Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 92

Fullerenes, graphenes, and carbon nanotubes share the same basic building block as graphite, a monolayer of hexagonally arranged carbon atoms. Graphene is the name given to a two‐dimensional flat monolayer of carbon atoms arranged in a hexagonal lattice (Figure 3.14a). The stacking of these flat monolayers gives the three‐dimensional structure of graphite. Graphene is a one‐atom thick, two‐dimensional crystal, often of size less than 1 mm (Geim and Novoselov 2007). The term graphenes is used in a more general sense to describe two‐dimensional crystals composed of one, two or a few (3–10) monolayers that are each distinguishable by their electronic structure. Structures thicker than ~10 monolayers are typically described as thin films of graphite.

Figure 3.14 Arrangement of carbon atoms in (a) graphene, (b) single‐walled carbon nanotube, and (c) buckminsterfullerene, C₆₀.

The structure of carbon nanotubes can be viewed as graphene rolled to form a tubular geometry. Carbon nanotubes may consist of single tubes (Figure 3.14b), called single‐walled carbon nanotubes (SWNTs), or concentric tubes called multi‐walled carbon nanotubes (MWNTs). SWNTs have a diameter from ~1 nm to a few tens of nanometers and lengths of hundreds of nanometers to a few millimeters (Iijima and Ichlhashi 1993). As the long‐range periodicity of the atomic arrangement is retained along the axial direction of the tube, SWNTs may be viewed as one‐dimensional crystals.

Fullerenes are large molecules with a structure that can be viewed as a monolayer composed of hexagonal and pentagonal arrangements of carbon atoms which has been used to form a cage‐like geometry (Giacolone and Martin 2006). The most commonly investigated fullerene is the buckminsterfullerene, C₆₀, also called a buckyball, composed of 60 carbon atoms joined together to form 20 hexagons and 12 pentagons (Figure 3.14c). This arrangement of hexagons and pentagons, called a truncated icosahedron, has a pattern similar to a soccer ball. The carbon atoms in the pentagons are bonded by single bonds whereas the bonds in the hexagons consist of resonant double bonds. Although classified as a molecule, the C₆₀ fullerene has a diameter (0.7 nm) close to the lower limit of nanoparticles. When dispersed in a solvent, fullerenes often form aggregates due to weak van der Waals attractive forces, showing properties characteristic of nanoparticles rather than molecules.

The strong covalent (σ) bonds due to the sp² hybrid orbitals, delocalized electrons of the π bonds due to the overlapping p orbitals in the hexagonally arranged carbon atoms and small size endow graphenes, carbon nanotubes, and fullerenes with unique physico‐chemical properties. These structures have received an enormous amount of scientific and technological interest since their discovery and are now receiving considerable interest for potential biomedical applications, such as drug delivery, gene delivery, biomedical imaging, biosensing, and tissue engineering (Eatemadi et al. 2014; Zhao et al. 2017).

As carbon is chemically inert and hydrophobic, a related area of investigation is the functionalization of graphenes, carbon nanotubes, and fullerenes with appropriate molecules for optimal use in these applications (Chapter 13). In common with nanostructured materials such as nanoparticles intended for use in vivo, the possible toxicity of graphenes, carbon nanotubes, and fullerenes has been widely studied. While the vast majority of studies have shown no serious adverse response from cells and tissues, some questions still remain about possible toxic effects.