ARTICLE https://doi.org/10.1038/s41467-022-30554-7 OPEN From radial to unidirectional water pumping in zeta-potential modulated Nafion nanostructures María J. Esplandiu 1 ✉, David Reguera 2,3, Daniel Romero-Guzmán 4,5, Amparo M. Gallardo-Moreno4,5 & Jordi Fraxedas 1 1234567890():,; Chemically propelled micropumps are promising wireless systems to autonomously drive fluid flows for many applications. However, many of these systems are activated by nocuous chemical fuels, cannot operate at high salt concentrations, or have difficulty for controlling flow directionality. In this work we report on a self-driven polymer micropump fueled by salt which can trigger both radial and unidirectional fluid flows. The micropump is based on the cation-exchanger Nafion, which produces chemical gradients and local electric fields capable to trigger interfacial electroosmotic flows. Unidirectional pumping is predicted by simulations and achieved experimentally by nanostructuring Nafion into microarrays with a fine tune modulation of surrounding surface zeta potentials. Nafion micropumps work in a wide range of salt concentrations, are reusable, and can be fueled by different salt cations. We demonstrate that they work with the common water-contaminant cadmium, using the own capture of this ion as fuel to drive fluid pumping. Thus, this system has potential for efficient and fast water purification strategies for environmental remediation. Unidirectional Nafion pumps also hold promise for effective analyte delivery or preconcentration for (bio)sensing assays. 1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain. 2 Departament de Física de la Matèria Condensada, Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain. 3 Universitat de Barcelona, Institute of Complex Systems (UBICS), C/Martí i Franquès 1, 08028 Barcelona, Spain. 4 University of Extremadura, Department of Applied Physics and University Institute of Biomedical Research (INUBE), Badajoz, Spain. 5 Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Badajoz, Spain. ✉email: mariajose.esplandiu@icn2.cat NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 I nterfacial diffusio-osmotic or/and electroosmotic fluid flows context, Fischer and coworkers devised a unidirectional water triggered by self-generated chemical gradients are the basis pumping system using an array of three-dimensional photo- behind chemically propelled micro/nanomotors or swimmers. chemically active Au/TiO2 Janus pillars acting as pumping The intense activity in this fascinating field in the last years has walls43. The pillars catalyze the water-splitting reaction under UV provided very appealing demonstrations of multitasking illumination, giving rise to local osmotic flows around them. The swimmers1–20. The immobilized counterparts of micro/nanomo- cooperative effect of the flows generated by an array of pillars tors are micropumps, sharing the same working principle of the leads to unidirectional macroscopic flow for a careful choice of swimmers, but driving the flow of the surrounding fluid instead of the geometry and arrangement of the pillars. self-propelling in a fluid at rest21. These immobilized versions of The scarcity of studies reporting wireless fluid flow uni- motors are also especially suitable for better experimental probing directionality without externally imposed gradients might be and understanding the phoretic mechanisms behind swimmers21. rooted in the difficulties to match the proper arrangement and Micropumps are also promising platforms for many applications pumping mechanism to accomplish a constructive effect using an such as mass release, transport, accumulation, and clearance5,22; array of pumps. Fluid flow unidirectionality is not simply material patterning at precise locations23–25; or in sensing achieved by a periodic repetition of active structures. Instead, a applications22,26,27. suitable geometrical layout and a controlled surface nanoengi- Self-powered micropumps of different material composition neering of the active materials are needed to sustain fluid flow and working principles have been investigated in recent years. motion in one direction. Most studies have focused on catalytic bimetallic or semi- Here, we report on a self-activated micropump based on conductor/metallic pumps governed by electroosmotic Nafion which can pump fluid using salts as chemical fuel and can flows5,21,28–34. Other type of pumps have combined passive be nanostructured to achieve unidirectional pumping. Nafion is a materials with metals, semiconductors, solid salts, polymers or perfluorinated sulfonic acid ionomer with high performance enzymes, switching on ionic, neutral diffusio-osmotic, or density- cation-exchanger capabilities apart from other attributes such as driven convective flows induced by the gradients of ions or very high ionic conductivity, excellent thermal and mechanical neutral species generated at the active part of the pump32,35–39. stability, biocompatibility, and antifouling characteristics44. We Fluid pumping has been also achieved by just illuminating a harness the ion-exchange capabilities of Nafion44 to induce both microfluidic chamber with UV light, initiating photochemical radial or unidirectional tangential electroosmotic flows in the reactions in the solution phase which drives flows by solute presence of salts45–48. To accomplish long-range unidirectional buoyancy (in the bulk fluid) and diffusion-osmosis (at the walls of fluid pumping, an array of Nafion microstrips are patterned with the chamber)27. highly controllable nanofabrication strategies. These active strips Many of the