Biomedical Engineering Reference
In-Depth Information
peptides were able to locate and track the tumors with imaging techniques in
mice. Using triblock copolymer coating, QDs modified with prostate tumor tar-
geting ligands were used to tag injected cancer cells as well as to locate tumors
and in mice using a whole-body illumination system with spectral QD imag-
ing.
59
Polyethylene glycol-phosphatidylethanolamine (PEG-PE) and PEG-
phosphatidylcholine modified CdSe/ZnS-QDs were used to image Xenopus
embryo development.
2.1.4 Ligand-Exchange Reaction to Make QDs Water Soluble
60
One way to convert the hydrophobic QDs into their water-soluble form is to
perform ligand exchange. The stepwise process is described as follows:
(1)
The TOPO-capped QDs in toluene are precipitated with methanol and re-
dispersed in CHCl
3
.
(2)
In a separate flask, dissolve 500 µL of 3-mercaptopropionic acid (3MPA) in
10 mL of CHCl
3
.
(3)
To this flask, add 1 g of the organic base tetramethylammonium hydroxide
pentahydrate (TMAH, (CH
3
)4N-OH·5H
2
O). The chemical combination
forms a two-phase mixture.
(4)
Collect the bottom organic phase, containing deprotonated 3MPA and add
1 mL of TOPO-capped QDs (1 µM) and incubate at RT for 48 h.
(5)
Remove excess 3MPA because alkyl thiols are deactivators of 1-ethyl-3-
(3-dimethylaminopropyl)-carbodiimide (EDC), which is the promoter used
to couple amine-modified biomolecules such as proteins, peptides, or DNA
onto the QDs. This can be carried out using dialysis against a phosphate-
buffered saline (PBS) buffer (10 mM, pH 7).
60
(6)
The water soluble QDs maintain their fluorescence and absorbance proper-
ties (
Figure 2.5
) and can be kept at 4-8 °C before conjugation with biomol-
ecules (see Chapters 3 and 4).
2.1.5 Characterization of QDs
56
One of the most important goal in the preparation of NMs is the production
of monodisperse particles. Monodisperse pertains to the narrow size distri-
bution of the NPs that is responsible for a narrow absorbance, which is mea-
sured through the full width at half the maximum (FWHM) absorption peak
and a single emission peak (
Figure 2.5
). QDs exhibiting such properties are
considered high quality NMs that are preferred for labeling studies where dif-
ferent sizes are used as different labels. Unlike fluorescent organic dyes, one
excitation wavelength causes different-sized QDs to fluoresce at their respec-
tive emission peak wavelengths with narrow and nonoverlapping emission
peaks enabling multiple labels to be simultaneously observed.
61,62
In general,
the experimental,
48
the FWHM of the QD fluorescence curve, is a measure of
the particle size dispersion in the sample. For multiple imaging applications,
