  We derive analytic expressions for the sensitivity and figure of merit for
refractive index sensing with plasmonic resonators in the quasistatic limit.
Based on these expressions the limits of detection and their origins are
investigated. The critical role of limited energy concentration within the
resonators surrounding is shown. The classical figure of merit is found to be
solely dependent on the ratio of real to imaginary part of the resonators
permittivity . Based on these findings we discuss the optimum resonance
wavelength and resonator size and shape for single molecule detection.
