Transfer of hydrophobic ZnO nanocrystals to water: an investigation of the transfer mechanism and luminescent properties

Abstract

We investigated the “interdigitated double layer” strategy, with the objective to transfer to water hydrophobic photoluminescent ZnO nanocrystals (Ncs) stabilized by a hydrophobic ligand (octylamine). This strategy relies on the formation of a double layer around the Ncs by interdigitation of an added surfactant within the alkyl chains of the pristine ligand. Various surfactants were evaluated and, surprisingly, transfer could only be achieved with a limited choice of molecular structures. Among them, the family of glycolic acid ethoxylate ethers surfactant yielded transfers up to 60%. The molecular organization of the organic coating in water was characterized using dynamic light scattering, photoluminescence and NMR (including DOSY and NOESY). Our results suggest that the success of this transfer strategy depends on a subtle interplay of interactions between the added surfactant, the ligand and the surface of the Ncs. The ZnO Ncs exhibit a strong luminescence in water.