Room-Temperature Negative Differential Resistance in Surface-Supported Metal-Organic Framework Vertical Heterojunctions.

The advances of surface-supported metal-organic framework (SURMOF) thin-film synthesis have provided a novel strategy for effectively integrating metal-organic framework (MOF) structures into electronic devices. The considerable potential of SURMOFs for electronics results from their low cost, high versatility, and good mechanical flexibility. Here, the first observation of room-temperature negative differential resistance (NDR) in SURMOF vertical heterojunctions is reported. By employing the rolled-up nanomembrane approach, highly porous sub-15 nm thick HKUST-1 films are integrated into a functional device. The NDR is tailored by precisely controlling the relative humidity (RH) around the device and the applied electric field. The peak-to-valley current ratio (PVCR) of about two is obtained for low voltages (<2 V). A transition from a metastable state to a field emission-like tunneling is responsible for the NDR effect. The results are interpreted through band diagram analysis, density functional theory (DFT) calculations, and ab initio molecular dynamics simulations for quasisaturated water conditions. Furthermore, a low-voltage ternary inverter as a multivalued logic (MVL) application is demonstrated. These findings point out new advances in employing unprecedented physical effects in SURMOF heterojunctions, projecting these hybrid structures toward the future generation of scalable functional devices.

Polydopamine nanofilms for high-performance paper-based electrochemical devices.

Since the discovery of polydopamine (PDA), there has been a lot of progress on using this substance to functionalize many different surfaces. However, little attention has been given to prepare functionalized surfaces for the preparation of flexible electrochemical paper-based devices. After fabricating the electrodes on paper substrates, we formed PDA on the surface of the working electrode using a chemical polymerization route. PDA nanofilms on carbon were characterized by contact angle (CA) experiments, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (topography and electrical measurements) and electrochemical techniques. We observed that PDA introduces chemical functionalities (RNH2 and RC═O) that decrease the CA of the electrode. Moreover, PDA nanofilms did not block the heterogeneous electron transfer. In fact, we observed one of the highest standard heterogeneous rate constants (ks ) for electrochemical paper-based electrodes (2.5 ± 0.1) × 10-3  cm s-1 , which is an essential parameter to obtain larger currents. In addition, our results suggest that carbonyl functionalities are ascribed for the redox activity of the nanofilms. As a proof-of-concept, the electrooxidation of nicotinamide adenine dinucleotide showed remarkable features, such as, lower oxidation potential, electrocatalytic peak currents more than 30 times higher when compared to unmodified paper-based electrodes and electrocatalytic rate constant (kobs ) of (8.2 ± 0.6) × 102  L mol-1  s-1 .

Enhanced Hydrophobicity in Nanocellulose-Based Materials: Toward Green Wearable Devices.

Nanocellulose is a promising material for fabricating green, biocompatible, flexible, and foldable devices. One of the main issues of using nanocellulose as a fundamental component for wearable electronics is the influence of environmental conditions on it. The water adsorption promotes the swelling of nanopaper substrates, which directly affects the devices' electrical properties prepared on/with it. Here, plant-based nanocellulose substrates, and ink composites deposited on them, are chemically modified using hexamethyldisilazane to enhance the system's hydrophobicity. After the treatment, the electrical properties of the devices exhibit stable operation under humidity levels around 95%. Such stability demonstrates that the hexamethyldisilazane modification substantially suppresses the water adsorption on fundamental device structures, namely, substrate plus conducting ink. These results attest to the robustness necessary to use nanocellulose as a key material in wearable devices such as electronic skins and tattoos and contribute to the worldwide efforts to create biodegradable devices engineered in a more deterministic fashion.

Versatile and Robust Integrated Sensors To Locally Assess Humidity Changes in Fully Enclosed Paper-Based Devices.

The synergic combination of materials and interfaces to create novel functional devices is a crucial approach for various applications, including low-cost paper-based point-of-care systems. In this work, we demonstrate the implementation of surface-modified polypyrrole (PPy) structures, monolithically integrated into a three-dimensional multilayered paper-based microfluidic device, to locally assess humidity changes. The fabrication and integration of the system include the deterministic incorporation of PPy into the paper-based structure by gas-phase polymerization and the modification of the polymer properties to allow local humidity monitoring. The functionalization of PPy changes both the wettability and the chemical composition of the interface, what is of fundamental importance for the sensor's operation. The PPy structure has excellent mechanical stability, enduring at least 600 bending cycles, what is of relevance on flexible electronics. The electrical resistance correlates with the local relative humidity (RH) inside of the sealed microfluidic system, and the sensor response is fully reversible. The integrated system capable of locally monitoring the RH allowed us to verify that inside the microfluidic channel, water molecules can diffuse across the wax barriers-a possibility disregarded so far. Our results attest that RH variations of 5-10% can affect the flow of extended channels (>5 cm) even when they are fully enclosed.

Flexible and Foldable Fully-Printed Carbon Black Conductive Nanostructures on Paper for High-Performance Electronic, Electrochemical, and Wearable Devices.

In this work, we demonstrate the first example of fully printed carbon nanomaterials on paper with unique features, aiming the fabrication of functional electronic and electrochemical devices. Bare and modified inks were prepared by combining carbon black and cellulose acetate to achieve high-performance conductive tracks with low sheet resistance. The carbon black tracks withstand extremely high folding cycles (>20 000 cycles), a new record-high with a response loss of less than 10%. The conductive tracks can also be used as 3D paper-based electrochemical cells with high heterogeneous rate constants, a feature that opens a myriad of electrochemical applications. As a relevant demonstrator, the conductive ink modified with Prussian-blue was electrochemically characterized proving to be very promising toward the detection of hydrogen peroxide at very low potentials. Moreover, carbon black circuits can be fully crumpled with negligible change in their electrical response. Fully printed motion and wearable sensors are additional examples where bioinspired microcracks are created on the conductive track. The wearable devices are capable of efficiently monitoring extremely low bending angles including human motions, fingers, and forearm. Here, to the best of our knowledge, the mechanical, electronic, and electrochemical performance of the proposed devices surpasses the most recent advances in paper-based devices.

Hybrid organic/inorganic interfaces as reversible label-free platform for direct monitoring of biochemical interactions.

The combination of organic and inorganic materials to create hybrid nanostructures is an effective approach to develop label-free platforms for biosensing as well as to overcome eventual leakage current-related problems in capacitive sensors operating in liquid. In this work, we combine an ultra-thin high-k dielectric layer (Al2O3) with a nanostructured organic functional tail to create a platform capable of monitoring biospecific interactions directly in liquid at very low analyte concentrations. As a proof of concept, a reversible label-free glutathione-S-transferase (GST) biosensor is demonstrated. The sensor can quantify the GST enzyme concentration through its biospecific interaction with tripeptide reduced glutathione (GSH) bioreceptor directly immobilized on the dielectric surface. The enzymatic reaction is monitored by electrical impedance measurements, evaluating variations on the overall capacitance values according to the GST concentration. The biosensor surface can be easily regenerated, allowing the detection of GST with the very same device. The biosensor shows a linear response in the range of 200pmolL-1 to 2µmolL-1, the largest reported in the literature along with the lowest detectable GST concentration (200pmolL-1) for GST label-free sensors. Such a nanostructured hybrid organic-inorganic system represents a powerful tool for the monitoring of biochemical reactions, such as protein-protein interactions, for biosensing and biotechnological applications.