Liquid crystal droplets functionalized with charged surfactant and polyelectrolyte for non-specific protein detection

Abstract

Monodisperse micrometer-sized 4-cyano-4′-pentylbiphenyl (5CB) droplets coated with ionic surfactant produced by microfluidic technique were electrostatically functionalized with polyelectrolytes (PEs) at the 5CB/aqueous interface, and the change in their configurational orientations was used for protein detection. Quarternized poly-4-vinylpyridine (QP4VP) and polystyrenesulfonate (PSS) (strong cationic and anionic PEs, respectively) were used for functionalization of the sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB)-coated 5CB droplets (5CB_SDS and 5CB_DTAB), respectively, on which the charge densities (determined from the measured zeta potential) were controlled at their maximum values by adjusting the surfactant concentration. The radial (homeotropic) orientation of the 5CB_SDS and 5CB_DTAB droplets was changed to the bipolar (planar) orientation when the oppositely charged PE was adsorbed on the surfactant-coated droplets at concentrations not higher than 0.1 wt%, although it was retained when a PE with the same charge was used. The adsorption of the PE may cause a decrease in the net electric charge on the droplet surface. However, over-coating with high PE concentrations (≥0.1 wt%) resulted in the radial orientation, probably due to the increased net electric charge generated on the droplet surface by the over-coated PE. The QP4VP-functionalized 5CB_SDS droplets were then successfully utilized for protein detection with hemoglobin and bovine serum albumin as the model proteins. This new approach for functionalization of the interface between the LC and water may introduce a simple approach for developing LC-based biosensors.