  Dynamical response of nanomechanical cantilever structures immersed in a
viscous fluid is important to in vitro single-molecule force spectroscopy,
biomolecular recognition of disease-specific proteins, and the detection of
microscopic dynamics of proteins. Here we study the stochastic response of
biofunctionalized nanomechanical cantilevers beam in a viscous fluid. Using the
fluctuation-dissipation theorem we derive an exact expression for the spectral
density of the displacement and a linear approximation for the resonance
frequency shift. We find that in a viscous solution the frequency shift of the
nanoscale cantilever is determined by surface stress generated by biomolecular
interaction with negligible contributions from mass loading.
