Sodium nitrate

Graphene oxide (GO) was prepared by the modified two steps Hummers’ method^{49 (link),50 (link)}. In a typical synthesis protocol, 4.0 g of 200 mesh natural graphite oxide flakes (Alfa Aesar), 48 mL of concentrated sulfuric acid (Beijing Chemical Works), 8.0 g of potassium permanganate (Shanghai Aladdin Biochemical Technology Co., Ltd), and 8.0 g of phosphorus pentoxide (Beijing Chemical Works) were added in turn to synthesize pre-oxidized GO, which was then washed by deionized water till the vacuum filtrate becomes neutral. Then, 3.0 g of the pre-oxidized GO was dried in an oven at 60 °C for 12 h and was further oxidized with 3.0 g sodium nitrate (Beijing Chemical Works) and 18.2 g potassium permanganate (Shanghai Aladdin Biochemical Technology Co., Ltd.) in an ice bath. The mixed solution was then transferred to a 35 °C water bath for 2 h. After that, the solution was diluted with 300 mL of deionized water, followed by adding 10 mL of 30% hydrogen peroxide (Shanghai Aladdin Biochemical Technology Co., Ltd.) to completely react the excess potassium permanganate. The mixture was purified in dialysis tubes for a week, and followed by centrifugation at 8000 r/min twice to remove the liquid supernatant. The retained sticky GO dispersion was diluted to a suspension with 0.06 g/L GO.

Graphite powder (99.95%, Shanghai Aladdin Reagent Co., Ltd., Shanghai, China); hydrochloric acid, sulfuric acid, sodium nitrate, and potassium permanganate (30%, 98%, 99%, 99%, Beijing Chemical Plant, Beijing, China); potassium ferricyanide, urea, ethanol, DA, and tetrachloroauric(III) acid trihydrate (99.5%, 99%, 99.5%, 98%, 99.9%, Beijing Innochem Technology Co., Ltd.,Beijing, China); potassium chloride and uric acid (99.5%, 99%, Beijing Oka Biotechnology Co., Ltd., Beijing, China); aptamers (Shanghai Sangon Biotech Bioengineering Co., Ltd., Shanghai, China); and 0.1 M phosphate buffered solution (PBS), which was self-made, were all used in this study.