An assay triggered by recognition-induced charge switching is developed for protein detection and quantification. Aptamer-functionalized silica nanoparticles (NPs) have been synthesized to capture lysozyme, resulting in an alternation of the surface charge from negative to partially positive. The binding event is then translated and monitored by the fluorescence signal of a highly fluorescent anionic poly(fluorene-alt-vinylene)(PFVSO3), which "stains" on protein/aptamcr-NP complexes via electrostatic interaction. Blue-greenish fluorescence of PFVSO3 is observed in the presence of lysozyme by the naked eye, while no fluorescence is obtained for NPs upon treatment with a mixture of foreign proteins. A linear relationship between NP fluorescence and lysozyme is observed in the concentration range of 0-22.5 mu g/mL, which gives a limit of detection as similar to 0.36 mu g/mL. This work demonstrates a convenient label-free conjugated polyelectrolyte (CPE)-based protein detection with high specificity and sensitivity, which has potential applications in medical diagnosis.