Nanoemulsions enable the fabrication of uniform polymeric nanoparticles for various applications, including drug delivery, nanomaterials synthesis, cosmetics, and food manufacturing; however, existing nanoemulsion preparation methods are limited to spherical production of nanoparticles.
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Recently, researchers from the Laboratory for Polymeric Materials at ETH Zürich developed a simple method for nanoemulsion formulation at room temperature that produces nanoparticles with different morphologies, such as spheres, vesicles, and worm-like structures.
Nanoemulsions are metastable dispersions of nanoscale droplets (with a size in the range of 20-500 nm) of one fluid into the continuous phase of another fluid. Such nanoscale dispersed systems are nearly transparent or translucent and inherently more stable than micro-and macroscale emulsions.
In nanoemulsions, because of the small droplet size, the Brownian motion dominates the gravitational forces. This then prevents coalescence or flocculation and improves the kinetic stability of the nanoemulsion. Still, the nanoemulsions are not entirely stable from the thermodynamics viewpoint and are subject to spontaneous phase separation, although over much longer time scales compared to microemulsions.
Formation and Application of Nanoemulsions
The phase separation of the two immiscible fluids (such as oil and water) in such biphasic nanoscale dispersions can be suppressed by adding thickening agents or surfactants to the emulsion to slow down droplet coalescence and to stabilize the interface between the fluids, respectively.
The surfactant compounds, which are amphiphilic molecules that feature hydrophobic and hydrophilic components, embed themselves at the droplet interface, thus stabilizing the nanoemulsion and avoiding phase separation.
Nanoemulsions can be synthesized either by high-energy or low-energy approaches. The high-energy methods, such as high-pressure homogenization and ultrasonication, use energy densities often in excess of 108 W/kg to break large droplets down to around 100 nm or less in size. Such methods provide a robust way to synthesize nanoemulsions with a volume fraction of the dispersed phase as high as 40%.
However, the excessive shear forces cause overheating of the nanoemulsion, which might alter the physical and chemical properties of the nanodroplets. In contrast, the low-energy methods exploit the low interfacial tension properties of the nanoemulsions to reduce droplet size and require much lower energy input (around 103 W/kg) that can be achieved simply by stirring or shaking.
Nanomaterial Synthesis Using Nanoscale Droplets
A particularly promising route is the combination of nanoemulsions with controlled radical polymerization strategies, such as reversible addition-fragmentation chain-transfer (RAFT) polymerization.
Such a combination enables the synthesis of uniform polymer nanoparticles. Compared to traditional solution polymerization, the compartmentalization and segregation of the precursor materials within nanoscale droplets facilitate higher polymerization rates at much lower concentrations of the reagents, thus increasing the efficiency of the polymerization process.
The inherent stability of the nanoemulsion suppresses the transfer of monomers and radicals between the nanodroplets. Besides, the huge number of nanodroplets in the emulsion means that each droplet contains at least one propagating radical, resulting in the conversion of all monomer droplets into polymer nanoparticles with a size identical to that of the original nanoemulsion droplets.
However, a major drawback of the existing nanoemulsion polymerizations strategies is the fact that they can produce only spherical nanoparticles (mimicking the shape of the nanodroplets).
Low-Energy Nanoemulsions for RAFT Polymerization
A research team from the Swiss Federal Institute of Technology in Zürich (ETH Zürich) led by Prof. Athina Anastasaki has developed a simple low-energy nanoemulsion polymerization method that allows the researchers to synthesize nanoparticles with complex morphologies.
The key to success was the combination of a low-energy nanoemulsion with a RAFT polymerization process mediated by a specially formulated chain-transfer agent (CTA). The CTA synthesized by Prof. Anastasaki’s team, when mixed with a small-molecule surfactant sodium dodecyl sulfate (SDS), facilitated the formation of 200 nm nanodroplets of hydrophobic styrene in water by simple hand-shaking of the mixture.
The synergistic action of the CTA and SDS dramatically reduced the surface tension at the interface of the nanodroplets.
The SDS molecules at the interface further stabilized the nanodroplets electrostatically, resulting in a stable nanoemulsion both at ambient and elevated temperatures for up to two days.
Polymer Nanoparticles with Tailored Shapes and Sizes
More importantly, the CTA was designed in such a way that at 70 °C it exhibited hydrophobic properties allowing it to copolymerize with styrene during the subsequent RAFT polymerization process, thus forming well-defined block copolymer nanoparticles that matched the size and shape of the initial nanodroplets.
At ambient temperature, however, the molecules of the CTA reverted to more hydrophilic behavior, thus forming a hydrophilic shell on the surface of the nanoparticles suspended in the aqueous solution.
By adjusting the ratio between the special thermoresponsive CTA and the styrene molecules, the researchers synthesized nanoparticles with different shapes and sizes. The variation of the polystyrene molecular weight and the optimization of the surface distribution of CTA resulted in a wide range of nanoparticle morphologies, including nanospheres, worm balls, nanoworms, and nanovesicles.
The size of the nanoparticles was much smaller than that of previously synthesized particles using traditional emulsion polymerization strategies.
The researchers envisage that further development of the novel low-energy nanoemulsion polymerization method would offer a simple and versatile strategy for synthesizing polymer nanoparticles with well-controlled sizes, shapes, surface properties, and core chemistries for a myriad of applications.
This would include catalysis, organic electronics, and biomedicine, where nanoparticle morphology plays an important role.
Continue reading: How Can Nanocrystal Size Affect X-Ray Diffraction Results?
References and Further Reading
Rolland, M., et al. (2021) Shape-Controlled Nanoparticles from a Low-Energy Nanoemulsion. JACS Au 1 (11), 1975-1986. Available at: https://doi.org/10.1021/jacsau.1c00321
Asmaa, E., et al. (2021) Nanoemulsions for synthesis of biomedical nanocarriers. Colloids Surf. B 203, 111764. Available at: https://doi.org/10.1016/j.colsurfb.2021.111764
Muñoz-Espí, R., Álvarez-Bermúdez, O. (2018) Application of Nanoemulsions in the Synthesis of Nanoparticles, in S. M, Jafari, D. J. McClements (Eds.), Nanoemulsions, Academic Press, p 477-515. Available at: https://doi.org/10.1016/B978-0-12-811838-2.00015-1