Stability of ZIF-8 nanopowders in bacterial culture media and its implication for antibacterial properties

The stability and degradation characteristics of metal–organic frameworks (MOFs) in liquid media dictate their use in biomedical engineering applications. In this study, the antibacterial property of a zinc-based MOF, zeolite imidazolate framework-8 (ZIF-8), against gram-negative Escherichia coli (E. coli) is studied in comparison with ZnO, a widely-used antimicrobial agent. Superior antibacterial property of ZIF-8 was demonstrated and the reason was elucidated by new insights on the stability of ZIF-8 in bacterial culture media. Contrary to the widely held notion that ZIF-8 is stable in aqueous media, inductively coupled plasma-optical emission spectrometry and UV–Vis adsorption spectroscopy revealed the release of zinc ions and 2-methyilimidazolate molecules in the culture media. Electron microscopy and X-ray diffractometry revealed that the degradation of ZIF-8 is enhanced by the reaction of released zinc ions with phosphate ions in phosphate-buffered saline (PBS) or with organic moieties in Luria Bertani (LB) media, resulting in stronger antibacterial properties of ZIF-8 than that of ZnO. In fact, due to the affinity of ions and organic constituents in the cell media towards the building block of ZIF-8 structure, a higher amount of ZIF-8 degrades in LB and PBS media (70–80 wt%) than in pure water (22 wt%) after 24 h of immersion. The results of the present investigation strongly advocate the importance of stability study on MOFs in developing their biomedical applications, in terms of its efficacy, biosafety and eco-safety.