Estimating microstructural length scales in κ-carrageenan hydrogels by PFG NMR nanoprobe diffusometry

Abstract

We use PFG NMR to measure hindered self-diffusion of spherical, monodisperse diffusional nanoprobes in a polysaccharide network. These nanoprobes have different diameters in the 1-10 nm range, but identical inert (PEG) surfaces. We use Johnson's model of particle self-diffusion in fibrous networks to estimate the polymer strand thickness and to obtain a measure for the network mesh size. Johnson's model takes into account the obstruction effect, and the degree of obstruction is independent from the arrangement of the polymer strands at low polymer concentrations. Probe particles with different sizes provide consistent predictions of strand thickness in this concentration regime. This allows us to infer a structural length scale close to the average mesh size of the polymer network.

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

“…This relation between the probe self-diffusion coefficient and the polymer concentration can be accurately fitted by Johnson’s obstruction model , (Figure D; 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. , …”

“…Our results show the existence of primarily two nanoprobe diffusional states, which we attribute to diffusion in the coarse bulk network and in dense network regions, along with occasional switching between these states. The relationship between nanoprobe diffusion in the bulk coarse network is consistent with Johnson’s obstruction model, ,, describing the coarse network as consisting of 3.2 ± 0.3 nm diameter κ-carrageenan fibers independent of sodium ion concentration. Within this coarse network, regions of dense networks with ∼1 μm diameter are embedded, irrespective of the sodium ion or κ-carrageenan polymer concentration.…”

“…This finding agrees with earlier NMR studies in which a fiber diameter of ∼3 nm was found. 10,27 Dense Networks Span 1 μm-Sized Regions. Next, we were interested in the spatial distribution of the network heterogeneity, which was not yet addressed in comparable experimental settings.…”

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

“…In previous work, we have shown that the self-diffusion coefficients of nonsticky 19 F-labeled G1-, G3-, and G5-nanoparticles with diameters of 2.8–6.9 nm decrease with the increased κ-carrageenan concentration. 11 , 19 In the current work, we carried out NMR diffusometry experiments using the same nanoparticles in gels induced at lower sodium concentrations (≤200 mM Na + )—it is well-established that this condition results in a more heterogeneous network structure. 13 For these experiments, performed with G1-, G3-, and G5- 19 F nanoparticles ( d = 6.9 nm), we used a single κ-carrageenan concentration of 2 wt %, a K + concentration of 20 mM, and a varied Na + concentration.…”

“…According to the obstruction model of Johnson, 18 the κ-carrageenan polymer strand radius r f can be obtained from the reduced diffusion coefficients D / D 0 using where r s is the nanoparticle radius and φ is the polymer volume fraction. A length scale for the mesh size d m can be estimated from 19 …”