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Manganese Oxide-Based Chemically Powered Micromotors Muhammad Safdar,† Owies M. Wani,† and Janne Jan̈ is* Department of Chemistry, University of Eastern Finland, FI-80101 Joensuu, Finland S Supporting Information *
ABSTRACT: Chemically powered micromotors represent an exciting research area in nanotechnology. Such artificial devices are typically driven by catalytic bubble formation, taking place at the solid−liquid interface. Platinum has been most frequently used for the fabrication of different micromotors due to its superior catalytic efficiency. Other materials typically suffer from slow speeds and require very high concentrations of chemical fuel. Here, we report preparation and characterization of fast moving micromotors based on manganese oxide (MnO2) with different geometrical shapes (tubes, rods, and spheres). On the basis of the results, the prepared micromotors reached the highest speeds (up to ∼900 μm s−1 in 10% H2O2) reported to date for any MnO2-based micromotors. Moreover, they moved by bubble propulsion even at very low concentrations of peroxide fuel. Thus, MnO2 represents a promising material for the preparation of micromotors for various biomedical or environmental applications, where high speeds are desired. KEYWORDS: micromotors, nanomotors, manganese oxide, template fabrication
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particles were reported to exhibit a similar kind of motion as Ptbased micromotors, but required very high concentrations of fuel to gain high speeds (ca. 120 um s−1 in 21% H2O2).22 MnO2 catalyzes the decomposition of hydrogen peroxide (H2O2) into oxygen gas (bubbles), which induces motion of the particles. In another report, MnOx/graphene crumples were synthesized via an evaporation-induced self-assembly and ultrasonic spray pyrolysis method from the graphene oxide sheets and potassium permanganate.23 The resulting MnOx based micromotors moved by bubble propulsion in the presence of H2O2. MnO2/graphene sandwichlike nanocomposite structures also moved with an average speed up to 111 μm s−1 in the presence of 2.5% fuel and had a lifetime of just over 2 min.24 However, practical biomedical or environmental implementation of MnO2 based micromotors will require high speed motion, operation in lowest possible concentration of the fuel and sufficiently longer operational lifetime. Hence, substantial research efforts are required to enhance their performance (speed, lifetime) by exploiting different geometrical designs, sizes, surface morphologies, and composition. Here, we explored novel strategies for the fabrication of high-speed MnO2 micromotors of different shapes. These included (1) electrochemical synthesis of MnO2 microrods and poly(3,4ethylenedioxythiophene) (PEDOT)/MnO2 microtubes, as well as (2) bulk synthesis of MnO2 based microspheres with a manganese carbonate core (MnO2@MnCO3). The prepared micromotors were characterized and their performance was studied with respect to the device design and the fuel (H2O2) concentration used.
hemically powered artificial micromotors constitute an exciting research area in materials science and nanotechnology. There have been considerable efforts to explore different materials and device designs for micromotors capable of converting chemical energy into mechanical force. The resulting motion can be utilized for cargo delivery, detoxification, or motion-based sensing.1−3 Different catalytic materials, such as platinum, iridium, and silver, have been tested for their performance.2,4−6 Platinum has been extensively used for the fabrication of chemically powered micromotors.7−10 Various geometries, such as microtubes, nanowires, microcaps, nanoshells, and Janus particles, have been prepared containing a thin Pt layer serving as a catalyst to produce gas bubbles or a chemical gradient, necessary to provide a driving force for motion.11−15 Pt-based micromotors show superior catalytic activity and can reach very high speeds but they also suffer from some serious limitations. Pt is a highly expensive rare earth metal, which makes its widespread use economically less viable. In addition, some small molecules, like blood metabolites, proteins, and salt tend to deactivate Pt micromotors.16−18 This seriously limits their use in physiological media. Hence, there is a growing need to explore new low-cost materials for micromotors. Silver is another catalytic material suitable for micromotors but its aquatic toxicity may be a concern in some applications.2,19 Enzyme-based micromotors may be a good alternate in some applications, but they typically suffer from short lifetimes because of rapid deactivation of the enzyme.20 Manganese oxide (MnO2) is another efficient catalytic material, which is easy to synthesize, significantly low cost, chemically stable, and nontoxic. Unlike Pt, which is a malleable nanocrystalline material, MnO2 is relatively amorphous, and can undergo degradation in the presence of dilute acid into harmless Mn2+ ions.21 Recently, commercial MnO2 micro© XXXX American Chemical Society
Received: September 17, 2015 Accepted: November 9, 2015
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DOI: 10.1021/acsami.5b08789 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
