Biomedical Engineering Reference
In-Depth Information
special attention to the surface composition and chemistry. The stoichiome-
try and structure of a nanoparticle's surface may significantly differ from its
bulk material. Most importantly, missing binding partners of surface atoms
may cause enlarged crystal lattice spacings, affect crystal plane orientations,
and induce secondary surface reactions such as oxidation or the formation
of radical surface states. This generally leads to the formation of enhanced
surface polarity and/or reactivity, which tend to affect dissolution processes
acting on the particle.
The chemical nature of surface groups also determines pH-dependent for-
mation of surface charges and governs particle agglomeration propensities and
surface adsorption phenomena. Surface chemistry therefore plays an impor-
tant role in biological systems, in particular with respect to particle transport
and biological interactions. Especially the presence of radical surface sites on
nanoparticles may induce reactive oxygen species and lead to biological stress
[10]. Polar or chemically reactive surface groups may interfere with biological
systems in many ways, for instance, by immobilizing toxic chemicals, nutri-
ents, or biomolecules such as proteins and enzymes on their surface.
The importance of a comprehensive characterization of the particle sur-
face chemistry must therefore not be underestimated. However, the reliabil-
ity of state-of-the-art techniques for the identification and quantification of
functional groups on nanoparticle surfaces requires further improvement.
Determination of the chemical composition of nanoparticle surfaces relies on
traditional analytical tools, originating from surface science. These techniques
generally require compact, laterally extended surfaces of known orientation,
conditions that are not necessarily satisfied by nanoparticles. Furthermore,
the information depth of those analysis methods may lie in the order of the
particle size.
For example, XPS probes core electrons of atoms and collects information
from a depth of a few nanometers below the surface. This may render XPS a
volume-sensitive technique if applied to nanoparticles. The same holds for
SEM/EDX, confocal microscopy, and Fourier-transform infrared (FTIR) spec-
troscopy, which exhibit an analysis depth of several micrometers. For TEM/
EDX, the information depth equals the sample thickness. Therefore, cross-
sectional views of surface layers are required for EDX analyses. Laterally and
vertically resolved surface chemical analysis is achievable by complex and
time-consuming high-resolution (HR) scanning TEM (HR-STEM) in combi-
nation with electron energy loss spectroscopy (EELS). Also, advanced AFM
techniques that explore nanoparticle surfaces with functionalized tips by
force-distance curve measurement allow surface chemical analysis. Titration
measurements and functional group labeling by chemical derivatization
techniques, including fluorescence staining and calorimetry, exhibit analy-
sis depth profiles that critically depend on, for instance, particle solubility,
porosity, as well as reagent size and permeability. Surface chemical analysis
therefore requires expertise for proper sample preparation and precise inter-
pretation of results.
