University of Tokyo develops wearable Raman sensor

27 Jun 2022

Gold nanomesh in skin patch allows detection of biomarkers, drugs and microplastics.

Wearable sensors detecting heart rate and motion are now commonplace, but versions of such sensors able to detect chemical molecules and biomarkers would be a significant step forward for the monitoring of complex biochemical conditions.

To date such advanced sensors have usually been sensitive to just one chemical biomarker at a time, with architectures capable of chemical multiplexing becoming too large and expensive to allow true wearable technology.

A project at the University of Tokyo has now developed a wearable patch capable of being fabricated in any shape and worn on any surface, designed to be interrogated by surface-enhanced Raman spectroscopy (SERS).

Raman techniques are likely to be significant for such wearable biomonitoring, thanks to their ability for sensitive and multiplexed chemical analysis without the need for molecular labeling. The hurdle has been the inherently faint Raman signals from biological systems, requiring the molecules of interest to bond onto a suitable substrate that amplifies the Raman response.

Gold is one material known to be effective as a SERS substrate, and multiple research projects have investigated different ways to use the metal in practical SERS platforms.

A Shanghai team recently employed nanoparticles combining gold with black phosphorus in its SERS platform assessing the suitability of donor kidneys for transplant. Elsewhere Wasatch Photonics and Nikalyte have partnered for the development of gold nanoparticles generated and deposited in vacuum, ensuring that they are ultra-pure and uniformly distributed for optimal SERS performance.

As reported in Advanced Optical Materials, the Tokyo project took its inspiration from recent advances in the manufacture of gold-coated polyvinyl alcohol (PVA) nanofibers, used in electronic sensors that can be worn on human skin for long periods of time.

"These PVA devices are spun from ultrafine threads coated with gold, so can be attached to the skin without issue as gold does not react with or irritate the skin in any way," commented team member Limei Liu. "As sensors, they were limited to detecting motion, and we were looking for something that could sense chemical signatures, biomarkers and drugs."

New generation of low-cost biosensors

The wearable sensor was created by making a nanomesh of PVA fibers; coating the fibers with a 150-nanometer layer of gold; attaching the coated fiber nanomesh to the target surface, including human skin; and then dissolving away the PVA with water to leave a gold nanomesh intact and attached to the surface.

The sharp edges of the nanowire act as "hot spots" for the localized SERS effect, and the project optimized the number of hot spots in a unit volume by decreasing the diameter of the nanowire while maintaining sufficient mechanical strength for wearability.

In proof of concept trials, the patch was worn by human volunteers and exposed to different chemical species, before being interrogated with a commercial 785 nanometer Raman spectrometer. The system was able to detect biomolecules such as urea and ascorbic acid, and to identify microplastic contamination in water. Common drugs of abuse could also be detected, indicating potential uses for the sensors in law enforcement.

At present the system requires an external light source and spectrometer, but the project will investigate the integration of a semiconductor nanolaser and a nanospectrometer into the wearable SERS sensor by direct bonding.

"There is also potential for the sensor to work with other methods of chemical analysis besides Raman spectroscopy, such as electrochemical analysis, but all these ideas require more investigation," said Keisuke Goda from the University of Tokyo. "I hope this research can lead to a new generation of low-cost biosensors that can revolutionize health monitoring and reduce the financial burden of health care."