New metastable interfacial synthesis of a silver-terephthalate metal organic framework: structure, morphology and antibacterial activities

In this work, a silver terephthalate-based metal organic framework, [Ag2(BDC)]n (where H2BDC is the terephthalic acid), has been obtained by new synthetic routes using terephthalic acid and silver salts dissolved in a dimethyl sulfoxide (DMSO)/water mixture in the presence of ammonia used to stabilize silver ions and to control their availability. Fast crystal growth was obtained at the metastable interface between the DMSO/water mixture and a water layer formed upon slow water addition. Chemical and morphological properties of the obtained MOFs depend on the adopted synthesis conditions (routes a-c), but in all cases [Ag2(BDC)]n formation and growth took place by the dissolution of an ammonia hydrogen terephthalate salt. Large crystals of the ([Ag2(BDC)]n) monocline phase (named a-AgMOF) were grown through a slow (10 h) crystallization process (route a), while fast (3 h) crystallization processes (route b) lead to small size crystals with two different morphologies, named b-AgMOF. In order to obtain single crystal phases with a squared morphology, the Ag+ availability was limited using potassium chloride (c(KCl)-AgMOF) and benzoic acid (c(BA)-AgMOF) as additives during the synthesis (route c). The chemical properties of the nanostructures were evaluated by transmission FTIR measurements, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The obtained results indicate that all the obtained crystals possess efficient antibacterial activity. In particular, the b-AgMOF exhibited the highest efficiency against Gram-negative (Pseudomonas aeruginosa and Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. In addition, the b-AgMOF showed good stability both in water (evaluated up to 84 days) and in different culture media (evaluated up to 24 hours), suggesting it as a promising candidate for use as a new antibacterial agent for several applications.