The grafting of stimuli-responsive polymer brushes on plasmonic structures provides a perfectly controlled two-dimensional active device with optical properties that can be modified through external stimuli. Herein, we demonstrate thermally induced modifications of the plasmonic response of lithographic gold nanoparticles functionalized by thermosensitive polymer brushes of (poly(N-isopropylacrylamide), PNIPAM). Optical modifications result from refractive local index changes due to a phase transition from a hydrophilic state (swollen regime) to a hydrophobic state (collapsed regime) of the polymer chains occurring in a very small range of temperatures. The refract:ye index of the polymer in aqueous solution is estimated in both states, deduced from the discrete dipole approximation (DDA) method. The combination of lithographic gold NPs and thermoresponsive polymer chains leads to a new generation of perfectly calibrated and dynamically controlled hybrid gold/polymer system for real-time nanosensors.