Spatially Resolved Diffusion NMR for Structurally Heterogeneous Materials

Understanding the internal architecture of hydrogel materials is essential for their effective use in biomedical and pharmaceutical applications, yet the use of noninvasive, spatially resolved methods remains limited. We report a robust analytical approach using spatially resolved pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy to quantify the depth-dependent self-diffusion of small molecular probes in intact hydrogel systems. By introducing probes post-gelation via passive downward diffusion, this method avoids perturbations associated with probe incorporation during gel formation and enables nondestructive profiling of internal gel architecture. Applied to high amylose maize starch, agarose, and calcium-triggered low-molecular-weight (LMWG) gels, the technique revealed vertical variations in network density and porosity in starch gels, corroborated by scanning electron microscopy, while the other gels exhibited uniform structure. In contrast, conventional nonselective PFG-NMR yields a single self-diffusion coefficient averaged over the entire sample and is unable to reveal the heterogeneity present. Our methodology broadens the NMR analytical toolkit for characterizing soft matter systems and offers promising utility in evaluating structurally complex biomaterials, where spatial heterogeneity is functionally relevant.