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Super control of nanocrystals
RESEARCH NEWS
Targeting cancer
Super control of nanocrystals NANOTECHNOLOGY
NANOTECHNOLOGY Karen L. Wooley and colleagues from Washington University have functionalized polymer nanostructures for targeting cancer cells [Chem. Comm. (2003) 19, 2400]. Shell crosslinked nanoparticles (SCKs) are attracting interest for biological and biomedical applications because of their amphiphilic core-shell morphology. The polymer nanostructures have the advantages of structural integrity and the ability to be functionalized with receptorrecognizing or receptor-specific ligands. The ability to control the size, shape, and composition of SCKs indicates the potential for transport and delivery of biologically active agents. Of particular interest in drug delivery is the ability to target specific tissues. Cancerous tissue is an obvious target, and the folate receptor (FR) has been identified as a marker of ovarian, colorectal, and breast tumors. Since folic acid is now known to be a high affinity ligand for FRs, Wooley and colleagues set out to create folic acidconjugated SCKs. The functionalization process is
(Left) Fabrication of the amorphous SiO/SiO2 superlattice and thermal-induced phase separation and crystallization. (Right) Crosssectional TEM image of the layers of Si nanocrystals separated by oxide. (Courtesy of Max Planck Institute of Microstructure Physics.)
Researchers from the Max Planck Institute of Microstructure Physics in Halle, Germany have devised a new approach for the fabrication of ordered Si quantum dots [Solid State Phenom. (2003) 94, 95]. By preparing a SiO/SiO2 superlattice, the researchers are able to fabricate highly luminescent Si nanocrystals in a simple and easy manner. First, the superlattices are fabricated on 4” n-type Si wafers by reactive evaporation of SiO powders under vacuum or in an O2 atmosphere. The superlattices consist of SiO layers 1-4 nm thick with 3 to 45 periods separated by SiO2 layers 3-4 nm thick. Annealing of the amorphous SiO/SiO2 films at 1100°C induces a phase separation in which SiO in the ultrathin layers is transformed into nanoscale Si clusters or nanocrystals coated with an amorphous SiO2 shell (as shown). The superlattice structure forces the nanocrystals
into a dense, layered arrangement. In the transmission electron micrograph (TEM) shown above, the dark spots indicate the denser Si nanocrystals. Their uneven shape is caused by unresolved nanocrystals in lower layers. The SiO layer thickness limits the crystallization process and controls the size of the nanocrystals, which are typically 2.5 nm in diameter. Varying the thickness of the SiO2 layer and the oxygen content controls the crystal density (in the range of 1019/cm3) in the layers and their separation. Most importantly, however, the Si nanocrystals show strong room temperature luminescence. Implanting the nanocrystals with Er shifts the luminescence to the technologically useful 1.54 µm range. The approach could be applicable to light-emitting diodes, which currently rely on ion implantation for Si-based devices, or memory devices.
relatively simple, according to the researchers, but various means were necessary to confirm the attachment
Bringing light into line
of the folate groups. In aqueous
OPTICAL MATERIALS
solution, dynamic light scattering and analytical ultracentrifugation reveal an increase in nanoparticle size, molecular weight, and degree of hydration, which are all to be expected with the addition of folate. Atomic force and transmission electron microscopy in the solid state reveal comparable particle sizes. The researchers are now exploring the
in vitro and in vivo properties of the folate-functionalized SCKs, in particular the ability to direct the nanostructures to folate receptor expressing cells and enable cell uptake.
A material that combines buckyballs with polyurethane has powerful signal processing properties, according to new research [Appl. Phys. Lett. (2003) 83 (11), 2115]. “In our high optical quality films, light interacts 10 to 100 times more strongly with itself, for all wavelengths used in optical fiber communications, than in previously reported C60-based materials,” explains University of Toronto researcher Ted Sargent. Together with colleagues from Carleton University, the researchers have shown that the conjugated crosslinked C60-containing polyurethane films meet commercial engineering requirements by performing well at 1550 nm, the wavelength used in optical communications. By engineering a molecular system consisting of densely-packed
buckyballs in a processable polymer matrix, the researchers have produced a strong material with ultrafast nonlinear optical response. “Our materials enable light to change its own phase through nonlinear interactions with minimal absorption of light within the materials,” explains Sargent. The material could be used for optical switching and signal processing, in devices such as nonlinear gratings, nonlinear interferometers, and nonlinear directional couplers. “Ultimately, the research is slated to enable an agile optical network: one in which signals can be routed and processed on-thefly within the optical domain,” says Sargent. The next stage of the research, he says, is to incorporate the materials into nonlinear periodic devices to demonstrate their potential.
