All chemicals and reagents used for experiments and analysis were analytical grade. Graphite oxide was synthesized from expandable graphite by a modified Hummers method [21]. Then graphite oxide (30 mg) was dispersed into deionized water (10 mL) followed by sonication for more than 3 h to obtain brown GO solution. Later, 15 g of periodic acid (H5IO6) were added into 10 mL of GO dispersion (concentration is 3 mg/mL) and the mixed solution was kept at 60 °C for 24 h. The precipitate was centrifuged out from solution at 15,000 rpm for 30 min, and then washed with deionized water until the supernatant was neutral. After that, PSS (0.1 g) was added into the above GO nanosheets solution (concentration is 1 mg/mL) and sonicated for 2 h. Finally, l-AA (250 mg) was added into the aqueous solution and stirred at 50 °C for 24 h. When the reaction ended, the color of the solution turned from yellow to dark black, which was a visual indication that the GO nanosheets were successfully reduced into GQDs. The resulting black suspension was filtered through a 0.22-μm microporous membrane. The colloidal solution still contained some large graphene nanoparticles (50–200 nm) that emitted weak blue fluorescence. So, the colloidal solution was further dialyzed in a dialysis bag (retained molecular weight: 3500 Da) overnight and GQDs that were strongly fluorescent through the bag were obtained. And the obtained GQDs solution was applied in the detection of heavy metal ions.