Hydrated soft matter presents property-determining morphology over a range of length scales, many of which cannot be resolved by light microscopies.  Electron-optical approaches provide sufficient resolution to characterize the nano/microscale gel morphology, but they bring challenges associated with the study of hydrated, non-conductive material.  Cryogenic fixation and imaging techniques have been developed over the past several decades, primarily by the biological community, and these are increasingly being translated to electron-optical studies of hydrated soft material.  We have applied these approaches to measure the mesoscale morphology of various hydrated polyelectrolyte gels and microgels.  For the most part, we use high-pressure-freezing (HPF) methods to avoid artefacts that can form due to water crystallization in place of water amorphization.  Water crystallization in highly hydrated soft matter can introduce mesoscale structure such as channels, which are often misinterpreted to reflect the underlying gel mesh size.  Water amorphization avoids this.  Studying microgels in frozen-hydrated colloidal solutions at various stages of amorphous ice sublimation enables us to quantify the microgel swell ratio and, hence, their true underlying mesh properties.  We are particularly interested in polyanionic microgels such as poly(acrylic acid) [PAA] whose net negative charge enables them to be electrostatically deposited on synthetic surfaces and subsequently loaded by complexation with small-molecule cationic antimicrobials.  Strong complexation interactions lead to the formation of a classical core-shell structure, and the nanoscale resolution of SEM imaging and microanalysis enables us to study the local core-shell morphology within individual microgels.  By following the relative carbon, oxygen, and nitrogen intensity ratios measured by energy-dispersive X-ray spectroscopy (EDS) using low-energy (2 keV) incident electrons, we show that the shell is highly enriched in complexed antimicrobial and is almost entirely dehydrated.  Furthermore, trace analysis that challenges the sensitivity of spatially resolved X-ray spectroscopy shows that the shell is depleted in Na, thus giving direct evidence of counterion release during complexation. <br/>

Cryo scanning electron microscopy of gels and microgels

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.