Varlei Rodrigues

Functional polymer coatings with tunable electrical and surface properties are central to applications in organic electronics, sensing, and practical coatings, where durability and self-healing (SH) capability are also critical for reliable operation. A good candidate for these applications is Poly­(ethylenimine)/poly­(acrylic acid) (PEI/PAA) layer-by-layer (LbL) films with intrinsic SH ability, whose properties are strongly influenced by the amount of water retained within the multilayer structure during growth. In this paper, we investigate PEI/PAA multilayers deposited under controlled relative humidity (RH = 50%, 70%, 90%) to clarify the role of water in film formation and performance. Films deposited at 50% and 70% RH exhibit capacitances of approximately 1.06 nF and 0.70 nF, respectively, while those grown at 90% RH reach approximately 5 nF. Electrical transport shows a pronounced negative differential resistance (NDR) peak at around 1.3 V, strongest in the 90% RH film. Through terahertz time-domain spectroscopy (THz-TDS), we independently confirm hydration differences through changes in the dielectric constant and, consequently, the refractive index, which manifest as relative delays of the transmitted ultrashort terahertz pulses. Surface characterization reveals a increase in roughness, from 1.8 ± 0.2 μm to 14.8 ± 3.1 μm, accompanied by enhanced hydrophobicity (contact angle rising from 68° to 90°). Only the 90% RH film exhibits complete self-healing within 10 min. Overall, humidity-controlled deposition provides a simple, scalable strategy to tune dielectric response, surface functionality, and SH performance in weak polyelectrolyte multilayers. Such control enables the design of reproducible, multifunctional coatings for next-generation soft-electronic and sensing applications.

Impedance measurements are crucial in various fields such as sensor transduction, process monitoring, and control systems, yet measuring low values of capacitance (~1 pF) in electrically noisy environments remains a challenge, particularly at an affordable cost. This paper presents a differential transimpedance amplifier (TIA)-based electronic circuit , which incorporates three blocks: two TIAs and a differential circuit with fine-tuning for null adjustment. The differential configuration improves common-mode noise rejection, allowing the system to operate reliably even in high-interference environments, which represents its main novelty. Detailed characterization of the circuit is provided, including its frequency-dependent response and baseline noise analysis. The circuit is suitable for frequencies below 20 kHz for samples with capacitance below 1 nF, though this capability varies depending on the capacitance being measured. The primary application demonstrated in this study is the real-time monitoring of silver nanoparticle deposition using a gas aggregation source. Capacitance measurements effectively tracked the deposition progress, even in electrically noisy environments, confirming the circuit’s ability to suppress external disturbances while maintaining picofarad-level sensitivity. Results show that the system can accurately monitor capacitance changes from the very initial stages of nanoparticle deposition, making it a valuable tool for in-situ real-time monitoring. The findings highlight the setup's precision and its potential for applications requiring detailed capacitance measurements. This work presents a robust and reliable method for nanoparticle deposition monitoring, emphasizing its differential noise-rejection performance and low-cost implementation, which is essential for advancing research and development in nanotechnology and materials science. Future improvements aim to enhance integration with other characterization techniques.

Research in supercapacitors is essential for driving innovation in energy storage, paving the way for a more sustainable and efficient future where technology and the environment coexist in harmony. Supercapacitors can instantly provide higher energy density than conventional capacitors and higher power density than batteries, despite limitations of low volumetric performance. Here, we show a simplified way to manufacture a hybrid all-solid-state supercapacitor operating at room temperature and dry conditions based on poly-(diallyldimethylammonium chloride) (PDDA)/graphene oxide (GO) multilayer assembly using the layer-by-layer technique. They display rapid discharge (relaxation time constant, τ₀ down to 1 μs), high energy (up to 7 Wh/kg) and power (up to 1400 W/kg) densities, and specific capacitance (up to 12 F/g). Molecular dynamics simulations of a PDDA/GO system are performed with and without water molecules, highlighting the crucial role of chlorine in the system's structure. The charge storage and fast discharge, LbL thickness control, and film conformability on practically any surface are attractive approaches in numerous practical applications. Besides simplifying the system, the exclusion of liquid electrolytes and the use of ultrathin films are advantageous in several applications, without compromising weight in structures.

