Architecting graphene oxide rolled-up micromotors: a simple paper-based manufacturing technology

We report a graphene oxide rolled-up tubes production process using wax printed membranes for the fabrication of on demand engineered micromotors at different levels of oxidation, thicknesses and lateral dimensions. The resultant graphene oxide rolled-up tubes can show magnetic and catalytic movement within the addition of magnetic nanoparticles or sputtered platinum in the surface of graphene oxide modified wax printed membranes prior to the scrolling process. As a proof of concept, the as-prepared catalytic graphene oxide rolled-up micromotors are successfully exploited for oil removal from water. This micromotor production technology relies on an easy, operator-friendly, fast and cost-efficient wax-printed paper-based method and may offer a myriad of hybrid devices and applications. Graphene oxide rolled-up tubes production process using wax printed membranes for the fabrication of on demand engineered micromotors is presented. This micromotor production technology relies on an easy, operator-friendly, fast and cost-efficient wax-printed paper-based method and may offer a myriad of hybrid devices and applications.

(´stamping´) technology presented herein which is extremely advantageous in term of costefficiency and versatility for future applications. [18,[30][31] Here, we report on a simple, cost-effective and straight forward paper-based, GO rolled-up process for the fabrication of on demand engineered micromotors. We stand the production of well-shaped GO based rolled-up tubes at different levels of oxidation, thicknesses and lateral dimensions for two different geometries (squares and rectangles). The resultant GO rolled-up tubes are further modified to show magnetic and catalytic movement. The reported micromotors have the ability to open and close reversibly as bubbles are formed/ejected from their internal cavities. Such cyclic process is easily modulated by the amount of fuel provided and can be observed by optical microscope due to GO optical transparency. The resultant rolled-up structures displace smoothly in water and oil environments. Indeed, as a proof of concept, the as-prepared GO rolled-up micromotors are successfully exploited for oil removal from water. Although janus and rolled-up microfabricated micromotors have been developed with graphene, this is the first report that relies on an easy, operator-friendly, fast and costefficient wax-printed paper-based method. Such method have been previously reported by our group for printed electronics into substrates, but not in connection to micromotors. [18,[32][33] A suitable modification of such process by inducing curvature on the GO sheet until rolled-up tube formation provides a novel route to build versatile on-demand engineered tubular GO structures. This technology allows for a mass production of GO rolled-up tubes which was further exploited for micromotors fabrication, thus expanding and boosting the use of GO tubular structures beyond the current limitations. The process to produce GO rolled-up tubes can be divided in three steps (see scheme in and dried in air for a least one hour ( Figure 1a). After drying, the bottom part of the GOcoated WPM is wetted in water and the excess of water removed with a tissue (Figure1b). In a third step, the membrane is transferred into ethanol ( Figure 1c) and subjected to one minute of manual lateral shaking (Figure 1c1 and 1c2). Noteworthy, to form rolled-up materials the wetting time in water must be as short as one second. Such an instantaneous wetting step generates latter enough stress to produce rolled-up tubes when the GO-coated WPM is soaked in ethanol (Figure 1c and 1d). Higher wetting times, such as 1 minute for instance, enables to release much easier the filtered GO structures but hinders the rolled-up process. As a result, on demand suspended structures such as patterned letters or medusa-like structures can be obtained ( Figure S1, Supporting Information). A further discussion of this process is described below.
The rolled-up process here reported does strongly depend on several experimental parameters, such as thickness, shape and GO oxidation level. For a thorough study of the roll-up process, 350x700 µm GO films were prepared at different concentrations. For example, GO concentrations of 10, 5 and 2.5 µg/mL lead to 350x700 µm GO films with thicknesses of 84.1±7.1, 60.7±7.9 and 57.2±1.7 nm respectively. After placing the WPM in ethanol and shaking the solution, rolled-up tubes were spontaneously formed. Ethanol was chosen in this work, among other solvents with higher apolarity constants (such as, isopropanol or 1octanol), because tends to precipitate GO and reduced GO (RGO) sheets, and does not dissolve neither the membrane nor the wax, while inducing the enough stress or shrinkage over the membrane that enable it to roll-up. We also note that absolute ethanol (99%) was more effective in the scroll formation than ethanol 96% ( Figure S2, Supporting Information).
The GO and RGO rolled-up structures tend to precipitate and agglomerate in ethanol medium after one hour. This provides stability to the rolled-up structures in terms of morphology and avoids π-π stacking interactions in the case of RGO structures or dissolution of GO structures (as in case of water medium), being the best condition for long-term storage and further dispersion by simple hand shaking. The resultant structures were stable for 9 months at room temperature storage conditions with the possibility of resuspension. The results and discussion below are related to the scrolling processes done using absolute ethanol. For sake of simplicity we will focus on the rolled-up process for standard rectangular 350x700µm GO sheets, of two different reduction levels of GO with a concentration of 10 µg/mL. Figure 2 summarizes the different rolled-up events that take place on a rectangular shaped film depending on the scrolling direction for three different levels of oxidation. Note that due to symmetry, squares have only two rolling-up directions while rectangles have three.
