Biomedical Engineering Reference
In-Depth Information
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luminescence peak in suspension
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Wavelength (nm)
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Fig. 17.8.
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signal from porous silicon NPs in water (relative to background):
insert shows the ultrastructure of these NPs
For Case A, a variety of NPs can be used that either encapsulate many pho-
tosensitizer molecules per NP or have many attached to the surface in order
to deliver a high payload of photosensitizer to the target cells/tissues. There
are several demonstrations in the literature of this approach. One advantage
that PDT has over, say chemotherapeutic drugs, is that generally the toxicity
of most photosensitizers is very low in the absence of light activation, so that
there is a higher tolerance for nontarget “leakage” in the delivery system.
For Case B, Fig. 17.8 shows an example where singlet oxygen is generated
upon light irradiation of porous silicon NPs, without any added molecular
photosensitizer. This is believed to be due to direct energy transfer to the
molecular oxygen in the liquid that permeates the NPs, which have an ex-
tremely large surface-to-mass ratio. It has not yet been shown that this can
actually be exploited to produce PDT cell killing: these studies are in progress.
Potential barriers are getting the NPs into the cells and having the
1
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2
dif-
fuse from the (inner) NP surface to the biological target. (Note that in the
case of NPs as photosensitizer delivery vehicles, it may not be necessary for
the NPs themselves to penetrate the cell wall if the payload can be released
within the target tissue, such as in the interstitial space, from which it would
then be taken up by the cells.) In the third scenario, Case C, Qdots are used
as the primary light absorber. The energy is then transferred to a photosen-
sitizer molecule that is conjugated to the Qdot, activating it to the S
1
state,
as in standard PDT. The generation of
1
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via Forster Resonant Energy
Transfer has been demonstrated in solution and should work in cells/tissues.
