# Biocompatible Dispersants for Carbon Nanomaterials
## 6 Biocompatible Dispersants  
### Cellulose  
Cellulose is an environmentally friendly, naturally occurring biopolymer. It is very cheap and ubiquitous, with an estimated 10\({}^{11}\)-10\({}^{12}\) tons naturally produced each year [21]. This renewable material is very hydrophilic due to its hydroxyl-rich surface. Cellulose consists of repeating \(\beta\)-1,4-linked anhydro-D-glucose units (Figure 2); these can form inter and intramolecular hydrogen bonds. Different polymorphs arise from distinct, hydrogen-bonding-induced packing and orientations of the glucose units [36]. Nanocrystalline cellulose (NCC), also known as cellulose nanocrystals, are rod-shaped cellulose crystals typically 5-15 nm wide and 100-300 nm long [21]. NCC was found to bind to CNM surfaces by dipole-induced dipole interactions, and the NCC/CNM complex is stabilised electrostatically [37]. There are four reported cellulose polymorphs, with type I and type II being the most well studied [38]. The attraction between CNMs and NCC can also be varied by changing polymorph composition. The type II polymorph has a higher surface charge due to residual sulphate groups present from acid treatment. Type I NCC exhibits a parallel arrangement of individual cellulose polymer chains, whereas the chains in type II NCC are arranged in an antiparallel fashion. While type I is the naturally occurring form of NCC, many methods exist to convert it to type II, the most industrially viable of which is sulphuric acid treatment [39]. This process is both time and cost-friendly. This method also allows for the production of type I and II NCC mixtures--the ratios of which may be varied by careful control of experimental parameters. Both types have different properties; for example, type II NCC exists in a shorter and wider crystal form than type I. Cellulose can be a highly effective surfactant, up to 20 times  
Figure 1: Schematic of a dynamic and static CNT dispersion [33]. _Figure reprinted with permission under CC 3.0._  
Figure 2: Monomer unit of cellulose. more effective than conventional dispersants such as sodium dodecyl sulphate (SDS) or hexadecyltrimethylammonium bromide (CTAB) in some cases [40]. The main drawback of cellulose, and particularly nano-cellulose materials, is that they are hygroscopic. Upon absorbing water, these materials swell, which affects their surface interactions [41]. This is particularly problematic for nano-cellulose materials such as NCC due to their high surface area, which can cause reproducibility issues [41]. In 2019, [PERSON] et al. demonstrated that NCC could be used to disperse single-walled carbon nanotubes (SWCNTs) in aqueous solutions at concentrations of approximately 0.1 mg/mL, with long-term dispersion stability (1.5 years) [38]. Details of this dispersion can be seen in Table 2 (1). This study revealed no significant difference between types I and II NCC dispersing ability for SWCNTs. However, TEM analysis (Figure 3) showed different morphologies of the hybrid systems. Type I NCC/SWCNT systems consisted of individual NCC particles binding to SWCNTs along their hydrophobic planes. In contrast, type II systems consisted of carbon nanotubes (CNTs) wrapped in a tangled mass of NCC [38]. Whilst very little information exists on the use of NCC for dispersing other CNMs, colloidal probe AFM studies with graphene suggest a synergetic NCC/CNM adsorption mechanism, which could potentially be applied to other sp\({}^{2}\) hybridised CNMs [37]. This study also proved that hydrogel and nano-paper preparations could be produced using SWCNTs and cellulose (Table 2 (2)). As is the case, [PERSON] et al. used methylcellulose to disperse graphene nanoplatelets [42] (Table 2 (3)). This study also revealed that NCC with a higher surface charge binds more efficiently to CNMs. The tuning of NCC mixtures could also prove advantageous. For example, type II dominant mixtures could be used to disperse carbon nano-onions (CNOs) due to their smaller size. The better SWCNT surface coverage of NCC type II over type I seen by [PERSON] et al.