Dual-Fuel-Driven Bactericidal Micromotor
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In this paper, we report fabrication of the bimetallic Janus microsphere, a magnesium microsphere with a silver surface coating, through thermal evaporation technique. Because of the Janus structure, this micromotor can be propelled in two different directions by the surface silver or magnesium ‘engine’ and hydrogen peroxide or water fuel. In addition, due to the bactericidal property of silver, this autonomous micromotor is capable of killing bacteria in solution. As compared to the static one, the micromotor is able to kill the bacteria at a much faster rate (about nine times of that of the static one), demonstrating the superiority of the motion one. We thus believe that the micromotor shown in the current study is potentially attractive for the environmental hygiene applications.
KeywordsMicromotor Self-propelling Bacterial killing Janus E. coli
In the past decade, self-propelling nano- or micro-motors have attracted more and more attentions [1, 2, 3, 4, 5, 6] due to their unique properties compared to the other nano/micro devices, such as their capability of achieving controlled cargo capture , transportation , release , and the more efficient delivery or faster recognition kinetics  caused by their autonomous motion process, which hold great promise in a wide range of applications, including drug delivery for biomedical field , oil droplet removal  for environmental remediation or detoxification . These tiny devices are able to harvest chemical energies from the surrounding environment and convert them into mechanical work to realize autonomous movement in a liquid environment. A variety of propulsion mechanisms have been developed in order to realize the self-propelling behavior of a micromotor, which include bubble recoil , diffusiophoresis , and interfacial tension-induced propulsion . In addition, since there is normally no control over the directionality of a moving micromotor, external fields such as magnetic field  or ultrasound  are generally applied to manipulate its moving direction. Although a variety of fuels such as hydrogen peroxide , acid , bromine, and iodine  have been developed to propel the micromotor, most of them are toxic for use in biomedical application and may even cause the environmental problems. For the purpose of protecting the environment and realizing better biocompatibility, it is desirable to develop micromotors that are environmentally friendly.
Recently, magnesium-based micromotor propelled by the magnesium and water reaction have been developed, which is capable of moving autonomously in a number of aqueous solutions, such as simulated body fluid, low concentration sodium carbonate, and sodium chloride solution [21, 22]. Because of the environmentally friendly nature of the magnesium-based micromotor, it has been demonstrated that they have potentials in a variety of biomedical or environmental applications. For example, Guan et al. have shown that the Pt/Mg bimetallic micromotor exhibits the excellent hemolytic property, which is an indication of its good biocompatibility . By capping part of the magnesium microsphere with a thermally responsive polymer, i.e., poly(N-isopropylacrylamide), a water-driven micromotor, which is capable of achieving controlled drug release upon heat treatment, has been developed . Recently, by depositing a thin layer of gold or titanium dioxide onto the surface of the magnesium microsphere, Wang et al. have developed the autonomous micromotor (Au/Mg and TiO2/Mg), which is able to collect spilled oil droplet for water cleaning (in the case of Au/Mg)  or degrading chemical or biological warfare agents for environmental remediation purpose (in the case of TiO2/Mg) . Despite these progresses, to the best of our knowledge, few studies have realized the bacteria killing utilizing the magnesium-based micromotor.
In this paper, we report the Ag/Mg bimetallic micromotor which exhibits bactericidal capability. The micromotor, which is obtained through thermal evaporation method, possesses a Janus feature and can be self-propelled in two directions based on hydrogen peroxide or water fuels. Since silver and its ions have inhibitory effects on the bacteria and can be utilized as bactericidal or antimicrobial agents, the resulting micromotor shows excellent anti-bacterial property for bacteria, such as E. coli. In addition, as compared to the static one, the moving micromotor exhibits a much faster killing rate. This can be attributed to the more silver ions released from the motion one as opposed to the static one and the motion-based solution mixing process, which cause the dissolved silver ions to reach the bacteria in a shorter time. The amount of bacteria killed in the case of the micromotor is about ninefold of that in the case of the static one, demonstrating the superiority of the autonomous micromotor. The self-propelling and fast bacterial killing property shown in this study makes current micromotor an attractive candidate for the environmental hygiene applications.
Magnesium microspheres with diameters of 20–30 μm are obtained from Tangshan Weihao Magnesium Company. Silver, 99 % purity, is obtained from Zhong Nuo Advanced Material (Beijing) Technology Company. Sodium bicarbonate is purchased from Sigma-Aldrich Company. E. coli is obtained from J&K Chemical Company. LIVE/DEAD Baclight staining kit is obtained from Molecular Probes Incorporation.
2.2 Fabrication of Ag/Mg Janus Microsphere
The Mg microsphere is first dispersed in ethanol solution with a concentration of 50 mg mL−1 under ultrasonication and then placed on a glass substrate. The substrate is then placed inside the vacuum chamber of a thermal evaporator (NANO36, Kurt J. Lesker Company), where 20-nm Ag is deposited as indicated by the quartz crystal microbalance inside the chamber. After the Ag deposition, the Mg microsphere with the Ag surface layer is released into solution through ultrasonication.
Scanning electron microscopy (SEM) images and energy-dispersive X-ray analysis (EDX) are obtained on a Carl Zeiss Supra 55 scanning electron microscope. For the autonomous movement study, the Ag/Mg micromotor is first placed in aqueous solution containing 1 M NaHCO3. The self-propelling behavior is then observed and captured by utilizing a Nikon Eclipse 80i optical microscope. The data analysis of the moving behavior of the micromotor is carried out by utilizing PhysVis software. For the catalytic micromotor, the Ag/Mg structure is placed in aqueous solution containing 3 % hydrogen peroxide. The released silver ion concentration in the solution is measured on a Varian AA240FS-GTA120 atomic absorption spectroscopy.
2.4 Anti-bacterial Activity of the Micromotor
In a typical experiment, 1 mL E. coli (1 × 1010 CFU mL−1) incubation solution is first centrifuged at 10,000 rpm. After removing the supernatant, 5 mg Ag/Mg micromotors (corresponding to ~3.5 × 105 in number) are then added to 200 μL E. coli suspension containing 1 M NaHCO3. After 10 min, the mixture is first centrifuged at 1000 rpm to remove the micromotor and then centrifuged again at 10,000 rpm to obtain the E. coli bacteria. The isolated E. coli bacteria are then mixed with 6 μM Syto-9 or 30 μM propidium iodide solution following the procedure shown in the LIVE/DEAD Baclight bacterial viability kit. After staining in the dark for 15 min, the E. coli bacteria are centrifuged at 10,000 rpm and re-dispersed in water. For the live and dead bacteria counting, 2 μL of solutions containing the bacteria is drop cast on a cover glass, which is then subject to fluorescence microscopic analysis. The captured fluorescence images are analyzed by utilizing ImageJ software.
3 Results and Discussion
In conclusion, we fabricate a Janus micromotor consisting of magnesium microsphere with asymmetric surface silver coating by thermal evaporation method. Not only can this micromotor be self-propelled by two different fuels, they are also capable of killing bacteria due to the bactericidal property originating from the silver material. Because of the solution mixing process induced by the continuous motion and the more released silver ion, the self-propelled micromotor exhibits more efficient anti-bacterial property than the static one. The features shown in the current study, such as the easy fabrication, water driven, and excellent anti-bacterial behavior, make current micromotor potentially attractive for environmental hygiene applications.
This work is supported by the National Natural Science Foundation of China (Grant No. 21304064), the Natural Science Foundation of Jiangsu Province (Grant No. BK20130292), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Fund for Excellent Creative Research Teams of Jiangsu Higher Education Institutions, and the project-sponsored by SRF for ROCS, SEM.
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