Hydration and bactericidal activity of nanometer- and micrometer-sized particles of rock salt-type Mg_1-xCu_xO oxides

Copper substitution together with nano-structuring are applied with the aim to increase the bactericidal performances of the rocksalt-type MgO oxide. The partial substitution of magnesium ions with Cu²⁺ has been successfully achieved in both micrometer- and nanometer-sized particles of MgO up to 20 mol% in increments of 5 mol%. Microstructural analyses using the Integral Breadth method revealed that the thermal decomposition of the single source precursor Mg_1-xCu_x(OH)_2-2y(CO₃)_y.zH₂O at 400 °C creates numerous defects in 10-20 nm-sized particles of Mg_1-xCu_xO thus obtained. These defects make the surface of nanoparticles highly reactive towards the sorption of water molecules, to the extent that the cubic cell a parameter in as-prepared Mg_1-xCu_xO expands by +0.24% as soon as the nanoparticles are exposed to ambient air (60% RH). The hydration of Mg_1-xCu_xO particles in liquid water is based on a conventional dissolution-precipitation mechanism. Particles of a few microns in size dissolve all the more slowly the higher the copper content and only Mg(OH)₂ starts precipitating after 3 h. In contrast, the dissolution of all 10-20 nm-sized Mg_1-xCu_xO particles is complete over a 3 h period and water suspension only contains 4-12 nm-sized Mg_1-xCu_x(OH)₂ particles after 3 h. Thereby, the bactericidal activity reported for water suspension of Mg_1-xCu_xO nanoparticles depends on the speed at which these nanoparticles dissolve and Mg_1-xCu_x(OH)₂ nanoparticles precipitate in the first 3 h. Only 10 mol% of cupric ions in MgO nanoparticles are sufficient to kill both E. coli and S. aureus with a bactericidal kinetics faster and reductions in viability at 3 h (6.5 Log₁₀ and 2.7 Log₁₀, respectively) higher than the conventional antibacterial agent CuO (4.7 Log₁₀ and 2 Log₁₀ under the same conditions). EPR spin trapping study reveals that "hydroxylated" Mg_0.9Cu_0.1O as well as Mg_0.9Cu_0.1(OH)₂ nanoparticles produce more spin-adducts with highly toxic hydroxyl radicals than their copper-free counterparts. The rapid mass adsorption of Mg_0.9Cu_0.1(OH)₂ nanoparticles onto the cell envelopes following their precipitation together with their ability to produce Reactive Oxygen Species are responsible for the exceptionally high bactericidal activity measured in the course of the hydroxylation of Mg_0.9Cu_0.1O nanoparticles.