Recent Advances in ZnO-Based Carbon Monoxide Sensors: Role of Doping.

Monitoring and detecting carbon monoxide (CO) are critical because this gas is toxic and harmful to the ecosystem. In this respect, designing high-performance gas sensors for CO detection is necessary. Zinc oxide-based materials are promising for use as CO sensors, owing to their good sensing response, electrical performance, cost-effectiveness, long-term stability, low power consumption, ease of manufacturing, chemical stability, and non-toxicity. Nevertheless, further progress in gas sensing requires improving the selectivity and sensitivity, and lowering the operating temperature. Recently, different strategies have been implemented to improve the sensitivity and selectivity of ZnO to CO, highlighting the doping of ZnO. Many studies concluded that doped ZnO demonstrates better sensing properties than those of undoped ZnO in detecting CO. Therefore, in this review, we analyze and discuss, in detail, the recent advances in doped ZnO for CO sensing applications. First, experimental studies on ZnO doped with transition metals, boron group elements, and alkaline earth metals as CO sensors are comprehensively reviewed. We then focused on analyzing theoretical and combined experimental-theoretical studies. Finally, we present the conclusions and some perspectives for future investigations in the context of advancements in CO sensing using doped ZnO, which include room-temperature gas sensing.

Implementation of the emulsification-diffusion method by solvent displacement for polystyrene nanoparticles prepared from recycled material.

From an integral perspective, nanotechnology can be used to care for the environment by improving current preparation methods and facilitating industrial scale-up. This article discusses the implementation of techniques for obtaining polystyrene nanoparticles (PSN), as an added value, using an emulsification and solvent displacement method (EDSD); the solvent displacement is a novel modification to the emulsion-diffusion methodology, where the diffusion is done directly on the emulsion, which allows concentrated dispersions that facilitate direct use to be obtained, eliminating the dilution with water and the recovery of water by additional processes. The solvent was recovered by reduced pressure, which is class 3, conforming to ICH, and making this method sustainable. The optimization of this process has not been reported elsewhere. This approach made it possible to obtain highly-concentrated nanoparticles while allowing the reuse of the solvent. A scaling proposal is presented that integrates the conditioning of the solid urban waste material called expanded polystyrene (EPS) foam that constitutes a serious environmental problem both nationally and globally. Hence, the article presents an alternative to the recycling of EPS, and a methodology in the context of green chemistry, because solvent is recovered to prepare other batches. The PSN obtained from this waste material had a minimum particle size of 225.8 nm, with a polydispersion index of 0.158. Process performance was 97.1%, and the solvent was recovered at a maximum rate of 85%. The morphology of the PSN was spherical and uniform, with a smooth surface.