A small charged channel can be used as pH sensor
Etched surfaces, immersed in salt solutions, carry immobile electric charges. The immobile charges attract mobile counter ions. Close to the surface the conductivity increases. The charge density depends on the acidity/basicity (pH) of the salt solution.
In small channels the surface conductivity dominates and can be used for pH sensors.
This work profited highly from the work of Dietrich Woermann, University of Cologne, Germany.