Controlling Color with Magnets New material can become any visible color
A solution of nanoscopic iron oxide particles changes color as a magnet gets closer to it, causing the particles to re¬arrange. The color changes from red to blue as the magnetic field’s strength increases.
SOURCE: “Highly Tunable Superparamagnetic Colloidal Photonic Crystals” Yadong Yin et al. Angewandte Chemie International Edition online, July 3, 2007
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RESULTS: Researchers at the University of California, Riverside, have demonstrated that a liquid containing suspended magnetite particles changes colors in the presence of an electromagnet. The liquid can be made to reflect any visible color and can switch colors at a rate of twice per second.
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WHY IT MATTERS: Others have made magnetically controlled color-changing materials, but the colors covered only small parts of the spectrum, and the materials took longer to switch colors than the Riverside researchers’ do. The new materials could be used as sensors that register changes in magnetic fields. And microcapsules full of the liquids could eventually be used as pixels in rewritable posters or other large displays.
METHODS: The researchers used a new high-temperature method to synthesize nanoscale, crystalline magnetite particles, which were then induced to form clusters. The researchers treated the clusters with a surfactant that creates an electric charge on their surfaces. This charge repels neighboring clusters. They then applied a magnetic field, counteracting the repellent forces; the stronger the field is, the closer together the clusters get. As the clusters rearrange themselves, the solution they’re suspended in reflects light of different colors.
NEXT STEPS: The researchers hope to increase switching speeds by confining smaller amounts of material in microscopic spaces. They are also developing applications such as sensors and displays.
Nanowire Microscope A tiny laser could reveal new details about the structure and behavior of living cells
SOURCE: “Tunable Nanowire Nonlinear Optical Probe” Jan Liphardt, Peidong Yang, et al. Nature 447: 1098-1101
RESULTS: Researchers have developed a nanowire-based laser smaller than a red blood cell. They incorporated the laser into a type of microscope that combines multiple microscopy techniques and achieves a resolution of about 100 nanometers.
WHY IT MATTERS: In addition to imaging by means of light, the microscope could eventually probe cells by applying finely controlled amounts of force with the nanowire; it could then monitor how these forces change the shape of cells and how the cells respond to such mechanical stimuli. This could give researchers a better understanding of how cells work.
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METHODS: Tiny forces exerted by light from an infrared laser hold the nanowire in place. The laser also serves as an optical pump, providing a source of energy that induces the nanowire to emit green light. Images can be obtained by measuring the light that either passes through or reflects off a sample as the nanowire moves over it. The device can also be used to trace the shape of a cell membrane by monitoring the displacement of the nanowire as it moves across the membrane.
NEXT STEPS: The researchers will modify the shape of the nanowire so that the laser can better hold it in place: the wire tends to slide around in the optical trap. A conical shape could give the device better resolution and give the researchers increased control over mechanical probing.
