Novel technological applications in catalysis and bactericidal formulation have emerged for zinc oxide (ZnO) nanoparticles owing to their ability to generate reactive oxygen species by fostering H2O dissociation. Rational improvement of those properties requires a mechanistic understanding of ZnO nanoparticle reactivity, which is currently lacking. Here, we determine the structural and electronic properties of nanometer-sized ZnO, determine the binding energetics of H2O adsorption, and compare to an extended macroscopic surface. We show that the electronic density of states of ZnO nanoparticles is size-dependent, exhibiting a decreasing bandgap with the increase of nanoparticle diameter. The electronic states near the Fermi energy dominantly arise from O 2p states, which are spatially localized on “reactive” surface O atoms on the nanoparticle edges that are doubly coordinated. The frontier electronic states localized at the low coordinated atoms induce a spontaneous dissociation of H2O at the nanoparticle edges. The surface Zn and O atoms have inhomogeneous electronic and geometrical/topological properties, thus providing nonequivalent sites for dissociative and molecular H2O adsorption. The free energy of H2O binding is dominated by the electronic DFT interaction energy, which is site-dependent and correlated with the Bader charge of surface Zn atom. Entropy is found to stabilize the bound form, because the increase in the vibrational contribution is greater than the decrease in the translational and rotational contribution, whereas solvation stabilizes the unbound state. The absence of rough edges on an extended, macroscopic ZnO surface prevents spontaneous dissociation of a single H2O. This study underlies the importance of coupling geometrical and electronic degrees of freedom in determining the reactivity of nanoparticles and provides a simple elucidation of the superior catalytic activity of ZnO nanoparticles compared to ZnO in macroscopic forms.

Copper (Cu) bactericides/fungicides are used extensively for crop protection in agriculture. Concerns for Cu accumulation in soil, Cu leaching into the surrounding ecosystem, and development of Cu resistance in phytopathogenic bacteria are evident. While there is no suitable alternative to Cu available to date for agricultural uses, it is possible to reduce Cu per application by supplementing with Zn and improving Cu bioavailability using nanotechnology. We have prepared a non-phytotoxic composite material consisting of generally recognized as safe ZnO 800 particles and nanocopper-loaded silica gel (ZnO-nCuSi). The morphology of the ZnO-nCuSi material was characterized using scanning electron microscopy, showing ZnO particles dispersed in the silica gel matrix. ZnO-nCuSi demonstrated strong in vitro antimicrobial properties against several model plant bacterial species. Two consecutive year field efficacy results showed that agri-grade ZnO-nCuSi was effective in controlling citrus canker disease at less than half the metallic rate of the commercial cuprous oxide/zinc oxide pesticide.

Environmental contextThere are great concerns around current wide usage of copper-based agrochemicals. We compare the fate of nano- and conventional forms of copper, in particular their resistance to wash-off by rain (rainfastness), following their application to citrus leaves. Results showing large differences between the formulations in the amount and forms of copper washed from the leaves provide essential information to optimise agrochemical efficacy while minimising the environmental impact. AbstractThis study compares the rainfastness of nine forms of Cu, including nano and conventional Cu-based fungicide formulations, as well as their salt or bulk equivalents. Rainfastness is the ability to resist wash-off; it is a key property for improving pesticide formulations and for assessing the potential transfer of pesticides to the soil. A new protocol was developed to characterise losses of Cu from treated leaves. It consisted of dipping the leaves in rainwater and then in an acid/ethanol mixture followed by size fractionation. The proportion of Cu lost by wash-off from citrus leaves ranged from <2% (Tribasic, nCuO or Cu(OH)2) up to 93% (CuSO4) of the initial amount of Cu applied. Intermediate Cu losses were observed for formulations with silica (nano)particles (9–14% of applied Cu), Kocide (22%), ChampDP (31%), and a formulation with graphene oxide (47%). Smaller particles generally resulted in less wash-off, possibly due to stronger attachment to the leaf surface, but other factors such as the particle shape and solubility also played an essential role. The retention of nCuO to the leaves was particularly high, and the exact mechanisms involved (e.g. foliar uptake) deserve further work. Most of the Cu was washed off in its ionic form (>74%). Two Cu formulations (one commercial formulation and the formulation with graphene oxide) also showed wash off in significant proportions of Cu (~17%) in the nano-sized fraction. This study provides essential information on the amounts and forms of Cu that may reach the soil after the application of Cu-based agrochemicals. The great diversity in behaviour across the range of formulations considered highlights the need for more systematic research to fully exploit the potential improvements of current agrochemicals through (nano)formulation technologies.

In agriculture, prolonged use of copper biocides increases the risk of development of Cu resistance and its accumulation in soil, demanding an alternative. In this paper, we report antimicrobial magnesium hydroxide nanoparticles (NPs) as an alternative to Cu biocides with low cytotoxicity. To improved bioavailability, Mg hydroxide NPs were synthesized followed by coating with water-soluble capping agents, trisodium citrate (zeta potential, ξ = -22 mV) or betaine (ξ = +35 mV). Electron microscopy study confirmed the formation of ∼10-nm-sized cubical NPs with citrate and ∼100-nm-sized lamellar NPs with betaine. As-synthesized Mg hydroxide NPs inhibited bacterial growth of X. alfalfae, P. syringae, and E. coli within 4 h. Significant bacterial growth inhibition and killing were observed at 24 h post-treatment. Phytotoxicity studies on tomato plants showed no significant tissue injury. Therefore, Mg hydroxide NPs have the potential to serve as a Cu alternative.

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