ABSTRACT
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.
ABSTRACT
Gold nanoparticles (AuNPs) are commonly synthesized using the citrate reduction method, reducing Au3+ into Au1+ ions and facilitating the disproportionation of aurous species to Au atoms (Au0). This method results on citrate-capped AuNPs with valence single states Au0. Here, we report a methodology that allows obtaining AuNPs by the dewetting process with three different valence states (Au3+, Au1+, and Au0), which can be fine-tuned with ion bombardment. The chemical surface changes and binding state of the NPs were investigated using core-level X-ray photoelectron spectroscopy (XPS). This is achieved by recording high-resolution Au 4f XPS spectra as a function of ion dose exposure. The results obtained show a time-dependent tuning effect on the Au valence states using low-energy 200 V acceleration voltage Ar+ ion bombardment, and the valence state conversion kinetics involves the reduction from Au3+ and Au1+ to Au0. Proper control of the reduction in the valence states is critical in surface engineering for controlling catalytic reactions.
ABSTRACT
Heavy metal pollution is posing a severe health risk on living organisms. Therefore, significant research efforts are focused on their detection. Here, we developed a sensing platform sensor for the selective detection of lead(II) acetate. The sensor is based on self-assembled polyethyleneimine-functionalized carbon nanotubes (PEI-CNTs) and graphene oxide films deposited onto gold interdigitated electrodes. The graphene-based nanostructure showed a resistive behavior, and the fabricated layer-by-layer film was used to detect Pb(II) acetate in an aqueous solution by comparison of three electrochemical methods: impedance spectroscopy, amperometry, and potentiometry stripping analysis. The results obtained from different methods show that the detection limit was down to 36 pmol/L and the sensitivity up to 4.3 µAL/µmol, with excellent repeatability. The detection mechanism was associated with the high affinity of heavy metal ions with the functional groups present in the PEI-CNTs and GO, allowing high performance and sensitivity. The achieved results are important for the research toward integrated monitoring and sensing platforms for Pb(II) contamination in drinking water.
