*3.1. Characterization of Graphene*

Graphene is a single layer of sp2-hybridized carbon atoms arranged into a two-dimensional honeycomb-like lattice (Figure 2a). In other words, graphene is a one-atom-thick layer of graphite. It is a basic building block for other carbon allotropes, such as fullerenes, carbon nanotubes and graphite. Graphene is a very thin, nearly transparent sheet, but it is remarkably strong (about 100 times stronger than steel), and highly electrically and thermally conductive (for a review, see [19,20,125,126]). Graphene can be prepared by various methods, which can be divided into two main categories, namely the top-down approach and the bottom-up approach. The top-down approaches include treatment of

graphite by mechanical or electrochemical exfoliation, intercalation or sonication, and also nanotube slicing. The bottom-up approaches include growth of graphene from carbon-metal melts, epitaxial growth of graphene on silicon carbide, the dry ice method, and deposition methods such as chemical vapor deposition or dip coating a substrate with graphene oxide (GO), followed by GO reduction (for a review, see [19,20,125,126]). Graphene can be prepared in the form of monolayer or bilayer sheets, nanoplatelets, nanoflakes, nanoribbons and nanoscrolls. Chemically, graphene-based materials include pure graphene sheets, GO or reduced graphene oxide (rGO). Pure graphene sheets can be produced by mechanical exfoliation of graphite or by chemical vapor deposition. GO, a highly oxidative and water-soluble form of graphene, can be obtained by the exfoliation of graphite oxide. Reduced GO can be prepared by chemical, thermal or pressure reduction, and even by bacteria-mediated reduction of GO, which improves its electrical properties (for a review, see [13,18,19,37,125,126]). Graphene and graphene-based materials hold a great promise not only for a wide range of industrial and technology applications, but also for biomedical applications, such as drug, gene and protein delivery, photothermal therapy, construction of biosensors, bioimaging, antimicrobial treatment, and also as scaffolds for tissue engineering (for a review, see [20]).

**Figure 2.** Scheme of graphene (**a**) and of the preparation and structure of nanocellulose/graphene composites (**b**).
