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 chemomechanical actuation of catalytic pumps. Our experimental approach could be used to study other electrochemical systems with heterogeneous electrodes. © 2013 American Physical Society.We acknowledge support from the European Union (ERC-carbonNEMS project), the Spanish government (FIS2009-11284, FIS2011-22603, MAT2012-31338), and the Catalan government (AGAUR, SGR).Peer Reviewe

Nanociencia y Nanotecnología: la gran revolución de lo pequeño

1 página.-- Charla divulgativa presentada en la semana de la ciencia 2010 celebrada en Barcelona (España) el 16/11/2010.Peer reviewe

Non-equilibrium thermodynamics of catalytic nanomotors and micropumps

Abstract of Twentieth Symposium on Thermophysical Properties; Boulder, Colorado (USA), June 24–29 (2018).The conversion of chemical energy into directed motion is a key point behind the high efficient operation of biomolecular machines. Inspired by nature, there is a growing interest in engineering novel artificial machines which can self-propel and carry out autonomous work “a la carte,” mimicking the impressive molecular machinery of living organisms. In this context, Janus particles, bimetallic nanomotors and micropumps, which are powered by catalytic reactions at two different metals, stand out as one of the most promising candidates. But how catalytic reactions end up driving particle or fluid motion is a rather complex non-equilibrium process involving the coupling of electrochemical reactions, electrostatics, mass and charge transport, and fluid dynamics. In this context, a theoretical description complemented by numerical simulations becomes a powerful tool to optimize and shed light on the complex chemo-mechanical actuation of catalytic objects. In this talk, we will discuss the non-equilibrium thermodynamics concepts controlling the operation of catalytic micropumps. The results of this study facilitate a better understanding of the whole scenario behind this process and also provide important clues on how to design more efficient catalytic micro/nano motors for future applications.Peer reviewe

Synthesis of polydopamine at the femtoliter scale and confined fabrication of Ag nanoparticles on surfaces

Nanoscale polydopamine motifs are fabricated on surfaces by deposition of precursor femtolitre droplets using an AFM tip and employed as confined reactors to fabricate Ag nanoparticle patterns by in situ reduction of a Ag+ salt.This work was partly funded through grants MAT2012-38318-C03-02 and MAT2012-38319-C02-01 of the Spanish MINECO. M.G. thanks the CSIC for a JAEPre predoctoral grant.Peer Reviewe

Silicon-based chemical motors: An efficient pump for triggering and guiding fluid motion using visible light

We report a simple yet highly efficient chemical motor that can be controlled with visible light. The motor made from a noble metal and doped silicon acts as a pump, which is driven through a light-activated catalytic reaction process. We show that the actuation is based on electro-osmosis with the electric field generated by chemical reactions at the metal and silicon surfaces, whereas the contribution of diffusio-osmosis to the actuation is negligible. Surprisingly, the pump can be operated using water as fuel. This is possible because of the large - Potential of silicon, which makes the electro-osmotic fluid motion sizable even though the electric field generated by the reaction is weak. The electro-hydrodynamic process is greatly amplified with the addition of reactive species, such as hydrogen peroxide, which generates higher electric fields. Another remarkable finding is the tunability of silicon-based pumps. That is, it is possible to control the speed of the fluid with light. We take advantage of this property to manipulate the spatial distribution of colloidal microparticles in the liquid and to pattern colloidal microparticle structures at specific locations on a wafer surface. Silicon-based pumps hold great promise for controlled mass transport in fluids.The authors acknowledge support from MINECO and the “Fondo Europeo de Desarrollo Regional” (FEDER) through Grant MAT2012-31338, the European Union (ERC-carbonNEMS project), and the Catalan government (AGAUR, SGR).Peer Reviewe

Impedimetric genosensing of DNA polymorphism correlated to cystic fibrosis: A comparison among different protocols and electrode surfaces

