Article pubs.acs.org/Langmuir
Magnetotactic Artificial Self-Propelled Nanojets Guanjia Zhao and Martin Pumera* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore S Supporting Information *
ABSTRACT: Self-propelled catalytic bubble-ejecting nanotubes (nanojets) are expected to perform a variety of autonomous tasks. Herein, we will show that with the introduction of a Ni segment into the Au/Ni/Pt nanotube design followed by consequent magnetization a permanent change in the magnetic domain orientation of the Ni segment can be induced. Consequently, this results in the presence of a permanent magnet within the nanojet with its north/south domains oriented along the tube axis. Such a magnetized nanojet orients itself according to the external magnetic field and propels itself toward or away from the source of the magnetic field depending on its orientation. This behavior is similar to that of the magnetotactic bacteria. The ability to sense the magnetic field is expected to have a strong impact on future applications of autonomous self-propelled nanojets.
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magnetic field, light, or chemicals, and the resulting behavior is correspondingly called thermotaxis, magnetotaxis, phototaxis, or chemotaxis. To date, there are only a few examples of tactic behavior of artificial self-propelled objects. These include the chemotaxis or magnetotaxis of multisegmented metallic nanomotors.32−34 The term “taxis” means that these nanodevices can sense the chemical or magnetic gradient and proceed to move toward or away from the source of this gradient. Chemotaxis is well known in nature because organisms can sense chemicals, and magnetotaxis has been observed in magnetotactic bacteria (i.e., Magnetospirillum magnetotacticum). These bacteria contain Fe3O4 crystals with magnetic domains and can orient themselves according to the external magnetic field. In fact, these bacteria can still orient themselves according to the external magnetic field even when they are dead, acting similarly to a compass needle.35−37 Going back to the nanomotors, magnetotaxis was shown on rod-based self-electrophoretically propelled devices.15,38 Bubbleejection-based micrometer-sized devices underwent dramatic development in the past few years,20−29 and a new subcategory of such devices in the submicrometer range (nanojet engines) has recently emerged.39,40 However, magnetotaxis has not been studied on these bubble-ejection-propelled nanomotors so far. In our previous work, it was demonstrated that rolled-up microsized jet engines that contain Pt, Fe, and Au metal layers can be magnetized, and the resulting orientation of the magnetic field of the magnetized devices is parallel to its longitudinal axis. It is important to highlight that microjets fabricated by rolled-up
INTRODUCTION Self-propelled microsystems and nanosystems are at the center of research interest in nanotechnology.1−6 It is envisioned that these nanodevices and microdevices will be employed in various tasks such as biomedical applications, resource discovery, and environmental cleanup.7−10 Two key properties of such systems are self-propulsion and autonomous navigation. Self-propulsion can be achieved in various ways for both the microsized and nanosized devices, and external energy sources are typically utilized, such as the conversion of chemicals presented in the solution11,12 or the electromagnetic field.4 The propulsion of the micro/nanoobjects via the transformation of the chemicals in the solution can take place by several routes: (i) selfelectrophoresis, (ii) bubble ejection (jet mechanism), or (iii) selfdiffusiophoresis. (i) The self-electrophoretic mechanism is based on the reduction/oxidation of fuel (most often hydrogen peroxide but also hydrazine) on opposite parts of the micro/ nanoobject. Because of the difference in chemical potentials of the object, oxidation and reduction reactions of the fuel take place at the opposite segments of the object and electrons flow within it. The flow of electrons within the material is compensated for by the flow of hydronium ions on the surface of the object, leading to its self-propulsion.11−19 (ii) The bubble jet mechanism is based on the catalytic conversion of fuel (typically H2O2) to oxygen gas that is asymmetrically released from the device, propelling itself forward.20,29 (iii) The selfdiffusiophoresis is based on the site-specific conversion/release of the “fuel”, which creates an asymmetric concentration gradient in the vicinity of the micro/nanomotor and it in turn leads to the asymmetric diffusion-based propulsion of the object.30−32 In addition to “self-propulsion”, the key element for the operation of these devices is its autonomy. The devices should be able to sense and act in response to its environment. Examples of such a response can be due to changes in the temperature, © XXXX American Chemical Society
Special Issue: Interfacial Nanoarchitectonics Received: September 19, 2012 Revised: October 5, 2012
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technology have a “Swiss roll” structure, and thus the iron segment is presented along the whole device.41 In this article, we wish to follow up on our previous work on magnetotactic microjets,41 and the major differences are 2-fold. First, the question is whether it is possible to create a permanent magnetic moment in the nanomotors. Second, because the nanojets are fabricated by electrodeposition technology, they have individual metallic segments along the axis (contrary to the Swiss roll structure of microjets in ref 41), and thus the question is whether such segments can also be magnetized axially because the direction of magnetization has a profound influence on the magnetotactic behavior of the nanojets. Here we investigate the magnetotactic behavior of bubble-ejection-based nanojets.
