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to other nanostructures that have unique optical properties, they could play an
important role in this new class of therapeutics.
As previously discussed, SWCNTs intrinsically emit in the NIR region [53].
Most biological material is fairly transparent in this spectral range, and very little
endogenous fluorescence is generated in the tissue. In addition, emission from
SWCNTs can penetrate though relatively thick tissue. Furthermore, CNTs do not
blink and photo bleach. The first example of the use of nanotubes as imaging
agents involved surfactant encapsulated SWCNTs that were internalized by live
phagocytic cells [54]. The samples were excited at 660 nm and emission from the
CNTs was detected at 1125-1600 nm, producing NIR images of SWCNT within
the cells. More recently, a similar study with 3T3 fibroblast and myoblast stem
cells used SWCNTs functionalized with DNA; the Raman and fluorescence
spectra inside the cells was monitored for a period of three months. The group
demonstrated unprecedented stability of CNTs as fluorescent probes; spectral
changes in the CNT emission over the period of incubation were also noticed.
The observed behavior changes were attributed to changes in the local environ-
ment surrounding the nanotubes. Thus, it may even be possible to use nanotubes
for real-time, in vivo imaging of changes taking place inside of cells.
In addition to their intrinsic fluorescent properties, it is also possible to
fluorescently label CNTs such that they emit in the visible spectrum and can
therefore be imaged through more conventional fluorescence microscopy. Many
of the CNT-based drug and gene delivery applications mentioned above have
specifically introduced fluorophores onto the CNTs in order to determine their
internalization through confocal fluorescence microscopy [55]. Also, conventional
fluorophores have been found to associate nonspecifically with MWCNTs
through hydrophobic interactions; this has allowed for the fluorescent imaging
of MWCNTs. Fluorescent polymers have been used as a noncovalent means of
introducing fluorescence onto CNTs as well. Quantum dots, another emerging
nanomaterial with promising applications in biomedical applications, have been
conjugated to CNTs and have even been grown from functional groups attached
to their surfaces. Although devices comprised of nanotubes and quantum dots
have been developed primarily for electronic and photonic applications in mind,
new work is focused on using such devices as intracellular fluorescent probes [56].
18.4.4. Biosensors for New Diagnostics
While the future of medical treatment lies in targeted therapeutics, the future of
clinical diagnosis is likely to involve a shift to early detection of disease. Today,
when a patient observes symptoms of a possible disease, she visits the doctor, and
a sample of body fluid is taken and sent out for testing. The results are generally
not available for days. Further, because existing tests are largely based on systemic
physiological changes rather than local or cellular changes, disease at an early
stage often goes undetected. The development of new, minute and ultrasensitive
biosensors could lead to personalized diagnostic kits that allow patients to
determine their state of health from the comfort of their own homes. By placing
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