pumps outlined above are triggered by the are integrated, in turn, into an array of adjoining strips with hydrogen peroxide (H2O2) decomposition reaction which can different zeta potentials (ζ). Thus, the novelty of this wireless be toxic for some applications, especially in the biological pump system lies in the design of an ion-exchange powering context. Therefore, there is always a need to search for more system integrated in a nanofabricated array with fine-tuning innocuous chemical fuels or novel reaction mechanisms to modulation of zeta potential surrounding the active Nafion to overcome such limitations. The use of motors or pumps fed by control the direction of the flow. We prove fluid pumping in a enzyme substrates is a very elegant way to expand applications wide range of salt concentrations covering more than four orders in the biomedical field9,10,37–39. Another alternative using of magnitude, from micromolar to the millimolar range. We also innocuous fuels are self-powered pumps or swimmers made of show that these pumps can be easily regenerated for reusability ion-exchange polymers. In this context, a pump based on without almost no loss of performance. More importantly, we immobilizing particles of ion-exchange resins on glass with the demonstrate that these pumps can be self-driven using different capability of triggering electroosmotic flows with micromolar cations as fuel. In particular, we show that these Nafion pumps salt concentrations has been reported40. However, most bio- can work by efficiently capturing heavy metal cations such as medical and environmental remediation applications need to cadmium ions present in contaminated water samples. Therefore, operate at higher salt concentrations. these systems might have promising potential applications in Another desirable feature of self-powered pumps is to control water remediation, where one could devise micropump channels the directionality of the flow. The vast majority of previous stu- in which contaminated water would be self-driven by the own dies have focused on demonstrating fluid pumping through local contaminant, delivering purified water at the end of the pumping recirculated flows towards and away from active patches. But channel. achieving fluid flow unidirectionality will expand their applica- This study also expands the versatility of Nafion material from tions in the field of drug delivery, biosensing or environmental the well-known application areas of the chlor-alkali industry, fuel remediation. An initial strategy of originating directional flows cell technology44,49–52 or biosensor technology53,54 to the was carried out by Hess and coworkers with a pumping mem- appealing field of wireless micro/nanofluidic networks and self- brane trying to imitate the transport of analytes in propelled micro/nanomotors, promoted by the strategies achieved biomembranes41. In that work a porous membrane coated at both in Nafion nanopatterning. sides with two different noble metals could induce electroosmotic flows through the pores triggered by the decomposition of hydrogen peroxide at the metallic layers when both metal layers Results and discussion were connected by an external switch. Later on, the group of Sen Nafion pumps driving radial flows. We first demonstrate the and coworkers came up with a pumping system made of an array capability of Nafion to trigger fluid pumping in a simple con- of metal/enzymatic catalytic strips that could achieve unidirec- figuration which activates radial flows. The pump layout consists tional flows by setting-up a gradient of chemical fuel, in this case of a 100 µm diameter and about 600 nm thick Nafion disc sur- of H2O2 with a gel soaked in H2O222. However, it would be rounded by a region in which Nafion has been mostly deacti- desirable to achieve self-driven unidirectional pumping without vated. The fresh Nafion material is in its protonated form and the application of external gradients (electrical, chemical, or deactivation is achieved by electron-beam lithography which pressure). Recently the same group has achieved self-organization scissors Nafion −SO3− moieties decreasing its ion-exchange of fluid flows with a multienzymatic pump system42. In a similar capabilities47. 2 NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 ARTICLE Fig. 1 Radial Nafion micropumps. a (top) Schematic picture of the cross section of a Nafion micropump, illustrating the pumping mechanism. The ion- exchange generates an ion concentration gradient (in green) and an electric field (E) schematized by the red lines, mimicking those obtained from the simulations as described in the Supplementary Figure 3. The charges in the deactivated Nafion illustrate the negative zeta potential of this surface (negative charges at the active Nafion interface are not shown for simplicity). The positive charges in the liquid interface denote the counterions that will be moved by the tangential component of the ion-exchange self-generated electric field, dragging a fluid flow (Vf) towards the active Nafion disc. Near the disc edge, the flow goes up by fluid continuity and finally bends outwards due to the presence of the top wall. b, c Top view snapshots of the actual Nafion pump taken at different heights above the surface. The polystyrene tracers dragged by the fluid approach the Nafion structure as shown in the image captured at a plane near the surface b and then they are lifted by the fluid and move away from the Nafion patch, as shown by the image taken at a z-plane located at 100 µm from the surface c. d Average radial velocity of polystyrene tracers dispersed