Biomedical Engineering Reference
In-Depth Information
states strongly affects the luminescence efficiency of the semiconductor NMs
but because of stability issues, surface state passivation with ligands is not the
way to create highly luminescent NMs with a robust emission. However, this
change in luminescence can be used to monitor the ligand-NM interaction.
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The use of bifunctional ligands that contain one functional group that
attaches at the NMs surface and a second group that is exposed to the surround-
ings is a very useful development. This is demonstrated by the use of thiols
that make CdSe NMs adsorb at gold surfaces or by polymeric materials such
as PEG that carry carboxyl, amine, hydroxyl, or a combination of these func-
tional groups (
Figure 3.1
). An excellent application is for NM sensors that are
modified with functional groups that provide affinity to a specific analyte that
results in a change in the NMs' optical properties. One such NM prepared by
Nath and Chilkoti
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gave gold NMs (AuNMs) affinity for streptavidin through
functionalization with biotin. Changes in the AuNMs plasmon resonance were
used to indicate the binding of streptavidin to biotin. Colloidal CdSe NMs were
engineered through encapsulation with an amphiphilic polymer upon which a
pH-sensitive squaraine dye was conjugated.
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In this NM structure, the CdSe/
ZnS nanocrystal may either fluoresce or undergo fluorescence energy transfer
(FRET) to the squaraine dye. The FRET efficiency in these nanostructures is a
function of the surroundings that results from the dye's absorption profile which
is controlled by pH while the ratio of the NM to dye emission is a measure of the
pH of the environment. An alternative to long-chain ligands in the preparation
of nanocrystal solids is short bifunctional ligands, such as hydrazine, forming
a tightly packed nanocrystal solid that has sufficient conductivity to be used in
different charge-transfer applications.
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3.2.2 Thiols for Ligand Exchange
Monothiols,
12,21,28
bidentate thiols,
12,24
silanes/silanols,
12,24,25
oligomeric phos-
phines,
12,27
amine box dendrimers,
12,24,26
dithiothreitol,
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amphiphilic sac-
charides,
12,30
proteins and peptides, etc.
12,31,32
are used to prepare NMs for
medical applications. For example, CdSe nanocrystals coated with hydrophilic
deprotonated thiol (thiolate) ligands were studied systematically in a pseudo-
steady-state titration, and introduced for determining the precipitation pH of
nanocrystals coated with electron-donating ligands.
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For comparison, CdTe
and CdS nanocrystals coated with the same types of ligands were also exam-
ined. The results showed that the precipitation of the nanocrystals was caused
by the dissociation of the nanocrystal−ligand coordinating bonds from the nano-
crystal surface. The ligands were removed from the surface due to protonation
in a relatively low pH range, between 2 and 7 depending on the size, approxi-
mately within the quantum confinement size regime, and chemical composition
(bandgap) of the nanocrystals. In contrast, the re-dispersion of the nanocrystals
was found to be solely determined by the deprotonation of the ligands. The
size-dependent dissociation pH of the ligands was tentatively used as a means
