  Here we report a random lasing based sensor which shows pH sensitivity
exceeding by 2-orders of magnitude that of a conventional fluorescence sensor.
We explain the sensing mechanism as related to gain modifications and lasing
threshold nonlinearities. A dispersive diffusive lasing theory matches well the
experimental results, and allow us to predict the optimal sensing conditions
and a maximal sensitivity as large as 200 times that of an identical
fluorescence-based sensor. The simplicity of operation and high sensitivity
make it promising for future biosensing applications.
