Chitosan-Guided Directed In Situ Formation of Zinc- and/or Manganese-Based Phosphate Nanostructures

In the present work, metal phosphate nanostructures were synthesized in situ using a natural polymer-assisted route based on chitosan (CS) and employing zinc and/or manganese as metal precursors. CS acted as a multifunctional matrix capable of regulating the nucleation, growth, and colloidal behavior of the formed nanostructures through electrostatic and coordination interactions with zinc and/or manganese phosphates, also highlighting the decisive role of the type of metal in these processes. The formation mechanism and structural properties of the systems obtained were investigated using FTIR, FE-SEM/EDS, DLS and molecular docking. FTIR analysis confirmed the participation of CS protonated amino groups in acid-base and coordination interactions with metal ions, while characteristic phosphate vibrations evidenced the formation of phosphate phases. Morphological analysis revealed that Mn-containing systems generate hemispherical structures embedded in the polymer matrix, while Zn-containing systems exhibit larger, oblong morphologies with a greater tendency towards aggregation and precipitation. DLS results showed broad size distributions and time-dependent colloidal instability, particularly in zinc-containing systems. Molecular docking simulations provided mechanistic information, showing that phosphate groups have a significantly higher binding affinity for CS compared to isolated metal ions. These results demonstrate that metal-polymer affinity and metal-phosphate coordination rigidity are key parameters in the structural and colloidal control of in situ synthesized metal phosphate nanostructures.