Chemistry Reference
In-Depth Information
t of surfactant exchange is the ability to prepare
particles that are suitable for di
The foremost bene
ering environments and applications. The
obvious example is the use of semiconductor nanoparticles in biological
labelling, where particles are required to be water-soluble. Nie described the
simple reaction of TOPO-capped CdSe/ZnS nanoparticles and mercaptoacetic
acid in a chloroform solution. The thiol group coordinated to the nano-
particle surface and the deprotonated carboxylic acid group imparted water
solubility. The resulting mercaptoacetic acid-capped particles were then
isolated by extraction into water.
127
This simple reaction was one of the
seminal phase-transfer methods for preparing QDs for use in biology. The
particles exhibited signi
d
n
1
y
4
n
g
|
6
cantly reduced emission, and further investigations
into the size and charge of thiols have shown that smaller thiols increase
emission quenching.
128
The availability of the pendant carboxylic acid
functional group provided a simple anchor point for further functionalities
to be attached to the luminescent particle, a common starting point for the
synthesis of more complex biological labels.
129
It is worth noting that the
coupling of a carboxylic acid group with an amine containing biological
entity is a simple way of functionalising QDs with biological vectors,
although the common use of 1-ethyl-3-(3-dimethylaminopropyl)carbodii-
mide (EDC) as coupling reagent is sometimes problematic, occasionally
resulting in the irreversible precipitation of the particles due to the interac-
tion of the dimethylamine functionality with the nanoparticles. The use of
polyethylene glycol (PEG) carbodiimide coupling agents is an elegant way of
circumventing this problem, as the particles are stable even at high loading
of the reagent, and can even be used as a method of adding PEG to a nano-
particle surface.
130
PEG has become arguably one of the most useful mole-
cules available to nanoparticle chemists interested in biological applications,
as materials functionalised with PEG are found to be non-antigenic, non-
immunogenic and protein resistant;
131
and, importantly, nanoparticles cap-
ped with PEG show limited non-speci
.
c binding
132
(a major problem with
early labelling studies), prolonged circulation times in biological systems
and reduced toxicity.
133
In related work mercaptoundecanoic acid-capped particles have had the
terminal carboxylic acid groups cross-linked with lysine, giving a stable
hydrophilic shell composed of carboxylic acid and amine functional groups
available for further conjugation.
134
In this way, the thiolated molecule can be
thought of as a bridge, linking the particle to further entities such as
proteins
135
or antibodies,
136
which are usually capable of further interaction
with the biological material of interest. It is worth noting that pendant
carboxylic acid groups are not exclusively used; thiols with protonated amine
groups have also been used to transfer TOPO-capped CdTe to water.
137
An important part of bioconjugation is rendering the particle hydrophilic
while maintaining the smallest possible hydrodynamic diameter; Liu
et al.
have prepared CdSe/ZnCdS QDs which were made water-soluble by phase-
transfer using cysteine, a thiol-based amino acid zwitterion. Interestingly,
the resulting material possessed a quantum yield of 40%, due to the e
ective
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