Fractionating MOF crystals unlocks their potential: retaining electrical properties with enhanced sensitivity

31.03.2025

Illustrative diagram of the fragmentation of a nanometric crystal of a metal organic framework (MOF) with molecular switching properties, called spin transition (SCO). Credit: Patricia Bondía.

• Researchers at IMDEA Nanociencia develop porous materials with switchable electronic properties.
• They observe that the properties of MOF materials are maintained when they are nanometric in size; could be miniaturized without compromising their functionality.

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Madrid, 31^st March, 2025. IMDEA Nanociencia's "Switchable Materials" Group is working on the development of materials whose properties can change as easily as we flip a switch. It is led by Dr. José Sánchez Costa, and one of his pioneering works focuses on joining porosity properties, electrical properties and molecular switching (spin transition) in the same material. Metal Organic Frameworks (MOFs) are interesting materials due to their high porosity, which would give them enormous potential to host gas molecules, such as carbon dioxide or hydrogen. They are not only passive materials, they can also be designed to feature responsive properties in the presence of these gases. For example, some MOFs exhibit spin crossover (SCO) behavior, i.e., they can change their magnetic state in response to an external stimulus. This is of enormous interest for the development of electronic applications such as data storage, or sensors.

The change in the MOF conductive state is triggered by an external stimulus

This work, that reports the spin transition phenomenon in a conductive MOF material, was published in the journal Chemistry of Materials. This collaboration with Dr. Enrique Burzurí (Universidad Autónoma de Madrid) showed that the electrical transport properties depend directly on the spin state of the material. The change in the conductive state is triggered by an external stimulus, a change in temperature, in the range of temperatures close to room temperature. In addition, the change is accompanied by a striking change in the colour of the glass, from orange to blue.

In a subsequent work, published in the journal Small, the researchers studied in depth the relationship between spin state and temperature. When molecules as ubiquitous as water can significantly alter the properties of MOF materials, it becomes crucial to understand the interaction between the spin state and host molecules. A combined experimental and computational approach was employed to explore how the uptake and release of host molecules influences the dynamics of the spin transition phenomenon. Unlike in most cases, the results revealed a solid-state mechanism in which water loss facilitates the transition to the low-spin state, in a reverse-spin transition phenomenon.

Nanosctructuring transforms the MOF crystals into better sensors

The Switchable Materials Group's research on these SCO-MOF molecular crystals has also shown that their properties are maintained when the crystals are fractionated down to nanometer sizes. It may seem trivial for a nanocrystal to have the same properties as its macroscopic analogue, but it is not; Nano-scale quantum effects are relevant enough to change the properties of materials. However, the researchers have found that the conductive properties of the macroscopic crystal SCO-MOF are the same in nanocrystals, which is a very encouraging result because it means that the electronic properties already observed can be combined with the fact that nanostructuring the crystals increases the reaction surface and makes these crystals more sensitive to external chemical stimuli such as, for example, the capture of gases. In other words, nanosctructuring transforms the crystals into better sensors.

This research, also published in Small, is the result of a collaboration between scientists led by Dr. Sánchez Costa (IMDEA Nanociencia) and Dr. Sañudo (University of Barcelona). To confirm the structural integrity of the SCO-MOF nanocrystals, the team used an advanced technique based on electron diffraction (MicroED), performed at the National Center for Biotechnology (CNB-CSIC). This technique allowed them to analyse the atomic arrangement of the nanocrystals and compare them with their macroscopic counterparts. Their findings revealed that the nanocrystals retained both their crystal structure and their charge-carrying properties, suggesting that MOFs could be miniaturized without compromising their functionality.

This scientific advance opens up exciting possibilities for MOF networks in next-generation nanotechnologies. This study represents a significant step towards the integration of MOF materials into cutting-edge technological applications, where precise control over the properties of the material at the nanoscale is crucial.

Glossary:

• Metal Organic Framework (MOF): A class of materials that consist of the bonding of metal ions to each other using organic ligands. These repetitive coordination units extend into structures that may contain pores.
• Spin crossover (SCO): A phenomenon that occurs in some coordination compounds, such as MOFs, in which the spin state of the metal changes due to an external stimulus.
• Spin state: magnetic angular momentum of electrons. Unlike the classical interpretation of a particle spinning on its own axis, spin does not imply that the electron is actually rotating. Spin is a fundamental property that describes a type of angular momentum associated exclusively with subatomic particles.

Reference

A. Martinez-Martinez, S. Gullace, E. Resines-Urien, L. Martín-Pérez, J. Collado, R. Arranz, E. Burzurí, C. Santiago, E. C. Sañudo, J. Sanchez Costa, Conversion of Flexible Spin Crossover Metal–Organic Frameworks Macrocrystals to Nanocrystals Using Ultrasound Energy: A Study on Structural Integrity by MicroED and Charge-Transport Properties. Small 2024, 2408966. https://doi.org/10.1002/smll.202408966

 Link to IMDEA Nanociencia Repository: https://hdl.handle.net/20.500.12614/3915

Contact:

José Sánchez Costa
Switchable Nanomaterials Group
https://www.imdeananociencia.org/switchable-nanomaterials/home
@josescostalab.bsky.social

Oficina de Divulgación y Comunicación en IMDEA Nanociencia
divulgacion.nanociencia [at]imdea.org
  

Source: IMDEA Nanociencia.

IMDEA Nanociencia Institute is a young interdisciplinary research Centre in Madrid (Spain) dedicated to the exploration of nanoscience and the development of applications of nanotechnology in connection with innovative industries.