Biomedical Engineering Reference
In-Depth Information
with various components of the environment. Whereas, in aggregation where engineered NPs
tend to agglomerate with neighbors due to high surface energy, engineered NPs lose their reactiv-
ity as well as catalytic nature by forming large-sized particles. However, aggregation is a kinetic
process; size may easily change with the passage of time or due to natural geothermal weather-
ing processes. But reverting back to nanosized particles is a time-taking process and is also
influenced by a variety of environmental factors, for example, pH, ionic strength, salinity, and so
on. Engineered NPs also show the tendency to interact with the natural organic matter (NOM)
or artificial organic compounds, which further direct their stability or aggregation. Aggregation
of engineered NPs is enhanced in the presence of high-molecular-weight NOM compounds,
whereas, low-molecular-weight NOM compounds increase the mobility of engineered NPs in
colloidal solution.
Analytical procedures for assessing dissolution and structural transformation of metallic NPs
include chemical assays and the use of techniques for structural characterization. Atomic arrange-
ment can be studied in terms of the long-range (crystal parameters) and short-range order of the
NPs' atomic structure. Of particular interest is the arrangement of atoms at the surface of metallic
NPs. The various characterization techniques that are available for these purposes may be non-
element specific (x-ray diffraction, total scattering, Raman spectroscopy, etc.) or element specific
(x-ray absorption spectroscopy, Mo¨ssbauer spectroscopy, nuclear magnetic resonance, etc.).
Particular attention has also been given to the interaction of the surface of metallic NPs with
nutritive solutions such as cell growth media. Owing to their surface reactivity and affinity for ions
in solution, metallic NPs have a high capacity to adsorb molecules (amino acids, proteins, sugar,
or salts) within biological media. These strong interactions and the neutral pH (close to the point of
zero charge of metallic NPs) can destabilize the colloidal suspension or passivate their surface. The
agglomeration state must be determined using scattering techniques based on light or x-ray. During
the toxicological study, the nature of culture broth plays an important role. A strong cytotoxicity
is observed for CeO
2
NPs toward
E. coli
in 0.1 M KNO
3
, whereas no significant effects occur in
Luria-Bertani medium. Moreover, in particular applications, metallic NPs are functionalized on
the surface with (in) organic compounds affecting their behavior in biological media. For instance,
functionalization of magnetite NPs with dextran to enhance their blood circulation time induces
an inhibition of BrdU incorporation with disruption of the F-actin and viculin filament of human
cells. However, when magnetite NPs are functionalized with albumin to be recognized by specific
cellular receptors, no significant biological effects are reported. The mechanisms of adsorption of
the coating at the surface are crucial to understand NP modifications and ultimately reactivity. For
instance, in neutral solutions, albumin is hydrolyzed and negatively charged that allows for chemi-
sorptions at the surface of the positively charged iron oxide, whereas dextran is electrically neutral
in similar conditions and the binding at the surface of iron oxide is more labile (physisorption).
Consequently, once adsorbed, the albumin can persist at the surface of iron oxide and prevent a
direct contact with the toxic magnetite NPs. On the contrary, dextran can be desorbed yielding to a
direct contact between magnetite NPs and biological compounds.
Since NPs are typically engineered or postprocessed for specific applications, their physico-
chemical properties and reactivity can vary considerably. The two key physicochemical properties
of ZnO NPs are relevant to or predictive of their ecotoxicity. The first is solubility and the second
is photoreactivity of NPs. The solubility is determined by both intrinsic properties of the NPs (such
as particle size, chemical composition) and environmental parameters of the exposure media (such
as pH, temperature, organic matter, etc.). The second key property is photoreactivity of the NPs.
This photoreactivity can cause photoinduced toxicity that is enhanced hundreds of times under
environmentally relevant ultraviolet (UV) radiation (such as sunlight) as compared to laboratory
fluorescent lighting. This phototoxicity has been demonstrated for both ZnO and TiO
2
NPs. It is
suggested that photocatalytic activity of TiO
2
NPs, measured with a simple chemical ROS assay,
may be a predictor for phototoxicity of TiO
2
NPs. Similar investigation on ZnO NPs is warranted to
fully understand their potential hazard and risk to the environmental biota.
