Biomedical Engineering Reference
In-Depth Information
vins, and the like), which have absorption bands in the near ultraviolet or
blue and characteristic spectra in the visible region.
2. Fluorophores synthesized in the tissue after external administration of a
precursor molecule, such as protoporphyrin IX induced by 5-aminolevulin-
ic acid (ALA).
3. Fluorophores administered as exogenous drugs, including fl uorescein, in-
docyanin green (ICG), and photosensitizers such as hematoporphyrin de-
rivative (HpD) and tetra(m-hydroxyphenyl)chlorin (mTHPC).
These materials have been used in clinical studies of fluorescence diagnostics.
The exogenous fluorophores may also be subdivided according to whether or not
a delivery vehicle is used to target them to specific tissue/cell compartments (e.g.,
coupling to monoclonal antibodies or encapsulation into liposomes). Exogenous
fluorescent markers with a high quantum yield can also be used to achieve a stron-
ger extrinsic fluorescence contrast. In this case, information is obtained from the
change of the fluorescence properties of the marker during interaction with the
different tissue components, or by selective localization of the marker in certain
tissue constituents.
There has been increasing interest in fluorescence-based techniques with the
discovery of colloidal semiconductor nanocrystals called quantum dots (QDs).
QDs are generally composed of atoms from groups II-VI or III-V of the peri-
odic table, and an example is zinc sulfide-capped cadmium selenide (CdSe-ZnS).
Their size range of 2-10 nm or 10-50 atoms, often referred to as a size less than
the Bohr radius in physics, leads to a quantum confinement effect dictated by the
rules of quantum mechanics. This effect endows QDs with unique optical and elec-
tronic properties such as: exceptional photochemical stability, high photobleach-
ing threshold, continuous absorption profiles, readily tunable emission properties
(from the UV to the IR) allowing simultaneous excitation of several particle sizes at
a single wavelength, and size-tunable narrow spectral line widths. For example, in
comparison with commonly used organic fluorophore rhodamine, QD luminescent
label is 20 times brighter (high luminescence), 100 times more stable against pho-
tobleaching, and one-third as wide in spectral line width. In addition, large surface
area-to-volume ratio QDs makes them appealing for the design of targeted molecu-
lar probes. Such fluorophores allow disease demarcation by fluorescence imaging
and, in some cases, can also be exploited for photodynamic therapy. However, side
effects of extrinsic fluorophores on the processes studied and toxicity issues make
autofluorescence microscopy more desirable.
8.5.7.1 Targeted Imaging Using QDs
To minimize side effects, targeted molecular probes are very useful for diagnos-
tic purposes or as a drug delivery system, and antigen/antibody immunoassays.
Specificity in targeting could be obtained by conjugating the surface with small
molecules, such as receptor ligands or enzyme substrates (described in Chapter
7), or higher-molecular-weight
affinity ligands, such as monoclonal antibodies or
recombinant
proteins. Targeting specific tissues is possible by tagging specific pep-
tides to QDs and imaging the tissue. Although producing stable QD-biomolecule
