Natural graphite flake

GO sheets were obtained from natural graphite flakes (99%, Aldrich) using the modified Hummers and Offman method. 50 mg of GO was dispersed in 50 mL of deionized water via sonication treatment for 60 min at room temperature, and resulting GO sheets were stripped to be single sheet structure, affording a brown liquid, which contributed to AgNPs adsorption on the surface of GO sheet. Then, 0.01, 0.1, or 1 mmol of a AgNO₃ solution was added to the graphene solution, followed by NaBH₄ as the reducing agent, and the resulting mixture was stirred at a speed of 1500 rpm at 25 °C for 1 h, producing Ag-GO nanoparticles. This procedure was performed in a photo area, which can prevent the decomposition of AgNO₃. Then, 10 mL of a solution of ascorbic acid, which is an innocuous green reducing agent, was introduced to the solution of silver-modified GO. In order to enhance the reduction effect, the reaction was stirred at a speed of 1500 rpm at 95 °C for 1 h. In order to remove impurities from the solution, it was washed repeatedly using deionized water, and then filtered. Ag-rGO powders were obtained after drying in a vacuum oven at 25 °C for 24 h.

Tens of grams of CMG were reproducibly and safely prepared using a modified Tour et al.26 (link) synthesis in a custom-built rig designed to manipulate up to 10 l of concentrated acids. In a typical synthesis, a 9:1 mixture of concentrated H₂SO₄/H₃PO₄ (3:0.3 l) was added to 24 g of natural graphite flakes (150–500 μm sieved, Aldrich), followed by the addition of 144 g of KMnO₄ (6 wt. equiv.). This reaction was slightly exothermic and the temperature rose to 35–40 °C. The reacting suspension was then heated to 50 °C and vigorously stirred at 400 rpm for 18 h. Next, it was cooled to room temperature and the oxidation was stopped by adding dropwise 1.72 l of aqueous H₂O₂ (2 wt.%). The graphene oxide suspension was washed using repeated centrifugation at 9,000 rpm (Thermo Scientific Sorvall LYNX 6000 Superspeed Centrifuge) and redispersion in double-distilled water. The work-up was carried out until the supernatant water of the centrifuged CMG was close to pH 6, typically occurring after 16 washing cycles. Low speed (<1,000 r.p.m.) centrifugation cycles were performed to remove any un-exfoliated graphite particles.

N-isopropylacrylamide (NIPAm), N,N’-methylenebisacrylamide (BIS), ammonium persulfate (APS), and N,N,N’,N’-tetramethylethylenediamine (TEMED), dopamine (DA), uric acid (UA), and ascorbic acid (AA) were purchased from J&K Scientific Ltd. (Beijing, China). Carboxymethyl chitosan (CMC, degree of substitution greater than 80%), hydrogen peroxide (H₂O₂, 30% aqueous solution), disodium hydrogen phosphate (Na₂HPO₄), and sodium dihydrogen phosphate (NaH₂PO₄) were obtained from Beijing Chemicals Co., Ltd. (Beijing, China). Natural graphite flakes (99.8% purity) were purchased from Sigma-Aldrich (Milwaukee, WI, USA).