Undulatory locomotion of magnetic multi-link nanoswimmers Bumjin Jang, Emiliya Gutman, Nicolai Stucki, Benedikt F. Seitz, Pedro D. Wendel-García, Taylor Newton, Juho Pokki, Olgaç Ergeneman, Salvador Pané, Yizhar Or, and Bradley J. Nelson*, Institute of Robotics and Intelligent Systems, ETH Zurich, Zurich, CH-8092, Switzerland Faculty of Mechanical Engineering, Technion Israel Institute of Technology, Israel Table of contents S1 Experimental procedures S2 Movies of 1-, 2-, and 3-link nanoswimmers References 1
S1. Experimental procedures Fabrication of 1-, 2- and 3-link nanoswimmers was achieved by adapting the fabrication concept of Ozin and co-workers 1. Our substantial contribution to this fabrication approach consisted of developing a flexible tail made of polypyrrole (PPy) for 1-link swimmer as a strategy to break time-reversible motion under planar oscillatory magnetic field. Besides, we developed a robust layer-by-layer deposition protocol to increase the reproducibility for the creation of hinges. The structures we made for our 1-, 2- and 3-link nanoswimmers are Ni/Au/PPy, Ni/hinge/PPy, Ni/hinge/Ni/hinge/PPy multi-segments, respectively. Experimental details are described as follows. Electrodeposition of multisegmented nanowires. Multisegmented nanowires (MSNWs) were fabricated by electrochemical deposition in the pores (200 nm in diameter) of a commercially available anodic aluminum oxide (AAO) template (Anodisk 25, Whatman). The deposition was achieved through a multistep process described below. In the first step, a 350 nm-thick Au layer was evaporated on the filtration side of the AAO template to serve as the working electrode. Next, the AAO template was placed on an electrically conductive Cu sheet and mounted onto a homemade holder. The holder had a 19 mm-sized circular opening so that only the portion of the Cu sheet overlaid with AAO was exposed to an electrolyte bath. The holder was displaced in the electrolyte bath parallel to a Pt counter electrode. The electrolyte baths for the deposition of each material were prepared as follows. The Ni electrolyte contained nickel (II) sulfate hexahydrate (NiSO 4 6H 2 O, 300 g L 1 ) and boric acid as a buffer agent (H 3 BO 3, 45 g L 1 ). The cyanide Au electrolyte was composed of gold (I) potassium cyanide (AuK(CN) 2, 8 g L 1 ), potassium citrate (C 6 H 5 K 3 O 7, 90 g L 1 ), citric acid 2
(C 6 H 8 O 7,90 g L 1 ), and 10 ml L 1 of a brightener concentrate which contained cobalt(ii) carbonate as a grain refiner (0.05 g L 1 ). The PPy electrolyte consisted of 24.842 g L -1 of dodecylbenzene sulfonate (DBS) and 6.936 ml L -1 of pyrrole. We used DBS as an additive to fabricate negatively doped PPy segments for further layer-by-layer (LBL) deposition. Prior to the deposition of the MSNWs that constituted the body of our swimmers, an Au plug layer was grown at a DC current density of 2 ma cm 2, a ph of 3.5, and a temperature of 35 C. The plug layer was necessary to increase the cathode efficiency for the further deposition by avoiding leakage of electrolytes through the openings of the pores. Three different MSNWs were used in the construction of the swimmer bodies. The polymer PPy was used to synthesize high aspect-ratio filaments that served as the swimmers tails. We chose this material for its advantages in terms of fabrication and manipulation. These benefits are detailed in the following paragraphs. First, to fabricate a hinge between the tail segment and the body segments of 2- and 3-link swimmers, polymers were deposited using a layer-by-layer process so as to fully cover the surface of the swimmers, including Ni body segments, thiol-functionalized Au sacrificial segments and the PPy tail segments. To achieve this, the surface charge of the tail segments must possess either a thiol-functionalizable neutral surface charge (similar to the Au segments) or a negative surface charge (like the Ni body segments). In this way, an electrostatic force is generated that binds the positively charged polymers (PAH) onto the surface of the swimmers when the first layer of the layer-by-layer process is applied. Second, for swimmers operating in low Reynolds number environments, non-time-reversible motion is essential for creating net displacement over each stroke cycle. 2 We hypothesize that when a magnetically susceptible nanostructure is subjected to a planar oscillatory magnetic field, a flexible tail may help to break 3
