Michael J. Solomon, Mark A. Burns and Joanna Mirecki Millunchick, University of Michigan
The assembly of nanocolloids into useful structures is key to emergent applications of nanoscale science and engineering. Yet, the initial impact of assembly has been limited because of the disappointingly small number of unit cells assembled to date. Because of their reduced symmetry relative to spheres, ready availability of nanocolloids with anisotropic shape and interactions would improve the scope for assembly of ordered arrays with novel morphology. However, the development of methods to produce anisotropic particles is in its infancy. Methods available are laborious and/or capital intensive and often neither flexible nor scalable. Here we will develop a versatile nanofluidic platform for the rapid manufacture of nanocolloids with anisotropic shape and interactions. This exploratory project combines an interdisciplinary team with strong capabilities in particle synthesis, lithography, fluidics and focused ion beam (FIB) patterning in pursuit of this aim.
On chip fluidic processing is an efficient way to direct, mix, react and analyze minute amounts of reactants and products. We aim to apply nanofluidic technology to direct, order and link chains of individual nanocolloids in nanoscale channels. The ordering and linking will provide the desired shape and interaction anisotropy. Nanofabrication and FIB methods will be used to construct a nanofluidic processor that can be replicated into parallel processors for large-scale production. Through syntheses of monodisperse nanocolloids of poly(methyl methacrylate) and silica with different steric layer chemistry, we have available particles with differing potential interactions, here labeled as nanocolloids "A" and "B." Their size is controlled and as small as 40 nm. The proposed nanofluidic processor will yield linked, permanently bonded chains of nanocolloids with ordering of A and B that is variable and specifiable. Our initial aim, for example, will be to produce a "nano-surfactant" of type B-B-A-A-A-A. The device function will be: By a software-programmed sequence of pressure actuations, A and B particles are alternately fed single-file to a narrow channel in the desired sequence. Matching the channel cross-section to the particle diameter yields single-file motion that conserves the sequence order. The particles are then conveyed into a reaction zone where a flow constriction immobilizes and compresses the sequence so that adjacent particles are in contact. A surface coupling reaction (initiated by UV excitation) then covalently binds the particles into the anisotropic sequence specified by the fluidic ordering. The device will first be developed at scales of ~ 500 nm so that fluidic control and operation can be optimized through in situ visualization with two-channel confocal fluorescence microscopy. The device will then be scaled down for manufacturing with ~ 50 nm building blocks. The products of the manufacturing will be used to perform initial assembly studies.
Our idea to apply nanofluidics for nanocolloid synthesis has intellectual merit because: (1) fluidic processing allows precise control of the relative orientation of particles needed for high fidelity synthesis; (2) the scheme uses continuous (rather than batch) processing and thus achieves a true manufacturing capability. The materials will be used for initial fundamental engineering science studies of anisotropic particle assembly that could not be accomplished in any other way. In addition to the outcomes of graduate education and outreach to middle school girls, the broader impact of the work will be to establish that nanofluidics and nanopatterning can be applied to solve a difficult problem in nanocolloid synthesis. The project will: (1) make possible routine synthesis of anisotropic particles in academic laboratories, thereby increasing the scope for assembly studies nationwide; (2) render feasible the possibility of large-scale, commercial manufacturing of specialty anisotropic nanocolloids for such applications as chemical sensing and photonic band gap materials. The principal research and education theme that this proposal addresses is "Manufacturing processes at the nanoscale."
