Detection of arginase through the optical behaviour of liquid crystals due to the pH-dependent adsorption of stearic acid at the aqueous/liquid crystal interface

Abstract

The authors here developed a stearic acid-doped liquid crystal (LC)-based sensor for the label-free and real-time detection of arginase (ARG1). This sensor was based on the orientational transition of LCs due to the adsorption of stearic acid at the aqueous/LC interface. The shift in the optical image of LCs from bright to dark was obtained after transferring an aqueous solution of arginase, ʟ-arginine, and urease onto the aqueous/stearic acid-doped LC interface. Arginase catalyzes ʟ-arginine hydrolysis to produce urea, which is further catalyzed by urease to produce ammonium, which in turn increases the pH of the aqueous bulk. This increase in pH causes the deprotonation and adsorption of stearic acid and the formation of a stearate self-assembled monolayer at the interface, which induces a change in the orientation of the LC film from planar to homeotropic anchoring. ʟ-norvaline can inhibit the enzymatic activity of arginase; therefore, the LC optical image stayed bright after adding a mixed solution of arginase and ʟ-norvaline to the optical cell incubated with ʟ-arginine and urea, indicating no orientation change of LCs. ARG1 could be quantitatively detected by the proposed sensor in the range from 1 ng/mL to 100 ng/mL, with the detection limit as low as 0.6 ng/mL. The effect of the human serum matrix on the performance of proposed sensors was also investigated to confirm the applicability in ARG1 detection. The proposed sensor shows great potential for the development of pH-sensitive instruments for the monitoring of enzymatic reactions. © 2021 Elsevier B.V.