Alignment of nematic liquid crystals decorated with gemini surfactants and interaction of proteins with gemini surfactants at fluid interfaces

Abstract

A series of cationic gemini surfactants with diverse chemical structures, that is, imidazolium-based gemini surface active ionic liquids (gemini IM-SAILs) with different alkyl chain length or spacer length, viz. 1,s-bis(3-alkylimidazolium-1-yl) ethane bromide ([Cn-s-Cnim]Br2; s = 2, n = 6, 8, 10, 12, 16; n = 12, s = 2, 4, 6, 10), and quaternary ammonium-based gemini surfactants (gemini QASaa) with different symmetries, viz. 1,2-bisalkylquaternary ammonium bromide (m-2-n; m = 12, 14, 16, n = 8, 10, 12, m + n = 24), were synthesized and utilized to decorate aqueous/liquid crystal interfaces (ALI). Initially, the optical response of the LCs changed from bright to dark after incubation with gemini IM-SAILs (except [C6-2-C6im]Br2) or gemini QASaa aqueous solutions, due to the formation of stable surfactant monolayers at the ALI. We verify that gemini IM-SAILs with shorter spacer or longer hydrophobic chains are more conducive to adsorption onto the interface, and gemini IM-SAILs form monolayers more easily than the corresponding monomers or gemini QASaa. Interestingly, a dark-to-bright shift in the optical image of the LCs subsequently occurred after the fluid interface decorated with the gemini surfactants came into contact with Bovine serum albumin (BSA), a negatively charged protein in neutral environments, whereas the optical appearance of LCs did not change upon addition of two other proteins with positive charge (viz. lysozyme and trypsin). Therefore, based on the different action mechanisms, a low-cost, label-free, and convenient LC-based sensing platform using the gemini surfactant-decorated LC interface was constructed for identification of the proteins with opposite charges.