# Biocompatible Dispersants for Carbon Nanomaterials
## 6 Biocompatible Dispersants
### Bile Acid Salts  
Bile acid salts (BASs) are another class of efficient biomolecular dispersants that are found in a range of vertebrates; including humans [45]. They are used to solubilise a range of hydrophobic biomolecules such as lipids, proteins, vitamins, and fatty acids [45]. BASs are easy to obtain and environmentally friendly [46]. The general structure of these anionic biosurfactants is shown in Figure 4. Groups R\({}_{1}\)-R\({}_{4}\) are hydrophilic groups--typically OH or COOH--and the hydrophobic region is comprised of four rings. The steroid backbone provides a unique rigidity to the molecule [19]. The structure and charge distribution of these amphiphiles result in one face of the molecule being hydrophobic whilst the other is hydrophilic, thus maximising the surface coverage efficiency when adsorbed onto CNMs. The main contribution to BAS/CNM adsorption is hydrophobic interactions between the steroid backbone of the BAS and the CNM surface. The disadvantages of BAS are that they are known to denature proteins, damage DNA, chelate ions, and interfere with bacterial membranes in high concentrations [47]. This could limit their in-vivo usage and affect in-vitro studies. Unlike cellulose-based surfactants, BAS has been tested with 0D CNMs. In 2020, [PERSON] and [PERSON] non-covalently functionalised CNOs with sodium deoxycholate (SDC) [48] (Table 3 (1)). The dispersion was produced through a simple process involving sonication and mixing. The aqueous dispersion was found to be stable for six weeks, with a CNO: SDC mass ratio of 2:1 found to be ideal for surfactant efficiency. This CNO functionalisation was driven by hydrophobic interactions [48]. BAS can also disperse carbon quantum dots (CQDs), as demonstrated by [PERSON] et al. in 2020 [49]. SDC was used to functionalise CQDs via hydrophobic interactions. The resulting dispersions were used to produce films via electrophoretic deposition (EPD). In solution, the deoxycholate (DC) anion is water-soluble. However, as protons are generated at the anode, they combine with the deoxycholate moiety to form an insoluble protonated adduct. As BAS act as dual dispersants and EPD agents, they are ideal for producing CNM-containing films [49] (Table 3 (2)). [PERSON] et al. used a similar approach to produce a range of stable graphene dispersions [46] (Table 3 (3)). SDC, tautrodeoxycholic acid sodium salt hydrate, and sodium cholate (SC) were all employed to non-covalently functionalise graphene through the chemical reduction of graphene oxide in the presence of BAS solutions. These all displayed fantastic stability, remaining in solution for over 80 days. More recently, SC has also been used to disperse SWCNTs to facilitate chirality purification [50] (Table 3 (4)). In another study, [PERSON] et al. proved that SC could exfoliate CNTs to produce stable aqueous dispersions, with no noticeable agglomeration or sedimentation after 30 days [51]. This was achieved using high-power sonication and centrifugation steps. The different dispersions were compared through a range of quality metrics such as CNM concentration, aggregate size, and mean CNT length. It was found that SC-based dispersions were not as stable as those produced with other common dispersants--such as sodium dodecyl sulphate (SDS) or sodium dodecylbenzene sulphonate (SDBS). This was found to be due to the low charge density of the BAS molecule, which was not as high compared to an alkyl chain surfactant [51] (Table 3 (5)). Figure 4: The general structure of a bile acid salt (BAS); R groups are hydrophilic. In an attempt to improve the dispersion efficiency of BAS, [PERSON] et al. functionalised a range of BASs with aromatic substituents at varying positions [52] (Table 3 (6)). It was postulated that adding aromatic groups to the BAS structure would increase the molecules' affinity SWCNTs by inducing \(\pi\)-\(\pi\) stacking interactions. However, these modifications result in a lower charge density and increased hydrophobicity.