Precise determination of the nanoparticle concentration and ligand density of engineered water-soluble HgSe fluorescent nanoparticles
HgSe nanoparticles (NPs) have been recognized as the ultimate product of mercury detoxification in biological systems. However, the exact mechanism of their formation and distribution in animals is not understood, and thus well-characterized engineered NPs would be invaluable to investigate such metabolic products. In this work, HgSe NPs were sonochemically synthesized and transferred for the first time to aqueous media using dihydrolipoic acid (DHLA) as a capping ligand. A thorough characterization of the engineered HgSe NPs was carried out. Different complementary analytical techniques, including high resolution-transmission electron microscopy (HR-TEM) and X-ray powder diffraction (XRD), provided information about the morphology and crystal structure of the NPs. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectrometer (EDX) analysis were used in order to assess the chemical composition and purity of the same HgSe NPs. Moreover, Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), zeta potential determinations and asymmetric flow field-flow fractionation (AF4) coupled on-line to fluorescence and inductively coupled plasma-mass spectrometry (ICP-MS) detectors confirmed that the DHLA ligands were bound onto the surface of the NPs making them stable in aqueous solutions. Fluorescence measurements evidenced that the engineered NPs exhibit a photoluminescence emission centred at 575 nm. Finally, the Se/Hg molar ratio of the NPs was accurately and precisely determined by ICP-MS to be 0.96 ± 0.03. Such valuable information was used to precisely determine the nanoparticle concentration in aqueous solution (in terms of number of nanoparticles per unit volume) and the ligand density, in terms of number of DHLA molecules per nanoparticle (2500 ± 125).