Biomedical Engineering Reference
In-Depth Information
2.1.8 Photostability
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Various applications of QDs require exposure to UV irradiation; therefore, they
must remain stable over longer periods of time under UV. Most fluorescent
organic dyes lose brightness after a few hours of exposure to UV. The following
protocol is used to measure photostability of QDs:
(1)
Place a 2 µM water soluble QD in a vial.
(2)
Expose to UV lamp for 12-h recording the fluorescence every hour.
(3)
Plot the fluorescence signal intensity against time.
(4)
Repeat the process over three days for a total of 36-h exposure.
2.1.9 Atomic Absorption Spectroscopy
Atomic absorption spectroscopy (AAS) is another technique that can be used
to characterize NMs in terms of elemental composition. For example, a known
amount of CdSe/ZnS core/shell QDs can be used to determine the concentration
of cadmium and zinc. The following protocol is used to determine the amount
of Cd and Zn in a CdSe/ZnS core/shell:
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(1)
Take a known mass of the organic soluble QD powder and dissolve with
16 M nitric acid in a fume hood. Sonicate for 10-15 min and allow it to
dissolve overnight in the fume hood.
(2)
Add a few drops of 12 M HCl to dissolve any precipitates.
(3)
Dilute the solution to 25 mL with DI water (resistivity ≥ 18 MΩ-cm).
(4)
Prepare standard solutions of Cd and Zn in DI water (resistivity ≥ 18 MΩ-
cm) following manufacturer's recommendation.
(5)
Take the signal of the blank solution (DI water, resistivity ≥ 18 MΩ-cm)
in triplicate.
(6)
Take the signals of the standard solutions (in triplicate) using the lamp
corresponding to each element.
(7)
Prepare the calibration curves for Cd and for Zn after subtracting the
blank from the recorded signals.
(8)
Calculate the absorption constant for each element using the Beer law
equation:
A
= abC
where
A
is the absorbance,
a
is the molar absorptivity constant,
b
is the
light path length, and
C
is the concentration.
(9)
Take the absorption of the CdSe/ZnS core/shell solution using the respec-
tive elements.
(10)
Calculate the concentration of Cd from the Cd calibration curve.
(11)
Similarly, calculate the concentration of Zn from the Zn calibration curve.
Note: You can find the concentration of any other element in an NM with
AAS following the same principle that was used above.