in 1.0 × 10−4 M of LiCl near the surface as a function of the radial distance and at different pumping times. Data of the different curves have been obtained by evaluating and averaging the trajectory of more than 50 polystyrene particles. The yellowish shaded area represents the presence of the Nafion disc from the center of the disc (0 µm) to the edge (50 µm). e Time evolution of the maximum radial velocity of tracers averaged over 25–50 trajectories in presence of aqueous solutions containing different salt cations. Error bars in the figures represent the standard deviation. Source data of d, e are provided as a Source Data file. Figure 1a schematizes a cross section of the micropump under retained on the disc when the speed of the fluid is not large enough study depicting the pumping mechanism. Fluid pumping is to lift them, a phenomenon that is more probable to occur at large triggered when the patterned Nafion system is immersed in times. Maximum tracer velocities of around 30 µm/s have been aqueous salt solutions due to the exchange of protons by the salt detected at very short times. These are large values as compared to cation, in this case exemplified with LiCl aqueous solutions. In the those obtained with other catalytic pumps combining metals in experimental device, the fluid flow is followed by tracking the H2O2 solutions30 or metals/semiconductors in water31,32. The motion of polystyrene-based tracer particles. Figure 1b and c maximum velocity was decreasing to values around 2 µm/s when show the typical motion behavior of the colloidal tracers close (b) reaching half an hour of actuation. The pump was able to continue and far away (c) from the surface, respectively. Particles close to running for more than 45 minutes, which is remarkable consider- the surface move toward the Nafion disc, however, as they ing that the layer of Nafion is only 600 nm thick. approach the disc edge their trajectory bends upward in the Nafion pumps can be activated with different salts as fuel since direction perpendicular to the disc and finally bend outwards the Nafion can exchange protons with different cations47 (Supple- disc due to fluid continuity and the presence of the top wall of mentary Movies 3 and 4). In fact, Fig. 1 e shows comparatively the cell (See Supplementary Movie 1 and 2). the time evolution of the pumping velocity triggered with LiCl, The spatial velocity and time evolution of the pump performance NaCl, and CdCl2 of same concentration (1.0 × 10−4 M) and with was evaluated by tracking the average radial velocities of the CdCl2 of the same ionic strength than the alkali metal chlorides polystyrene tracers dispersed in 1.0 × 10−4 M of LiCl at the region (CdCl2 3.3 × 10−5M). Very high velocities have been observed at close to the surface (Fig. 1b). In general, particles increase their shorter times being those obtained with the divalent cations the radial velocity when approaching to the disc, exhibiting maximum higher ones and also those that decay more abruptly with time, values at 60–80 µm from the disc center (radius of the Nafion disc especially the one at 1.0 × 10−4 M of CdCl2. is 50 µm). Then the radial velocity decays inside the Nafion region The effect of salt concentration on the pump performance has because the particles are pushed up by the fluid or because they are also been evaluated experimentally as a function of different NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 concentrations of LiCl as depicted in Supplementary Fig. 2a. The tangential component of the electric field is needed. This is radial fluid velocity of the tracers exhibits a non-monotonic achieved by nanostructuring Nafion into patches, as the one behavior; it increases with salt concentration at very low depicted in Fig. 1a, representing a Nafion disc surrounded by concentrations of salts but then starts decreasing at higher deactivated Nafion. For this finite Nafion patch the electric field concentrations. Fluid pumping remarkably persists even at mM will still point towards the Nafion, but it will now have a salt concentrations. Thus, these Nafion pumps are very promising component parallel to the surface that, when this surface is for applications since they can be operative in a wide range of salt charged, will lead to fluid flow by multi-ion diffusiophoresis47,55. concentrations, in contrast with catalytic pumps. In a multi-ion solution the diffusio-osmotic velocity of the fluid is Another very interesting feature of these pumps is their given by55: reusability after regeneration. The fresh Nafion pump is in its protonated form and after use with LiCl electrolytes the pump is ε ∑i z 2i ∇C i 2 ε v do ¼  ζ  Eζ ; ð3Þ loaded with Li+ ions. The Nafion pump was regenerated to the 8η ∑i z 2i C i w η w protonated form by immersion in 10−2 M HCl for 6 h. Then the pump was reused in 1.0 × 10−4 M LiCl. Supplementary Figure 2b where the first term represents the chemi-osmotic contribution compares the radial velocity of tracers in 1.0 × 10−4 M LiCl on a and the second one the electroosmotic term that is proportional freshly prepared pump with that obtained when using the same to the electric field given by Eq. (2), being ζw the surface zeta pump for a second time after a regeneration step. The pump potential and ε and η the permittivity and the viscosity of the performance was almost the same, with only a slight decrease in fluid, respectively. We found that the electroosmotic contribution speed after regeneration. is the dominant term47. Accordingly, the direction of the fluid flow is dictated by the sign of the surface’s zeta potential56. We