What You Will Learn
What is a natural nanomaterial?
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.
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.
- Nature runs on sunlight and uses only the energy it needs. Natural nanomaterials are extremely energy eﬃcient!
- Nature ﬁts form to function and recycles everything — waste products are minimized in nature!
- 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
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).
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.