▎ 摘　　要

NOVELTY - Nitrogen-doped graphene quantum dot comprises graphene nanosheet layer having carbon atoms attached with hydroxy group, alkylamino group and carboxy group, and 75-85 mole% carbon, 24-12 mole% oxygen and 1-3 mole% nitrogen. The average diameter of the quantum dot is 1.5-5 nm, preferably 3 nm and average thickness is 0.5-1 nm. The alkylamino group is dimethylamino or diethylamino. USE - The quantum dot is useful for fluorescence imaging and two-photon imaging of living cells (Hela cells) (all claimed), and as two-photon fluorescence probe. ADVANTAGE - The quantum dot has X-ray photoelectron energy spectrum and infrared absorption spectrum (claimed), broad application prospects, excellent photostability of up to 24 months, two-photon fluorescent imaging effect over a wide pH range, and two-photon absorption cross section surface of up to 48000 GM; and is easy to dissolve in water and non-toxic. The quantum dot preparation method is simple. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for: (1) producing nitrogen-doped graphene quantum dot, comprising oxidizing graphite sheet as starting material, ultrasonic stripping, solvothermal nitrogen doping and purifying; and (2) fluorescence imaging of Hela cells, comprising (a) co-culturing Hela cells and quantum dot (with concentration of 80-120 mu g/ml culture medium, preferably 100 mu g/ml culture medium) in culture medium for 2-12 hours, and (b) observing the cultured cells under two-photon microscopy at an excitation wavelength of 800 nm.