Biomedical Engineering Reference
In-Depth Information
4.2 Role of Surfactants
The most challenging issue is the synthesis of well-dispersed and stable colloidal
synthesis of nanoparticles for biomedical applications. The fundamental problems
in use of the nanoparticles is the lack of stability of their dispersions and the
generation of spacious aggregates in the dry state, which leads to loss of their
special nanoscale properties and also making them unsuitable for biomedical
applications. In aqueous media, the nanosized particles are primarily separated by
the ionic repulsion forces produced due to adsorption on their surface [
41
,
42
].
Nanoparticles capped by surfactants stay well dispersed in solution for a longer
time. The surfactants act as microreactors for inorganic reactions and also act as
steric stabilizers to inhibit aggregation of nanoparticles. Surfactants modify the
nanoparticle's shape, size, and other surface properties differently depending on
their molecular structure, i.e., nature of the head group, length of hydrophobic tail,
and type of counterions.
Nanocrystals such as CdS, CdSe, CdTe, or CdSe/ZnS, ZnS, ZnO etc. have been
synthesized in organic solvents at high temperatures in the presence of surfactants
to yield monodisperse and stable particles [
43
]. Surfactant molecules coat the
surface of nanoparticles, as shown in Fig.
5
. The polar surfactant head group is
attached to the inorganic particle surface, while the hydrophobic chains protrude
into the organic solvent, mediating colloidal stability. Then, to make the particle
water soluble or dispersible, the surfactant layer is replaced or a coating is made,
with an additional layer introducing either electric charge or hydrophilic polymers
for mediating solubility in water. Coulomb repulsion between nanocrystals with
surface charge of the same polarity prevents aggregation in water. Hence, synthesis
Fig. 5 Surfactant molecules on the surface of nanoparticles
