Structure-function relationships in chitosan hydrogel spheres as a platform for silver nanoparticle stabilization and catalytic performance driven by macromolecular architecture.

Chitosan (CH) is a renewable, metal-chelating polysaccharide widely explored for its ability to form functional macromolecular networks with tunable structural and interaction properties. In this work, chitosan hydrogel spheres were developed via ionic gelation without the use of chemical crosslinkers or external stabilizing agents, providing a well-defined three-dimensional matrix rich in amino and hydroxyl functionalities. Silver incorporation was achieved through two different strategies, coprecipitation and precipitation, allowing control over nanoparticle spatial distribution within the macromolecular network. Comprehensive characterization (FTIR, UV-vis, TEM, XRD, and atomic absorption spectroscopy) confirmed the in situ formation of ultrasmall and uniformly dispersed Ag nanoparticles (1-3 nm). Spectroscopic analysis revealed that -NH2 and -OH groups play a key role in metal coordination and stabilization, while structural results indicated preservation of the semicrystalline organization of chitosan after metal incorporation. The resulting hybrid spheres exhibited tunable morphology (2-3 mm diameter) and long-term stability under ambient conditions. The relationship between macromolecular architecture and functional behavior was further assessed using nitroaromatic reduction as a model reaction. Differences in nanoparticle localization and bead size influenced accessibility and reaction kinetics, with surface-confined AgNPs showing enhanced apparent activity. Overall, this study demonstrates how the intrinsic chemical functionality and structural organization of chitosan govern metal-polymer interactions, nanoparticle stabilization, and resulting properties, providing insight into the rational design of functional polysaccharide-based materials for environmental and technological applications.