Bimetallic nanoparticles are used in numerous applications in catalysis, plasmonics or fuel cell technology. The addition of the second metal to the nanoparticles allows enhancing and fine‐tuning their properties by choosing their composition, size, shape and environment. However, the crucial additional parameter of chemical structure within the particle is difficult to predict and access experimentally, even though segregated core–shell structures and random alloys can have drastically different physicochemical properties. This is highlighted by the vast literature on the most studied bimetallic system, gold‐silver, for which the controversy on whether gold and silver are miscible on the nanoscale or segregate persists. Here, these contradictions are solved by determining quantitatively the coexistence of an alloyed core and a 1–2 nm thick shell with gradual silver enrichment as the chemical ground state structure. Chemical reactions with the environment and meta‐stable structures are furthermore identified as responsible for the contradictions in the literature. This method is applicable to other multi‐metallic systems, provides benchmark input for theoretical models, and forms the basis for studying chemical rearrangements under reactive conditions in catalysis.

Silver nanoparticles (AgNPs) possess unique characteristics ideal for enhancing device sensitivity, primarily due to their high surface-to-volume ratio facilitating heightened interaction with analytes. Integrating AgNPs into polymers or carbon-based materials results in nanocomposites with synergistic properties, enabling the detection of minute changes in the environment across various applications. In this study, we investigate the adsorption kinetics of AgNPs within multilayered layer-by-layer (LbL) structures, specifically examining the impact of AgNPs concentration in the LbL film formation that is further explored as sensing units in an impedimetric microfluidic e-tongue. Although absorption kinetic studies are infrequent, they are crucial to optimize the AgNPs adsorption and distribution within LbL structures, significantly influencing upcoming applications. Through systematic variation of AgNPs concentration within identical LbL architectures, we applied the films as sensing units in a microfluidic e-tongue capable of distinguishing food enhancers sharing the umami taste profile. Across all tested scenarios, our approach consistently achieves robust sample separation, evidenced by silhouette coefficient, principal component analyses, and long-term stability. This work contributes to exploring controlled nanomaterial-based developments, emphasizing the importance of precise parameter control for enhanced sensor performance across diverse analytical applications.

Self-healing materials inspire the next generation of multifunctional wearables and Internet of Things appliances. They expand the realm of thin film fabrication, enabling seamless conformational coverage irrespective of the shape complexity and surface geometry for electronic skins, smart textiles, soft robotics, and energy storage devices. Within this context, the layer-by-layer (LbL) technique is versatile for homogeneously dispersing materials onto various matrices. Moreover, it provides molecular level thickness control and coverage on practically any surface, with poly(ethylenimine) (PEI) and poly(acrylic acid) (PAA) being the most used materials primarily employed in self-healing LbL structures operating at room temperature. However, achieving thin film composites displaying controlled conductivity and healing ability is still challenging under ambient conditions. Here, PEI and PAA are mixed with conductive fillers (gold nanorods, poly(3,4-ethylene dioxythiophene): polystyrenesulfonate (PEDOT:PSS), reduced graphene oxides, and multiwalled carbon nanotubes) in distinct LbL film architectures. Electrical (AC and DC), optical (Raman spectroscopy), and mechanical (nanoindentation) measurements are used for characterizing composite structures and properties. A delicate balance among electrical, mechanical, and structural characteristics must be accomplished for a controlled design of conductive self-healing composites. As a proof-of-concept, four LbL composites were chosen as sensing units in the first reported self-healing e-tongue. The sensor can easily distinguish basic tastes at low molar concentrations and differentiate trace levels of glucose in artificial sweat. The formed nanostructures enable smart coverages that have unique features for solving current technological challenges.