Shedding Light on the Dark Corners of Metal–Organic Framework Thin Films: Growth and Structural Stability of ZIF-8 Layers Probed by Optical Waveguide Spectroscopy

RAFTI, MATÍAS; KNOLL, WOLFGANG; ALLEGRETTO, JUAN A.; MENGES, BERNHARD; DOSTALEK, JAKUB; AZZARONI, OMAR

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Lugar: Washington; Año: 2018 vol. 123 p. 1100 - 1109

1089-5639

Metal−organic framework (MOF) thin filmsare promising materials for multiple technological applications, such as chemical sensing. However, one potentiallimitation for their widespread use in different settings is theirstability in aqueous environments. In the case of ZIF-8(zeolitic imidazolate framework) thin films, their stability inaqueous media is currently a matter of debate. Here, we showthat optical waveguide spectroscopy (OWS), in combinationwith surface plasmon resonance (SPR) spectroscopy, offers aconvenient way for answering intriguing questions related tothe stability of MOF thin films in aqueous solutions and,eventually provide a tool for assessing changes in MOF layersunder different environmental conditions. Our experimentsrelied on the use of ZIF-8 thin films grown on surface-modified gold substrates, as optical waveguides. We have found a linearthickness increase after each growing cycle and observed that the growing characteristics are strongly influenced by the nature ofthe primer layer. One of our findings is that substrate surface modification with a 3-mercapto-1-propanesulfonate (MPSA)primer layer is critical to achieve ZIF-8 layers that can effectively act as optical waveguides. We observed that ZIF-8 films arestructurally stable upon exposure to pure water and 50 mM NaCl solutions but they exhibit a slight swelling and an increase inporosity probably due to the permeation of the solvent in the intergrain mesoporous cavities. However, OWS revealed thatexposure of ZIF-8 thin films to phosphate-buffered saline solutions (pH 8) promotes significant film degradation. This poses animportant question as to the prospective use of ZIF-8 materials in biologically relevant applications. In addition, it wasdemonstrated that postsynthetic polyelectrolyte modification of ZIF-8 films has no detrimental effects on the structural stabilityof the film