Magnetic probes read the chemistry inside COF pores, in the lab

Inside a porous crystal whose pores are only a few nanometres wide, no optical instrument can see what is happening. At Martin Luther University Halle-Wittenberg (MLU), the doctoral students Simona Bassoli and Sebastian Michler have found another way to look: they add tiny magnetic probes to the pores and read how those probes respond. The work is published in the journal Small, and it is still at the laboratory stage.

The materials at stake are covalent organic frameworks, or COFs: porous, crystalline compounds whose regular pores make them useful for filtering pollutants out of water or for acting as catalysts in chemical reactions. Before any of this can be used, the chemical environment inside the pores must be finely adjusted, and that requires measuring it precisely. The standard method measures how many nitrogen molecules become adsorbed in the pores at about -196°C. At that temperature, all molecular motion freezes. The result is a snapshot that says nothing about chemical interactions.

The Halle team replaces nitrogen with a special type of magnetic probe that dissolves in water at room temperature. Stimulated by microwaves inside a strong magnetic field, the probes send back a response signal, read by a technique called electron spin resonance (EPR) spectroscopy. The principle is simple: when a probe touches a pore wall, its signal changes. That signal shows how many probes are bound inside the pores, how concentrated they are, the type of bond that holds them to the wall, and whether the molecular chains lining the pores attract water (hydrophilic) or repel it (hydrophobic).

To check that the method really detects chemical differences, the team made two COF variants with contrasting pores, one with water-attracting chains and one with water-repelling chains. The method complements established analytical techniques, since it provides different information about the same material. No precision figures have been published yet.

For laboratories that design porous materials, the gain is a way to check what a pore actually does before using it for filtration or catalysis. The team has not given a timeline. Its next steps are to determine whether the probes can reveal other material properties, and whether the approach works on other porous materials. Until those questions are answered, the method stays a research tool.