The rolling direction of the WPM when soaked into ethanol defines the type of GO based rolled-up tube obtained after hand shaking. The shaking direction is inverse to the scrolling direction and thus, defines the direction of the GO based rolled-up tubes as represented in rolled-up tubes per WPM. It is known that reduction methodologies (such as the experimentally used in our process) increase structural defects in the structure at the nanoscale, affecting the mechanical properties of GO based films and thereby the scrolling process. [34] Yet, no visual defects on the structure, such as cracks, were visualized by SEM ( Figure 2d).
At this point, it is worthy to underline that the WPM used for producing rolled-up tubes was custom designed to boost the scrolling process. The design consists on the formation of edges along the vertex introducing small triangles (see Figure 3a), which facilitate the release of the GO structures from the WPM. Hydrophobicity and e) thickness of the different films at different levels of reduction. Figure 2 shows that we were able to roll-up GO films at two levels of reduction (RGO1 and RGO2) with C/O ratios of 2.97 and 3.37 respectively (note that GO has a C/O ratio of 2.38, see Figure 3b and c). The reduction was performed by placing the GO filtered through WPMs in 1mg/mL ascorbic acid for one (RGO1) and three (RGO2) days, respectively. X-ray diffraction (XRD) analysis showed how the film structure changes from a d-spacing of 0.82 nm of GO to 0.79 nm in RGO1, while two attributed peaks of 0.77 nm and 0.36 nm were observed for RGO2 ( Figure 3c). Such decrease in d-spacing is in agreement with the GO reduction process, where the GO sheets spacing decreases as a result of the elimination of intercalated oxygen-carbon by carbon-carbon structure bonds. [34] Such reduction process is not complete and oxygen functional groups still remain. This evolution of the structure increases the hydrophobicity of the material (Figure 3d) alone and decrease the thickness from 84.1 ± 7.1 to 73.5 ± 1.7 nm for GO and RGO2, respectively ( Figure 3e).
It is worthy to underline that the properties of RGO are considerably different compared to GO and hence the scrolling process. On one hand, GO-coated membranes have the ability to absorb water molecules on their structure by using the oxygen open venues, but they cannot absorb ethanol molecules. [13,[35][36] For this reason, and in order to have only absorption of water molecules on the bottom surface, the wetting process must be fast. In this way, the rolled-up tube formation is achieved by using the interior dried layers from the GO based films as elastic precursors for scrolling. On the other hand, RGO has less venues for the water absorption process as the stacked and hydrophobic carbon-carbon layers prevent water adsorption inside the structure, thereby staying only at its surface. As a result, the initial water wetting step is not as crucial as for the GO sheets. This hydrophobicity seems to be a crucial issue for the formation of thinner rolled-up tubes, that is, the formation of rolled-up tubes with smaller radius in comparison to the rolled-up tubes produced with the pristine GO. In other words, reduced sheets appear easier to scroll than pristine GO sheets. We latter confirm this hypothesis studding the rolling-up process by filtering 5 mL (10µg/mL) of a GO solution thorough 350x350 µm squares. Interesting, reduced squares (RGO1 and RGO2) form totally rolled-up structures unlike his pristine GO counterparts of same size and shape ( Figure S3, Supporting Information). This control experiment highlights the role of the GO reduction on the scrolling process. Nonetheless, this is not only the parameter to be taken in consideration.
A combination of thickness, lateral dimensions and oxygen content are the crucial parameters that enable or disable the rolled-up tubes to form at a given energy provided by the ethanol/water interface. We hypothesize that the above mentioned parameters directly define the mechanical properties of the sheet and hence the energy necessary to scroll them. We would like to highlight the relationship between the supplied energy and the mechanical properties of the sheets. Rigid sheets (reduced sheets) are more feasible to be scrolled which means that the energy provided by the water to ethanol interface might be stored in the sheet as mechanical stress and later released during the scrolling process. Instead, flexible sheets (GO sheets) might absorb this energy for instance as a plastic deformation which will not be available for the roll-up process. Noteworthy, these rolled-up tubes can also be opened or un- After forming the tubular structures with a platinum layer on the inner cavity, catalytic motion was tested by using hydrogen peroxide as fuel. Figure 4c shows a diagonal flexible scroll which can autonomously and cyclically go from a completely rolled-up structure, with formation of big bubbles, to the corresponding opened initial rectangular shape upon bubbles release. This behavior, thus far never observed for graphene based rolled-up tubes, reassembles the features of shape memory alloys or stimuli-responsive shape memory polymers/composites (see movie 1). [37][38] Structures of short-side scrolled rectangular shapes, opened and closed rather fast. Such actuation mechanism enables a mechanical energy to displace the structures in addition to the catalytic one (see figure 3d and movie 2). These soft micromotor actuation systems where observed when navigated in a solution containing 2.5% sodium cholate (NaCH) and 1% H 2 O 2 moving at a speed of ~400 µm/s. NaCH is a surfactant added to the solution to change its surface tension. Such a change helps to modulate the size and frequency of the O2 bubbles ejected from one of the microtubular structure sides, which in the last stay is responsible for its motion. [24] By changing the H2O2 concentration, the fold and un-fold process can be tuned up.