Ion-exchange driven fluid pumping. The mechanism respon- have measured experimentally the zeta potential of Nafion and sible for fluid pumping is illustrated schematically in Fig. 1a. deactivated Nafion from the streaming potential /current of Pumping originates from the Nafion capability to exchange ions. planar surfaces of these materials using 1.0 × 10−4 M LiCl as When a protonated Nafion sample is immersed in a salt- electrolyte and pH of 5.7. The Nafion region is the one with containing electrolyte, such as LiCl, protons are released from largest negative zeta potential (−73 ± 3 mV). The deactivated Nafion and in exchange, lithium ions from the electrolyte are Nafion exhibits less negative zeta potential (−37 ± 3 mV) with incorporated into the Nafion film. As a consequence, gradients in respect to Nafion which is expected due to the removal of sulfonic the concentration of protons and lithium ions built-up near the groups. Since the zeta potential of the surface (both deactivated interface47. These gradients will generate an ion current governed Nafion and Nafion) is negative, tangential electroosmotic and by the Nernst-Planck equation chemi-osmotic fluid flows towards the Nafion disc are switched on with the consequent formation of convection rolls. z eD ji ¼ Di ∇Ci þ i i Ci E þ vCi ; ð1Þ This mechanism has been verified by simulations mimicking kB T the experimental setup. The details of the implementation are where the terms on the right-hand side represent the diffusion explained in the Supplementary methods. Supplementary Figure 3 (due to concentration gradient), migration (due to an electric field shows the proton concentration, the electric field, and the fluid E), and convection (due to a fluid velocity v) contributions, flow streamlines resulting from the simulation of the radial respectively. Di and Ci stand for the diffusion coefficient and Nafion pump. The simulation shows that the exchange of protons concentration of the ion i, zi is the ion valence, e is the elementary by Li+ ions taking place at the Nafion disc generates a charge, kB is Boltzmann’s constant, and T is the temperature. This concentration gradient and an electric field pointing radially ion exchange does not generate a net electric current Je . There- towards the Nafion disc. The tangential component of this electric fore, by imposing Je ¼ ∑ z i eji = 0, one obtains that an electric field acting on the positive mobile counterions accumulated on i the negatively charged deactivated Nafion surface, drags the fluid field towards the Nafion disc generating convection rolls. Supplemen- ∑ z D ∇C tary Figure 4 shows the value of the radial component of the kB T i i i i electric field as a function of the radial distance, measured from E¼ ð2Þ e ∑ z2i Di Ci the center of the Nafion disc, depicting a strong peak at the disc i edge. The simulations also show that this tangential component builds up in the presence of concentration gradients in order to of the electric field is almost insensitive to the value of the zeta guarantee that there is no electric current. The generation of an potential of the material surrounding the active Nafion. The zeta electric field of similar origin is the basic principle behind ionic potential of the surface sets the sign of the charge and distribution diffusiophoresis, upon which many catalytic motors and pumps of mobile counterions that move in the presence of the tangential operate. (The convective term is usually negligible and actually electric field, dragging the fluid along. Simulations also confirm does not contribute to the net charge current if the fluid is that there is no fluid flow in the absence of ion-exchange, and that electroneutral. Electroneutrality also yields a zero electric field if there is pumping even if the zeta potentials of both surfaces the diffusion coefficient of the ions Di are all the same). Hence, a would be the same. concentration gradient and an electric field are spontaneously Supplementary Figure 5 shows the behavior obtained in the built-up near the interface due to the unequal diffusion coeffi- simulation for the pumping velocity as a function of salt cients of the exchanged ions as proved by previous studies47,55. concentration. If Nafion acts as a pure sink for cations, the Among the salts containing alkali metal chlorides, Li ions were electric field and the pumping velocity are expected to increase chosen for the experiments due to their low diffusion coefficient, with salt concentration up to reaching a constant value at high thus providing higher difference of diffusion coefficients with salt concentrations (see Supplementary Fig. 5a and b). In fact, respect to protons, which results in larger electric field generation Eq. (2) helps explaining this behavior and the fact that these near the interface. pumps can still work at high salt concentrations, unlike catalytic When a continuum layer of Nafion is used, the concentration pumps. In catalytic pumps, the current and the self-generated gradients and the electric field are perpendicular to the surface, electric field are dictated by the fuel concentration, typically and do not generate any fluid flow. For triggering fluid motion a hydrogen peroxide. The addition of salt increases the ionic 4 NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 ARTICLE strength and screens the electric field, thus reducing the driving convective rolls could be suppressed, achieving the desired uni- force and making this kind of motors and pumps almost directional pumping. inoperative at high salt concentrations. Contrarily, for ion- Accordingly, we have fabricated