Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

https://www.mdpi.com/1424-8220/10/6/5469/pdf

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

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Nanoparticles of conjugated polymers.

The grand vision of manufacturing large-area emissive devices with low-cost roll-to-roll coating methods, akin to how newspapers are produced, appeared with the emergence of the organic light-emitt ...

/pdf/ambient-fabrication-of-flexible-and-large-area-organic-light-2okku9iqap.pdf

Ambient fabrication of flexible and large-area organic light-emitting devices using slot-die coating.

In vitro and in vivo model systems to study microbial biofilm formation

Inkjet printed surface cell light-emitting devices from a water-based polymer dispersion

A new gas sensor based on ultrathin SnO 2 -films with high sensitivity for humidity and carbon monoxide has been developed. The SnO 2 -sensing films are fabricated by a spray pyrolysis process with a thickness between 50 and 100 nm on oxidized silicon substrates. The SnO 2 -sensing films are finally processed in arrays of parallel bars to achieve the sensor device. The structure of SnO 2 -layers has been characterized by SEM, AFM and XPS analysis. The sensors are operated at temperatures of 250–400 °C, show high sensitivity to humidity and are able to detect carbon monoxide down to a concentration of less than 5 ppm. The fabrication process of the gas sensor is fully compatible with silicon process technology. The spray pyrolysis technique is a simple and flexible process and thus is suitable for the cost efficient fabrication of a new gas-sensing device. Strategies to improve sensitivity and selectivity of the sensor toward different gas species are discussed.

Ultrathin SnO2 gas sensors fabricated by spray pyrolysis for the detection of humidity and carbon monoxide

Microfabricated biochips are developed to continuously monitor cell population dynamics in a non-invasive manner. In the presented work we describe the novel combination of contact-less dielectric microsensors and microfluidics to promote biofilm formation for quantitative cell analysis. The cell chip consists of a polymeric fluidic (PDMS) system bonded to a glass wafer containing the electrodes while temperature and fluid flow are controlled by external heating and pumping stations. The high-density interdigitated capacitors (μIDES) are isolated by a 550 nm multi-passivation layer of defined dielectric property and provide stable, robust and non-drifting measurement conditions. The performance of this detector is evaluated using various bacterial and yeast strains. The high sensitivity of the developed dielectric microsensors allows direct identification of microbial strains based on morphological differences and biological composition. The novel biofilm analysis platform is used to continuously monitor the dynamic responses of C. albicans and P. pastoris biofilms to increased shear stress and antimicrobial agent concentration. While the presence of shear stress triggers significant changes in yeast growth profiles, the addition of 0.5 µg mL−1 amphotericin B revealed two distinct dynamic behaviors of the C. albicans biofilm. Initially, impedance spectra increased linearly at 30 Ω h−1 for two hours followed by 10 Ω h−1 (at 50 kHz) over 10 hours while cell viability remained above 95% during fungicide administration. These results demonstrate the ability to directly monitor dielectric changes of sub-cellular components within a living cell population.

Development of a microfluidic biochip for online monitoring of fungal biofilm dynamics.

The invention relates to a device, particularly a bio-chip, for the phenotypical analysis and for the identification and characterization of microorganisms, by means of dielectric spectroscopy without the addition of markers by using a microsensor with a metal comb structure and a measuring chamber, which is arranged above the microsensor and receives the microorganism, and a unit for feeding high-frequency alternating fields and for impedance change detection, characterized in that the measurement chamber (5) to receive the organisms (8), or the measuring surface (50) thereof to be provided with the organisms (8), and the organisms (8) per se are separated by a preferably multilayer shielding layer (40), which is permeable only to the electric alternating fields emitted by the microsensor (4) and made of otherwise non-permeable, electrically non-conductive, non-magnetic and non-dielectric material from the plating (41) of the comb structure (42) of the micro sensor (4) applied to a glass substrate (1) and facing the measuring chamber (5) or the measuring surface (50) thereof.

Device, particularly a biochip, for identifying microorganisms

Microfabricated biochips are developed to continuously monitor cellular phenotype dynamics in a non-invasive manner. In the presented work we describe the novel combination of contact-less micro-dielectric sensors and microfluidics for quantitative cell analysis. The cell chip consists of a polymeric fluidic (PDMS) system bonded to a glass wafer containing the electrodes while temperature and fluid flow are controlled by external heating and pumping stations. Additionally, the cell chip contains an integrated reference arm providing a low-noise detection environment by eliminating background signals and interferences. The high-density interdigitated capacitors (µIDC) are designed to monitor living cells in a space of approximately 10 nL volume by controlling critical electrode characteristics, such as size, shape and passivation composition as well as thickness. The integrated µIDCs are isolated by a 300 nm multi-passivation layer of defined dielectric property and provide non-invasive, stable, robust and non-drifting measurement conditions. The performance of this detector is evaluated using various bacterial, yeast and mammalian cells.

Christoph Stepper

Papers

Inkjet printed surface cell light-emitting devices from a water-based polymer dispersion

Ultrathin SnO2 gas sensors fabricated by spray pyrolysis for the detection of humidity and carbon monoxide

Development of a microfluidic biochip for online monitoring of fungal biofilm dynamics.

Device, particularly a biochip, for identifying microorganisms

Development of a Lab-on-a-Chip for the Characterization of Human Cells