Design of Optical Transducer for Recognition of Biomolecular Interactions between Bacterial Lipopolysaccharides and Amino Acids

Recognition of biomolecular phenomena at interfaces is a key scientific challenge with fundamental and practical importance. In this work, we discover two structurally similar amino acids, glutamic acid (Glu) and aspartic acid (Asp), showing different biological functions, have a significant but distinct orientational coupling with liquid crystals (LCs). Combining experiments with density functional theory calculations, we reveal that Glu and Asp exhibit unique adsorption-desorption dynamics at the LC-aqueous interfaces, producing distinct macroscopic optical signals. These interfacial behaviors are specifically governed by pH condition of the aqueous solution because the pH variations cause significant changes in charge states of the amino acids and thus their intermolecular interactions with LCs. Additionally, the dynamics are also modulated by the presence of other biological molecules (e.g., bacterial byproducts of Salmonella lipopolysaccharide, Staphylococcus aureus lipoteichoic acid, and Escherichia coli lipopolysaccharide) forming complexes with the amino acids, which dramatically change the LC's optical response. Leveraging this specific recognition mechanism for the interfacial dynamics of biomolecules and their interactions with other biological species, we also demonstrate the practical LC systems that autonomously recognize and optically report Salmonella at trace concentrations (10² cfu/ml) within 1 min, significantly faster than the conventional methods (PCR and ELISA) requiring several hours.