(38b) Enhancing the Selectivity of Gas Sensors By Pre-Separation with Membranes or Powder Filters

A challenge in gas sensing (e.g. breath analysis or indoor air quality monitoring) is the accurate detection of trace-level gases in complex mixtures. While modern chemoresistive gas sensors can be extremely compact, inexpensive and highly sensitive, their success is still limited by selectivity. Here, we dramatically enhance their selectivity by pre-separating gas mixtures with zeolite membranes or activated alumina powder for detection of formaldehyde or isoprene at the ppb-level at 90% relative humidity. Formaldehyde is a potential breath marker for lung cancer and a tracer for indoor air quality monitoring while isoprene is a versatile breath marker for non-invasive monitoring of high blood cholesterol levels as well as for influenza, end-stage renal disease, muscle activity, lung cancer and liver disease with advanced fibrosis. As proof-of-concept, a zeolite MFI/Al₂O₃ membrane is placed upstream a highly sensitive but weakly selective Pd-doped SnO₂ sensor. Their combination exhibits exceptional selectivity (>100) for formaldehyde down to 30 ppb at 90% relative humidity, outperforming state-of-the-art detectors by more than an order of magnitude (1). Similarly, we present a pre-separation concept based on activated alumina powder that retains hydrophilic compounds (e.g. ketones, alcohols, ammonia) representing major interferants in breath while hydrophobic isoprene is not affected. This results in fast (10 s) measurement of isoprene down to 5 ppb at 90% relative humidity using the same sensor with outstanding selectivity (>100) to breath-relevant acetone, ammonia, ethanol and methanol, that is superior to state-of-the-art isoprene sensors. Most importantly, when exposed continuously to simulated breath mixtures (four analytes) for eight days, this filter–sensor system showed stable performance (2). Both pre-separation systems can be interfaced readily with portable breath detectors or compact indoor air monitors.

1. A.T. Güntner, S. Abegg, K. Wegner, S.E. Pratsinis, Zeolite membranes for highly selective formaldehyde sensors, Sensors and Actuators B: Chemical, 257, 916-923 (2018).
2. J. van den Broek, A.T. Güntner, S.E. Pratsinis, Highly Selective and Rapid Breath Isoprene Sensing Enabled by Activated Alumina Filter, ACS Sensors, 3, 677-683 (2018).