Nano-optomechanical resonator detects low-frequency bacteria vibrations

Researchers in Spain and France have measured the vibrations of individual bacteria by coupling them to a nanomechanical device with a similar resonance frequency. This new optomechanical spectrometry technique could offer an alternative to current methods of detecting and classifying bacteria and other biological particles.

Proteins, viruses and bacteria all vibrate at frequencies in the terahertz and gigahertz range. Their vibrations carry valuable information about their structure and mechanical properties, but efforts to study these using optical inelastic scattering techniques are extremely challenging because the bioparticles change shape and deform as they vibrate.

The new method, developed by a team led by Javier Tamayo and Eduardo Gil-Santos from the Instituto de Micro y Nanotecnología (IMN-CSIC) in Madrid, involves coupling the mechanical vibrations of bacteria to an ultrahigh frequency (UHF) nano-optomechanical resonator made from a GaAs microdisk. Such coupling is only possible when the resonance frequencies of the disk and the bacteria are similar, Tamayo explains. In contrast, previous experiments relied on the biological particles vibrating much faster than the micro- and nanomechanical resonators or microcantilevers used to measure the particles’ mass and stiffness.

Going beyond mass and stiffness measurements

The Madrid team’s disks support two different types of vibration: radial breathing modes (RBMs), which correspond to a radial expansion and contraction of the disk (and therefore depend on its diameter), and so-called optical whispering gallery modes, which correspond to resonances within the disk’s structure. In both cases, the disk vibrates at frequencies in the GHz range, which can actually surpass the low-frequency vibration modes of bioparticles.

In their experiments, Tamayo and colleagues deposited a single S. epidermis bacterium onto their optomechanical microdisks using an electrospray ionization technique. The bacteria are round in shape and have a radius of roughly 400 nm. The researchers measured the fundamental RBM frequencies of the disks before and after depositing the bacteria, then used a general theoretical framework to describe the coupling between the bacteria and the disks. This framework allowed them to calculate the resonant frequencies of a bacterium’s low-frequency vibration modes based on their “before and after” measurements of the disks’ RBM frequencies.

To determine the mechanical coupling between a bacterium and their nanomechanical resonator, the researchers had to measure very tiny fluctuations – a few picometres (10^-12 m) – at ultrahigh frequencies. Such measurements were only possible thanks to the strong optomechanical coupling of the disks to the bacterium. Indeed, the coupling is so strong that these devices can measure displacements of just attometres (10^-18 m) – a value similar to the precision of the kilometre-sized interferometers used to measure gravitational waves, Tamayo says.

The work was done within the framework of the EU FE VIRUSCAN project, which aims to use optomechanical resonators to detect viral particles based on their physical parameters. The idea is to build up a “library” of the mechanical and vibrational properties of different viruses and bacteria.

Members of the Madrid team, who report their work in Nature Nanotechnology, are now planning to use their technique to measure the vibration modes of viruses, which are much smaller than bacteria. “This future work will be more challenging,” Tamayo tells Physics World.