Dextran fluorophor- labeled

The quantification of fluorescent particles in cellular systems is difficult because several aspects such as autofluorescence, bleaching (see below), and quenching hamper analysis. Keep in mind that many fluorophores show a pH-dependent change in emission spectrum and intensity fluorescein-labeled dextrans (FITC-dextran) and calcein are strongly quenched upon acidification. If available, one should read the fluorescence intensity at its isosbestic point, where the intensity is not pH dependent. 

The PAH polymeric layer played an important role in our fluorescence sensor design. First, its positive charges enabled the deposition of anionic dextran that was labeled with the pH indicator fluorescein on the surface of the nanoparticles. More importantly, the PAH polymeric layer separated between the fluorescein molecules and the metal particle. In fact, the thickness of the polymeric layer was over 10 nm, which is larger than the Forster distance required for efficient energy transfer between the fluorophore and the metallic gold particles. 

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