Cyclodextrin-covered organic nanotubes derived from self-assembly of dendrons and their supramolecular transformation

Abstract

The dendritic building blocks with a focal pyrene unit self-organize into vesicles in aqueous phase. The in situ inclusion of the focal pyrene units into the cavity of beta- or gamma-cyclodextrin (CD) induces self-assembled organic nanotubes with an average outer diameter of approximately 45 nm and inner diameter of 22 nm. The surface of the nanotube is covered with CD. Therefore, the functional group on the surface of the nanotube is controlled simply by modifying the functionality of CD. The removal of CD from the nanotube with poly(propylene glycol) reversibly generates vesicles. This work provides an efficient methodology not only to create an additional class of CD-covered organic nanotubes but also to exhibit reversible transformation of nanotubes and vesicles triggered by the motifs of dendron self-assembly, CD inclusion, and pseudorotaxane formation.

Fig. 1.

Schematic route to organic nanotubes and the reversible transformation of supramolecular assemblies of dendrons triggered by the motifs of CD inclusion and pseudorotaxane formation. (A) Amphiphilic dendron-pyrene conjugate. (B) Vesicular organization of dendron-pyrene conjugates in water. (C) The CD-covered nanotubes obtained by the supramolecular transformation after addition of CDs to the vesicular solution. The reverse transformation from nanotube to vesicle can be accomplished by removal of CDs on the surface of the nanotube by using PPG.

Fig. 2.

Structure of the amide dendron-pyrene conjugates.

Fig. 3.

TEM and E-SEM images. (A and B) TEM (A) and E-SEM (B) images of vesicles of 3. (C) E-SEM image of vesicle of 4.

Fig. 4.

Fluorescence spectra and TEM analysis. (A) Fluorescence emission spectra of vesicular solutions of 2 (a), 3 (b), 4 (c), and 5 (d). (B) Fluorescence emission spectra of 3 in water (e), γ-CD/3 complex in water (f), and γ-CD/3/PPG1000 in water (g). (C) TEM image of the nanotube from γ-CD/3 complex with phosphotungstic acid encapsulated in the inner hollow of the tube (see Materials and Methods and Fig. 11). (D) EF-TEM image (zero-loss image) of the nanotube from the per-6-thio-γ-CD/3 complex in water. (E) EF-TEM image for sulfur component with electron energy loss spectroscopy of the nanotube from the per-6-thio-γ-CD/3 complex in water.

Fig. 5.

Images of nanotubes. (A) E-SEM image of γ-CD/3 tube. E-SEM image in upper right shows the open end of nanotube. (B) TEM image of γ-CD/3 tube (uranyl acetate 2%). (C) EF-TEM image of β-CD/3 tube. (D) E-SEM image of β-CD/3 tube.

Fig. 6.

EF-TEM and E-SEM images. (A) EF-TEM image shows the transformation of the supramolecular structures from nanotube to vesicle after the addition of PPG1000 to the tubular solution of γ-CD/3. (B) E-SEM image also shows spherical vesicles.