# Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications
## 3 Surface Properties and Modifications
### Modification of the Inner Cavity  
Two strategies have been applied for the inner modification of hydrophilic imogolite nanotubes, namely (i) post-synthesis or (ii) direct synthesis chemical procedures. The list of the different coupling agents is summarized in Table 1.  
Johnson & Pinnavaia were the first to report the functionalization of imogolite nanotubes in an attempt to obtain organosilane derivatives of INTs for their intercalation as pillaring agents with montmorillonitees [71]. Silanyzation reactions are carried out in aqueous solutions. Post-functionalization has also been completed after a perfect dehydration of the hydrophilic INT samples in presence with different silane agents (MTMS, TCIS, AcCl) [73]. However, in both cases, the degree of inner surface substitution was lower than 35%.  
A more convenient way is the direct, template-free synthesis of imogolite nanotubes with the desired functional moieties by simply replacing the initial alkoxide (prefiguring the tetrahedral layer) by a functionalized one. [PERSON]'s group was the first to succeed in synthesizing methyl-modified Si-SWINT by using methyltriethoxysilane (MTES) [75; 76], a method that is still the most widely used currently [77; 30; 74; 78]. In a similar manner, [PERSON] et al. reported a single-step approach to synthesize hybrid Si-SWINT with a 15% aminomethyl group substitution on the inner cavity, via the design of a specific organosilane precursor (Figure 4a) [72]. In both cases (-CH\({}_{3}\) and -CH\({}_{2}\)NH\({}_{2}\)), the chemical modification leads to a large enhancement in molecular adsorption selectivity.  
It is worth noting that all these studies concern only Si-based imogolite nanotubes. As stated before, the diameter of INT increases continuously with the substitution of Si by Ge [53; 54]. Similar behavior could therefore be expected in the case of methyl-modified INTs by using a mixture of MTES and MTEG (methyltriethoxysgermane). Such an assumption has been confirmed by [PERSON] et al. [77]. It was shown by X-ray scattering measurements (XRS) that the continuous methylation of INTs can be obtained from Si to Ge endmembers (Figure 4b), but most importantly that the inner diameter size increases progressively (1.8-2.5 nm). In addition, [PERSON] et al. recently revealed that methylation of the inner cavity not only offers control over the surface properties, but it also leads to a drastic structural change of the chiral vector of the nanotube, with the armchair configuration being energetically favored over the zigzag structuring [30].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Compound** & **Acronym** & **Strategy a ** & **INT** & **Refs.** \\ \hline (3-aminopropyl)triethoxysilane & APTES & post. & Si & [71] \\ \hline (aminomethyl)triethoxysilane & AMTES & direct & Si & [72] \\ \hline Methyltrimethoxysilane & MTMS & post. & Si & [73] \\  & direct & Si & [74] \\ \hline Methyltriethoxysilane & MTES & direct & Si & [74; 75; 76; 77; 78] \\ \hline Trichlorosilane & TCIS & post. & Si & [73] \\ \hline Acethyl chloride & AcCl & post. & Si & [73] \\ \hline Methyltriethoxysgermane & MTEG & direct & Ge & [77; 30] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coupling agents used for inner modification (tetrahedral sites) of imogolite nanotubes (INT).