Environmental Engineering Reference
In-Depth Information
microorganism produces compounds that enhance the precipitation of the NM or its
intermediate (e.g., metals); the precipitation may take place both in the solution and on
the cell surface. However, precipitation is not dependent on microbial metabolism, if it
occurs as a result of a chemical interaction between the constituent and cell surface.
In general, cellular membranes control the passage of dissolved organics and
nutrients into the cells and the waste materials and metabolic by-products out of the cell.
The transport of NMs or their transformation intermediates across microbial cell
membranes can lead to intracellular accumulation, which depends on microbial cell
metabolism. Based on analogy to physical-chemical properties of larger molecules of
the same material, it may be possible to estimate the tendency of NMs to cross cell
membranes and bioaccumulation. However, NMs have some unique characteristics that
must be considered to study their crossing cell membranes. For example, in organic
media, the low-lying and degenerate LUMOs of Ceo permit its active participation in
many free-radical and electron-transfer processes (Guldi et al., 1999). Nano-Ceo (even
as low as 20 ppb) produces oxygen radical species that induce peroxidation of the lipid
bilayers of cells and cause cell death (Sayes et al., 2004). Chang et al. (2007) reported
that silica or composite NPs of silica NPs were nontoxic at low dosages but cell viability
decreased at high dosages due to membrane damage; the cytotoxicity of these NPs
strongly depended on the metabolic activity type of the cell line.
One of the key advantages that NMs offer is the possibility of effectively
crossing biological barriers, which would allow their use in the delivery of
therapeutically active molecules. Recently, several studies have been conducted on
cellular uptake of various types of functionalized carbon nanotubes (/"-CNTs). The major
categories of fuctionalization include non-covalent, Ji-stacking (complexation,
adsorption), endohedral intercalation, and defect and covalent sidewall functionalization.
For example, Kostarelos et al. (2007) covalently functionalized CNTs with small
molecules such as ammonium, acetamido, fluorescein isothiocyanate (FITC) and
bifuctionalized CNTs with ammonium groups and FITC (via or not via an amide
linkage), methotrexate (MTX) and FITC, and amphotericin B (AmB) and FITC. Water-
soluble /CNTs interact with mammalian cells, leading to their cytoplasimic
translocations. They found that/-CNTs possess a capacity to be taken up by mammalian
and prokaryotic cells and to intracellularly traffic through the different cellular barriers
by energy-independent mechanisms. The cylindrical shape and high aspect ratio of f-
CNTs allow their penetration through the plasma membrane, similar to a 'nanosyringe',
as has been experimentally reported and theoretically simulated (Kostarelos et al., 2007).
Moreover, Kostarelos et al. (2007) reported that cellular uptake of functionalized carbon
nanotubes is independent of the functional group and cell type. Other studies also
indicate that CNTs coated with proteins, polymers, ammonium, and single-stranded
DNA also interact with mammalian cells, leading to their cytoplasmic translocations
(see references in Kostarelos et al., 2007).
Search WWH ::




Custom Search