Spatially resolved diffusion NMR for structurally heterogeneous materials

Understanding internal heterogeneities in hydrogel materials is essential for their effective use in biomedical and pharmaceutical applications, yet non-invasive, spatially resolved methods remain limited. We report a robust analytical approach using spatially resolved pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy to quantify 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 non-destructive profiling of internal gel architecture. Applied to high amylose maize starch and agarose gels, the technique revealed vertical variations in network density and porosity in starch gels, corroborated by scanning electron microscopy, while agarose gels exhibited uniform structure. In contrast to conventional non-selective PFG-NMR, which yields averaged diffusion data, our spatially encoded approach differentiates local microstructural environments with high sensitivity. This methodology broadens the analytical toolkit for characterizing soft matter systems and offers promising utility in evaluating structurally complex biomaterials and tissues where spatial heterogeneity is functionally relevant.