We worked with monodisperse gold NPs with >20 distinct sizes varying between 2.6 and 8.5 nm (the size refers to the diameter of the NP's metallic core). These NPs were synthesized in toluene in the presence of oleylamine (OLA) as the capping ligand. To endow Au•OLA NPs with negative and positive surface charges, we performed a ligand exchange reaction, in which the weakly binding OLA was replaced with strongly bound thiols: 11-mercaptoundecanoate (MUA) and (11-mercaptoundecyl)-N,N,Ntrimethylammonium (TMA), respectively (Fig. , bottom). Au NPs functionalized with a singlecomponent monolayer of MUA (i.e., Au•MUA) exhibited excellent solubility in basic water (pH=11) for all NP sizes studied. To render Au•TMA soluble in water, a small amount of a short, electrically neutral thiol (we used hexanethiol; ~20 mol%) was included during the ligand exchange (TMA alone proved to be unable to displace all the OLA ligands, even when used in excess). Both Au•MUA and Au•TMA bear hundreds of electric charges (estimated at ~370 negatively and ~290 positively charged groups for Au•MUA and Au•TMA for a typical NP size of 5 nm), with charge densities far exceeding those in both natural and artificially supercharged proteins.