Ionic component sensor and method for making and using same

A composition of matter useful in a sensor for sensing the concentration of an ionic component in a medium is disclosed (e.g. PH value of blood). This composition comprises a first material (24) which includes a sensing component (e.g. fluorescence indicator) 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 which is permeable to the ionic component; and a second material (22) which includes a second matrix material which is permeable to the ionic component, and an opaque agent in an amount sufficient to render the second material effectively opaque. The first matrix material and the second matrix material are covalently bonded together forming a sensing layer (24) and a opaque layer (22). Sensing apparatus including such composition of matter and methods for making and using such composition of matter are also disclosed.