María J. Esplandiú

Asymmetric Hybrid Silica Nanomotors for Capture and Cargo Transport: Towards a Novel Motion‐Based DNA Sensor

An innovative self-propelled nanodevice able to perform motion, cargo transport, and target recognition is presented. The system is based on a mesoporous motor particle, which is asymmetrically functionalized by the attachment of single-stranded DNA onto one of its faces, while catalase is immobilized on the other face. This enzyme allows catalytic decomposition of hydrogen peroxide to oxygen and water, giving rise to the driving force for the motion of the whole system. Moreover the motor particles are able to capture and transport cargo particles functionalized with a noncomplementary single-stranded DNA molecule, only if a specific oligonucleotide sequence is present in the media. Functionalization with characteristic oligonucleotide sequences in the system implies a potential for further developments for lab-on-chip devices with applications in biomedical applications.

Electrocatalytic tuning of biosensing response through electrostatic or hydrophobic enzyme–graphene oxide interactions

The effect of graphene oxidative grades upon the conductivity and hydrophobicity and consequently the influence on an enzymatic biosensing response is presented. The electrochemical responses of reduced graphene oxide (rGO) have been compared with the responses obtained from the oxide form (oGO) and their performances have been accordingly discussed with various evidences obtained by optical techniques. We used tyrosinase enzyme as a proof of concept receptor with interest for phenolic compounds detection through its direct adsorption onto a screen-printed carbon electrode previously modified with oGO or rGO with a carbon-oxygen ratio of 1.07 or 1.53 respectively. Different levels of oGO directly affect the (bio)conjugation properties of the biosensor due to changes at enzyme/graphene oxide interface coming from the various electrostatic or hydrophobic interactions with biomolecules. The developed biosensor was capable of reaching a limit of detection of 0.01 nM catechol. This tuning capability of the biosensor response can be of interest for building several other biosensors, including immunosensors and DNA sensors for various applications.

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 ζ-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.

Real time protein recognition in a liquid-gated carbon nanotube field-effect transistor modified with aptamers.

The combination of optimized and passivated Field Effect Transistors (FETs) based on carbon nanotubes (CNTs) together with the appropriate choice and immobilization strategy of aptamer receptors and buffer concentration have allowed the highly sensitive and real time biorecognition of proteins in a liquid-gated configuration. Specifically we have followed the biorecognition process of thrombin by its specific aptamer. The aptamer modified device is sensitive enough to capture a change in the electronic detection mechanism, one operating at low protein concentrations and the other in a higher target concentration range. The high sensitivity of the device is also sustained by the very low detection limits achieved (20 pM) and their high selectivity when other target proteins are used. Moreover, the experimental results have allowed us to quantify the equilibrium constant of the protein-aptamer binding and confirm its high affinity by using the Langmuir equation.

Carbon Nanotubes and Electrochemistry

Rigid carbon nanotube (CNT) polymer composites were characterized as bare electrochemical transducers and as support for (bio)functionalization. In order to accomplish that, cyclic voltammetry and impedance spectroscopy in presence of benchmark redox systems were performed. The response and sensitivity can be enhanced if the density of edges is increased in the CNT systems. Bamboo-like CNTs seem to be the best choice for composite electrodes. Additionally, the CNT electrodes result in a very robust support for chemical functionalization. Direct electron transfer of myoglobin has been verified without the need of electrochemical mediators and without compromising the biomolecule activity. Finally, some other exciting applications of CNT electrochemistry at the nanoscale regime are also highlighted.

Sequential Tasks Performed by Catalytic Pumps for Colloidal Crystallization

Gold-platinum catalytic pumps immersed in a chemical fuel are used to manipulate silica colloids. The manipulation relies on the electric field and the fluid flow generated by the pump. Catalytic pumps perform various tasks, such as the repulsion of colloids, the attraction of colloids, and the guided crystallization of colloids. We demonstrate that catalytic pumps can execute these tasks sequentially over time. Switching from one task to the next is related to the local change of the proton concentration, which modifies the colloid ζ potential and, consequently, the electric force acting on the colloids.

