Biomedical Engineering Reference
In-Depth Information
graphene sheets, respectively. Kang et al. developed an alkali-assisted electro-
chemical method to prepare C-dots with graphite rods as both anode and cathode
and NaOH/EtOH as electrolyte. The judicious cutting of a graphite honeycomb
layer into ultrasmall particles leads to tiny fragments of graphite, producing
C-dots (Fig.
2.2
a) [
22
]. Pan et al. prepared GQDs with bright blue photolumines-
cence by hydrothermal cutting of oxidized graphene sheets. The starting material,
micrometer-sized ripped graphene sheets obtained by thermal reduction of gra-
phene oxide sheets, was first oxidized in concentrated H
2
SO
4
and HNO
3
. After
that, the hydrothermal treatment of the oxidized graphene sheets at 200 °C was
performed, resulting in the dramatical decrease in the size of graphene sheets.
Ultrafine GQDs were isolated by a dialysis process (Fig.
2.2
b) [
36
]. Bottom-up
approaches typically refer to solution chemistry methods during which C-dots and
GQDs are formed from molecular precursors. For example, Li et al. reported a
solution-chemistry-based approach to large, stable colloidal GQDs with uniform
size and shape based on oxidative condensation reactions. The oxidation of poly-
phenylene dendritic precursors led to fused graphene moieties, which were then
stabilized by multiple 2′,4′,6′-trialkyl phenyl groups. The crowdedness on the
edges of the graphene cores twists the substituted phenyl groups from the plane
of the core, leading to alkyl chains closing the latter in all three dimensions and
reducing face-to-face interaction between the graphenes to produce large, stable
GQDs [
97
]. Puvvada et al. [
98
] prepared water-soluble C-dots through microwave-
assisted pyrolysis of an aqueous solution of dextrin in the presence of sulfuric
Fig. 2.2 a
Schematic diagram of electrochemical fabrication of C-dots. Reproduced with per-
mission from Ref. [
22
]. Copyright 2012, Royal Society of Chemistry.
b
Mechanism for the
hydrothermal cutting of oxidized graphene sheets into GQDs. A mixed epoxy chain composed
of epoxy and carbonyl pair groups (
left
) is converted into a complete cut (
right
) under the hydro-
thermal treatment. Reproduced with permission from Ref. [
36
]. Copyright 2010, Wiley
