Abstract: OBJECTIVE To stain glioma cells by ultrasmall blue nanoparticles which were prepared through crosslinking with genipin.METHODS Various amino compounds including lysine (Lys) were cross-linked by genipin in reversed microemulsion.Color change, particle size and distribution of the products were characterized to screen out the specific amino compound.Next, the factors such as concentration, molar ratio between –NH₂ and genipin, and crosslinking time in preparation were optimized.To investigate the stability, the nanoparticles were placed different pH and temperature solutions.Finally, the nanoparticles were used to stain U87 cells and cytotoxicity to CHO cells was tested.RESULTS Lys-g NPs were the smallest, with the deepest color and good polydispersity.The optimal conditions for Lys-g NPs preparation were 30 mg·mL^-1 of Lys, 1.5∶1 of –NH₂ to genipin, and 2 min of crosslinking time.At the condition, 3.8-6.8 nm of Lys-g NPs were observed under TEM.Lys-g NPs maintained dark blue and stable in different pH solutions (3.0–12.0) and temperatures (4–60 ℃).More importantly, both the cell pellets(10⁵) and the single cells showed visible blue after incubated with the Lys-g NPs.Moreover, the Lys-g NPs were non-toxicity to cells even at very high dosages(≤2.5 mg·mL^-1).CONCLUSION Ultrasmall dark blue, color stable, and non-toxicity Lys-g NPs are prepared successfully and hold great potential for tumor staining in vivo.