Table 4.

The GQDs-based optical sensor for lead ion detection.

Type of GQDs	Synthesis Method	Starting Materials	Optical Method	Linear Range	LOD 1 (nM)	References
GQD-DMA	hydrothermal	GO-DMA	Fluorescent probe	0.01–1 nM	0.009	[69]
GQDs/L-Cys	chemical oxidation	carbon black	ECL	100–1000 nM	70	[70]
rGQDs	oxidation/reduction	graphite powder	Fluorescence “turn ON”	9.9–435 nM	0.6	[71]
S-GQDs	hydrothermal	pyrene/1,3,6-trinitropyrene	Fluorescent probe	0.1–140.0 µM	30	[72]
GQDs and AuNPs	purchased	-	FRET	0.05–4 µM	16.7	[73]
GQDs@GSH	pyrolysis	citric acid/glutathione	FRET	2.4–11.5 nM	2.2	[74]
NPS-GQDs	electrochemical oxidation	anthracite coal	Fluorescent probe	1–20 µM	750	[75]
DDTC-GQDs	pyrolysis	citric acid/DDTC	RLS	4.83–48.3 nM	3.86	[76]

¹ where LOD is limit of detection. AuNPs: Au nanoparticles, DDTC: diethyl dithiocarbamate, DMA: 3,9-dithia-6-monoazaundecane, FRET: fluorescence resonance energy transfer, GO: graphene oxide, GSH: glutathione, L-Cys: L-cysteine, NPS- nitrogen, phosphorus and sulfur, RLS: resonance light scattering, S: sulfur.