Biomedical Engineering Reference
In-Depth Information
manipulation of individual atoms became possible,
5,6
significantly contributing
to the rapid discovery and development of fullerenes (carbon60 molecules),
7
carbon nanotubes,
8
and semiconductor quantum dots (QDs).
9-11
The core QDs are usually prepared in high-temperature solvents that often
involve a mixture of trioctylphosphine and trioctylphosphine oxide (TOP/
TOPO)
12-16
followed by a layer of wide-bandgap semiconductor materials that can
be coated on the surface of the QD core/shell.
12,17,18
These QDs are either used
as hydrophobic nanocrystals or are made water soluble by ligand exchange or by
adsorption of heterofunctional organic coating on the QD surface.
12,17-20
A compre-
hensive review by Medintz et al.
12,21
summarizes the group of compounds that give
functionality to the QD surface. These include thiols,
12,21-23
silanes/silanols,
24,25
bidentate thiols,
12,24
amine box dendrimers,
12,26
oligomeric phosphines,
12,27
phos-
phatidyl compounds,
12,28,29
amphiphilic saccharides,
12,30
proteins and peptides,
etc.
12,31,32
In a similar manner, iron oxide magnetic nanoparticles (IOMNPs) and
other magnetic nanoparticles can be modified like the QDs to make them biocom-
patible. This chapter is devoted to a discussion of various ways of modifying the
NM surface to make them water soluble and ultimately biocompatible.
33-45
3.2 NANOMATERIAL CONVERSION INTO THE
WATER-SOLUBLE FORM
Most nanomaterial (NMs) including quantum dots (QDs),
46
iron oxide mag-
netic nanoparticles (IOMNPs), gold or silver nanoparticles (NPs) are usu-
ally synthesized in nonpolar organic solvents. In general, NM synthesis uses
long-chain ligands such as oleic acid (OA) (C18) to control nucleation and
particle growth. However, an OA capping introduces a 2-nm thick tunneling
barrier on NMs that is detrimental for applications that rely on charge trans-
fer. To make these useful for biological and medical applications, they have
to be converted into water-soluble form by replacing the hydrophobic surface
ligands with amphiphilic ligands. Different NM water solubilization processes
have been developed over the past few years. These are (i) ligand exchange with
simple thiol-containing molecules
20,47-49
or more sophisticated ones such as
oligomeric phosphines,
27
peptides,
50
and dendrons,
26
and (ii) encapsulation by a
layer of amphiphilic diblock
51
or triblock copolymers
52
or in silica shells,
25,53,54
polymer shells,
19
phospholipid micelles,
29,55
or amphiphilic polysaccharides,
30
and (iii) a combination of layers of different molecules.
56-58
NP surface modification is often necessary for specific applications. While
QDs synthesized in organic solvents tend to have higher quality due to lower den-
sity of surface trap states leading to higher photoluminescence (PL) yield, their
hydrophobic surfaces are not compatible with applications that require hydrophilic
surfaces such as various medical applications. In order to convert the hydropho-
bic into hydrophilic surfaces, a number of strategies have been developed. Three
types of modifications will be discussed in this chapter, which include
59-61
ligand
exchange,
48
silanization,
25,53,62
and surface coating using amphiphilic polymers
