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Long-life labels for imaging
RESEARCH NEWS
Long-life labels for imaging NANOTECHNOLOGY
Noninvasive fluorescence image showing major sites of quantum dot deposition in a mouse 24 hours after injection. (Courtesy of Lauren Ward.)
Quantum dots (QDs) have the potential to act as stable, bright fluorophores for labeling biological specimens. However, preparations of QDs have not so far been optimized for in vivo studies. If QDs are to useful for be in vivo imaging, they must have an adequate circulation lifetime, show minimal nonspecific deposition, and retain fluorescence over a long period. Researchers at Carnegie Mellon University and Quantum Dot Corp. have engineered the surface coating of QDs for long-term, in vivo imaging in mice [Ballou et al., Bioconjugate Chem. (2004) 15 (1), 79].
The researchers coated core-shell CdSe-ZnS QDs with amphiphilic poly(acrylic acid). After injecting the coated QDs into mice, noninvasive fluorescence imaging showed a circulation lifetime of ~12 minutes. To improve this, different poly(ethylene glycol) molecules of increasing lengths were coupled to the initial polymer coat. This increased circulation lifetimes up to ~70 minutes. Fluorescence imaging also reveals where the QDs are deposited, including the lymph nodes, bone marrow, liver, and spleen. Fluorescent signals were still observed after eight months, demonstrating the long-term stability of the QDs in vivo. “The new coatings allowed us to observe QDs much longer than previously demonstrated,” says Byron Ballou of Carnegie Mellon. “We were pleased with the long persistence of fluorescence, as well as the large increase in circulating time caused by increasing the thickness of the outer polymer coat.” These results could enable QDs to target specific tissues or track the trafficking of labeled molecules over long periods. “Our findings are a promising step towards using QDs for noninvasive imaging in humans to monitor and treat diseases such as cancer.”
On the blink no longer NANOTECHNOLOGY Two researchers at the University of Illinois, UrbanaChampaign have suppressed blinking in quantum dot (QD) emission under conditions relevant to biological imaging experiments [Hohng and Ha, J. Am. Chem. Soc. (2004) 126 (5), 1324]. The broad absorption and narrow emission of semiconductor QDs, combined with their brightness and photostability, makes them far better fluorophores for biological labeling than organic dyes. However, the emission from single QDs blinks on and off, limiting their use for labeling individual biological molecules. The off-state is thought to occur when a charged dot loses an electron to a surface trap state. If blinking could be suppressed, quantum information processing using single photon sources might also benefit. Sungchul Hohng and Taekjip Ha show that passivation of QD surfaces with small thiolcontaining molecules can significantly reduce blinking. Streptavidin-coated CdSe/ZnS QDs were immobilized on a biotinylated surface and the
10
March 2004
emission intensities from hundreds of dots were recorded over time. Concentrations of β-mercaptoethanol greater than 1 mM showed a reduction in blinking. At 1 mM, 60% of dots were blink-free (fewer than two on-off events in 80 s), and at 10 mM, 80% were blink-free. Larger molecules containing thiol groups did not suppress the on-off blinks in emission. Hohng and Ha believe that the QD polymer coating has holes that allow only small molecules to pass through to bind onto the ZnS surface. The researchers suggest that the thiol moiety, an electron donor, can transfer electrons to the surface traps. This prevents loss of electrons from the QD to the traps and reduces the blinking frequency. Hohng and Ha suggest that uninterrupted observations of the continuous movement of labeled motor proteins and nucleic acids may now be possible. However, live cell imaging applications may not be compatible with the presence of 1 mM β-mercaptoethanol.
Headache for nanoparticles NANOTECHNOLOGY The discovery that inhaled nano-sized particles can make their way into rats’ brains poses a number of questions for further study of environmental exposure to nanomaterials, say researchers [Oberdörster et al. Inhalation Toxicol. (2004), in press]. Ultrafine particles (UFPs), <100 nm in size, from multiple natural and artificial sources, permeate our environment. UFPs have been associated with respiratory and cardiovascular disease in a number of studies. Oberdörster and colleagues at the University of Rochester, University of New Mexico, National Institutes of Health, and the National Research Center for Environment and Health (GSF) in Germany have now shown that rats exposed to 13C-labeled UFPs for six hours show an increase in 13C concentration in parts of the brain. There is a persistent increase in 13C in the olfactory bulb and some inconsistent evidence of increases in the cerebellum and cerebrum. The researchers suggest that UFPs deposited in the nasal region of rats are translocated along the olfactory nerve to the olfactory bulb. This pathway for the transport of inhaled particles is not generally considered and its significance in humans is unknown. A number of issues are yet to be resolved: the mechanism of neuronal uptake; how physical and chemical properties of the particles affect transport; how far the particles can penetrate the central nervous system beyond the olfactory bulb; and the long-term toxic effects of accumulation in the brain. The answers will be important for assessing the risks of environmental and occupational exposure to ambient nanoparticles, as well as the usefulness of nanoparticles and quantum dots in biomedicine.
