Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water

Heavy metal contamination in water is a serious risk to the public health and other life forms on earth. Current research in nanotechnology is developing new nanosystems and nanomaterials for the fast and efficient removal of pollutants and heavy metals from water. Here, we report graphene oxide-based microbots (GOx-microbots) as active self-propelled systems for the capture, transfer, and removal of a heavy metal (i.e., lead) and its subsequent recovery for recycling purposes. Microbots’ structure consists of nanosized multilayers of graphene oxide, nickel, and platinum, providing different functionalities. The outer layer of graphene oxide captures lead on the surface, and the inner layer of platinum functions as the engine decomposing hydrogen peroxide fuel for self-propulsion, while the middle layer of nickel enables external magnetic control of the microbots. Mobile GOx-microbots remove lead 10 times more efficiently than nonmotile GOx-microbots, cleaning water from 1000 ppb down to below 50 ppb in 60 min. Furthermore, after chemical detachment of lead from the surface of GOx-microbots, the microbots can be reused. Finally, we demonstrate the magnetic control of the GOx-microbots inside a microfluidic system as a proof-of-concept for automatic microbots-based system to remove and recover heavy metals.


Materials and reagents
Graphene oxide, sodium dodecyl sulfate (SDS), lead nitrate, nitrate acid, hydrochloric acid and sodium hydroxide were purchased from Sigma-Aldrich (Germany). Hydrogen peroxide 30%, potassium nitrate, methylene chloride and ethanol were purchased from Merck (Germany). Ultrapure water (Millipore Corporation, USA) was used for the preparation of all aqueous solutions.

Fabrication of graphene oxide-based multilayer microbots
The graphene oxide-based multilayer microbots were fabricated using a common template directed electrodeposition protocol. A cyclopore polycarbonate membrane, SDS to the commercial Ni solution and sonicating using an ultrasound bath for 15 min.
The first metallic layer, which uses a 1:1:1 Pt:Ni:water solution, was deposited galvanostatically at -2 mA for 300 s to provide a smooth surface and to improve the deposition of the next metallic layers. After washing three times with water, the Ni layer was deposited amperometrically at -1.0 V for 2.4 C to achieve the ferromagnetic properties that allows the microbot guidance by properly orienting the magnetic field created by a simple neodymium magnet. Finally, after other three washings, the catalytic inner Pt layer was deposited galvanostatically at -2 mA for 300 s. To release the GOxmicrobots from the template, the sputtered gold layer was completely removed by mechanical hand polishing with 5 m alumina slurry (Electron Microscopy Sciences, Hatfield, PA). The membrane was then dissolved in methylene chloride for 10 min to completely release the microtubes. Finally, the microbots were washed two times more with methylene chloride, followed by ethanol and ultrapure water, two times of each, and collected by centrifugation at 9000 rpm (Eppendorf 3409) for 3 min after each wash.

Equipment
Template electrochemical deposition of microtubes was carried out with using a potentiostat (AUT50101, Metrohm Autolab B.V. with an output power of 5 W was used as the excitation source. The spectra were collected in ranges of 100 to 4000 cm -1 with exposure time of 1 second and 10 magnification. All Raman experiments were conducted at room temperature and ambient pressures. X-ray photoelectron spectroscopy (XPS) from SPECS system (Germany) was used to identify functional groups on the GOx-microbots surface. The instrument was equipped with XR50 duel anode source (Al operated at 150W) and a Phoibos MCD-9 detector. All measurements were done under the vacuum (pressure 5x10 −9 mBar) and the hemispherical analyzer was set at the pass energy 25 eV while the high resolution spectra step size was set at 0.1 eV.
Casa XPS program (Casa Software Ltd., UK) was used for the data analysis. Inductively coupled plasma optical emission spectrometry (ICP-OES), was employed as analytical technique for the detection of Pb(II) ions. Origin Pro 9.0 and Microsoft Excel 2010 were employed for the analysis of the experimental data.

Experimental procedure
The GOx-microbots were characterized using different characterization techniques, such as, Raman, SEM and EDX. After characterization, the GOx-microbots fabricated were transferred together in a falcon tube and they were observed by using an inverted microscope for estimating the concentration of GOx-microbots in water. Once the GOx-microbots have been characterized and counted, they were used for decontamination experiments, which are carried out in a glass beaker containing total 3 ml of heavy metal polluted water consisted of 1ppm of lead (1µg/ml or 1000 ppb), hydrogen peroxide (1.5% v/v) and SDS (0.1 % w/v) at the pH (5.7). The assay was carried out by triplicate (n=3) for each time (5, 10, 30, 60 min and 24 hours) that the motors are swimming in the contaminated solution. Control experiments were also carried out by triplicate.
Swimming of microbots was recorded at different time interval by inverted microscope and tracking was done by a custom made python script which used open CV libraries. Lead concentration was measured by ICP-OES and the GOx-microbots were kept in the glass beakers using a magnet due to their magnetic properties. Then, the GOx-microbots were washed once with water and characterized by SEM and EDX analysis. After that, they were exposed for 1 hour to different treatments, such as water presence, water 2 presence under low temperature, basic pH, acid pH by using HCl (pH=1) 3 and HNO 3 (pH=0.3) 4, 5 , for the lead recovery and washed once with water. The supernatant from these different treatments were measured by ICP-OES to probe the lead content and thus, the lead recovery from the GOx-microbots. On the other hand, the GOx microbots were characterized by SEM and EDX and washed several times with