Resolving the internal morphology of core-shell microgels with super-resolution fluorescence microscopy

Abstract

We investigate the internal morphology of smart core-shell microgels by super-resolution fluorescence microscopy exploiting a combination of 3D single molecule localization and structured illumination microscopy utilizing freely diffusing fluorescent dyes. This approach does not require any direct chemical labeling and does not perturb the network structure of these colloidal gels. Hence, it allows us to study the morphology of the particles with very high precision. We found that the structure of the core-forming seed particles is drastically changed by the second synthesis step necessary for making the shell, resulting in a core region with highly increased dye localization density. The present work shows that super-resolution microscopy has great potential with respect to the study of soft colloidal systems.

Fig. 1. The temperature dependent swelling behaviour of microgels made of PNIPMAM, which are subsequently used as cores, is here characterized by PCS shown in (A). In (B) a dSTORM reconstruction of the microgels is presented, where (C) is the zoom-in of the cyan highlighted area in (B).

Fig. 2. In (A) the 2D localization density of the core microgel (N = 1048) is plotted following the calculation model shown in Fig. S5. In (B) the recalculated 3D localization density is shown.

Fig. 3. (A, D and G) show the swelling curves of the three core–shell microgels with nominally increasing shell thickness from thin, intermediate to thick. The shell thickness is varied according to the synthesis described in methods under 2.1. The reconstructed dSTORM images (B, E and H) for the microgels with different thick shells are shown in the remaining images with increasing shell thickness from the top to the bottom. Images on the right hand side (C, F and I) are the zoomed versions of the white highlighted regions of interest.

Fig. 4. The microgels are immersed in R6G solutions and afterwards spincast on PEI coated cover slips. To remove excess dye molecules the sample is washed with purified water while spincasting. The remaining dye molecules in the microgels are localized. We investigated the localization densities, acquired by dSTORM, of core–shell microgels with three different shell thicknesses termed as “thin” (green triangle, N = 314), “intermediate” (yellow circle, N = 608) and “thick” (blue triangle, N = 1512) with a cross-linker content of 10 mol% for the core and 2 mol% for the shell during the synthesis. Therefore, the presented scheme for data evaluation (see Fig. S5†), after getting the localization table, is used. Localizations are counted and assigned to circular rings with a width of 10 nm. From this it's possible to calculate a localization density for each ring. This was done for the core and the three different core–shell microgels. The 2D localization density shown in (A) shows a higher localization density for all the core–shell microgels over the whole radius in comparison to the core microgel. In (B) the recalculated 3D density for all measured microgels is shown. The hydrodynamic radius R_H of the collapsed core microgels is indicated by the dashed line in magenta in both graphs.