7 páginas, 5 figuras, 1 tabla.In this work, a genosensor for the impedimetric detection of the triple base deletion in a cystic fibrosis-related DNA synthetic sequence is presented. Screen-Printed Carbon Electrodes containing Carboxyl functionalised multi-walled carbon nanotubes were used for the immobilization of an amino-modified oligonucleotide probe, complementary to the Cystic Fibrosis (CF) mutant gene. The complementary target (the mutant sequence) was then added and its hybridization allowed. The change of interfacial charge transfer resistance (Rct) between the solution and the electrode surface, experimented by the redox marker ferrocyanide/ferricyanide, confirmed the hybrid formation. A non-complementary DNA sequence and a three-mismatch sequence corresponding to the wild DNA gene (present in healthy people) were used as negative controls. A further step employing a signalling biotinylated probe was performed for signal amplification using streptavidin-modified gold nanoparticles (strept-AuNPs). In order to observe by SEM the presence and distribution of strept-AuNPs, a silver enhancement treatment was applied to electrodes already modified with DNA–nanoparticles conjugate. The developed protocol allowed the very sensitive detection of the triple base deletion in a label-free CF-related DNA sequence, achieving a LOD around 100 pM. Results were finally compared with those obtained using different protocols for immobilization of DNA capture probe.Financial support for this work has been provided by the Ministry of Science and Technology (MCyT, Madrid, Spain) trough projects Consolider-Ingenio 2010 CSD2006-00012, and by the Department of Innovation, Universities and Enterprise (DIUE) from the Generalitat de Catalunya.Peer reviewe

Key Parameters Controlling the Performance of Catalytic Motors

The development of autonomous micro/nanomotors driven by self-generated chemical gradients is a topic of high interest given their potential impact in medicine and environmental remediation. Although impressive functionalities of these devices have been demonstrated, a detailed understanding of the propulsion mechanism is still lacking. In this work, we perform a comprehensive numerical analysis of the key parameters governing the actuation of bimetallic catalytic micropumps. We show that the fluid motion is driven by self-generated electro-osmosis where the electric field originates by a proton current rather than by a lateral charge asymmetry inside the double layer. Hence, the surface potential and the electric field are the key parameters for setting the pumping strength and directionality. The proton flux that generates the electric field stems from the proton gradient induced by the electrochemical reactions taken place at the pump. Surprisingly the electric field and consequently the fluid flow are mainly controlled by the ionic strength and not by the conductivity of the solution, as one could have expected. We have also analyzed the influence of the chemical fuel concentration, electrochemical reaction rates, and size of the metallic structures for an optimized pump performance. Our findings cast light on the complex chemomechanical actuation of catalytic motors and provide important clues for the search, design, and optimization of novel catalytic actuators

Electrostatic and hydrophobic interactions involved in CNT biofunctionalization with short ss-DNA

This work is aimed at studying the adsorption mechanism of short chain 20-mer pyrimidinic homoss-DNA (oligodeoxyribonucleotide, ODN: polyC20 and polyT20) onto CNT by reflectometry. To analyze the experimental data, the effective-medium theory using the Bruggemann approximation represents a suitable optical model to account for the surface properties (roughness, thickness, and optical constants) and the size of the adsorbate. Systematic information about the involved interactions is obtained by changing the physicochemical properties of the system. Hydrophobic and electrostatic interactions are evaluated by comparing the adsorption on hydrophobic CNT and on hydrophilic silica and by modulating the ionic strength with and without Mg2+. The ODN adsorption process on CNT is driven by hydrophobic interactions only when the electrostatic repulsion is suppressed. The adsorption mode results in ODN molecules in a side-on orientation with the bases (nonpolar region) toward the surface. This unfavorable orientation is partially reverse by adding Mg2+. On the other hand, the adsorption on silica is dominated by the strong repulsive electrostatic interaction that is screened at high ionic strength or mediated by Mg2+. The cation-mediated process induces the interaction of the phosphate backbone (polar region) with the surface, leaving the bases free for hybridization. Although the general adsorption behavior of the pyrimidine bases is the same, polyC20 presents higher affinity for the CNT surface due to its acid-base properties.Authors acknowledge the financial contributions of FONCyT, SeCyT-UNC, CONICET, the International Exchange Collaboration between CAPES (Brazil) and SPU (Argentine) (Grant No. 025/05), and National Institute of General Medical Sciences (NIGMS)/National Institutes of Health (1SC3GM081085) (C.D.G). M.J.E. thanks the Ministry of Education and Science of Spain (Project NAN2004-093006-C05-03) and the “Ramón and Cajal” Program. M.L.C. thanks CONICET for the fellowship granted.Peer Reviewe

Effect of temperature on the growth of single crystalline monolayer graphene by Chemical Vapor Deposition (CVD)

