Researchers at the Institute of Optoelectronic Systems and Microtechnology at the University of Madrid (UPM) have designed a biosensor that can identify proteins and peptides in quantities as small as a single monolayer. To achieve this, surface acoustic waves (SAW), a type of electrically controlled nanoseismicity on a chip, are generated via an integrated transducer and act on a 2D material stack coated with the biomolecules to be sensed.
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As they report in the journal Biosensors and Bioelectronics (Surface acoustic wave-driven graphene plasmonic sensors for fingerprinting ultrathin biolayers down to the monolayer limit), SAW will cause the surface of the graphene-based stack to ripple. Confining mid-infrared light to very small volumes enhances light-matter interactions at the nanoscale. In particular, quasiparticles (surface plasmon-phonon polaritons), which are part light (photons) and part matter (electrons and lattice vibrations), form in ripple stacks that interact strongly with the molecules on top.
Organic molecules absorb light at specific wavelengths in the mid-infrared range that are characteristic of their chemical composition and structure. Organic compounds can therefore be identified through this set of absorption resonances, called vibrational fingerprints. ““By enhancing the interaction between light and the biomolecules deposited on the sensor, we will be able to identify analytes that require lower amounts and reach levels as low as a single monolayer.” says Raúl Izquierdo, first author of the study.
According to Jorge Pedrós, the study's lead scientist: “One advantage of this mechanism is that the SAW is actively controlled via high-frequency voltages, allowing it to easily switch between ON and OFF configurations, which increases interaction without signal enhancement. “This measurement method increases sensor resolution.”
In addition to sensor design and performance calculations, the authors also provide mathematical methods to extract quantitative information hidden in plain sight to further increase the sensor's sensitivity. For this purpose, the analyte molecules and the surface plasmon-phonon polaritons are modeled as oscillators interacting with each other, both driven by an external force (light incident on the sensor). Despite its simplicity, this model has been shown to reproduce computational results well.
In conclusion, the authors are confident that this study will contribute to the development of new lab-on-chip devices by combining the chemical fingerprinting capabilities of this new SAW-driven biosensor with other acoustic capabilities such as SAW-based mass sensing or droplet streaming. Mixing in a microfluidic circuit.
Source: https://www.upm.es/