Biomedical Engineering Reference
In-Depth Information
profile. The high surface area (>900 m2/g), tunable pore diameter and
uniform mesoporous structure of the mesoporous silica NPs offer unique
advantages for loading and releasing large quantities of biomedical agents.
Mesopores loaded with guest molecules were capped by inorganic NPs, or
large organic molecules, via a chemically cleavable disulfide linkage to the
mesoporous NP surface. Since drug molecules are effectively physically
trapped, they are unable to leach out of the mesoporous NP host thus pre-
venting any premature release. Compared with many current biodegradable
polymer-based drug delivery systems, that rely on the hydrolysis-induced
erosion of the carrier structure, the mesoporous NP structure provides the
ability to release the cargo in a controlled manner [172].
5.11 Quantum dots
Quantum dots (QDs) (Figure 2-2) are structurally colloidal semiconductor
nanocrystals, ranging from 2 to 10 nm in diameter. QDs can be synthesized
from various types of semiconductor materials via colloidal synthesis or elec-
trochemistry. The most commonly used QDs are cadmium selenide (CdSe),
cadmium telluride (CdTe), indium phosphide (InP), and indium arsenide
(InAs). These NPs have unique photophysical properties, such that upon
excitation they emit fluorescence that is brighter and more stable than that
of traditional fluorophores, and their size can be varied to achieve excita-
tion and emission at different wavelengths. QDs can be used as probes for
high-resolution molecular imaging of cellular components, for tracking cell
activities and movement inside the body, for specific targeting interaction
through antibodies linked onto NPs surface, with specific tumor-associated
antigens expressed on cancer cell surface. This interaction permits penetra-
tion inside targeted cancer cells of specific drugs, protein, siRNA, genetic
materials, and antisense oligonucleotides and modulates genic expression
into the cancer cells genome. In vivo cancer targeting and imaging in liv-
ing animals by QDs was first demonstrated by Gao et al., [173] wherein
both subcutaneous injection of QD tagged cancer cells (prostate cancer)
and systemic injection of multifunctional QD probes were used to achieve
sensitive and multicolor fluorescence imaging of cancer cells. The utility of
the quantum dot (QD)-aptamer(Apt)-doxorubicin (Dox) conjugate [QD-
Apt(Dox)] as a novel targeted cancer imaging, therapy, and sensing system
has been demonstrated in a recent a study. The targeted QD imaging sys-
tem (QD-Apt) was capable of differential uptake and imaging of prostate
cancer cells that express the prostate specific membrane antigen (PSMA)
[174]. The Tan et al. [175] study group conjugated siRNA targeting the
gene encoding human epidermal growth factor receptor-2 (HEGFR-2) to
QDs by using these last ones not only as carriers, but also as a means to
monitor the transfection efficiency. By directing antibodies against EGFR-2
over-expressed by breast cancer cells, it was possible to induce a selective
interaction of siRNA-QDs conjugates with cancer cells, and a receptor-
