Biomedical Engineering Reference
In-Depth Information
from the reflected colors of quantum dots with varying diameters, shifting from blue to red with
an increase in size. However, for metallic NPs with diameters ca. >2 nm, the operating principle is
different from semiconductor quantum dots since there is no band gap between valence and con-
duction bands and the energy states form a continuum analogous to bulk metals. For these metallic
NPs, another phenomenon known as surface plasmon resonance (SPR) is active for these structures,
involving specific scattering interactions between the impinging light and the nanostructures. For
the smallest of metallic NPs with dimensions ca. <2 nm, the surface plasmon absorption disap-
pears. Quantum confinement effects also cause a change in the other optical properties of metallic
NPs, because the spacing between intraband energy levels increases with decreasing particle size.
This change will lead to changes in light emission characteristics. The reason that this is mentioned
is because there is no direct, material-independent relationship between the size and novel effects.
Thus, case-by-case studies are necessary for every NM, and we emphasize that more attention is
needed toward the novel nanoscale properties when the potential impact of NMs is evaluated [3].
8.2.2 p
hysIcocheMIcal
c
haracterIzatIoN
of
p
rIMary
NM
s
The characterization of NMs in their dry states is the first step for the assessment of their potential
hazards. The characterizations include chemical composition and impurities; geometric properties
such as size distribution, shape, surface morphology, and surface area; crystalline structure; and
optical, electronic, and magnetic properties. The chemical composition, crystalline structure, shape,
surface morphology, and surface area of NMs can be characterized by conventional techniques
developed in materials science, including x-ray photoelectron spectroscopy (XPS), x-ray diffrac-
tion (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and
atomic force microscopy (AFM). Here, we do not address these techniques in detail and pay no
attention to standardized characterization techniques, such as crystalline structure by XRD, mor-
phology by SEM, and TEM and AFM, which is discussed elsewhere. We only emphasize what one
should pay attention to when a physicochemical property is under investigation for hazard assess-
ment by using common and mature techniques.
Equipped with energy dispersive spectrometer (EDS) microanalysis system, SEM/EDS, and
TEM/EDS can locally obtain the elemental composition of NMs, that is, their relative propor-
tions (atomic% for specimen). EDS can be used as a semiquantitative mode to determine chemical
composition by peak-height ratio relative to a standard. In an EDS spectrum, there may be energy
peak overlaps among different elements, particularly those corresponding to x-rays generated by
emission from different energy-level shells (K, L, and M) in different elements. EDS cannot detect
the lightest elements, typically below the atomic number of Na, by detectors equipped with a Be
window. Despite the high spatial resolution, the sensitivity of the EDS technique is lower than that
of XPS and the detection limit is about 0.1 wt%, which prevents the detection of trace impuri-
ties. Thus, for the testing of low concentration elements (less than 1 wt%), the accuracy and preci-
sion is about 10%. Electron backscatter diffraction (EBSD), an accessory system of SEM, provides
quantitative microstructural information about the crystallographic nature, such as grain size, grain
boundary character, grain orientation, texture, and the phase identity of a specimen. In the case of
the measurement of poor-conduction specimens by SEM and TEM, a conducting layer (2-5 nm
thick) of Au, Pt, or C is needed to solve the problem of the charge-up effect for the imaging.
XPS utilizes photoionization (using soft x-rays with a photon energy of 200-2000 eV to examine
core levels) and the analysis of the kinetic energy distribution of emitted photoelectrons to study the
composition and electronic states of the surface region of a sample. XPS is a quantitative spectroscopic
technique that measures the elemental composition, empirical formula, chemical state, and electronic
state of the elements that exist at a surface, generally from the topmost 1-12 nm of a material. While
XPS is a surface-sensitive technique, a depth profile of the sample in terms of XPS quantities can
be obtained by combining a sequence of ion gun etching cycles interleaved with XPS measurements
from the current surface. The detection limit is approximately 0.1-1.0 at% (0.1 at% = 1 part per
