An example of the different surface chemistry design of nanoparticles for biomedical applications. The base particles are usually designed with organic functional groups (eg, amines and carboxylic acids) that allow the particles to be conjugated to targeting agents (eg, antibodies, aptamers), polymers (eg, polyethylene glycol [PEG]), cell-penetrating peptides (eg, TAT peptides), or imaging agents (eg, fluorophores, radiolabels). In other nanoparticle designs, the nanoparticles has a porous interior that can store therapeutic agents.