1：Experimental Design and Method Selection:

GQDs were synthesized using graphene oxide (GO) as a precursor via thermal treatment at high temperature. The synthesis involved the use of concentrated HNO3 and H2SO4, heating and refluxing at 120°C for 24 h, followed by pH adjustment, centrifugation, and dialysis.

2：Sample Selection and Data Sources:

Graphite flakes were used to prepare GO, which was then converted to GQDs. Metal perchlorates were used for the detection of heavy metal ions.

3：List of Experimental Equipment and Materials:

Equipment included a Varian (Cary 500) UV-Vis-NIR spectrophotometer, JEOL, JEM 2100 microscope for TEM images, Edinburgh (F900) spectrofluorometer, and Perkin Elmer (model G-FTIR) spectrometer. Materials included graphite flakes, H2SO4, HCl, and metal perchlorates.

4：Experimental Procedures and Operational Workflow:

The synthesis process involved mixing GO with acids, heating, sonication, pH adjustment, centrifugation, and dialysis. For detection, emission spectra were recorded for aqueous GQDs upon addition of various metal ions.

5：Data Analysis Methods:

The formation of GQDs was confirmed by XRD, FT-IR, Raman spectroscopy, and TEM. The optical characteristics were studied by UV-Visible and Fluorescence spectroscopy.