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Copper (Cu)-silica nanocomposite containing valence-engineered Cu: a new strategy for improving the antimicrobial efficacy of Cu biocides.

Copper (Cu) compounds are widely used as antibacterial/antifungal agents for protecting food crops. Prolonged use of Cu biocides would lead to undesirable Cu levels in agricultural soil. In the absence of a suitable alternative, prudent use of Cu biocides is required. This paper reports for the first time a composite material of sol-gel silica host matrix loaded with mixed-valence Cu as an alternative to conventional biocides. In this composite material, Cu is present in different oxidation states. The hydrophilic silica matrix serves as a water-dispersible delivery vehicle for antimicrobial Cu. It is hypothesized that a mixed-valence Cu system, specifically enriched with Cu(0) and Cu(I), will exhibit enhanced antimicrobial efficacy over traditional Cu(II) compounds. Materials were characterized by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy for the determination of particle size, morphology, crystallinity, and Cu oxidation states. Antimicrobial studies against Xanthomonas alfalfae and Escherichia coli (minimum inhibitory concentration) showed improved efficacy in MV-CuSiNG treatment compared to CuSiNG and other controls. Phytotoxicity studies performed (in Vinca sp. and Hamlin orange) under greenhouse conditions showed that the newly prepared nanocomposite is safe for plants, demonstrating potential usefulness of the material in agricultural biocides.