Figure 1

Illustration of the pH modulation assay in detection by either decreasing or increasing the fluorescence response. (a) A positively charged protein (e.g., avidin) recruits hydroxide ions upon binding and renders the membrane surface more basic, decreasing the fluorescence response of the dye. (b) By contrast, a negatively charged protein (e.g., streptavidin) recruits hydronium ions upon surface binding and enhances the fluorescence response of nearby dye molecules.