Completely irreversible of more rigid rolled-up tubes demonstrated to be typical tubular micromotors with potential for creating a micro-vortex effect that accelerate the kinetics of a variety of chemical reactions as reported 39 (figure 3e and movie 3). [39] As a result of the catalysis on top of the platinum layer, the wax-assisted micromotors self-propelled with a rocket-like configuration for diagonal scrolled structures. [40] The formation of oxygen bubbles at the inside of the RGO rolled-up tubes provides a directional threat at a speed of ~50 µm/s when navigated in a solution containing 2.5% NaCH and the very low concentration of 0.2% H2O2. Square based RGO Platinum-modified based rolled-up tubes show similar features as discussed above ( Figure S8, Supporting Information and movie 4).
Further work could be developed in order to determine the mechanical stability of the RGO rolled-up micromotors at higher thicknesses and their behavior under bubble formation. It is known that the flexibility of the RGO is lower at thicker films, which could be interesting for the production of more stacked tubes, more resistant to bubble formation at higher H2O2 concentrations, for specific applications.  To demonstrate a practical application of the wax-printed rolled-up RGO micromotors, they were successfully applied for oil collection from water. Taking advantage of the hydrophobic nature of RGO tubular-shaped micromotors, oil droplets were removed from water. The high surface to volume ratio associated with the motor material and their self-propelled movement offer favorable conditions to collect oil droplets present in water through hydrophobic interactions. After 5 min incubation of RGO2 platinum-modified micromotors in a solution containing 2.5 % NaCH, 0.2 % H2O2 and 50% oil, successfully capture and transport of oil droplets was observed (Figure 5 and movie 6). Considering the capability of RGO to adsorb and release oil, the micromotors open a door for renewable cleaning features. [41] This result highlights the great potential of such micromotors for the dynamic removal of large amounts of pollutants from water. Herein, the oil clean up application was presented only as a proofof-concept. In a practical scenario, a judicious scaling up of the technology must be necessary.
GO sheet-based micormotors have demonstrated to have potential to perform remediation tasks. For example, persistent organic pollutants and heavy metals got efficiently stacked and adsorbed on GO sheet-based micromotors, whereas, they were able to carry and transport cargo, even several times higher than their size, owing to their powerful towing force. [18,30,41] Such well-demonstrated properties along with the easiness of the fabrication process open-up a myriad of opportunities in the environmental field. Overall, this work showed a simple and cost-effective way to produce on demand GO-based structures. Among them, those modified with platinum and magnetite nanoparticles holds great promise as micromotors with potential for the transport of different cargos. It opens an avenue for their use not only in cleaning processes but also in release applications such as drug delivery or self-repairing/healing; both in the environmental and biomedical fields.
In conclusion, we have developed on demand GO-based rolled-up tubular structures, by using a very simple and cost-effective WPM-assisted approach. We studied the roll-up tubes formation conditions and probed the ability to produce on-demand shaped structures either one-sided or diagonal rolled-up tubes of two different geometries and three levels of GO oxidation. The planar structures were successfully decorated with platinum and NPs and the resultant modified rolled-up structures self-propelled either by catalysis or magnetization.
Upon bubble formation resulting from the reaction of platinum and H2O2, rolled-up tubes were able to move in a typical rocket-like architecture or to open and close rapidly with shape memory. As a proof-of-concept, we used RGO platinum-modified rolled-up tubes for oil removal in water. Such wide range of actions can be exploited for instance to capture or encapsulate different cargos for its further release or elimination. Overall, this work shows for the first time a user-friendly and cost-effective process to produce flat or rolled-up tubes structures of GO-based flexible composite for a wide range of applications. Optical properties of graphene quantum dots, amenable with paper-based manufacturing technology, are worthy of being explored for the development of rolled up tubular micromotors. The presented structures combined with their electrical features can be of profit for enhanced micromotors, soft micromachines, biomimetics, kirigami-like structures and developing of micro-optoelectro-mechanical devices. [42][43][44][45]  XRD measurements were performed with a X'pert MPD difractometer (Multipurpose Diffractometer) at room temperature using a Cu Kα radiation (l=1.540 Å). This diffractometer has a vertical theta-theta goniometer (240 mm radius), where the sample stages are fixed and do not rotate around omega axis as in omega-2theta diffractometers. The detector used is an X'Celerator which is an ultra-fast X-ray detector based on Real Time Multiple Strip (RTMS) technology. The diffraction pattern was recorded between 4 and 30º using an step size of 0,03º and a time per step of 1000s.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.