Page 451 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

428 Potential Impacts of Nanomaterials

Depending upon the applications, hydrophilic coatings may be added
to increase the solubility for biological media or conjugated to macro-
molecules for drug delivery and diagnostics (Michalet et al., 2005). These
nanometer sized conjugates are water soluble and biocompatible. When
QD are prepared in organic solvents, they are not compatible for bio-
logical applications. By using mercaptoacetic acid for solubilization and
covalent protein attachment, they become water-soluble. This allows for
free carboxyl groups to be available for covalent coupling to biomolecules
such as proteins, peptides, and nucleic acids. When labeled with the pro-
tein transferrin, they undergo a receptor-mediated endocytosis in cul-
tured HeLa cells, and when conjugated to immunomolecules, they can
recognize specific antibodies or antigens (Chan and Nie, 1998). Other
studies have shown that CdSe-core/ZnS-shell QD conjugated to a tar-
geting peptide could be internalized by HeLa cells and tracked in live
cells for more than ten days with no signs of toxicity (Jaiswal et al.,
2003). This study suggested that QD were nontoxic at doses suitable for
long-term imaging studies and as vectors for drug delivery.
The effect of two different dosages of PEG-silane-QD (8 and 80 nM)
on phenotypic and whole genome expression of exposed human fibrob-
lasts were examined. These QD did not induce any statistically signif-
icant changes in cell cycle and were associated with minimal
apoptosis/necrosis in human skin fibroblast at both doses. There was a
down-regulation of genes involved in controlling the mitotic M-phase
spindle formation and cytokinesis. Importantly, they did not activate
genes associated with immune and inflammatory responses (Zhang
et al., 2006).
Recently, we reported that exposure of skin to commercially available
QD differing in core/shell shape, hydrodynamic size, and surface coat-
ings resulted in penetration of the intact stratum corneum barrier with
localization of QD in the underlying epidermal and dermal layers as
early as eight hours after topical application (Ryman-Rasmussen et al.,
2006). This study has indicated that skin is a potential route of expo-
sure for QD. However, the toxicity of QD to skin cells is unknown and
the potential for QD to cause inflammation had not been addressed.
Therefore, we conducted a study to determine the cytotoxic and inflam-
matory potential of QD in skin cells (human epidermal keratinocytes
[HEK]). Soluble QD of two sizes, QD 565 and QD 655, were obtained
with three different surface coatings: polyethylene glycol (PEG), PEG-
amines, or carboxylic acids. The QD were studied with laser scanning con-
focal microscopy on live human epidermal keratinocytes and transmission
electron microscopy to verify QD uptake in cells by 24-hour MTT via-
bility and inflammatory cytokines IL-1β, IL-6, IL-8, IL-10, and TNF-α
was assessed at 24 and 48 hours. The results indicated that all QD
were intracellularly located in HEK within 24 hours, independent of QD