Skip to content
Search
Generic filters
Exact matches only

Nanomaterials Tutorial

What is a natural nanomaterial?

All materials can, in principle, be described at the nanoscale. In this text, by ‘natural nanomaterials’ we mean materials that belong to the natural world (animal and mineral), without human modification or processing, and that have remarkable properties because of their inherent nanostructure.

The chemical identity and properties of a substance depend upon its molecular structure. The nanostructure of biological material is due to its supramolecular organization — the arrangement of tens to hundreds of molecules into shapes and forms in the nanoscale range. The interaction of light, water and other materials with these nanostructures gives the natural materials some remarkable properties which can be appreciated at the macroscale.

Natural nanomaterials provide an inspiring way of bringing nanoscience into the classroom. Many natural materials, with which students will be very familiar, owe their properties to nanostructures in their composition. It can be really enlightening to discover that common, natural materials, such as feathers and spider silk, or materials that we use every day, such as paper and clay, have properties that depend not only on their chemistry but also on their nanostructure.

Overview of natural nanomaterials

We see hundreds of examples of nanoscience under our eyes daily, from geckos that walk upside down on a ceiling, apparently against gravity, to butterflies with iridescent colors, to fireflies that glow at night. In nature, we encounter some outstanding solutions to complex problems in the form of fine nanostructures with which precise functions are associated.

A shortlist of some natural nanomaterials follows: it is not exhaustive, but the interested teacher can find more information in the bibliography at the end of this module.

  • Nanoparticles from natural erosion and volcanic activity: nanoparticles are part of our mineral world since they are naturally produced during erosion and volcanic explosions.
  • Minerals, such as clays, are nanostructured: clays are a type of layered silicate characterized by a fine 2D crystal structure. Among clays, mica has been the most studied. Mica is made up of large sheets of silicate held together by relatively strong bonds. Smectic clays, such as montmorillonite, have relatively weak bonds between layers. Each layer consists of two sheets of silica held together by cations such as Li+, Na+, K+and Ca2+. The presence of the cations is necessary to compensate for the overall negative charge of the single layers. The layers are 20–200 nm in diameter laterally and form into aggregates called tactoids, which can be about 1 nm or thicker. Naturally occurring clays include montmorillonite (MMT) and heritage. Thefine nanostructure of clays determines their properties. When water is added, the clay swells, but the volume change is rather unusual — it is several times the original volume due to the ‘opening’ up of the layered structure by the water molecules that replace the cations. Clay swelling is a significant factor in soil stability and must be taken into account when building roads etc.
  • Natural colloids, such as milk and blood (liquid colloids), fog (aerosol type), gelatin (gel type): in these materials, nanoparticles are dispersed in the medium (liquid or gas) but do not form a solution, rather a colloid. All these materials have the characteristic of scattering light and oſten their color (as in the case of blood and milk) is due to the scattering of light by the nanoparticles that make them up.
  • ineralised natural materials, such as shells, corals and bones: many of these materials are formed by calcium carbonate crystals that self-assemble together with other natural materials, such as polymers, to form fascinating three-dimensional architectures. For instance, a shell is grown by a layer of cells that first lays down a coating of protein supported by a polysaccharide polymer like chitin. The proteins act like a nano-assembly mechanism to control the growth of carbon carbonate crystals. Around each crystal remains a honeycomb-like matrix of protein and chitin. This relatively ‘flexible envelope’ is fundamental for the mechanical properties of the shell and mitigates cracking. The size of each crystal is around 100nm. The result is that the nacre of mollusk shells has extraordinary physical properties (strength, resistance to compression, etc.).
  • Materials like skin, claws, beaks, feathers, horns, hair: these materials are made largely of very flexible proteins like keratin, elastin and collagen. Keratins have a large glycine and alanine content. This leads to β-sheets that can bond strongly one with another in an aligned fashion. Fibrous keratin molecules can twist around each other to form helical intermediate filaments. Similarly, collagen (not related to the keratin in terms of primary structure) has a high percentage of glycine and forms flexible triple-helix structures. In addition to intra and inter-molecular bonds, keratins have numerous cysteines that can form stable disulfide bonds. The amount of cysteine in the protein determines the strength and rigidity of the material: keratin in human hair, for instance, contains about 14% cysteine. Materials like nails, hooves and claws have a higher percentage of cysteine.
  • Paper and cotton: both are made mainly of cellulose. The high strength, durability and absorbency of cotton are due to the nanoscale arrangement of the fibres.
  • Insect wings and opals: the colors seen in opals and butterflies are directly related to their fine structure, which reveals packed nanostructures that act like a diffraction grid and induce iridescence. In the case of opals, this is due to packed silica spheres in the nanometre range, uniform in size and arranged in layers. Butterflies oſten owe the color of their wings to pigments that absorb specific colors; in some species, such as the beautiful Morpho the tenor, colors are due to the presence in the wings of nanostructures which are photonic crystals. This example is discussed in more detail in the next section of this chapter.
  • Spider silk: silk is the material with the greatest known strength — about five times that of steel of the same weight. The extraordinary properties of spider silk are due to the proteins that make up the silk (mainly fibroin) and its supramolecular organization which is at the nanoscale level.
  • Lotus leaves and similar(nasturtium): the nanostructure of the leaves of these plants is responsible for their extraordinary surface properties and their ability to ‘self-clean’. This example is discussed in more detail in the next section of this chapter.
  • Geckos’ feet: the structure of the gecko foot is an amazing example of the relationship between function and nanostructure. The ability of geckos to walk upside down, against gravity, even on wet or dirty surfaces, is intimately connected to the nanostructure of their feet. This example is also discussed in more detail in the next section of this chapter.

