Biomedical Engineering Reference
In-Depth Information
for determining the size-dependent free energy associated with the formation of
a nanocrystal−ligand coordinating bond.
To solve the instability of NMs that are converted into water-soluble form
through thiol ligand exchange, two methods have been developed. One is based
on the direct adsorption of bifunctional ligands on the nanocrystal surface
20
and
the other is based on surface coating with a silica layer.
25
The direct adsorption
of bifunctional ligands on the nanocrystal surface is carried using ligands that
have organic soluble and water-soluble functional groups. In this process, the
organic soluble group orients toward the hydrophobic NMs, while the hydro-
philic groups project away from the organic soluble region and toward the aque-
ous solution making the modified NM water soluble. Surface coating with a
silica layer encapsulates multiple NMs inside a silica NP of 100 nm or greater
diameter.
Mercaptoacetic acid is one of the thiols used to make QDs water soluble,
because the mercapto portion has a large affinity to Zn atoms, while the car-
boxylic acid group is hydrophilic and reactive to biomolecules.
20
However, this
process causes the fluorescence QYs to drop below 10% after water solubiliza-
tion.
80
Another issue is the slow desorption of mercaptoacetic acid on the QD
surface often leads to aggregation and precipitation of the QDs. A modified ver-
sion of this process involves attachment of engineered proteins to QDs through
electrostatic interactions,
58
and the use of dithiothreitol has been adopted for
nanocrystal stabilization and bioconjugation.
47
3.2.2.1 Protocol for Ligand Exchange
81
The following method was used for the preparation of water-soluble CdSe/CdS/
ZnS QDs for imaging and other biomedical applications.
(1)
Precipitate the organic soluble CdSe/CdS/ZnS QDs with acetone two times
to remove free octadecylamine (ODA) in the solution.
(2)
Re-disperse in chloroform.
(3)
Add 40 mg of polyethylene imine (PEI,
M
w
10 kDa) to 0.5 nmol of CdSe/
CdS/ZnS QDs in chloroform.
(4)
Shake the mixture for 2 h at room temperature (RT).
(5)
Evaporate the solvent under argon.
(6)
Dissolve the dried film in ultrapure water (18 MΩ).
(7)
Centrifuge at 6000
g
for 10 min to yield a clear supernatant with white
deposits.
(8)
Dialyze with
M
w
50 kDa to remove the unbound PEI molecules against
water or borate buffer solution (10 mM, pH 7.4).
(9)
Store at 2-8 °C to avoid bacterial and mold contamination.
Characterize the water-soluble QDs with transmission electron microscopy
(TEM) to establish the diameter and shape. Use a Zeta tracker to establish the
zeta potential and the hydrodynamic size. Run a scan to establish the absorption
and emission wavelength.
