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Zirconia/Graphene Oxide Hybrid Micromotors for Selective Capture of Nerve Agents Virendra V Singh, Aida Martin, Kevin Kaufmann, Severina D. S. de Oliveira, and Joseph Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03960 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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Chemistry of Materials

Zirconia/Graphene Oxide Hybrid Micromotors for Selective Capture of Nerve Agents Virendra V. Singh, Aida Martin, Kevin Kaufmann, Severina D. S. de Oliveira, Joseph Wang* Department of Nanoengineering, University of California San Diego, La Jolla, CA 92093 (USA)

*Correspondence to: [email protected] ABSTRACT Self-propelled

zirconia-graphene/Pt

hybrid

tubular

micromotors,

prepared

by

electrochemical template synthesis, are used for both high-affinity capture and isolation of nerve agents. The simultaneous electrochemical deposition of zirconia and reduced graphene oxide leads to high surface area with a needle-like zirconia microstructure. The attractive surface properties of graphene sheets are used as growth directing template for the electrochemical synthesis of high surface area of zirconia nanostructures for effective and selective binding of organophosphate compounds. Such selective binding is dramatically enhanced by the rapid movement of the motors and the corresponding bubble-induced solution mixing. The greatly increased fluid transport leads to a 15 fold faster remediation compared to the use of the static counterpart structures. The unique material properties allow the convenient alkaline regeneration of the micromotor surface, improving the cost effectiveness of this methodology. The new strategy provides an opportunity to develop reusable micromotors for high-affinity capture-based separation of nerve agents and can be extended to verification analysis of chemical weapon convention. Such coupling of advanced surface materials with rapidly moving micromotors holds considerable promise for diverse defense and sustainable environmental applications.

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INTRODUCTION The escalating threat of weapons of mass destruction have generated major demands for rapid, field-deployable, and cost effective methods for their destruction in a fast,

selective,

efficient

and

simple

manner.

Of

particular

concern

are

organophosphorous nerve agents due to their ability to irreversibly inhibit acetylcholinesterase activity, which lead to neuromuscular paralysis and eventually death.1-2 Although different methodologies have been used for combating chemical warfare agents (CWA), such as chemical, enzymatic, photocatalytic degradation or incineration, the majority of these methods are slow, energy intensive, and result in a strong negative environmental impact.3-7 Recent advances in chemically-powered micromotors have addressed these drawbacks and have led to attractive routes for environmental remediation of hazardous chemicals.8,9 Today’s micro and nanomotors offer attractive capabilities that hold considerable

promise

for

important

biomedical,

environmental

or

security

applications.10-16 Thus, electrochemically synthesized tubular micromotors usually prepared by depositing a conducting polymer outer layer;10,17,18 or by rolled-up approaches

19,20

and Janus particles

21,22

have been used with these purposes. Several

innovative micromotor concepts have been reported recently for the removal of pollutants,

toxic 10,13-16

micromotors.

waste

degradation, 19

Soler et al.

or

killing

of

harmful

pathogens

by

described the degradation of rhodamine 6G by

coupling the Fenton’s process in the presence of hydrogen peroxide. TiO2-based Janus particles micromotors have shown promise for effective photocatalytic degradation of biological and chemical agents.22 Orozco et al. utilized the movement and bubbleformation aspects of micromotors for accelerated decontamination via a built-in selfmixing.15 While these studies indicate the potential of synthetic micromotors for environmental remediation, new developments based on functionalizing self-propelled micromotors with advanced reactive materials are highly desired. In this present work, we advance the material science aspect of the micromotor surface chemistry to fully realize self-propelled reusable functional materials. In particular, we describe the preparation and characterization of graphene-ZrO2 hybrid reusable tubular micromotors, fabricated by simultaneous electrodeposition of ZrO22


