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Using just one kind of nanoparticle (gold) the researchers built two common but very different crystalline structures by merely changing one thing -- the strands of synthesized DNA attached to the tiny gold spheres. A different DNA sequence in the strand resulted in the formation of a different crystal.
The technique, to be published in the journal Nature, and reflecting more than a decade of work, is a major and fundamental step toward building functional "designer" materials using programmable self-assembly. This "bottom-up" approach will allow scientists to take inorganic materials and build structures with specific properties for a given application, such as therapeutics, biodiagnostics, optics, electronics or catalysis.
DNA would appear to be the best option for guiding the assembly of the nanoparticle bricks into the desired construction. DNA strands can be attached to the nanoparticles, with sequences programmed to zip up with complementary DNA strands on a neighboring particle.
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Using a single DNA linker sequence results in a close-packed, face-centered cubic crystal structure. But using two different linker sequences that bind to each other but not themselves gives a binary system, which crystallizes in an open, body-centered cubic structure.
“We are now closer to the dream of learning, as nanoscientists, how to break everything down into fundamental building blocks and reassemble them into whatever structure we want,” says Mirkin.
Using a similar method, researchers at Brookhaven National Laboratory (BNL) have further investigated the interactions between complementary DNA-functionalized nanoparticles [Nykypanchuk et al., Nature (2008) 451, 549].
Just 17 piconewtons, or 60 trillionths of an ounce, is the force it takes to push a cobalt atom across a copper surface. This is one of the findings of a group of IBM scientists who have been testing a new kind of atomic force microscope (AFM), which has made the first measurements of the force required to move an individual atom.
The February 1, 2008 edition of the journal Science includes a brief article focusing on the futuristic sounding topic of Adaptive Composites. Richard Vaia and Jeffery Baur write that materials are now under development that can respond dynamically to changes in their environment. Concepts such as suppleness to squeeze through crevices, aircraft skin that regulates temperature and self-healing composites are now being studied. These ideas go beyond the idea of advanced composites and into the field of adaptive composites. This same focus in materials science are "driving the development of adaptive composites that mimic biological responsive functionality while operating in extreme environments." Today's design includes ideas of "structural efficiency" with "active functions such as sensing, energy harvesting and propulsion are added by attaching components to the structure." But changes may be on the way soon.
Currently "advanced passive material technologies such as continuous-fiber organic-matrix composites" are used in various applications. New innovations are aiming to incorporate "flexible, jointed frameworks and complex materials [that] impart active functionality at multiple length scales within the materials." This requires synthetics that combine materials that have active properties, autonomic response and "new computational tools that enable design, analysis and optimization of the collective and hierarchical dynamic character." The aerospace industry seems to be most interested in the "transformation of rigid substructure to a dynamic, articulated structure." Uses include "large-scale antennae in space and the development of morphing wings on unmanned aerial vehicles."
Other goals include "energy-efficient locomotion and concealment" which are top priorities. By mimicking organisms it is hoped that "highly deformable networks can be created from cellular materials, bistable composite laminates and bimorph strips." It is also hoped that using CNTs(carbon nanotubes) will be incorporated into mats and arrays that stress energy can be stored and recovered. From all appearances the greatest hurdle is not the lack of constituents for material composites but a real need for better and more "streamlined computational design tools that capture the properties of the many possible configurations or states." The complexity in coupling multiple active materials with evolutionary, emergent and morphogenic attributes is with not an obstacle. Indeed it remains only a challenge yet to be fulfilled. Some real challenges do include weakness at interfaces, bonding and reverse flexibility.
The contributors Vaia and Baur say that elegant outlines have been proposed in earnest since 2004. They also believe utilization of micro- and nano-robotics as well as fluidics will also contribute much to these novel adaptive materials. In these two ideas we not only face the challenge of design but also that of durability.
Imagine a search-and-rescue robot that can change shape to squeeze through crevices with the suppleness of an octopus, or an aircraft skin with a circulatory system that enables temperature regulation, cooling, and self-healing similar to an animal. Such concepts are driving the development of adaptive composites that mimic biological responsive functionality while operating in extreme environments. Traditionally, the high-performance, load-bearing substructures of aircraft, satellites and robots are designed for structural efficiency.
Thus, they are rigid and passive; active functions such as sensing, energy harvesting, and propulsion are added by attaching components to the structure. This compartmentalization of functions into attached subsystems streamlines manufacturing and maintenance and facilitates upgrades. Advanced passive material technologies, such as continuous-fiber organic-matrix composites, have revolutionized applications from sporting equipment and prosthetics to satellites and aircrafts.
For example, Baughman and co-workers used a coating of catalytic nanoparticles on a shape-memory alloy wire to convert enough chemical energy to thermal energy to trigger a shape recovery of the wire (7). Advances in understanding natural systems are also high-lighting chemical triggers. For example, addition of acetylcholine and calcium ionophores shifts the iridescence of the mantle iridophores of the cephalopod Lolliguncula brevis from red to blue, providing dynamic coloration. Recombinant derivatives of comparable proteins have been self-assembled in vitro to form fibers and diffraction gratings with dynamic coloration (8).
Radiation can also be used as a trigger for adaptive response. Recent advances in optically triggered, reversible colloid surface chemistry (9) may lead to rheological fluids that do not require a bias to maintain an ordered electrical or magnetic state. Photoisomerization on the molecular scale can alter liquid-crystal phase stability or control domain orientation, resulting in substantial changes in mechanical and optical characteristics (10). Koerner and co-workers used carbon nanotubes dispersed in shape-memory polymers to trigger shape recovery both electrically and optically (11).
Materials that directly convert radiation into electricity could produce a new era of spacecraft and even Earth-based vehicles powered by high-powered nuclear batteries, say US researchers. Electricity is usually made using nuclear power by heating steam to rotate turbines that generate electricity. But beginning in the 1960s, the US and Soviet Union used thermoelectric materials that convert heat into electricity to power spacecraft using nuclear fission or decaying radioactive material. The Pioneer missions were among those using the latter, "nuclear battery" approach. Dispensing with the steam and turbines makes those systems smaller and less complicated. But thermoelectric materials have very low efficiency. Now US researchers say they have developed highly efficient materials that can convert the radiation, not heat, from nuclear materials and reactions into electricity.