Regardless of scientific field, temperature is always of fundamental importance. Thermocouples and thermistors constitute accurate, affordable and easy-to-install experimental probes; however, such traditional thermometers are unsuited to length scales of under 10 microns. Several methods now exist for measuring temperatures over such small distances, including spectroscopic techniques and approaches based on filled nanotubes.
An alternative approach is to use materials whose luminescence is temperature dependent. This means that the temperature can be read optically, in a relatively unobtrusive manner. Researchers based at the University of Aveiro and the University of Zaragoza have recently demonstrated the most accurate luminescent lanthanide thermometer to date, which also possesses a number of other unique features [Brites et al., Adv. Mater., 22, 4499-4504, (2010)].
In contrast to previous single lanthanide thermometers, this novel approach consists of two lanthanide compounds, which are imbedded onto nanoclusters measuring up to 400 nm. The emission of light is the result of electrons moving from higher to lower energy states. By using two different lanthanides, the researchers have managed to create a material with two separate light emitting channels. By comparing the relative intensities of light from the two channels the thermometer can be calibrated without external references.
The light emitted from the nanoclusters is detected using an optical microscope, which also determines the resolution of the thermometer. In this case a resolution of 35 microns was obtained. However, the researchers hope that this can be brought closer to the ultimate resolution, which is determined by the cluster size, by using a more sophisticated measurement technique.
The thermometer has been tested in the critical 15-300 K temperature range, and found to have a maximum sensitivity of 4.9%/K. By imposing a minimum sensitivity of 1%/K the researchers found that the thermometer is effective over a large region, of up to 80 K. The thermometer may be processed into a film, and is also highly stable, meaning that two-dimensional temperature mapping can be achieved over an extended period of time.
As the technique is both accurate and easily readable, the teams involved are hopeful that their molecular thermometer may have medical applications. Professor Luis Carlos reveals that the teams are hoping to “develop a prototype for magnetic hyperthermia”, such that the temperature of tissues can be mapped while being treated with magnetic nanopartcles in an AC field. In addition to this they will also be working to “design different host hybrid matrices and lanthanide ligands” and “internalize the nanoparticles into a cellular medium”.
Stewart Bland