The shape of a particle can be described as a topology (sphere, cylinder, doughnut, sheet), while the surface of a particle exhibits either homogeneous chemistry, chemical surface patterns or even asperities (topography). The behavior of particles in solution is largely determined by their interaction potential with other particles often on distances larger than the particle diameter. For the recognition by interfaces at close range, the surface structure of the particles becomes more relevant. Virus particles are probably the most prominent example from nature, where the surface structure is well-optimized for nanoparticle/membrane interaction and internalization. In our laboratory, we develop concepts to equip nanoparticles with well-defined chemical surface patterns and/or topographies using self-assembly and block copolymers.
Figure 1: Isotropic and anisotropic particles with homogeneous surface, patterned surface chemistry and nanotopography.In the following, find a short list of projects and concepts that allow the patterning of nanoparticle surfaces with few-manometer resolution.
The self-assembly of block copolymers in bulk and solution has been the focus of interest for several years. More recently, assembly in confinement has emerged as an additional direction to create non-equilibrium morphologies. Typically a diblock copolymer is emulsified in a water-immiscible solvent and allowed to dry out inside the spherical confinement of the emulsion droplet. This process then leaves behind microparticles with inner microphases according to the combined effect of microphase separation and curvature.
Figure 2: Confinement assembly. The polymer is dissolved in a low-boiling organic solvent and emulsified in water. The organic solvent evaporates leaving behind the water-insoluble polymer. During concentration the block copolymer starts to microphase separate. The final morpholopgy is influenced by the block lengths and the curvature.
We study the confinement assembly of special polymer architectures and block chemistries, e.g., triblock terpolymers, polymer brushes, miktoarm stars, biodegradable or crystalline blocks.
Inverse Morphologies: Block copolymer hexosomes
In surfactant and block copolymer assembly, the packing parameter p is a crude estimation for the resulting topology of the formed superstructure based on volume ratios of core and corona forming parts and their induced surface curvature. The most frequent topologies are spheres, cylinders and bilayer sheets/vesicles with the packing parameter steadily increasing from p < 1/3 (sphere) to p = 1 for planar assemblies such as the bilayer sheet. For a special liquid crystalline class of small molecular surfactants also packing parameter p>1 are possible, which lead to fascinating inverse morphologies such as the inverse spherical morphology or cubosome and the inverse hexagonal cylinders or hexosomes. These particles have unusual large interface and open channel systems to the surrounding medium.
Soft Patchy Nanoparticles
Particles with two hemispheres of different chemical properties are the simplest patchy particle and referred to as Janus particle. Theoretical and experimental works have shown that this type of particle displays a very affinity to interfaces with high stabilization capability, making Janus particles potentially attractive for a variety of applications (also called colloidal surfactants). Soft Janus nanoparticles usually consist of polymer phases and have been realized as spherical Janus micelles, cylinders and sheets (also discs). In a recent work, we expanded the family of soft Janus nanostructures about perforated Janus nanomembranes featuring a PS and PtBMA side, a thickness of only 13nm and homogeneous pore size distribution centering around 25nm.
Figure 3: Soft Janus nanoparticles. Spherical Janus micelle, cylinder, perforated Janus nanomembrane and Janus sheet (color code: grey and green are chemically different coronas; the dark part is the cross-linked core).
Topography through Interpolyelectrolyte Complexes
In collaboration with researchers from Aalto University and University of Jena we prepared block copolymer micelles and structured the corona through ionic complexation to a brush-on-brush system. High chain crowding within the particle periphery resulted in unusual microphase separation to morphologies perpendicularly oriented to the particle core. The snapshots in Figure 2 were recorded on a 300kV cryogenic transmission electron microscope that visualizes the particles in the near-native solvent-swollen state. The relatively high acceleration voltage thereby reduces beam damage and allows recording a series of images at varying tilt angles. After image alignment a mathematical algorithm calculated a 3D reconstruction of the observed area as depicted in the schematics.
Figure 2: Introducing topography to nanoparticles via IPEC formation. Cryo-TEM images of nanoparticles in the near-native state: (from left to right) lamellar, cylindrical and spiral microphase oriented perpendicular to the particle core. The schematic images were 3D reconstructions calculated from cryo-tomographies of the respective nanoparticle.
Block Copolymer Polymorphs with Topography
Since block copolymers are prone the microphase separate on the length scale of few to dozen nanometres (depends on block lengths), they are ideal materials for the patterning of surfaces. through the self-assembly of properly designed ABC triblock terpolymers, soft polymer-based solution nanostructures can be generated whose surface is decorated with a geometric pattern. In Figure 4 examples of recently developed solution nanostructures are given with unprecedented complexity.
Figure 4: block copolymer assemblies with topography.Novel solution morphologies of ABC triblock terpolymers as exemplified on spirals-on-discs, double helix-on-cylinders, striped sheets, patchy spheres and striped vesicles (polymersomes). Here, block A forms the inner core or substrate, block B (golden) the topographic pattern and block C the stabilizing corona that has been omitted for clarity.
20. Rational design of ABC triblock terpolymer solution nanostructures with controlled patch morphology
T. I. Löbling, O. Borisov, J. S. Haataja, O. Ikkala*, A. H. Gröschel*, A. H. E. Müller*
Nat. Commun., 7:12097 doi: 10.1038/ncomms12097 (2016).
19. “Patchy" carbon nanotubes as efficient compatibilizers for polymer blends
T. Gegenhuber, M. Krekhova, J. Schöbel, A. H. Gröschel*, H. Schmalz*
ACS Macro Lett., 2016, 5, 306–310.
18. Noncovalent grafting of carbon nanotubes with triblock terpolymers: Toward patchy 1D hybrids
T. Gegenhuber, A.H. Gröschel, T. I. Löbling, M. Drechsler, S. Ehlert, S. Förster, H. Schmalz*
Macromolecules, 2015, 48, 1767–1776. Selected as Cover for issue 6 (2015).
15. Hidden structural features of multicompartment micelles revealed by cryogenic transmission electron tomography
T.I. Löbling, J. Haataja, C. Synatschke, F.H. Schacher, M. Förtsch, A. Hanisch, A. H. Gröschel* and A. H. E. Müller*
ACS Nano, 2014, 8, 11330–11340.
12. Janus micelles as effective supracolloidal dispersants for carbon nanotubes
A.H. Gröschel*, T.I. Löbling, P.D. Petrov, M. Müllner, C. Kuttner, F. Wieberger, A.H.E. Müller*
Angew. Chem. Int. Ed., 2013, 52, 3602–3606 Selected as frontispiece of issue 52 (2013).