Magnetic manipulation of self-assembled spherical nanocapsules

Bottom-up nano-structures are made from molecular building blocks designed to assemble automatically into object of predefined shape. This area of macromolecular chemistry offers the perspective make tailor-made physical and chemical properties in molecular self-assemblies. There are attractive possibilities of further controlling those objects by external fields, for instance by the high magnetic fields available in Nijmegen. For instance such high magnetic fields can deform the molecular nano-structures and these magnetically deformed objects can be trapped in a compatible organogel making them suitable for further investigations.

Figure 1. a) Schematic representation of thespherical assembly of sexithiophene molecules and SEM image of such a shpere fixed in a matrix. b) Schematic representation and SEM image of the magnetically deformed nanocapsules.

The self-assembly behavior of sexithiophene molecules in 2-propanol represents an interesting case where the molecule/solvent combination leads to spherical architectures (nanocapsules) as shown schematically in fig. 1. Their diameter varies from 55 to 125 nm between 20 ⁰C and 70 ⁰C. These nanocapsules can be fixed in organogel and visualized by scanning electron microscopy (SEM). Without magnetic field perfect spherical objects were observed (fig. 1a). When a magnetic field is applied all molecules within the nanocapsules will experience a torque as a consequence of their diamagnetic anisotropy resulting in all molecules trying to orient themselves parallel to the field direction. To minimize the obtained magnetic energy the sphere will deform into an oblate ellipsoid. Compelling evidence for the magnetic deformation was obtained by trapping the deformed nanocapsules in a matrix in situ 20 Tesla. Gelled samples were further investigated by SEM showing images of deformed capsules (fig. 1b). This experiment demonstrated for the first time the possibility to influence the shape of “non-magnetic” nano-objects by high magnetic fields. Moreover, data analysis in combination with additional experiments provided an insight into physical properties of such an objects, like the determination of the elastic constant of a nanocapsules. The method is not restricted to thiophene-based materials and we, therefore, anticipate an increased use of magnetic forces as a promising tool for manipulation of supramolecular structures, like sizable nano-containers and reactors, or capsules for magnetically controlled drug delivery.

This work was published in:

I.O. Shklyarevskiy, P.C.M. Christianen, P. Jonkheijm, A.P.H.J. Schenning, E.W. Meijer, O. Henze, W.J. Feast, A.F.M. Kilbinger, A. Del Guerzo, J.-P. Desvergne and J.C. Maan,
Magnetic Deformation of Self-Assembled Sexithiophene Spherical Nanocapsules
J. Am. Chem. Soc. 127, 1112-1113 (2005)