# Stability of Halloysite, Imogolite, and Boron Nitride Nanotubes in Solvent Media
## 4 Boron Nitride Nanotubes  
To improve dispersion stability, numerous methods to functionalize BNNTs were used [10; 45; 73]. These methods can be organized into three groups: (1) noncovalent, (2) covalent modifications, and (3) alternation in BNNTs (fabrication of defect sites, insertion of the amino groups, and transformation of the amino groups) (Figure 4). Figure 4: Different approaches for functionalization of the boron nitride nanotubes (BNNTs). Noncovalent reactions are the most frequently used approach for the surface functionalization of BNNTs [73]. However, only positively charged agents (e.g., poly-l-lysine and poly-ethyleneimine), organic amines, or chemical species able to make \(\pi\)-stacking and hydrophobic interactions showed good results [73]. Synthetic structures with aromatic rings (2-naphthalencarboxylic acid, 9-anthranecarboxylic acid, 1-pyrenecarboxylic acid, 1-aminopyrene, and 1-hydroxypyrene) [74] and polymers containing aromatic subunits, polyaniline, poly(p-phenyleneethynylene), polythiophene, poly(xylyldienetetrahydrothiophene) and poly(sodium styrene sulfonate) [75; 73] interacted with the sidewall of BNNTs via \(\pi\)-\(\pi\) stacking and created stable dispersions. It was shown that poly(p-phenylene) derivatives have the most interesting potential to disperse BNNTs among studied polymers [75]. Another simple way to prepare stable colloidal dispersions of BNNTs in organic solution or aqueous media is to obtain boron nitride nanotubes with NH\({}_{3}\) or organic amines [76] or amino acids (glycine), and coat them subsequently with biopolymers [77]. The glycine has two roles: its amine group interacts with the B-sites of nanotubes, binding with them; meanwhile, its carboxylic acid group is an ionic site for anchoring polyelectrolytes. Unexpectedly, BNNTs were effectively dispersed in water using arabic gum (hydrophilic polymer), where the hydrophobic part of the polymers had strong hydrophobic interactions with BNNTs and the hydrophilic part was exposed to interactions with water molecules [78]. Other examples of the fabrication of the stable BNNT dispersions are modification of the BNNTs, using peptides [79], nucleotide [80], DNA [81], doxorubicin and folate [82], and lipids [83]. The most stable dispersions in water were obtained for the flavin mononucleotide, a derivative of vitamin B\({}_{2}\) containing an aromatic structure interacting with BNNT via \(\pi\)-\(\pi\) stacking [80]. In addition, flavin mononucleotide-functionalized BNNTs showed high visible light emission, and were stable for different pH and temperature values. A new approach to disperse the BNNTs was recently demonstrated [6], in this case via a layer-by-layer deposition of hydroxylated BNNTs with polyelectrolytes onto _Saccharomyces cerevisiae_ cells. Different methods of covalent modification of BNNTs have been developed to create colloidal dispersions in both aqueous media and organic solvents. This functionalization can be done by exploiting the -NH\({}_{2}\) and -OH groups of boron atoms [46; 84; 85; 86]. Covalent modification of the hydroxylated BNNTs with glutaraldehyde, followed by functionalization with oligonucleotides [87] and carbohydrates [88] are described. [PERSON] developed a new functionalization route by linking stearoyl chloride with amino groups onto BNNTs [84]. A similar approach was realized with hydroxylated BNNTs esterified by perfluorobutyric acid or a thioglycolic acid [88]. Another easy approach for covalent derivatization of nanotubes with organic peroxides was proposed [89], and the functionalized BNNTs were able to form the stable dispersion in chloroform. Nowadays, one of the most interesting procedures for nanomaterial functionalization is to prepare grafted polymer brushes [7; 90] (Figure 5a). In this route, the nanotubes are covalently functionalized with polymer brushes through surface polymerization. In particular, BNNTs were covalently modified with hydrophobic polystyrene or polyglycidyl methacrylate polymer brushes [90].