By Will Soutter
Nanostructures are usually assumed to be a relatively new phenomenon, but in fact some nanoparticles have been present in animals and minerals for millions of years.
Now that scientists have the tools to examine samples on this scale, they are discovering more and more naturally-occuring functional structures which rely on nanoscale effects.
Some types of bacteria use magnetic nanocrystals to orient themselves in a magnetic field.
Nano navigation systems
A group of bacteria, known as magnetotactic bacteria, have been found to produce strings of magnetite nanocrystals to help them navigate using the earth's magnetic field. These bacteria are thought to be some of the oldest bacteria in existence, having existed for around 2 billion years.
Many migratory birds also use magnetic structures for navigation. Homing pigeons have nanostructures in their beaks which amplify magnetic fields, making it easier for them to sense the earth's weak magnetic field.
A common type of marine algae called diatoms have silica nanostructures in their cell walls which strongly diffract incoming light, helping them use solar energy more efficiently. Researchers have suggested that imitating this natural nanostructure could be useful in developing new technologies, like light-activated drug delivery tools.
Moths can see much better than humans in dim or dark conditions, because of the nanostructures in their eyes, which can absorb light very efficiently. The surface of the moth's eye is patterned on a scale of around 100nm, which is smaller than the wavelength of visible light (350-800nm). This reduces the reflectance of the surface, enabling the moth's eye to absorb more of the incoming light.
Homing pigeons and other migratory birds use nanostructures to help them navigate using the earth's magnetic field.
Another amazing phenomenon commonly seen in nature is the hydrophobicity of surfaces, such as lotus leaves. In 2006, researchers at the University of Michigan studied the lotus to understand exactly how its hydrophobicity occurs.
They found that the leaves of lotus plants are coated in hydrophobic wax crystals measuring about 1 nm across. As the water rolls off, the dirt on the surface is also removed, thereby making the lotus a "self-cleaning" plant.
Some species of beetle, like the opal weevil, possess an exoskeleton made of nano-sized photonic crystals, giving their backs a sparkling shimmer. These 3D crystals can reflect light at any angle and produce bright, brilliant colors that appear metallic or iridescent. This phenomenon was first studied by researchers at the University of Utah in 2008.
The surfaces of butterfly wings have multilayer nanoscale patterns. These structures cause an optical interference pattern, which filters light by reflecting mostly one wavelength. This means that we see a single bright color, like the striking blue wings of the Brazilian Morpho Rhetenor butterfly, without pigments.
The edelweiss is an alpine flower found at high altitudes of about 3,000 m (10,000 ft), where UV radiation is strong. The flower is protected from this radiation, as it is coated with a layer of thin hollow filaments.
These filaments are in turn covered in nanostructures around 100-200 nm in size, which is roughly the wavelength of UV radiation. These structures absorb the UV, protecting the inner parts of the flower from damage. They also reflect all visible light, which gives rise to the flowers' bright white colour.
Edelweiss flowers contain a protective nanotructured layer which shields them from the UV radiation they are exposed to at high altitudes.
The discovery of natural nanostructures has opened up a whole new world of possibilities for scientists, who can work at replicating them in the lab. Studying how these structures are naturally produced will give a clearer picture about how materials behave on the nanoscale, which can pave the way for more exciting applications of nanotechnology.
Sources and Further Reading
Image credits: Photos.com