pH-responsive cholesteric liquid crystal double emulsion droplets prepared by microfluidics

Functionalized cholesteric liquid crystal (CLC) double emulsion droplets (DEDs) were prepared from nematic LCs of MLC-2132 doped with (S)-4-cyano-4′-(2-methylbutyl)biphenyl (CB15) by using a microfluidic method, employing glass capillaries with a combined co-flow and flow-focusing geometry. Both planar (P) and homeotropic (H) anchoring on the surfaces of the CLC DEDs could be achieved using poly(vinyl alcohol) (PVA) and a mixture of sodium dodecyl sulfate (SDS)/polysorbate 80 (TWEEN 80), respectively. The H configuration was unstable when used with SDS alone, and hence required the use of a mixture of SDS and TWEEN 80. The P/P, P/H, H/P, and H/H configurations on the inner/outer surfaces exhibited different surface morphologies, which were observed in reflection-mode bright-field image patterns. The P/P configuration displayed a distinct, circular central spot with a reflection color that represented the helical pitch of the CLC, since the planar orientation of the mesogenic groups caused a parallel orientation of helical layers relative to the droplet surface, (which meets the Bragg reflection condition). The P/H, H/P, and P/P images observed from the same mode exhibited small, black circular spots assembled in a pseudo-hexagonal fashion, with reflected colored light coming from the boundary regions between the circular spots. This result indicated that the small, circular spots were due to the perpendicular orientation of the helical layers that resulted from the homeotropic orientation of the mesogenic groups (which does not meet the Bragg reflection condition). The CLC DEDs coated with poly(acrylic acid)-b-poly(4-cyanobiphenyl-4′-oxyundecylacrylate) (PAA-b-LCP) on the outer surface (CLC DED_PAA) were pH-responsive. When the inner surface was controlled to have a P configuration with water, the central, circular reflection color pattern changed to a small, black circular spot pattern with increasing pH. This indicated that protonation and deprotonation of the carboxylic group of PAA occurred, which in turn led to planar (tangential) and perpendicular (homeotropic) orientations, respectively. These pH-sensitive, mono-dispersed CLC DEDs have potential use as biosensors, where immobilization of the ligands causes local changes in pH in response to analytes.