Learning from nature

Natural nanomaterials are of interest not only to understand (and appreciate) the amazing proper-ties of biological materials but also to gather inspiration for the design and engineering of new materials with advanced properties.

The physical origins of the remarkable properties of many biological materials are due to complex, oſten hierarchical structures (3). They are characterized by a surprising level of adaptability and multifunctionality. These materials can provide a model for designing radically improved artificial materials for many applications, such as solar cells, fuel cells, textiles, drug delivery systems, etc.

What is even more inspiring is the notion that in nature, some very simple laws apply.

  1. Nature runs on sunlight and uses only the energy it needs. Natural nanomaterials are extremely energy efficient!
  2. Nature fits form to function and recycles everything — waste products are minimized in nature!
  3. Nature rewards cooperation although it encourages diversity and local expertise.

The field of materials engineering devoted to trying to fabricate artificial materials that mimic natural ones is conventionally called biomimetics. Nanoscience is a fundamental component of biomimetics.

Detailed description of some natural nanomaterials

Now, a few fascinating natural nanostructures are described in some detail and how their natural nanostructure is responsible for their properties (adhesiveness, strength, flexibility, color, etc.) is explained.

Bone

Thinking about it, the unique properties of bone are a list of apparent contradictions: rigid, but flexible; lightweight, but solid enough to support tissue growth; mechanically strong, but porous. Bone can withstand weight without breaking. Its compressive strength is about twice its tensional strength. These outstanding properties are the result of bone’s complex hierarchical structure and composition: bone material is made of a composite of collagen (mainly Type I collagen) fibrils reinforced with calcium phosphate particles (hydroxyapatite).
From a mechanical point of view, many bones such as the femoral head can be described as a ‘sandwich’ structure with a dense external shell (cortical bone) and a spongy interior (cancellous bone). In cancellous bone, only about 20% of the volume is filled with bone material, the rest is made of bone marrow. Cortical bone is made of fibrils regularly arranged(Figure 1).
The fibrils consist of an assembly of collagen molecules 300 nm long and 1.5 nm thick which are deposited by the osteoblasts (bone-forming cells) into the extracellular space and self-assembled into fibrils. Adjacent fibrils molecules are staggered along the axial direction by about D=67nm (Figure 2), generating a characteristic pattern of gap zones of length 35nm and overlap zones of length 32nm within the fibril.
Collagen fibrils are filled and coated by tiny mineral crystals of hydroxyapatite. These are mainly flat plates mostly arranged parallel to each other and parallel to the fibril main axis. Crystals occur at regular intervals along the fibrils, with an approximate repeat distance of 67nm. In mammalian species, bone mineral crystals have a thickness of 2–4nm (Figure 3).
To summarise, bone is formed from a soſt organic matrix (collagen) reinforced by an anisotropic stiff inorganic component (crystals of hydroxyapatite). These two components are assembled in a hierarchic structure that is organized at the nanoscale level. It is this nanoscale hierarchic organization that allows the bone to tolerate small microfractures that arise from normal activity and dissipate deformation energy without propagation of the crack. Hydroxyapatite is a rigid material that is not capable of dissipating much energy; therefore, collagen is believed to have a major role in the structural properties of bone (elastic and plastic deformation). Figure 4illustrates the role of collagen during bone deformation. Older bone, which is more mineralized and thus has a larger percentage of hydroxyapatite, is stiffer and breaks more easily.

