L-Tryptophan/Levofloxacin-Based Ratiometric Fluorescence Biosensor for <i>Escherichia coli</i> by a Portable Device

A cost-effective ratiometric biosensor was developed for on-site detection of Escherichia coli (<i>E. coli</i>) using readily available l-tryptophan (l-trp) and levofloxacin. The detection was based on the degradation of the enzyme tryptophanase in <i>E. coli</i> toward l-trp and its fluorescence quenching of levofloxacin. The biosensor achieves the integrated effect of bacteria detection and sterilization. The detection and quantification limit were 102–158 and 308–442 cfu mL⁻¹, respectively. The feasibility of the method was validated by analyzing fruit juice, green tea, blueberry peels, and lettuce leaves samples with recoveries from 96.1% to 106.3%. Moreover, a self-designed portable device was constructed.