Fluorescent nanoparticles for ion sensing In this work the synthesis and characterization of small stable and fluorescent poly (acrylamide) NPs for sensing of chloride ions and pH is presented. The NPs were prepared by inverse microemulsion polymerization and characterized regarding their structural and fluorescence properties. Nearly all of the described NPs formed stable colloidal solutions in water and possessed a small diameter sufficient monodispersity and a neutral surface potential. Two different kinds of sensor particles were prepared: chloride sensitive NPs with a physically entrapped indicator fluorophore and pH sensitive NPs with a covalently bound indicator dye. The co polymerization of a reference dye enabled ratiometric measurements in suspensions with both types of nanosensors. The nanosensors showed good response to the respective analyte although the sensitivity was reduced by incorporation of the fluorophores inside the polymer matrix. The polymer matrix protected the fluorophores from undesired interaction with proteins and at the same time allowed small ionic analytes to pass through the polymer. The chloride nanosensors were transferred into living cells and their response was evaluated using fluorescence microscopy. After transfection of the cells the NPs were located partially in small cellular organelles (endosomes) as well as in the cytosol. In order to enhance the cellular uptake and to force the endosomal escape of the NPs different methods of functionalization were evaluated. Therefore novel core shell NPs with pH independent size and a positive  potential were developed. Both a pH indicator fluorophore and a reference dye were co polymerized to enable ratiometric pH sensing. The signal magnitude in suspension was not as good as for the poly(acrylamide) NPs although a clear correlation between the fluorescence intensity and the pH was obtained.