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Background and Aim Gold nanoparticles (GNPs) have been widely used as one of the most versatile nanomaterials due to their unique physico-chemical properties.The aqueous-organic phase transfers of metal nanoparticles in miscible two-phase systems usually necessitates the use of typical capping molecules for morphology-controlled synthesis with special shapes, sizes and surface modifications.In recent years, postsynthesis purification and separation of NPs has received increased attention as another crucial step for achieving stable and size-uniform nanoparticles to expand their applicabilities.A variety of postsynthesis techniques have been introduced for the separation and fractionation of GNPs in miscellaneous solutions or mixed colloidal systems, such as centrifugation,electrophoresis, chromatography, filtration and extraction.However, the majority of these commonly used methods are limited because they are quite expensive, energy intensive, or environmentally pollutive.Herein, we report a facile and green approach for the direct and complete phase separation of GNPs from water-miscible organic solutions to aqueous medium by using cheap and readily available inorganic salts.Methods This method involved the addition of salts in the mixtures to produce the salting-out of acetonitrile from water, followed with a low-speed centrifugation to accelerate direct-in-plate precipitation of GNPs remaining in the supernatant.In this study, the salt-induced effects of different amounts of salt additives, including MgSO4, Na2SO4, MgCl2 and NaCl, on the separation of GNPs with different diameters ranging from 5.6 to 100 urn, were compared.Characterization of the particles before and after phase separation was performed by using UV-Vis spectroscopy, digital camera imaging, transmission electron microscopy (TEM) and capillary electrophoresis.Results and conclusion A combination of salts (0.05 g MgSO4 plus NaCl) demonstrated the best results, where the acetonitrile phase (supernatant) could be directly injected into the high performance liquid chromatography system This new "green" strategy, by taking advantage of the salt-induced effects on liquid-liquid phase separation between two miscible solvents leading to aggregation and subsequent precipitation of the nanoparticles, allows for rapid recovery of water-soluble GNPs that are present in a homogeneous mixture under ambient conditions, and also for highly effective removal of the coexisting nanoparticle contamination in aqueous biological fluids for their direct HPLC-based analysis in human plasma and liver microsomes for drug metabolism.The effects of nanoparticle concentration and interactions of GNPs with bio-sample components on the degree of partitioning, and the mechanism of salt-triggered phase separation/nanoprecipitation was also investigated.