the time-reversibility through time-dependent deformations due to interactions with its viscous surroundings. Therefore, we chose PPy to fabricate the tail for its tunable surface charge (modified through the application of DBS additives during electrodeposition) and its mechanical flexibility. 3, 4 For 1- and 2-link swimmers, we fabricated Ni, Au and PPy MSNWs of lengths 5 µm, 1.5 µm and 9 µm, respectively. 2-link swimmers were constructed with Ni-Au-PPy topology, whereas 1- link swimmers were of PPy topology. Our 3-link swimmers were fabricated with Ni-Au-Ni-Au- PPy topology, and link lengths of 1.75 µm, 1.5 µm, 1.75 µm, 1.5 µm and 9 µm, respectively. Deposition conditions for Ni, Au and PPy were as follows. Ni segments were pulse-deposited at a pulse current density of -75 ma cm -2 with a fixed on time of 8 ms, a rest time of 600 ms, and a ph of 5 at room temperature. PPy segments were galvanostatically deposited at a DC current density of 1 ma cm -2 at room temperature. The same conditions for the Au plug layers were also used for the deposition of the Au segments. Stirring was kept constant at 250 rpm during the entire electrodeposition process. The overall length for 1-, 2- and 3-link swimmers was approximately limited to 15.5 µm to control for the effects of body length on swimming performance. Liberation of MSNWs for the preparation of layer-by-layer deposition. Following deposition, the MSNWs must be liberated from the AAO template. To this end, a positive photoresist (AZ1512) was spin-coated on the top of the AAO template, and subsequently softbaked for 120 seconds at 120 C. The spin-coated template was then immersed in Au etchant for 10 min at room temperature to remove the unwanted Au plug layer. During this etching step the spin-coated layer of photoresist prevented the etchant from diffusing into the network of PPy 4
segments and etching the adjacent Au segments. After etching the Au plugs, the spin-coated layer was removed in acetone under sonication over 10 minutes, and rinsed with DI water to remove any organic residuals. Next, the swimmer bodies were etched from the cleaned AAO template in a 5 M NaOH solution for 10 min at room temperature. The liberated MSNWs were cleaned three times each with DI water and ethanol. A magnetic bar was used to collect MSNWs during this cleaning process. Layer-by-Layer (LBL) deposition of polyallylamine hydrochloride (PAH) and polystyrene sulfonate (PSS). At this stage in the process, the Ni and PPy links of the nanoswimmers were fully formed. First, the ethanol solvent was discarded and completely evaporated in N 2 ambient, leaving only MSNWs. The MSNWs were then re-suspended in 1 mm mercaptoethanesulfonic acid (MESA) solution and continuously shaken for 40 minutes to render negative surface charges on the Au surfaces. LBL deposition was performed by suspending and shaking MSNWs alternatively in 1 mg/ml PAH and 0.5 M NaCl, and 1 mg/ml PSS and 0.5 M NaCl solutions each for 30 minutes. This process was repeated for three cycles and four cycles to create three and 4 bilayers of PAH/PSS for the 3-link swimmer and 2-link swimmer, respectively. It is worth noting that the PAH and PSS LBL processes were carried out after a thorough rinse with DI water to prevent any unwanted reactions generated by polymer residuals. All the solutions were prepared with DI water and used without further purification. 5
The formation of 1-, 2- and 3-link swimmers. 1-link swimmers were obtained after liberation Ni/Au/PPy from the AAO template; no further LBL process was conducted. The 2-link and 3-link swimmers were obtained by etching the Au segments in Au etchant for five minutes after the LBL process. Finally, all swimmers were cleaned with DI water several times. Manipulation. To investigate their swimming behavior, the swimmers were immersed in 65 % glycerol solution and placed in a test chamber comprising an acrylic plate with a cylindrical hole 5mm in diameter and 2mm in height, covered by cover glass panels on each side. The chamber was placed in the center of two electromagnetic coil pairs each wound on a soft magnetic core. By dynamically varying the magnetic field produced by these coil pairs, we were able to actuate our swimming nanostructures. An oscillating magnetic field was achieved by superimposing two sinusoidal fields on x and y; B x = B A cos(θ sin(2πft)), B y = B A sin(θ sin(2πft)). The three input parameters relevant to the manipulation were the field strength (B A ), oscillation angle (θ) and frequency (f). S2. Movies of 1-, 2-, and 3-link nanoswimmers All optical microscopy videos were taken at 50 Hz with 50X objective lens. Movie S1. The dynamic motion of 1-link nanoswimmer Movie S2. The dynamic motion of 2-link nanoswimmer Movie S3. The dynamic motion of 3-link nanoswimmer Movie S4. The propulsion of 1-link nanoswimmer at the maximum speed 6
Movie S5. The propulsion of 2-link nanoswimmer at the maximum speed Movie S6. The propulsion of 3-link nanoswimmer at the maximum speed References (1) Mirkovic, T.; Foo, M. L.; Arsenault, A. C.; Fournier-Bidoz, S.; Zacharia, N. S.; Ozin, G. A. Nat. Nanotechnol. 2007, 2, 565. (2) Purcell, E. M. Am. J. Phys. 1977, 45, 3. (3) Pedrosa, V. A.; Luo, X.; Burdick, J.; Wang, J. Small 2008, 4, 738. (4) Maw, S.; Smela, E.; Yoshida, K.; Sommer-Larsen, P.; Stein, R. B. Sens. Actuators, A 2001, 89, 175. 7