Scheme 1 shows the diagram for the synthesis of GQDs for electrochemical detection heavy metal ion. The detailed process will be discussed next. Firstly, expandable graphite was used to synthesize GO by a modified Hummers method [21]. Secondly, periodic acid, based on the relatively strong acidic and oxydic properties, which can quantitatively oxidize the carbon–carbon bonding of vicinal diol to the corresponding structure of aldehyde or ketone [24]. Because the epoxy group and ring-opening structure of GO can be regarded as a vicinal diol structure. Meanwhile, experimental and theoretical studies have shown that the epoxy groups tend to form a line on a carbon lattice of GO during acid oxidization [25], [26] and [27]. Thus, the periodic acid cut the carbon–carbon bond of vicinal diol along the cooperative alignment of epoxy groups and brought on the cutting of GO into nanosheets. After that, the GO nanosheets were reduced by the green approach to GQDs, using l-AA as a reducing agent and PSS as the dispersant. Following, GQDs modified electrode were used to detect Pb2+ solution, it is well known that the defective structure of GQDs and the existence of negative charge of PSS are beneficial for the adsorption of metal ions. In addition, the GQDs with high specific surface area could improve the interaction opportunity of active sites and target ions greatly, which endowed the capability of accumulating metal ions. Under optimal conditions, the GQDs showed ultrahigh sensitivity in the detection of heavy metal ions.