*2.1. Characterization of Fullerenes*

Fullerenes are spheroidal cage-like molecules composed entirely of carbon atoms (Figure 1a). Fullerenes with 60 and 70 carbon atoms (C60 and C70) are the most stable molecules, and they are therefore most frequently used in industrial and biomedical applications. Fullerenes were discovered in 1985 by Sir Harold Walter Kroto (1939–2016) and his co-workers Richard Smalley, Robert Curl, James Heath and Sean O'Brien. Kroto, Smalley and Curl were awarded the Nobel Prize in 1996. Fullerenes were named after Richard Buckminster "Bucky" Fuller (1895–1983), an American architect, designer, futurist, inventor, poet and visionary, who designed his geodesic dome on similar structural principles (for a review, see [12,13,18,119,120]). Fullerenes are carbon nanoparticles with diverse biological activities. This is due to the fact that they can act as either acceptors or donors of electrons (for a review, see [121]). The acceptor activity can lead to oxidative damage to cell components, such as DNA, cell membrane, mitochondria and various enzymes, to the activation of inflammatory reactions, and to cell apoptosis. These harmful effects of fullerenes can, however, be utilized for photodynamic therapy against tumors and pathogenic microorganisms (for a review, see [122]). The electron donor activity is associated with quenching oxygen radicals, which can be used in protecting skin against UV irradiation, in anti-inflammatory therapy against osteoarthritis, in cardioprotection during ischemia, in neuroprotection during amyloid-related diseases, damage by alcohol or heavy metals, in obesity treatment and in the treatment of diabetes-related disorders. Due to their structural analogy with clathrin-coated vesicles, fullerenes are also promising candidates for drug and gene delivery (for a review, see [12,13,18,119]).

**Figure 1.** Scheme of fullerene C60 (**a**) and of the preparation and structure of nanocellulose/fullerene composites (**b**).

However, fullerenes have low solubility in many solvents, especially in water. This is a major drawback for their wider application in biomedical applications. The water solubility of fullerenes can be achieved by functionalizing them with hydrophilic groups, but this approach does not solve problems arising from the aggregation and clustering of fullerenes. In addition, the formation of singlet oxygen, which is needed for photodynamic therapy, decreases after functionalization due to the perturbation of the fullerene π system. These problems can be mitigated by the complexation of fullerenes with water-soluble agents, including nanocellulose [35].
