Reducing N2O induced cross-talk in a NDIR CO2 gas sensor for breath analysis using multilayer thin film optical interference coatings

• Published in: Surface & coatings technology Vol. 336; pp. 9 - 16
• Main Authors: Fleming, Lewis, Gibson, Des, Song, Shigeng, Li, Cheng, Reid, Stuart
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 25.02.2018
• Subjects: Breath analysis; Capnography; CO2 sensor; Cross-talk; Microwave-assisted DC magnetron sputtering; Mid infrared light emitting diodes; Mid infrared photodiodes; Mid-IR; Multilayer optical interference filter; NDIR; Nitrous oxide; Surgical anaesthesia; Thin films; Mid infrared photodiodes; Surgical anaesthesia; CO2 sensor; Thin films; Mid infrared light emitting diodes; Mid-IR; Capnography; Cross-talk; Breath analysis; Microwave-assisted DC magnetron sputtering; Nitrous oxide; NDIR; Multilayer optical interference filter
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2017.09.033

Carbon dioxide (CO2) gas sensing is an important aspect in the biomedical field of capnography, where cheap, fast and accurate measurement of exhaled CO2 vs. time is crucial in the evaluation of lung and tracheal function during surgical anaesthesia and is an under-used bio-marker for underlying respiratory conditions. Current detection methods do not adequately meet these requirements and suffer from considerable cross-talk associated with the commonly used anaesthetic gas nitrous oxide (N2O). In this work, we report how cross-talk can be reduced in a commercially available, low power (35mW) non-dispersive infrared (NDIR) CO2 gas sensor using thin film multilayer optical filters. Current sensor spectral response, spans 2500nm–5000nm via use of a pentanary alloy LED/photodiode optopair grown by molecular beam epitaxy (MBE), resulting in sensor sensitivity to gases with absorption bands in this region, including N2O. To reduce the effective spectral response of the sensor, capturing only CO2, a multilayer thin film optical interference bandpass filter has been designed and deposited directly onto the diode epi-structures using microwave plasma assisted DC magnetron sputtering. Three different coating configurations have been explored; LED-only coated, photodiode only coated and both coated. Gas sensor response to N2O for each coating configuration has been explored. It was found that application of an optical bandpass filter onto both the sensor LED and photodiode only was the most effective method of reducing sensor response to N2O, however no signal was observed in one of the two “LED and PD coated”, therefore optimal coating configuration for cross-talk reduction is subject to further investigation. •N2O cross-talk in CO2 gas sensing for breath analysis is an obstacle to accurate CO2 monitoring during surgical anaesthesia.•Multilayer thin film optical filters can be used to tune spectral response shape for the NDIR gas sensing technique.•Ge/Nb2O5 multilayer bandpass filters can be deposited directly onto planar LED/photodiode opto-pair hetero-structures.•Cross-talk reduction is dependent on which optical sensor element (LED, photodiode or both) is coated.•Coating both LED and photodiode was the most effective in reducing sensitivity to N2O, however at a cost of reducing sensor SNR.