Biomedical Engineering Reference
In-Depth Information
Fig. 1.1
Annual trends in the
number of publications for
QDs. The terms “Quantum
Dots” and “QDs” have been
considered. The literature
search was done using
PubMed
1.2.1 Optical Property
The size-tunable absorption and luminescence spectra of QDs arise from the quan-
tum confinement effect. Excitons of QDs are confined in all three spatial dimen-
sions and divided into discrete energy levels, which are similar to the energy
levels in atoms. Electrons are first excited from the occupied level to the unoc-
cupied energy level and fluorescence occurs when the excited electron relaxes to
the ground state and combines with the hole. In a simplified model system, the
energy of the emitted photon can be deemed as the sum of the bandgap energy
between the occupied level and the unoccupied energy level, the confinement ener-
gies of the hole and the excited electron, and the bound energy of the exciton (the
electron-hole pair). A decrease in the size of the QDs will bring an increase in the
bandgap energy which represents a hypsochromic shift of the absorption and pho-
toluminescence (PL) spectra [
20
,
21
].
Since the emergence of QDs, comparison of pros and cons of QDs and organic
fluorophores have always been a hot topic. Organic fluorophores have a narrow
absorption spectrum which results in a narrow range of emission and they do not
have a sharp symmetric emission peak which is further broadened by a red tail. In
contrast, QDs have a broader excitation spectra and a more sharply defined emis-
sion peak. QDs are several thousand times more stable against photobleaching
than organic dyes and are thus well suited for continuous tracking studies over a
long period of time. One order of magnitude longer excited-state lifetime of QDs
than that of organic dyes provides a feasible approach to separate the QD fluores-
cence from background fluorescence to achieve an accurate signal readout [
22
,
23
].
Meanwhile, the large Stokes shifts of QDs (measured by the distance between the
excitation and emission peaks) can be used to further improve detection sensitivity.
As reported, the Stokes shifts of QDs can be as large as 300-400 nm, depending
on the wavelength of the excitation light. Organic dye signals with a small Stokes
shift are often buried by strong tissue autofluorescence, whereas QDs signals with
