Images of science: new materials to trap or screen molecules

This image was taken during the formation of a membrane of ultra-porous material, which can "sift" molecules or trap them. These materials are notably developed for the separation of gas mixtures, the depollution of air or water, or the administration of the active ingredients of drugs more efficiently.

Martin Drobek, University of Montpellier; Anne JULBE, University of Montpellier and Didier Cot, University of Montpellier

An intermediate step in the synthesis of a membrane, before the gaps between the crystals are filled in to create a compact, continuous layer of ultraporous, metal organic framework (or MOF) material. Martin Drobek, Provided by the author

These materials are composed of elementary bricks that form meshes of impressive diversity. The bricks can be assembled as a real "Lego set" at the nanometric scale.

They are made up of metal ions linked together by organic molecules acting as cement and spacer. These networks, called "metal organic frameworks " (MOFs), allow the formation of molecular structures in almost unlimited numbers and with adjustable physical and chemical properties. Some MOFs can accommodate and transport small gas molecules, such as hydrogen, while others can trap and release large molecules, such as the active ingredients of drugs.

Their hybrid nature, both organic and inorganic, gives "MOFs" a very flexible structure. Their extremely high porosity, with a mesh of small, regular and well-ordered pores, explains their low density and their large accessible internal surface, the size of a soccer field for a single gram of material! Such a surface area - several thousand square meters - far exceeds that of reference porous materials, such as zeolites or activated carbons.

Most MOFs are, by default, prepared and used as powders, but in order to exploit their application potential on a large scale and at an industrial level, a "shaping", e.g. in granules or thin layers, is generally required and the production costs must be competitive.

Screening molecules through a highly selective porous network

At the European Membrane Institute in Montpellier, we are interested in these materials for the development of membranes allowing the separation of gas mixtures by a "molecular sieving" effect. Deposited on the surface of gas sensors, such membranes can improve the selectivity of the detection of toxic or explosive gases, thanks to their preferential transport in the pores.

By modulating the length of the organic molecules that connect the metal centers, the pore size can be adjusted. Most MOFs are microporous (pore size less than 2 nanometers) and their pores can accommodate and transport, in addition to hydrogen, other small gas or vapor molecules such as water, oxygen or carbon dioxide.

Currently, we are paying particular attention to hydrogen sensors in relation to the safety problems posed by the production, transport, storage and use of this gas. The strategy consists in covering the sensitive material of the sensor with a layer of a specific type of "MOFs". The separating effect of this molecular sieve allows hydrogen to diffuse easily to the sensor while rejecting the other gases in the mixture.

Trapping molecules as in cages

However, given the diversity of possible structures and functionalities, the field of application of "MOFs" is much wider. Indeed, their pores correspond to the dimensions of a large diversity of common molecules and they can be used as nanocage sponges for the selective adsorption of these molecules.

For these applications, we are also interested in mesoporous "MOFs " (diameter greater than 2 nanometers). Their main advantage lies in the possibility of using them to encapsulate large molecular systems, for example proteins or drugs, nanoparticles or "macromolecular" assemblies (groupings of giant molecules).

We can therefore envisage the development of complex architectures of the "MOFs" type to store and produce energyto clean up the air or water by the selective adsorption of harmful compounds; or in the health field, for example for the controlled distribution of active ingredients.The Conversation

Martin Drobek, CNRS Research Fellow, University of MontpellierAnne JULBE, Research Director at CNRS, specialist in ceramic and hybrid membranes, University of Montpellier and Didier Cot, Engineer, head of the electron and photonic microscopy unit, European Membrane Institute, University of Montpellier

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