Chemistry Reference
In-Depth Information
moieties for dispersion in aqueous media. Hydrophilic ligands are
commonly used to functionalize the nanocrystals surface prior
to dispersing in aqueous media or attachment with biomolecules.
Different strategies have been explored to provide nanocrystal with
properties such as water dispersity and biomolecule conjugation for
biomedical applications and biodetection schemes. Table 2.1 shows
some of the major strategies reported in the literature, including
ligand exchange, ligand oxidation, ligand attraction, layer by layer
assembly, and surface silanization.
Ligand exchange has been demonstrated by Yi
et al.
[72] for
upconverting NaYF
nanocrystals where they used polyethylene
glycol 600 diacid (HOOC-PEG-COOH) to replace the surface amine
ligand (oleylamine) with carboxyl functional groups. The carboxyl
functional groups on the nanoparticle surface render them water
dispersible. Chen
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[122] developed a ligand oxidation technique
in which Lemieux-von Rudloff reagent was used to oxidize the
surface oleic acid to azelaic acid. The oxidized ligands have carboxyl
groups on the surface, which provide them water dispersity. The
same group later reported a strategy based on epoxidation of the
double bond followed by reaction with PEG-amines to render the
nanocrystals water dispersible [123]. A limitation of this process
is that it can only be applied to ligands containing unsaturated
carbon-carbon bonds. A ligand attraction process was reported by
Yi
et al.
. [73] where they coat the core-shell nanocrystals with 25%
octylamine and 40% isopropylamine modified poly(acrylicacid)
(PAA). Wang
et al
. [124] reported layer-by-layer assembly technique,
which is based on the adsorption of alternatively charged polyions
on the nanocrystals surface. They reported sequential adsorption of
poly(allylamine hydrochloride) and negatively charged poly(sodium
4-styrenesulfonate) onto the NaYF
et al
surface to generate water-
4
dispersible nanocrystals.
In comparison, the surface silanization technique is versatile
and applicable to both hydrophobic and hydrophilic nanocrystals.
The Stöber method [125] is a popular silanization technique for
hydrophilic nanoparticles, while reverse-microemulsion technique
has been adopted for silanization of hydrophobic nanoparticles. The
silanization process is based on the hydrolysis and polycondensation
of silica precursor (e.g., tetraethoxysilane, TEOS) in the presence
of alkali hydroxide (e.g., ammonium hydroxide, NH
OH). In the
surface silanization process, organosilanes with amine functional
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