Chemists at Sheffield University have demonstrated a 10–12-fold mechanical expansion in their porphyrin film material upon exposure to acid vapour.
Porphyrins are intensely coloured compounds — naturally found in the haem of blood cells and chlorophyll of plants — that have the ability to readily form complexes.
The team has been investigating the use of synthetically created porphyrin as thin layers floating on a water surface — referred to as Languier films.
‘When you expose them to an acid vapour you protonate the porphyrin rings, they go from a neutral state to [being] charged,’ project lead Dr Tim Richardson of Sheffield told The Engineer.
‘Imagine a load of two pennies stacked close together and suddenly they repel to become facedown and flat on the surface.’
Furthermore, this expansion can be reversed through deprotonation allowing contraction to recur, Richardson added.
The group has now verified the phenomenon in the lab using ammonia gas as the anylate to create a 10–12-fold mechanical expansion, validated with a reflectometer at the ISIS facility at the Rutherford Appleton Laboratory.
It is now looking for ways to create a working device from the material, which is currently restricted to a layer on water. Richardson envisaged deploying it onto a cantilever system akin to bi-metallic strip that can be probed with a laser.
As he explained, there would potentially be distinct advantages to using a porphyrin-based device for gas sensing.
‘The initial sticking of anylate molecules to the surface occurs very quickly so you’ve got a potentially very fast sensor,’ said Richardson.
‘Whereas if you’ve got a thick crystal the surface-to-volume ratio is relatively small and to get a big signal you then have to wait for the anylate molecules to diffuse into the bulk and find the binding sites — it’s sluggish and it takes a lot longer to recover.’