Autonomously Moving Local Nanoprobes in Heterogeneous Magnetic Fields Prajnaparamita Dhar, ² Yanyan Cao, ‡ Timothy Kline, ‡ Priya Pal, ² Cheryl Swayne, ² Thomas M. Fischer,* ,² Brian Miller, ² Thomas E. Mallouk, ‡ Ayusman Sen, ‡ and Tom H. Johansen § Department of Chemistry and Biochemistry, The Florida State UniVersity, Tallahassee Florida 32306 4390, Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Physics, UniVersity of Oslo, P.O. Box 1048, Blindern, Norway ReceiVed: NoVember 6, 2006; In Final Form: January 2, 2007 Different strategies of navigation in heterogeneous magnetic fields are investigated using natural and synthetic autonomously moving micro- and nanonavigators on top of a magnetic garnet film with uniaxial anisotropy creating a dense stripe domain structure. The use of ferromagnetic navigators leads to dynamic frustration, but synergy is achieved between the autonomous motion and magnetic orientation of paramagnetic navigators. The use of differently magnetized ferromagnetic and paramagnetic nanonavigators enables one to change from a roving to a guided motion. These different modes of motion can be utilized for distinct processes, such as the delivery and distribution of molecular cargo attached to synthetic navigators. Introduction Properly functioning nanomachines 1-4 operate in the unex- plored realm of nanoscience. The motion of micro- and nanoscale objects is being used in nature for cell trafficking and the delivery of metabolized products. In nanoscience, one mimics strategies from nature and applies them to the nanoscale motion of roving sensors, drug delivery, and effective transport systems. Depending on the application, one tries to either guide the motion along well-established paths, or one uses a roving statistical motion for the fast spreading of components over a broad range. In either case, nanoshuttles on the colloidal scale 5,6 depend on reliable navigational data to fulfill their tasks in a usually heterogeneous environment. Navigational data will tell the nanoshuttles where they are, or where they should go, and will prevent the nanomachines from getting lost. Here, we present a variety of autonomously moving natural and synthetic para- or ferromagnetic nanoshuttles 2 that respond with their orientation and motion to heterogeneous magnetic fields in their surroundings. We show that the motion of ferromagnetic magnetotactic bacteria and the motion of catalytic para- and ferromagnetic nanorods can be understood by control- ling the relative strengths of their magnetism, their propulsion, and their thermal fluctuating properties. There is a surprisingly complex interplay of these interactions when the nanoshuttles are placed in a heterogeneous magnetic field on top of a magnetic garnet film. This allows us to continuously vary the motion from an enslaved guided motion, via a partially controlled anomalous diffusion, toward an autonomous statistical roving motion. While performing their autonomous motion, the navigators report information on the local direction of the magnetic field via the orientational order of their long axis. Catalytic nanorod and bacterial propulsion compete with the magnetic forces acting in the magnetic heterogeneities. The nanonavigator distribution in the heterogeneities is therefore dominated by the interplay of propulsion and orientation and significantly differs from an equilibrium Boltzmann distribution. The navigators are also able to sample the magnetic field in regions that are energetically unfavorable by multiples of the thermal energy. Four different nanoprobes, magnetotactic bacteria (Magnetotacticum gryphiswaldense), and three types of catalytic magnetic nanorods were released in an aqueous solution on top of a magnetic Y 2.5 Bi 0.5 Fe 5-q Ga q O 12 (q ) 0.5- 1) garnet film where they moved according to the constraints set by the inhomogeneous magnetic field of the stripe domain pattern in the uniaxial anisotropy garnet film. Experimental Magnetotacticum Gryphiswaldense. Some of the most excit- ing magnetic navigators in nature are magnetotactic bacteria, which are guided by the geomagnetic field. Magnetotacticum gryphiswaldense (Figure 1 top) are bacteria of length 6.5 µm and diameter 0.7 µm that contain membrane-encapsulated vesicles (magnetosomes) filled with 40-100 nm Fe 3 O 4 par- ticles. 7 The vesicles are aligned along the long axis of the bacterium. The magnetic moment of the bacteria 8 is of the order m ≈ 1.5 × 10 -15 Am 2 . The bacterium is propelled forward by flagella with a typical power 9 of P ≈ 10 4 k B Ts -1 . An oxygen concentration dependent forward or backward motion together with their magnetic nanocompass helps these bacteria to find their favored environment, the oxic/anoxic boundary layer 10 in the sediment at the bottom of lakes and rivers. Synthetic Autonomous Magnetic Navigators. With our catalytic nanorods, we try to both mimic and vary the magnetic properties found in magnetotactic bacteria, but we use an entirely different propulsion mechanism. Three different structures were investigated for cylindrical nanorods. 2 Paramagnetic nanorods (type P nanorods, Figure 1) of diameter 2r ) 400 nm and length l ) 2.3 µm are subdivided into two segments, one consisting of gold l Au ) 0.9 µm followed by another consisting of polypyrrole l PP ) 1.4 µm. The polypyrrole segment contains 5-20 nm diameter Fe 3 O 4 particles. 11 Because of their smaller size as compared to the magnetite in magnetotactic bacteria, * Corresponding author. E-mail: tfischer@chem.fsu.edu. ² The Florida State University. ‡ The Pennsylvania State University. § University of Oslo. 3607 J. Phys. Chem. C 2007, 111, 3607-3613 10.1021/jp067304d CCC: $37.00 © 2007 American Chemical Society Published on Web 02/10/2007