Revisiting Mg solubility in CuO nanorods: limit probed by neutron diffraction and effect on the particle toxicity towards bacteria in water

Both nanometer-sized CuO and MgO particles exhibit bactericidal activities against Staphylococcus aureus and Escherichia coli, two bacteria causing healthcare-associated infections. The solid solution Cu_1-xMg_xO is potentially interesting for biomedical applications as one of the compositions could have a much higher bactericidal activity than the parent CuO and MgO oxides considered separately. But, to date, no Vegard's law proves the real existence of such a solid solution. This study was aimed at shedding light on the solubility of Mg²⁺ ions in CuO nanoparticles and its impact on the free oxygen radicals they produce, the quantity of which determines their bactericidal performance. The solid solution Cu_1-xMg_xO does exist and particles were synthesized as nanorods of 50-60 nm length by thermally decomposing at 400 °C the single source precursors Cu_1-xMg_x(OH)₂. Vegard's laws exist only in the compositional range 0 ≤ x ≤ 0.1, due to the low capacity of the distorted NaCl-type structure to accommodate regular coordination [MgO₆] octahedra. Only neutron diffraction allowed the detection of the small amount of MgO nanoparticles present as impurity in a 10 g sample beyond the solubility limit of x = 0.1. In this series, CuO nanorods remain the most active against E. coli and S. aureus with reduction in viability of 99.998% and 98.7% after 180 min in water, respectively. Our synthesis route has significantly increased the activity of pure CuO nanoparticles beyond the values reported so far, especially against E. coli. The bactericidal performances of CuO and the magnesium-substituted counterparts (i.e. Cu_1-xMg_xO) are not linked to cupric ions they release in water since their mass concentrations after 180 min are much lower than minimal concentrations inhibiting the growth of E. coli and S. aureus. These CuO nanorods kill bacteria in water because they produce a large quantity of free oxygen radicals in the presence of H₂O₂ only, the majority of which are highly toxic HO˙ radicals. Mg²⁺ ions have a detrimental effect on this production, thus explaining the lowest bactericidal performance of Cu_1-xMg_xO nanorods. Definitive knowledge of the toxicity of Cu_1-xMg_xO nanoparticles towards bacteria in water is now available.

Understanding the bactericidal mechanism of Cu(OH)2 nanorods in water through Mg-substitution: high production of toxic hydroxyl radicals by non-soluble particles

To date, there is still a lack of definite knowledge regarding the toxicity of Cu(OH)₂ nanoparticles towards bacteria. This study was aimed at shedding light on the role played by released cupric ions in the toxicity of nanoparticles. To address this issue, the bactericidal activity of Cu(OH)₂ was at first evaluated in sterile water, a medium in which particles are not soluble. In parallel, an isovalent substitution of cupric ions by Mg²⁺ was attempted in the crystal structure of Cu(OH)₂ nanoparticles to increase their solubility and determine the impact on the bactericidal activity. For the first time, mixed Cu_1-xMg_x(OH)₂ nanorods (x ≤ 0.1) of about 15 nm in diameter and a few hundred nanometers in length were successfully prepared by a simple co-precipitation at room temperature in mixed alkaline (NaOH/Na₂CO₃) medium. For E. coli, 100% reduction of one million CFU per mL (6 log₁₀) occurs after only 180 min on contact with both Cu(OH)₂ and Cu_0.9Mg_0.1(OH)₂ nanorods. The entire initial inoculum of S. aureus is also killed by Cu(OH)₂ after 180 min (100% or 6 log₁₀ reduction), while 0.01% of these bacteria stay alive on contact with Cu_0.9Mg_0.1(OH)₂ (99.99% or 4 log₁₀ reduction). The bactericidal performances of Cu(OH)₂ and the magnesium-substituted counterparts (i.e. Cu_1-xMg_x(OH)₂) are not linked to cupric ions they release in water since their mass concentrations after 180 min are much lower than minimal concentrations inhibiting the growth of E. coli and S. aureus. Finally, an EPR spin trapping study reveals how these nanorods kill bacteria in water: only the presence of hydrogen peroxide, a by-product of the normal metabolism of oxygen in aerobic bacteria, allows the Cu(OH)₂ and its magnesium-substituted counterparts to produce a lethal amount of free radicals, the majority of which are the highly toxic HO˙.

Hydration and bactericidal activity of nanometer- and micrometer-sized particles of rock salt-type Mg1-xCuxO 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.