Accurately measuring blood sodium levels is crucial for diagnosing and managing various health conditions, including dehydration, kidney disease, and certain neurological and endocrine disorders. Terahertz radiation, located between microwaves and the mid-infrared region of the electromagnetic spectrum, is particularly suitable for biological use due to its low energy, tissue safety, minimal scattering compared to near-infrared and visible light, and its sensitivity to biological structure and function. Despite these advantages, terahertz spectroscopy encounters two major challenges in biomedical settings: the difficulty of detecting molecules beyond water in complex samples and limited penetration through thick tissue layers needed for internal detection. To address these issues, researchers have now created a new system that merges optoacoustic detection with terahertz spectroscopy for long-term, non-invasive blood sodium monitoring.

Researchers at Tianjin University (Tianjin, China) have developed a multispectral terahertz optoacoustic system capable of monitoring sodium concentrations in live mice over extended periods without requiring any labeling. Their findings, published in Optica, also include encouraging results from preliminary testing with human volunteers. This work is part of a broader initiative to apply terahertz technology in biomedical settings through optoacoustic methods. A significant objective of this initiative is to address signal interference caused by water’s strong absorption of terahertz waves. To tackle this, the team designed a modular system that directs terahertz radiation at the sample. As the sample absorbs this energy, sodium ions linked to water molecules vibrate and generate ultrasound waves, which are then captured by an ultrasonic transducer. This process, known as optoacoustic detection, translates the absorbed terahertz energy into measurable sound signals.

In validating their system, the researchers demonstrated that it could track rising sodium levels in the blood vessels beneath the skin of live mice in milliseconds and maintain this tracking for over half an hour. Measurements were conducted via the ear, with the skin cooled to 8 °C to minimize background optoacoustic noise from water. The team also showed that the system could rapidly differentiate between high and low sodium concentrations in human blood samples. Furthermore, they successfully performed non-invasive measurements of sodium ion levels in the hand blood vessels of healthy individuals. The resulting optoacoustic signal correlated with the amount of blood flow under the skin surface, even without skin cooling.

Although further development is required, these findings indicate that the system has potential for real-time, non-invasive monitoring. The researchers note that adapting this technology for clinical use will involve identifying optimal detection areas on the body — such as the inside of the mouth — that can tolerate rapid cooling and produce strong signals with minimal interference from water. Additionally, they are investigating alternative signal processing techniques that could eliminate the need for cooling, potentially making the method more viable for routine clinical diagnostics.

“Terahertz optoacoustic technology represents a groundbreaking advancement for biomedical applications by effectively overcoming the water absorption barrier that has historically limited these applications,” said research team leader Zhen Tian from Tianjin University. “The broader significance of this work extends far beyond blood sodium detection. This technology has the capability to identify various biomolecules—including sugars, proteins, and enzymes—by recognizing their unique terahertz absorption signatures.”

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