Biomedical Engineering Reference
In-Depth Information
thousand = 1000 ppm), and the minimum analysis area ranges from 1 to 200 μm. Importantly, XPS
allows for the determination of the valence band structure of a specimen. We recently studied the
cytotoxicity of SiO
2
, Al
2
O
3
, CeO
2
, ZnO, CuO, Al-doped ZnO, and Al-doped CuO NPs [3]. We found
that the cytotoxicity in the A549 and NIH3T3 cell lines was associated with the electronic properties
of the nano-oxides by measuring the density of states (DOS) near Fermi level of the nano-oxides
with core-level XPS. Although there is other evidence showing the influence of electronic properties
of metal oxides [12] and carbon nanotubes [22] on the toxicological potential [23] at the cellular and
whole animal levels, the effect of the electronic characteristics of NMs on biological systems has not
yet gained great attention. It is known that most of the intracellular and
in vivo
toxicities from NMs
arise from the production of excess reactive oxygen species (ROS) [24,25]. ROS and other radicals
are involved in a variety of biological phenomena, such as mutation, carcinogenesis, degenerative
diseases, inflammation, aging, and development [26]. ROS are well recognized for playing a dual
role as deleterious and beneficial species. It is also known that many important biological processes
involve redox reactions. For example, the process of cell respiration also depends heavily on the
reduction of NAD
+
to NADH and the reverse reaction (the oxidation of NADH to NAD
+
). All cellu-
lar membranes are especially vulnerable to oxidation due to their high concentrations of unsaturated
fatty acid. The redox potential is defined as the ratio between the oxidant and reductant, for example,
ROS and scavengers [26]. The redox state of a biological system is kept within a narrow range under
normal conditions—similar to the manner in which the biological system regulates its pH parameter.
Under pathological conditions, the redox state can be changed toward lower (redosis) or higher (oxi-
dosis) values. Although redox is a thermodynamic parameter, reducing power is not and, therefore,
can be calculated in biological systems to supply valuable information concerning cellular responses
to oxidative stress. This parameter represents and encompasses the overall capability of the cell,
biological fluid, or tissue to donate electrons (oxidation potential) and the overall concentration of
the reducing equivalents responsible for this ability. Therefore,
electrons
play an important role in
biological redox reactions, and all the factors that influence electron concentrations, electron trans-
fers, and electron transport may have effects on biological redox reactions. The links connecting the
electronic properties of NMs to toxic effects need more investigations.
Belonging to the same family as scanning probe microscopy, AFM is one of the most powerful
tools in nanotechnology. It enables us to quickly obtain three-dimensional, topographic images of
various materials and structures with a resolution at the nanoscale or near the atomic level. AFM
is extensively used in a wide range of disciplines, such as materials science, solid-state physics,
electronics, and life science [15,27], for academic research as well as for industrial fabrication and
inspection. AFM can be operated in ambient air and in liquid; it works under extreme circumstances,
such as variable temperature, high magnetic field, and ultrahigh vacuum. Furthermore, AFM has
evolved into one of the most important tools in nanotechnology with its unmatched capabilities of
monitoring biological events in their native environments [28]. AFM forms images by sliding a
sharp tip over a sample surface through a raster scanner and subsequently recording relevant sig-
nals. Because each tip has its own three-dimensional shape with a finite size, the acquired image
does not reflect the actual shape of the specimen but a dilation or convolution of the sample topogra-
phy, dependent on the shape of the tip [13]. This issue will become significant when the dimensions
of NMs are comparable to the radius of curvature of the tip's apex (normally about 5-10 nm) and,
thus, the tip effect should be taken into account if accurate size information is critical for the study.
An ultrasharp AFM tip can be used as a simple way to mitigate the tip effect [13]. In spite of the
tip effect, AFM allows for the study of electrical [29], magnetic [30], and other properties of NMs.
One of the origins of toxicities in NMs is assumed to be the small size feature of NMs. Thus, the
availability of techniques for the measurement of size (size distribution, agglomerates/aggregates)
is crucial for nanotoxicity studies. Several advanced techniques are available as different tech-
niques based on different measurement principles. However, these may yield different results when
measuring the very same object, and the measured results are normally not comparable [31,32].
