MANDAL ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Independent Positioning of Magnetic Nanomotors Pranay Mandal, * ,† Vaishali Chopra, † and Ambarish Ghosh * ,†,‡,§ † Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India, ‡ Department of Physics, Indian Institute of Science, Bangalore 560012, India, and § Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore 560012, India P recise, fuel-free and remote manip- ulation 1À4 of nanoscale objects in flui- dic media, including biological tissues and organs, can revolutionize 5 various as- pects of nanomedicine, 6À9 such as micro- surgery, in vivo sensing and drug delivery. 10 The most popular method of micromanipu- lation in fluidic environments is to use an optical tweezer, 11,12 which allows multiple objects to be maneuvered and positioned independently. In spite of the tremendous success of the optical tweezer in aiding various biological 13 measurements, the technique requires intense laser beams and close proximity to a focusing lens, and therefore not useful for remote operations in living systems. Furthermore, the manip- ulation technique only works well for di- electric objects larger than few hundred nanometers, although there are current ef- forts toward trapping metallic 14 particles of smaller dimensions. Some of these limita- tions do not exist when magnetic fields are used to maneuver small objects, since most living systems are compatible with strong magnetic fields, and the method works naturally with metals. Similar to optical tweezers, conventional magnetic manipula- tion 15 mostly relies on gradient 16 forces, as a result of which para- and ferromagnetic objects move toward the poles of a perma- nent magnet. The strength of the force scales with volume of the magnetic object and the gradient of the magnetic field, which renders the method useless 17 for remote manipulation of nanoscale objects. The solution is to use time varying homo- geneous magnetic fields with nanostruc- tures of various symmetries, including flexible filaments, 18 helices 19À21 and colloi- dal doublets. 22,23 For example, rotating magnetic fields are commonly used to ro- tate ferromagnetic helical nanostructures about their long axes, which due to their inherent chirality translate, similar to various flagellated bacteria. 24,25 In the proximity of a surface, even achiral structures such as magnetic nanorods 26 and colloidal dou- blets can be rotated and thereby translated, owing to the time varying surface induced drag experienced by the rotating object. This strategy of inducing translational mo- tion by time varying homogeneous mag- netic fields have been used by many groups to design and develop various nanostruc- tures, and study their motion in a wide variety of media including biologically important fluids, 27À31 such as undiluted * Address correspondence to pranaymandal13@gmail.com, ambarish@ece.iisc.ernet.in. Received for review September 29, 2014 and accepted March 30, 2015. Published online 10.1021/acsnano.5b01518 ABSTRACT There is considerable interest in powering and maneuvering nanostructures remotely in fluidic media using noninvasive fuel-free methods, for which small homogeneous magnetic fields are ideally suited. Current strategies include helical propulsion of chiral nanostructures, cilia-like motion of flexible filaments, and surface assisted translation of asymmetric colloidal doublets and magnetic nanorods, in all of which the individual structures are moved in a particular direction that is completely tied to the characteristics of the driving fields. As we show in this paper, when we use appropriate magnetic field configurations and actuation time scales, it is possible to maneuver geometrically identical nanostructures in different directions, and subsequently position them at arbitrary locations with respect to each other. The method reported here requires proximity of the nanomotors to a solid surface, and could be useful in applications that require remote and independent control over individual components in microfluidic environments. KEYWORDS: nanopropulsion . magnetic actuation . glancing angle deposition . active matter . independent control . nanomotors ARTICLE