Biomedical Engineering Reference
In-Depth Information
could induce distinct cytotoxicity and in vivo behaviors. The surface modifica-
tions of QDs could also greatly affect its interaction between the cellular mem-
brane and subsequent uptake into the cells. Taken CdSe as an example, a common
surface modification to reduce the cytotoxicity of the core material is coated with
a ZnS shell. In one hand, the additional shell semiconductor layer could increase
the QDs' photoluminescence. In the other hand, the ZnS shell protects the core
CdSe from oxidation and other environmental factors that contribute to cadmium
release. Besides, ligands with terminal carboxylic acid, hydroxyl, or amine groups
have been used as the charged surface coatings for the QDs protection, which could
effectively prevent the core oxidation, cell death, and inflammatory responses.
Another important problem, concerning the body clearance of these nanoparti-
cles, is attracted more and more attention. When employing living mouse for the
in vivo imaging study by QDs injection, Ballou et al. [
42
] found that methoxy-
terminated poly(ethylene glycol) amine QDs (mPEG-QDs) remained for at least
one month in liver, lymph nodes, and bone marrow. Therefore, the use of QDs in
vivo must be critically examined.
Considering these biotoxicities, various new kinds of QDs are emerged in
recent years such as silicon QD [
43
], carbon dot [
44
], graphene QD [
45
]. Owing
to their special cadmium free property, excellent biocompatibility, and environ-
mentally friendliness, these novel nanomaterials gained significant consideration
after being successfully prepared.
4.1.2 ECL of GQDs
As new type of QD, graphene QDs has been widely studied nowadays. In 2008,
Bard's group [
46
] first reported the ECL from electrochemically oxidized highly ori-
ented pyrolytic graphite (HOPG) and from a suspension of graphene oxide platelets,
whose results were presented in Fig.
4.2
. They supposed that the smaller aromatic
hydrocarbon-like domains formed on the “graphitic” layers by interruption of the
conjugation could form the oxidized and emitting centers. In the case of individual
graphene oxide NPs, ECL signal could also be detected by using tri-n-propylamine
(TPrA) as a coreactant at relatively high concentration. At positive potential
of
∼
1.15 V, oxidation of graphene oxide NPs takes place either directly on the work-
ing electrode during collision or via TPrA radical cations. The highly reductive radi-
cal intermediate could be generated by the deprotonation reaction of TPrA, and the
excited state of graphene oxide for the generation of ECL could be formed between
the radiative recombination of this radical and the graphene oxide NP.
Afterward, luminescent property of graphene was studied widely. In 2011, Li
and co-workers [
47
] observed the cathodic ECL response of luminol at a positive
potential with a strong light emission on a graphene-modified glassy carbon elec-
trode. With the utilization of the excellent electrocatalytic properties, graphene
could facilitate the reduction in O
2
in a solution dissolved with trace oxygen,
which was critical on the cathodic ECL behavior of luminol on the GR-CHIT/GC
