The synthesis, complexation, and photophysical properties of the Eu(III)-based quinoline cyclen conjugate complex Eu1 and its permanent, noncovalent incorporation into hydrogels as sensitive, interference-free pH sensing materials for biological media are described. The Eu(III) emission in both solution and hydrogel media was switched reversibly “on−off” as a function of pH with a large, greater than order of magnitude enhancement in Eu(III) emission. The irreversible incorporation of Eu1 into water-permeable hydrogels was achieved using poly[methyl methacrylate-co-2-hydroxyethyl methacrylate]-based hydrogels, and the luminescent properties of the novel sensor materials, using confocal laser-scanning microscopy and steady state luminescence, were characterized and demonstrated to be retained with respect to solution behavior. Water uptake and dehydration behavior of the sensor-incorporated materials was also characterized and shown to be dependent on the material composition.