Gold nanoparticles (GNPs) have unique electronic, photonic, and catalytic properties, making their application attractive. One of the most promising applications of gold nanoparticles is targeting individual cells to treat them by using lasers and gold nanoparticles. This is done by zapping gold nanoparticles inside cells with lasers. Being visible under a microscope, these can be used to diagnose sick cells or, when the power of the laser is increased, destroy the cells. The use of functionalized gold nanoparticles for biological and biomedical applications includes bioimaging, single molecule tracking, biosensing, drug delivery, transfection and diagnostic. For example, through proper functionalization, the particles can be engineered to accumulate preferentially in tumor cells using targeting ligands, providing a tool for cancer diagnosis and gene therapy. Sensor arrays have been developed to differentiate normal, cancerous, and metastatic cells using the fluorescence quenching properties of gold nanoparticles. Here the detection of Gold nanoparticles is important.

Gold nanoparticles can be detected by fluorescence in two ways. By a fluorescent label attached to the gold core or a fluorescent gold core itself. But it is difficult to detect their intracellular localization, efficiency of delivery, and integrity of the surface capping. The second method is widely used taking care of the quenching effect of the gold core. Depending on the distance between the fluorescent label and the gold core, the size of the nanoparticle, and the loading of the fluorescent dye on the nanoparticle, energy transfer may prevent or enhance photons from being emitted. The molecule should therefore be far enough from the gold core. If the fluorescent signal is used to report on the localization of the particle, independent proof should be provide information that the conjugate is intact. It is possible that a ligand can be removed on or after cell entry either by ligand exchange or proteolysis. In all these cases the fluorescence can be measured by wild field microscopy or, for a better resolution, by laser scanning confocal microscopy which provides an image of a thin section of the sample giving better localization information.