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reduced graphene oxide (erGO)-hybrid followed by Pt deposition, for the selective removal of nerve agents from the environmental matrices. Such coupling of the remarkable selectivity of zirconia for phosphate groups with the use of nano-size high surface area graphene oxide template,23 and the dynamic movement of the composite micromotors results in a dramatically accelerated selective detoxification of CWA. Recently, graphene-based micromotors,24-28 have offered an efficient propulsion behavior. Furthermore, the chemical structure of graphene represents an attractive platform for the incorporation of other subunits to generate self-propelled graphenebased hybrid materials.29-33 In particular, erGO-based hybrid materials increase the overall surface area of the co-synthesized zirconia, in comparison with other polymermetal hybrid materials, because the graphene increases the active sites and functionality of the hybrid material.29-33 The remaining oxygen functionalities and defects in the synthesized erGO act as nucleation sites for the growth of nano-sized zirconia particles on the outer micromotor surface. The graphene layer plays also an important role for the growth of highly porous inner Pt or Au layers, and hence leads to enhanced propulsion behavior.24 Thus, the synthesis of hybrid graphene materials increases the overall surface area of the internally and externally co-deposited materials. The unique properties of zirconia, such as specific surface chemistry, chemical inertness, thermal stability,34-37 make it an attractive reactant for the efficient and preferential binding of nerve agents. The phosphate groups of the CWA share electrons with the electronically deficient zirconia via acid-base Lewis interaction, allowing the effective removal of nerve agents and enrichment of phosphopeptides,34-37 an important step towards verification analysis of chemical weapon convention.34,35 In the following sections, we will demonstrate for the first time the synthesis and application of graphene-inorganic composite micromotors. The resulting hybrid motors combine the attractive properties of graphene and zirconia particles along with efficient motor movement to offer a powerful and selective capture and isolation of nerve agents within 5 minutes (with ~15-fold enhanced efficiency compared to static conditions), without external agitation (as desired for large bodies of water). Furthermore, the new micromotor-strategy was demonstrated to be reusable via basic pH adjustment to

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remove and degrade captured nerve agent, making it an attractive nanovehicle for the effective decontamination of organophosphate (OP) nerve agents. EXPERIMENTAL SECTION Synthesis of ZrNPs/graphene micromotors. The electrosynthesis of the zirconia nanoparticles-reduced graphene oxide (ZrO2/erGO) micromotors was performed in a single step involving the simultaneous electrochemical reduction of zirconium (IV) oxychloride octahydrate, ZrOCl2, (Sigma-Aldrich, USA) and graphene oxide (GO, graphene supermarket, New York, USA) into 5 µm diameter conical pores of a polycarbonate membrane (Catalog No. 7060-2513; Whatman, Maidstone, UK). A thin gold film was first sputtered on the branched side of the membrane to serve as a working electrode. The membrane was assembled in a Teflon plating cell with aluminum foil serving as an electrical contact for the subsequent electrodeposition. Graphene oxide (GO, 0.1 mg/mL) was first dispersed in a solution containing 0.1 M H2SO4 and 0.5 M Na2SO4 by ultrasonication for 15 min. 7.5 mg ZrOCl2 were then added to the resulting homogeneously dispersed yellow-brown GO solution. The simultaneous electrochemical reduction and deposition of ZrO2 and erGO was carried out using cyclic voltammetry (CV) over the +0.2 to -1.5 V range, at 10 mV s-1, for five cycles (n=5), using Ag/AgCl (3 M) and Pt wire as reference and counter electrodes, respectively. Subsequently, a Pt inner layer was galvanostatically deposited at -2 mA for 500 s from a commercial platinum plating solution (Platinum RTP; Technic Inc, Anaheim, CA). The control polyaniline (PANI)-Pt micromotors were prepared following a similar procedure. Briefly, aniline was freshly distilled before use at a vapor temperature of 100 °C and a pressure of 13 mm Hg. PANI micromotors were electropolymerized by cyclic voltammetry (CV, +1.5 to -1.5 V vs. Ag/AgCl, 10 mV s-1, n=5) from a plating solution containing 0.1 M aniline, 0.1 M H2SO4 and 0.5 M Na2SO4. Subsequently, the inner Pt layer was deposited galvanostatically at -2 mA for 500 s. The control graphene-Pt micromotors were prepared without ZrOCl2 using the same potential window from +0.2 to -1.5 V range, at 10 mV s-1. The ZrO2/erGO-Ni-Pt micromotors were synthesized by the simultaneous electrodeposition of ZrOCl2 and GO in a first step (described before), a Pt layer was galvanostatic deposited at -2 mA for 250 s, a Ni layer was deposited at -1.3 V using a charge of 4 Coulombs (C) to achieve the 4


