Biomedical Engineering Reference
In-Depth Information
beacons for more direct analysis. The surface adsorption of proteins to nanowires
has also been of recent interest, when Meyer and coworkers adsorbed palmitic
(pentadecanoic) acids onto Ni segments of Au/Ni nanowires to selectively target
immunoglobulin G (IgG) to the magnetic segments of the wire by hydrophobic/
hydrophobic interactions [57]. Here, adsorption was found to take place following
a classical Langmuir isotherm, with an equilibrium constant on the order of
10
6
M
− 1
[57] .
14.3.2
Ligand Exchange
Nanoparticles can alternatively be synthesized in the presence of species that
covalently bind to their surfaces, acting as ligands that passivate the surface
through ionic or van der Waals interactions. The majority of these surface ligands
contain little to no chemical functionality for further reactivity or interaction with
species other than the particle surface. Thus, to impart chemical and/or biological
functionality, these ligands must be “exchanged” for other molecules that contain
some chemically or biologically relevant functionality. While the debate of the exact
mechanism by which this process occurs is ongoing and beyond the scope of this
chapter, ligand exchange is broadly classifi ed as the addition of a surfactant or
molecule which has a favorable covalent interaction at the particle surface which,
upon addition in excess, eventually reaches an equilibrium state with the existing
surface moiety. This approach was originally established as a displacement style
functionalization technique of two-dimensional Au monolayers [50b, 58]; the most
widely recognized studies have been those of Murray and coworkers on small Au
nanoclusters [3, 59]. These fundamental investigations of the mechanism of ligand
exchange on Au, and its use in laying the groundwork for further Au monolayer
reactivity, are relevant to magnetic particles and should be considered as core
materials for any investigator considering this approach, even if the mechanisms
and specifi c chemistries differ.
In this chapter, we will highlight some of the more recent uses of ligand
exchange to form functional
magnetic
nanoparticle systems, noting that there are
inherent diffi culties in characterizing the products of these reactions. Many of the
early reports on ligand exchange on Au nanoclusters used nuclear magnetic reso-
nance (NMR) spectroscopy as the primary characterization tool for confi rming the
identity and number of the molecules on the surface following exchange. Unfor-
tunately, magnetic nanoparticles do not lend themselves to NMR as a method of
analysis, because their magnetic core causes inhomogeneities in the magnetic
fi eld, leading to broadening, large paramagnetic shifts, and/or a loss of signal.
Although several reports have described the use of NMR spectroscopy to analyze
magnetic nanoparticles [60], these methods must be supported with additional
analytical evidence of surface attachment versus colocation in solution. Confi rma-
tion of the molecular composition of nanoparticle surfaces poses a signifi cant
challenge, as most analytical methodologies provide an ensemble average of
the bulk sample and cannot distinguish between a molecule attached to the
