Electrolyte Solutions Improve Nanoparticle Transfer from Oil to Water

Many techniques to transfer NPs from the oil phase into the water phase have been developed, but all have limitations that remain to be addressed, specifically low transfer yields and partial NP aggregation. One of the transfer processes involves emulsifying an oil suspension of NPs in water with surfactants, followed by evaporation of the oil to produce water suspensions of surfactant-encapsulated NPs. We show here that using salt solutions instead of deionized water significantly improves this emulsion-based transfer process. With oleate-coated CdSe quantum dots as a model NP system, hexane as the oil phase, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as the emulsifying surfactant, and NaCl solution as the water phase, the resultant aqueous suspensions exhibit months-long photoluminescence stability, near 100% phase-transfer yield, and nonaggregation. These benefits are attributed to a more laterally packed AOT layer surrounding the NP, as supported by zeta-potential measurements, surface-charge calculations, thermogravimetry, and Nile Red fluorescence analysis. The new phase-transfer method is general for a variety of NP, surfactant, and salt types.