Colloidal stability of silver nanoparticles with layer-by-layer shell of chitosan copolymers

The influence of polymeric shell formed by the layer-by-layer (LbL) assembly technique using chitosan or its grafted copolymers with dextran or poly(ethylene glycol) and dextran sulfate on physico-chemical properties and aggregation kinetics of silver nanoparticles (AgNP) was investigated. It is shown that the hydrodynamic diameter of encapsulated AgNP increases linearly with increasing the number of polyelectrolyte layers without sufficient aggregation. The critical concentrations of coagulation (CCC) and Hamaker constants (H) were determined in dependence on electrolyte charge, structure of polycation, and number of bilayers in the shells. While a shell consists of more than two bilayers, the values of critical electrolyte concentration and energy of interparticle interactions H are influenced mainly by copolymer structure. In all cases, the CCC value is more than doubled after formation of two or more bilayers over silver nanoparticle surface. The shells based on chitosan-poly(ethylene glycol) graft copolymer were proven to be the most effective in increasing aggregation stability in NaCl solutions, while chitosan-dextran graft copolymer effectively decreases van der Waals interaction between nanoparticles. Application of grafted copolymers allows one to obtain encapsulated colloids that are stable over a wide range of NaCl concentrations including the isotonic one and improve biocompatibility and targetability of nanovehicles engineered for therapeutic or imaging purposes.