Biomedical Engineering Reference
In-Depth Information
(20)
Pass the solution of silanized nanocrystals in methanol through a solvent
exchange column (either NAP columns or a homemade 20-cm long col-
umn with a 0.7 cm diameter filled with ~5 g of Sephadex G25 medium
and equilibrated with 10 mM phosphate buffer (PB), pH ~7.
(21)
Only the fluorescent eluted fractions are collected and filtered with a
22-µm pore size acetate filter.
(22)
Centrifuge at 20,000
g
for 30 min and discard the precipitate.
(23)
Store the solutions in air.
(24)
Establish the OD using the extinction coefficient as 10
5
M
−1
cm
−1
.
Characterize the water-soluble QDs with 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.
Another process of preparing silane-coated QDs leads to organic soluble
ones that are treated with ligand exchange before they become water soluble.
Similar to the procedure above, the process starts with CdSe@ZnS NMs that are
synthesized according to a previously published procedure.
87
The resulting QD
core/shell nanocrystals are encapsulated with an SiO
2
shell using the following
process.
88
(1)
Place 10 mL of cyclohexane, 1.3 mL of NM-5, 400 µL of CdSe@ZnS
stock solution in chloroform, and 80 µL of tetraehtylorthosilicate (TEOS)
in a flask with vigorous stirring.
(2)
Thirty minutes after the microemulsion system is formed add 150 µL of
ammonia aqueous solution (33 wt%) to initiate the polymerization process.
(3)
Allow the process to grow silica over 24 h with vigorous stirring.
(4)
Isolate the NMs from the microemulsion by adding acetone followed by
centrifugation.
(5)
Collect the precipitate consisting of CdSe@ZnS@SiO
2
composite particle.
(6)
Sequentially wash with 1-butanol, 1-propanol, ethanol, and water to
remove any surfactant and unreacted molecules. Completely disperse the
precipitate in each solvent to remove any physically adsorbed molecules
on the particle surfaces.
The resulting silica-encapsulated NMs are resuspended in the desired solvent
and characterized for size, QY, concentration, and maximum wavelengths of
emission and absorption. These CdSe@ZnS@SiO
2
are converted into their
water-soluble form using ligand exchange discussed in previous section.
3.2.4 Polymer Coating
Polymer coating on various organic synthesized NMs has been used to con-
vert these into their water-soluble form. Polymer-coated nanocrystals have been
used in biological studies.
89-93
These hydrophobic nanocrystals are modified
with amphiphilic polymer shell making them hydrophilic, thus water soluble.
This is achieved by the reaction of the amphiphilic polymer, bis(6-aminohexyl)
