*5.2. Preparation and (Bio)Application of Nanocellulose*/*Nanodiamond Composites*

There is greater use of diamond nanoparticles than of fullerenes in nanocellulose/nanocarbon composites, but diamond nanoparticles are used less than graphene and carbon nanotubes. This may be because a nanodiamond is more expensive and is electrically nonconductive in its pristine state. In composites with nanofibrillated cellulose (Figure 4b), a nanodiamond was used for constructing highly thermally conductive, mechanically resistant and optically transparent films with potential application as lateral heat spreaders for portable electronic equipment [70]. A highly mechanically resistant and optically transparent nanopaper was made of cationic CNFs and anionic nanodiamond particles by filtration from a hydrocolloid and subsequent drying [159]. Moreover, a diamond can be rendered electrically semiconductive by doping it with boron, and then can be used for constructing biosensors. For example, a sensor for biotin was developed by the adsorption of captavidin, a nitrated avidin with moderate affinity to biotin, on a carboxymethylcellulose layer stabilized on a boron-doped diamond electrode by a Nafion film. This biosensor was used for analyzing biotin in blood plasma [66] (Table 1).

The reinforcing effect of diamond nanoparticles, coupled with their optical transparency, has also been used advantageously for other biomedical applications, particularly for wound dressing. Nanocellulose/nanodiamond composites are more mechanically resistant than purely nanocellulose-based materials, but they retain their flexibility and stretchability. In addition, their optical transparency enables direct inspection of wounds without the need to remove the dressing. For example, incorporating diamond nanoparticles in a concentration of 2 wt % into chitosan/bacterial nanocellulose composite films resulted in a 3.5-fold increase in the elastic modulus of these films. These composite films were transparent, but their transparency can be modulated by the concentration of diamond nanoparticles, turning them gray and semitransparent at higher nanodiamond concentrations (3 and 4 wt %). The viability of mouse subcutaneous L929 fibroblasts in cultures on these films, evaluated by an MTT test of the activity of cell mitochondrial enzymes, was more than 90% at 24 h after seeding. However, at 48 h, it had dropped to about 75%, which indicated that diamond nanoparticles are slightly cytotoxic [62]. A similar result was obtained on L929 fibroblasts grown on electrospun composite nanofibrous mats containing chitosan, bacterial cellulose and 1–3 wt % medical-grade nanodiamonds [58]. The viability of these cells, estimated by the MTT assay, dropped from approx. 90% on day 1 to approx. 75% on day 3. Nevertheless, the addition of nanodiamonds facilitated the electrospinning process, reduced the diameter of the nanofibers in the mats, regulated the water vapor permeability of the mats, enhanced their hydrophilicity and improved their mechanical properties to a similar level as in native skin [58].

Adding diamond nanoparticles per se did not significantly increase the antibacterial activity of chitosan/bacterial nanocellulose composites [62]. This activity can be further enhanced, e.g., by adding silver nanoparticles [160]. Nanocellulose/nanodiamond composites can also act as a suitable platform for drug delivery. This was demonstrated on transparent doxorubicin-loaded carboxylated nanodiamonds/cellulose nanocomposite membranes, which are promising candidates for wound dressings. These membranes are porous, transparent, with appropriate mechanical properties, and without doxorubicin they are noncytotoxic for HeLa cells [97].