Resumen del póster presentado a la 6th edition of Graphene Conference series, the largest European Event in Graphene and 2D Materials, celebrada en Genova (Italia) del 19 al 22 de abril de 2016.The ever increasing interest in graphene properties and its applications has motivated the controlled growth of high-quality graphene and fabrication of graphene-based devices. The growth of graphene via CVD using metal catalysts depends on both the intrinsic properties of the metal catalysts and the growth parameters. Here we demonstrate that the structure of single layer graphene flakes grown on a copper substrate by low pressure CVD depends dramatically on the furnace temperature, within a few tens of degrees Celsius. Optical microscope analysis of as-grown and transferred graphene onto SiO2/Si shows that growth at 1000ºC results in dendritic shapes while growth at 1040ºC gives a compact graphene flake. The low temperature growth was extended over a long time (1 hour) in order to check if there was a change in the structure towards a compact flake as the one in Figure b, which was obtained after just 10 minutes of growth time at 1040ºC. However, the size of the dendrites increased without merging. Although still poorly understood, the dendritic growth may be due to the poor smoothening of the copper at the lower annealing temperatures and to the low carbon attachment/detachment kinetics at the graphene growth fronts. We have characterized the charge and spin transport properties of the graphene grown at low temperatures. We have fabricated non-local spin valve devices with 3 μm graphene channel length and found a spin life time of 0.2 ns and spin diffusion length of 2.5 μm at room temperature. The mobility of the device was of 1000 cm2 /Vs, which is typical for CVD grown graphene on SiO2/Si. Future work will focus on comparing these results with the spintronic performance of graphene grown at higher temperatures.Peer Reviewe

Unraveling the operational mechanisms of chemically propelled motors with micropumps

The development of effective autonomous micro- and nanomotors relies on controlling fluid motion at interfaces. One of the main challenges in the engineering of such artificial machines is the quest for efficient mechanisms to power them without using external driving forces. In the past decade, there has been an important increase of man-made micro- and nanomotors fueled by self-generated physicochemical gradients. Impressive proofs of concept of multitasking machines have been reported demonstrating their capabilities for a plethora of applications. While the progress toward applications is promising, there are still open questions on fundamental physicochemical aspects behind the mechanical actuation, which require more experimental and theoretical efforts. These efforts are not merely academic but will open the door for an efficient and practical implementation of such promising devices. In this Account, we focus on chemically driven motors whose motion is the result of a complex interplay of chemical reactions and (electro)hydrodynamic phenomena. A reliable study of these processes is rather difficult with mobile objects like swimming motors. However, pumps, which are the immobilized motor counterparts, emerge as simple manufacturing and well-defined platforms for a better experimental probing of the mechanisms and key parameters controlling the actuation. Here we review some recent studies using a new methodology that has turned out to be very helpful to characterize micropump chemomechanics. The aim was to identify the redox role of the motor components, to map the chemical reaction, and to quantify the relevant electrokinetic parameters (e.g., electric field and fluid flow). This was achieved by monitoring the velocity of differently charged tracers and by fluorescence imaging of the chemical species involved in the chemical reaction, for example, proton gradients. We applied these techniques to different systems of interest. First, we probed bimetallic pumps as counterparts of the pioneering bimetallic swimmers. We corroborated that fluid motion was due to a self-generated electro-osmotic mechanism driven by the redox decomposition of H2O2. In addition, we analyzed by simulations the key parameters that yield an optimized operation. Moreover, we accomplished a better assessment of the importance of surface chemistry on the metal electrochemical response, highlighting its relevance in controlling the redox role of the metals and motion direction. Second, we focused on metallic and semiconductor micropumps to analyze light-controlled motion mechanisms through photoelectrochemical decomposition of fuels. These pumps were driven by visible light and could operate using just water as fuel. In these systems, we found a very interesting competition between two different mechanisms for fluid propulsion, namely, light-activated electro-osmosis and light-insensitive diffusio-osmosis, stemming from different chemical pathways in the fuel decomposition. In this case, surface roughness becomes a pivotal parameter to enhance or depress one mechanism over the other. These examples demonstrate that pumps are practical platforms to explore operating mechanisms and to quantify their performance. Additionally, they are suitable systems to test novel fuels or motor materials. This knowledge is extensible to swimmers providing not only fundamental understanding of their locomotion mechanisms but also useful clues for their design and optimization.This research was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under Contract Nos. MAT2015-68307-P, FIS2015-67837, and M-ERA.NET Project PCIN2016-093. The ICN2 is funded by the CERCA program/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa program of MINECO (Grant SEV-2013-0295).Peer reviewe