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magnetic properties and another Pt layer was galvanostatically deposited at -2 mA for 250 s. The nickel solution contained a mixture of 20 g/L NiCl2·6H2O, 515 g/L Ni(H2NSO3)2·4H2O, and 20 g/L H3BO3. The sputtered gold layer was gently removed by hand polishing with 3–4 µm alumina slurry. The membrane was then dissolved in methylene chloride for 15 min to completely release the micromotors. The micromotors were collected by centrifugation at 7000 rpm for 3 min and washed once with isopropanol, once with ethanol, and three times with ultrapure water (18.2 Ώ cm), with a 3 min centrifugation following each wash. All micromotors were stored in ultrapure water at room temperature when not in use. Such template preparation method resulted in reproducible micromotors. Propulsion of Zr/erGO micromotors

The propulsion of ZrO2/erGO micromotors was performed by adding a solution containing 1% SDS (Sigma–Aldrich, USA) and 1.5% hydrogen peroxide (Sigma– Aldrich, USA). Videos were captured by an inverted optical microscope (Nikon Eclipse Instrument Inc. Ti-S/L100), coupled with 20× and 10× objectives, a Hamamatsu digital camera C11440, and NIS Elements AR 3.2 software. The speed of the micromotors was tracked using an NIS Elements tracking module.

Spectrophotometric Measurement of Chemical Warfare Agent

All of the measurements were performed with a UV–Vis spectrophotometer (UV-2450, SHIMADZU). For adsorptive detoxification experiment, the efficiency in remediation was determined by monitoring the absorption curves of methyl paroxon (MP), ethyl paraoxon (EP) and bis-4 nitrophenyl phosphate (b-NPP) (Sigma Aldrich, USA). The 1.5 x 10-5 M aqueous solution of OP gives an absorption peak at ~400 nm. In the presence of the motors, OPs react with the Zr-based motor surface, creating an acid-base Lewis adduct, which results in a decrease in the absorbance. The remediation efficiency was calculated by measuring the decrease in absorbance signal. For adsorptive detoxification experiments, 1 mL aliquots of 1.5 x 10-5 M MP, EP or b-NPP aqueous solutions were used, in the presence of different amounts of micromotors (1 × 105 to 10 × 105 motors mL−1) along with 1% SDS and 1.5% hydrogen peroxide as the propulsion medium. 5



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Samples were filtered before spectrophotometric measurements to avoid possible interferences of the motors or the generated bubbles. After the remediation (adsorptive detoxification experiments), the reaction mixture was allowed to react with the micromotors for 5 min, after which the change in the absorbance signal at 400 nm, proportional to the amount of the remaining OP, was monitored.

