Divalent metal release and antimicrobial effects of layered double hydroxides

Resistance to antimicrobial agents is responsible for major social and economic losses. The World Health Organization estimated 700,000 global deaths a year due to antimicrobial resistance. The use of layered double hydroxides (LDHs) as antibacterial materials could present a way to reduce the risk of bacterial infections and antibacterial resistance, by partial release of metallic ions in aqueous dispersion. The partial dissolution of different synthetic LDHs MII-AlIII (M = Zn, Cu, Ni, Co, Mg) was studied in Lysogeny Broth (LB) and Tryptic Soy Broth (TSB), growth media of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria, respectively. The influence of several parameters (crystallinity, MII:AlIII ratio, type of intercalated anion {CO32−, Cl−, NO3−, ClO4−} and nature of MII cations) was investigated. In the absence of any post-synthetic hydrothermal treatment, ZnII-AlIII LDH showed a release of ZnII ions 6 times enhanced. Upon increasing ZnII:AlIII molar ratio to 3: 1 and exchanging carbonate anions with other anions having lower intercalating affinities, a similar effect was observed. The dissolution properties of MII-AlIII LDHs were correlated with the thermodynamic stability of their MII(OH)2 hydroxide counterparts with the exception of CuII-based LDH, which showed an amplified release of CuII due to its irregular structure presenting defects. Finally, the antibacterial activity was only noted for ZnII and CuII-based LDHs. The antimicrobial effect of the studied LDHs was linked in the first place to the nature of divalent metal itself, and to the amount of released MII ions into the culture media in the second place. This effect was more easily identified in ZnII-AlIII LDHs whose minimum inhibitory concentration was decreased significantly from 12 to 0.375 mg.mL−1 when higher amounts of ZnII ions were released. [Display omitted] •LDHs with low crystallinity and many defects release more ions into broth media.•LDHs having intercalating anions with low affinity are more soluble.•Copper LDHs release amplified amounts of CuII due to its defected structure.•The antimicrobial activity of LDHs is primary related to nature of divalent metal.•The antimicrobial effect of Zn LDHs is dependent on the amount of ZnII released.