μm s−1 (Figure 2). The microtubes and microrods slowed down after a few minutes and the bubble formation eventually disappeared. A careful examination of recorded videos suggested that the electrodeposited MnO2 undergoes selfdestruction during the catalytic decomposition of H2O2. This was difficult to visualize in the case of microtubes due to the external PEDOT layer, but for the microrods, we observed an intense tail of bubbles, generated by MnO2 microrod, diminished in few minutes. The lifetimes of the microtubes and microrods were up to 3 and 13 min, respectively, which is merely dictated by the quantity of MnO2 deposited. The synthesis of the spherical MnO2@MnCO3 micromotors was performed by following a bulk synthesis approach (see the Experimental Section for details). For this purpose, manganese carbonate (MnCO3) microparticles as template were synthesized first and subsequently oxidized using potassium permanganate (KMnO4), to form an outer shell of MnO2. However, MnCO3 was left undissolved afterward. The thickness of the MnO2 shell can be tuned by the concentration of the oxidant or the reaction time. The SEM image of the microspheres is presented in Figure 1, whereas EDX mapping was performed to determine elemental distribution (Figure S3). The surface of MnO2@MnCO3 microparticles contains a nanostructured MnO2 (Figure 1, bottom right). The average particle size was ∼5 μm, as estimated by SEM. To study their motion, we chose 1% H2O2 fuel concentration to start with and added 0.5% sodium dodecyl sulfate (SDS) as surfactant. Initially, the bubble formation was accompanied by a slow random motion, which gradually increased to 142 ± 27 μm s−1. A gradual acceleration of the MnO2@MnCO3 micromotors is expected to be due to slow wetting of MnO2 nanostructures. When the concentration of fuel was increased to 2%, the micromotors exhibited a more rapid motion with intense tails of bubbles. The average speed from seven individually tracked micromotors was recorded to be ∼250 ± 30 μm s−1 (∼50 body lengths s−1). A progressive increase in the average speed was recorded as the concentration of fuel was further increased (Figure 3). At 10% fuel concentration, the speed reached ∼665 μm s−1 on average, with the maximum speed of an individual motor tracked to be ∼900 μm s−1. To the best of our knowledge, these are the highest speeds observed to date for any catalytic micromotors based on a material other than Pt, which could be propelled by H2O2 as fuel. These speeds are remarkably higher than the results reported recently, i.e., for commercial MnO2 particles and MnO2/ graphene micromotors.22,24 There can be more than one reasons behind this notable difference in speeds, such as (i) presence of different phases of MnO2, (ii) influence of particle size and shape, (iii) surface morphology. These variables can lead to different decomposition rates of H2O2, and a varying extent of drag force experienced by different geometrical designs. As compared to electrochemically prepared motors, MnO2@MnCO3 micromotors can operate for up to 40 min in 1% fuel. Increased concentration of fuel leads to faster motion but decreases overall lifetime, e.g., 15 min in the presence of 5% H2O2. This implies that the degradation rate is fast in the presence of higher concentration of H2O2, which results in shorter lifetime. A possible reason behind the difference in speed and operational lifetime of electrochemically prepared and MnO2@MnCO3 micromotors can be the differences in phase forms and composition. On the basis of the literature, electrochemical synthesis of manganese oxide results in a mixture of different phases such as MnO, MnO2, Mn3O4.25 On
The MnO2 microrods and PEDOT/MnO2 microtubes were synthesized by a template-assisted electrochemical deposition method. The method requires a porous membrane template (polycarbonate or alumina). The dimensions of micromotors are dictated by the pore sizes of the membrane. Because the membranes are nonconductors, a thin conductive layer of Pt or gold has to be applied on one side of the membrane, serving as a working electrode. The metal film can be deposited by using a simple sputter coater and the membrane is then assembled into an electrochemical cell. Description of the cell design can be found in Figure S1. Scheme 1 depicts the steps involved in the fabrication of the MnO2 micromotors in this study. Scheme 1. Schematic Illustration of (A) Electrochemical Preparation of the PEDOT/MnO2 Microtubes and MnO2 Microrods by a Template-Assisted Electrochemical Deposition, (B) Synthesis of MnO2@MnCO3 Microspheres
If deposited without a conductive support material, MnO2 tends to form solid microrods within the membrane template. To obtain a tubular geometry, we first electropolymerized the supporting PEDOT layer, onto which MnO2 was subsequently deposited. The morphology of the prepared micromotors was analyzed by scanning electron microscopy (SEM) (Figure 1). The elemental composition was confirmed by performing energy dispersive X-ray spectroscopy (EDX) (Figure S2 A, C). The amount of charge needed for the deposition of PEDOT and MnO2 was carefully optimized to obtain smooth tubular shapes. Often, a slightly excessive charge led to the asymmetric overgrowth of MnO2, forming mushroomlike structures at the rear end of the growing tubes, without blocking the pore openings (Figure S2B). In such a case, gently polishing the membrane template from the top using alumina slurry could remove the overgrown MnO2, without damaging the microtubular structures. A further increase in charge led to the complete blockage of the openings. The motion of the electrochemically prepared microtubes and microrods was then studied in the presence of different concentrations of H2O2. For each experiment, surfactant was added to facilitate bubble formation. At low fuel concentrations (
