Subcellular optical pH nanoscale sensor

Coated gold nanoparticles bearing a pH-sensitive molecule serve as nanoscale optical sensors for non-invasive pH quantification of their endocytosis through surface-enhanced Raman scattering. Our sensor consists in colloidal gold spheres coated either with (polyethylene glycol) PEG molecules or a silica shell. They carry a sensor molecule that specifically recognize protons. The read out for monitoring changes in the pH is the Raman shift of the sensor molecule that is enhanced on the surface of the plasmonic spheres. Sensing was performed along the way of internalization from the extracellular site through different endo/lysosomal compartments where they are closely packed. The creation of hot spots favored by particle agglomeration inside cells was responsible for the enhancement of changes in signal intensity and was dependent on the surface chemistry. We establish a correlation between the physicochemical properties of the nanoscale sensor (shape, surface chemistry) and its ability to monitor the different pH along its cellular internalization. The PEGylated spheres can sensitively track the pH along their cellular internalization whereas the silica coated ones fail