Tunable metamaterial opens new window for molecule sensing at trace levels

Cavendish researchers have shown that tiny clusters of metal nanoparticles can be tuned to interact with light in unusual ways, offering a new route to detecting very small concentrations of molecules in real time. The findings could improve non-invasive sensing technologies for healthcare and environmental monitoring such as molecular signatures of biomolecules, fingerprinting of different nanoplastics and real-time biosensing of metabolites in biofluids.

“The next step is to use these structures in a wide range of applications. We are now exploring how these mid-IR metamaterials could be applied in electrochemical systems, by studying interface effects on the nanoscale and detecting very small amounts of molecules, including biomolecules.”  Jeremy J. Baumberg

Mid-infrared (mid-IR) light is a powerful tool for identifying the unique “fingerprints” of molecules. However, poor sensitivity and strong background interference restrict its ability to detect molecules at trace levels in real-time sensing. Surface-enhanced infrared absorption (SEIRA) is a technique that makes it easier to detect molecules by amplifying the way light interacts with them. This is done using specially designed tiny metal structures that concentrate mid-infrared light in very small spaces, creating “hotspots” where the light is much stronger than usual. However, its static design limits the ability to tune it for different molecules or applications, and its tight-packing impedes larger analytes from entering the hotspots.

In order to overcome this limitation, the researchers created tunable metamaterials.

In this research, published in Optica, the Cambridge researchers, in collaboration with the University of Birmingham and Astra-Zeneca, studied how changing the shape and arrangement of tiny structures affects how they bend light, known as the refractive index. “To study this, we created self-assembled arrays of nanoparticles packed less than a billionth of a metre apart. These are so small that millions could fit on the tip of a pin,” said PhD student Nicolas Spiesshofer from the Cavendish Laboratory.

“These arrays of nanoparticles resonate strongly in the mid-infrared range, where important molecular vibrations from things like proteins or nanoplastics can be detected. Because of the way they interact with light, these arrays act as metamaterials, which can bend and control light in unusual ways that normal materials can’t.”

To build them, the scientists combined gold and silver nanoparticles, so they formed dense aggregates, and then dissolved the silver. This allowed them to adjust how tightly packed the structure was, which in turn controlled both its resonance and its porosity. A more porous metamaterial makes it easier for molecules, such as therapeutic proteins, to reach the sensing sites. Working in collaboration with Birmingham’s colleagues, the researchers also ran simulations to understand how porosity and other design features influence the material’s resonance.

“We found that even though this material is mostly made of gold nanoparticles, it behaves more like a non-conducting material and can trap mid-infrared light without losing much energy,” explained Dr Rakesh Arul from the Cavendish Laboratory.

“By changing the balance of metal and air in the structure, we can control how the material resonates in the mid-infrared, which changes how it interacts with bound analytes.”

By altering porosity and geometry, the researchers were able to drastically tune the refractive index across the mid-infrared. They found that further tuning can lead to refractive indices above 10, far higher than those found in most natural materials, potentially unlocking new electromagnetic phenomena.

What surprised the researchers the most was just how sensitive these nanoparticles metamaterials are. Even small changes to the particle shape or spacing lead to dramatically different optical responses. Yet, despite the disorder in the nanoparticles, we can model their behaviour with simple tools.

This sensitivity makes these nanoparticles ideal for sensing applications. By optimising their structure, the same material could be used for both SEIRA (surface-enhanced infrared absorption) and SERS (surface-enhanced Raman spectroscopy). Potential applications include identifying environmental pollutants such as the critically challenging nanoplastics, real-time biosensing of metabolites in biofluids and more. Because the metamaterials are made using straightforward self-assembly methods, they also offer a cost-effective and scalable alternative to existing technologies.

“The next step is to use these structures in a wide range of applications. We are now exploring how these mid-IR metamaterials could be applied in electrochemical systems, by studying interface effects on the nanoscale and detecting very small amounts of molecules, including biomolecules,” concludes Prof Jeremy J. Baumberg from the Cavendish Laboratory.

This research contributes to EPSRC-funded programmes such as Ubiquitous Optical Healthcare Technologies (UbOHT) and Mid-infrared sensing (MIRVALS)

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Reference:

Nicolas Spiesshofer et al., ‘Tailoring ultrahigh index plasmonic combinatorial metamaterials for SEIRA and SERS by tuning the fill fraction‘, Optica 12, 1357-1366 (2025), DOI:10.1364/OPTICA.567324

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Contributors to this research:

• Experimentalists: Nicolas Spiesshofer, Elle Wyatt, Caleb Todd, James W. Beattie, Rowena Davies
• Theorist: Zoltan Sztranyovszky

• Group Leads: Dr Rakesh Arul, Title A Research Fellow | St. John’s College Cambridge; Viv Lindo, Senior Director, Analytical Sciences | Astra Zeneca; Prof. Thomas Krauss, Professor | Dept of Physics, University of York; Prof. Angela Demetriadou, Professor | Dept of Physics & Astronomy, University of Birmingham; Prof. Jeremy Baumberg, Harold Aspden Professor of Fundamental Physics | Cavendish Laboratories, University of Cambridge.