From the Lab: Weighing viruses to to find out how infectious they are

A team of researchers at the French Alternative Energies and Atomic Energy commission (CEA) in Grenoble has built a device that characterises viruses by weighing them.

Sebastien Hentz 'left) and Christophe Masselon at the CEA lab in Grenoble. © Dhananjay Khadilkar

The device, called an optomehanical mass spectrometer and developed by Christophe Masselon and Sebastien Hentz, can already detect certain types of viruses.

According to Hentz, who is a deputy director of Science at CEA, the device works similar to a guitar string.

“If you place a finger along the string, its tune or frequency will change. In our device, there are nano strings made of silicon that are kept vibrated permanently on a particular frequency. When a particle lands on the string and adds mass, this frequency changes and we detect the change,” he said.

The frequency change is detected with the help of light. “We use an optical fiber to bring the light very close to the resonators. Due to their proximity, the light interacts with the strings, resulting in the modulation of the power of light,” Henz added. A detector measures the light modulation to calculate the frequency. Once the frequency is known, the mass is calculated by using a mathematical formula.

The reason why knowing mass of nanoparticles is important is that it reveals their structural details.

“Mass is an important parameter in the nano world,” says Masselon, who is a senior research scientist at CEA.

“For instance, if a virus contains DNA, its mass will be more. The genome size can induce a shift in mass by the number of DNA-based pairs it has. Thus, we can determine if a particular capsid is empty and therefore harmless or contains DNA and is therefore potentially infectious,” he adds.

The reason why the CEA device is able to detect the mass in a matter of hours is because of a key modification that improves the efficiency with which the particles reach the nano resonator.

“In a conventional system, the nanoparticles get ionised before getting guided by electrostatic, electrodynamic or magnetic fields to the nano resonator.

"However, as the particles get bigger, they have more inertia. This makes it more difficult to guide them with these fields.”

The CEA team replaced the conventional guiding mechanism by a new system.

“In our device, we are using the inertia of the particles by confining them to gas flows. This keeps the particles on target to reach the nano resonator,” he adds.

The device has already been used to characterise a bacteriophage virus capsid by weighing it when it was empty and when it contained genome.

While the empty capsid weighed 27 megadalton, the one with genome measured over 100 megadalton. A dalton is a unit used to indicate mass on the nano scale. One megadalton roughly corresponds to one billion billionth of a gram.

One of the potential applications of the device is to check the integrity of viral particles in vaccines.

According to Masselon, the team is working on improving the sensitivity of the device in order to detect pathogens in cultures.