Herein, we report newly developed smart hydrogel nanoparticles with thermoresponsive photoluminescence (PL) at
a broad change of temperatures. Such thermoresponses of our bright hydrogels were detected to be linear, reversible,and sensitive regarding the emission intensity but with little change in the emission peak position. The photoluminescent source was the water-soluble europium organic complex Eu(III) phthalate, which was designed to coordinate in the shell of core/shell hydrogel nanoparticles. We designed the core to be poly[styrene- co -( N -isopropylacrylamide)] (PS- co -PNIPAM) and the shell PNIPAM. Thus, our thermoresponsive photoluminescent hydrogels were denoted as Eu-doped PS- co -PNIPAM/PNIPAM nanoparticles. Their temperature-stimulated PL response is considerably different from that of the previously reported PNIPAM hydrogels. These novel photoluminescent nanoparticles with linear and reversible temperature sensitivity should be very promising in various applications. To the best of our knowledge, these thermoresponsive photoluminescent hydrogels are the first to have their PL intensity reversible, linear, and sensitive to the broad temperature range of 10–50 ° C.