Preparation of rGO–COOH

GO was synthesized by modified Hummers’ method using graphitic powders.57 Firstly, 0.5 g graphitic powder and 25 mL concentrated H₂SO₄ were mixed under ice bath and stirred for 2 hours. Then, 2.5 g KMnO₄ was added, heated to 35°C for 4 hours. Next, with the addition of 50 mL water, the reaction was maintained for 40 minutes at 98°C. Further, 150 mL water was added into the system to dilute it, and 30% H₂O₂ was added to stop the reaction. Finally, the mixture was filtered through 0.45 µm filter membrane and washed repeatedly until the pH of the filtrate was within the range of 5–6. The filter was redispersed into water and stored until further use. Next, GO-COOH was obtained as described previously.58 Briefly, NaOH (1.4 g) and Cl–CH₂–COOH (1 g) were added to GO (2 mg/mL, 10 mL) suspension and sonicated for 2 hours at 80°C. The reaction can convert –OH on GO to –COOH via conjugation of acetic acid moieties and can partially reduce GO into rGO. The resulting rGO–COOH solution was quenched with HCl (20%), washed until neutral pH was obtained, and purified by repeated centrifugation (4,000 rpm, 20 minutes).