micropump units consisting of exchange pumps, since the salt itself is the fuel, increasing the salt three adjacent strips made of deactivated Nafion, Nafion, and concentration will increase the concentration gradient (the Al2O3 with 25, 25, and 30 µm width, respectively, and 500 µm numerator in Eq. (2)) and the ionic strength (the denominator) long (Fig. 3a). These elements constitute the repetitive unit in a in a similar way, leading to an electric field and a corresponding patterned array extended in space. The self-pumping system was velocity flow which stay almost constant for high salt concentra- patterned using standard electron-beam lithography, electron- tions, as observed in the simulations. beam evaporation, and spin coating as detailed in Methods and The decay of the velocity for high salt concentrations (Supple- Experimental Section. The process is also compatible with mentary Fig. 2a) as well as the decline in performance in time photolithography or the use of stencil lithography. The zeta observed in the experiments (Fig. 1 e) is due to the limited proton potential of alumina was also measured at 1.0 × 10−4 M LiCl and capacity of the thin Nafion layer used. A simple order of magnitude pH of 5.7, verifying that it has a positive value of 17 ± 2 mV. As estimate can be done to illustrate this effect. The ion exchange before, the fluid motion is followed by tracking the motion of capacity of Nafion is about 4 × 103 mols of H+/m3 47, which for a polystyrene particle tracers in 1.0 × 10−4 M LiCl. 0.6 µm thick Nafion film yields 2.4 × 10−3 mols of H+/m2. The Figure 3b shows a bunch of trajectories of the polystyrene average diffusion current of the Li+ cation at the Nafion interface tracers when interacting with the patterned pump, proving their obtained in the simulations is 3.8 × 10−6 mol/m2 s. That means that unidirectional motion. The average velocity of tracers along the after roughly 630 s, all the protons will be exhausted if the exchange patterned surface during the first ten minutes of actuation is proceeds at the steady state rate. Presumably, as protons get scarced depicted in Fig. 3c. In general, the tracers move with average the exchange current will get smaller, reducing the electric field and velocities above 3 µm/s, which can be speed up at the boundaries the pump velocity. Since the exchange current is proportional to the of the different materials. The motion of particle tracers can be salt concentration, this saturation process will occur faster for higher observed in Supplementary Movie 5. salt concentrations. Accordingly, due to a faster saturation, the We have also designed another pump array by replacing the pumping speed will vanish quicker and at a fixed time, the velocity is deactivated Nafion with SiO2 structures. Silicon oxide exhibits a expected to be smaller as the salt concentration gets higher. more negative zeta potential than the deactivated Nafion Simulations (see Supplementary Fig. 6) also show that the structure with a value of −66 ± 3 mV. The repetitive pumping pumping works with similar velocities with different monovalent unit consists in this case of alternating strips of SiO2/Nafion/ cations as Li+, Na+, K+ and also with divalent ions as Cd+2. In Al2O3 with dimensions of 25, 25, and 30 µm width, respectively, the case of the divalent ion, 2H+ are exchanged by one Cd2+. and 500 µm long (see Fig. 4a). Unidirectional fluid flow has been Therefore, saturation would be reached at shorter times, which accomplished as can be proved from the tracer trajectories explains the rapid drop in pump speed observed in the collected in Fig. 4b and Supplementary Movie 6. We have experiments with Cd+2. extracted the average velocity along the patterned array by Supplementary Figure 8 shows the experimental verification evaluating the motion of the tracers during the first ten minutes that large pumping speeds can be sustained longer times by of the pump actuation as depicted in Fig. 4c. Similar pump increasing the thickness of the Nafion layer, since its saturation is performance has been achieved as compared to the deactivated delayed. Thus, we believe these polymeric pumps could have the Nafion/Nafion/Al2O3 with average velocities above 2 µm/s capability to operate at concentrations of salts above the together with velocity peaks mainly at the SiO2/Nafion bound- millimolar range by tuning the thickness of the polymeric layer. aries. Control experiments in absence of salt show only Brownian Future work will be done to address specifically this issue, and the motion in the tracer particles (Supplementary Movie 7). details of the ion-exchange mechanism that go beyond the scope Another cross-shaped pump configuration was fabricated using of the present paper, where the goal was to demonstrate the four arrays of these strips made from repetitive units of SiO2/ feasibility to use ion-exchange to generate fluid pumping and Nafion/Al2O3 as depicted in Supplementary Fig. 9. This layout achieve unidirectional flow. serves to demonstrate that the fluid can be redirected simulta- neously in different orthogonal directions, as can be seen in the Supplementary Movie 8. Nafion pump driving unidirectional flows. The most common We have also contrasted these experimental data with finite strategy to sustain unidirectional fluid pumping over long dis- element simulations. The implementation details are discussed in tances is to repeat periodically the basic structure that generates the Supplementary Information. A micropump array of 5 fluid motion. However, in this case the combination of alternating repeating units, each