Hydrophobic colloidal nanoparticles (NPs) have been synthesized in organic phase with attractive physicochemical properties with high surface quality. These NPs covered by a layer of hydrophobic surfactant cannot be solubilized or dispersed in the aqueous phase, then their biological applications in vitro or in vivo are quite limited. We have designed a simple ligand exchange method to transfer hydrophobic gold nanoparticle (Au NP) into aqueous phase using catecholamine molecules including dopamine, epinephrine, norepinephrine. These are positively charged molecules and efficiently bound to surface of the Au NPs to make them water-soluble. We examined interaction of the hydrophilic Au NPs with model bacteria cells. We also demonstrated how Au NPs effectively catalyze the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride. We propose that catecholamine can be used for phase transfer of various hydrophobic metallic NP and these water-soluble NPs can be used in a variety of bioanalytical and biomedical applications. (C) 2020 Elsevier B.V. All rights reserved.