Biomedical Engineering Reference
In-Depth Information
the stability of nanoparticle suspensions used for toxicological studies and
material characterization. Since agglomeration shifts particle size distri-
butions to higher medians and may result in microparticle formation, it
affects the stability of suspensions and may lead to sedimentation. Larger
particles may show less nano-size-specific toxicity and sedimentation
may become a clearance effect that changes the effective dose. This under-
lines the importance of a comprehensive characterization of nanoparticle
suspensions.
Further, it has to be considered that nanotoxicological studies working
with the most dispersed nanomaterials may represent worst-case scenar-
ios that may not be representative for nanoparticles in their usual agglom-
eration state. The preparation step for the separation of agglomerates or
aggregates into primary nanoparticles may require very high energies
or shear forces and may be able to affect the nanoparticle's properties.*
Especially, breaking of aggregates by sonication may generate surface radi-
cals that would not have been created under milder dispersion conditions.
Such radicals may be quantified by means of electron paramagnetic reso-
nance (EPR) spectroscopy.
Many researchers add surfactants to facilitate dispersion and stabilization.
Such additives coat nanoparticles with a film of nanometric thickness that
generally change the character of the nanoparticles by suppressing surface-
chemical features. Their application therefore requires elaborate control
experiments to understand their effects and verify the nontoxicity of the
additives themselves.
2.2.4 Morphology and Porosity
Experimental techniques for the determination of the structure, shape, and
size of primary particles, agglomerates, and aggregates have been discussed
in Section 2.2.1.1. Here, methods to determine their porosity will be briefly
presented.
The porosity of a nanomaterial can either be an intrinsic particle property or
a collective property of particle aggregates. High porosity reduces the appar-
ent density of the nanomaterial and enhances its surface area. Nanoscale and
mesoscale pores may induce trapping of molecules, as known from molecu-
lar sieves. Surface-functionalized porous materials were successfully used
for selective sorption of contaminants from waste streams. The toxic effects
of molecule trapping in biological environments are to be studied.
* Grinding or ultrasonic treatment may create radicals on particles and fiber fragments [13].
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