Mixed-Valence Core-Shell Copper Loaded Silica Nanoparticle – A Powerful Antimicrobial Composite Material For Agricultural Crop Protection

Antimicrobial activity of Copper (Cu) is known for centuries. Cu compounds are used in agriculture for protecting crops from bacterial and fungal diseases. Prolonged use of Cu as agricultural pesticides is detrimental to the environment due to undesired toxicity issues. In absence of suitable Cu alternative, an effort towards reducing the amount of Cu pesticide release in the environment is desirable. In this paper, we present a strategy to load Cu on the surface of a solid silica (inert) support with high surface area, thus forming a core-shell (C-S) Cu loaded silica particle (CuSiNP). It is hypothesized that Cu coated on an inert support will be still bioavailable and exhibit antimicrobial efficacy. In this design, the core volume of the Cu particle is replaced with an inert material (silica). Since Cu is contact killing pesticide, similar level of antimicrobial efficacy (per unit amount of metallic Cu) is expected from C-S CuSiNP material when compared with traditional Cu compounds. We have presented in this paper two different design of CuSiNPs. The silica core (inert) was synthesized using a modified Stöber sol-gel method. Cu SEM and TEM techniques were used to estimate particle size and morphology. Particle dispersibility in aqueous suspension was measured using Dynamic Light Scattering (DLS) technique. Antimicrobial efficacy was evaluated against E. coli (a gram negative bacteria) B. subtilis (a gram positive bacteria) using disc diffusion assay (DDA) and growth inhibition assay. Based on the bacterial growth inhibition (OD measurements), minimum inhibitory concentration (MIC) value of the CuSiNP material was estimated. Results showed that C-S CuSiNP materials possess improved efficacy in comparison to a commercial pesticide (Kocide®3000 fungicide/bactericide). Plant tissue injury potential (phytotoxicity) was evaluated using ornamental Vinca sp. as a model system. Results show that C-S CuSiNP material was non-phytotoxic