Copper- and nickel-plated ball bearings. Credit: Hustvedt, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons.
Researchers have developed an alternative method for the production of a new kind of copper needed for medical applications.
The copper isotope Cu-64 plays an important role in medicine: it is used in imaging processes and also shows potential for cancer therapy.
However, it does not occur naturally and must be produced artificially – a complex and costly process.
Until now, Cu-64 has been generated by bombarding nickel atoms with protons. When a nickel nucleus absorbs a proton, it is transformed into copper.
At TU Wien, however, a different pathway has now been demonstrated: Cu-63 can be converted into Cu-64 by neutron irradiation in a research reactor.
This works thanks to a special trick – so-called ‘recoil chemistry’.
From nickel to copper
Copper atoms contain 29 protons, while the number of neutrons can vary.
The most common naturally occurring variant is Cu-63, which has 34 neutrons and is stable. Cu-64, by contrast, contains one additional neutron and is radioactive, decaying with a half-life of about 13 hours.
This makes Cu-64 attractive for medical use: it remains stable long enough to be transported to its target location inside the body, but decays quickly enough to keep the patient’s radiation exposure low.
TU Wien’s Veronika Rosecker said: “Today, Cu-64 is typically produced in a cyclotron; you can produce Cu-64 by taking Ni-64 and bombarding it with protons.
“The nickel nucleus absorbs the proton, ejects a neutron, and is thereby transformed into copper-64.”
This method works very well, but it is expensive – and it requires access to both a cyclotron and enriched Ni-64, itself a rare isotope.
Veronika Rosecker in the reactor hall at TU Wien. Credit: TU Wien.
Copper with one extra neutron
It is therefore natural to consider a simpler alternative: producing Cu-64 from Cu-63 directly.
All that is needed is to add a single neutron – something a research reactor can provide. But this approach comes with a challenge.
TU Wien’s Martin Pressler advised: “When Cu-63 is irradiated with neutrons, Cu-64 nuclei are indeed produced, but it is almost impossible to separate them chemically from the ordinary copper atoms.
“You end up with a mixture that consists mostly of ordinary copper, with only tiny traces of the desired Cu-64.”
Now, however, this problem has been solved using recoil chemistry. This effect has been known for nearly a century, but has not previously been used for the production of medically relevant radioisotopes.
Before irradiation, the copper atoms are built into specially designed molecules.
Rosecker said: “When a Cu-63 atom within such a molecule absorbs a neutron and becomes Cu-64, it briefly holds a large amount of excess energy, which it releases as gamma radiation.”
The emission of this high-energy photon gives the atom a recoil – much like a rocket recoils when expelling exhaust. This recoil is strong enough to eject the copper atom from the molecule.
Rosecker continued: “This means that Cu-63 and Cu-64 can now be cleanly separated. The Cu-63 atoms remain bound within the molecules, while the newly formed Cu-64 atoms are released.
“This makes it easy to separate the two isotopes chemically.”
Finding the right molecule
A key challenge was identifying a suitable molecule. It needed to be stable enough to withstand conditions inside a nuclear reactor, yet soluble enough to allow efficient chemical processing afterward.
Pressler explained: “We achieved this using a metal-organic complex that resembles haem – the molecule found in human blood.”
Similar substances had been studied before, but were not soluble. The new complex was chemically modified to make it soluble, enabling straightforward recovery of the Cu-64 after neutron irradiation.
The method can be automated, the molecules can be reused without loss, and – instead of requiring a cyclotron – it only needs a research reactor such as the one at TU Wien.
The study is published in Dalton Transactions.
