A liquid crystal (LC) based method is described for the sensitive determination of the activity of urease and of heavy metal ions which acts as inhibitors. Stimulus-responsive surfactant-encapsulated phosphotungstate clusters (SECs) were fabricated and deposited onto octadecyltrichlorosilane-coated glass. A copper TEM grid filled with LCs was placed on the substrate to construct the LC optical cell. Upon addition of water to the LC interface, the optical appearance of LCs on the glass undergoes a bright-to-dark shift due to an orientational transition of the LCs from a planar to a homeotropic state. However, the LCs display a bright appearance if they are pretreated with an aqueous solution containing urea and urease. This is caused by the disassemby of the SECs from the glass surface due to an increase of the pH value that is induced by the enzymatic hydrolysis of urea by urease. The method is highly sensitive and can detect urease activities as low as 0.03 mU/mL. It can also be applied to the determination of heavy metal ions which exert an inhibitory effect on the activity of urease. For example, Cu(II) can be quantified via urease inhibition in 1 nM concentration. Graphical abstract Schematic presentation of a liquid crystal-based sensor for detection of urease and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters.
Keywords: Ionic liquid; Ionic self-assembly; Liquid crystal; Nanocomposites; Polyoxometalates; Sensor layer; Stimulus-responsive material; Surfactant; Urease.