*2.2. Preparation of Cellulose Nanofibrils*

Pure cellulose was prepared from rice straw by extracting wax and dissolving lignin, hemicellulose and silica [31]. Cellulose nanofibrils (CNFs) were prepared by TEMPO mediated oxidation employing 5 mmol NaClO per gram of cellulose followed by mechanical blending at 37,000 rpm for 30 min [32]. TEMPO-mediated oxidation converted the C6 primary hydroxyls on the surface of the cellulose into carboxyls [33]. The successive mechanical treatment disintegrated oxidized cellulose fibers into individual nanofibrils that were 1–5 nm in width and hundreds of nanometers to 2 μm in length [32]. The carboxylate content of the CNF was measured using a conductometric titration (Oakton CON 6+, Cole-Parmer Instrument Company, Vernon Hills, IL, USA) following a previous method (Gu and Hsieh, 2015) (Figure S1). The carboxylate (COOH + COO−) content was determined to be 1.36 mmol/g at neutral pH (COOH content: 0.137 mmol/g).
