Silica nanoparticles and their interaction with cells : a multidisciplinary approach

Silica nanoparticles are increasingly used as drug delivery systems and for biomedical imaging.
Therapeutic and diagnostic agents can be incorporated into the silica matrix to improve the stability
and solubility of hydrophobic drugs in biological systems. However, the safety of silica
nanoparticles as drug carriers remains controversial. To date, no validated and accepted nanospecific
tests exist to predict the potentially harmful impact of these materials on the human body.
The mechanism proposed for hemolysis of unmodified silica nanoparticles is based on the electrostatic
interaction between the silanol surface groups and the quaternary ammonium in the choline
head group of the phospholipids. However, a detailed understanding of this process is missing.
In this thesis, different silica nanoparticles where synthesized, characterized, and tested in two cell
lines regarding viability and oxidative stress. Hemolysis was assessed using red blood cells. Furthermore,
the hemolytic mechanism of a chosen silica nanoparticle type was investigated in depth
using a biophysical chemistry approach. We used the dye-leakage assay, isothermal titration
calorimetry, solid state nuclear magnetic resonance, and flow cytometry to elucidate this mechanism.
Our results revealed that silica nanoparticles with a porous surface and negative surface charge
had the strongest impact on viability in a concentration dependent manner. This is in contrast to
non-porous silica nanoparticles. None of the studied particles caused oxidative stress in either cell
lines. Particles with a negative surface charge induced hemolysis. The mechanism responsible for
the hemolysis for silica nanoparticles had no electrostatic component. The nuclear magnetic resonance
data revealed no interaction with the choline group. However, nuclear magnetic resonance
data suggested the presence of faster tumbling species.
Our toxicological and mechanistic studies showed potential hazards of spherical amorphous silica
nanoparticles. Physico-chemical properties mediating toxicity in living cells were identified.
We propose that our standardized silica nanoparticles may serve as a readily available reference
material for nanotoxicological investigations. Mechanistic data did not support an electrostatic
interaction as postulated in the literature, but rather a strong adsorption process that may lead
to hemolysis. Furthermore, the presence of faster tumbling species suggested the formation of
smaller lipid bilayer structures upon silica nanoparticles exposure. Flow cytometry data revealed
that their size is about 100 nm. It remains to be proven if the bilayer wraps around the hemolytic
silica nanoparticles, if an exclusive formation of smaller species without wrapping is present, or
both of the aforementioned.