Magnetic birefringence gives vesicle shape information

Researchers from the HFML and the Bio-Organic Chemistry group of the Radboud University Nijmegen have demonstrated how magnetic fields of only 2 Tesla can be used in probing shape changes of polymeric vesicles. They changed the shapes of the vesicles with osmotic pressure (induced by different organic solvents) and measured the magnetic birefringence of the sample during this dialysis process in a 2 Tesla magnetic field. Investigation with electron microscopy showed a clear correlation: the ‘flatter' the shape, the larger the measured birefringence. The results are published online on 24 October in Chemical Communications and can be used by other researchers that work with vesicles, for example in the area of nano-reactors or drug carriers.

Figure 1
Shape transformation of spherical polymersomes into ellipsoids, disks and stomatocytes can be probed by magnetic birefringence.

Magnetic properties of block co-polymers
The polymeric nanostructures are self-assembled from amphiphilic poly(ethylene glycol)-polystyrene (PEG-PS) block co-polymers. The block co-polymers are diamagnetic anisotropic, which means they have a preferential direction of alignment in a magnetic field. Since the nanostructures are made of these diamagnetic anisotropic polymers, the nanostructures themselves can be diamagnetic anisotropic as well, depending on their shape (or more precise: the distribution of polymer orientations). By aligning the polymeric structures, a measurable difference in refractive index (birefringence) is induced between directions parallel and perpendicular to the magnetic field. This birefringence only occurs when the nanostructures are aligned by the magnetic field. The size of the magnetic birefringence depends not only on the degree of alignment (magnetic field strength), but also on the shape of the nanostructures.

Magnetic birefringence during dialysis
The dialysis was started on spherical polymersomes with no preferential direction of alignment, which is in agreement with the absence of birefringence (point a and b of Fig 2). During dialysis, organic solvent is introduced to the polymersome sample. The organic solvent also diffuses into the polymer membrane which becomes more swollen and therefore more permeable to water. At some critical point (point c), the water inside the polymersomes diffuses outwards due to the difference in concentration over the membrane. At this point the polymersomes deflate, creating ellipsoids (point d) and finally disks (point e).

Figure 2
I: Magnetic birefringence during dialysis of a polymersome sample at 2 Tesla.

II: Cryo-SEM images of samples at points a-h. Note that the birefringence increases from the moment the polymersomes start to deflate and become anisotropically shaped (c-e). The decrease in birefringence occurs when the disks at point e fold and partly inflate to form stomatocytes (f-h).

Electron microscopy showed that the highest birefringence occurred for disks, which makes sense since almost all polymers have an identical orientation in a disk and will therefore all contribute constructively to a maximal birefringence. At this point the disks start to bend and partly inflate into bowl-shaped structures (points f, g and h). The shape becomes less flat and the birefringence decreases again. The measured birefringence curve allows the dialysis to be stopped at well defined points which provides different shapes in a predictable manner.

Drug delivery and nano rockets
The next step is to employ this technique at high magnetic fields. Initial measurements have already shown that high magnetic fields of 10 T and higher can lead to reversible deformations of polymersomes and stomatocytes. This means that the shapes of several different structures can be fine-tuned in a high magnetic field. Since polymersomes and stomatocytes can be used as nano containers, it might very well be possible that magnetic fields can be used to influence the uptake and release of nano particles. This might prove useful in applications like drug delivery. Stomatocytes have also been employed as nano rockets when loaded with platinum nano particles. High magnetic fields might also be useful to optimize their function as rockets by magnetic deformation (enhance speed) and magnetic alignment (steering).

R.S.M. Rikken, H.H.M. Kerkenaar, R.J.M. Nolte, J.C. Maan, J.C.M. van Hest, P.C.M. Christianen and D.A. Wilson, Probing morphological changes of polymersomes with magnetic birefringence. Chem. Commun., 2013, DOI: 10.1039/C3CC47483F