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

Abstract

Metal–organic framework (MOF) thin films are promising materials for multiple technological applications, such as chemical sensing. However, one potential limitation for their widespread use in different settings is their stability in aqueous environments. In the case of ZIF-8 (zeolitic imidazolate framework) thin films, their stability in aqueous media is currently a matter of debate. Here, we show that optical waveguide spectroscopy (OWS), in combination with surface plasmon resonance (SPR) spectroscopy, offers a convenient way for answering intriguing questions related to the stability of MOF thin films in aqueous solutions and, eventually provide a tool for assessing changes in MOF layers under different environmental conditions. Our experiments relied on the use of ZIF-8 thin films grown on surface-modified gold substrates, as optical waveguides. We have found a linear thickness increase after each growing cycle and observed that the growing characteristics are strongly influenced by the nature of the 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 are structurally stable upon exposure to pure water and 50 mM NaCl solutions but they exhibit a slight swelling and an increase in porosity probably due to the permeation of the solvent in the intergrain mesoporous cavities. However, OWS revealed that exposure of ZIF-8 thin films to phosphate-buffered saline solutions (pH 8) promotes significant film degradation. This poses an important question as to the prospective use of ZIF-8 materials in biologically relevant applications. In addition, it was demonstrated that postsynthetic polyelectrolyte modification of ZIF-8 films has no detrimental effects on the structural stability of the films

Stereochemically Active and Inactive Lone Pairs in Two Room-Temperature Phosphorescence Coordination Polymers of Pb2+ with Different Tricarboxylic Acids

“…Luminescent materials have a variety of practical applications in optical devices, e.g., optical waveguide used as the transmission medium in local and long haul optical communication systems or the components in integrated optical circuits, , light sources (fluorescent lamps and LEDs), − disinfection, medical imaging, − horticulture, temperature sensors, − and chemical sensors, − and so on; thus, lots of research attention has always been paid to exploring a new family of luminescent materials. − …”

Layer-by-layer integration of conducting polymers and metal organic frameworks onto electrode surfaces: enhancement of the oxygen reduction reaction through electrocatalytic nanoarchitectonics

“…20,52 ZIF-8 can be described as an infinite network of Zn 2+ ions tetrahedrally coordinated with 2-methylimidazolate N-bidentate organic linkers, featuring high BET surface area (≈ 1500-1800 m 2 g -1 ), and good resistance towards hydrolysis in moderate pH aqueous environments and ionic strengths. 53,54 We show for the first time that a LbL architecture is achievable using these presynthesized nanostructures; and moreover, that these films feature electrochemical activity and can act as enhancers of the neutral pH electrochemical ORR due to a synergetic effect between high surfacearea porous material and electroactive conductive polymer, offering thus a cost-effective alternative to precious metal catalysts usually employed in energy conversion devices.…”

Influence of the Synthesis and Storage Conditions on the Activity of Candida antarctica Lipase B ZIF-8 Biocomposites

“…For example, we have shown that the immobilization of catalase using ZIF-8 generates an inactive biocomposite, attributed to protein unfolding upon interaction with the hydrophobic framework . There are examples within the literature, however, of catalase@ZIF-8 remaining active when stored and tested in phosphate-buffered saline (PBS). − Recently, it was demonstrated that pure ZIF-8 particles can rapidly decompose in the presence of phosphate anions. − Accordingly, we were motivated to develop our understanding of how the synthesis, and test conditions, affect the particle size, chemistry, and properties of enzyme@ZIF-8 biocomposites.…”