Metal oxide semiconducting nanomaterials for air quality gas sensors: operating principles, performance, and synthesis techniques.

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of  a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring.

Towards new carbon-neutral food systems: Combining carbon capture and utilization with microbial protein production.

Alternative protein sources such as microbial protein (MP) are currently considered to alleviate the burden that food production exerts on the environment. Even though MP production is highly efficient in land and nutrient utilization, their carbon footprint should be improved. Here we propose the use of CCU as a driver for heterotrophic MP production. By comparing different MP production routes starting from liquid substrates derived from CO2 (i.e., formate, acetate, methanol, and ethanol) and their respective metabolic pathways, the potential of this concept as a carbon-neutral food or feed production process was estimated. Acetate and ethanol appear to be the most beneficial substrates for the integrated CCU-to-MP process in terms of electricity demand (acetate: 25 - 54 kWh/kgproduct, ethanol: 28 - 56 kWh/kgproduct). Moreover, recycling CO2 enables a carbon-negative protein production process by 2030 (considering the projected CO2 emissions from electricity in the EU: 0.096 kgCO2-eq/kWh) for formate, acetate, and ethanol (-1.1 up to 13 kgCO2-eq/kgproduct).

Effect of biomass concentration on methane oxidation activity using mature compost and graphite granules as substrata.

Reported methane oxidation activity (MOA) varies widely for common landfill cover materials. Variation is expected due to differences in surface area, the composition of the substratum and culturing conditions. MOA per methanotrophic cell has been calculated in the study of natural systems such as lake sediments to examine the inherent conditions for methanotrophic activity. In this study, biomass normalised MOA (i.e., MOA per methanotophic cell) was measured on stabilised compost, a commonly used cover in landfills, and on graphite granules, an inert substratum widely used in microbial electrosynthesis studies. After initially enriching methanotrophs on both substrata, biomass normalised MOA was quantified under excess oxygen and limiting methane conditions in 160ml serum vials on both substrata and blends of the substrata. Biomass concentration was measured using the bicinchoninic acid assay for microbial protein. The biomass normalised MOA was consistent across all compost-to-graphite granules blends, but varied with time, reflecting the growth phase of the microorganisms. The biomass normalised MOA ranged from 0.069±0.006µmol CH4/mg dry biomass/h during active growth, to 0.024±0.001µmol CH4/mg dry biomass/h for established biofilms regardless of the substrata employed, indicating the substrata were equally effective in terms of inherent composition. The correlation of MOA with biomass is consistent with studies on methanotrophic activity in natural systems, but biomass normalised MOA varies by over 5 orders of magnitude between studies. This is partially due to different methods being used to quantify biomass, such as pmoA gene quantification and the culture dependent Most Probable Number method, but also indicates that long term exposure of materials to a supply of methane in an aerobic environment, as can occur in natural systems, leads to the enrichment and adaptation of types suitable for those conditions.