Core-shell ZIF-8@polydopamine nanoparticles obtained by mitigating polydopamine coating induced self-etching of MOFs: prototypical metal ion reservoirs for sticking to and killing bacteria

Metal–organic frameworks (MOFs) have the potential to boost the undervalued biomedical applications of metal ions. Such endeavor has been hindered by the challenge of how to avoid the (cyto)toxicity of the prematurely released metal ions to healthy cells or organisms and amorphous precipitates of ions formed with components in biological fluids before achieving targeting applications. The current work aimed to partially address such challenges by developing a prototypical metal ion reservoir. We prepared well-defined core–shell ZIF-8@PDA nanoparticles, with zeolitic imidazolate framework-8 (ZIF-8) as the reservoir of Zn2+ and polydopamine (PDA) with inherent metal ion chelating capabilities as the shell. The well-known self-etching problem of MOF templates upon polydopamine coating was mitigated by controlling the surface properties of cubic ZIF-8 nanoparticles. As regulated by the PDA shell that can partially trap the prematurely released metal ions, ZIF-8@PDA nanoparticles had a unique Zn2+ releasing behavior consisting of a limited releasing stage followed by pH-dependent burst releasing that could quickly increase the local Zn2+ concentration to harmful levels. Enhanced antibacterial efficiency of ZIF-8@PDA against Gram-positive Staphylococcus aureus was demonstrated. This could be attributed to the strong interaction of ZIF-8@PDA with bacteria as driven by inherent bioadhesion of PDA, which could promote direct interactions of released zinc ions with bacteria by avoiding potential precipitations.