November 2003
9
Targeting cancer
Super control of nanocrystals NANOTECHNOLOGY
NANOTECHNOLOGY Karen L. Wooley and colleagues from Washington University have functionalized polymer nanostructures for targeting cancer cells [Chem. Comm. (2003) 19, 2400]. Shell crosslinked nanoparticles (SCKs) are attracting interest for biological and biomedical applications because of their amphiphilic core-shell morphology. The polymer nanostructures have the advantages of structural integrity and the ability to be functionalized with receptorrecognizing or receptor-specific ligands. The ability to control the size, shape, and composition of SCKs indicates the potential for transport and delivery of biologically active agents. Of particular interest in drug delivery is the ability to target specific tissues. Cancerous tissue is an obvious target, and the folate receptor (FR) has been identified as a marker of ovarian, colorectal, and breast tumors. Since folic acid is now known to be a high affinity ligand for FRs, Wooley and colleagues set out to create folic acidconjugated SCKs. The functionalization process is
(Left) Fabrication of the amorphous SiO/SiO2 superlattice and thermal-induced phase separation and crystallization. (Right) Crosssectional TEM image of the layers of Si nanocrystals separated by oxide. (Courtesy of Max Planck Institute of Microstructure Physics.)
Researchers from the Max Planck Institute of Microstructure Physics in Halle, Germany have devised a new approach for the fabrication of ordered Si quantum dots [Solid State Phenom. (2003) 94, 95]. By preparing a SiO/SiO2 superlattice, the researchers are able to fabricate highly luminescent Si nanocrystals in a simple and easy manner. First, the superlattices are fabricated on 4” n-type Si wafers by reactive evaporation of SiO powders under vacuum or in an O2 atmosphere. The superlattices consist of SiO layers 1-4 nm thick with 3 to 45 periods separated by SiO2 layers 3-4 nm thick. Annealing of the amorphous SiO/SiO2 films at 1100°C induces a phase separation in which SiO in the ultrathin layers is transformed into nanoscale Si clusters or nanocrystals coated with an amorphous SiO2 shell (as shown). The superlattice structure forces the nanocrystals
into a dense, layered arrangement. In the transmission electron micrograph (TEM) shown above, the dark spots indicate the denser Si nanocrystals. Their uneven shape is caused by unresolved nanocrystals in lower layers. The SiO layer thickness limits the crystallization process and controls the size of the nanocrystals, which are typically 2.5 nm in diameter. Varying the thickness of the SiO2 layer and the oxygen content controls the crystal density (in the range of 1019/cm3) in the layers and their separation. Most importantly, however, the Si nanocrystals show strong room temperature luminescence. Implanting the nanocrystals with Er shifts the luminescence to the technologically useful 1.54 µm range. The approach could be applicable to light-emitting diodes, which currently rely on ion implantation for Si-based devices, or memory devices.
relatively simple, according to the researchers, but various means were necessary to confirm the attachment
Bringing light into line
of the folate groups. In aqueous
OPTICAL MATERIALS
solution, dynamic light scattering and analytical ultracentrifugation reveal an increase in nanoparticle size, molecular weight, and degree of hydration, which are all to be expected with the addition of folate. Atomic force and transmission electron microscopy in the solid state reveal comparable particle sizes. The researchers are now exploring the
in vitro and in vivo properties of the folate-functionalized SCKs, in particular the ability to direct the nanostructures to folate receptor expressing cells and enable cell uptake.
A material that combines buckyballs with polyurethane has powerful signal processing properties, according to new research [Appl. Phys. Lett. (2003) 83 (11), 2115]. “In our high optical quality films, light interacts 10 to 100 times more strongly with itself, for all wavelengths used in optical fiber communications, than in previously reported C60-based materials,” explains University of Toronto researcher Ted Sargent. Together with colleagues from Carleton University, the researchers have shown that the conjugated crosslinked C60-containing polyurethane films meet commercial engineering requirements by performing well at 1550 nm, the wavelength used in optical communications. By engineering a molecular system consisting of densely-packed
buckyballs in a processable polymer matrix, the researchers have produced a strong material with ultrafast nonlinear optical response. “Our materials enable light to change its own phase through nonlinear interactions with minimal absorption of light within the materials,” explains Sargent. The material could be used for optical switching and signal processing, in devices such as nonlinear gratings, nonlinear interferometers, and nonlinear directional couplers. “Ultimately, the research is slated to enable an agile optical network: one in which signals can be routed and processed on-thefly within the optical domain,” says Sargent. The next stage of the research, he says, is to incorporate the materials into nonlinear periodic devices to demonstrate their potential.
November 2003
9