Optical Gas Sensing with Liquid Crystal Droplets and Convolutional Neural Networks

• Published in: Sensors (Basel, Switzerland) Vol. 21; no. 8; p. 2854
• Main Authors: Frazão, José, Palma, Susana I C J, Costa, Henrique M A, Alves, Cláudia, Roque, Ana C A, Silveira, Margarida
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 18.04.2021; MDPI
• Subjects: Air pollution; Automation; CNN; Droplets; gas sensing; Gas sensors; Humidity; liquid crystal; Liquid crystals; LSTM; Microscopy; Neural networks; Nitrogen; Optical microscopy; Pattern analysis; Pattern recognition; Pattern recognition systems; Sensors; Support vector machines; Texture recognition; VOCs; volatile organic compound; Volatile organic compounds; YOLO; United States > US; Germany; YOLO; gas sensing; CNN; LSTM; volatile organic compound; liquid crystal
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s21082854

Liquid crystal (LC)-based materials are promising platforms to develop rapid, miniaturised and low-cost gas sensor devices. In hybrid gel films containing LC droplets, characteristic optical texture variations are observed due to orientational transitions of LC molecules in the presence of distinct volatile organic compounds (VOC). Here, we investigate the use of deep convolutional neural networks (CNN) as pattern recognition systems to analyse optical textures dynamics in LC droplets exposed to a set of different VOCs. LC droplets responses to VOCs were video recorded under polarised optical microscopy (POM). CNNs were then used to extract features from the responses and, in separate tasks, to recognise and quantify the vapours exposed to the films. The impact of droplet diameter on the results was also analysed. With our classification models, we show that a single individual droplet can recognise 11 VOCs with small structural and functional differences (F1-score above 93%). The optical texture variation pattern of a droplet also reflects VOC concentration changes, as suggested by applying a regression model to acetone at 0.9-4.0% ( ) (mean absolute errors below 0.25% ( )). The CNN-based methodology is thus a promising approach for VOC sensing using responses from individual LC-droplets.