  Nanoelectromechanical Systems (NEMS) have emerged as a promising technology
for performing the mass spectrometry of large biomolecules and nanoparticles.
As nanoscale objects land on NEMS sensor one by one, they induce resolvable
shifts in the resonance frequency of the sensor proportional to their weight.
The operational regime of NEMS sensors is often limited by the
onset-of-nonlinearity, beyond which the highly sensitive schemes based on
frequency tracking by phase-locked loops cannot be readily used. Here, we
develop a measurement architecture to operate at the nonlinear regime and
measure frequency shifts induced by analytes in a rapid and sensitive manner.
We used this architecture to individually characterize the mass of gold
nanoparticles and verified the results by performing independent measurements
of the same nanoparticles based on linear mass sensing. Once the feasibility of
the technique is established, we have obtained the mass spectrum of a 20 nm
gold nanoparticle sample by individually recording about five hundred single
particle events using two modes working sequentially in the nonlinear regime.
The technique obtained here can be used for thin nanomechanical structures
which possess a limited dynamic range.
