Imaging the proton concentration and mapping the spatial distribution of the electric field of catalytic micropumps

Catalytic engines can use hydrogen peroxide as a chemical fuel in order to drive motion at the microscale. The chemo-mechanical actuation is a complex mechanism based on the interrelation between catalytic reactions and electro-hydrodynamics phenomena. We studied catalytic micropumps using fluorescence confocal microscopy to image the concentration of protons in the liquid. In addition, we measured the motion of particles with different charges in order to map the spatial distributions of the electric field, the electrostatic potential and the fluid flow. The combination of these two techniques allows us to contrast the gradient of the concentration of protons against the spatial variation in the electric field. We present numerical simulations that reproduce the experimental results. Our work sheds light on the interrelation between the different processes at work in the chemo-mechanical actuation of catalytic pumps. Our experimental approach could be used to study other electrochemical systems with heterogeneous electrodes.

Despite the large number of tasks that have been demonstrated, the mechanism of the chemo-mechanical actuation has been less studied. The actuation mechanism is based on electrochemical processes at the liquid-surface interface of spatially heterogeneous electrodes. It has a lot in common with the physics of basic electrochemical systems [25], corrosion processes [26], energy related devices (such as batteries and fuel cells) [27,28], ionexchange membranes [29,30], and biological systems (such as biomembranes and ion pumps) [31]. New experimental methods are needed to enable quantitative studies at the microscale of these electrochemical processes.
The actuation mechanism of bimetallic motors/micropumps is based on the oxidation and reduction of hydrogen peroxide (  Fig. 1(a)]. The reactions are [13] at the anode: at the cathode: Overall, there is a net flux of  H from the anode to the cathode. The electric field generated in this process is believed to drive motors through electrophoresis and, in the case of micropumps, to induce the flow of the liquid through electro-osmosis [32][33][34][35][36][37].
The production and the consumption of  H are related to the electric field in the liquid (and to the electrostatic potential) in an intricate way. On the one hand, the electrochemical reactions, which produce and consume  H , depend, in principle, on the electrostatic potential difference between the liquid and the metal surface as well as on the local concentration of The interrelation between the catalytic reactions and the electro-hydrodynamics phenomena was analyzed by solving the governing Nernst-Planck, Poisson and Navier-Stokes equations using different approximations [15,[32][33][34][35]. The spatial variations in the concentration of  H and in the electric field were found to depend critically on a number of parameters that are difficult to quantify, such as the rate of electrochemical reactions, the zeta potential of metal surfaces, the concentration of ion impurities in the liquid, and their diffusion coefficient. Given the number of ill-defined parameters, and the complexity of the chemo-mechanical actuation, it is important to measure independently the concentration of  H and the electric field in order to establish the role played by the different processes.
In this Letter, we report on a new method to study the chemo-mechanical actuation of catalytic pumps; it combines two techniques based on optical microscopy. We employed fluorescence confocal microscopy to image and quantify the concentration of  H , a technique used before in biology to measure the local pH [38][39][40][41]. The second technique consists in monitoring the velocity in the liquid of particles with different charges in order to map the spatial variations in the electric field, the electrostatic potential, and the fluid flow. Previously, only the magnitudes of the electric field and the fluid flow were estimated [13,15]. The combination of these two techniques allows us to contrast the gradient of the concentration of  H against the spatial variation in the electric field. It also establishes the zeta potential of metal surfaces. By comparing our experimental findings to numerical simulations, we estimate the concentration of ion impurities and the constant rates of the electrochemical reactions at the anode and cathode. This study provides a quantitative understanding of the chemomechanical actuation of catalytic pumps.
Micropumps were fabricated by patterning 30-50 μm diameter platinum disks on gold surfaces using electron-beam lithography and electron-beam evaporation. Platinum and gold were chosen because their electrochemical reactivity in whereas the mixed potential of Au is at a lower voltage. This is consistent with the measurements discussed below where the platinum disk acts as the cathode, and the gold film as the anode (Fig. 1a). This finding is opposite to what has been observed in gold/platinum motors [5]. This difference may originate from the fact that the cleaning treatment alters the electrochemical properties of the metals by adding some oxygen functionalities to the surface, as demonstrated by X-ray photoelectron spectroscopy measurements (supplementary section II).
We first characterised the electric field and the flow of the liquid [13]. We added positively charged, negatively charged, and quasi-neutral particles to the solution, and then tracked their summarises the various types of motion we observed. Particles  p moved towards the cathode disk, whereas particles  p did not: they remained more than 20 μm away from its edge. This indicates that the electric field points towards the disk (Fig. 1a). Particles 0 p also moved toward the disk; however, once they arrived there, they tended to drift upwards in the direction normal to the disk (Fig. 1f). Since 0 p particles interact weakly with the electric field, due to their low charge, their motion reproduces to a good approximation the liquid's flow.
The motion of the fluid can be understood as follows [13]: due to electro-osmosis [37], the fluid is driven by the electric field towards the disk in the plane parallel to the surface; it then moves upwards in the direction normal to the disk because of fluid continuity (Fig. 1a).
The spatial variations in the electric field, the electrostatic potential, and the fluid velocity can be estimated from the velocities of  p and 0 p particles measured as a function of the radial coordinate ( r ) along the disk's radius (Fig. 2). The particle velocity has two contributions: one coming from the electrophoretic force ( eof v ) and the other arisen from the fluid flow ( where  is the fluid permittivity,  the fluid viscosity and  the zeta potential of the particle. (  (Fig. 3b), because it controls the rate of electrochemical reactions.
We estimate the zeta potential of the substrate (ξ w ) from the standard expression of the electrosmotic velocity, . Inserting the data of Fig. 3 a and c for the electric field and the fluid velocity in the previous equation, we obtain a value of the zeta potential that remains nearly constant as a function of the radial distance, with an average value of ξ w = -33 mV. This is close to the values considered for the zeta potential of Au in previous studies [13,15].
We imaged the concentration of  H using confocal fluorescence microscopy (Fig. 4).