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Custom-made nanotubes


Nanotubes are ideal platforms for studying the novel properties of molecules confined at the nanoscale. Compared to carbon nanotubes, the most emblematic system of nanosciences, in which water slips without friction, inorganic imogolite nanotubes have the particularity to own perfectly defined diameter after their synthesis. By adding an organic moiety to the initial precursor prefiguring the inner cavity, these hydrophilic nanotubes become hydrophobic, as carbon nanotubes. The possibility to tune both their diameter and the water affinity in the inner cavity allow these modified imogolite to be a model custom-made nanotubes system.

Imogolite nanotubes, having a nominal composition (OH)3Al2O3SixGe1-x(OH), are hydrophilic nanochannels. The possibility to synthesize methyl-modified imogolite nanotubes (OH)3Al2O3SixGe1-x(CH3), while tuning the Si/Ge substitution ratio, has been demonstrated by a team of Laboratoire de Physique des Solides in close collaboration with a CEA laboratory (LIONS). Infra-red spectroscopy measurements reveal that hydroxyl groups on the inner surface have been replaced with methyl groups. Moreover, the nanotube diameter can be tuned through the substitution ratio. This effect, evidenced by X-ray scattering (XRS) measurements, is evidenced by a progressive shift to lower scattering vector modulus Q of the positions of the minima of the scattered intensity (figure).


Figure : (Left) X-ray scattering diagrams on methyl-modified imogolite nanotubes (m-INT) suspensions at different substitution ratio x = [Si] / ([Si] + [Ge]). (Right) Atomic structure and inner diameter of hydrophobic imogolite nanotube for x = 0 and x = 1. ƞ correspond to the maximal amount of confined bromopropanol per gram of nanotubes.

The development of a simulation method to analyze XRS diagrams allows us to demonstrate that nanotube diameters remain monodisperse and vary from 1.8 (x = 1) to 2.4 nm (x = 0). Moreover, the position of the first minimum is strongly dependent on the electronic density contrast between the inner and outer surface of the nanotube. We show that, in solution, the inner water density for methylated nanotubes is decreased by a factor of 3 compared to bulk water density, illustrating the hydrophobic character of the inner cavity of methylated imogolite nanotubes. Spontaneous confinement of organic molecules (bromopropanol) inside these nanotubes, when added to the solution, is demonstrated (figure). Controlling both hydrophobicity and nanotube diameter should open the route towards novel experimental and theoretical works on molecular nanoconfinement. Our results on these newly synthesized nanotubes could also find applications for water filtration or water decontamination.

Reference :
Amara M.S., Paineau E., Rouzière S., Guiose B., Krapf, M.EM., Taché O., Launois, P. and Thill A.
Hybrid, Tunable-Diameter, Metal-Oxide Nanotubes for Trapping Organic Molecules
Chem. Mater. 27, 1488-1494 (2015).

Contacts :
Erwan Paineau
Pascale Launois