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Natural answer for photonic materials
RESEARCH NEWS
Split decision on quantum dot toxicity NANOTECHNOLOGY
Surface-coated CdSe QDs track the migration of hepatocyte cells over long periods. (Reprinted with permission from Nano Letters. 2004 © American Chemical Society).
CdSe quantum dots (QDs) are promising alternatives to organic dyes for biological labeling because of their bright fluorescence, narrow emission, and high photostability. In the future, QDs could also be used as smart particles for diagnostic and therapeutic use in the body. However, concerns about the potential toxicity of Cd-containing nanoparticles must first be addressed. “Some researchers have considered QDs to be inert structures where Cd is safely passivated,” explains Sangeeta Bhatia of the University of California at San Diego. “Others have dismissed the use of CdSe nanocrystals in vivo because
of their potential toxicity.” In order to settle this question, Bhatia’s research group in California has systematically investigated the cytotoxicity of CdSe QDs in cultures of primary hepatocyte cells [Derfus et al., Nano Lett., published online 10 Dec 2003, DOI: 10.1021/nl0347334]. These cells serve as a model for the liver, which is the primary site of acute injury within the body on exposure to Cd. The results show that surface oxidation of CdSe QDs in air and under exposure to ultraviolet light leads to a dramatic decrease in cell viability. This could be because free Cd is progressively released from the QDs. The addition of surface coatings decreases the oxidation and, therefore, the cytotoxicity of the particles. By adding a ZnS capping layer, followed by small organic ligands and a cross-linked organic shell, the biocompatibility of the CdSe QDs can be progressively increased. Coated CdSe QDs can be used to label hepatocyte cells in culture and track their migration over a period of one week (as shown). There is no deleterious effect on cell function. “We see our findings as a split decision,” says Bhatia. “On the one hand, QDs are great labels for long-term tracking of live cells in vitro. On the other, for in vivo applications, the outlook is more bleak.” Even with surface coatings, there is still some release of Cd from the QDs through oxidation. Since there are mechanisms in the body that could cause oxidation or breakdown of the particles, this process could occur in vivo, releasing Cd into the body.
Natural answer for photonic materials FABRICATION AND PROCESSING Two studies have revealed examples of nature’s ability to manufacture nanostructured, photonic materials. Understanding the structure and production of these materials could inspire advances in biomimetic fabrication methods. Wendy J. Crookes and colleagues at the University of Hawaii-Manoa and the University of California-Los Angeles have found that reflective tissues in squids contain unusual proteins, which they have named ‘reflectins’ [Crookes et al., Science (2004) 303, 235]. Animals often use reflective tissues for camouflage, modulating incident sunlight or bioluminescence. Such tissues consist of layers of flat platelets of high refractive index alternating with layers of low refractive
index. The Hawaiian bobtail squid, Euprymna scolopes, possesses a light organ in which reflective tissue directs the luminescence of a population of symbiotic bacteria ventrally. Unlike other aquatic animals, the platelets in the squid’s reflective tissue are proteinaceous. Furthermore, the proteins have a highly unusual amino acid composition. Crookes suggests that protein nanoreflectors such as these could be used to enhance the power and efficiency of bioelectronic devices. The biological world is an arena of nanofabrication, one that can be tapped for information about overcoming the constraints on the design and production of small-scale materials, according to the researchers.
The reflective tissues in the eye and skin of the Hawaiian bobtail squid are silvery because of the structure of reflectin proteins. (Courtesy of Margaret McFall-Ngai.)
Researchers at the University of Oxford have discovered an opaline photonic crystal in the Australian weevil, Pachyrhynchus argus [Parker et al., Nature (2003) 426, 786]. The metallic color, which is thought to be important for its behavior and recognition, derives from its scales. The inner structure is a
solid array of transparent spheres 250 nm in diameter. The spheres show hexagonal close-packing, analogous to that of opal. “If we can decipher and emulate the weevil’s means of opal production,” says Andrew R. Parker, “it would represent a technological breakthrough.”
