  Biological sensory systems generally operate out of equilibrium, which often
leads to their improved performance. Here, we study the sensitivity of ligand
concentration for a general receptor model, which is generally in the
non-equilibrium stationary state, in the framework of a stochastic diffusion
equation. We derived a general formula of the maximum sensitivity.
Specifically, the sensitivity is limited universally by the Berg-Purcell limit
[Biophys. J ., 1977], regardless of whether the receptor is in an equilibrium
or non-equilibrium state.
