Fluorescent sensors developed for in vivo cadmium detection
Michael J. Lander
Cadmium is toxic, yet it frequently finds use in electroplating and in the production of dry-cell batteries. As a result of such widespread application, the element can find its way into the food chain and can accumulate in human tissue. The similar interaction of cadmium and zinc ions with most fluorophores, however, has hampered efforts to use fluorescence methods to detect the former in cells.
Researchers led by Xiaojun Peng of Dalian University of Technology in China have developed a fluorescent sensor that demonstrates cadmium selectivity. They synthesized the molecule by condensation of a boradiazaindacene derivative with 4-(bis(pyridin-2-ylmethyl)amino)-benzaldehyde. A description of their work appears in the Jan. 19 online issue of the Journal of the American Chemical Society.
To determine the maximum fluorescence emission from the free sensor, the scientists used a fluorescence spectrometer from Photon Technology International Inc. They found the maximum to occur near 656 nm — the result of an internal charge transfer mechanism. Gradual addition and subsequent binding of cadmium ions prevented charge transfer from the molecule’s amino group to the fluorophore — in this case, boradiazaindacene — and yielded a blueshift in the emission intensity peak to 597 nm. Continued addition of cadmium steadily increased the intensity at this wavelength and the ratio of 656- to 597-nm emission.
Further titration experiments demonstrated that, of common metal ions, only chromium (III) ions slightly enhanced the molecule’s fluorescence. In competition experiments, neither zinc ions nor other metal ions increased the intensity ratio.
As a test of the sensor in living systems, the scientists incubated pheochromocytoma cells with the sensor. After excitation with green light from a mercury lamp with a filter block, they imaged the cells before and after cadmium addition with a Nikon fluorescence microscope. They repeated the procedure with dendritic cells but used a Leica confocal microscope in single- and double-channel modes with excitation from a Melles Griot HeNe laser at 543 nm. Universal Imaging Corp. (now Molecular Devices) software was used to analyze the data from the HeNe laser to obtain standard confocal and ratio fluorescence images.
For both cell types, cadmium exposure significantly increased fluorescence emission. Images of the dendritic cells, however, showed the change more clearly and also revealed an increase in the intensity ratio.
The team suggests that, based on these results, the fluorescent molecule could be used to selectively detect the heavy metal in living cells.
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