Long-life labels for imaging NANOTECHNOLOGY
Noninvasive fluorescence image showing major sites of quantum dot deposition in a mouse 24 hours after injection. (Courtesy of Lauren Ward.)
Quantum dots (QDs) have the potential to act as stable, bright fluorophores for labeling biological specimens. However, preparations of QDs have not so far been optimized for in vivo studies. If QDs are to useful for be in vivo imaging, they must have an adequate circulation lifetime, show minimal nonspecific deposition, and retain fluorescence over a long period. Researchers at Carnegie Mellon University and Quantum Dot Corp. have engineered the surface coating of QDs for long-term, in vivo imaging in mice [Ballou et al., Bioconjugate Chem. (2004) 15 (1), 79].
The researchers coated core-shell CdSe-ZnS QDs with amphiphilic poly(acrylic acid). After injecting the coated QDs into mice, noninvasive fluorescence imaging showed a circulation lifetime of ~12 minutes. To improve this, different poly(ethylene glycol) molecules of increasing lengths were coupled to the initial polymer coat. This increased circulation lifetimes up to ~70 minutes. Fluorescence imaging also reveals where the QDs are deposited, including the lymph nodes, bone marrow, liver, and spleen. Fluorescent signals were still observed after eight months, demonstrating the long-term stability of the QDs in vivo. “The new coatings allowed us to observe QDs much longer than previously demonstrated,” says Byron Ballou of Carnegie Mellon. “We were pleased with the long persistence of fluorescence, as well as the large increase in circulating time caused by increasing the thickness of the outer polymer coat.” These results could enable QDs to target specific tissues or track the trafficking of labeled molecules over long periods. “Our findings are a promising step towards using QDs for noninvasive imaging in humans to monitor and treat diseases such as cancer.”
On the blink no longer NANOTECHNOLOGY Two researchers at the University of Illinois, UrbanaChampaign have suppressed blinking in quantum dot (QD) emission under conditions relevant to biological imaging experiments [Hohng and Ha, J. Am. Chem. Soc. (2004) 126 (5), 1324]. The broad absorption and narrow emission of semiconductor QDs, combined with their brightness and photostability, makes them far better fluorophores for biological labeling than organic dyes. However, the emission from single QDs blinks on and off, limiting their use for labeling individual biological molecules. The off-state is thought to occur when a charged dot loses an electron to a surface trap state. If blinking could be suppressed, quantum information processing using single photon sources might also benefit. Sungchul Hohng and Taekjip Ha show that passivation of QD surfaces with small thiolcontaining molecules can significantly reduce blinking. Streptavidin-coated CdSe/ZnS QDs were immobilized on a biotinylated surface and the
10
March 2004
emission intensities from hundreds of dots were recorded over time. Concentrations of β-mercaptoethanol greater than 1 mM showed a reduction in blinking. At 1 mM, 60% of dots were blink-free (fewer than two on-off events in 80 s), and at 10 mM, 80% were blink-free. Larger molecules containing thiol groups did not suppress the on-off blinks in emission. Hohng and Ha believe that the QD polymer coating has holes that allow only small molecules to pass through to bind onto the ZnS surface. The researchers suggest that the thiol moiety, an electron donor, can transfer electrons to the surface traps. This prevents loss of electrons from the QD to the traps and reduces the blinking frequency. Hohng and Ha suggest that uninterrupted observations of the continuous movement of labeled motor proteins and nucleic acids may now be possible. However, live cell imaging applications may not be compatible with the presence of 1 mM β-mercaptoethanol.
Headache for nanoparticles NANOTECHNOLOGY The discovery that inhaled nano-sized particles can make their way into rats’ brains poses a number of questions for further study of environmental exposure to nanomaterials, say researchers [Oberdörster et al. Inhalation Toxicol. (2004), in press]. Ultrafine particles (UFPs), <100 nm in size, from multiple natural and artificial sources, permeate our environment. UFPs have been associated with respiratory and cardiovascular disease in a number of studies. Oberdörster and colleagues at the University of Rochester, University of New Mexico, National Institutes of Health, and the National Research Center for Environment and Health (GSF) in Germany have now shown that rats exposed to 13C-labeled UFPs for six hours show an increase in 13C concentration in parts of the brain. There is a persistent increase in 13C in the olfactory bulb and some inconsistent evidence of increases in the cerebellum and cerebrum. The researchers suggest that UFPs deposited in the nasal region of rats are translocated along the olfactory nerve to the olfactory bulb. This pathway for the transport of inhaled particles is not generally considered and its significance in humans is unknown. A number of issues are yet to be resolved: the mechanism of neuronal uptake; how physical and chemical properties of the particles affect transport; how far the particles can penetrate the central nervous system beyond the olfactory bulb; and the long-term toxic effects of accumulation in the brain. The answers will be important for assessing the risks of environmental and occupational exposure to ambient nanoparticles, as well as the usefulness of nanoparticles and quantum dots in biomedicine.