Biomedical Engineering Reference
In-Depth Information
of monodisperse and colloidal suspension in aqueous media can be accomplished.
However the exact mechanism that governs formation and stabilization is still not
very well understood.
5 Synthesis of Colloidal Nanoparticles
Colloidal nanoparticles are a class of nanomaterials synthesized by wet chemical
methods. The reaction chamber is a reactor containing a mixture of liquids that
control the nucleation and the growth. The precursors are introduced according to
the desired atomic species for the growth of the nanocrystals. The precursors
decompose, forming new reactive species or ions that are needed for the nucleation
and growth of the nanocrystals. The key factor in the colloidal synthesis of
nanocrystals is the surfactant used. A surfactant is dynamically adsorbed to the
surface of the growing QDs under the reaction conditions. The surfactant must be
mobile enough to provide access for the addition of the reaction precursor atomic or
ionic species. It should prevent aggregation of the nanocrystals. A surfactant that
binds very strongly to the surface of the QD would not allow the nanocrystal to
grow. A weakly co-ordinating molecule would yield large particles or aggregates.
Some suitable surfactant molecules include alkyl thiols, phosphines, phosphine
oxides, phosphates, phosphonates, amides, amines, carboxylic acids etc. The sur-
factant molecule should be stable if the reaction is carried out at higher
temperatures. By controlling the mixture of surfactant molecules that are present
during the generation and nucleation of QDs, control of their size and shape
becomes possible. Colloidal nanocrystals are dispersed in solution; hence, they
can be functionalized easily with molecules such as proteins and oligonucleotides.
6 Different Types of Nanoparticles
6.1 Functionalized Nanoparticles
6.1.1 Lipid-Functionalized Nanoparticles
Nanoparticles, such as iron oxide particles and QDs, are mostly synthesized in
nonpolar organic solvents and capped with a surfactant. If they are to be solubilized
in aqueous buffers, their hydrophobic surface components must be replaced by
amphiphilic ones. An alternative strategy was developed by Dubertret et al. for
TOPO-coated QDs [
44
]. The hydrophobic particles were dissolved in chloroform
together with PEGylated phospholipids. After evaporating the solvent and hydrat-
ing the mixed film of QDs and lipids, QD-containing micelles were formed. The
same method can be used for encapsulating hydrophobic iron oxide particles in
