(361e) The Effect of Nanoparticle Loading on Morphology and Function of Nanoparticle-Loaded Micelles

Nanoparticle-polymer composites combine the unique properties of nanoparticles and the mechanical strength of polymers to build tunable, multifunctional devices for healthcare, energy storage, and catalysis. Amphiphilic block co-polymers self-assemble in water, forming micelles with a variety of shapes and sizes that possess internal hydrophobic cores, which are stabilized by hydrophilic shells. The micelle cores serve as reservoirs for entraining hydrophobic nanoparticles, rendering them water-dispersible, whereas encapsulant properties are preserved and toxicity is minimized. High nanoparticle loading within micelles is desirable to improve functionality. However, loading nanoparticles into micelles can affect the shape, size and stability of the host micelles. Here, we studied the influence of nanoparticle packing within micelles to determine how these bulky cargoes effect final micelle morphology.

Quantum Dot (QD)-loaded micelles composed of poly(styrene-b-ethylene oxide) (PS-b-PEO) were synthesized using an emulsion-based approach developed by the Discher[1] and Hayward[2] groups. In this method, an organic solution of polymers and nanoparticles in a water-immiscible solvent is emulsified with water in the presence of a surfactant. Gradual extraction of the organic solvent by evaporation triggers co-assembly of the nanoparticles and polymers, yielding nanoparticle-loaded micelles. The QDs used in our study were non-spherical, with aspect ratios between 2.5 and 3 and were also larger than the polymer radius of gyration. We synthesized micelles with increasing QD loading by varying the proportion of QDs in the initial organic phase at constant polymer concentration. With increasing nanoparticle loading, there was a change in shape, size, deformation, and internal nanoparticle packing. The number of spherical micelles in the system decreased and more elongated structures, such as ellipses and cylinders, formed at higher QD loadings. Interestingly, the average sizes of the spherical micelles formed remained the same, whereas longer ellipses and cylinders formed with increasing QD loading. Beyond a critical QD loading, QDs appeared densely packed within nearly spherical composites as seen in transmission electron microscopy (TEM). Small angle x-ray scattering (SAXS) spectra of these densely packed composites revealed a Bragg shoulder, suggesting that the QDs adopt an ordered arrangement at high packing density.

As the use of QDs for labelling and tracking applications grows, non-spherical QDs such as nanorods, nanoplates, and tetrapods are being used for their unique shape-dependent properties. Here, we found that, despite their geometric dissimilarity, non-spherical QDs attempt to align within the spherical cores and deform overall micelle structures. Thus, non-spherical QDs can elongate spherical micelles, yielding ellipsoidal and cylindrical micelles, which could be detrimental in applications requiring uniform particle morphology. Such studies provide a general guideline for selecting optimum nanoparticle loadings for downstream applications and further the existing knowledge of nanoparticle packing within densely loaded micelles.

References

1. Geng, Y. and D.E. Discher, Hydrolytic degradation of poly(ethylene oxide)-block-polycaprolactone worm micelles. Journal of the American Chemical Society, 2005. 127(37): p. 12780-12781.

2. Zhu, J. and R.C. Hayward, Spontaneous generation of amphiphilic block copolymer micelles with multiple morphologies through interfacial instabilities. Journal of the American Chemical Society, 2008. 130(23): p. 7496-7502.