ERA NanoSci - Directed and Autonomous Motion of Hybrid Magnetic Nano-objects
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
A main goal of this still on‐going cooperation is to achieve a fundamental step forward towards nano‐object interaction or exchange; namely the active propulsion of individual “nano‐ swimmers” with respect to their environment and towards each other. Theory. In the theoretical part of the project we focused on a category of micro‐ and nano‐ swimmers propelled by a magnetic bead rotated by an oscillatory magnetic field and propelled by the consequent wagging of an attached flexible filament. The dynamics of the swimmers is found to be governed by three dimensionless parameters (apart from the Reynolds number, safely taken to be zero); the driving strength β = mB/(µa3ω); the sperm number Sp = L(ωζ/κ)1/4; and the Péclet number Pe = mB/(kBT). The analysis indicates that the particle is propelled along the direction of the driving field while having enhanced Brownian motion in the plane perpendicular to the field. For swimmers in the envisaged size range, the motion is found to be subject to strong thermal effects even for strong driving, since only one of its two rotational degrees of freedom is coupled to the field with the other fluctuating freely. Nanoswimmer Design. On the experimental side we followed different approaches based on magnetic and non‐magnetic nanostructures. In order to achieve a symmetry break on the structure side, dumbbell‐like particles and chain structures are realized. The effective chain length in the latter can be tailored by the zeta potential influenced by the ratio of cosurfactants present during the particle synthesis. Catalytic propulsion by “chemical fuel” is identified as a promising alternative approach. Dumbbells are realized which contain a catalytically active component attached to the ferromagnetic one. The particles can be functionalized with other useful functional groups ‐ e.g. the attachment of fluorophores along with the catalyst has proven effective. Also orthogonal functionalization is possible. Generation of nanostructure motion. The original concept for propulsion is based on the interaction of an anisotropic ferromagnetic particle component with an alternating magnetic field. The main method for observation of such a mechanical nanoobject in the present state of the project is based on dynamic magnetic susceptometry, and is readily confirmed. On the methodological side, a combination of the magnetic field with dynamic light scattering analysis is developed to allow the determination of (apparent) diffusion coefficients under the influence of resonant magnetic fields. With respect to catalytic propulsion, we investigated systems based on the catalytic decomposition of H2O2 (reductive) or NaBH4 (oxidative). While on the one hand a ferromagnetic, dumbbell‐based system involving a metallic Pt, Au, or Ag component has the advantage of a robust and well‐investigated propulsion principle, a catalase enzyme mimic was used for catalytic decomposition of hydrogen peroxide as a fuel. While for micro‐sized objects a bubble formation mechanism is predominant, small micro‐ and nano‐objects are propelled by self‐diffusiophoresis. Tracking. The motion was studied with three different techniques; based on a) dynamic light scattering, giving information on the (apparent) average diffusion coefficients of an ensemble of particles, b) fluorescence‐based single particle tracking, allowing the observation of time‐ dependent mean‐square displacement and directionality of dye‐labeled nanoobjects under propulsion, and c) a novel method based on dark‐field light scattering microscopy, allowing similar determinations for small‐object scatterers below the Abbé limit. The latter two furthermore allow a numerical classification of movement type, ranging from pure diffusive to purely propelled motion of the nano‐object by statistical analysis. We covered spherical, shape and functionality anisotropic (Janus) particles based on ferrite, silica and zeolite (aluminum silicate). All methods show consistent results for the different particle systems and propulsion methods investigated, and a clear tendency can be noticed. Conclusion. What we have achieved is in fact the first systematic study of catalytic particle motility covering a variety of sizes and shapes of particles. The striking observation is that when the size of the particles is decreasing, a surface catalytic reaction decreases Brownian motion rather than producing autonomous propulsion. We rationalize that this is because the rotational diffusion coefficient is growing exponentially with the decreasing size of the particles. While a theoretical model for the description of the particles behavior is under development, the results have to be taken into account whenever nanoswimmers are designed. Shape anisotropy is more important than functionality anisotropy. Decreasing size of the swimmers is limited for fixed aspect ratios.
Publications
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"Determination of core and hydrodynamic size distributions of CoFe2O4 nanoparticles suspensions using ac susceptibility measurements", J. Appl. Phys. 108, 033918 (2010)
F. Ludwig, A. Guillaume, M. Schilling, N. Frickel, A. M. Schmidt
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“Functional Silanes as Surface Modifying Primers for the Preparation of Highly Stable and Well‐Defined Magnetic Polymer Hybrids”, Langmuir 26(4), 2839 – 2846 (2010)
N. Frickel, R. Messing, T. Gelbrich, A. M. Schmidt
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"Hybrid nanocomposites based on superparamagnetic and ferromagnetic particles: A comparison of their magnetic and dielectric properties", Polymer 52, 1781‐1787 (2011)
N. Frickel, M. Gottlieb, A. M. Schmidt
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"Magnetic Properties and Dielectrical Relaxation Dynamics in CoFe2O4@PU Nanocomposites", J. Phys. Chem: C 115(22), 10946‐10954 (2011)
N. Frickel, A. G. Greenbaum, M. Gottlieb, A. M. Schmidt
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“Cobalt Nanoparticles with Tuned Interaction Potential: Towards Worm‐ and Chain‐like Aggregates”; Magnetohydrodynamics 47(2), 183‐190 (2011)
C. Dobbrow, M. Vaidyanathan, L. Belkoura, D. Guin, A. M. Schmidt
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"Improvement of the oxidation stability of cobalt nanoparticles", Beilstein J. Nanotechnol. 3, 75‐81 (2012)
C. Dobbrow, A. M. Schmidt
