Quantum dots can be shifty fluorophores
How well do you know your quantum dots? Those that behave well in solution may act differently
in living cells. Researchers at Fudan University in Shanghai, China, examined thiol-capped
CdTe quantum dots in mouse myoblast cells and human primary liver cancer cells.
The quantum dots’ optical properties in these cells differed dramatically
when compared with their behavior in solution.
The investigators employed a laser-scanning confocal
microscope from Olympus and a 405-nm semiconductor laser from Coherent as the excitation
source. They directed the beam through a 60x objective onto an area approximately
1 μm in diameter.
This image shows quantum dots in a mouse myoblast cell. They exhibited
a blueshift upon excitation with a 405-nmsemiconductor laser.
Based on the resulting images, they
chose points at which they measured microphotoluminescence spectra. To make those
measurements, they used the bundled microscope software to focus the laser on a
fixed point, and they adjusted their system to an excitation power density of about
250 W/cm2. They focused the photoluminescence output from the side exit of the microscope
on the entrance slit of a spectrometer with a nitrogen-cooled CCD, both from PI/Acton
of Trenton, N.J.
Upon continuous excitation, the quantum
dots exhibited an initial peak wavelength of 602 nm and gradually shifted to shorter
wavelengths. After 15 minutes of excitation, they had manifested a 35-nm blueshift
immediately before they were entirely photobleached. The photoluminescence increased
within 10 seconds and subsequently decreased, as reported in the Oct. 4 online publication
of the Journal of the American Chemical Society.
The researchers performed additional
tests that demonstrated that the blueshift was caused by singlet-state oxygen molecules
that formed on the quantum dots’ core surface. They said that might be interesting
to investigators who study singlet-state oxygen, which can cause oxidative damage
and is the active chemical in photodynamic therapy.
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