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Magnetics
The Whitesides group is pursuing several projects involving magnetism. In general we use magnetism as a handle for physical manipulation of objects that are too small to be easily manipulated directly (e.g. with tweezers or micromanipulators). Much of this work is in collaboration with Professors Donald Ingber (HMS) and Mara Prentiss (Physics).
Multifunctional Micro- and Nano-Rods
This project involves the fabrication of multifunctional anisotropic structures through electrodeposition inside porous templates. For example, we have demonstrated the synthesis of metallic rods with submicron diameters that contain disk-like ferromagnetic sections (Figure 1) [1]. The metallic sections of these nanorods can be easily functionalized using thiol chemistry, while the magnetic portions provide a handle for manipulation with external magnetic fields. These rods also self-assemble into highly stable, hexagonally close-packed arrays (Figure 2). This configuration minimizes the energy of the bundle and does not generate a net dipole for the structure. This work provides a simple demonstration that magnetic interactions between ferromagnetic objects can direct and stabilize the formation of ordered, 3D structures by self-assembly.
Magnetic Spheres
We are currently developing methodologies for generating homogeneous ferromagnetic nanoparticles coated with a uniform thin layer of gold. Similar to the multifunctional rods, these core-shell structures could be easily modified with functional bio-molecules (e.g. proteins, DNAs, etc) and then manipulated with external magnetic fields. We are also exploring the synthesis and use of functionalizable metallic/magnetic spheres in the form of half-shells (Figure 3) [2]. We have demonstrated that it is possible to use spherical colloids (e.g. silica or polystyrene) as templates for vapor-phase metal deposition. In this case, we deposit colloids in a monolayer on a flat substrate and evaporate first a magnetic layer, then a metallic layer. The colloids can be either resuspended in solution to give half-coated spheres, or dissolved to give half-shells.
Magnetic Separations
We are actively examining the potential for using functional magnetic micro- and nano-structures in microfluidics [3,4]. One potential use for such structures is in microfluidic separations. We can use functional magnetic particles to bind to certain components of a mixture selectively. We can flow this mixture down a microfluidc channel with multiple outlets. Application of a magnetic field gradient across the channel can be used to direct the magnetic labeled components in the mixture into a specific outlet. The factors that determine the efficiency of this system include: strength of the magnetic field, magnetic susceptibility of the particles, viscosity of the liquid, and flow rate.
Magnetic Traps
This project involves the fabrication of three-dimensional magnetic traps for diamagnetic objects in an aqueous solution of paramagnetic ions [5]. We have demonstrated trapping of polystyrene spheres, and of various types of living cells: mouse fibroblast (NIH-3T3), yeast (Saccharomyces cerevisae), and algae (Chlamydomonas reinhardtii). The trapped particle and location of the magnetic trap can be translated in three dimensions by independent manipulation of the magnets that contribute to the overall magnetic field.
Magnetic Tweezers
We have recently begun a project to measure the rates of protein-ligand dissociation in a single-molecule format, using magnetic forces. We have been able to extrapolate to the rate constant for dissociation in the absence of an applied force and have obtained values that are in good agreement with rate constants from other techniques for a representative protein-ligand pair. We will extend this technique to protein-ligand complexes that exhibit complicated energy landscapes that cannot be followed adequately using ensemble averaging techniques and other interesting biological systems.
Select Publications:
1. Love, J. C. et al. "Three-Dimensional Self-Assembly of Metallic Rods with Sub-Micron Diameters Using Magnetic Interactions". Journal of the American Chemical Society 125, 12696-12697 (2003).
2. Love, J. C. et al. "Fabrication and Wetting Properties of Metallic Half-Shells with Sub-Micron Diameters. Nano Letters 2, 891-894 (2002).
3. Deng, T. Prentiss, M. and Whitesides, G. M. "Fabrication of magnetic microfiltration systems using soft lithography". Applied Physics Letters 80, 461-463 (2002).
4. Deng, T. et al. "Manipulation of magnetic microbeads in suspension using micromagnetic systems fabricated with soft lithography." Applied Physics Letters 78, 1775-1777 (2001).
5. Winkleman, A., et al. "A magnetic trap for living cells suspended in a paramagnetic buffer". Applied Physics Letters 85, 2411-2413 (2004).
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