European researchers develop luminescent nanothermometer

Device may be used to map temperatures at the molecular level and help in the treatment of cancer.

By Elton Alisson, in João Pessoa (PB)

Agência FAPESP – Conventional thermometers in use today to measure body temperature may soon be replaced by nanometric-scale (one billionth of a meter) devices capable of measuring temperature variations at a molecular level.

A group of researchers from the Center for Research in Ceramics and Composite Materials (CICECO) at the University of Aveiro in Portugal, together with colleagues from the Institute of Materials Sciences of Aragon at the University of Zaragoza in Spain, has developed a prototype of a luminescent nanothermometer with potential biomedical applications.

Described in an article published in the journal Advanced Materials, the device was presented at the XIII Meeting of the Brazilian Materials Research Society (SBPMat), held September 28-October 2, 2014 in João Pessoa, Paraíba.

“We have already filed a patent for the device in Europe and the United States, and some companies have indicated interest in the idea,” said Luis António Dias Carlos, CICECO researcher and one of the prototype designers, in comments to Agência FAPESP.

According to Dias Carlos, the device is based on the concept of using luminescent (light-emitting) materials – such as nanoparticles of trivalent lanthanide europium (Eu3+), terbium (Tb3+), ytterbium (Yb3+) and erbium (Er3+) ions – to measure temperature.

When excited by ultraviolet radiation – raised to higher energy – the Eu3+ and Tb3+ ions emit light in the red and green spectral regions at intensities that vary according to the temperature of the material over which they are dispersed.

In this way, once light is propagated in the space, it is possible to measure the temperature by analyzing the variations of intensity of the light emission from the distant ions without any need for physical contact between the thermometer and the material it seeks to analyze.

Because the lanthanide ions can be dissolved or dispersed in biological fluids (such as blood), the nanothermometer can be used in liquid media, says the Portuguese researcher.

Dias Carlos has already conducted research in the field together with colleagues from the Institute of Chemistry at São Paulo State University (Unesp), Araraquara campus, with support from FAPESP.

“The luminescent nanothermometer enables temperature to be measured in a non-invasive way with high spatial resolution,” the researcher said. “We were one of the first research groups in the world to propose the concept of nanothermometry based on the emission of light from lanthanide ions.”

The prototype of the luminescent nanothermometer developed by the researchers consists of a micrometer-scale plate composed of overlapping layers of nanoparticles of Eu3+ and Tb3+ ions dispersed in polymer films, covered by one layer of silicon dioxide (SiO2) and another layer of magnetic iron oxide ions at the nanometric scale.

When exposed to a heat source, the device’s magnetic layer heats up and increases the temperature around the nanoparticles of Eu3+ and Tb3+ ions.

“Depending on the temperature they are exposed to, the nanoparticles emit different intensities of visible light and change the color of the material, enabling a determination of temperature at the site where the device was placed,” Dias Carlos explained.

Possible applications

According to the University of Aveiro researcher, one possible use for the luminescent nanothermometer is in special 1 micron-resolution temperature mapping such as that used with tumor cells.

“We know that the temperature of cancer cells is higher than that of normal cells and that cancer cells cannot withstand temperatures above 42°C,” said Dias Carlos.

The idea is to inject luminescent nanoparticles into cancer patients so that the nanoparticles could be attracted by the tumor cells. By exposing these tumor cells to a radiation source at temperatures above 42°C, it would be possible to eliminate them selectively without affecting normal cells.

“The limitation of this type of therapy today is that there is no device capable of locally measuring the temperature of the cells,” Dias Carlos said. “If we can make a nanothermometer that can precisely measure the temperature of the cells, it would be possible to precisely control heat distribution around the cells we are particularly interested in.”

According to the researcher, temperature measurements are crucial for numerous scientific studies and technological developments and represent 75% to 80% of the world’s sensor market.

Traditional thermometers are not generally suited to measuring temperatures at scales below 10 microns (close to 10-times smaller than the average width of a human hair).

This intrinsic restriction has encouraged the development of new thermometers that do not require contact with the surface measured and that have spatial precision on the order of microns or even nanometers, according to Dias Carlos.

“The field for developing luminescent molecular thermometers is wide open, and there is still an enormous amount of new ground to be broken,” he said.