  We propose a design for tuning the resonant spectra of Fabry-P\'{e}rot
nanocavities mediated by the Casimir force. The system involves a suspended
gold nanoplate approaching to a dielectric-coated gold substrate in a univalent
electrolyte solution. The gold nanoplate can be stably suspended due to the
delicate balance between repulsive and attractive components of the Casimir
forces. In an electrolyte solution, the presence of ionic-charge fluctuations
can partially or totally screen the thermal $n$=0 Matsubara term, resulting in
strongly modified interactions. As a result, the separation between the gold
nanoplate and the substrate experiences a significant modulation in response to
variations in salt concentration. Under proper conditions, we find that the
modulation of the Casimir force would strongly shift the resonances of
Fabry-P\'{e}rot nanocavities at the optical frequencies, when the Debye length
of the electrolyte decreases from 1000 nm to 10 nm. Finally, the temperature
dependence of the thermal Casimir force would provide an additional modulation
of Fabry-P\'{e}rot nanocavity resonances for their eventual fine tuning. These
results open up a promising venue for general tuning of the optical resonances
with potential applications in re-configurable microfluidic nanophotonics.
