The goal of the proposed research is to synthesize and study hybrid nanoparticles 10-100 nm in diameter constituted of an inorganic superparamgnetic core and a mixed polymer brush shell. Mixed polymer brushes refer to monolayers of two unlike polymers end-grafted to the same solid substrate. Mixed brushes grafted onto nanoparticles were successfully used by the PIs to design responsive colloidal systems which change their properties (e.g., interfacial energy and composition, adhesion, adsorption, aggregation, stability, etc.) according to external signals such as solvent quality, pH and temperature. The mechanism of this switching/responsive behavior was shown to originate from the microphase segregation of unlike polymers in the shell, where outside conditions may strongly affect the phase segregation. In this proposal, we suggest the further development of the responsive particle approach to a challenging new system - hybrid nanoparticles with dual responses due to the nanoparticles' specially tailored core-shell structure. The superparamgnetic nanoparticle's core will be coated by the mixed brush shell composed of a water soluble polymer and a hydrophobic weak polyelectrolyte. Solubility of the latter polymer in aqueous solutions can be tuned by pH, ionic strength, and temperature. For example, at room temperature and at pH 7 this polymer segregates to the particle core and the mixed brush forms radially segregated shell. The particles with a stratified mixed brush shell will demonstrate non-sticky properties and form stable suspensions in aqueous solutions in a broad range of pH values and ionic strengths. They will be stabilized due to the steric repulsion mechanism by the polymer brush (forming the outer shell) prepared from a water soluble polymer. The situation can be dramatically changed by applying an external magnetic field. The magnetic forces will overcome the steric repulsion and the particles will interact via the inner brush shells. Thus, this external magnetic field can turn on interactions between particles themselves, or between particles and the targeted substrate. The interaction remains unchanged even after removal of the external magnetic field due to the strong interactions between the inner shells. This mechanism is termed here the "locking mechanism". The interaction between the inner shells can be tuned and switched by the pH and temperature in their host environment. Thus, the particles can be unlocked by applying external stimuli. The nanoparticles proposed here will respond to an external magnetic field and, at the same time, they will respond to changes in pH, ionic strength and temperature. Properties of the particles' colloidal dispersion will be tuned/switched by a combination of a magnetic field and chemical/physical stimuli. In the proposed research we will aim: (1) developing the synthesis of the nanoparticles with the dual response; (2) studying the responsive behavior of the particles, and (3) switching between an adhesive and a non-adhesive particle shell to turn the adsorption of particles on various surfaces and their aggregation on and off. Broader Impact The obtained results are expected to substantially impact nanoscience and nanotechnology fields involving nanoparticle technologies and the design of complex functional materials and devices. The magnetically responsive particles will be used for a range of important technical, biological and medical applications where the specific versus nonspecific particle interactions can be switched on in an external magnetic field. Another priority of the proposed project is the involvement of the brightest high school, undergraduate and graduate students in modern nanostructured materials research. The project will train these students in nanoscience, nanotechnology, particulate science and surface science. Students will benefit greatly from this project's interdisciplinary nature. Significant effort will be directed to increasing the number of students, especially minorities and women, who pursue advanced degrees in science and engineering.

Project Report

Major Outcome: Researchers at Clarkson and Clemson Universities have successfully synthesized magnetic liquids that can undergo structural reconfiguration upon application of a pulse of magnetic field. Impact/benefits: Suspensions of tiny particles that self-assemble into complex and controllable microstructures in confined areas of limited access, e.g. inside capillary, living cells, or microfluidic channels, have important applications in medical diagnostics, drug delivery, and energy efficient separation processes. Of particular interest are smart fluids whose structures can be locked or unlocked externally. One example of potential commercial application is for magnetic hypothermia, where magnetic nanoparticles constituting the liquid can be concentrated/locked in a selected parts of human body (e.g. in cancer cells) with specific temperature and/or pH. Background/Explanation: The commercially available magnetic liquids form self-assembled structures in the presence of magnetic field. Upon removal of the field, however, the structures are often destroyed due to thermal fluctuations. The new magnetic liquid developed by the Clemson and Clarkson researchers form self-assembled structures that are stabilized and remain locked by strong interactions between particles (see figure uploaded) even after the field is removed. The structures may be unlocked, if desired, by varying temperature or acidity level (pH) of the solution. This is achieved by a novel process of coating magnetic nanoparticles with copolymers or mixed polymer brushes whose structure changes with the changes in acidity (pH) or temperature. Many new commercial applications are possible with the new programmable liquid. Scientific Uniqueness: The developed magnetic material has been unavailable previously. The known magnetic liquids form self-assembled structures in magnetic field. Integrity of the structures is preserved by the external magnetic field and, thus, by consumption of energy. The ability of the developed materials to form and stabilize the structure by a pulse of magnetic field is unique. The developed novel materials have promising applications in engineering, biotechnology and medicine. In the long term, the proposed magnetic material could serve as a platform for the development of a wide range of new energy-efficient functional materials, processes and devices which explore the discovered locking mechanism for colloidal dispersions of the hybrid magnetic nanoparticles. Intellectual merit of this project is related to the development of responsive nanoparticles capable of reorganization in an external magnetic field, which can turn on interactions between the particles themselves or between particles and their environment. The interaction remains unchanged even after removal of the external magnetic field due to the specially tailored polymer shell of the nanoparticles. This mechanism is termed here the "locking mechanism." The "locking particles" can be unlocked by applying external stimuli such as temperature, changes in pH, chemical reaction, or strong shear forces. In terms of Broader Impacts, this work is notable because the obtained results are expected to have substantial impact on nanoscience and nanotechnology fields involving nanoparticle technologies and design of complex energy-efficient functional materials and devices. The magnetic responsive particles will be used for a range of important medical and technical applications where the specific versus nonspecific particle interactions can be switched on in the external magnetic field. The project has an important educational role for training undergraduate and graduate students. The project helped for strengthening of the training of students in the areas of magnetic nanoparticles and surface science. The project provided opportunities to integrate research and education through cross-disciplinary student training in research labs, and scientific seminars. The PIs involved both undergraduate and graduate students in the research and train them to gain (i) expertise in nanofabrication techniques, (ii) familiarity with modern concepts in materials and colloidal chemistry, (iii) the ability to synthesize and characterize nano-materials, (iv) the ability to run experiments, gather data, and make discoveries, and (v) the ability to write scientific papers and make effective technical presentations. The project helped to strengthen collaboration between Clarkson University (NY) and Clemson University (SC) and develops partnerships between faculty and students. PIs of the project organized two symposia supported by NSF: in August 2008 "Responsive and interactive polymer materials and systems" at 236st ACS National Meeting in Philadelphia, PA; and in August 2009 "Hybrid Smart Micro- and Nanoparticles" at 238th ACS National Meeting in Washington, DC. The symposia were very well attended and attracted significant number of specialists in the field of the tunable materials. The symposia resulted in a publication in Nature Materials that was coauthored by a group of invited speakers.

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Clemson University
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