RESULTS AND DISCUSSION Figure 1A illustrates the template electrodeposition protocol for the fabrication of zirconia oxide (ZrO2)-electrochemically reduced graphene oxide (erGO)/Pt bilayer micromotors. As shown in Figure 1A, the graphene oxide (GO) and ZrOCl2 were simultaneously electrodeposited in one-step on the inner wall of a polycarbonate (PC) membrane by using cyclic voltammetry (CV) technique. The simultaneous electrochemical deposition of zirconia with graphene was achieved by cycling the potential between +0.2 V to -1.5 V at a scan rate of 10 mVs-1 for 5 consecutive scans. As shown in Figure 1B (a), the CV exhibits two peaks, one at -0.7 V which is attributed to the deposition of zirconia particles and another peak at -1.2 V due to the reduction of the oxygen functionalities of GO. In order to confirm the electrochemical potential of ZrOCl2 reduction peak, the electrochemical deposition of erGO/Pt micromotors was carried out, as shown in Figure 1B (b). The absence of the peak at -0.7 V confirmed the deposition of ZrO2 on the graphene layers. Briefly, the simultaneous electrochemical synthesis of ZrO2 and graphene can be expressed by the following reaction mechanism: (1) ZrOCl2 + GO + n e− → ZrOCl + Cl− +erGO (2) 2 ZrOCl + 2 H2O → 2 ZrO2 + 2 HCl + H2 Zirconyl chloride, ZrOCl2, in its highest oxidation state, Zr (IV), is first reduced to Zr (III) as it is shown in (1); however, the aqueous conditions are sufficient for the reoxidation of the Zr (III) to a more stable compound, zirconia, ZrO2.38-40 While these nanocrystals are being synthesized, the oxygen functionalities of graphene oxide are reduced at a more negative potential of -1.2 V, enhancing the conductivity of the external layer.

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Figure 1. Template-based fabrication of zirconia-functionalized graphene/Pt micromotors and their application toward effective and selective “on-the-move” CWA removal from the complex matrix: (A) Schematic of the template-based fabrication procedure: (a) simultaneous electrodeposition of zirconia and graphene oxide, (b) galvanostatic deposition of Pt, and (c) dissolution of the membrane template; (B) Cyclic voltammograms corresponding to the (a) simultaneous electrochemical reduction of ZrOCl2 (-0.7 V) and graphene oxide (-1.2 V), and (b) electrochemical reduction of graphene oxide (-1.2 V) as a control experiment using one scan at 10 mVs-1. (C) Schematic representation of the structure of zirconia-graphene micromotors: functional micromotors swimming in a complex sample and preferentially removing organophosphate compounds. The reduction of the oxygen functionalities in the graphene surface increased the hydrophobicity and conductivity of the graphene film and the interaction between the layers, permitting their stacking within the PC wall membrane.24 This electronically conductive ZrO2-erGO composite structure favors the secondary deposition of the inner catalytic Pt layer (Figure 1A (b)), using previously optimized galvanostatic conditions 7



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(-2 mA, 500 s). Finally, the graphene-hybrid micromotors were released by dissolving the membrane for their designed application (Figure 1A (c)). Figure 1C shows the strategy of the present for “on-the-move” removal of CWA based on coupling the advantages gained by the structure of graphene and the properties of ZrO2. The high content of defects and functionalities in water soluble GO, allowed the co-synthesis of small size zirconia particles on the outer surface of the micromotor.41-44 These oxygencontaining groups and defects in GO layers are potential nucleation sites for growing the ZrO2 particles; therefore, the Zr4+ ions tend to be adsorbed at these specific sites and generate a large number of graphene active sites, increasing the surface area of the zirconia on the outer layer of the micromotor. The inherent structure of CWA based on phosphate functional groups is key for the presented approach. This interaction is an acid-base Lewis interaction between electronically rich phosphate groups (Lewis base) of the CWAs and the electron-deficient zirconium cation (Lewis acid) of the zirconia presented in the outer layer of the micromotor.41-44 The propulsion of the ZrO2functionalized micromotors in the contaminated solution facilitates the capture and removal of the CWA from the solution through improved contacts and increased fluid transport. On the other hand, molecules with similar structures, but lacking phosphate groups, are not captured by the zirconia surface functionality of these propelled micromotors.

