L-Tryptophan assisted construction of fluorescent and colorimetric dual-channel biosensor for detection of live Escherichia coli

Detection of hygiene indicator Escherichia coli (E. coli) is usually employed for assessing the hygiene and safety of water and food supply. Herein, an L-tryptophan (L-Trp)-based fluorescent and colorimetric dual-channel sensing system was developed for detection of live E. coli. The fluorescent channel is based on the stereo-specifically degradation of L-Trp by tryptophanase excreted by E. coli, while L-Trp can be served as a fluorescent indicator. The colorimetric channel is based on the chromogenic reaction between the degradation product indole and 4-dimethylaminobenzaldehyde. Importantly, the dual-channel sensing system can distinguish live E. coli from dead bacteria. Also, the system demonstrates excellent selectivity toward E. coli due to the highest specific activity of tryptophanase in E. coli. In addition, a smartphone-based potable device was designed and used to capture the images, and ImageJ software was used for the analysis of the color thresholds and gray values of images. The feasibility of the sensing system was verified by successful detection of E. coli in river water and milk samples with recoveries in the range of 90.2% to 108%. The sensing system provides an alternative platform for convenient and selective detection of E. coli in water and food products.