Page 444 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

424 Potential Impacts of Nanomaterials

1.9 nm in diameter, gold cleared the blood more slowly than iodine
agents did. Retention in the liver and spleen was low with elimination
by the kidneys. With 10 mg/ml of gold initially in the blood, mouse
behavior was unremarkable and neither blood plasma analytes nor
organ histology revealed any evidence of toxicity 11 days and 30 days
after injection (de la Fuente et al., 2005).
Globally, any high level of toxicity associated with the gold nanopar-
ticles is expected because gold as well as silver colloids have a history
in medicines and natural therapeutics. Nevertheless, the smaller size
merits investigation, as do the ligand shells that can be combined with
the metal core. Table 11.2 lists a number of reports on metal nanoma-
terials toxicity.

Quantum Dots
Quantum dots (QD) have widespread potential applications in biology
and medicine, including semiconductor nanocrystals used as fluorescent
probes in ultrasensitive bioassays, biological staining, and diagnostics.
QD are ideal for fluorescent biolabeling because they have long-term
photostability, high brightness, and they can be labeled with several
colors for multi-target studies. QD can be synthesized with different
core semiconductor materials, including cadmium selenide (CdSe), cad-
mium telluride, indium phosphide, or indium arsenide, however, the
latter three have not been shown to be useful conjugates for biological
purposes. CdSe core QD has been used in biological labeling due to their
bright fluorescence, narrow emission, broad UV excitation, and high
photostability (Bruchez et al., 1998; Jovin, 2003; Watson et al., 2003).
Compared to organic dyes such as rhodamine, they can be 20 times as
bright and 100 times more stable against photobleaching. When QD are
embedded in biological fluids and tissues, their excitation wavelengths
can be compromised, so their excitation and wavelength should be
selected based on their specific application (Lim et al., 2003). Customized
QD may have multiple layers with one or more surface coatings. The core
can consist of CdSe with a shell or “cap,” such as ZnS, that will reduce
leaching of the toxic core metals (Derfus et al., 2004). The unique prop-
erties of QD have shown promise for numerous biological applications for
detection and imaging, however, their toxicology in humans is not known.
There are numerous reports of QD cytotoxicity in the literature, some
being conducted by the laboratories that synthesize QD for customized
applications. Interpretation of these studies make it difficult because of
the heterogeneity of these QD preparations from different laboratories
(core composition, coatings, size, etc.), the use of different cell lines, and
a variety of endpoints of cytotoxicity. QD can be purchased commer-
cially, but their toxicity in cells is unknown.