Exogenous chemically-driven electromagnets

Abstract

Magnetically-driven dynamic systems have gained considerable attention in multiple applications ranging from cargo delivery to environmental remediation. However, they commonly require ferromagnetic components or sophisticated electromagnetic equipment. In this work we take advantage of the synergy between exogenous bipolar electrochemistry and the classic geometry of a solenoid in order to design an externally driven chemo-electromagnet. By wirelessly triggering redox reactions at each extremity of a solenoid-shaped swimmer, the generated electric current follows the helical path of the coil, thus generating a concentric magnetic field in its center. Such an externally induced redox current generates magnetic fields in the range of μT which are proportional to the applied electric field. The on-board chemically induced magnetic dipole allows the swimmers to perform rotational motion in the presence of an external magnetic field, without the use of traditional ferromagnetic materials. Additionally, when exposing these devices to alternating electric and magnetic fields, well-defined oscillatory motion is produced, demonstrating the efficient electromagnetic control of the dynamic displacement. This opens up novel and, so far, unexplored possibilities for localized chemical conversion via magnetically-driven "chemistry on-the-fly".

Scheme 1. Illustration of the current produced by the wirelessly triggered redox reactions taking place at the extremities of a clockwise or anticlockwise solenoid, with a representation of the currents produced by the chemical reactions and the exogenous chemically-induced magnetic field. The blue and red colored arrows represent the north and south pole of the generated magnetic fields, respectively.

Fig. 1. (a) Schematic illustration and (b) optical pictures of the rotational displacement of a clockwise and anticlockwise coiled exogenous chemo-electromagnet in the presence of an external magnetic field, with a representation of the chemical reactions, the direction of the associated electron flow, the orientation of the induced and external magnetic fields, the external electric field, the torque force and the trajectory of the rotation. The blue and red color represent the north and south pole of the magnetic fields, respectively. Global time of the experiment was 20 seconds. (c) Average angular progression as a function of time, measured during the displacement of a clockwise (black dots) and anticlockwise (red dots) Au/Ti solenoid moving at the air/water interface of a 5 mM LiClO₄/10 μM DBS solution containing a 1 : 1 ratio of HQ/Q (10 : 10 mM) in the presence of an external magnetic field and a constant applied electric field (4.2 V cm⁻¹).

Fig. 2. (a) Angular progression as a function of time observed during the rotation of a clockwise Au/Ti solenoid moving at the air/water interface of a 5 mM LiClO₄/10 μM DBS solution containing a 1 : 1 ratio of HQ/Q (10 : 10 mM) in the presence of an external magnetic field, for different applied electric fields (indicated in the figure). (b) Maximum angular velocity of the swimmer (left axis black dots) as a function of the applied electric field and the average B_swim value estimated from amperometric measurements (right axis green dots). The red dot refers to a measurement carried out with the anticlockwise Au/Ti solenoid. The blue dots refer to the maximum angular velocity of the swimmer as a function of the applied electric field for the oxygen and hydrogen evolution reactions.

Fig. 3. (Top) Angular progression and (bottom) angular velocity as a function of time observed during the rotation of a counter clockwise Au/Ti solenoid moving at the air/water interface of a 5 mM LiClO₄/10 μM DBS solution containing a 1 : 1 ratio of HQ/Q (10 : 10 mM) in the presence of an alternating external electric (3.15 V cm⁻¹) and magnetic field (indicted in the figure).