Supramolecular organic nanotubes: how to utilize the inner nanospace and the outer space

Organic nanotubes (ONTs) are tubular nanostructures prepared from small organic molecules or macromolecules. These structures have attracted growing attention because their inner and outer spaces exhibit unique properties that may be exploited for potential applications. In the first part of this review, we describe methodologies to construct well-defined ONTs: how to control the dimensions, discriminate the inner and outer surfaces, and functionalize the nanostructures. The well-defined ONTs contain cylindrical nanospaces that can capture, store, and release various nanomaterials, from small molecules to macromolecules. The ONTs' outer spaces and surfaces play critical roles in dispersibility, organization, and manipulation of the ONTs. In the second part, we describe the ONTs' physicochemical properties and utilization of the inner and outer spaces, emphasizing the advantages of ONTs over other types of nanomaterials. Smaller nanomaterials can be efficiently captured in the nanospaces of the ONTs via selective surface interactions. For example, encapsulation of proteins in the ONT nanospaces prevents them from chemical or thermal denaturation. Furthermore, the encapsulated materials can be released in response to external stimuli, such as pH or temperature, which can alter the surface charge and/or fluidity. These unique properties of ONTs allow them to be utilized for biomaterials and drug delivery applications.