Gold nanoparticles have emerged as attractive nanomaterials for biological and biomedical applications because of their physical and chemical properties. The use of functionalized gold nanoparticles for biological and biomedical applications includes bio-imaging, single molecule tracking, biosensing, drug delivery, transfection, and diagnostic. Particularly for cancer cell targeting and killing, the requirement is proper cell recognition and uptake independent of the ultimate localization (e.g. vesicular localization is not a problem in this context). However, for intracellular imaging and sensing the ultimate intracellular localization of the nanomaterial is crucial and needs to be fully addressed. However to understand the status of gold nanoparticles which entered the live cells and their interactions with intracellular molecules or compartments, complementary techniques are used to obtain a detailed view, which includes quantitative assessment of uptake, intracellular localization, biochemical environment, and the status of the capping layer.

TEM allows direct visualization of metal nanoparticles and it is used for intracellular detection due to their high electron density. Conventional TEM can detect nanoparticles inside cells, of size more than 5 nm in cellular compartments and organelles due to good contrast provided by the high electron density of metal nanoparticles. But it has rather low throughput since it necessitates time-consuming method of preparation of the samples (cell fixation and resin embedding). Single representative image is not sufficient to make a decision on number of nanoparticles found, number of images analyzed and number of different cells imaged. It requires many images taken from a large number of sliced cells to obtain significant results about localization. In the case of gold nanoparticles TEM has been used to quantify the number of citrate-coated gold nanoparticles in vesicles after uptake in mammalian cells. TEM results can also be further corroborated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to get results on the count of the number of gold atoms, thereby providing a more precise quantification of the number of gold spheres. As for ICP-AES, inductively coupled plasma mass spectrometry (ICP-MS) gives an elemental analysis to estimate of the number of nanoparticles in the sample. Examples include study the uptake of GdIII-enriched polyvalent Cy3–DNA–gold nanoparticles conjugate (Cy3–DNA–GdIII@Au-NP) in NIH/3T3 and HeLa cells. These methods require cell lysis of a large number of cells which provide, quantitatively, the average population uptake at a given time point, but do not reveal cell-to-cell variations, dynamics and intracellular localization.