Nanoparticles have tremendous potential in fields ranging from energy to biomedicine. However, much nanoparticle research remains at the bench scale, with little translation to industrial applications. Many of the most promising nanoparticles, such as semiconductor quantum dots (QDs) and superparamagnetic iron oxide nanoparticles (SPIONs) are manufactured via organic phase synthesis. High throughput manufacturing strategies for rendering inorganic nanoparticles soluble in water are thus sorely needed.
Here, we describe a promising method to synthesize inorganic-polymer composite nanoparticles that combines a scalable electrospray technique with an interfacial instability process to form micellar structures. In the electrospray method, aqueous poly(vinyl alcohol) is used as an outer flow, and poly(styrene-block-ethylene oxide) (PS-b-PEO) in chloroform comprises the inner. Solutions flow through a thin coaxial needle assembly, and a voltage is applied until a cone-jet spray forms. Aerosol particles are ejected from the spray and collected in water as micron-scale emulsion droplets. Transient interfacial surface tension changes then result in micelle formation. This technique has been utilized to encapsulate QDs and/or SPIONS, effectively co-localizing fluorescent and magnetic material in a ~30 nm diameter particle.
To more fully explore the utility of the coaxial electrospray process, we varied operational parameters to achieve higher polymer processing and throughput, thus accessing greater nanoparticle yields. In particular, the influence of electrospray volumetric flow rates, organic block copolymer loading, and system processing temperature were independently evaluated. Conditions of stable electrospray with increased volume flow were identified, thus permitting greater production of emulsion particles without compromising the fine, monodisperse aerosol formed in a cone-jet electrospray. Furthermore, polymer loading was increased while still generating an electrospray-enabled emulsion. Changes to operational settings also altered the rate of the interfacial instability process. With increasing organic block copolymer concentration, a general decrease in the time required for the interfacial instability process to take place was observed. Similarly, increasing processing temperature also increased the rate of particle production.
Finally, process changes also allowed access to differing PS-b-PEO assembly morphologies. Increases in polymer concentration resulted in the production of both spherical and wormlike micelles. Choice of nanoparticle shape is important for eventual utility as different applications require specific sizes or loadings. This micelle production technology holds promise for use in the formation of empty and loaded micelles of different phases with potential application in drug deliver, imaging, diagnostics, and separations.