Abstract: A composition of matter useful in a sensor for sensing the concentration of an ionic component a medium is disclosed. This composition comprises a first material including a sensing component in an amount effective to provide a signal which varies as the concentration of the ionic component in the medium varies, and a first matrix material, and a second material which includes a second matrix material which is compatible with the first matrix material, and an opague agent in an amount sufficient to render the second material effectively opaque. The first material and the second material are partially intermingled. Apparatus including such composition of matter and methods for making and using such compositions of matter are also disclosed.