Biomedical Engineering Reference
In-Depth Information
8.2.3 c
haracterIzatIoN
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atrIces
At present, to analyze NMs in the environment [41-43] and cosmetics [44], a generally used scheme
includes isolation, purification, and concentration steps (if necessary) before the identification of the
NMs contained in the original matrices [37]. There are many methods for separating NMs from
the complex surroundings. These techniques include ultrafiltration, nanofiltration, dialysis, field-
flow fractionation (FFF), and size exclusion chromatography (SEC) [37,40]. Once extracted from
the complex matrices, the NMs can be characterized by the conventional analytical techniques
addressed above. However, the state of characterized NMs that are isolated by the schemed process-
ing is certainly different from that in complex matrices, and the analysis is extremely susceptible to
artifacts; even chemical compositions may be altered. Such
ex situ
analysis provides less informa-
tion to understand the biological behaviors of NMs for nanotoxicology.
The characterization of NMs in a biological matrix is more difficult and challenging as com-
pared to the characterization of primary NMs, because the surrounding environment can have a
substantial impact on the behaviors of NMs. The physicochemical properties of NMs in biological
environments tend to change with time. The interaction of NMs with biological surroundings is
especially important for the hazard assessments of NMs. It is well known that in a biological envi-
ronment, NMs can adsorb proteins via physical or chemical interactions, forming a protein corona.
The formation of the protein corona dynamically alters the size and surface composition of the NM
[34]. This gives a biological identity of NMs distinct from the original ones. The protein corona's
behavior is believed to be very important for the physiological response, including signaling, kinet-
ics, transport, uptake, transport, accumulation, and toxicity [2,45]. It has been suggested that the
NM itself does not constitute the effective unit for interaction with biological systems but, rather, it
is the combination of the NM with its specific protein corona that reacts with the biological system.
Compared to the intensive study of the protein corona, there are a few reports on the interaction
of NMs in the biological environment with the ions in the biological system [1,46]. The effective
surface charge of a NM will be changed due to the consequences of the interactions with the bio-
logical matrix. Thus, deliberate attempts at surface modifications in NMs may not show effects
as expected. Owing to these matrix effects, physicochemical properties, such as size, agglomera-
tion/aggregation states, surface charge and coatings, dissolution, and other related properties, may
change in different solvents, test media, and biological environments. Characterizations in a biologi-
cal matrix should provide information on any changes in the NM's characteristics in order to make
correlations with its biological outcome.
8.2.4 c
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NM
s
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By
e
lectroN
M
Icroscopy
By TEM and XPS, we found that ZnO and CuO NPs can selectively adsorb ions in biological
environments, forming a bio-nanomaterial complex, also called an ion corona [1]. Owing to the
high energy of the electron beam used in the TEM study (200 keV), we did not detect signals from
key components of serum proteins, such as N, by the EDS technique. However, we did observe
the disappearance of certain matters during the TEM observation because biomolecules can be
easily damaged by high-energy electron beams, and detected an N signal by XPS. This is the first
clear observation of the adsorption of various ions, not biological molecules, by NMs, after being
dispersed in biological matrices. Without an elemental identification, this kind of ion corona may
be mistakenly considered as a protein corona. Walczyk et al. [45] studied the proteins surround-
ing modified polystyrene and silica particles by TEM operated 80 keV as well as other relevant
techniques. We believe that proteins and ions are mingled together when surrounding NMs in bio-
logical environments, and that the identification of the constituents of nanomaterial-bio complexes
will significantly improve our understanding of what is selectively acquired by NMs in biological
environments; how those acquired constituents influence the reactivity, transport, transformation,
bioavailability, and toxicity of NMs; and how the NMs interact with living cells.
