Spot the difference: Optical micrographs before gold coating (1, 3) and after removal of same (2, 4). On the left are teeth from Wurmiella excavate, an eel-like chordate called a conodont; on the right is a scale from a lobe-finned fish (probably Onychodus). |
Microscopic
fossils, by their nature, contain microscopic detail—as do some
macrofossils if they are extremely well preserved. To see that detail
under a scanning electron microscope, the standard technique is to coat
the fossil with a conductive metal such as silver, palladium or gold,
which ensures that the surface of the specimen conducts evenly and no
charge builds up. Gold-coating is also used in optical microscopy to
provide a reflective surface for transparent or translucent specimens.
The
problem is: how do you remove the gold afterwards so that you can
further study the specimen itself? The gold has to react with something,
but it is a very inert metal (one of the reasons why it is so useful)
so you need either a very powerful oxidizing agent, which is potentially
liable to damage your specimen, or something which will stabilise the
gold ions so that a mild oxidising agent can be employed. Which sounds a
better idea, except that the standard procedure is to stabilise the
gold using cyanide, with all the dangers and problems inherent in that
infamously unpleasant chemical.
At
Leicester, chemists have been making great strides in the use of ionic
liquids as solvents; that is, salts which are liquid at room
temperature. These ionic liquids are safe to handle, easy to store and
environmentally friendly, and much of theIonic Liquids Group’s research
has been into industrial processes such as metal-plating. But now Dr.
Gero Frish and PhD student Jennifer Hartley from our Department of
Chemistry have teamed up with Professor Mark Purnell and research
associates Laurent Darras and David Jones from our Department of Geology
to apply similar techniques to fossil microscopy. The results of their
research have been published in the online journal Palaeontologia Electronica.
The
Leicester team used iodine as an oxidizing agent, dissolved in a
chemical called Ethaline which is a ‘deep eutectic solvent’, ie. a
mixture of two chemicals that has a melting point much lower than either
of its ingredients. Ethaline is produced by mixing choline chloride
with ethylene glycol at about 60 C. It can easily be made in large
quantities and stored at room temperature, and it dissolves iodine
nearly 700 times better than water: at 60 C, a kilogram of ethaline will
happily dissolve 200 g of iodine—and the resulting solution is stable
for a week.
Before and after. This SEM image of a tiny part of the Onychodus scale (indicated on the left) clearly shows how well-preserved the detail is after the removal of gold-coating using an Ethaline-iodine solution. |
A
range of specimens from the Department of Geology’s fossil collections
were used, including calcareous fossils of plankton and phosphatic
fossils of lungfish scales and mineralised muscle fibres. Micrographs
taken before the application of a 30 nm layer of gold, and after its
removal, show how effective the process is. But it’s not just efficient,
it’s also very cheap, very quick (less than 12 hours to fully remove
the gold) and very safe. The cyanide technique requires a fume cupboard
whereas the ionic liquid process can be done on a workbench without even
opening a window.
Finally
the specimens are given a quick rinse in dilute potassium iodide and
then deionised water. The solution contains so much iodine that it can
be used several times—after which it can be simply tipped down the drain
without any environmental concerns.
On
top of all these advantages, the Leicester technique won’t affect the
adhesive used to attach the specimen to the microscope slide (provided
it isn’t water-soluble), which makes handling so much easier.
In
their paper, the team point out that the wide range of ionic liquids
available means that bespoke solutions can be easily determined for
other types of fossil and/or metallic coatings. But they do recommend a
‘trial run’ of any new solvent before applying it to any particularly
rare or valuable specimen, just in case.
Non-destructive, safe removal of conductive metal coatings from fossils: a new solution
