Biomedical Engineering Reference
In-Depth Information
FIGURE 2.5
Emission spectrum of ~6 nm water-soluble QDs emitting at 620 nm with FWHM
of ~26 nm.
Courtesy of Ocean NanoTech.
(For color version of this figure, the reader is referred to
the online version of this topic)
a small FWHM is necessary to detect the multiple analytes of interest. Thus,
during synthesis, the absorption and emission wavelengths are measured continu-
ously. When the QDs are not properly purified, the absorption spectra show peak
maxima with additional absorption at lower wavelength due to starting materials
and oleic acid polymerization.
56
The separation between the excitation and emis-
sion wavelengths is called the Stokes shift. The Stokes shift indicates the ability to
separate the emission from excitation peak that is necessary in multiple analytes'
labeling and detection. The presence of nearby energy labels allows semiconduc-
tor NMs to be excited by photons with greater than or equal to the minimum
absorption energies. In
Figure 2.4
, the same excitation wavelength of 364 nm
was used to observe the emission colors for all different sizes and optical colors,
which indicates that a single light source can be used to excite multiple QDs of
different sizes and all the labels can be excited and detected simultaneously.
2.1.6 UV Absorption and Emission
The synthesis of QDs is followed by measurements of UV absorption and emis-
sion (
Figure 2.6
) from the preparation of the precursor to the core/shell QD. This
protocol is a step-by-step process of measuring the absorption and emission of a
QD during and after synthesis.
(1)
Place dilute QD in a quartz cuvette.
(2)
Measure the absorbance with a UV spectrophotometer.
