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Biocompatible nanotubes
RESEARCH NEWS
Biocompatible nanotubes POLYMERS Wolfgang Meier and colleagues at the University of Basel, Switzerland have prepared stable nanotubes in aqueous solution through the self-assembly of amphiphilic ABA-triblock copolymers [Grumelard et al., Chem. Commun. (2004) (13), 1462]. The water-filled nanotubes could find applications in drug delivery, as templates for inorganic synthesis, or as nanofluidic connections. While carbon nanotubes have a wide range of applications, they are difficult to purify, nonbiocompatible, and cannot be broken down by living organisms. Nanotubes made from biocompatible polymers could be used in biomedicine but, so far, such nanotubes have only been fabricated in organic solvents. Meier’s group prepared nanotubes of poly(2-methyloxazoline-blockdimethylsiloxane-block-2methyloxazoline) or (PMOXA-b-PDMS-
b-PMOXA) using a chloroform solution of the copolymer in a test tube. The solution is dried under nitrogen to leave a polymer film. The film is then rehydrated by adding water and stirring. Transmission electron microscopy reveals a mixture of selfassembled polymer vesicles and nanotubes. The nanotubes are tens of microns long and have remarkably uniform diameters of 40 nm. The stability of the nanotubes can be increased by introducing chemical cross-links. The ends of the copolymer are functionalized with methacrylate groups that polymerize on ultraviolet light irradiation. Since the nanotubes are based on synthetic polymers, a wide variety of modifications should be possible, says Meier. “For example, use of amphiphilic ABC triblock copolymers should lead to nanotubes with chemically different inner and outer surfaces.” Jonathan Wood
Designer defects show light touch OPTICAL MATERIALS
Two research groups at Kyoto University, Japan and Massachusetts Institute of Technology have independently reported the fabrication of three-dimensional photonic crystals (PCs) that use point defects to control light propagation at telecommunications wavelengths. The groups make use of different material systems, fabrication methods, and PC structures to achieve the same result. Periodic variations in the refractive index of PCs form a photonic band gap (analogous to the electronic band gap in semiconductors) in which light propagation is prohibited. If a defect mode is introduced into the band gap, light is strongly confined. Controlling light in this way could enable optical devices to be miniaturized, low threshold lasing, and optical quantum information processing. Susumu Noda and colleagues in Kyoto used a wafer-bonding technique to fabricate a ‘wood pile’ structure consisting of layers of stacked GaAs logs with a light-emitting layer of InGaAsP/InP multiple quantum wells inserted in the center [Ogawa et al., Science (2004) 305, 227]. Light emission at 1.55 µm is suppressed by up to 20 dB, but including defects in the wood pile lattice results in strong light emission from the defect locations. In contrast, researchers at MIT used a lithographic layer-by-layer approach to
Scanning electron micrographs of three-dimensional PCs fabricated using a layer-by-layer lithography technique. (Courtesy of Minghao Qi, MIT.)
fabricate three-dimensional PCs in Si [Qi et al., Nature (2004) 429, 538. The novel structure consists of layers of vertical rods alternating with layers of cylindrical holes. The unetched cylindrical holes can be considered microcavities or point defects with resonant signatures between 1.3 µm and 1.5 µm. “We are currently focusing on low-cost, efficient fabrication techniques that produce three-dimensional PCs with designed defects in large areas,” says Minghao Qi. “Noda’s paper is ‘scientifically’ more interesting since it deals with light-emitting materials, while our paper is ‘technologically’ more interesting since our processes can be extended to mass production in a straightforward way,” says Qi. Jonathan Wood
Nanotubes light up the home NANOTECHNOLOGY When Thomas Edison made his incandescent lamp, he used a carbon filament in a high-vacuum enclosure made of glass. But the filaments were fragile, burnt out prematurely, and bulbs rapidly darkened as carbon was deposited on the glass. Now light bulbs may have come full circle with the demonstration of carbon nanotube filaments that perform better than standard W filaments [Wei et al., Appl. Phys. Lett. (2004) 84 (24) 4869]. Researchers at Tsinghua University, China and Louisiana State University have made light bulb filaments from either strands of single-walled nanotubes (SWNTs) or double-walled nanotube (DWNT) films prepared by chemical vapor deposition. The SWNTs or DWNTs are immersed in alcohol where they assemble into long filaments under surface tension as the alcohol evaporates.