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Figure 2: SEM images illustrating the surface of the hybrid micromotor: (A) Pt roughness in Zr-erGO/Pt, (B) Zirconia pattern on ZrO2-erGO/Pt micromotors, (C) erGO surface on erGO/Pt micromotors, and (D) Zirconia microstructure on PANI/Pt micromotors. Scale bars, 500 nm. (E) SEM and corresponding EDX spectroscopy images of a Zr-erGO/Pt illustrating the distribution of zirconia (green), carbon (grey), and Pt (red), respectively. Scale bars, 2 µm. (F) Propulsion images of ZrO2-erGO/Pt micromotors showing different trajectories and movement patterns in 1.5 % H2O2 solution containing 1% SDS (based on SI Video 1). Figure 2 displays the scanning electron microscopy (SEM) images of the prepared ZrO2-erGO/Pt micromotors (Reviewer 2: comment 2). Figure 2A shows the bottom side-view of the conical ZrO2-erGO/Pt microtube synthesized using optimized conditions. This image indicates an outer diameter of 5 µm and reveals the nanoparticlebased porous platinum inner structure, as was reported previously.24 The outer graphene layer provides oxygen functionalities in the erGO. These moieties within the high number of boundaries and defects in graphene’s thin shell-layer promote the generation of the porous Pt inner layer with a diameter of ~200 nm. This Pt structure increases greatly the catalytic surface area of the inner layer and improves the propulsion behavior of the micromotor, leading to enhanced CWA removal efficiency. SEM characterization was performed to examine the outer surface of the prepared micromotors. The image of Figure 2B shows the needle-like microstructure present on ZrO2-erGO micromotors with sizes around 20-30 nm and the uniform decoration of the graphene layers. To compare and confirm the presence of zirconia particles, control graphene motors (without ZrO2) were synthesized under similar conditions (see Experimental Section). Figure 2C shows the smooth surface of the outer layer resulted from the erGO-based micromotors. This image demonstrates that the previous needlelike microstructure was due to the formation of ZrO2 nanoparticles. Furthermore, other conductive polymers, such as polyaniline (PANI), were also co-deposited with ZrOCl2 under similar conditions instead of erGO (see Experimental Section). The resulting ZrO2 structure on PANI is shown in Figure 2D. In the case of ZrO2-PANI/Pt micromotors, ZrO2 microspheres tend to agglomerate together since the PANI has a lower number of active sites for growing ZrO2; thus, it is hard to define the exact size of each ZrO2 nucleation. The outer structure is rougher in the ZrO2-erGO/Pt micromotors, imbuing the ZrO2-erGO/Pt with higher and more uniform zirconia surface area, which would enable a greater number of contact sites with CWA. Figure 2E shows the cross9



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section of the ZrO2-erGO/Pt micromotors, a 7 µm length conical structure, along with an EDX mapping analysis that corroborates the presence of zirconia (from ZrO2 particles), carbon (from erGO), and Pt (inner layer). As expected and was demonstrated earlier,24 the inner porous Pt structure provides an enlarged catalytic surface area and improves the efficiency of the fuel decomposition reaction and hence of the propulsion process. Figure 2F and corresponding SI Video 1 examine the propulsion behavior and illustrate several trajectories and movement patterns of ZrO2-erGO/Pt micromotors at a very high speed of 450±80 µm s−1 (represents mean value ± standard deviation for 10 micromotors) in 1.5% H2O2 fuel. The microbubble tails resulting from the hydrogen peroxide decomposition reaction reveal unique trajectories with snake-like, spiral, and flower-like moving behaviors. These different movement patterns reflect the asymmetric inner porous Pt catalyst structure, which provide anisotropic distribution of unbalanced oxygen production along the tube, leading to the distinct motion patterns.24

Figure 3. “On-the-move” effective removal of the chemical warfare agent simulant methyl paraoxon (MP) (at 1.5×10-5M) using the Zr-erGO/Pt micromotor. Effect of the (A) number of micromotors: (a) standard MP solution, (b) 1×105, (c) 2.5×105, (d) 5×105, (e) 7.5×105, and (f) 10×105 motors; (B) Effect of the remediation time: (a) standard MP, (b) 1 min, (c) 2 min, (d) 3min, (e) 4 min, and (f) 5 min. Reaction conditions: 1.5% H2O2 and 1% SDS.

