Sensitive Detection of Trypsin using Liquid-Crystal Droplet Patterns Modulated by Interactions between Poly-L-Lysine and a Phospholipid Monolayer

Liquid‐crystal (LC) droplet patterns are formed on a glass slide by evaporating a solution of nematic LC dissolved in heptane. In the presence of an anionic phospholipid, 1,2‐dioleoyl‐sn‐glycero‐3‐phospho‐rac‐(1‐glycerol) (DOPG), the LCs display a dark cross pattern, indicating a homeotropic orientation. When LC patterns are incubated with an aqueous mixture of DOPG and poly‐L‐lysine (PLL), there is a transition in the LC pattern from a dark cross to a bright fan shape due to the electrostatic interaction between DOPG and PLL. Known to catalyze the hydrolysis of PLL into oligopeptide fragments, trypsin is preincubated with PLL, significantly decreasing the interactions between PLL and DOPG. LCs adopt a perpendicular orientation at the water–LC droplet interface, which gives rise to a dark cross pattern. This optical response of LC droplets is the basis for a quick and sensitive biosensor for trypsin. Counting crosses: Liquid crystals (LCs) of 4‐cyano‐4′‐pentylbiphenyl form a bright fan shape (planar orientation at the water–LC interface) when in contact with a mixture of 1,2‐dioleoyl‐sn‐glycero‐3‐phospho‐rac‐(1‐glycerol) (DOPG) and poly‐L‐lysine (PLL). The LCs take on a dark cross appearance (homeotropic orientation) in a DOPG–PLL mixture preincubated with the protease trypsin. This optical response can be used to measure trypsin activity. OTS=octyltrichlorosilane.