A simple method for the fabrication of nanomotors based on a gold nanosheet decorated with CoPt nanoparticles

Abstract

In this communication we present an extremely rapid, simple and template-free method for the electrochemical fabrication of CoPt/gold nanosheet motors (NSMs) via a three-step applied potential process. The oxygen-propelled NSMs can be guided by a magnetic force in an aqueous solution containing 1% hydrogen peroxide as fuel. Finally, the catalytic activity of the prepared NSMs toward the degradation of rhodamin B (RB) and methylene blue (MB) was investigated as a model for environmental remediation.

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Catalytic nanomotors, first discovered by Mallouk and Sen in 2004,¹ are nanoscale manufacturing devices that can be propelled by different mechanisms.^2,3 In recent years, an increasing amount of research in the field of nanoscience has been devoted to the synthesis of nanomotors, opening the door for new applications in nanotechnology, including drug delivery,^4–7 surgery,^8–10 bio-isolation,^11–14 chemical sensing^15–17 and environmental remediation.^18–21 Until now, various methods have been reported for the synthesis of nanomotors.^22–26 However, the fabrication process in these methods (especially template-based^{1,4,17,22,25,26} and rolled-up-based methods^8,27–29) are generally time-consuming, expensive and require operators with a high level of experience. Therefore effective, low-cost and simple methods for the synthesis of nanomotors are still desirable.

Herein, for the first time, we report a simple and rapid method for the fabrication of oxygen-propelled magnetically-guided CoPt/gold nanosheet motors that have a non-uniform structure. Furthermore, we exploit the fact that the fabricated nanomotors, in the presence of 1% hydrogen peroxide as fuel, can be magnetically guided to pollution regions for the degradation of organic pollutants such as rhodamin B (RB) and methylene blue (MB).

The characterization of the NSMs was carried out using scanning electron microscopy (SEM) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The obtained images are shown in Fig. 1. It can be seen that the prepared NSMs have a sheet structure of gold, upon which are deposited the CoPt alloy nanoparticles (Fig. 1A–C). However, it can also be seen that the prepared NSMs have different structures compared to each other. The average thickness of the gold nanosheet is approximately ∼70 nm, and the average size of the deposited CoPt alloy nanoparticles is approximately ∼ 35 nm. The elemental analysis of the obtained NSMs was carried out by EDX and the results are shown in Fig. 1D. The EDX results clearly indicate that the prepared NSMs contain 93.31 wt% gold, 4.64 wt% cobalt and 2.05 wt% platinum. Fig. S1† also shows a typical SEM image of a gold nanosheet film.

Fig. 1 FE-SEM (A and B) and SEM/EDX (C) images of prepared NSMs, (D) EDX spectra obtained for NSMs.

Remote control of nanomotors is very important. In this work, the prepared NSM was guided through a solution by a magnetic force. Fig. 2 shows a series of optical images demonstrating the route of the NSM through a solution containing 1% hydrogen peroxide as a fuel, via magnetically guided movement, with the elapse of time (from A to F in a duration of about 3 s). The corresponding movie is shown in the ESI as Videos S1 and S2.† As shown in these videos, the production of oxygen bubbles from the surface of the NSM led to its movement in solution (Video S1,† Fig. 2A–C). After applying an external magnetic force, the NSM quickly moved toward the magnet (Video S1,† Fig. 2D–F).

Fig. 2 Remote magnetic guidance of an oxygen-propelled NSM. The arrows show the movement of the NSM in the absence (A–C) and the presence (D–F) of a magnetic field (0.5 Tesla). The magnification of the images is 640 times. These figures are snapshots from Video S1 and S2.†

Fig. 3 displays schematically the oxygen bubble-propelled magnetically guided movement of the NSM. The presence of CoPt magnetic nanoparticles not only results in catalytic decomposition of H₂O₂ to oxygen bubbles, which makes the movement of the NSM possible, but also allows the remote control of the NSM with a magnetic field.

Fig. 3 Schematic of the movement of an oxygen-propelled magnetically guided NSM.

The time-lapse images in Fig. S2,† captured from Video S3,† display the movement of the oxygen-propelled NSM under the irradiation of a focused NIR laser in 0.1% (v/v) H₂O₂. It can be seen that the increase in temperature surrounding the NSM due to NIR irradiation rapidly induces higher rates of catalytic decomposition of H₂O₂ to O₂, producing oxygen bubbles and a propulsion force to move the NSM accordingly. It should be noted that in the absence of the irradiation of a focused NIR laser, the catalytic decomposition rate of H₂O₂ is too low to power self-propulsion of the NSM at this concentration level.

