Biomedical Engineering Reference
In-Depth Information
synthesis of hydrophilic QDs. Gao et al. prepared water-soluble CdTe QDs
through the reaction between Cd
2
+
and NaHTe, with TGA as the stabilizer. They
controlled the ratio of Cd
2
+
, NaHTe, and TGA and adjusted the pH value to 4.5-
5.0. The combination between thiol group of TGA and Cd
2
+
promoted not only
the hydrophilic stability but also the photoluminescent quantum yield [
84
,
85
].
Besides, microwave-assisted green synthesis has been popular for the preparation
of QDs. In comparison with conventional thermal techniques, microwave dielec-
tric hearting has a few merits, such as fast heating and 1-2 orders of magnitude
increase in the kinetics of the reaction rate. Specifically, microwave dielectric heat-
ing could realize the rapid and homogeneous growth of nanocrystals, which is
extraordinarily beneficial for preparing high-quality NCs. With the help of micro-
wave irradiation, CdTe, CdTe/ZnS, and CdTe/CdS/ZnS nanocrystals with high
photoluminescent quantum yield and excellent photostability were synthesized
successfully by He et al. [
86
-
88
].
2.3.2 Cadmium-Free Quantum Dots
Compared with cadmium-based QDs, available protocols for the synthesis of Si
QDs are limited. As a whole, strategies for the preparation of Si QDs are gener-
ally composed of solution-phase-based methods [
89
-
91
], microemulsion synthe-
sis [
92
], thermally induced disproportionation of solid hydrogen silsesquioxane
in a reducing atmosphere, and so on [
93
]. Swihart's group successfully prepared
water-dispersible Si QDs with blue, green, and yellow photoluminescence by the
functionalization with acrylic acid in the presence of HF. However, as illustrated,
these Si QDs still cannot satisfy the colloidal and spectral stability in biological
environments [
94
]. Then, they further proposed a method for the preparation of
water-dispersible and biocompatible Si QDs using phospholipid micelles. They
were prepared by laser-driven pyrolysis of silane, followed by HF-HNO
3
etching,
and the obtained Si QDs were dispersible in chloroform because of the surface
functionalization of styrene, octadecene, or ethyl undecylenate. For water solubil-
ity, phospholipid micelles were then introduced and a hydrophilic shell with PEG
groups was formed on the surface of Si QDs. Such micelle-encapsulated Si QDs
displayed good application as biological luminescent probe in in vitro cell labeling
[
95
]. Kauzlarich proposed a microwave-assisted reaction to produce hydrogen-
terminated Si QDs. Two different methods were developed for the water-soluble Si
QDs: hydrosilylation produced 3-aminopropenyl-terminated Si QDs, and a modi-
fied Stober process produced silica-encapsulated Si QDs. Both of them exhibited a
maximum emission at 414 nm with intrinsic fluorescence quantum yield efficien-
cies of 15 and 23 %, respectively [
96
].
Synthetic methods for C-dots and GQDs are generally classified into two cat-
egories: top-down method and bottom-up method. Top-down methods com-
monly make use of laser ablation and electrochemical oxidation, where C-dots
and GQDs are formed or “broken off” from a larger carbon structure and larger