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EXPERIMENTAL SECTION
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METHODS
Materials. Aluminum oxide templates (AAO template) with a pore size of 200 nm diameter were purchased from Whatman (cat. no. 6809-6022, Germany). Silver conductive ink (lot no. L18U007), NiCl2·6H2O (lot no. 10169999), Ni(H2NSO3)2·4H2O (lot no. C12W070), H3BO3 (lot no. 10144053), diethylene glycol monoethyl ether acetate (99%, lot no. 10152398), NaOH, H2O2 (37%, lot no. 10151507), and sodium dodecyl sulfate (SDS) were purchased from Alfa Aesar, Singapore. Pt electrodes with 1 mm diameter and Ag/ AgCl/1 M KCl were from CH Instruments, Inc., USA. CuSO4·5H2O (98+%) and HNO3 (≥65%) were purchased from Sigma-Aldrich. The platinum plating solution (lot no. 20251001) and gold plating solution (lot no. TA10061402) were from Technic Inc., USA. Apparatus. Electrochemical deposition was carried out with a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a computer and controlled by General Purpose Electrochemical Systems version 4.9 software (Eco Chemie). The deposition procedure was conducted at room temperature (25 °C) using a three-electrode arrangement. A platinum electrode was utilized as a counter electrode, and Ag/AgCl was used as a reference electrode. The ultrasonication process was carried out with a Fisherbrand FB 11203 ultrasonicator, and centrifugation was carried out with a Beckman Coulter Allegra 64R centrifuge. Scanning electron microscopy (SEM/ EDX) analysis was obtained with a JEOL JSM 7600F instrument. Optical microscope videos and images were obtained with a Nikon Eclipse TE 2000-E microscope. Video sequences (100 fps) were processed with Nikon NIS-Elements software.
Synthesis and Characterizations of Au/Ni/Pt Magnetic Nanotubes. The Au/Ni/Pt magnetic nanotubes were synthesized with an improved electrochemical deposition procedure on an aluminum oxide template. Silver conductive ink was applied to the branched side of the AAO template with commercial cotton swabs. A piece of flattened aluminum foil was attached to the ink immediately, which serves as the working electrode. The template was assembled into a customized electrochemical deposition cell. A platinum counter electrode and Ag/AgCl reference electrode were utilized. Electrochemical deposition was carried out with a μAutolab type III electrochemical analyzer connected to a computer and controlled by General Purpose Electrochemical Systems version 4.9 software. The template was rinsed with 5 mL of ultrapure water (18.2 MΩ cm) four times, and a Cu sacrificial layer was deposited galvanostatically at −10 mA for 900 s. The deposition solution contained 1 M CuSO4. Consequently, after removing the solution, the template was rinsed five times with 8 mL of water. Platinum, nickel, and gold segments were subsequently electrodeposited. Pt and Au were deposited at −5 mA for 2700 s each using the commercial plating solutions. The Ni segment was also deposited at −5 mA for 2700 s with a nickel plating solution42 (20 g L−1 NiCl2·6H2O, 515 g L−1 Ni(H2NSO3)2·4H2O, and 20 g L−1 H3BO3 (buffered to pH 3.4)). When the nanotube deposition was finished, the electrochemical cell was disassembled and the template was washed five times with 8 mL of water each. After that, the template was ultrasonicated three times in 2 mL of diethylene glycol
Figure 1. Microscopic characterization of Au/Ni/Pt nanojets. (A) Scanning electron microscopy (SEM) image of the Au/Ni/Pt nanojet. (B) SEM/EDX elemental analysis of the Au/Ni/Pt nanojet with an indication of the elemental composition. (C) Optical microscopy of the Au/Ni/Pt nanojet showing the difference in reflected light from different segments of the nanojet. monoethyl ether acetate for 60 s each time. The silver ink layer was removed completely during the sonication procedure. The sacrificial copper layer was removed by the mechanical polishing of the surface using cotton swabs soaked with 6.5% HNO3. After being washed with water, the template was placed in an Eppendorf tube with 2 mL of NaOH and ultrasonicated until no shards remained. The solution was washed and centrifuged 10 times at 1500 rpm for 1 min with 2 mL of B
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Figure 2. Movement of magnetized nanojets in (A) the presence of fuel (H2O2) and (B) the absence of fuel. Also shown is the orientation of the magnetized microjet changes with reversing/turning polarity of the external magnetic field, leading to a reversing of the movement. The red/blue bar in the vicinity of nanojet indicates the magnetic polarity of the nanojet. Note that the schematic of the external magnet is inserted only in order to explain the orientation of the external magnet; the distance of the external magnet from the nanojet was ∼3 cm. Conditions: (A) 9 wt % hydrogen peroxide with 1 wt % SDS; (B) distilled water with 1 wt % SDS. water to remove the salt impurities completely. The final aqueous solutions of nanotubes were stored at room temperature. Nanojet Operations. The motion of the nanojet engines was investigated in an aqueous solution containing 9 wt % hydrogen peroxide at constant surfactant concentrations (1 wt % of SDS) in a Petri dish (3.4 cm diameter). A magnetic field was applied by placing a permanent magnet about 3 cm from the Petri dish in the horizontal plane. To reverse the magnetic field, the permanent magnet was rotated 180° at the same distance. The neodymium permanent magnet (∼270 mT) was used for magnetic manipulations at a distance of ∼3 cm.
