As nanotechnology applications and nanomaterials slowly move into mainstream manufacturing, there will have to be an increasing focus on the environmental footprint that the production of various nanomaterials creates. A growing research body promises to lead to green(er) nanomanufacturing technologies. However, as we discussed in a Nanowerk Spotlight last year, this emerging field of green nanoscience faces considerable research challenges to achieve the maximum performance and benefit from nanotechnology while minimizing the impact on human health and the environment. As it stands now, it remains to be seen what the environmental footprint of nanotechnologies will be. So far, the message is mixed.
Silver has long been recognized for its infection-fighting properties and it has a long and intriguing history as an antibiotic in human health care. In ancient Greece and Rome, silver was used to fight infections and control spoilage. In its modern form, silver nanoparticles have become the promising antimicrobial material in a variety of applications because they can damage bacterial cells by destroying the enzymes that transport cell nutrient and weakening the cell membrane or cell wall and cytoplasm. For instance, an increasingly popular applications is to use pure silver, or silver-coated, nanoparticles in food packaging materials such as plastic bags, containers, films or pallet. A new study has found that silver nanoparticles can bind with double-stranded DNA and, possibly in this way, result in compromised DNA replication fidelity both in vitro and in vivo. But the study could not conclusively determine whether silver nanoparticles directly interact with DNA polymerases.
Back in 2006, researchers introduced the concept of a carbon nanotube (CNT) knife that, in theory, would work like a tight-wire cheese slicer. In the meantime, other research groups have developed similar approaches, for instance for cutting and sharpening carbon nanotubes (see for instance: Nanotechnology grinders). Now, the group that introduced the CNT nanoknife in 2006 has refined their design and demonstrated the feasibility of fabricating a nanoknife (compression-cutting tool at the nanoscale) based on an individual CNT. The researchers stretched an individual nanotube between two tungsten needles in a manner that allowed them to test the mechanical strength of assembled device. A force test on the prototype nanoknife indicated that failure was at the weld while the CNT was unaffected by the force we applied. In situ load tests on the nanoknife indicated maximum breaking force to be in micro Newton range.
Just a few days ago we covered the exciting and quickly developing world of nanotechnology machinery, specifically nanomotors. In this previous Nanowerk Spotlight we focused on one approach to nanomotors, which is copying nature's catalytic biomotors. Today we will look at an example of mechanical approach that works with carbon nanotubes (CNTs). Some experimental work concerning CNT motor system has already been reported, but new work coming out of Japan could very well be the smallest motor so far. Researchers in Japan investigated the linear and rotary motions of a CNT capsule at room temperature when it is sealed by other CNTs in a hollow space of a host CNT. It is the first observation of linear motion of CNT capsules. Such a system can be obtained by simply heating C60 peapods, and its size is comparable or much smaller than well-known protein-based molecular motors in the bioengineering field.
One of the many application areas that carbon nanotubes (CNTs) are experimented with is as a drug carrier in nanomedicine. Typically, nanoparticles have been used for drug delivery and it is only recently that carbon nanotubes have gained attention as potential drug delivery vehicles. Current research has shown the ability of CNTs to carry a variety of molecules such as drugs, DNA, proteins, peptides, targeting ligands etc. into cells - which makes them suitable candidates for targeted delivery applications. Polyethylene glycol (PEG) with molecular weight between 1 and 40 kDa is usually used to coat drug or imaging nanocarriers with the purpose of reducing non-specific interactions with cells and thus increasing the nanocarriers circulation time in vivo. However, when PEG in the form of PL-PEG (adsorbing phospholipid-PEG) has been used to functionalize single-walled CNTs, the finding were not consistent, sometimes even contradictory.
Nearly every chemical or physical property of materials depends upon temperature, and researchers are only beginning to understand the huge breadth of applications that nanoscale heaters could facilitate. For example, scientists have previously demonstrated that a micro-heater built into an atomic force microscope can be used instead of a large furnace that is normally used to grow nanotubes as part of the chemical vapor deposition process. The tiny device provided highly-localized heating for only the locations where researchers wanted to grow the nanostructures. While most previous research on this kind of microcantilever heaters and thermometers used device elements that were several micrometers in size, researchers have now reported an approach to fabricate a 100 nanometer-sized heater/thermometer using contact photolithography and controlled anneal conditions. A deep understanding of nanomaterials requires nanoscale probes. With such nanoscale heater/thermometer devices it becomes possible to test the temperature dependence of materials properties at the very smallest scale and perform thermal diagnostics of nanomaterials.
Platinum nanoparticles are widely used as the cathode material in hydrogen/oxygen fuel cells due to their efficiency in catalyzing the oxygen reduction reaction (ORR), the process that breaks the bonds of the oxygen molecules. Although platinum is still considered the state-of-the-art ORR catalyst, it does not exhibit good stability. Typically, catalyst performance degradation begins as soon as the catalyst is introduced into a fuel cell and continues until it is no longer active. Platinum can lose its effectiveness either by clumping together or by becoming 'poisoned' by carbon monoxide, requiring high hydrogen purity or higher catalyst densities for the fuel cell to stay effective. This, together with the high cost of platinum is seen as one of the major showstoppers to producing mass market fuel cells for commercial applications. Researchers now found that vertically-aligned nitrogen-containing carbon nanotubes could be used as effective ORR electrocatalysts.
While the concept of a 'machine' can be extended to the nano-world, these nanomachines can not be built by just further miniaturizing machine blueprints from the macro-world. On the nanoscale, the nanomachine components would be atomic or molecular structures each designed to perform a specific task which, all taken together, would result in a complex function. The problem is that functional nanomachinery will need to take into account the quantum effects that dominate the behavior of matter at the nanoscale, affecting the optical, electrical and magnetic behavior of materials. An alternative approach to miniaturizing machines down to the nanoscale is to borrow from the highly successful design shop of Mother Nature. Until a few years ago, catalytic micro- and nanomotors have been more or less unknown outside biology. With the explosive growth in nanotechnology, however, catalyzed nanoscale motion has become a heavily researched phenomenon. Scientists find that there is much to be learned from nature's motor systems for the development of artificial nanoscale machinery. Joseph Wang, who runs the Laboratory for Nanobioelectronics at the Department of NanoEngineering at UC San Diego, has just published a review where he addresses the question: Can Man-Made Nanomachines Compete with Nature Biomotors?