Biomedical Engineering Reference
In-Depth Information
Heidelberger et al. 2005 ) at rapid timescales (<1 ms) (Burgoyne and Barclay 2002 ;
Heidelberger et al. 2005 ). The interplay between ligand binding to receptor chan-
nels is a dynamic process, with rapid insertion and clearance of receptors from
subcellular synaptic domains occurring on millisecond time scales (Triller and
Choquet 2003 ; Thomas et al. 2005 ). Ligand-receptor binding events produce down-
stream modifi cation of postsynaptic subcellular constituents in highly confi ned cal-
cium ion microdomains, enclosed nanovesicles, and along cytoskeletal transport
elements. Moreover, in neural tissue, the complex connectivity of the different types
of neurons makes it diffi cult to identify and distinguish specifi c synapses for study.
In vivo imaging and drug targeting of brain structures present an additional chal-
lenge of bypassing the blood-brain barrier (BBB). Thus, highly sensitive, rapid,
cell-accessible and cell-specifi c means are needed to detect and target chemical
events occurring in nanometer-sized subcellular structures.
3
Historical Development of Quantum Dots
Nanocrystals, nanoparticles with crystalline cores, can be synthesized with precise
shape and sizes (1-10 nm) using wet-chemical methods. As a general rule, the opti-
cal, electrical, and magnetic properties of these nanocrystals are largely determined
by their shape and size and can thus be tailored by their dimensions (Vanmaekelbergh
and Liljeroth 2005 ). Presently, technological interest in nanoparticles stems from
the powerful implication that novel materials with distinct physical properties can
be created by controlling the size and shape of nanoparticles during synthesis. Thus,
nanocrystals can serve as versatile building blocks for designing nanoscale devices
with desired physical properties.
Colloidal nanocrystals can be synthesized from noble metals, transition metals,
and many semiconductor compounds (Murray et al. 2000 ). QDs are single spherical
nanocrystals (1-100 nm) that are made from semiconductor materials. Semiconductor
QD research was originally motivated by understanding electronic structure on a
molecular scale and the use of these materials to develop new opto-electronic solid
state devices. Recently, it has been realized that the size and unique optical proper-
ties of QDs also have useful implications in biology and medicine. In 1998, initial
studies using QDs for biological labeling were reported (Bruchez et al. 1998 ; Chan
and Nie 1998 ) and since then have generated an exponential growth in the develop-
ment of QD-based biomedical applications (Pinaud et al. 2006 ) .
4
Physical Properties of Quantum Dots
The unique size and semiconductor composition of QDs confer upon them unusual
photophysical properties. The size of a QD is comparable to that of the bulk Bohr
radius (~56 Ä for CdSe). This results in confi ned movement of free charge carriers,
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