New approaches in the design of magnetic tweezers–current magnetic tweezers Valentina Bessalova a,n , Nikolai Perov a,b , Valeria Rodionova b,c a Lomonosov Moscow State University, Leninskie Gory 1-2,119991 Moscow, Russia b Immanuel Kant Baltic Federal University, Nevskogo 14, 236004 Kaliningrad, Russia c National University of Science and Technology "MISiS", Leninsky Prospect 4,119049 Moscow, Russia article info Article history: Received 15 September 2015 Received in revised form 3 March 2016 Accepted 8 March 2016 Available online 10 March 2016 Keywords: Magnetic tweezers Magnetic particles Electrical current magnetic field abstract The main advantages of the magnetic tweezers are the low price and simplicity of use. However the range of their application is reduced due to shortcomings like, for example, the remanent induction of the core and interaction between ferromagnetic cores. We present the new design of magnetic tweezers–Current Magnetic Tweezers (CMT) that allow particle manipulation by means of the magnetic field generated by the electric currents flowing through the non-magnetic wires. Arranging wires in different geometric shapes allows the particle movement either in two or three dimensions. Forces acting on the magnetic particles with the magnetic moment of 2 Á 10 À11 А m 2 at distances up to 1 mm had been experimentally measured. It is established that a current of about 1 A at a 1 mm distance generates force of (approximately) 3 pN which is consistent with theoretical estimates. & 2016 Published by Elsevier B.V. 1. Introduction Advances in nanotechnology gave rise to the growing interest in the study of methods of manipulation of micro and nano-ob- jects. The main attention is focused on the following systems: optical tweezers [1], atomic force microscope (AFM) [2] and magnetic tweezers [3–5]. Each of them provides unique oppor- tunities to study micro objects, including their mechanical prop- erties (behavior under the influence of tension and/or torsion forces). These forces are applied either AFM, or by means of ex- ternal electromagnetic field manipulators (optical tweezers and magnetic tweezers). Let us present a brief overview of the prin- ciples of listed above methods. To manipulate a single molecule, one end of the molecule is attached to a magnetic microparticle (in the case of magnetic and optical tweezers) [6] or to the AFM cantilever [7]. The other end is then attached to the surface of a coverslip. Once the molecule is attached one can carry out the experiments and study its mechanical properties. These “single- molecule” methods are very sensitive, they allow to measure the displacement of the object down to nanometers and apply forces in the order of piconewtons. On one hand, all of the above methods are based on the same principles, on the other hand, each technique can be used to perform unique operations that are not possible with the others. Still, all these methods have limits to their application. Optical tweezers are used only for partially transparent particles. The size of such particle has to be between 0.2 and 5 μm (it cannot be smaller than the half (quarter) of the visible light or bigger than a few wavelengths). Optical tweezers manipulate objects in liquid environment [8,9]. AFM allows to manipulate atom-sized objects and observe objects up to several nanometers in size. With the help of the AFM the sample surface can be scanned and small atomic clusters can be moved. However, this method is not suitable for manipulating objects bigger than several hundred atoms [3]. Magnetic tweezers have various de- signs and are used to work with large organic molecules like DNA and RNA. They provide opportunities to study elastic properties of DNA and dynamic properties of various biological macro- molecules, for example, polymerases, helicases, topoisomerases, DNA-binding proteins [10]. In comparison with other micro- tweezers magnetic tweezers have a number of advantages: there are no restrictions on transparency and the size of particles (unlike optical tweezers) and they can work in various environments: gas, liquid, vacuum. [4]. In general, magnetic tweezers consist of the ferromagnetic cores magnetized with winded coils. However, the presence of ferromagnetic cores in existing models of magnetic tweezers causes two serious drawbacks: 1. presence of a remanent magnetic field of the core means the magnetic particle is affected by this field and the effect cannot be canceled by merely switching off the electric current in the coils; Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials http://dx.doi.org/10.1016/j.jmmm.2016.03.038 0304-8853/& 2016 Published by Elsevier B.V. n Corresponding author. Journal of Magnetism and Magnetic Materials 415 (2016) 66–71