ABSTRACT
Concentrations of biomolecules within cells are typically of the order of nM and therefore a highly sensitive technique is required for intracellular studies of intact and living cells. Fluorescence imaging has been the predominant platform for such biological studies. The key to successful utilisation of nanoparticles inside cells for surface-enhanced Raman scattering and other measurements is understanding and manipulating their mechanism of uptake and cellular distribution. Internalisation methods generally fall into three categories: involuntary delivery by physical methods, passive diffusion, and active uptake. Endocytosis refers to commencement of the vesicular endolysosomal pathway through the cell for the active intake of macromolecules and proteins which are too large to pass directly through the plasma membrane. In its initial step, cargo for internalisation is engulfed by invagination or pit formation of the plasma membrane. Early endosomes are initially relatively small, have a mildly acidic pH and are the main sorting station in the endosomal pathway.
