Polyelectrolyte Capping As Straightforward Approach toward Manipulation of Diffusive Transport in MOF Films

Abstract

We present experimental results demonstrating the suitability of polyelectrolyte capping as a simple and straightforward procedure to modify hydrophilic/hydrophobic character of porous films, thus allowing additional control on transport properties. In particular, we synthesized ZIF-8 metal organic framework (MOF) films, an archetypal hydrophobic zeolitic imidazolate framework, constituted by Zn ions tetrahedrally coordinated with bidentate 2-methylimidazolate organic linker, and poly(4-styrenesulfonic acid) as capping agent (PSS). MOF films were synthesized via sequential one pot (SOP) steps over conductive substrates conveniently modified with primer agents known to enhance heterogeneous nucleation, followed by dip-coating with PSS aqueous solutions. Crystallinity, morphology, and chemical composition of ZIF-8 films were confirmed with traditional methods. Continuous electron density depth profile obtained with synchrotron light X-ray reflectivity (XRR) technique, suggest that PSS capped-films do not adopt segregated configurations in which PSS remains surface-confined. This affects functional properties conferred by PSS capping, which were assessed using cyclic voltammetry with both positively and negatively charged redox probe molecules. Furthermore, taking advantage of the control attained, we successfully carried in situ synthesis of film-hosted d-block metal nanoparticles (Au and Pt-NPT@5x-ZIF-8+PSS) via direct aqueous chemical reduction of precursors (diffusion-reaction approach).

Modulation of Hydrophilic/Hydrophobic Character of Porous Environments in Metal–Organic Frameworks via Direct Polymer Capping Probed by NMR Diffusion Measurements

“…Polymer capping was demonstrated previously to be a suitable strategy for conferring hydrophilicity to an otherwise hydrophobic surface in a porous network. 28 Moreover, similar confinement effect has been observed for porous polymer matrices with surface affinity toward water adsorption. 50 Figure 4 shows schematically the proposed structures for ZIF-8/PABA composites allowing for a small fraction of water molecules to be adsorbed on top of the polymer-coated nanocrystals, which would continuously exchange with the vapor phase in contact with the intercrystallite space, leading to the observation of a fast decay in the STE experiment.…”

“…Synergy between an electroactive component (polyaminobencylaminePABA, a conducting polymer (CP) featuring −NH 2 moieties) and a high surface area material capable of enhancing gas adsorption (e.g., ZIF-8 MOF, constituted by tetrahedrally coordinated Zn 2+ ions with 2-methylimidazolate) ,, was explored and proved to be successful to create a soft composite electroactive surface, featuring both meso- and microporosity. , The observed enhancement of the ORR can be understood in terms of an interplay between the eminently hydrophobic (and capable of oxygen adsorption) microporosity present in ZIF-8 crystals (the reported Brunauer–Emmett–Teller surface areas range between 1500 and 1700 m 2 g –1 , with micropores of ∼0.3 nm window, and ∼1.1 nm diameter size) and the PABA-coated interparticle mesoporosity . It was hypothesized that the hydrophilicity endowed by PABA allows the (otherwise hindered) diffusion of ionic species intervening in the ORR, in line with what was reported for similar systems of polymer-capped MOF films …”

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

“…It was previously reported that PSS can modify the polar character of intergrain mesoporous cavities in ZIF-8 films, which in turn, alters effective diffusion coefficients of the probe molecules going through the film. 19 This opens the path for the use of the diffusion−reaction approach for the synthesis of filmembedded metal nanoparticles. The successful polyelectrolyte capping of films constituted by porous materials depends on the nature and strength of the interactions between capping agent and the pore walls.…”

“…ZIF-8 optical waveguides were constructed on surface-modified gold substrates, in order to characterize film growth process and to determine structural stability toward the exposure to different aqueous environments. The suitability of this approach to elucidate the interaction of ZIF-8 films with modifying agents such as polyelectrolyte solutions (postsynthetic modification allows for the application of many different fine-tuning steps in terms of material's structureresponse as recently reported) 19,56 is demonstrated. In conjunction with effective medium theory, the optical response of ZIF-8 films can be quantified in terms of thickness, surface mass density, and porosity variations, which are of the utmost importance for any sensor-related application.…”

“…ZIF-8 thin films were synthesized on the top of the gold surface according to previously reported protocols. 19 Briefly, a gold-coated substrate was placed overnight in MPSA aqueous solution. Then, the substrate was washed with Milli-Q pure water and dried under a stream of N 2 .…”