Terahertz Optoacoustic Method Images Water-Rich Samples

A research team led by Zhen Tian and Jiao Li at Tianjin University developed a method for analyzing water-rich samples using time-domain terahertz (THz) optoacoustics. The method overcomes a bottleneck that prevented the use of THz radiation in the investigation of biophysical and biochemical processes deep within tissues.

THz radiation can be used successfully to analyze the structural dynamics of water and biomolecules, though applying the technique to aqueous solutions and tissues has been challenging; THz radiation is strongly absorbed by water. That property helps in certain instances, such as analyzing the structure of water and its interactions with biological solutes, though it limits the thickness of samples that can be analyzed and drowns out weaker signals from biomolecules of interest.

Manipulable water sensing and muting with time-domain THz optoacoustics. Courtesy of Li et al.

The system that the researchers developed is able to overcome this by dampening the optoacoustic signal of water by altering temperature to enable the sensitive detection of other molecules of interest. The nondestructive method enables the precise detection of optoacoustic signals in water-rich samples over a 104-fold thickness, ranging from microns to centimeters (102-fold larger than traditional THz detection methods such as THz time-domain spectroscopy). The team successfully obtained the time-domain THz optoacoustic signals from water in different samples of agar-in-water phantoms, bio-tissues, and aqueous solutions with different solutes.

By adjusting the temperature to alter the THz optoacoustic signal of water, the researchers improved the sensitivity with which it can be analyzed. Conversely, the signal of water can be reduced or even silenced.

The method achieves an order of magnitude greater sensitivity than commercially available THz spectroscopy systems.

The authors noted that the technique can be extended to the study of other biological molecules and tissues, such as sugars, proteins, DNA, and RNA. Additional temperature and concentration parameters related to both THz absorption and ultrasonic propagation can be provided by time-domain THz optoacoustics, potentially contributing to studies of biological and chemical properties such as the hydration number of ion solutions.

“We aim to inspire long-term research in THz spectroscopy and imaging in order to harness the biophysical, structural, and functional insights that cannot be obtained using radiation of other frequencies,” Tian said.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.3.2.026003).