Physico-chemistry of a successful micro-reactor: Random coils of chitosan backbones used to synthesize size-controlled silver nanoparticles.

The synthetic system to produce silver nanoparticles (AgNP) based on the thermally activated reduction of aqueous silver ions by chitosan (CS) polysaccharide is investigated to unravel the physicochemical processes controlling AgNP nucleation and growth. An anomalous preeminence of AgNP nucleation over growth is found for conditions under which the opposite trend is obeyed for AgNP synthesized from soluble precursors in homogeneous media. The behavior is modeled assuming the formation of a tridimensional supramolecular structure from silver ions / CS´s amino groups coordination complexes, driving the crosslinking within polymer folding and aggregation in shaping random coils. These structures become reactive precursor structures, that behave as microreactors during thermal treatment, and AgNP size are modulated by controlling the amino groups to silver ion ratio. Stabilized AgNP of high quality are easily produced from an environment-friendly synthetic system, which requires low cost reagents and demands simple and safe laboratory procedures.