Tunable core–shell MOF nanoparticles

We are delighted that Kieran’s paper based on his MChem Part II research project in Oxford has now been published in Chemical Science!

The article describes how, by shortening the length of reaction, Zn/Cd-based ZIF-8 nanoparticles form with a Cd-rich core and Zn-rich shell. We collaborated with Sean Collins, whose beautiful scanning transmission electron microscopy showed us the core–shell structures, which we then used as the basis for a new model, first suggested by Andrew Goodwin to fit high-resolution X-ray diffraction data. This model allowed Kieran to quantify for numerous bulk samples the amount of Cd-rich material and Zn-rich material in the particles, as well as where the core–shell interface lay and how diffuse it was. He performed 99 syntheses at a range of temperatures and Zn/Cd ratios to map out how the nanoparticles’ internal interface and structure varied as a function of reaction conditions. Finally, we showed using in situ X-ray diffraction that the particles form first with a Cd-rich core followed by Zn-rich shell and the interface becomes increasingly diffuse the longer the reaction goes on.

By developing this simple synthesis and powerful new analysis method, and understanding the underlying formation mechanism, we have shown that it is indeed possible to control the spatial distribution of different components in metal–organic frameworks (MOFs) such as ZIF-8, which is really important to enable researchers to tap into their enormous potential as gas storage, separations and catalysis materials.

See the citation and all our publications here.

This work could not have been performed without several amazing co-authors: thank you Sean Collins for the STEM–EDS, Andrew Goodwin for co-supervision, Emily Reynolds (now at ISIS), Frank Nightingale, Hanna Boström (now at the Max Planck Institute for Solid State Research, Germany) and Simon Cassidy in the Goodwin group for help with all aspects of the XRD, Daniel Dawson and Sharon Ashbrook for NMR insights, Oxana Magdysyuk at Diamond beamline I12 for help with the in-situ beamtime, and Paul Midgley at Cambridge for support with the microscopy – Well done and thank you!

Sensing the strain, quickly

When metal–organic frameworks (MOFs) adsorb small molecules their structures often change, sometimes really noticeably, sometimes in such small ways that it’s hard to see. In a collaboration with Kota Shiba, Genki Yoshikawa and Kosuke Minami at the National Institute for Materials Science, Japan, we put MOF nanoparticles on a unique sensor device, the membrane-type surface stress sensor (MSS), by inkjet printing and spray-coating, and found that the MSS can detect these changes really well. The volatile organic compounds (VOCs) that we tested can be detected even at parts-per-million levels (one hundred times more dilute that CO2). The response is different for different VOCs and different MOFs, which enables them to be easily discriminated. What’s more, because of the high external surface area of the MOF nanoparticles, the response of the MOF-MSS sensor is really quick–– it takes just seconds to get a reading. This could make such technology really useful for real-time monitoring of chemical processes or biomarkers in healthcare.

The paper, “Strain-based chemical sensing using metal–organic framework nanoparticles” is published in the Journal of Materials Chemistry A. See our publications for more details.