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The ‘quantum dot’ nanoparticles used by Van Manen and Otto replace
existing fluorescent labels that are employed to enable the cell’s
biomolecules to light up under the microscope. While fluorescence
microscopy continues to be instrumental in unraveling the intricate
biological processes that take place inside living cells, it would be
even more informative to combine it with the intracellular chemical
analysis capabilities of vibrational spectroscopy techniques such as
Raman microscopy. Common fluorescent labels are not suitable for this
combination, however, because the much stronger fluorescence
overshadows the intrinsic weak Raman signals coming from cells. By
taking fluorescent quantum dots that emit light in a wavelength region
that is well-separated from Raman signals, the Dutch researchers now
show that fluorescence microscopy can indeed be combined with Raman
microscopy on the same cell.
Vibrations inside cells
Techniques based on vibrational spectroscopy are able to detect the
specific vibrations that occur inside the cell’s biomolecules (such as
DNA, proteins, and lipids), making them very powerful tools for
‘chemical fingerprinting’ of cells. In contrast to fluorescence
microscopy, vibrational spectroscopy does not require the biomolecules
of interest to be labeled, which is a great advantage. The Biophysical
Engineering Group at the University of Twente, headed by prof. Vinod
Subramaniam, has pioneered the application of Raman spectroscopy to
investigate the chemical make-up of single cells, and this group is
now worldwide at the front of high-resolution chemical mapping of
cells by Raman microscopy.
In their Nano Letters article, the researchers demonstrate two
applications of the hybrid fluorescence Raman technique. By
illuminating white blood cells with UV light at a wavelength of 413
nm, the Raman signal from an enzyme that is critical in the innate
immune response can be detected and visualized across the cell. The
fluorescence signal of quantum dot nanoparticles that have been
ingested by the cells can be visualized separately. The second
application employs light at a wavelength of 647 nm, which results in
the separate detection of Raman signals from cellular proteins and
lipids and the fluorescence signal from the nanoparticles.
Van Manen and Otto expect that the fluorescence Raman microscopy
combination will provide exciting new possibilities: the nanoparticles
might be coated on their surface with antibodies against, for example,
marker proteins for cancer cells. In this way the quantum dots will
serve as a torch for specific cells, which can subsequently be
subjected to a detailed chemical analysis by using Raman microscopy.
The research described in the Nano Letters article was funded by the
Landsteiner Foundation for Blood Transfusion Research (Amsterdam, The
Netherlands) and the MESA+ Institute for Nanotechnology at the
University of Twente.
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