of them composed of alternating strips of strips of active and deactivated Nafion would not accomplish SiO2/Nafion /Al2O3 with the same stripe dimensions and zeta unidirectional flow since multiple convection rolls would be potentials than in the experiment, was simulated. The rest of produced instead. The electric field pointing towards the active parameters are listed in Supplementary Table 1. Nafion pad together with the symmetric surface zeta potential at Figure 5a shows the value of the horizontal component of the both sides would always direct the fluid flow towards the active velocity of a polystyrene tracer particle obtained from the Nafion patches as illustrated in Fig. 2a. To rectify the fluid motion simulations at 3 µm above the surface. The simulations reveal in one direction it is important to break the surface charge the net motion of the tracer along the micropump array with symmetry around Nafion. A simple way to accomplish that is by average velocities above 2 µm/s and support the generation of a adding a third stripe of a material with positive ζ potential. The directional fluid flow. The tangential velocity exhibits periodic electric field generated by the ion-exchange in the Nafion pad variations with large peaks at the SiO2/Nafion boundaries and acting on the region of positive zeta potential would drive an small peaks at the Nafion/Al2O3 interface. This simulation results electroosmotic flow now pointing outwards from the Nafion agree qualitatively with the experimental data shown in Fig. 4c. A patch, redirecting the fluid flow to the following pump unit of the direct, quantitative comparison is hard to perform since the tracer array as illustrated in Fig. 2b. Therefore, by periodically repeating particles do not follow a trajectory of constant height due to the a basic structure formed by alternating strips of deactivated influence of gravity, Brownian motion, and the vertical Nafion (negative ζ) / Nafion / surface with positive ζ, the component of the velocity. The velocity peaks coincide with the NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications 5 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 Fig. 2 From convective rolls to unidirectional pumping. a Scheme of a pump made of alternating strips of deactivated and active Nafion, leading to the formation of multiple convection rolls along the patterned array. b Scheme of the design of a pump based on the periodic repetition of a basic unit made of alternating strips of deactivated Nafion (negative ζ)/Nafion/Al2O3 (positive ζ) which would lead to unidirectional fluid flow along the patterned surface. The charged interface in the Nafion has been omitted to lighten the content of the figure. The Al2O3 patches with positive ζ accumulate negative counterions that in the presence of the tangential component of the electric field generated by the ion-exchange will move also to the right, dragging the fluid along to the next repeating unit, achieving unidirectional flow. Fig. 3 Unidirectional pumping with deactivated Nafion/Nafion/Al2O3 nanostructures. a Patterned array of strips made from the repetitive unit composed of deactivated Nafion (25 μm, ζ = −37 ± 3 mV) /Nafion (25 μm, ζ = −73 ± 3 mV) /Al2O3 (30 μm, ζ = 17 ± 2 mV). b Tracer trajectories along the patterned surface proving directional motion (DN and N refer to deactivated Nafion and Nafion, respectively). c Average tangential velocity of polystyrene tracers along the patterned structure. Blue, green and red shaded areas represent deactivated Nafion, Nafion and Al2O3 strips, respectively. Error bars in the figure represent the standard deviation. Source data of b, c are provided as a Source Data file. 6 NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 ARTICLE Fig. 4 Unidirectional pumping with SiO2/Nafion/Al2O3 nanostructures. a Patterned array of strips made from the repetitive unit composed of SiO2 (25 μm, ζ = −66 ± 3 mV) /Nafion (25 μm, ζ = −73 ± 3 mV) /Al2O3 (30 μm, ζ = 17 ± 2 mV). b Tracer trajectories along the patterned surface proving directional motion (N refers to Nafion). Source data are provided as a Source Data file. c Average tangential velocity of tracers along the patterned structures. Polystyrene tracers in 1.0 × 10−4 M LiCl have been used to track the fluid flow. Violet, green and red shaded areas represent SiO2, Nafion, and Al2O3 strips, respectively. Error bars in the figure represent the standard deviation. Source data of b, c are provided as a Source Data file. a 10 SiO2 Nafion Al2O3 SiO2 Nafion Al2O3 SiO2 Nafion Al2O3 b 8 Tracer velocity (µm/s) Velocity µm/s 65 µm 6 pH 4 2 SiO2 Nafion Al2O3 SiO2 Nafion Al2O SiO2 Nafion Al2O3 3 0 0 40 80 120 160 200 240 x ( µm) Fig. 5 Simulations of unidirectional micropumps. a Horizontal component of the expected velocity of a polystyrene tracer particle along the strip array composed by repetitive micropumps of SiO2/Nafion/Al2O3. The tracer velocity was obtained from the simulations at 3 µm above the surface and was smoothed using a 5 µm moving average. Source data are provided as a Source Data file. b pH mapping (color scale) and streamlines of the fluid flow over the first three repetitions of the periodic structure. The streamlines are colored by the local value of the fluid velocity, and illustrate the variations in speed caused by the local pumping triggered by the Nafion patches. regions of higher chemical gradients, as can be observed in the negative electric field acting above the positive Al2O3 surface, pH mapping of Fig. 5b. The pH map depicts an increase of proton both drive a fluid flow in the same direction (towards the