"Each protein or particle adsorbing onto the NEMS is detected as a real-time frequency shift that is proportional to the mass of the protein," team member Ashkay Naik told nanotechweb.org. "This type of system shows great promise for massive parallel mass measurements of proteins."

The new Caltech device measures mass in the same way as a quartz crystal oscillator, where a load on the oscillator changes its resonant frequency. The new device is better in many ways though, including the fact that it is more sensitive and much smaller. It can be scaled up because each device acts like an individual mass spectrometer.

The range of masses that can be detected is larger too – masses of a dalton to tens of megadaltons can be measured without changing anything in the system. "And while conventional mass spectrometers measure the mass to charge ratio, our system is directly sensitive to the mass of a particle or molecule itself," said Naik.

The researchers continually track the resonant frequency of the NEMS device using a feedback system, and each protein molecule or nanoparticle landing on the NEMS changes the resonant frequency of the device. This decrease in frequency is proportional to the mass of the adsorbed molecule.

The current set-up tracks the resonant frequency of a single mode in the device. As such, the researchers cannot determine the exact position of the protein on the device and they need to record hundreds of frequency shifts coming from individual adsorption events to determine the mass of a particle. The team hopes to overcome this problem in a next-generation system in which the resonant frequency of multiple modes of the device can be measured.

As well as proteins and other biomolecules, the new device could be used to measure the mass of air pollutants in the atmosphere, for example.

The work was published in Nature Nanotechnology.