Nanoscale optical imaging of individual and densely packed microgel colloids
Prof. Frank Scheffold, University of Fribourg
We report on advances in the study of nanoscale structured soft polymeric materials. We employ dSTORM superresolution microscopy to study the internal density profile and shape of microgels 'in-situ' - meaning in their natural state in solution [1,2]. As we demonstrate this allows us to monitor 'live' the change in microstructure across the volume phase transition when particles are swelling and deswelling.
Microgels are among the most studied colloidal and polymeric systems of the past two decades. Swelling thermosensitive, poly(N- isopropylacrylamide) microgels by lowering the temperature provides a unique mechanism for controlling the porosity and size of colloidal particles on the nanoscale. As a consequence, these smart microgel particles are being considered for applications ranging from viscosity modifiers and sensing to drug delivery and as models for the glass and the jamming transition. Here, we present results from in-situ two-color superresolution microscopy of dye-labelled submicron sized pNiPAM microgels. We first demonstrate direct Stochastic Optical Reconstruction Microscopy (dSTORM) to image single microgels in two and three dimensions, at different stages of the volume phase transition, with a lateral resolution of 30nm. We find that the swelling behaviour observed in real space matches quantitatively with results from traditional light scattering measurements in reciprocal space. Next, we study dye labelled tracer microgels embedded in dense microgel suspensions. As we increase the packing density we map out the different contributions that allow the dense packing of the soft microgels, due to deformation, interpenetration and compression. Moreover, we can observe shape changes, such as faceting, in dense microgel systems where particles are compressed by the presence of its neighbours . This is all but impossible with other nanoscale imaging techniques used to date (AFM, Cryo-TEM). Based on the detailed understanding of the local structure and morphology we can model the macroscopic elastic properties of dense suspensions over a large range of densities .
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 G. M. Conley, S. Nöjd, M. Braibanti, P. Schurtenberger and F. Scheffold, Superresolution Microscopy of the Volume Phase Transition of pNIPAM Microgels, Colloids and Surfaces A 499, 18–23 (2016)
 G. M. Conley, Philippe Aebischer, S. Nöjd, P. Schurtenberger and F. Scheffold, Jamming and overpacking fuzzy microgel spheres: deformation, interpenetration and compression, Science Advances, 3(10),1700969 (2017)
 G. M. Conley, and F. Scheffold, in preparation.