PhD Discussions Session: Javier Burgués and Jemish Parmar

Metal Oxide Gas Sensors for mHealth Applications

Volatile organic compounds (VOCs) in exhaled breath carry valuable information for the diagnosis of various diseases related to respiratory and gastrointestinal dysfunction, including cancer. There is an urgent need of portable breath VOCs detection devices that provide immediate point-of-care diagnoses of the patient health status that can support important medical decisions. As the market of medical health practice supported by mobile devices (mHealth) grows, smartphones or wearables equipped with miniaturized chemical sensors would provide the ideal platform to provide real-time healthcare data to the clinic at a very low cost.

The evolution of microelectromechanical systems (MEMS) have resulted in miniaturized metal oxide semiconductor (MOX) gas sensors which are promising for smartphone integration. A potential limitation of MEMS MOX sensing mechanisms may be the ability to accurately report the presence of a compound if the concentration in the sample is lower than the detection limit of the device or if chemical interferences are present in the sample. Current developments in micro-structured hot plates have reduced the power consumption of MOX sensors to the mW range per sensor but still further reductions might be necessary to meet the requirements of mobile manufacturers.

In this work, we compare conventional univariate and multivariate models in their ability to analyze complex data sets from MOX sensors and provide low detection limits in a scenario of carbon monoxide detection under chemical interferences. Elevated levels of exhaled carbon monoxide can be associated to chronic obstructive pulmonary disease (COPD), asthma or smoking habits. We will also propose a low power mode which can reduce the power consumption of the device by one order of magnitude without compromising the stability of the sensor.

Micromotors for environmental applications

Water contamination is one of the most persistent problems in public health. Recently, researchers have reported that micromotors can act as an efficient tool for water remediation because of the enhanced mass transfer by active motion. We developed different types of micromotors for water cleaning applications such organics degradation, heavy metal removal and bactericidal activity.

Among the myriad of existing motors, bubble propelled micromotors, that move due to the movement and release of gas bubbles, provide a promising platform for water remediation applications because of the added micro-mixing capability. The surface of the bubble propelled micromotors can be modified to target a wide variety of pollutants. For instance, rolled-up micromotors (Fe/Pt) with iron as the outer surface can degrade organic pollutants via Fenton-like reaction and the inner platinum layer can act as the engine, decomposing hydrogen peroxide to oxygen for bubble propulsion. These micromotors are capable of swimming continuously for hours for long term cleaning applications, are stable for weeks and can be reused in multiple cycles with low sacrifice of their activity. To overcome the higher fabrication cost and mass synthesis issue associated with Fe/Pt micromotors, we also developed inexpensive cobalt iron oxide based micromotors aiming at removal of pharmaceutical waste.

Furthermore, upon surface modification with Graphene oxide and with inner layers of nickel and platinum, magnetically guided micromotors can capture, transfer, and remove heavy metals from water. Mobile GOx-micromotors remove lead 10 times more efficiently than non-motile GOx-micromotors, cleaning water to below 50 ppb in less than one hour. These micromotors can be also recycled and reused after the recovery of the heavy metal from their surface.

Regarding bactericidal applications, silver nanoparticles (AgNPs) decorated Janus micromotors can efficiently disinfect and remove Escherichia coli (E. coli) bacteria from contaminated water, taking advantage of the bubble propulsion by oxidation of magnesium by water and AgNP’s bactericidal properties.

Our results show the multifunctionality of self-propelled micromotors, demonstrating their potential as efficient tools for water remediation.

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