A simple and fast fabrication method to create high-performance pencil-drawn electrochemical sensors is reported for the first time. The sluggish electron transfer observed on bare pencil-drawn surfaces was enhanced using two electrochemical steps: first oxidizing the surface and then reducing it in a subsequent step. The heterogeneous rate constant was found to be 5.1 × 10 cm s, which is the highest value reported so far for pencil-drawn surfaces. We mapped the origin of such performance by atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Our results suggest that the oxidation process leads to chemical and structural transformations on the electrode surface. As a proof-of-concept, we modified the pencil-drawn surface with Meldola's blue to electrocatalytically detect nicotinamide adenine dinucleotide (NADH). The electrochemical device exhibited the highest catalytic constant (1.7 × 10 L mol s) and the lowest detection potential for NADH reported so far in paper-based electrodes.

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.

The possibility of current injection and transport in eumelanin films, humans' most common natural pigment, has fascinated physicists and materials scientists since the early '60s. Nowadays, it is accepted that...

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Annealing of sugarcane bagasse cellulose or lignin biocarbons under isopropanol vapor has induced an improvement in electrical conductivity of these materials. Remarkably, the sheet resistance dropped nearly three times for lignin biocarbon treated with isopropanol vapors. The use of isopropanol vapor annealing has increased sp2 carbon and decreased oxygenated functionality contents of these biocarbons. These chemical changes were confirmed by X-ray photoelectron and Raman spectroscopy analyses. Transmission electron microscopy images revealed the formation of graphitic domains on samples pyrolyzed in the presence of isopropanol, while electron energy loss spectroscopy mapping at a nanoscale showed an increase in graphitic characteristics of the particles. These chemical and structural changes of biocarbons have improved their electrical conductivity and decreased sheet resistance values of conductive tracks prepared with such materials. As a proof of concept, we fabricated flexible electronic circuits and paper-based electrochemical devices using conductive lignin-based inks prepared with our method.

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.

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The investigation of enhanced Raman signal effects and the preparation of high-quality, reliable surface-enhanced Raman scattering (SERS) substrates is still a hot topic in the SERS field. Herein, we report an effect based on the shape-induced enhanced Raman scattering (SIERS) to improve the action of gold nanorods (AuNRs) as a SERS substrate. Scattered electric field simulations reveal that bare V-shaped Si substrates exhibit spatially distributed interference patterns from the incident radiation used in the Raman experiment, resulting in constructive interference for an enhanced Raman signal. Experimental data show a 4.29 increase in Raman signal intensity for bare V-shaped Si microchannels when compared with flat Si substrates. The combination of V-shaped microchannels and uniform aggregates of AuNRs is the key feature to achieve detections in ultra-low concentrations, enabling reproducible SERS substrates having high performance and sensitivity. Besides SIERS effects, the geometric design of V-shaped microchannels also enables a “trap” to the molecule confinement and builds up an excellent electromagnetic field distribution by AuNR aggregates. The statistical projection of SERS spectra combined with the SIERS effect displayed a silhouette coefficient of 0.83, indicating attomolar (10 –18 mol L –1 ) detection with the V-shaped Si microchannel.