Researchers have unveiled a highly sensitive detector capable of identifying molecules by analyzing their infrared vibrational “fingerprints.”
This groundbreaking device works by transforming incoming infrared light into ultra-confined “nanolight” through phonon polaritons within its active area. This dual-purpose mechanism significantly increases the detector’s sensitivity while amplifying the vibrational signals of nanometer-thin molecular layers placed on its surface. These amplified molecular fingerprints can then be detected and analyzed with greater precision. The detector’s compact design and ability to operate at room temperature open the door to developing ultra-compact platforms for molecular and gas sensing in various applications.
Molecular Identification Through Vibration
Molecules possess unique “fingerprints” that can be used to distinguish them from one another. When exposed to specific types of light, each molecule vibrates at a characteristic frequency—known as its resonance frequency—typically within the infrared spectrum, and at a specific intensity.
Just as human fingerprints are used to identify individuals, these molecular fingerprints can be exploited to differentiate between various types of molecules or gases. This capability is not only crucial for scientific analysis but also has practical applications, such as identifying harmful or toxic substances.
Advancements in Infrared Spectroscopy
One conventional approach is infrared fingerprint spectroscopy, which uses infrared reflection or transmission spectra to identify different molecules. However, the small size of organic molecules compared to the infrared wavelength results in a weak scattering signal, making it challenging to detect small quantities of material.
In recent years, this limitation has been addressed using Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy.
SEIRA spectroscopy leverages infrared near-field enhancement provided by rough metal surfaces or metallic nanostructure to amplify the molecular vibrational signals. The main advantage of SEIRA spectroscopy is its ability to measure and study minute material quantities.
Enhancing Detection With Phonon Polaritons
Recently, phonon polaritons — coupled excitations of electromagnetic waves with atomic lattice vibrations — particularly hyperbolic phonon polaritons in thin layers of hexagonal boron nitride (h-BN), have emerged as promising candidates for boosting the sensitivity of SEIRA spectroscopy.
“Previously, we demonstrated that phonon polaritons can be applied for SEIRA spectroscopy of nanometer-thin molecular layers and gas sensing, thanks to their long lifetimes and ultra-high field confinement,” says Prof. Rainer Hillenbrand from CIC nanoGUNE.
Towards On-Chip Molecular Detection
However, SEIRA spectroscopy remains a far-field technique that requires bulky equipment, such as light sources, SEIRA substrates, and typically nitrogen-cooled infrared detectors. This reliance on large instruments limits its potential for miniaturization and on-chip applications.
In parallel, “we have been investigating graphene-based infrared detectors that operate at room temperature, and we have shown that phonon polaritons can be electrically detected and can enhance detector sensitivity,” adds Prof. Frank Koppens from ICFO.
Miniaturization and Future Applications
By combining these two progresses, a team of researchers has now successfully demonstrated the first on-chip phononic SEIRA detection of molecular vibrations. This result was made possible through the joint experimental efforts of Nanogune and ICFO researchers, along with theoretical support from the groups of Dr. Alexey Nikitin at the Donostia International Physics Center and Prof. Luis Martín-Moreno at the Instituto de Nanociencia y Materiales de Aragón (CSIC- Universidad de Zaragoza).
The researchers employed ultra-confined HPhPs to detect molecular fingerprints in nanometer-thin molecular layers directly in the photocurrent of a graphene-based detector, eliminating the need for traditional bulky IR detectors.
Integration and Future Possibilities
“One of the most exciting aspects of this approach is that this graphene-based detector opens the way towards miniaturization,” comments ICFO researcher Dr. Sebastián Castilla. He continues, “By integrating this detector with microfluidic channels, we could create a true ‘lab-on-a-chip’, capable of identifying specific molecules in small liquid samples—paving the way for medical diagnostics and environmental monitoring.”
Outlook on Portable Detection Technologies
In a longer-term picture, nanoGUNE researcher and first author of the study, Dr. Andrei Bylinkin, believes that “on-chip infrared detectors operating at room temperature could enable rapid molecular identification, potentially integrated into smartphones or wearable electronics.” He further believes that “this would offer a platform for compact sensitive, room-temperature infrared spectroscopy.”
Reference: “On-chip phonon-enhanced IR near-field detection of molecular vibrations” by Andrei Bylinkin, Sebastián Castilla, Tetiana M. Slipchenko, Kateryna Domina, Francesco Calavalle, Varun-Varma Pusapati, Marta Autore, Fèlix Casanova, Luis E. Hueso, Luis Martín-Moreno, Alexey Y. Nikitin, Frank H. L. Koppens and Rainer Hillenbrand, 16 October 2024, Nature Communications.
DOI: 10.1038/s41467-024-53182-9
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