A photoinduced electron transfer-based nanoprobe as a marker of acidic organelles in mammalian cells.

Photoinduced electron transfer (PET)-based molecular probes have been successfully used for the intracellular imaging of the pH of acidic organelles. In this study, we describe the synthesis and characterization of a novel PET-based pH nanoprobe and its biological application for the signaling of acidic organelles in mammalian cells. A fluorescent ligand sensitive to pH via the PET mechanism that incorporates a thiolated moiety was synthesized and used to stabilize gold nanoparticles (2.4 ± 0.6 nm), yielding a PET-based nanoprobe. The PET nanoprobe was unambiguously characterized by transmission electron microscopy, proton nuclear magnetic resonance, Fourier transform infrared, ultraviolet-visible absorption, and steady-state/time-resolved fluorescence spectroscopies which confirmed the functionalization of the gold nanoparticles with the PET-based ligand. Following a classic PET behavior, the fluorescence emission of the PET-based nanoprobe was quenched in alkaline conditions and enhanced in an acidic environment. The PET-based nanoprobe was used for the intracellular imaging of acidic environments within Chinese hamster ovary cells by confocal laser scanning microscopy. The internalization of the nanoparticles by the cells was confirmed by confocal fluorescence images and also by recording the fluorescence emission spectra of the intracellular PET-based nanoprobe from within the cells. Co-localization experiments using a marker of acidic organelles, LysoTracker Red DND-99, and a marker of autophagosomes, GFP-LC3, confirm that the PET-based nanoprobe acts as marker of acidic organelles and autophagosomes within mammalian cells.