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Research Description: The Park Group, University of Pennsylvania
The combination of nanoparticles and polymers offers a powerful route to generate soft materials possessing unique optical, electrical,
and magnetic properties of nanoparticles and excellent processibility of polymers.
The primary focus of our research is to construct functional assemblies of inorganic
nanoparticles and amphiphilic polymers (e.g., insulating polymers, conjugated polymers, and biopolymers) with controllable structure and properties through self-assembly.
1. Self-Assembly of Nanoparticles and Amphiphilic Polymers
Inspired by the way nature forms functional supramolecular assemblies using lipid bilayers as architectural skeletons, our group utilizes the self-assembly of amphiphilic block-copolymers and nanoparticles to control the organization of nanoparticles. Using this strategy, we have self-assembled various types of nanoparticles (e.g., CdSe, Fe3O4, Au) into unique hierarchical self-assembly structures. This work demonstrated for the first time that ordered arrays of nanoparticles can be formed through the cooperative self-assembly of nanoparticles and block-copolymers in solution phase. Moreover, we found that the incorporation of nanoparticles can drastically impact the self-assembly behavior of amphiphilic block-copolymers and identified key factors that affect the co-assembly formation. These findings are our unique contributions to the area; prior to our study, little was known about what controls the morphology of nanoparticle-encapsulating block-copolymer assemblies formed in solution phase while a substantial progress had been made for thin film studies. We believe that these results will lead to general design principles for the solution phase self-assembly of nanoparticles and amphiphilic polymers.
Based on the understanding, we have fabricated various types of well-defined block-copolymer assemblies including unique radial nanoparticle assemblies and polymer vesicles packed with magnetic nanoparticles (superparamagnetic polymersomes). Furthermore, we demonstrated that the morphology of nanoparticle-encapsulating polymer assemblies significantly affects their magnetic relaxation properties, emphasizing the importance of the self-assembly structure and nanoparticle arrangement. We believe that the superparamagnetic polymersomes will open up many exciting opportunities in the field of nanomedicine owing to their ability to load both hydrophilic and hydrophobic substances, the controllable nanoparticle density, and the high magnetic relaxivity rate.
[Publications]
1. Hickey, R.; Haynes, A; Kikkawa, J. M.; Park, S. -J.*, “Controlling the Morphology of Nanoparticle-Encapsulating Block-copolymer Assemblies: from Micelles to Vesicles” JACS, 2011, In press.
2. Kamps, A; Sanchez-Gaytan, B. L.; Hickey, R.; Clarke, N.; Fryd, M.; Park, S. -J.* “Nanoparticle-directed Self-Assembly of Amphiphilic Block-Copolymers” Langmuir 2010, 26, 14345-14350.
3. Hickey, R.; Sanchez-Gaytan, B. L.; Cui, W.; Composto, R; Fryd, M.; Wayland, B. B.; Park, S. -J.* “Morphological Transitions of Block-copolymer Bilayers via Nanoparticle Clustering” Small, 2010, 6, 48-51.
4. Sanchez-Gaytan, B. L.; Cui, W; Kim, Y; Mendez-Polanco, M. A.; Duncan, T. V.; Fryd, M.; Wayland, B. B.; Park, S. -J.* “Interfacial Assembly of Nanoparticles in Discrete Block-Copolymer Aggregates”, Angew. Chem. Int. Ed., 2007, 46, 9235.
2. Self-Organizing Conjugated Polymers with Tunable Optical Properties
We developed a novel class of amphiphilic conjugated block-copolymers composed of polyalkylthiophene and polyethylene oxide (PAT-b-PEO). Significantly, the photoluminescent color of PAT-b-PEO can be drastically tuned from blue to red by simply controlling their self-assembly structure.
Various rod-coil block-copolymers containing ?-conjugated oligomers and polymers have been previously synthesized by other groups. However, most previously reported amphiphilic molecules showed spectral red shifts associated with polymer aggregation, which were accompanied by significant PL intensity drops. We believe our work is the first to report that a wide range of photoluminescence colors obtained through the self-assembly of conjugated block-copolymers without altering the molecular structure or the physical size of materials. This work clearly demonstrated that self-assembly can be used as a way to manipulate materials properties in a highly controllable fashion.
