In this image, purple balls represent radioactive iodine-125 atoms on a gold surface; green balls are atoms that have undergone nuclear decay into tellurium-125. [Sykes and Michaelides Labs]
In this image, purple balls represent radioactive iodine-125 atoms on a gold surface; green balls are atoms that have undergone nuclear decay into tellurium-125. [Sykes and Michaelides Labs]

For centuries, alchemists tried to accomplish transmutation—the morphing of one element to another—but they always failed. Now transmutation happens every day, many times in the context of radiation oncology, where decaying atoms are valued as a source of low-energy electrons, the bane of cancer cells. Going forward, transmutation could fight cancer even more effectively, say scientists at Tufts University.

Gold, they point out, is a factor, which might pique the interest of the alchemically minded. But the scientists also make it clear that their approach is not at all mystical.

The Tufts scientists, led by E. Charles H. Sykes, Ph.D., collaborated with PerkinElmer and University College London to study one-atom-thick films of the radioactive isotope iodine-125. They had in mind the direct observation of iodine-125’s transmutation to tellurium-125. Also, they hoped to assess the stability of their two-dimensional radioactive films.

To create the films, the scientists infused a single droplet of water with iodine-125 and deposited it on a thin layer of gold. The water evaporated, and the iodine-125 and the gold bonded. Then individual atoms of the iodine-125 underwent decay, a process the scientists observed by means of scanning tunneling microscopy.

The results of this work were presented June 15 in Nature Materials, in an article entitled, “Enhancement of low-energy electron emission in 2D radioactive films.” As the article's title indicates, the results were somewhat serendipitous.

“The metal interface geometry induces a 600% amplification of low-energy electron emission compared with atomic iodine-125,” the authors wrote. “This enhancement of biologically active low-energy electrons might offer a new direction for highly targeted nanoparticle therapies.”

It occurred to the scientists to try measuring the electrons emitted by the sample without prodding from X-rays in the photoelectron spectrometer. This idea was suggested by first author Alex Pronschinske, Ph.D., who was particularly interested in the emission of low-energy electrons, which have been shown to be very effective in radiation oncology because they break cancer cells' DNA into pieces. Because these electrons can travel only 1 to 2 nanometers—a human hair is about 60,000 nanometers wide—they do not affect healthy tissue and organs nearby.

“Low-energy electrons are the most important component of radiation damage in biological environments because they have subcellular ranges, interact destructively with chemical bonds, and are the most abundant product of ionizing particles in tissue,” explained the authors of the Nature Medicine article. “However, methods for generating them locally without external stimulation do not exist.”

The team calculated the number of low-energy electrons they expected would be emitted by the sample, based partly on data from simulations used by the medical community. They found that the gold-bonded iodine-125 emitted six times as many low-energy electrons as plain iodine-125.

The gold, said Dr. Sykes, “was acting like a reflector and an amplifier. Every surface scientist knows that if you shine any kind of radiation on a metal, you get this big flux of low-energy electrons coming out.”

The finding suggests a new avenue for radiation oncology: make nanoparticles of gold, bond iodine-125 to them, then affix the nanoparticles to antibodies targeting malignant tumors and put them in a liquid that cancer patients could take via a single injection. Theoretically, the nanoparticles would attach to the tumor and emit low-energy electrons, destroying the tumor's DNA. The gold-based nanoparticles would be flushed out of the body, Dr. Sykes noted, unlike free iodine-125, which can accumulate in the thyroid gland and cause cancer.

If proven, this approach could be a potential improvement over current radiation therapy protocols, in which doctors treat some cancers by putting radioisotopes, including iodine-125, into tiny titanium capsules and implanting them in tumors. Instead of emitting large amounts of low-energy electrons as the gold-bound iodine does, the titanium capsules inhibit radiation, Dr. Sykes pointed out, making such therapies less effective than they could be. He has applied for a patent on the new technique.

Researchers in Dr. Sykes' lab are now assessing precisely how the low-energy electrons travel through biological fluids.

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