Stimuli-responsive nanoparticles have attracted considerable interest due to their enormous potential applications. In this paper, nanosized bifunctional spherical polyelectrolyte brushes (BSPBs) were prepared through grafting random copolymer chains from N-isopropylacrylamide (NIPA) and acrylic acid (AA) onto polystyrene core by photoemulsion polymerization. The pH- and thermo-responses of the BSPB with different ratio of NIPA and AA were investigated by dynamic light scattering. Atomic force microscopy (A-FM) was employed to in situ monitor the pH response of the morphology and surface adhesion of BSPB as well as to visualize BSPB individually or as monolayer in air and liquid. The brush like shell was illegible in electron microscopy images but clearly observed in AFM images. The results demonstrated that AFM was a powerful tool for in situ observing the stimulus-response of brushes on the surface of nanoparticles.