This CAREER project will use the hydrophobic effect as a means to assemble condensed matter into discrete functional objects. Specifically, covalent attachment of Y-shaped amphiphiles to metallic clusters of platinum (Pt), palladium (Pd), and gold (Au) will impart the amphiphilicity to the resulting hybrid structure. Because every particle will be the junction point of all hydrophobic and hydrophilic arms, the micellization will lead to a dense packing of particles exactly at the boundary between the core and corona of the micelle. Thus, the individual particles will organize spontaneously into finite and well-defined nanoarrays. Importantly, each micelle will be a carrier of densely packed catalytic sites (Pt, Pd, or Au nanoparticles) and will combine the best features of homogeneous and heterogeneous catalysts. We hypothesize that the catalytic activity may not only be a function of size and the nature of particles, but will also depend on the curvature of nanoarrays and the degree of order in them. The curvature can be dramatically changed as cylinders transform into vesicles, and vice versa. Thus, the catalytic activity and/or selectivity may be manipulated by changing the morphology of the proposed assemblies. The expected outcomes are strongly supported by the feasibility studies on gold nanoparticles, but the approach can be applied to any spherical or rodlike carrier of functionality.
NON-TECHNICAL EXPLANATION
The broader impacts of the project are in developing new methods to bridge the gap between the nano- and mesoscale, and to strengthen the nanoscience education of the general public in Texas and nationwide. Of particular importance will be the collaboration with the NSF-funded educational outreach program NanoKids. The PI will introduce a Materials Studio Visualizer Program in the middle schools that are already using NanoKids educational materials. This 3D Molecular Graphics Program will allow children to learn and better understand the structure of various molecules, crystals, and DNA. In a parallel effort, three on-line mini-courses in nanotechnology, which will be used by 58 science teachers from the greater Houston area, will be developed. Unique hybrid structures and advanced catalysts proposed in this project will be excellent examples to help K-12 teachers understand the origin of unusual properties of matter at the nanoscale, and to learn about the industrial and societal implications of nanotechnology. This project will open up a new avenue for organizing functional matter and creating better catalysts, which will directly benefit society.
Intellectual Merit Our work was focused on the synthesis and self-assembly of various structures ranging from small organic molecules to mesoscale inorganic crystals. Harnessing this phenomenon is of great importance as it brings an opportunity to create new matter with desired properties. In the framework of the NSF CAREER grant entitled "Amphiphilicity Driven Organization of Nanoparticles into Discrete Assemblies" (DMR-0547399, 07/2006-06/2012) the PI’s group utilizes the amphiphilicity of block copolymer chains as a tool to drive self-organization of gold and silver nanoparticles into well-defined assemblies in aqueous solutions. Mesoscopic structures with various morphologies ranging from cylinders to spheres to vesicles have been produced. Their optical and catalytic properties have been studied and the work currently continues on the assemblies that can form reversibly as a function of pH and the ionic strength of solution. Analogous assemblies have been generated from catalytically-active palladium and platinum nanoparticles. Overall, the scientific findings of the PI’s group sponsored by the NSF CAREER grant during 2006-2012 are described in 28 papers (Fig. 1). Our research over the past several years brought new knowledge in the synthesis of branched amphiphilic macromolecules (JACS 2003, JACS 2003, Angew. Chem. 2004, Nano Lett. 2005). In the course of this work we realized that the same principles of self-assembly can be applied to amphiphilic hybrid structures and therefore, can be utilized for the organization of inorganic nanocrystals in solution (JACS 2006, JACS 2006). We reported on the first example of gold nanoparticles that can undergo reversible assembly-disassembly process in water and remain in solution in the form of tubular arrays and 2D vesicular assemblies (Fig. 2). In the next stage of the project we shifted our attention to low-symmetry nanocrystals. Polymer-functionalized gold nanorods were found to have an incredible ability to self-organize into ring-like arrays when water microdroplets are used as soft and self-removable templates (Angew. Chem. 2007). Finding new ways to assemble nano-objects into finite superstructures is an important task because at the nanoscale the properties depend on the arrangement of individual building blocks. Ring-like assemblies have been documented for polymers, organic molecules, and spherical nanoparticles. However, rings composed of anisometric nanocrystals have never been observed. We discovered a spontaneous assembly of hybrid gold-polymer nanorods into ring-like arrays (Fig. 3). Ring-like structures represent a critical component of metamaterials, which have an enormous potential in opto-electronics, communications, and military applications. More recent efforts were focused on the synthesis of taxol-functionalized gold nanoparticles, which possess a very high drug loading capacity and well-defined core-shell structure (JACS 2007, Highlighted by C&E News). The synthetic strategies developed in our lab allowed for the preparation of hybrid nanostructures and offered a new opportunity to create biologically active hybrids. We reported on the first synthesis of taxol-functionalized gold nanoparticles that carry up to 70 drug molecules (Fig. 4). Such structures have a very high drug-loading capacity and may offer a greater selectivity toward cancer cells and targeting of tumor tissue due to the enhanced permeation and retention (EPR) effect. The PI has actively participated in the educational program NanoKids at Rice University. His graduate students supported by this CAREER grant visited a local high school in Houston and presented on-site demonstration of nanomaterials and chemistry experiments. In addition, the PI actively used Molecular Graphics Program for this presentation and training of science teachers from participating schools. Six graduate students have worked on this project over the past 5 years. Three students successfully synthesized and studied the properties of amphiphilic nanoparticles, nanorods, and nanowires. The other three students were working on biologically active taxol-functionalized gold nanoparticles with amphiphilic V-shaped arms. Twelve presentations have been delivered at the national meetings by the PI and his graduate students. The broader impacts of the project were in successful development of new methods to bridge the gap between the nano- and mesoscale, and to strengthen the nanoscience education of the general public in Texas and nationwide. Of particular importance was the collaboration with the NSF-funded educational outreach program NanoKids. The PI introduces Materials Studio Visualizer Program in local middle schools that were using NanoKids educational materials. This 3D Molecular Graphics Program allowed children to learn and better understand the structure of various molecules, crystals, and DNA. In a parallel effort, the PI has developed an on-line course in nanotechnology which was used by science teachers from Houston schools. Unique hybrid structures and advanced nanomaterials developed in this project served as excellent examples to help K-12 teachers understand the origin of unusual properties of matter at the nanoscale, and to learn about the industrial and societal implications of nanotechnology. Overall, this project has opened up a new avenue for organizing functional matter and creating functional nanomaterials, which can directly benefit the society.