different configuration from that of all microjets fabricated either by rolled-up technology or electrochemical deposition.9,13,20,21,39 In these systems, the tubular jet engine interior is made up of one uniform Pt element. In our case, the element consists of ∼1/3 the length located at the end of the nanojets. We found out (as seen in Figure 2, A) that even at this configuration where only the small segment is catalytic, the jet engine is still able to function. The hydrogen peroxide is decomposed at the Pt end and the oxygen bubble is expelled at this end. This creates a suction force that is responsible for the intake of H2O2 at the Au end and its decomposition in the Pt segment, in a similar manner as for the tubular microjets.13,20,21 We will show here for the first time that the segment of a nanojet engine can be magnetized and that such a nanojet responds to an external magnetic field in a similar manner as that of the magnetotactic bacteria. Figure 2A,a (and corresponding Video S1) shows that the movement of the magnetized nanojet does not exhibit any particular directionality in the absence of an external magnetic field. However, when the magnetized nanojet is exposed to an external magnetic field, it will orient itself to align with the magnetic field. In Figure 2A,b, it can be seen that the nanojet is aligned such that the Pt end is facing toward the magnet and the nanojet moves in the direction that is away from the external permanent magnet. When the external magnet is rotated and the orientation of the external magnetic field is switched (Figure 2A,c), the magnetized nanojet moves toward the external magnet. When the external magnetic field changes direction again (Figure 2A,d), the magnetized bubble-propelled nanojet reorients accordingly and moves again in a direction away from the source of the magnetic field. The reversing of directionalities can be repeated as long as motion persists. When the nanojet is running against the magnet, the bubblepropulsion force overcomes the magnetic attraction force, leading to a smaller net driving force, which in turn generates a slower velocity (∼30 μm s−1 on average). When the nanojet is running toward the magnet, the combination of the bubble-propulsion force and magnetic-attraction force allowed the nanojet to attain a much higher velocity (∼88 μm s−1 on average). As mentioned above, the magnetotactic bacteria reorients according to the external magnetic field even if it is dead, and this is due to the presence of magnetite nanoparticles inside its
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RESULTS AND DISCUSSION The trimetallic tubular nanojets were fabricated with the electrodeposition method as described in the Experimental Section.40 These jets consists of three segments: a Pt segment that is responsible for the catalytic decomposition of hydrogen peroxide and bubble propulsion, a Ni segment available for magnetization, and a Au segment for possible future functionalizations. The diameter of the nanojet is ∼300 nm, and it has a typical length of 5 μm. Figure 1 shows the scanning electron (SEM) image that provides the morphology of the nanojets, an energy-dispersive X-ray spectrum (EDX) giving the elemental breakdown, and a microscopic analysis of the individual nanojet engine. From the SEM image (Figure 1A), one can distinguish the different contrast of the various metallic components of the nanojet with similar lengths. SEM/EDX analysis (Figure 1B) was carried out to identify the composition of our nanojet. As indicated in Figure 1B, the Ni segment (green color) is electroplated between the Au segment (red color) and the Pt segment (blue color). From the optical image (Figure 1C), it is also possible to recognize the three different metallic segments on the basis of different reflectivity. The trimetallic nanojets were dispersed in water and exposed to a magnetic field of 270 mT for 30 s. This exposure resulted in the reorientation of the magnetic domains in the Ni segment and its magnetization. After the magnetization, the Ni segment behaves as a small magnet, having north and south poles. Because the Ni segment has a larger dimension along the axis of the nanojet than across its axis, the north/south pole is oriented along the nanotube axis.43 As shown Figure 1, the catalytically active metal (Pt) consists of only one segment in the Au/Ni/Pt jet; Ni and Au do not catalytically generate O2 bubbles when immersed in a solution containing H2O2. This is a significantly C
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body.37 We wish to demonstrate here that the magnetized nanojet behaves similarly. We expose the magnetized nanojet in the absence of fuel to the external magnetic field as shown in Figure 2B and in corresponding Video S2. Figure 2B,a shows that in the absence of the external magnetic field, the nanojet remains stationary. However, when an external magnetic field is present, the nanojet will reorient itself to face the magnet (Figure 2B,b). When the external magnet is rotated, the nanojet reorients (Figure 2B,c) and ultimately, when the external magnetic field reverses its direction (Figure 2B, d), the orientation of the nanojet changes by 180°. No self-propulsion is observed because no fuel is present.
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CONCLUSIONS
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ASSOCIATED CONTENT
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We have demonstrated that self-propelled nanojet tubelike engines containing Ni segments can be magnetized so that they can behave like small magnets, having north and south magnetic poles along the longitudinal axis of the nanotube. The magnetized nanojets move randomly in the presence of fuel and in the absence of an external magnetic field. However, when an external magnetic field is present, the nanojets will align according to the external magnetic field, propelling themselves either toward or away from the source of the magnetic field. The magnetotaxis of self-propelled nanojet engines is an important feature for the future applications of these devices. S Supporting Information *
Videos S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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