Fluorescent Polysaccharide Nanoparticles for pH-Sensing

The exploration of chemical microenvironments in organisms as well as on the cellular level is of great interest in medical and biological research. Therefore, dextran nanoparticles, which are labelled with both a pH-indicator dye (fluorescein isothiocyanate, FITC) and a reference dye (sulforhodamine B acid chloride) as an internal standard, were developed as biocompatible nanosensores that can be used for ratiometric pH measurements. The ratio of the fluorescence dyes can easily be tuned during the nanoparticle formation by choosing the appropriate mixture of the labelled derivatives prior the nanoprecipitation. The fluorescence intensity of FITC increases with increasing pH value, whereas the signal of the reference dye remains constant. The polysaccharide based pH-nanosensors are characterized in terms of fluorescence lifetime, autoclaving stability, response to ionic strength, oxidation and photobleaching. The resulting pKa of 6.45 is appropriate for most of the measurement purposes. Fluorescence lifetime measurements indicate that energy transfer between the dyes takes places. However, no negative influence on the performance of the pH value measurements could be observed.