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their collective magnetic properties based on precise control of interparticle spacing
(Frankamp et al. 2005). The transition from superparamagnetic to ferromagnetic
(i.e., the blocking temperature) for iron oxide (Fe
2
O
3
) nanoparticle assemblies
shifted over a substantial range with increasing dendrimer generations. This result
can be explained by an increase of the activation energy for spin flipping associated
with an increase of interparticle decoupling. The ability to substantially reduce
dipolar coupling between particles could have a significant impact on applications
such as magnetic storage, which requires the placement of densely packed magnetic
nanoparticles in a minimum space.
In addition to dendrimers, charged biomacromolecules such as DNA or proteins
have also been utilized as “mortar” to assemble nanoparticles. DNA has been demon-
strated to be a particularly versatile construction material because of its flexible length
scale, rigidity, and duplex helical structures (Storhoff and Mirkin 1999; Fu et al.
2004; Becerril et al. 2005; Ongaro et al. 2005). Nanoparticle-DNA nanocomposites
can be formed because of the affinity of the anionic phosphate linkage of DNA
strands and cationic modified nanoparticles. The DNA-mediated assembly of iron
platinum (FePt) nanoparticles exhibited an increase in spacing from 7.8 nm for the
particle alone up to 8.9 nm for the DNA-particle assembly (Fig. 6.4; Srivastava,
Arumugam, et al. 2007). Although a certain degree of denaturation in the bonded
duplex DNA was observed through circular dichroism, the reduced dipolar coupling
between each particle due to the insertion of DNA molecules still allowed fine-tuning
of the magnetic behavior of the material.
In a similar fashion, the Rotello group reported several protein-mediated assemblies
of nanoparticles (Srivastava, Verma, et al. 2005; Verma et al. 2005). Unstable proteins
such as chymotrypsin are readily denatured upon prolonged interaction with functiona-
lized nanoparticles because of the exposure of proteins to the hydrophobic layer
(Srivastava, Verma, et al. 2005). Addition of a hydrophilic portion to the monolayer
by inserting a short tetraethylene glycol between the charged terminal group and hydro-
phobic aliphatic chain circumvented this denaturation problem (Hong et al. 2004).
A ferritin-FePt nanoparticle bionanocomposite was fabricated in later studies,
which shows the integrated magnetic behavior of the synthetic and biological com-
ponents (Fig. 6.5; Srivastava, Samanta, Jordan, et al. 2007). A transmission electron
Figure 6.4 (a) Self-assembly of magnetic FePt nanoparticles by DNAs and (b) a TEMmicro-
graph of networklike FePt-DNA aggregates; scale bar ¼ 100 nm. Reprinted with permission
from Srivastava, Samanta, Arumugam, et al. (2007). Copyright 2007 RSC Publishing.