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.

Elucidation of the wettability of graphene through a multi-length-scale investigation approach

Univocal conclusions around the wettability of graphene exposed to environmental conditions remain elusive despite the recent efforts of several research groups. The main discrepancy rests on the question of whether a graphene monolayer (GML) is transparent or not to water and more generally what the role is that the substrate plays in determining the degree of wetting of the GML. In this work, we investigate the water transparency of GML by means of a multi-length-scale approach. We complement traditional static contact angle measurements and environmental scanning electron microscopy experiments with atomic force microscopy based force spectroscopy to assess the role that intermolecular interactions play in determining the wetting of GML. To gain deeper insight into the wetting transparency issue, we perform experiments on inert metals, such as gold and platinum, covered or not covered by GML. The comparison of the results obtained for different systems (i.e. GML covered and uncovered inert metals), provides unambiguous evidence that supports the non-wetting transparency theory of GML. This work aims to assist the development of technologies based on graphene–water interaction, such as graphitic membranes for water separation processes.

Mechanical detection and mode shape imaging of vibrational modes of micro and nanomechanical resonators by dynamic force microscopy

We describe a method based on the use of higher order bending modes of the cantilever of a dynamic force microscope to characterize vibrations of micro and nanomechanical resonators at arbitrarily large resonance frequencies. Our method consists on using a particular cantilever eigenmode for standard feedback control in amplitude modulation operation while another mode is used for detecting and imaging the resonator vibration. In addition, the resonating sample device is driven at or near its resonance frequency with a signal modulated in amplitude at a frequency that matches the resonance of the cantilever eigenmode used for vibration detection. In consequence, this cantilever mode is excited with an amplitude proportional to the resonator vibration, which is detected with an external lock-in amplifier. We show two different application examples of this method. In the first one, acoustic wave vibrations of a film bulk acoustic resonator around 1.6 GHz are imaged. In the second example, bending modes of carbon nanotube resonators up to 3.1 GHz are characterized. In both cases, the method provides subnanometer-scale sensitivity and the capability of providing otherwise inaccessible information about mechanical resonance frequencies, vibration amplitude values and mode shapes.

Development of Tunable Nanocomposites Made from Carbon Nanotubes for Electrochemical Applications

The advent of the nanotechnology field has brought about novel devices and materials at the nanometer scale triggered by the demand of miniaturizing electronic, optical, actuating and sensing systems. In this nanotechnology era, the concept of nanocomposite has been brought to light though it has been present in nature and used from historical times. A nanocomposite can be defined as a material made of more than one solid phase where at least one of the constituent parts has a nanometer scale dimension. Building blocks with dimensions in the nanosize range make possible to design and create new materials with unprecedented versatility and improvement in their physical and chemical properties. The promise of nanocomposites lies in their multifunctionality for different applications and the possibility of unique combinations of properties unachievable with conventional materials. As mentioned before, the concept of enhancing properties and improving characteristics of materials through the creation of multiple phase nanocomposites is not recent. Mother Nature has a lot of examples of nanocomposites such as the structure of seashells and the bones. The idea has also been practiced since civilization started and humanity began producing more efficient materials for functional purposes. The Maya blue pigment, an ancient nanostructured material which was found to be very resistive to acids or (bio)corrosion phenomena, is a composite of organic and inorganic constituents, primarily leave dyes combined with a natural clay (Jose-Yacaman et al., 1996). Nanocomposites comprise a wide scope ranging from metal/ceramic nanocomposites, polymer-based nanocomposites to natural or biomimetic nanobiocomposites. They still share tremendous challenges, especially in the control over the distribution in size and dispersion of the nanosize constituents and in the tailoring and understanding of the role of the interfaces (Ajayan et al., 2003; Harris, 2004; Moniruzzaman et al., 2006; Spitalsky et al., 2010). Among the broad world of nanocomposites, the conducting ones attract special attention because of their special applications in electronics and electrochemistry (Grossiord et al.,