Genetically engineered bacterium-modified magnetic particles assisted chiral recognition and colorimetric determination of D/L-tryptophan in millets

•An MP@PDA-E. coli-based colorimetric method was developed to detect L-Trp.•The method exhibited excellent chiral selectivity for L-Trp.•The method was successfully applied to detect L-Trp in millets.•The detection of percentage of L-Trp in the enantiomeric mixture can be realized.•A potable device was fabricated to make detecting L-Trp more convenient.