The growing concern for sustainability and environmental preservation has increased the demand for reliable, fast response, and low-cost devices to monitor the existence of heavy metals and toxins in water resources. An electronic tongue (e-tongue) is a multisensory array mostly based on electroanalytical methods and multivariate statistical techniques to facilitate information visualization in a qualitative and/or quantitative way. E-tongues are promising analytical devices having simple operation, fast response, low cost, easy integration with other systems (microfluidic, optical, etc) to enable miniaturization and provide a high sensitivity for measurements in complex liquid media, providing an interesting alternative to address many of the existing environmental monitoring challenges, specifically relevant emerging pollutants such as heavy metals and toxins.

Highly sensitive and selective gas sensors with low energy consumption and amenable to miniaturization are required for real-time gas monitoring applications. The challenge is to produce sensing units at a sufficiently low cost to allow for broad deployment, which can only be reached with efficient materials and fabrication procedures. In this context, metal oxides are promising for next-generation gas sensors due to their high surface-area-to-volume ratio, efficient electron transfer, enhanced and tunable surface reactivity, fast response, and short recovery time. However, sensors from metal oxides are normally not sufficiently selective and have to be operated at high temperatures. These limitations have been overcome with strategies such as doping with other oxides, UV-illumination, and noble metal decoration. This review concentrates on the design and mechanisms of heterostructures for gas sensors, which are obtained with metal oxides in conjunction with other materials (e.g. other metal oxides and 2D materials). The key topics discussed include: (a) synthesis of metal oxide heterostructures (MOHs); (b) semiconducting heterostructures comprising n–n, n–p, and p–p heterojunctions, with emphasis on their sensing mechanisms; (c) sensors produced with heterostructures involving 2D materials. The challenges and prospects for gas sensors based on MOHs are also discussed.

Precision agriculture is crucial for increasing food output without expanding the cultivable area, which requires sensors to be deployed for controlling the level of nutrients in the soil. In this paper, we report on a microfluidic electronic tongue (e-tongue) based on impedance measurements which is capable of distinguishing soil samples enriched with plant macronutrients. The e-tongue setup consisted of an array of sensing units made with layer-by-layer films deposited onto gold interdigitated electrodes. Significantly, the sensing units could be reused with adequate reproducibility after a simple washing procedure, thus indicating that there is no cross-contamination in three independent sets of measurements. A high performance was achieved by treating the capacitance data with the multidimensional projection techniques Principal Component Analysis (PCA), Interactive Document Map (IDMAP), and Sammon’s Mapping. While an optimized performance was demonstrated with IDMAP and feature selection, during which data of a limited frequency range were used, the distinction of all soil samples was also possible with the well-established PCA analysis for measurements at a single frequency. The successful use of a simple microfluidic e-tongue for soil analysis paves the way for enhanced tools to support precision agriculture.