Flame-made chemoresistive gas sensors and devices

• Gas sensors are made by wet- or direct-deposition of flame-made particles. • The performance of almost all flame-made gas sensors is compared quantitatively. • Flames can deposit very porous sensing films with metastable phase composition. • Flame deposition of sensors can be monitored in situ through their resistance. • Embedded noble metals into metal oxides enhance their gas sensitivity. • Flame-made devices facilitate breath analysis, food & air quality assessment. Combustion aerosol technology has distinct advantages for the assembly of chemoresistive gas sensors compared to their traditional wet chemistry synthesis. These advantages are traced to combustion's steep temperature gradients and high particle concentrations during sensing particle formation and film deposition. This gives direct access to a plethora of material compositions (e.g. metastable phases, solid solutions, mixed oxides) and fractal-like porous but rigid structures that can lead to unique sensor selectivity, sensitivity and stability along with short response and recovery times. Here, flame-made gas sensors are reviewed tutorially and compared quantitatively. First, their basics are introduced focusing on the relationship between gas sensing and particle morphology (e.g. agglomerated vs. aggregated) and heterogeneity (e.g. noble metal surface clusters) including the embedding of noble metals into the chemoresistive metal oxides, a unique feature of flame-made particles. Then, sensors are distinguished between those made by conventional wet-deposition of flame-made sensing particles and those made by direct flame deposition onto sensing substrates. The fundamentals of combustion synthesis of sensing particles are traced to those of ceramic particles with emphasis on direct flame deposition of sensing films as their assembly can be monitored in situ, another unique feature of combustion processes. This is followed by a presentation of the evolution of flame-made gas sensor compositions (e.g. based on SnO 2 , WO 3 , ZnO, TiO 2 and other materials) with respect to selective sensing of key analytes (ethanol, NO 2 , CO, acetone, isoprene, H 2 etc.). Finally, sensor systems (arrays and catalytic or gas chromatographic filters) and their integration into devices with validation under realistic conditions are presented. Examples like carcinogenic formaldehyde monitoring in indoor air, fat metabolism monitoring in human breath or the distinction of toxic methanol from ethanol in alcoholic beverages and hand sanitizers are elaborated to demonstrate the immediate practical impact of flame-made gas sensors.

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