Cationic-anionic polyelectrolyte interaction as a tool to graft silver nanoparticles on hydroxyapatite crystals and prevent cytotoxicity

This paper reports an easy, green and low cost method to support silver nanoparticles (AgNPs) onto functionalized hydroxyapatite (HA) crystals. The process involves low molecular weight poly(ethylenimine) (PEI) as a reducing and stabilizing agent of AgNPs and polyacrylic acid (PAA) as a functionalizing agent of hydroxyapatite (HAPAA). With respect to HA, the negatively charged surface of HAPAA significantly improves the interaction with the positively charged surface of PEI-stabilized AgNPs. AgNPs content up to about 6.5 wt% in the composite HAPAA–AgNPs crystals has been obtained by varying the volume of the AgNPs suspension submitted to interaction with HAPAA. The cytotoxicity of the crystals, evaluated through in vitro tests on MG63 osteoblast-like human cells, increases on growing Ag content, but it is reduced by the presence of PAA. The HAPAA–AgNPs crystals display similar dosedependent antibacterial activity towards gram-positive and gram-negative bacteria. The comparison of data indicates that an AgNPs content of about 2 wt% allows for combining absence of cytotoxicity with a significant long-standing antibacterial activity towards both Staphylococcus aureus and Escherichia coli.