Morphology and Mechanical Stiffness of Soft Phytoglycogen Nanoparticles Revealed by AFM Force Spectroscopy
Phytoglycogen is a glucose-based polymer that is produced naturally by sweet corn in the form of highly branched nanoparticles with a dendritic or tree-like architecture. Their distinctive structure combined with their simple chemical composition results in soft, deformable, porous, nontoxic and biodegradable nanoparticles that make phytoglycogen ideal for applications in personal care, nutrition, and biomedicine. In the present study, I investigated the morphology and mechanical properties of native and modified phytoglycogen nanoparticles using atomic force microscopy (AFM) force spectroscopy. In this technique, the AFM tip is pressed into the sample, allowing the collection of high-resolution maps of local particle height and mechanical stiffness on a large number of individual phytoglycogen nanoparticles. The simultaneous acquisition of high-resolution height and stiffness maps provided unique insights into the morphology and mechanical stiffness of the nanoparticles at the single-nanoparticle level. Fully hydrated phytoglycogen nanoparticles were soft and deformable, showing a branched internal structure at modest values of the force setpoint. The corresponding high-resolution stiffness maps revealed a larger Young’s modulus within the internal structure region compared to that in the softer outer region of the nanoparticles. Phytoglycogen nanoparticles dried in air were approximately half the radius of fully hydrated particles, with a Young’s modulus that was three orders of magnitude larger than that of fully hydrated particles, indicating the intimate relationship between particle hydration and mechanical properties. I also used AFM force spectroscopy to characterize size-reduced particles produced using mechanical extrusion and acid- and enzyme-hydrolysis. The Young’s modulus of extruded phytoglycogen was comparable to that of the native particles, whereas the modulus of hydrolyzed phytoglycogen was significantly smaller with clear differences between the results for acid- and enzyme-hydrolyzed phytoglycogen. The softness and deformability of phytoglycogen nanoparticles, and the ability to tune their properties through mechanical and chemical modification, provides new insights at the single-nanoparticle level for this novel sustainable nanomaterial.