A simple assay for azide surface groups on clickable polymeric nanoparticles

The surface azide content of polymeric nanoparticles (latex) was measured in three steps: 1) conjugating an alkyne derivative of Rhodamine (Rho-Alk); 2) using paper chromatography to separate the immobile latex particles from unreacted fluorophore; and, 3) on-paper fluorescence quantification of the latex-bound Rho-Alk. Paper chromatography with methanol elution was effective because the nanoparticles did not migrate during elution, whereas the unreacted dye did. This assay can be used to estimate the coupling yield of any alkyne simply by conducting a second stage click reaction with Rho-Alk to determine the content of azides not consumed in the first click reaction.