right). concentration just above Nafion due to the ion-exchange building The streamlines of the resulting fluid velocity field are also up a strong chemical gradient at the neighboring strips. This depicted in Fig. 5b, colored by the modulus of the velocity. The gradient generates an electric field (see Supplementary Fig. 7) fluid velocity is large and shows strong intensity variations near with a tangential component which is positive (i.e., pointing to the surface as can be observed from the changes in color along the the right) at the SiO2/Nafion boundary and negative (i.e., pointing streamlines. The flow becomes slower and more uniform along to the left) at the Nafion/Al2O3 border. This positive tangential the array at higher z distances, with average velocities still electric field acting above the negatively charged SiO2 and exceeding 1 µm/s at 50 µm above the surface. NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications 7 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 Table 1 Changes of ion concentration of different aqueous solutions containing sodium and cadmium cations after interacting with Nafion devices. Sample Initial concentration (Co) Final concentration (Cf) Removal percentage (a) CdCl2 11.0 mg/L 0.3 mg/L 97% (b) CdCl2+NaCl [Cd2+] = 11.0 mg/L [Cd2+] = 0.2 mg/L 98% [Na+] = 2.3 mg/L [Na+] = 2.0 mg/L 13% (c) CdCl2+NaCl [Cd2+] = 11.0 mg/L [Cd2+] = 0.4 mg/L 96% [Na+] = 23.0 mg/L [Na+] = 21.0 mg/L 9% Potential application of Nafion pumps for water remediation. increased ionic strength is also balanced by the increase of the As a proof of concept, we have tested experimentally the cap- electric field generated by salt concentration gradient. In abilities of these Nafion micropumps to scavenge Cd2+ from particular, we have found that these Nafion micropumps can be water samples. Nafion samples of 2 × 2 cm2 and 600 nm thickness operative in a wide range of salt concentrations covering more were immersed in 3 ml of different ion-containing aqueous than four orders of magnitude, a remarkable result considering solution for 45 min. The initial and final ion concentrations after the thin Nafion layer used in this study. Moreover, they can be Nafion interaction were analyzed through optical emission easily regenerated for reusability. All these properties can become spectrometry with an inductively coupled plasma excitation very relevant for optimizing many applications such as mass source (ICP-OES). Table 1 shows the changes in ion concentra- transport and material patterning at precise locations, (bio) tion of different aqueous samples containing only Cd2+ (a) or a sensing, drug delivery or, environmental applications. mixture of Cd2+ and Na+ (b and c). In the case of sample (c) the Our results encourage further work on these systems since concentration of Na is the typical one found in drinking water. there is still a lot of room for device optimization. By adjusting Surprisingly, the Nafion pump is quite selective to the removal Nafion dimensions, it is possible to increase the operation time of cadmium ions and exhibits a very good removal efficiency of and to extend its applicability to fluids with higher saline more than 95% in very short times of operation with a capacity concentrations. Other lithography techniques could be used such removal of more than 78 mg of Cd ions per gr of Nafion, as stencil lithography or lithography based on high power lasers equivalent to 1200 moles of Cd2+/m3 and close to the nominal with cutting capacity. These are faster fabrication tools and would Nafion ion-exchange capacity. The selectivity of Nafion to Cd2+ leave less residues which could increase fluid pumping velocities. ions, its reusability, and the potential to work at high salt Moreover, cost effective plastic materials can be also used as concentrations, encourage further investigations to optimize the Nafion support which have already shown good adherence and design of these unidirectional self-driven pumps for water versatility for patterning. The integration of Nafion with adjacent decontamination applications. These studies can be expanded to metal pads could be used to change the surrounding zeta other multivalent ions of interest in biomedicine or environ- potential at will in real time via external stimuli, so that pumping mental remediation. can be turned on and off, or even switched between different In summary, we have introduced a chemically powered directions. All these ingredients are very promising to devise micropump based on Nafion which is self-activated by its ion- prototypes with practicality and scalability for real applications. exchange capabilities using innocuous salts as fuel. The Moreover, this study holds promise to expand the versatility of exchange of ions with different diffusion coefficients generates Nafion material from well-known application areas (e.g., fuel an electric field which, by properly arranging the Nafion into cells, biosensors, filtration/separation/purification technologies, patches, can drive fluid flow by electroosmosis. We have novel antifouling coatings, etc.) to the appealing field of wireless demonstrated high-speed fluid pumping forming convection micro/nanofluidic networks or self-propelled micro/nanomotors. rolls but also unidirectional pumping upon adequate micro/ In addition, the strategies achieved in Nafion nanopatterning nanostructuring of Nafion by modulating the zeta potential of open the possibility to integrate Nafion with metals, insulators, the adjacent strips. Unidirectional flow is achieved with a and semiconductors in nanofabricated devices. All these