Graphene has long been celebrated as a wonder material. Just one atom thick and made entirely of carbon, it’s incredibly strong, highly conductive, and almost perfectly impermeable. In fact, it blocks nearly everything except the very smallest particle, hydrogen. But what if you carve out a hole that’s exactly the size of a single benzene ring? Niyas and his colleagues have discovered that benzene holes in nanographene can not only let halides permeate through but also have selectivity towards certain halides.
What are benzene holes?
Benzene holes, or benzene pores, refer to a void or missing hexagonal rings – essentially a hole the size of one benzene unit – created within the graphene lattice. This hole has a precise diameter of 1.4Å – this means that a single strand of hair is roughly half a million times thicker than a benzene pore!
Figure 1: Benzene hole in graphene
How did they do this?
A sheet of graphene is not only extremely difficult to handle on its own; trying to drill a hole into it would be nearly impossible if not extremely messy. To ameliorate this issue, the researchers decided to build a molecule sized hexagonal lattice, which is called nanographene, with a benzene hole in it from organic molecules.
Then they stacked two of these nanographene together in just the right way to create a stable bilayer nanographene with a little cavity in the middle. The kinetically stable bilayer was formed by using specific combination of solvents.
The cavity in the middle can only be reached through that single benzene window due to π-π interactions elsewhere. There should also be an electrostatic factor that comes to play as the hydrogens at the hole are slightly positively charged due to electron withdrawing effects from the substituents on the nanographene.
They used this simple logic: If the dimer somehow captures a halide, the halide must have gone through the hole since it cannot enter the cavity otherwise. Thus, the permeation affinity could be assumed with the binding affinity of the halide to the nanographene bilayer. They utilized spectroscopic analysis in order to determine whether a halide has bound to the cavity.
What did they find?
It was found that halides could permeate through the hole. Out of the four halides they tested, fluoride showed an excellent proclivity to permeate through the hole which was highlighted by its highest binding affinity (K_f = 6.8 \times 10^7 M^{-1}). In fact, when computational calculations were performed, fluoride showed zero resistance when passing through the hole.
Chloride and bromide both showed that they experience a much greater barrier to permeate due to their bigger size. Out of the two, chloride was able permeate through much quicker. While chloride required only 0.5 equiv. of its source to saturate all the binding sites, bromide required over 3 equiv. of its source to do the same. This is expected since bromide is significantly larger than chloride.
Iodide did not show any permeation through the benzene hole. Itself having a diameter of around 2Å, it is simply too large to permeate through the hole.
Why does it matter?
Being able to design and control ion transport at the atomic level has huge potential
- Water desalination: ultra-precise graphene filters that let some ions through but block others.
- Energy storage: halide-ion batteries could benefit from selective membranes.
- Molecular machines: synthetic “ion channels” mimicking biology’s clever designs.
By proving that ions can sneak through such a tiny, well-defined hole, this work opens a path to making graphene-based materials act like artificial ion channels — something nature has perfected in cell membranes.
Author’s note: I couldn’t help but notice some interesting discrepancies. I am currently learning how to compute topologies and execute energy minimisation to answer some questions and prove my postulates. Stay tuned!
References:
Niyas, M.A., Kazutaka Shoyama, Matthias Grüne and Würthner, F. (2025). Bilayer nanographene reveals halide permeation through a benzene hole. Nature, [online] 637. doi:https://doi.org/10.1038/s41586-024-08299-8.
Shannon, R.D. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica Section A, 32(5), pp.751–767. doi:https://doi.org/10.1107/s0567739476001551.
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