Biomedical Engineering Reference
In-Depth Information
a quantum confinement effect occurs, making the bandgap energy dependent on
particle size.
42
Hence, at the nanoscale level, optical properties, such as fluores-
cence excitation and emission, can be “tuned” by altering the size.
42
QDs are significantly brighter and more stable than their corresponding
organic fluorophores (similar emission wavelength). Unlike organic fluoro-
phores, fluorescence of QDs is dependent on size and, therefore, a single light
source can be used for excitation and emission, which is tuned via particle size
to various wavelengths from the UV, visible, and near-mid infrared regions of
the electromagnetic spectrum.
42
Unlike organic fluorophores, QDs have larger
diameters allowing easy addition of targeting groups and small/big molecules
to the surface.
Due to the high surface area of QDs, a large number of atoms are exposed at
the surface.
42
These atoms have molecular orbitals that lack the full complement
of electrons necessary for stability. These “defect” sites can be highly reac-
tive, so another semiconductor with a wider bandgap is grown over the CdSe
core.
49
ZnS is usually used for this purpose serving as the “shell” around the
core because it enhances fluorescence efficiency
42,50
and also reduces cadmium
toxicity. The ZnS shell, shown in
Figure 2.2
, also decreases QD oxidation and
photobleaching while increasing chemical stability.
42
Other shells have also
been used.
43,44,47
Synthesis of QDs has been reviewed by Biju et al.
49
The QD core is mostly
responsible for all the physical attributes such as the absorption and emission
wavelengths. The shell structure protects the QD from losing its physical prop-
erties. After the QD is converted into its water-soluble form the external coating
introduces its biocompatibility. The external coating is also responsible for the
attachment of various molecules on the QD surface. The core is composed of
R
Functional group (-NH2, -
COOH, -SH, -OH, etc)
Polymer coating
QD
R
R
Shell
R
X
Core
Biomolecule
(Antibody, receptor,
DNA)
R
R
FIGURE 2.2
Schematic diagram of core/shell QDs.
(For color version of this figure, the reader is
referred to the online version of this topic)