We are further investigating the structure-property relationships of conjugated block-copolymers in order to use them as building blocks for optical and electrical device fabrications (e.g., photovoltaics, sensors). The key advantage of our approach is the ability to control the organization of polymers and incorporated nanomaterials over large area based on self-assembly.
[Publications: Conjugated polymers]
1. Park, S. –J.; Kang S. –K.; Fryd M.; Saven J. G.; Park, S. -J.* “Highly Tunable Photoluminescent Properties of Amphiphilic Conjugated Block-Copolymers” JACS, 2010, 132, 9931–9933.
2. Jung, Y.; Hickey, R. J.; Park, S. -J.* “Encapsulating Light-Emitting Polymers in Blockcopolymer Micelles”, Langmuir, 2010, 26, 7540-7543.
3. Duncan, T. V.; Park, S. -J.* “A New Family of Color-Tunable Light-Emitting Polymers with High Quantum Yields via the Controlled Oxidation of MEH-PPV” J. Phys. Chem B. 2009, 113, 13216.
3. Enhancing DNA Hybridization Properties through Self-Assembly of DNA Block-copolymers and Nanoparticles.
We developed a new and general synthetic method to functionalize nanoparticles with a high density DNA layer, based on the self-assembly of DNA block-copolymers and nanoparticles. Remarkably, the self-assembled hybrid structure possesses extremely high binding capability to complementary DNA even at very low salt concentrations, where plain DNA strands do not form duplex structure. This unusual binding property makes these hybrid structures ideal candidates for duplex DNA detection. We are investigating the origin of the enhanced DNA binding properties, which will allow us to optimize the structure for their applications in gene delivery and detection.
[Publications]
1. Chen, X.-J.; Sanchez-Gaytan, B. L.; Hayik, S. E. N.; Fryd, M.; Wayland, B. B.; Park, S. -J.*, “Self-Assembled Hybrid Structures of DNA Block-copolymers and Nanoparticles with Enhanced DNA Binding Properties” Small, 2010, 6, 2256–2260.
4. Spiky Metal Nanoshells
We developed a high yield synthetic method for a new type of metal nanostructures, spiky gold nanoshells, which combine the morphological characteristics of hollow metal nanoshells and nanorods. The spiky gold nanoshells strongly absorb and scatter light in near-IR region, and exhibit enhanced near field and far field scattering compared to smooth shells. The combination of spiky features and the hollow structure opens up many exciting possibilities for multifunctional applications such as simultaneous imaging and drug delivery, and potentially plasmon-controlled drug delivery.
Our method utilizes block-copolymer assemblies or polymer beads as templates for the growth of spiky nanoshells. Various shapes of spiky metal nanoshells were prepared in addition to spherical nanoshells by using diverse polymeric nanostructures obtained through the self-assembly of block-copolymers as templates. Furthermore, the spiky gold nanoshells can be combined with other types of functional nanoparticles to generate multicomponent nanostructures utilizing the capability of block-copolymers to encapsulate various types of small molecules and nanoparticles.
[Publications: Synthesis of nanomaterials]
1. Sanchez-Gaytan, B. L.; Park, S. -J.*, “Spiky Gold Nanoshells”, Langmuir 2010, 26, 19170–19174.
2. Duncan, T. V.; Mendez-Polanco, M. A.; Kim, Y; Park, S. -J.* “Improving the Quantum Yields of Semiconductor Quantum Dots through Photoenhancement Assisted by Reducing Agents” J. Phys. Chem. C. 2009, 113, 7561.
3. Park, S, -J; Kim, Y; Park, S. -J.* “Size-dependent Shape Evolution of Silica Nanoparticles into Hollow Structures” Langmuir, 2008, 24, 12134.
4. Park, S, -J; Duncan, T. V.; Sanchez-Gaytan, B. L.; Park, S. -J.* “Bifunctional Nanostructures Composed of Fluorescent Core and Metal Shell Subdomains with Controllable Geometry” J. Phys. Chem. C., 2008, 112, 11205.
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