A set of functionalized nanoparticles (PEGylated dendrimers, d = 2.8−11 nm) was used to probe the structural heterogeneity in Na+/K+ induced κ-carrageenan gels. The self-diffusion behavior of these nanoparticles as observed by 1H pulsed-field gradient NMR, fluorescence recovery after photobleaching, and raster image correlation spectroscopy revealed a fast and a slow component, pointing toward microstructural heterogeneity in the gel network. The self-diffusion behavior of the faster nanoparticles could be modeled with obstruction by a coarse network (average mesh size <100 nm), while the slower-diffusing nanoparticles are trapped in a dense network (lower mesh size limit of 4.6 nm). Overhauser dynamic nuclear polarization-enhanced NMR relaxometry revealed a reduced local solvent water diffusivity near 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-labeled nanoparticles trapped in the dense network, showing that heterogeneity in the physical network is also reflected in heterogeneous self-diffusivity of water. The observed heterogeneity in mesh sizes and in water self-diffusivity is of interest for understanding and modeling of transport through and release of solutes from heterogeneous biopolymer gels.
de Kort, D. W., Schuster, E., Hoeben, F. J. M., Barnes, R., Emondts, M., Janssen, H. M., Lorén, N., Han, S., Van As, H., & van Duynhoven, J. P. M. (2018). Heterogeneity of Network Structures and Water Dynamics in κ‑Carrageenan Gels Probed by Nanoparticle Diffusometry. Langmuir, 34, 11110-11120. https://doi.org/10.1021/acs.langmuir.8b01052