Biomedical Engineering Reference
In-Depth Information
neural tissue. In addition, immunolabeling techniques are often performed in fi xed
tissue, while the ability to labeling and monitor neuronal structures in live tissue
would yield more information in dynamic situations. Intracellular dyes, such as
Lucifer Yellow, carbocyanine dyes such as DiI, DiO, and DiD, or neurobiotin, have
been delivered by microinjection as well as ballistic labeling and have successfully
been used to fi ll target neurons. Although this allows for selective labeling in live
neural in vitro preparations; toxicity and photobleaching continue to limit the use-
fulness of these dyes (Mobbs et al.
1994
; Morgan et al.
2005
) . Although QDs have
not yet been used as neuronal tracers, this area of research may yield fruitful results.
The potential to modify size, charge, and other QD surface properties could yield a
new class of neural tracers that are brighter, more mobile, and can interact with
specifi c intracellular constituents to track intracellular neural events.
A current challenge in using QDs for visualizing and monitoring subcellular
structures is delivery of QDs to the cellular organelles of interest. The size of these
QDs precludes diffusion through the plasma membrane, unlike organic dyes, which
are used to label intracellular organelles. Derfus et al. showed that intracellular deliv-
ery of QDs to cells by incubation of QDs conjugated to PEG and peptide transfection
agents as well as using electroporation have successfully resulted in QD delivery
to the cellular cytosol without being captured in endosomes (Derfus et al.
2004a
) .
A drawback to both these methods, however, is that they produce QD aggregates of
up to 500 nm in diameter. Alternatively, microinjected delivery of QDs complexed
with PEG and a nuclear or mitochondrial localization sequence produced specifi c
labeling and the absence of QD aggregates (Derfus et al.
2004a
) .These results indi-
cate that it is possible to deliver single QDs to the cellular cytosol.
In the future, by successful pairing of a chosen QD delivery technique with specifi c
surface-conjugated biomolecules, it may be possible to not only prevent aggregation
of QDs but also to control interaction of QDs with specifi c intracellular neural con-
stituents. By conjugating QD surfaces with suitable proteins or peptides, it is conceiv-
able that QDs can be developed as quantitative neuroactive probes. For example, by
designing QDs that could bind to specifi c synaptic vesicle proteins, it may be possible
to track neural activity with lower toxicity or high background fl uorescence associated
with existing styryl dyes such as FM-143 (Brumback et al.
2004
) .
9.2
Quantum Dots for Bioimaging Brain and Retinal Function
Potential QD imaging techniques may yield exciting future research directions for
studying brain and retinal function in intact, live tissue. QDs have effi cient multipho-
ton excitation cross sections and can emit infrared or near-infrared light and thus they
are compatible with multiphoton imaging techniques used for in vivo imaging of cells
in thick tissue sections (Larson et al.
2003
; Kim et al.
2004
; Ballou et al.
2005
) . In vivo
imaging studies in a variety of biological systems have shown a number of promising
results. Larson et al. intravenously injected water-soluble QDs in the tail vein of mice
and showed that these probes can be used to successfully image skin and adipose
