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 M ^II-Al ^III (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, M ^II:Al ^III ratio, type of intercalated anion {CO ₃ ²⁻, Cl ⁻, NO ₃ ⁻, ClO ₄ ⁻} and nature of M ^II cations) was investigated. In the absence of any post-synthetic hydrothermal treatment, Zn ^II-Al ^III LDH showed a release of Zn ^II ions 6 times enhanced. Upon increasing Zn ^II:Al ^III 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 M ^II-Al ^III LDHs were correlated with the thermodynamic stability of their M ^II(OH) ₂ hydroxide counterparts with the exception of Cu ^II-based LDH, which showed an amplified release of Cu ^II due to its irregular structure presenting defects. Finally, the antibacterial activity was only noted for Zn ^II and Cu ^II -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 M ^II ions into the culture media in the second place. This effect was more easily identified in Zn ^II-Al ^III LDHs whose minimum inhibitory concentration was decreased significantly from 12 to 0.375 mg.mL ⁻¹ when higher amounts of Zn ^II ions were released.