February 2004
23
Split decision on quantum dot toxicity NANOTECHNOLOGY
Surface-coated CdSe QDs track the migration of hepatocyte cells over long periods. (Reprinted with permission from Nano Letters. 2004 © American Chemical Society).
CdSe quantum dots (QDs) are promising alternatives to organic dyes for biological labeling because of their bright fluorescence, narrow emission, and high photostability. In the future, QDs could also be used as smart particles for diagnostic and therapeutic use in the body. However, concerns about the potential toxicity of Cd-containing nanoparticles must first be addressed. “Some researchers have considered QDs to be inert structures where Cd is safely passivated,” explains Sangeeta Bhatia of the University of California at San Diego. “Others have dismissed the use of CdSe nanocrystals in vivo because
of their potential toxicity.” In order to settle this question, Bhatia’s research group in California has systematically investigated the cytotoxicity of CdSe QDs in cultures of primary hepatocyte cells [Derfus et al., Nano Lett., published online 10 Dec 2003, DOI: 10.1021/nl0347334]. These cells serve as a model for the liver, which is the primary site of acute injury within the body on exposure to Cd. The results show that surface oxidation of CdSe QDs in air and under exposure to ultraviolet light leads to a dramatic decrease in cell viability. This could be because free Cd is progressively released from the QDs. The addition of surface coatings decreases the oxidation and, therefore, the cytotoxicity of the particles. By adding a ZnS capping layer, followed by small organic ligands and a cross-linked organic shell, the biocompatibility of the CdSe QDs can be progressively increased. Coated CdSe QDs can be used to label hepatocyte cells in culture and track their migration over a period of one week (as shown). There is no deleterious effect on cell function. “We see our findings as a split decision,” says Bhatia. “On the one hand, QDs are great labels for long-term tracking of live cells in vitro. On the other, for in vivo applications, the outlook is more bleak.” Even with surface coatings, there is still some release of Cd from the QDs through oxidation. Since there are mechanisms in the body that could cause oxidation or breakdown of the particles, this process could occur in vivo, releasing Cd into the body.
Natural answer for photonic materials FABRICATION AND PROCESSING Two studies have revealed examples of nature’s ability to manufacture nanostructured, photonic materials. Understanding the structure and production of these materials could inspire advances in biomimetic fabrication methods. Wendy J. Crookes and colleagues at the University of Hawaii-Manoa and the University of California-Los Angeles have found that reflective tissues in squids contain unusual proteins, which they have named ‘reflectins’ [Crookes et al., Science (2004) 303, 235]. Animals often use reflective tissues for camouflage, modulating incident sunlight or bioluminescence. Such tissues consist of layers of flat platelets of high refractive index alternating with layers of low refractive
index. The Hawaiian bobtail squid, Euprymna scolopes, possesses a light organ in which reflective tissue directs the luminescence of a population of symbiotic bacteria ventrally. Unlike other aquatic animals, the platelets in the squid’s reflective tissue are proteinaceous. Furthermore, the proteins have a highly unusual amino acid composition. Crookes suggests that protein nanoreflectors such as these could be used to enhance the power and efficiency of bioelectronic devices. The biological world is an arena of nanofabrication, one that can be tapped for information about overcoming the constraints on the design and production of small-scale materials, according to the researchers.
The reflective tissues in the eye and skin of the Hawaiian bobtail squid are silvery because of the structure of reflectin proteins. (Courtesy of Margaret McFall-Ngai.)
Researchers at the University of Oxford have discovered an opaline photonic crystal in the Australian weevil, Pachyrhynchus argus [Parker et al., Nature (2003) 426, 786]. The metallic color, which is thought to be important for its behavior and recognition, derives from its scales. The inner structure is a
solid array of transparent spheres 250 nm in diameter. The spheres show hexagonal close-packing, analogous to that of opal. “If we can decipher and emulate the weevil’s means of opal production,” says Andrew R. Parker, “it would represent a technological breakthrough.”
February 2004
23