With the aim of investigating the catalytic performance of the prepared NSMs toward the degradation of organic pollutants, we selected RB and MB as models for environmental remediation in the presence of 1% hydrogen peroxide and 0.1 M NaOH as a peroxide activator, with no external stirring. After guiding NSMs to the polluted region with a magnetic force, the color intensity of the solution containing RB and MB decreased rapidly (see Fig. S3A† for RB and Fig. S3B† for MB), due to the catalytic properties of the prepared NSMs in the degradation of the organic water pollutant components. Based on a previous report, Pt nanoparticles, along with the H₂O₂ and peroxide activator, generate HOO˙ radicals that degrade organic pollutants.³⁰ Further, the movement of the NSMs and their bubble generation contributes to the substantial fluid motion and to the accelerated decontamination process. The changes in the absorption spectra of RB at 554 nm and MB at 656 nm in aqueous solution exposed to NSMs in the presence of hydrogen peroxide are also shown in Fig. 4A and B, respectively. It can be seen that the absorption peaks for RB and MB completely disappear after degradation by NSMs over a time period of 2 h. However, in the absence of the NSMs, no appreciable degradation of the RB and MB solutions was observed after 2 h (Fig. S4†). Also, in the absence of the magnetic field the NSMs could not travel to the pollution region within 2 h (Fig. S5†).

Fig. 4 UV-vis absorption spectra of 60 ppm RB (A) and MB (B), before (a) and after (b) degradation by NSMs. The inset shows the visual color changes of the RB (A) and MB (B) before (a) and after (b) degradation.

Conclusions

This study describes a rapid and simple method for the fabrication of nanomotors. The prepared nanomotors (NSMs) can be propelled by catalytic oxidation of hydrogen peroxide to produce oxygen bubbles. Due to the magnetic and catalytic properties of the CoPt nanoparticles present, the NSMs can be guided to a polluted region of water and subsequently can degrade the corresponding pollutants. We believe that the proposed nanomotor system could provide a unique opportunity for application in the broad field of nanotechnology, such as in environmental remediation, drug delivery, surgery, bio-isolation, and chemical and bio-sensing etc.

Experimental section

Synthesis of CoPt/gold nanosheet motors

First, the surface of a gold disk electrode (3.0 mm diameter) was polished successively using 0.3, 0.1 and 0.05 μm alumina slurry, and then cleaned in ethanol and water under ultrasonication. The freshly polished gold electrode was anodized at 6 V (versus Ag|AgCl|saturated KCl) in a 0.5 M phosphate buffer (PB) solution of pH 7.0 for 20 min and then reduced at −0.3 V for 5 min to obtain the gold nanofilm. Subsequently, the gold nanofilm electrode was immersed in a solution of 1 M NH₄Cl + 0.1 M H₃BO₃ at pH 4.5, containing 0.1 mM CoCl₂ and 0.02 mM Na₂PtCl₆, and then a −1.15 V potential was applied for 20 min. Finally, the obtained electrode was washed with water for 10 s, then immersed in doubly distilled water (2.0 mL) and sonicated for 5 min, in order to release the prepared CoPt/gold nanosheets with non-uniform structure into the solution. During the sonication process, the nanofilm decorated with CoPt nanoparticles broke into nanosheets with different structures. Finally, the solution was centrifuged at 2000 rpm for 5 min, and the products separated for subsequent studies.

Apparatus

Scanning electron microscopy (SEM) was performed on a Philips instrument, Model XL-30 with an accelerating voltage of 26 kV. A neodymium (NdFeB) magnet (0.5 Tesla) was used for magnetic control of the movement of the nanomotor. UV-visible absorption spectra were recorded using a single beam Agillan UV-vis spectrophotometer (model G1103A). The electrochemical experiments were performed using the μAutolab electrochemical system (Eco-Chemie, Utrecht, The Netherlands) equipped with GPES and FRA 4.9 software. A three-electrode assembled cell was employed, consisting of the gold disk electrode (3.0 mm diameter) as the working electrode, a platinum wire as a counter electrode and a Ag|AgCl|saturated KCl electrode as the reference electrode. Optical microscope videos (Videos S1–S3†) and images (Fig. 3 and S2†) were obtained with an Olympus IX71 microscope coupled to an Olympus DP72 digital camera. The NSM movement was tracked using Cellsens Olympus image software. The free software XiLi 3GP (video converter) was used to edit the video. A near infrared (NIR) laser beam at 780 nm with a power of 200 mW was used. All reagents were of analytical grade.

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Footnote

† Electronic supplementary information (ESI) available: Figures and Videos. See DOI: 10.1039/c5ra08552g

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