Terahertz Radiation to Monitor Heat

An emerging tech­nology involving tiny particles that absorb light and turn it into localized heat sources shows great promise in several fields, including medicine. For example, photo­thermal therapy, a new type of cancer treatment, involves aiming infrared laser light onto nano­particles near the treatment site. Localized heating in these systems must be care­fully controlled since living tissue is delicate. Serious burns and tissue damage can result if unwanted heating occurs in the wrong place. The ability to monitor tempera­ture increases is crucial in developing this technology. Several approaches have been tried, but all of them have drawbacks of various kinds, including the need to insert probes or inject addi­tional materials.

Illustration of sensing the size-dependent light-to-heat conversion efficiency of nanoparticles by terahertz radiation. (Source: R. Morandotti)

Now, scientists from the Canadian Institut National de la Recherche Scientifique INRS, Centre Énergie, Matériaux et Télé­communications (EMT) in Varennes report the development of a new method to measure tempera­tures in these systems using tera­hertz radiation. The study involved suspensions of gold nanorods of various sizes in water in small cuvettes, which were illu­minated by a laser focused on a small spot within the cuvette. The tiny gold rods absorbed the laser light and converted it to heat that spread through the water by convection. “We are able to map out the temperature distri­bution by scanning the cuvette with terahertz radia­tion, producing a thermal image,” Junliang Dong said.

The study also looked at the way the tempera­ture varied over time. “Using a mathe­matical model, we are able to calculate the efficiency by which the gold nanorod sus­pensions converted infrared light to heat,” said Holger Breitenborn. The smallest gold particles, which had a diameter of 10 nano­meters, converted laser light to heat with the highest effi­ciency, approxi­mately 90%. This value is similar to previous reports for these gold particles, indicating the measurements using tera­hertz radiation were accurate.

Although the smaller gold rods had the highest light-to-heat conversion effi­ciency, the largest rods — those with a diameter of 50 nanometers — displayed the largest molar heating rate. This quantity has been recently intro­duced to help evaluate the use of nano­particles in biomedical settings.

“By combining measure­ments of tempera­ture transients in time and thermal images in space at terahertz frequencies, we have developed a noncontact and noninvasive technique for charac­terizing these nano­particles,” co-author Roberto Moran­dotti said. This work offers an appealing alter­native to invasive methods and holds promise for biomedical appli­cations.

Reference: H. Breitenborn et al.: Quantifying the photothermal conversion efficiency of plasmonic nanoparticles by means of terahertz radiation, APL Phot. 4, 126106 (2019); DOI: 10.1063/1.5128524

Link: Institut National de la Recherche Scientifique (INRS), Centre Énergie, Matériaux et Télécommunications (EMT), Varennes

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