One of the foundations of modern capitalistic society is the opportunity for continued economic growth, with the potential for ever-increasing material wealth for its citizens. For many, before the present global economic downturn, that growth seemed to stretch out of sight. But the recent real-estate slump and related financial troubles are merely potholes. The real roadblock is energy. The way forward must be paved with innovation.
The world's energy challenge has three components. Foremost is the shortage of resources. Approximately 80 percent of the energy consumed worldwide comes from fossil fuels. At our current levels of production and consumption, we will run out of known reserves in the lifetime of our grandchildren, if not sooner.
Second, those resources are concentrated in politically unstable regions. That makes the quest for energy security a matter of national security.
Third, the effects of fossil fuels on the environment cannot be denied. Petroleum and coal each contribute 40 percent of the global emissions of carbon dioxide, the most abundant "greenhouse" gas and the main cause of global warming.
Can we stabilize greenhouse gas emissions without sacrificing our standard of living? Yes, if we take immediate action to reduce individual consumption of energy; improve the efficiency of energy conversion, transport and storage; and develop renewable sources via transformational science and technology.
The study and design of materials at the nanoscale—€” on the order of billionths of a meter—€”has the potential to address the energy challenge because nanomaterials have different chemical and physical properties than bulk materials. Understanding these properties will allow scientists to tailor materials for specific uses.
Here's one such game-changing approach: using DNA to guide the assembly of new materials from the bottom up. Such reliable, reproducible nanofabrication techniques could be applied to create new energy-related materials, from catalysts to solar cells.
A team led by physicist Oleg Gang at Brookhaven National Laboratory, using the lab's Center for Functional Nanomaterials, has used DNA to link up nanoparticles in various arrangements, including 3-D nanocrystals and clusters. The idea is that nanoparticles coated with complementary strands of DNA—€”segments of genetic code sequence that bind only with one another like highly specific Velcro—€”help the nanoparticles find and stick to one another in highly specific ways. By varying the use of complementary DNA and strands that don't match, scientists can exert precision control over the attractive and repulsive forces between the nanoparticles to achieve the desired construction.
The Brookhaven team's latest advance, reported recently in Nature Materials, has been to use DNA linkers to attach some of the DNA-coated nanoparticles to a solid surface to further constrain and control how the nanoparticles can link up. This yields even greater precision and, therefore, a more predictable, reproducible high-throughput construction technique for building clusters from nanoparticles.
When a particle is attached to a support surface, it cannot react with other molecules or particles in the same way as a free-floating particle. This is because the support surface blocks about half of the particle's reactive surface. Attaching a DNA linker or other particle that specifically interacts with the bound particle then allows for the rational assembly of desired particle clusters. Controlling the number and length of DNA linkers also regulates interparticle distances and a cluster's architecture, permitting precise assembly of nano-objects into more complex structures.
Instead of assembling millions and millions of nanoparticles into 3-D nanocrystals, as was done in the earlier work, this technique allows the assembly of much smaller structures from individual particles. In the Nature Materials paper, the scientists describe the details for producing symmetrical, two-particle linkages, known as dimers, as well as small, asymmetrical clusters of particles—€”both with high yields and low levels of other, unwanted assemblies.
By arranging a few nanoparticles in a particular structure, new properties can emerge. Here, nanoparticles are analogous to atoms, which, when connected in a molecule, often exhibit properties not found in the individual atoms. The properties of these new materials may be advantageous for many potential applications.
For example, Gang's team describes an optical effect that occurs when nanoparticles are linked as dimer clusters. When an electromagnetic field interacts with the metallic particles, it induces a collective oscillation of the material's conductive electrons. This phenomenon, known as a plasmon resonance, leads to strong absorption of light at a specific wavelength. By adjusting these parameters, scientists might engineer clusters for absorbing a range of wavelengths in solar-energy conversion devices. Modulations in the plasmonic response could also be useful as a new means for transferring data, or as a signal for a new class of highly specific biosensors.
Asymmetric clusters, which were also assembled by the Brookhaven team, allow an even higher level of control, and therefore open new ways to design and engineer functional nanomaterials.
Because of its reliability and precision control, Brookhaven's nano-assembly method would be scalable for the kind of high-throughput production that would be essential for commercial applications.