Design of Anisotropic Functional Colloids
Thermoresponsive microgels are cross-linked polymeric structures that can undergo reversible volume transitions upon variations of the temperature. Pure microgels and composite core-shell microgels made of a polystyrene core and a microgel shell have extensively been studied as model systems for investigating the phase diagram and dynamics of colloidal particles interacting with a soft potential [1-3]. As a result of the thermosensitive shell which undergoes a temperature induced volume phase transition, their overall size and interactions can be controlled via the temperature (see [1,3-5]. As versatile colloidal systems that allow for a delicate control of their conformation, size and interaction potential, they exhibit a rich phase behaviour [3,5,6] and proved to be ideal building blocks for directed self-assembly [3,6,7]. Whereas all these applications were done with spherical particles, the interest recently turned to non-spherical systems.
Colloidal Design© Crassous
Figure 1: Nanoengineering composite microgels into different shapes. Ellipsoidal, facetted and bowl-shape composite microgels with a polystyrene core are obtained by post-processing spherical core-shell microgels. Figure reproduced from Ref. 10.
The key requirement towards complex assembled structures with desirable properties implies accurate engineering of colloidal particles as suitable building blocks with anisotropy in shape and interactions that need to be highly monodisperse in size and highly uniform in anisotropy. Our synthetic approach aims to design more complex shapes and describe the main strategies to establish microgel-based particles as the next key players for the future developments in directed self-assembly. We are following two main approaches to create the responsive building blocks. The first one relies on nanoengineering composite microgel into different shapes as shown in Figure 1 [8,9,10]. The other is a stepwise synthetic approach where anisotropic building blocks are functionalized with a microgel shell as shown in Figure 2 for anisotropic hybrid magnetic “nano-compass” microgels .
Figure 2: Creating multifunctional hybrid responsive colloids. A. CryoTEM micrograph of hybrid microgels consisting of a silica coated maghemite spindle core and a thermosensitive microgel shell. B. Control of the particle orientation under applied magnetic field as shown by SAXS (cSAXS, PSI Switzerland). Hematite spindle (red) align perpendicular to the field, whereas maghemite spindle (grey) align along the field .
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