Graphene Plasmonic Sensor Uses Surface Acoustic Wave for Fingerprinting Proteins and Peptides

Scientists have recently learned to utilize the potential of surface acoustic waves (SAW) in creating devices that can work as filters, actuators, and signal processing units. In a recent breakthrough, a team of experts discovered a way to use sound waves to identify ultrathin biolayers.

Sensing Ultrathin Biolayer

The mid-infrared range plays an important role in spectroscopy because it contains the vibrational fingerprint of molecules. This fingerprint refers to the absorption resonances associated with the molecular bonds that uniquely identify a substance. The position and relative intensity of these resonances are also useful in interpreting the conformation of the molecules.

In the case of biomolecules such as proteins and peptides, the mid-infrared range provides access to their secondary structures and their local interactions, adsorption, or binding processes. However, scientists find it challenging to sense ultrathin biolayers due to the low absorption cross-section that results from the strong size mismatch between free radiation and biomolecules.

Harnessing the Potential of Sound Waves

Researchers from the Institute for Optoelectronic Systems and Microtechnology at Universidad Politécnica de Madrid (UPM) have made it possible to design a biosensor that can identify biomolecules in quantities as low as a single monolayer. The result of their study is discussed in the paper "Surface-acoustic-wave-driven graphene plasmonic sensor for fingerprinting ultrathin biolayers down to the monolayer limit."

This research generates surface acoustic waves with an integrated transducer to act on a stack of 2D materials coated with the biomolecules that must be detected. The scientists reported that the SAW would ripple the surface of a graphene-based stack by confining mid-infrared light to very small volumes. This enhances light-matter interactions at the nanoscale.

Organic molecules absorb specific wavelengths of light in the mid-infrared spectrum, which are characteristics of their chemical composition and structure. This means that this set of absorption resonances, known as their vibrational fingerprint, enables the identification of the organic compound.

Study first author Raúl Izquierdo reported that by strengthening the interaction between light and biomolecules deposited above the sensor, they can identify analytes that require smaller quantities, even reaching levels as low as a single monolayer. Lead scientist Jorge Pedrós also noted that one advantage of this mechanism is the active control of surface acoustic waves through a high-frequency voltage. This allowed them to switch between an ON configuration, where interaction is increased, and an OFF configuration, without any improvement to the signal. Such a measurement scheme increases the resolution of the sensor.

Aside from the design of the sensor and the calculations of its performance, the researchers also provide a mathematical method to extract hidden quantitative information, further increasing the sensor's sensitivity. This way, the analyte and the surface plasmon-phonon polariton molecules are modeled as oscillators that interact with each other while being driven by an external force.

The researchers conclude that the result of their study can contribute to the development of new lab-on-chip devices. It combines the chemical fingerprinting capability of SAW-driven biosensors with other acoustic functions like SAW-based mass sensing or droplet streaming and mixing in microfluidic circuits.