Biomedical Engineering Reference
In-Depth Information
and alloys such as Mn
3
O
4
[35] , Fe [24] , Co [36] , Ni [37] , FePt [22] , and FePd [38]
are less commonly employed, in part because of their rapid oxidation in air and/
or potential cytotoxicity in biomedical applications. Unlike well- known monolayer -
protected Au clusters [2, 3], the preparation methods for magnetic nanoparticles
do not tolerate the presence of reactive termini on the stabilizing ligands (i.e.,
−
Br,
−
SH, etc.), either because of the thermal instability or the bonding of transition
metals to the ligands. Consequently, the synthesis of magnetic nanoparticles in
the presence of functional groups has been largely unsuccessful. As a result, the
as-prepared particles often must undergo further modifi cations and post-synthetic
reactions to render them chemically functional.
14.3
Nanoparticle Functionalization
The functionalization and characterization of the surfaces of magnetic nanopar-
ticles continue to be important and challenging tasks. Surface functionalization
provides the means to add chemical stability,
in situ
and
in vivo
function, and
specifi c molecular recognition to these materials, allowing advantage to be taken
of their intrinsic magnetic properties in a wide variety of systems. In this chapter,
some recent accomplishments in this area are highlighted to provide a sense of
breadth of the fi eld; however, for additional information more detailed information
is available, including reviews by Latham and Williams [39] , Sch ü th
et al.
[40] , and
Sun [41] .
14.3.1
Surface Adsorption
Surface adsorption can be considered one of the simplest functionalization strate-
gies. As the fi eld of nanotechnology matures, adsorption continues to be a widely
used technique for appending a variety of functional molecules to particle surfaces.
Surface adsorption is defi ned as a process by which ionically stabilized nanopar-
ticles are coated by a functional moiety through either electrostatic interactions or
via a covalent attachment to the
naked
particle surface. This must be differentiated
from ligand exchange, which is the replacement of one covalently attached surface
coating for another, and will be discussed later in the chapter.
Like most nanoparticle functionalization methods, surface adsorption strategies
derive from routes developed for monolayer formation on two - dimensional ( 2 - D )
surfaces [42], and were followed shortly thereafter by a modifi cation of citrate-
stabilized Au colloids [43]. To date, the general method for this approach is to add
a surfactant or molecule of interest bearing a charge (i.e., ionic), either a carboxylic
acid (
CO
−
) [44] , phosphate (
PO
−
) [45], ammonium (
NR
+
, where R is either H
or C) [46], or other surface-binding group (e.g., OH [47] or SH [48]), to a solution
of ionically stabilized particles. The particles can then be separated by magnetic
decantation or centrifugation to remove any unassociated material and to yield
