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.

Since its discovery in 2007, polydopamine nanofilms have been widely used in many areas for surface functionalization. The simple and low-cost preparation method of the nanofilms with tunable thickness can incorporate amine and oxygen-rich chemical groups in virtually any interface. The remarkable advantages of this route have been successfully used in the field of electrochemical sensors. The self-adhesive properties of polydopamine are used to attach nanomaterials onto the electrode’s surface and add chemical groups that can be explored to immobilize recognizing species for the development of biosensors. Thus, the combination of 2D materials, nanoparticles, and other materials with polydopamine has been successfully demonstrated to improve the selectivity and sensitivity of electrochemical sensors. In this review, we highlight some interesting properties of polydopamine and some applications where polydopamine plays an important role in the field of electrochemical sensors. Graphical Abstract

The simple, fast, scalable, and integrative preparation of sustainable electrodes using earth-abundant materials toward energy applications is a long-standing challenge. In this work, we attempted to achieve such features by developing a binary cooperative thermal process using cellulose sheets and molybdenum disulfide (MoS 2 ) toward hydrogen evolution reaction (HER). The thermal process converts cellulose into a highly conductive hydrophobic carbon-based material while generating chemical defects on MoS 2 . The latter is of fundamental importance to improve the catalytic activity for HER through activating the wellknown inert MoS 2 basal planes. Thermal desulfurization was confirmed by X-ray photoelectron spectroscopy, Kelvin probe force microscopy, Raman spectroscopy, and energy-dispersive Xray spectroscopy. Interestingly, a desulfurization gradient was observed at the MoS 2 particles, where the edges are more defective than the basal planes. The resulting defect-like MoS 2 particles are highly active toward HER. In addition, the porosity of paper enables simple filtration of catalysts and the possibility to tune the electrochemically active surface area by simply adding isopropanol before the electrolysis. Finally, we showed an ultrafast coating method using a commercial MoS 2 spray on sheets of paper that enables H 2 bubble generation at specific regions of the electrode. The overpotential (η) positively shifted more than 300 mV when compared with nontreated MoS 2 , reaching η = 240 mV to obtain 10 mA cm −2 of HER current density.