulose and *x* is the concentration of m-phenylenediamine in precursor solution. For comparison, the product without pretreatment was denoted as BC-Polym-*x*.

More importantly, the difference on the wettability of raw and pretreated bacteria cellulose changed the loading mass of poly Schiff base. As shown in Figure 1h, the polymer loading on pretreated bacteria cellulose increased very fast and it took 30 min for saturation. However, for the raw bacteria cellulose, it spent ~120 min for the complete loading. Typically, the content of poly Schiff base on pBC was at least 20% higher than that on raw materials, which can be vividly distinguished by the color of the product (Supplementary Section S2). This should be related to the improvement of the wettability of bacteria cellulose by the pretreatment.
