(A) Absorption, excitation and emission spectra of glutathione-coated luminescent gold nanoparticles (G-AuNPs) and glutathione/cysteamine-coated luminescent gold nanoparticles (GC-AuNPs) in aqueous solution. G-AuNPs: One adsorption shoulder peak at 400 nm, λexmax = 300 nm, λemmax = 605 nm; GC-AuNPs: Three adsorption shoulder peaks at 305, 375 and 450 nm, λexmax = 315 nm, λemmax = 575 nm. Inset: Pictures taken under 365 nm UV excitation. (B) Typical transmission electron microscopy image of GC-AuNPs showing (C) a core size of 2.7±0.5 nm, and dynamic light scattering analysis showing a hydrodynamic diameter (HD) of 3.1±0.4 nm in aqueous solution.