# Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications
## 3 Surface Properties and Modifications
### Modification of the Outer Surface of Imogolite  
Modification of the external surface of imogolite has been strongly explored, mainly to render them compatible with another phase. As with the chemical modification of the INT inner cavity, the large density of hydroxyl groups offers a wide range of possibility for surface anchoring and numerous chemical pathways have been proposed with varying degrees of success (Table 2). The first attempt was performed by [PERSON] and [PERSON] via silanization of Si-SWINT with APTES [71; 79]. [PERSON] et al. evidenced a selective functionalization of APTES towards only the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Functional Group** & **Compound** & **Acronym** & **INT** & **Refs.** \\ \hline \multirow{4}{*}{Silane} & (3-aminopropyl)triethoxysilane & APTES & Si & [71; 76; 79; 80] \\  & (3-chloropropyl)triethoxysilane & CTES & Si & [81] \\  & (3-mercaptoethanolpropyl)trimethoxysilane & MPTMS & Si & [82] \\ \hline \multirow{4}{*}{Phosphonate} & Octadecylphosphonic acid & ODPA & Si & [83; 84; 85; 86; 87] \\  & Tetradecylphosphonic acid & TDPA & Si & [85] \\  & Vinylphosphonic acid & VPA & Si & [86] \\  & Dodecylphosphate & DDPO4 & Si & [89] \\  & 2-Acidphosphoxyethyl methacrylate & P-HEMA & Si & [90] \\  & Terthiophenes derivatives & HT3P HT3 OP & Si & [91] \\  & 8-(2-bromo-2-methylpropanoyl)octylphospholate & BMPOPO4 & Si & [92] \\ \hline \multirow{2}{*}{Carboxylate} & Stearic acid & SA & Si & [84] \\  & Dicarboxylic acid & DA & Si & [93; 94] \\ \hline \multirow{2}{*}{Sulfonate} & Poly[disodium 2,5-bis(3-sulfonatoprooxy)-1,4-phenylene-_all_-1,4-phenylene) & WS-PPP & Si & [95] \\ \hline \multirow{4}{*}{Others} & 4-(hydroxymethylthioacetyl)catechol & HETAC & Si & [96] \\  & Benzaldehyde & BA & Si & [97] \\  & Polyyrrole a & ppy & Si & [98; 99] \\  & \(\gamma\)-ray irradiation (peroxides) & - & Si & [100; 101] \\  & Isomorphic substitution (Al \(\rightarrow\) Fe) & Fe-INT & Si & [102; 103; 104; 105; 106] \\ \cline{1-1}  & & Ge & [107] \\ \hline \hline \end{tabular}  
* Graded by chemical vapor deposition (CVD).  
\end{table} 
Table 2: Main functionalization routes of the outer wall of imogolite nanotube (INT). Figure 4: (**a**) Direct synthesis of hybrid imogolite nanotubes with functionalized alkoxides M-R(OC2H5)3 with M = Si, Ge and R = -CH3\({}_{3}\)–CH2 NH2. (**b**) X-ray scattering (XRS) diagrams obtained on methyl-functionalized single-walled (OH)3 Al2O3 SiGe1\({}_{x}\)(CH3) at different substitution ratio \(x\) = [Si]/([Si] + [Ge]). XRS diagrams shift progressively upon substitution of Si by Ge, indicating an increase of the nanotube diameter. Adapted with permissions from [72; 77]. outer surface of the nanotubes by employing INTs with a methyl-modified inner cavity, while the silane anchoring groups remain unreactive with the Si-CH\({}_{3}\) entities [76]. However, APTES moieties remain hydrolytically labile and are totally removed after several days of dialysis [71; 79]. Such behavior may explain why the reusability of APTES-OsO4 complex grafted on Si-SWINT is severely degraded after their first use [80]. In contrast, the use of other silanes does not seem to induce such drawbacks [81; 82]. The most common way to modify a metal oxide surface is likely by using phosphonates, sulfonates and carboxylates derivatives [108]. [PERSON]'s group was one of the first to modify the external surface of an INT with different alkyl phosphonic acids (Table 2) [83; 84; 85; 89; 90]. As a proof-of-concept, such functionalized INTs are easily dispersed into hydrophobic solvents (Figure 5a) [83; 86; 88; 90], display weaker adhesion forces [83; 84], present self-organization behaviors either at the air/water [85] or water/oil interfaces [87] as well as higher static contact angle than unmodified nanotubes [89] and may display enhanced conductivity [91].