▎ 摘　　要

Ultrafine GO nanosheets with the lateral dimension of 20-40 nm are prepared first by a modified Hummers' method to obtain GO microsheets, then by using periodic acid (HIO4) to oxidize the GO microsheets. The ultrafine GO nanosheets are so small that they cannot be separated by centrifugation but by dialysis. However, the HIO4 with strong oxidization renders dialysis tubing easily broken, leading to the failure of purification. A solution is found by the employment of ethylene glycol (EG) to reduce the HIO4 before dialysis. The ultrafine GO nanosheets have the O content of 73.5% with a large edge-to-area ratio, dispersing very well in water. These ultrafine GO with quinoid structure emit fluorescence under acidic condition and the fluorescence can be tuned by changing the pH of the dispersion solution. Green fluorescence with an emission peak centred at 530 nm was given off at pH 1.0. The fluorescence color changes from green to yellow with the emission peak red-shifting to 590 nm when the pH is increased to 6.0. No fluorescence is observed at pH 12.0. A schematic fluorescence of the GO under acidic condition is proposed. This work on one hand presents a method of regulating the fluorescence of GO, on the other hand might be promising for providing guidance to apply GO in the fields of fluorescence immunoassay, biosensors and drug delivery.