Nanothermometers for cancer

No techniques previously existed for measuring temperature in spaces any smaller than a few millimeters, explained researcher Nicholas Kotov, a chemical engineer at the University of Michigan at Ann Arbor. His team's nanothermometers, which are roughly 30 nanometers wide, can gauge temperatures in volumes only 200 nanometers across, he said.

Each nanothermometer is made of a gold particle core 20 nanometers in diameter surrounded by a flexible springy polymer layer topped off with an envelope of several dozen semiconductor particles each about four nanometers wide. The polymer layer contracts with the cold and relaxes in the heat, bringing the nanoparticles closer together or farther apart.

When scanned with lasers, the semiconductor nanoparticles in the nanothermometers get excited. These excitations cause the cloud of electrons surrounding the gold nanoparticles to vibrate in sync. Such vibrations can boost the amount of light the semiconductor nanoparticles emit. The closer the gold and semiconductor nanoparticles are, the more this light boost increases.

By measuring the amount of light the nanothermometers emit in response to a laser scan, scientists can detect temperature changes down to 1 or 2 degrees. The lasers themselves do not significantly heat the nanothermometers, Kotov added. He and his colleagues, theorist Alexander Govorov at Ohio University in Athens and experimentalist Jim Lee at the University of Michigan, presented their findings in the international scientific journal Angewandte Chemie.

While other nanothermometers made with biomolecules and fluorescent compounds exist, those effectively get destroyed after "a few tens of seconds," while Kotov's thermometers ought to prove durable "for very long periods of time," said biophysicist Jan Liphardt at the University of California at Berkeley.

Such nanothermometers could help enhance the accuracy of anti-cancer treatments that rely on heat or cold to kill cancer cells, Kotov said. "The efficacy of these treatments depends on temperature differences of a few degrees, so cells will not die if the difference is as little as 1 degree Fahrenheit," he explained. "Nanothermometers could help doctors adjust their treatments accordingly to carve away the cancer completely or to not attack normal cells."

Nanothermometers could also help scientists regulate temperatures in microfluidic arrays, which include miniaturized genetic and protein analysis devices. "If you visited a lab, you would see probably 20 to 50 devices a room designed to carefully control or measure the temperature," Kotov said. "If you want to shrink everything down dramatically to a lab-on-a-chip, as is commonly getting done nowadays, you would need to shrink the thermometers down as well.

"If the temperatures across these arrays vary by several degrees, this will result in quite substantial changes to the reaction rates," Kotov added.

Moreover, as electronics researchers invent new ways to draw heat from microchips to keep them from overheating, they could use nanothermometers to ensure they come up with the best designs. "The energy density of nanocircuits is approaching the energy density of a nuclear reactor. It's amazing how much heat there can be in such small volumes," Kotov said.

In the future, Liphardt noted the springy heat-sensitive polymer Kotov and his colleagues used in their nanothermometers could be replaced by other polymers that respond in other ways, such as to pH levels or certain genetic sequences, for any number of different kinds of sensors.

Copyright 2005 by United Press International