*2.2. Preparation and (Bio)Application of Nanocellulose*/*Fullerene Composites*

Cellulose nanocrystals (CNCs) were used to create nanocellulose/fullerene composites. These nanocrystals are typically produced by acid hydrolysis of cellulose fibers, employing either sulfuric acid or hydrochloric acid in order to destroy the amorphous regions of the cellulose, while the crystalline segments remain intact. CNCs can have a needle-like or rod-like morphology, and are also referred to as nanowhiskers or nanorods. This morphology is characterized by a high aspect ratio (i.e., high length to diameter ratio), and thus by a relatively large surface area. In addition, CNCs have a wide range of other advantageous properties, such as high mechanical resistance, broad chemical-modifying capacity, renewability, biodegradability and low cytotoxicity [34,35]; for a review, see [2]. From these points of view, CNCs were considered ideal for immobilization of fullerene nanoparticles [92]. A scheme of preparation of nanocellulose/fullerene composites is depicted in Figure 1b. Composites of CNCs with fullerenes C60 were prepared by amine functionalization of CNCs and by subsequently grafting C60 onto the surface of amine-terminated CNCs [34]. Conversely, functionalized fullerenes, e.g., polyhydroxylated fullerenes C60(OH)30, were conjugated with the surface of CNCs [92]. Both of these composites showed a higher radical scavenging capacity in vitro than fullerenes alone, and therefore are promising for biomedical application in antioxidant therapies, e.g., as components of skin care products. In the third type of composites, both cellulose nanocrystals and fullerenes were functionalized, i.e., amino-fullerene C60 derivatives were covalently grafted onto the surface of 2, 2, 6, 6-tetramethylpiperidine-1-oxylradical (TEMPO)-oxidized nanocrystalline cellulose [35]. These composites hold promise for photodynamic cancer therapy (Table 1). When these composites were added to the culture medium of human breast cancer MCF-7 cells in the dark, they were taken up by these cells without changes in the cell viability, as revealed by a resazurin assay. However, when irradiated with light, these composites showed dose-dependent toxicity for MCF-7 cells [35].

However, fullerenes are less widely used in nanocellulose/nanocarbon composites than other carbon allotropes, particularly graphene and carbon nanotubes. More frequently, fullerenes are incorporated into a non-nanostructured cellulose matrix. For example, fullerene C70, characterized by a strong thermally activated delayed fluorescence at elevated temperatures, which is extremely oxygen sensitive, was incorporated into ethyl cellulose, i.e., a highly oxygen-permeable polymer. This composite was used for construction of an optical dual sensor for oxygen and temperature [115]. An oxygen sensor was constructed using isotopically enriched carbon-13 fullerene C70, dissolved in an ethyl cellulose matrix [116]. Mixed-matrix membranes, consisting of ethyl cellulose as a continuous matrix and fullerenes C60 as a dispersed phase, were prepared for propylene/propane separation [78]. Electrospun cellulose acetate nanofibers reinforced with fullerenes were used in the construction of dry-type actuators [123]. Cellulose impregnated with fullerenes C60 dissolved in o-xylene showed greater extraction efficiency for Cu2<sup>+</sup>, Ni2<sup>+</sup> and Cd2<sup>+</sup> ions from an aqueous environment than the pure polymer [124]. Biocompatible composites containing polysaccharides (cellulose, chitosan and gamma-cyclodextrin) and fullerene derivatives (amino-C60 and hydroxy-C60) were developed for various applications ranging from dressing and treating chronically infected wounds to nonlinear optics, biosensors, and therapeutic agents [118].
