A method for making a sensing element, and an assembled sensing element, useful in sensing the concentration of a gas, e.g., carbon dioxide, in a medium, e. g. blood, are disclosed. In one embodiment, the method comprises placing a quantity of a sensing composition precursor between and in contact with a transparent, gas impermeable solid disc and an opaque, gas permeable film, the sensing composition precursor comprises a sensing component, a polymer precursor and a cross-linking agent; and forming a gas sensing composition from this quantity of sensing composition precursor.