Gold nanoparticles have several advantages for cellular imaging compared to other agents. They scatter light intensely and they are much brighter than chemical fluorophores. They do not photobleach and they can be easily detected in as low as 10−16 M concentration [107]. Due to SPR gold nanoparticles have the ability to resonantly scatter visible light and NIR radiations upon excitation of their surface plasmon oscillation. This property can be used for intracellular trafficking using gold nanoparticles as contrast agents in dark field optical microscopy. The scattering cross section of gold nanoparticles is high when compared to the polymeric nanoparticles of same size and it increases per particle when the particle is agglomerated.

ref: https://www.sciencedirect.com/topics/chemistry/gold-nanoparticle