Lotus leaf

The lotus plant (Nelumbo nucifera) is a native Asian plant that has the distinct property of being able to maintain its leaves particularly clean even if its natural habitat is muddy. For this reason, this plant is considered sacred in some cultures and a sign of purity. The leaves of the lotus plant have the outstanding characteristic of totally repelling water because they are superhydrophobic (Figure 5). The consequence is that water droplets roll off the leaf surface and, in doing so, drag dirt away from it, as shown in Figure 7. This effect, ‘self-cleaning’, renders the lotus leaf clean and resistant to dirt. The same effect is found in other leaves such as those of nasturtium (Tropaeolum) and some Cannas, and in some animals such as the water strider.

How is this ‘Nano’?

The surface properties of the lotus leaf were first investigated by Wilhelm Barthlott. In 1997, he published an important paper where he described for the first time the ‘Lotus effect’ (a term that he is later copyrighted) responsible for the self-cleaning properties of the lotus leaves. In his original paper, Barthlott showed that the self-cleaning properties of the lotus plant are produced by a combination of the microstructure of the leaves and the epidermal cells on their rough surfaces, which are covered with wax crystals (Figure 6). These crystals provide a water-repellent layer, which is enhanced by the roughness of the surface, making it a superhydrophobic surface, with a contact angle of about 150°. The consequence is that water droplets on the surface tend to minimize the contact between the surface and the drop, forming a nearly spherical droplet. Figure 6 shows the progressive magnification of a nasturtium leaf. In the last image on the right, nanocrystals a few tens of nanometres in size are shown.

The consequence is that water droplets roll off the leaf surface and, in doing so, drag dirt away from it, as shown in Figure 7. This effect, ‘self-cleaning’, renders the lotus leaf clean and resistant to dirt.

Contaminants on the surface (generally larger than the cellular structure of the leaves) rest on the tips of the rough surface and when a water droplet rolls over the contaminant, the droplet removes the particle from the surface of the leaf (Figure 8).

Gecko

 A gecko can cling to virtually any surface at any orientation; walk on smooth or rough surfaces, even upside down on a glass surface; and walk on a dirty or wet surface maintaining full contact and adhesion to it. As it walks, a gecko does not secrete any sticky substance, and its feet do not have any suction-like features (even at microscopic sizes). The reason for the gecko’s amazing properties lies in the nanostructures that are present on its feet.

The gecko foot has a series of small ridges called sensors that contain numerous projections called setae. Each seta is about 100 µm long and has a diameter of about 5 µm. There are about half a million of these setae on the foot of a gecko. Each seta is further subdivided into about a thousand 200nm-wide projections called spatulae (Figure 9). As a result, the total surface area of the gecko’s feet is enormous. The gecko spatulae are very flexible, so they essentially mold themselves into the molecular structure of any surface. The result is a strong adhesion which is entirely due to van der Waals forces. A single seta can resist a force of 200µN or approximately 10 atmospheres of stress. The gecko case is thus a very good example of the effect of large surface area on small forces.

Another very interesting property of geckos is that their feet don’t get dirty as they walk, even if they walk on a surface covered with sand, dirt, water, etc. Their feet stay clean even on dirty surfaces and full adhesion is maintained. The phenomenon has been investigated and it was found that the feet remain clean because it is more energetically favorable for particles to be deposited on the surface than to remain to adhere to the gecko spatulae. If a gecko walks over a dirty surface, it takes only a few steps for its feet to be totally clean again, and adhesion is not compromised.

Morpho rhetenor

The wings of butterflies oſten display extraordinary colors, a consequence of the wing surface and its interaction with light. The wings also exhibit iridescence: the shiſt in the color of an object when observed at different angles. The effect can easily be seen by observing a music CD.
Iridescence is a ‘physical color’ and it results from the interaction of light with the physical structure of the surface. To interact with visible light, those structures must be nano-sized (the visible spectrum corresponds to wavelengths between 380 and 750nm). The interaction of light with this nano-rough surface can lead to constructive or destructive interference. The color, intensity and angles of iridescence depend on the thickness and refractive index of the substrate, and on the incident angle and frequency of the incident light.
In materials like opals, natural iridescence is observed, due to packed silica spheres in the nanometre range, uniform in size and arranged in layers. This provides appropriate conditions for interference.
In the case of butterflies and moths, the iridescence is produced in a peculiar way. Scientists have studied the structure of the wings of Morpho rhetoric detail and have found that these are formed by rows of scales arranged like tiles on a roof. Each scale is about 70×200µm and has a smaller structure on its surface, a very intricate and highly ordered nanometre organization of ridges. Each ridge is about 800µm wide. The spaces between them form a natural photonic crystal that can generate constructive and destructive interference. The SEM analysis of the cross-section of the ridges on the wings shows and an even more intricate structure that looks like fir trees (last image in Figure 10).
These are called setae, are about 400nm long, and are responsible for producing constructive interference in the blue wavelengths which generate the strong blue color (Figure 11).