Optical Detection of Proteins Using Microgel-Stabilized Pickering Liquid Crystal-in-Water Emulsions

Herein, we present a novel liquid crystal (LC)-based sensing platform utilizing microgel-stabilized Pickering LC droplets dispersed in water for simple and label-free detection of proteins in an aqueous environment. This could be achieved by tailoring the surface of 4-cyano-4'-pentylbiphenyl (5CB) LC droplets dispersed in aqueous medium through the interfacial adsorption of poly(N-isopropylacrylamide) (PNIPAM) microgel particles, followed by the introduction of model surfactants, such as anionic sodium dodecyl sulfate and cationic dodecyltrimethylammonium bromide. These surfactant/microgel complex-coated LC droplets underwent a configurational transition from radial-to-bipolar under a polarized optical microscope, upon exposure to model proteins, namely bovine serum albumin and lysozyme. This transition stemmed from the interfacial adsorption of proteins, which was facilitated by their strong interaction with the preadsorbed microgel particles and surfactant molecules. The adsorption of proteins led to the disruption of the interfacial packing density of surfactant molecules, inducing a switch from homeotropic-to-planar surface anchoring of LCs within the droplets. In addition to providing remarkable Pickering stability to the LC droplets, the microgel coating significantly enhanced the sensitivity of the resulting emulsions to proteins. The dose-response behavior and detection limit of these modified LC droplets were strongly influenced by the microgel concentration, surfactant charge, pH of the medium, and the types of proteins. Notably, the droplets exhibited heightened responsiveness under conditions that favor attractive interactions between the proteins and interfacial surfactant molecules. Thus, this study opens avenues for engineering Pickering LC-based biosensors to discern biomolecular interactions, thereby facilitating various interfacial and sensing applications.