I am still trying to understand the nano-world. It’s a big world and there are many different kinds of very small particles. But I’ve had some trouble finding good definitions of the inhabitants of this new world. What, for example, are quatum dots? And what makes metallic
nanoparticles different from other kinds of nanoparticles?
As discussed earlier down in Whoville, we know that not all nanoparticles (particles smaller than 100 nanometers) are created equally, and, even better -- or worse, depending on your viewpoint and the material -- many nanoparticles aren’t even equal to their larger counter parts. And that really, is just the point, or one of the points at least, of all this technology.
Fortunately for me, the EPA, in their recent Nanotechnology White Paper, organizes nanoparticles into four categories, and though there may be other ways to categorize nanomaterials I found these groupings helpful in understanding the different kinds of nanoparticles that might one day enter our world – if they haven’t already.Below are EPA's catagorizes for nanomaterials along with some brief examples.
Carbon-based nanomaterials include things like fullerenes (cage-like carbon structures) which make up the single walled carbon nanotubes (those are the SWNTs I’ve referred to before) and buckyballs. All are made carbon. Just carbon. When there are 60 carbons involved, a sphere is formed, its a Buckyball. When there are more, the structure is tube – or cage-like, and is made of a single layer of carbons, almost like a tube of chicken wire, it’s a SWNT.
Metal-base nanomaterials include quantum dots, metal oxides and pure metal nanoparticles. Quantum dots are structures so small that their properties are susceptible to the removal of a single electron. Every living creature depends on a kind of quantum dot for energy production, as electrons are moved around by proteins so the cell can store or use energy.
Manufactured quantum dots can contain a small number of atoms, for example, from tens of atoms to a few hundred. Some manufactured quantum dots are nanosized crystals of various elements (silicon and germanium or cadmium and selenium are a couple of examples), and emit light when excited. What most interesting is that the color of the light, which is based on wavelength, will vary with the size of the crystal or the type of crystal, with smaller particles of a particular crystal emitting light of shorter wavelengths (towards the blue end of the visible light spectrum) and larger particles emitting light of longer wavelengths (towards the red end.)
Titanium dioxide, which you can find in your sunblock lotion, is an example of a metal oxide that is now manufactured as a nano-metal oxide. As explained in an earlier article, it's the nano formulation of this material that allows us to smear the sunblock but avoid looking like a clown.
Metals can also exist as single ions, or larger bulkier structures think gold, or silver. But, as with many nanomaterials, it seems that when metals occur as nanoparticles they may exhibit different properties than their larger counterparts. Nanoized silver (or silver ions), for example, is a potent antimicrobial, but apparently aggregates of silver particles tend to loose their antimicrobial ability.
Dendrimers are branched polymers (a polymer is made up of repeating units or monomers. Monomers are molecules that can combine – or polymerize - with similar or identical molecules.) These can be manufactured so that they can carry other molecules within them, such as certain drugs.
Composites refer to combinations of nanomaterials with other materials, for example DNA molecules may be combined with various nanomaterials to make a nanosized biocomposite.These examples just scratch the surface of the world of nanomaterials. But this revolutionary technology is sure to present those charged with protecting human health and the environment a future filled with both opportunity, (providing new materials to clean up and reduce distribution and use of hazardous materials, and new drug formulations) and challenges as health and environmental scientists race to understand the impact of materials that play by new rules.