The filaments are then connected to electrodes and sealed into a glass bulb under a high vacuum. The researchers demonstrate that both SWNT and DWNT filaments show a lower threshold voltage for incandescent light emission than W filaments. The light intensity is much stronger for the same applied voltage and the nanotubes show a higher electrical power efficiency. The light bulbs are also durable, operating continuously for over 360 hours and after being switched on and off over 5000 times. Household bulbs using carbon nanotubes as filaments could be available in the near future, the researchers believe. However, the nanotubes will need to be purified to remove amorphous carbon and catalyst particles to prevent the darkening of bulbs. Jonathan Wood
September 2004
11
Biocompatible nanotubes POLYMERS Wolfgang Meier and colleagues at the University of Basel, Switzerland have prepared stable nanotubes in aqueous solution through the self-assembly of amphiphilic ABA-triblock copolymers [Grumelard et al., Chem. Commun. (2004) (13), 1462]. The water-filled nanotubes could find applications in drug delivery, as templates for inorganic synthesis, or as nanofluidic connections. While carbon nanotubes have a wide range of applications, they are difficult to purify, nonbiocompatible, and cannot be broken down by living organisms. Nanotubes made from biocompatible polymers could be used in biomedicine but, so far, such nanotubes have only been fabricated in organic solvents. Meier’s group prepared nanotubes of poly(2-methyloxazoline-blockdimethylsiloxane-block-2methyloxazoline) or (PMOXA-b-PDMS-
b-PMOXA) using a chloroform solution of the copolymer in a test tube. The solution is dried under nitrogen to leave a polymer film. The film is then rehydrated by adding water and stirring. Transmission electron microscopy reveals a mixture of selfassembled polymer vesicles and nanotubes. The nanotubes are tens of microns long and have remarkably uniform diameters of 40 nm. The stability of the nanotubes can be increased by introducing chemical cross-links. The ends of the copolymer are functionalized with methacrylate groups that polymerize on ultraviolet light irradiation. Since the nanotubes are based on synthetic polymers, a wide variety of modifications should be possible, says Meier. “For example, use of amphiphilic ABC triblock copolymers should lead to nanotubes with chemically different inner and outer surfaces.” Jonathan Wood
Designer defects show light touch OPTICAL MATERIALS
Two research groups at Kyoto University, Japan and Massachusetts Institute of Technology have independently reported the fabrication of three-dimensional photonic crystals (PCs) that use point defects to control light propagation at telecommunications wavelengths. The groups make use of different material systems, fabrication methods, and PC structures to achieve the same result. Periodic variations in the refractive index of PCs form a photonic band gap (analogous to the electronic band gap in semiconductors) in which light propagation is prohibited. If a defect mode is introduced into the band gap, light is strongly confined. Controlling light in this way could enable optical devices to be miniaturized, low threshold lasing, and optical quantum information processing. Susumu Noda and colleagues in Kyoto used a wafer-bonding technique to fabricate a ‘wood pile’ structure consisting of layers of stacked GaAs logs with a light-emitting layer of InGaAsP/InP multiple quantum wells inserted in the center [Ogawa et al., Science (2004) 305, 227]. Light emission at 1.55 µm is suppressed by up to 20 dB, but including defects in the wood pile lattice results in strong light emission from the defect locations. In contrast, researchers at MIT used a lithographic layer-by-layer approach to
Scanning electron micrographs of three-dimensional PCs fabricated using a layer-by-layer lithography technique. (Courtesy of Minghao Qi, MIT.)
fabricate three-dimensional PCs in Si [Qi et al., Nature (2004) 429, 538. The novel structure consists of layers of vertical rods alternating with layers of cylindrical holes. The unetched cylindrical holes can be considered microcavities or point defects with resonant signatures between 1.3 µm and 1.5 µm. “We are currently focusing on low-cost, efficient fabrication techniques that produce three-dimensional PCs with designed defects in large areas,” says Minghao Qi. “Noda’s paper is ‘scientifically’ more interesting since it deals with light-emitting materials, while our paper is ‘technologically’ more interesting since our processes can be extended to mass production in a straightforward way,” says Qi. Jonathan Wood
Nanotubes light up the home NANOTECHNOLOGY When Thomas Edison made his incandescent lamp, he used a carbon filament in a high-vacuum enclosure made of glass. But the filaments were fragile, burnt out prematurely, and bulbs rapidly darkened as carbon was deposited on the glass. Now light bulbs may have come full circle with the demonstration of carbon nanotube filaments that perform better than standard W filaments [Wei et al., Appl. Phys. Lett. (2004) 84 (24) 4869]. Researchers at Tsinghua University, China and Louisiana State University have made light bulb filaments from either strands of single-walled nanotubes (SWNTs) or double-walled nanotube (DWNT) films prepared by chemical vapor deposition. The SWNTs or DWNTs are immersed in alcohol where they assemble into long filaments under surface tension as the alcohol evaporates.
The filaments are then connected to electrodes and sealed into a glass bulb under a high vacuum. The researchers demonstrate that both SWNT and DWNT filaments show a lower threshold voltage for incandescent light emission than W filaments. The light intensity is much stronger for the same applied voltage and the nanotubes show a higher electrical power efficiency. The light bulbs are also durable, operating continuously for over 360 hours and after being switched on and off over 5000 times. Household bulbs using carbon nanotubes as filaments could be available in the near future, the researchers believe. However, the nanotubes will need to be purified to remove amorphous carbon and catalyst particles to prevent the darkening of bulbs. Jonathan Wood
September 2004
11