Fluorescent silica nanoparticles decorated with a responsive shell, a mixed polymer brush, were synthesized. Specifically, a poly(2-vinylpyridine), P2VP, and poly(ethylene glycol), PEG, binary polymer brush was synthesized on silica nanoparticles via the "grafting to" technique. The selection of the components (PEG and P2VP) for the responsive brush shell was motivated by potential biomedical applications. Poly(glycidyl methacrylate), PGMA, labeled with Rhodamine B, RhB, was used to form a reactive and fluorescent shell on the nanoparticle surface. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change in pH. At lower pH, aqueous dispersions of the particles are stable, since PEG and P2VP are water-soluble, extended and contribute to the steric and electrostatic mechanisms of colloidal stability. An increase of pH to 6 causes a slow aggregation as a consequence of the hydrophobic attraction between the collapsed and almost nonprotonated P2VP macromolecules. The aggregation was well controlled and occurred within 90-120 min of the pH change. The aggregation was fully reversible after the decrease in pH. The pH variation did not quench the fluorescence of the colloidal suspensions. The pH-tunable aggregation of the fluorescent nanoparticles could find diverse applications for labeling and contrasting of cells and tissues when the size of the label and the intensity of the optical signals can be tuned by and related to the pH of the host environment.