Nanoprobing the acidification process during intracellular uptake and trafficking

Shedding light on time dependent pH processes inside cells gives valuable information, especially for the design of nanoparticle (NP) drug carriers with a pH sensitive release mechanism. This study showed that the acidification process of endosomal compartments can be monitored in a temporally and spatially resolved manner by SNARF-1-functionalized NP-based pH sensor (SNARF-1-NP). SNARF-1 (seminaphthorhodafluor-1) is a dual-wavelength pH-sensitive fluorescent dye which allows the ratiometric imaging of protonate (HA) and deprotonated (A-) forms. The functionality of the pH sensor was shown by an external calibration with buffers of different pH values and compared in the following with an intracellular pH calibration(left graph). Subsequently, we followed the pH changes inside the endolysosomal compartments over time and matched these with the co-localization of different endolysosomal markers. Before endocytosis, the majority of surface-bound carboxy SNARF-1 was deprotonated (blue stars). During maturation from early to late endosomes and lysosomes, the ratio of protonated (orange stars) to deprotonated (blue/green stars) SNARF-1-NP increased with the decrease of the pH.534