# Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications
## 1 Introduction  
Hollow cylinders with a diameter ranging between 1 and 100 nm, so-called nanotubes, are carving out prime positions in nanoscience and nanotechnology. Research into these systems started with [PERSON]'s 1991 seminal article on multi-walled carbon nanotubes (MWCNT) [1]. This was followed by the synthesis in 1993 of single walled carbon nanotubes (SWCNT) of truly nanometric diameters [2; 3]. In the meantime, large efforts have been devoted to the study of boron-nitride nanotubes [4] and many different inorganic nanotubes [5; 6; 7; 8; 9]. Their physicochemical properties are recognized to be appealing for next-generation devices that could outperform current technologies for nanoelectronics [10], nanofluidics [11], selective molecular sieving [12], energy conversion [13; 14], catalytic nanoreactors [15] and carriers for the sustained release of active molecules [16; 17; 18], to highlight a few. However, most of these applications require controlling at the nanometer scale, the nanotube dimensions (pore size and length), their interface properties as well as their organization, by using straightforward approaches if possible. In this context, having the possibility to synthesize nanotubular structures with desired size and interfaces would be a major step forward in exploring physical concepts in a controlled manner.  
Among the wide variety of inorganic nanotubes, imogolite nanotubes (INTs) can boast having these unique and versatile properties. Belonging to clay minerals, these aluminosilicate minerals consist of a curved octahedral [Al(OH)\({}_{3}\)] layer on which isolated [SiO\({}_{3}\)(OH)] tetrahedron units are connected upright to the octahedral vacancy via covalent bonding between three mutual oxygen atoms (Figure 1). INT structure can hence be described as a three-dimensional (OH)\({}_{3}\)Al\({}_{2}\)O\({}_{3}\)Si(OH) elementary unit arranged in a hexagonal lattice [19]. It is worth noting that imogolite structure is very different from the one of halloysite, another clay nanotube ubiquitous in soils and weathered rocks, where the curved octahedral [Al(OH)\({}_{3}\)] layer forms the internal surface of the nanotubes whereas the external surface is composed of Si-O-Si groups [7]. These differences are certainly at the root of the unique properties of imogolite nanotubes.  
With inner diameters in the nanometer range (1-3 nm), INTs represent one of the most promising inorganic analogs of SWCNTs. Naturally formed from weathered volcanic glass either on Earth [20] or Mars [21], INTs can also be synthesized easily by using low-temperature sol-gel methods [22]. The advantage of INTs originates from a well-defined minimum in the strain energy of the structure [23; 24; 25; 26; 27], allowing the production of samples with monodisperse diameter [28; 29] and chirality [30], contrarily to other nanotubes where the strain energy decreases monotonically with increasing diameters [31; 32]. Coupled with the possibility to adjust either their morphology (single (SW) vs. double-walled (DW) structures, aspect ratio) and surface properties by changing only the nature of the precursors, the synthesis of INT thus offers a convenient and simple way to obtain diameter-controlled nanotubes with functional interfaces. Therefore, they started to be applied in a wide range of potential applications.  
Through studies conducted over the last two decades, this review aims to demonstrate how the imogolite nanotubes represent a promising candidate for up-to-date nanotechnological applications. We will first focus here on the recent developments for INT synthesis and their unique properties, particularly in terms of morphological control and surface functionalization. The interested reader is referred to [33; 34] for recent reviews on the formation mechanisms, structure and properties of nanotubular clay minerals (imogolite and halloysite). The unique features of INTs will then be illustrated through recent applications and future prospects.