Biomedical Engineering Reference
In-Depth Information
that induced cell death [226]. Williams, Hancock, and coworkers used biotin-
terminated PEG-phospholipids on large CoFe
2
O
4
(15 nm - diameter) nanoparticles
for the magnetic labelling of microtubules, the transport of which over kinesin-
covered surfaces could be magnetically manipulated [227]. In these studies, either
streptavidin or neutravidin was used as a linker unit between the nanoparticle and
the biotinylated microtubule [227b] .
14.3.6.2
Dendrimers
Recently,
dendrimers
have been considered as functional groups for modifying the
surface chemistry of magnetic nanoparticles. Dendrimers are highly branched,
generational macromolecules with defi ned three - dimensional ( 3 - D ) structures
[228, 229] and synthetically controlled terminal reactive groups (i.e., surface chem-
istry). One major challenge for the functional nanoparticle community is to deter-
mine and control the exact number of surface molecules and reactive groups per
particle. As nanoparticle samples have a size distribution, so too will there be a
distribution in the number of attached molecules. The use of dendrimers to func-
tionalize nanoparticles would circumvent the issue of population dispersity,
however, as only a fi nite number of the macromolecules will stabilize a single
nanoparticle. Particles synthesized in the interior of a dendrimer template (i.e.,
one particle in one dendrimer) have an exactly known number of functional
groups that is dictated by the chemical constitution of the macromolecule.
Crooks and coworkers have pioneered the use of dendrimers to encapsulate
nanoparticles [230]. In one study, Ni nanoparticles were synthesized in the interior
of sixth generation poly(amidoamine) (PAMAM) dendrimers by adding Ni
2+
and
sodium triethylborohydride as the reducing agent [230a]. The size of the resultant
nanoparticles was seen to depend on the molar ratio of Ni
2+
to dendrimer, and
they were also protected from oxidation by the dendrimer shell [230a]. Subse-
quently, Atwater and coworkers demonstrated the use of amine- terminated,
fi fth-generation PAMAM dendrimers for Co nanoparticle synthesis [231], while
Chandler
et al.
synthesized NiAu nanoparticles in hydroxyl-terminated, fi fth-
generation PAMAM dendrimers [232].
The ligand exchange of dendrimers has also been recently investigated. Carboxyl-
terminated, third-generation dendrimers which were adsorbed to stabilize iron
oxide nanoparticles were characterized by a variety of techniques, including poly-
acrylamide gel electrophoresis (PAGE), to measure the binding strength of the
ligand and the ionic charge of the particles [233]. Among the several different den-
drimers tested, the PAGE results showed those bearing folic acid and succinic acid
termini to be the most effective [233]. Banaszak, Holl and Orr used fi fth - generation
PAMAM dendrimers terminating in folic acid and fl uorescent dyes to phase-
transfer iron oxide nanoparticles from organic solvents into water, and then char-
acterized their interaction with human KB cells [234]. In these experiments the folic
acid acted as both a nanoparticle surface ligand and as a substrate for the cell sur-
faces. When X - ray fl uorescence was used to monitor the iron content of KB cells
(which possess folic acid receptors) and UM-SCC-38 cells (which were folic acid
knock-outs), a signifi cant nanoparticle uptake was seen only in the KB cells [234].
