Biomedical Engineering Reference
In-Depth Information
successful synthesis of colloidal CdX (X
=
S, Se, Te) QDs with size-tunable
band-edge absorption and emissions by Murray et al. [
4
]. So far, CdX is the most
investigated QDs due to their excellent optical and electrochemical properties.
However, with the further application in biological area, the toxicity of cadmium
ion in CdX was paid more and more attention. In order to improve the biocom-
patibility as well as the PL quantum yield and stability of these core nanocrystals,
a layer of a few atoms with a higher bandgap semiconductor was introduced to
encapsulate the core nanocrystals to form core-shell nanocrystals. The lumines-
cence efficiency is significantly improved when the nanocrystals are passivated
on their surface by a shell of a larger bandgap semiconductor and the leaching of
metal ions from the core is blocked well by this structure [
5
,
6
]. At the beginning,
CdSe/ZnS and CdSe/CdS are the most intensively studied [
5
,
7
]. Later, more and
more other “core-shell” QDs were developed, such as CdSe/ZnSe [
8
], CdTe/CdS
[
9
], CdTe/ZnS [
10
], and even CdTe/CdS/ZnS “core/shell/shell” QDs [
11
]. Reiss et
al. proposed a simple synthetic route for the preparation of CdSe/ZnSe core/shell
nanocrystals applying zinc stearate as a zinc source. Based on the literature, they
firstly synthesized CdSe core nanocrystals in a mixed TOPO/HAD solvent with a
molar ratio of 60-80 % HAD and using CdO, complexed by dodecylphosphonic
acid, as cadmium precursor. Then, ZnO was complexed with dodecylphosphonic
acid and slowly injected together with TOPSe into a mixture of HAD-/TOPO-
containing CdSe core nanocrystals. After the formation of CdSe/ZnSe QDs,
mercaptocarboxylic acids were introduced to make them water soluble and the
photoluminescence efficiencies in organic solvents as well as in water after func-
tionalization with mercaptoundecanoic acid could reach 60-85 % [
8
]. In our group,
CdSeTe@ZnS-SiO
2
QDs were prepared with ZnS-like clusters filled into the SiO
2
shell via a microwave-assisted approach (shown in Fig.
2.1
). The mercaptopropi-
onic acid (MPA)-capped green-emitting CdSeTe alloy quantum dots were firstly
prepared and purified. After that, the CdSeTe QDs were coated with a silica layer
at room temperature in the presence of Zn
2
+
and glutathione (GSH). Lastly, the
silica-coated QDs were refluxed under microwave irradiation. With the increase in
reaction time, the fluorescence gradually changed from dim green to bright orange
under a 365-nm excitation and quantum yield was enhanced from 11.9 to 56.9 %
with rhodamine 6G as standard before and after the ZnS-SiO
2
coating [
12
].
2.2 New Emerging Quantum Dots
For traditional QDs, cadmium is the main element for their composition. However,
it is well known that leaked cadmium ions are culprits for the observed cytotoxicity
of cadmium-based QDs, which hampers their further applications to cellular or in
vivo study. With the demand for more biocompatible QDs as the signal reporter, the
emphasis has shift toward the fabrication of cadmium-free quantum dots (CFQDs)
for applications in biology, such as silicon QDs (Si QDs), carbon dots (C-dots),
graphene QDs (GQDs), Ag
2
Se, Ag
2
S, InP, CuInS
2
/ZnS. Strictly, some of them are
