Biomedical Engineering Reference
In-Depth Information
trioctylphosphine (TOP), triphenylphosphine, trioctylphosphine oxide (TOPO),
perfluorated lauric acid, OA, didodecylamine, trioctyl aluminum, and a mixture
of dodecanethiol and TOP. Of these, only thiols and OA result in increased air
stability.
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The other ligands precipitate NPs (perfluorated lauric acid and the
mixture of dodecanethiol and TOP) or show identical behavior to lauric acid
(TOP, triphenylphosphine, TOPO, didodecylamine, trioctyl aluminum). The lat-
ter may indicate that no exchange occurred, resulting from weaker adsorption
than that of lauric acid.
Most medical and health-related applications of NMs depend on specific
ligands and replacing the existing ligands with other ligands is often an essential
processing step called ligand exchange. Ligand exchange is typically accom-
plished by exposing the particles containing the original ligands to the new
ligands, which are usually present in excess. Ligand exchange can be achieved
in the following ways:
Dissolving the NPs in solid form (dried or precipitated through centrifugation
or other means) in a solution containing the new ligand, which is a method
used to functionalize CdSe and InP with pyridine.
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A phase-transfer approach where, for example, water-soluble particles are
transferred to an organic phase or vice versa by adding a suitable ligand that
acts as a phase-transfer agent—for example, dodecanethiol promotes the
transfer of charge-stabilized gold NPs to a toluene phase or mercapto acids
have been used to transfer CdTe NMs from an organic phase to an aqueous
phase.
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The outcome of ligand-exchange processes is difficult to assess except by indi-
rect methods such as a change in solubility or even electrophoretic mobility.
Verification is usually not an issue as long as the ligand exchanged NPs behave
differently and that they do their job. Ligand exchange is one of the successful
ways to integrate functionality that provides versatility for various applications
of NMs. Ligand exchange has serious effects on the properties of NMs espe-
cially their luminescence or fluorescence properties. For instance, semiconduc-
tor NMs that have absorbed light can relax back to its ground state through
luminescence and the efficiency of this process crucially depends on the NPs'
surface. Surface trap states or surface defects in the nanocrystal structure act
as a temporary hole for the electron preventing their radiative recombination.
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The alternate occurrence of ion trapping and untrapping events results in inter-
mittent fluorescence (blinking) that is visible at the single-molecule level,
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which reduces the overall quantum yield (QY) which is the ratio of emitted to
absorbed photons. To overcome these problems and to protect surface atoms
from oxidation and other chemical reactions, a shell consisting of a few atomic
layers of a material with a larger bandgap is grown on top of the nanocrystal
core. This shell can be designed to enhance photostability by several orders of
magnitude compared with conventional dyes
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and at the same time obtain a QY
close to 90%.
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Binding of ligands to surface atoms that can passivate these trap
