BioPhotonics spoke with Zhen Tian, a professor in the Department of Opto-Electronics Science Technology at Tianjin University. Tian was the corresponding author on a paper that described the use of terahertz optics to monitor blood sodium, as described in Optica. The technique shows the potential to measure blood sodium levels in real time without the need for sampling and time-consuming laboratory analysis.

Your work concerns the development of a real-time system to test blood sodium, an imbalance of which is associated with various health conditions. What are the limitations of existing techniques used for that purpose?

Noninvasive, real-time measurement of sodium (Na+) concentrations in blood without exogenous labels is critical for rapid diagnosis and clinical intervention of dysnatremia. Currently, measuring ion concentrations in blood requires invasive sampling, either through a finger prick, phlebotomy, or implantation of a monitoring device, followed by analysis in the clinical laboratory. These techniques impose a potentially long delay between blood sampling and analysis.

An effective solution would be a technique that can determine blood ion concentrations in real time and in a noninvasive manner. Terahertz spectroscopy holds potential value due to its rich fingerprint absorption spectra of ions; however, its in vivo application has historically been limited by strong water absorption. By employing the multispectral terahertz optoacoustics, the authors realized rapid and long-term detection of blood Na+ in live mice without labels or invasive sampling and showed the signal proportional to blood flow in human hands.

So, terahertz spectroscopy, which detects structural details, is combined with optoacoustics, which produces vibrations in specific molecules? And the system you assembled is made of a light source, broadband filters, temperature control, ultrasound transducer, and data acquisition card — is that right?

Sodium ions have characteristic absorption in the terahertz band. In addition, when ions, including sodium ions, dissolve in water, they will participate in the hydrogen bond network of the aqueous solution, and the terahertz frequency is highly sensitive to the hydrogen bond network.

We use a special kind of invisible light (terahertz waves) that is perfectly tuned to interact with sodium in the tissues. When this light shines on your skin, it gently vibrates the sodium ions connected to water molecules in the blood, creating tiny sound waves (ultrasound) that we can detect. The louder that these sound waves are, the more sodium is present in the blood.

The system consists of a terahertz light source module, temperature control module, sample module, and detection module. The terahertz wave serves as an excitation source to irradiate the sample, which absorbs terahertz radiation and triggers an adiabatic expansion, thereby emitting ultrasonic waves that are detected by an ultrasonic transducer.

What challenges remain to adapt this technique for widespread clinical use?

The current setup used mouse ears, which are relatively thin. When applied to humans, we need a suitable detection site that achieves both water background noise suppression and lossless signal detection. We will explore cooling-tolerant regions such as the oral mucosa, which can withstand rapid temperature reduction.

Research will also be conducted on how to create models of rapid changes in sodium ions in vivo and how to verify the processes of high sodium and low sodium in the human body. We will develop other methods to eliminate water background interference, such as muting strategies based on Grüneisen relaxation, which doesn’t require cooling and can achieve water background signal suppression at room temperature.