Biomedical Engineering Reference
In-Depth Information
central role for nanomaterials. BET measurement is a well-established stand-
ard technique. However, geometrical calculations of the surface area as a
substitute for BET may be valid only for regularly shaped, similar-sized,
nonporous particles such as spheres.
The tendency of adsorption and agglomeration of nanoparticles is gov-
erned by the surface free energy and the polarity of the particles. As the
reactivity of the surface, they all depend on the surface chemistry. However,
investigation of the surface chemistry of nanoparticles is the most challeng-
ing task in nanomaterial sciences. For comprehensive analysis, a number
of different techniques have to be combined. Practical characterization is
therefore often limited to surface charge estimations by particle charge siz-
ers (PCS) and zeta potential measurement, surface polarity assessment by
dynamic vapor sorption or inverse chromatography, and surface free energy
determination by tensiometry. For the identification and quantification
of surface functional groups, a number of surface analytical techniques have
to be combined. XPS is a valuable tool for chemical composition and oxida-
tion state determination. It can be combined with surface group labeling
by derivatization reactions. Also, fluorescent staining, coupling of isotope-
enriched reagents for NMR spectroscopic quantification, or radioactive
markers can be employed to label reactive surface groups. Potentiometric
titration has been employed successfully for acid-base quantification.
Temperature-induced cleavage of surface functionalities coupled to a bal-
ance and to infrared absorption spectroscopy or mass spectrometric detec-
tors (TGA/FTIR, TGA/MS) is used frequently for qualitative group analysis
by thermogravimetry. Ion-beam induced surface fragmentation patterns are
subject of time-of-flight scanning ion-beam mass spectroscopy (ToF-SIMS)
and may allow to derive surface-chemical information. Individual particle
surface-chemical investigations have been performed by scanning probe
microscopy with functionalized tips or electrochemical measurements by
a SPM tips.
2.2.7 Aging of Nanoparticles
The characterization of nanoparticles is further complicated by aging phe-
nomena. For the example of carbon nanotubes (CNTs), it has been reported
in the literature that similar aging-related decreases in three unrelated prop-
erties (surface area and pore volume, surface oxygen, and structural defects)
in multiple CNT samples were observed [14]. It is therefore suggested that
(certain) physicochemical properties for this particular class of nanoparticles
can be characterized with reliability only after the samples have sufficiently
aged (9-15 months).
Knowing about the possibility that certain nanoparticles alter in various
properties with time, makes the postsynthesis stability, storage (humidity,
temperature, exposure to light and atmosphere conditions) and aging of
materials under nonequilibrium and under ambient conditions an important