The high adsorption capability of the hybrid micromotors reflects their continuous movement and depends upon several interdependent parameters such as time, number of motors, and accessibility of adsorption sites by CWA (effect of substituent). Due to safety requirements and regulations related to the use of actual CWA, the reactive nerve-agent simulants methyl paraoxon (MP), ethyl paraoxon (EP), and bis (4nitrophenyl) phosphate (b-NPP) have been used here since they display a parallel 10


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reactivity to nerve agents. Optimization studies were performed prior to the remediation studies. Figure 3 examines the influence of different motor densities (ranging from 1×105 to 10×105 micromotors/mL) upon the removal of 1.5×10-5 M methyl paraoxon (MP) from a 1 mL solution during a 5 min movement. The UV absorbance spectra of Figure 3A shows that the extent of MP removal increases from 24% to 91% upon increasing the density of ZrO2-erGO/Pt micromotors from 1×105 to 10×105 micromotors/mL. Figure 3B displays the influence of the remediation time upon the removal of MP over the 1 to 5 min range (1 min intervals) using the optimized amount of micromotors (10×105 micromotors/mL). It was found that the MP removal efficiency increases from 6.5% to 91% upon increasing the navigation time of micromotors from 1 to 5 minute. Overall, the MP concentration is observed to decrease upon increasing remediation time and motor numbers.

Figure 4: “On-the-move” effective and selective removal of chemical warfare agents using Zr-erGO/Pt micromotor: Absorbance spectra of the (A) MP: (a) without any treatment, (b) moving PANI micromotor, (c) static hybrid motor, (d) graphene micromotor, and (e) moving ZrO2-erGO hybrid micromotor; (B) absorbance spectra of EP, (C) b-NPP: (a) without any treatment, (b) static ZrO2-erGO hybrid micromotor, (c) moving ZrO2-erGO hybrid micromotor, and (D) zirconia-mediated detoxification of nerve agent scheme illustrating the binding of the OP CWA to the Zr-containing motor surface. Reaction conditions: time, 5 min; H2O2 (1.5%) and SDS (1%). To demonstrate the practical utility of the hybrid ZrO-erGO/Pt micromotors associated with their hybrid material, movement and enhanced mixing - we examined the ability to remove a variety of OP pesticides with analogous molecular structures, but different steric hindrance namely methyl paraoxon (MP), ethyl paraoxon (EP) and bis(4nitrophenyl) phosphate (b-NPP). Figure 4 displays the absorbance signals of the para11



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nitrophenol (p-NP) hydrolysis product after NaOH treatment of MP (4A), EP (4 B) and b-NPP (4C) following the ZrO-erGO/Pt micromotor-based treatment in sample containing various levels of different OP compounds. The motors were propelled in the contaminated solution for 5 min in the presence of 1.5% H2O2 fuel. Figures 4A, B and C demonstrate the efficient micromotor-induced removal of different OP compounds, with corresponding UV-Vis absorbance signals showing 91, 70, and 58 % removal of MP, EP, and NPP, respectively. Apparently, the removal efficiency decreases upon increasing the steric hindrance, with the hidden phosphate group has limited access to the ZrO2. In contrast, only a negligible removal (∼1–5%) of these agents is observed in control experiments carried out using the static micromotors. These data indicate that the new dynamic adsorption platforms offer significantly shorter remediation times and that the unique motor composition along with movement of multiple micromotors, along with the corresponding bubble generation, across the contaminated samples (e.g., SI Video 2) provide a favorable hydrodynamic environment. Such motor-induced mixing (without external agitation) is crucial for the accelerated decontamination processes. Furthermore, as we can see from absorbance spectra of Figure 4A (b) with the case of conducting polymer (PANI) only 5.1% removal was obtained, which reflects the higher surface area of the ZrO2. A negligible (

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