We report on the immobilization of gold nanoparticles on end-functionalized and solvent responsive polystyrene brushes, grafted on an underlying substrate. The presence of gold nanoparticles on polystyrene brushes was confirmed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The resulting polystyrene-Au nanoassemblies have been used as the nanosensors for the detection of a variety of organic solvents in surrounding media. The sensing mechanism is based upon the change in the proximity of the immobilized gold nanoparticles as a consequence of the solvent induced reversible swelling-deswelling of polystyrene chains. The sensing ability was demonstrated by a simple analytic tool, i.e., UV-vis spectroscopy, through a shift in plasmon resonance band of immobilized Au nanoparticles. A dramatic blue shift of 32 nm in the surface resonance band was observed as the surrounding media of Au immobilized polystyrene brushes (Au-PS) was changed from air to the toluene. The described approach is facile and versatile in nature, which can be used for the fabrication of a variety of nanosensors based on the polymer brushes-nanoparticle assemblies.