Bilayer Approaches for Nanoparticle Phase Transfer

Abstract

Nanoparticles (NPs) are often synthesized in organic solvents due to advantages of superior size and shape control obtainable in a non-polar environment. However, many applications featuring NPs require them to be in aqueous media. To transfer NPs from oil to water, surfactants with amphiphilic (hydrophobic and hydrophilic) groups have been widely used. A popular phase-transfer approach involves formation of oil-in-water emulsions upon which the oil storing the NPs is boiled off. In the process, surfactants form bilayers with hydrophobic groups on the NPs rendering them water-dispersible. This transfer route however is limited in that NPs aggregate to form clusters which results in poor colloidal stability and for the specific case of quantum dots (QDs), adversely impacts optical properties. It has ever since remained a challenge to devise approaches that transfer NPs from oil to water as single particles without compromising NP stability and properties. We have discovered that by simple addition of salt to water during the step of emulsion formation, NP transfer efficiency can be greatly enhanced in "salty-micelles" of surfactants. The strength of this approach lies in its simplicity and generic nature in that the transfer scheme is valid for different NP, surfactant and salt types. Using a model system with cadmium selenide (CdSe) QDs as NPs, Aerosol-OT (AOT) as the surfactant and NaCl as the salt in water, we found >90% of CdSe QDs transferred in salty-micelles of AOT which was significantly higher than the 45-55% QDs that transferred in deionized-water (DI-water) micelles of AOT. In the salty-micelle environment, QDs were found to exist predominantly as single NPs with narrow size distribution, as established by light scattering, analytical ultracentrifugation and electron microscopy. The effects of salt were in lowering aqueous solubility of AOT through "salting-out" action and in screening repulsions between like-charged head groups of AOT molecules. Electrophoresis, thermogravimetric analysis and photoluminescence measurements using a solvatochromic dye established higher surfactant coverage with greater lateral compaction for QDs in salty-micelles over the DI-micelle counterpart. Single NP characteristics along with a hydrophobic environment in laterally compact salty-micelles resulted in better retention of optical properties of QDs. Observations of a secondary effect by salt in inducing spontaneous emulsification of a hydrocarbon (octane)/AOT/brine system were systematically investigated by tracking time-variant octane droplet size and charge. Salinity levels that determine the spontaneous curvature and phase behavior of AOT were seen to influence the initial nucleation of octane droplets and their subsequent growth. The smallest octane drops (sub 50 nm) were nucleated at the optimum cross-over salinity and emergence of the liquid crystalline phase of AOT resulted in slowest growth rates. These factors contributed towards higher transfer efficiency of NPs in salty-micelles. Two applications from formulating aqueous NP suspensions by the new phase-transfer approach are described. In the first, QD and carbon-dot (C-Dot) "nanoreporters" were formulated for oil-field reservoir characterization using Neodol 91-7 (nonionic) and Avanel S150 CGN (hybrid nonionic and anionic) as surfactants. These NPs were stable to aggregation under reservoir-representative conditions (salinities: 1M NaCl, 1M KCl and 0.55M synthetic seawater; temperatures: 70-100 °C) and demonstrated flow and transport through crushed-calcite and quartz-sand columns with high breakthrough and recovery (> 90%). In the second application, tandem assembly of a cationic polymer, multivalent salt, and NPs was investigated in a microfluidic channel where charge ratio of the polymer/salt and shear from flow and device geometry determined their assembly into higher ordered structures such as gels and capsules.