Liquid crystal-based optical aptasensor for the sensitive and selective detection of Gram-negative bacteria

The sensitive and differential detection of Gram-negative bacteria is essential in food processing, environmental monitoring, and the daily chemical industry. Herein, we propose and validate a liquid crystal (LC)-based aptasensor for the ultrasensitive detection of Escherichia coli ( E. coli ), a model of Gram-negative bacteria. The nematic liquid crystal of 4-cyano-4′-pentylbiphenyl (5CB) molecules can be orderly or disorderly arranged at the LC-aqueous interface via different stimuli, causing changes in optical texture due to birefringence. Bright schlieren texture is observed when a mixture solution of aptamer and hexadecyl trimethyl ammonium bromide (CTAB) is dripped onto the segmented LC films on a copper mesh. The specific binding of aptamers with target bacteria biomarkers liberates the CTAB molecules, which then self-assemble at the LC-aqueous interface to induce the vertical alignment of LCs. An optical transition from bright to dark is therefore achieved via the LC molecular orientation and serves as an aptasensor. Given the prominent affinity and specificity of the aptamer, the established sensitive and selective E. coli assay shows an ultralow detection limit of 27 cfu/mL. The prepared aptasensor can also be applied for the sensitive and selective determination of E. coli in fruit juice, soft drink, and cosmetic products, and shows great promise for the on-site detection of Gram-negative bacteria with high sensitivity and specificity for environmental monitoring, food safety assessment, and household chemical inspection.