Heterogeneity of Network Structures and Water Dynamics in κ-Carrageenan Gels Probed by Nanoparticle Diffusometry

Abstract

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.

Spatiotemporal Heterogeneity of κ-Carrageenan Gels Investigated via Single-Particle-Tracking Fluorescence Microscopy

“…10,38,39 The coarse network bulk is expected to have a polymer concentration-dependent mesh size of ∼100 nm, allowing slightly obstructed diffusion of the 28 nm diameter probes, in agreement with our first observed species (Figure 1C top). Meanwhile, distinct regions in the network exist with a much denser network with mesh size <∼20 nm 10 , leading to hampered entry of the probes in the dense, but inherently flexible hydrogel network. This finding is in agreement with probes getting trapped and showing largely decreased selfdiffusion (Figure 1C middle).…”

“…The mean diffusion coefficient of this obstructed population was ∼1 × 10 −12 m 2 /s in 2.5% κ-carrageenan gels and increased to ∼5 × 10 −12 m 2 /s in 0.5% κ-carrageenan gels. This relation between the probe self-diffusion coefficient and the polymer concentration can be accurately fitted by Johnson's obstruction model 10,26 (Figure 2D; Methods), indicating that the probe self-diffusion is obstructed by fibers of 3.2 ± 0.3 nm diameter in the coarse network. This finding agrees with earlier NMR studies in which a fiber diameter of ∼3 nm was found.…”

“…We note that we observed a higher self-diffusion than shown previously for dendrimers trapped in κ-carrageenan gels observed via PFG NMR (10 −14 m 2 /s). 10 We attribute this to the coating of the The population describing the probes in the coarse network (blue outline in Figure 2A,B) shows a decreasing mjd with increasing κ-carrageenan polymer concentration, while it is mostly unaffected by the NaCl concentration. The mean diffusion coefficient of this obstructed population was ∼1 × 10 −12 m 2 /s in 2.5% κ-carrageenan gels and increased to ∼5 × 10 −12 m 2 /s in 0.5% κ-carrageenan gels.…”

“…12−14 Currently used imaging techniques [e.g. electron microscopy (EM), 5,8 raster image correlation spectroscopy (RICS), 10 or fluorescence recovery after photobleaching (FRAP) 15,16 ] have indicated that dense network regions exist on a micrometer scale. However, these methods require invasive or destructive sample preparation (EM) or have limited power to quantitatively resolve spatial heterogeneity (RICS and FRAP).…”

Nuclear magnetic resonance/magnetic resonance imaging on lubricating greases: Observation of bleeding and aging

“…These mechanisms are currently also discussed in the context of hydrogels, for example, Junker et al [ 21 ] and Rauzan et al [ 22 ] While chemical compositions of hydrogels and lubricating greases are considerably different, the conception is similar: on the length scales of PFG‐NMR, the free mean path length of guest molecules is strongly influenced by the surrounding matrix in both cases, which can be measured by PFG‐NMR. [ 21,23–25 ]…”