cap- micropump array patterned with repetitive units consisting of abilities can pave the way to exploit the remarkable properties of alternating strips of materials with negative zeta potential (such this ion-exchange polymer in myriads of novel applications. as deactivated Nafion or SiO2), Nafion, and materials with positive zeta potential (e.g., Al2O3). The interplay of the tangential component of the electric field, generated by the Methods Micropump fabrication. ion-exchange and pointing always towards Nafion, with the fine tuning of the zeta potential of the surrounding strips redirects 1. Nafion micropump for triggering radial flows the fluid flow to the next micropump unit of the array, The fabrication of Nafion micropumps started by spinning a diluted Nafion dispersion (5%) in isopropanol from a commercial Nafion dispersion achieving the desired unidirectional flow. Numerical simula- in the protonated form (Aldrich, 10% in water, eq. weight 1100) on Si wafers tions of these pumps also support the experimental findings. As modified with a 50 nm Au layer and followed by a heating step at 100 °C for proof-of-concept application, we have used these Nafion pumps 5 min. The spinning process was repeated to achieve typical thicknesses of to scavenge selectively and efficiently cadmium ions from water about 600 nm as measured with an Atomic Force Microscope and a samples with the particularity that the pumping is self-driven by profilometer. After that, the Nafion film was subjected to electron-beam lithography to define a Nafion disc of 100 µm of diameter. The e-beam the own contaminant ion. lithography modifies Nafion composition by the scission of −SO3− moieties With this study, we have shown the possibility of driving and which yield to a large loss of its ion-exchange capabilities47. guiding self-sustained fluid flows at the microscale by combining 2. Nafion micropump for triggering unidirectional flows ion-exchange with a proper micropatterning of surfaces with The fabrication process starts by patterning a gold strip array on a SiO2 wafer adequate zeta potentials. In contrast to other chemically propelled with strip dimensions of 50 nm thick, 50 µm wide and 500 µm long, and a pitch of 80 µm. The patterning is made by standard electron-beam lithography plus gold pumps, ion-exchange pumps present high salt tolerance, rooted evaporation. Then the surface was subjected to an oxygen plasma process (400 W, in the fact that the screening of the electric field caused by the 2 minutes) followed by the spin coating of a 600 nm thick Nafion layer on the 8 NATURE COMMUNICATIONS | (2022)13:2812 | https://doi.org/10.1038/s41467-022-30554-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30554-7 ARTICLE whole surface as described above. Taking advantage of the good adhesion of Nafion concentrations of the standard solutions for both Na+ and Cd2+ were 0.02, 0.1, on Au, the sample was then immersed in water which removes the Nafion from the 0.25, 1, 2.5, and 5 ppm, all of them in 1% of HNO3. silicon parts remaining the Nafion layer only on the gold strips. A second lithographic step was performed to define an array of adjacent Al2O3 strips. The alumina strip was evaporated with an electron-beam source and had dimensions of 30 µm wide, 500 µm long, and 50 nm thick with a pitch of 80 µm. When doing this Data availability second lithographic step with the sample already containing Nafion, the All data needed to evaluate the conclusions in the paper are present in the paper and/or development was done in methylisobutyl ketone without being diluted in the Supplementary Information. The source data underlying Figs. 1d, e, 3b, c, 4b, c, 5a, isopropanol and for the lift-off dichloromethane was used instead of acetone to Supplementary Fig. 2a and b, Supplementary Fig. 4, Supplementary Fig. 5a and b, avoid Nafion degradation. After that, an area of 25 µm wide x 500 µm long of the Supplementary Fig. 6a and b, Supplementary Fig. 7, Supplementary Fig. 8 and gold/Nafion strip was subjected to an electron-beam lithography to deactivate Supplementary Fig. 9b and c are provided in the Source Data file. Source data are Nafion and thus obtain the pump unit consisting of alternating strips of deactivated provided with this paper. Nafion/Nafion/Al2O3. In the case of the SiO2/Nafion/Al2O3 micropump array, the fabrication process consisted of two lithographic steps. The first one was to define the Au strip patterns Received: 4 August 2021; Accepted: 25 April 2022; but in this case with a width of 25 µm and keeping the rest of the dimensions the same. 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Sci. 111, 6554–6559 (2014). Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in 46. Musa, S., Florea, D., Wyss, H. M. & Huyghe, J. M. Convection associated with published maps and institutional affiliations. exclusion zone formation in colloidal suspensions. Soft Matter 12, 1127–1132 (2016). 47. Esplandiu, M. J., Reguera, D. & Fraxedas, J. Electrophoretic origin of long- range repulsion of colloids near water/Nafion interfaces. Soft Matter 16, Open Access This article is licensed under a Creative Commons 3717–3726 (2020). Attribution 4.0 International License, which permits use, sharing, 48. Mercado-Uribe, H. et al. On the evolution of the exclusion zone produced by adaptation, distribution and reproduction in any medium or format, as long as you give hydrophilic surfaces: a contracted description. J. Chem. Phys. 154, 194902 appropriate credit to the original author(s) and the source, provide a link to the Creative (2021). 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