Real‐Time Liquid Crystal pH Sensor for Monitoring Enzymatic Activities of Penicillinase

Abstract

A liquid crystal (LC)‐based pH sensor for real‐time monitoring of changes in localized pH values near a solid surface is reported, along with its application for the detection of enzymatic activities. It is found that 4‐cyano‐4′‐pentylbiphenyl (5CB), when doped with 4′‐pentyl‐biphenyl‐4‐carboxylic acid (PBA), shows a bright‐to‐dark optical response to a very small change in pH (from 6.9 to 7.0). The pH‐driven optical response can be explained by using orientational transitions of 5CB induced by the protonation and deprotonation of PBA at the aqueous/LC interface. Because of its high pH sensitivity, the LC‐based sensor is further exploited for monitoring local pH changes resulting from enzymatic reactions. As a proof of concept, the hydrolysis of penicillin G by surface‐immobilized penicillinase is monitored using the system, even when the concentration of penicillin G is as low as 1 nM. This type of LC‐based sensor may find potential utilities in high‐throughput screening of enzyme substrates and enzyme inhibitors.

Liquid Crystal Droplet Patterns to Monitor Catalase Activity at Femtomolar Levels

“…When we use doped LC with functional materials instead of pure LC, this system, even more so, broadens the range of sensing applications. 23,24 Detection that could not be formulated using pure 5CB was enabled using PBA-doped 5CB and pH sensor was successfully built. 23 In another study, enzymatic activity of lipase was studied using glyceryl trioleate doped in 5CB.…”

Beyond displays: The recent progress of liquid crystals for bio/chemical detections

“…It was noted that LCs are fluid-like and possess mobility, combined with liquid crystallinity, assembling biological amphiphiles at the LC-aqueous interface promises a possibility for mimicking cell membrane, which is mobile and composed of a large amount of lipids in liquid crystalline phase. A series of more complex interfacial phenomena, such as specific binding events involving proteins [29,42] enzymatic reactions [43][44][45][46][47], hybridization of DNA [48], and the culture of human embryonic stem cells (hESCs) [49] at LC-aqueous interfaces have shown to trigger dynamic orientational transitions in LCs. Apart from decorating the LC-aqueous interface with amphiphiles, it is also possible to utilize synthesized LCs containing functional groups and tailor the interfacial properties for versatile uses [50].…”

Imaging the Enzymatic Reaction of Urease Using Liquid Crystal-Based pH Sensor

“…Bi et al developed a new LC-based pH sensor to monitor small amounts of H + released from enzyamtic reactions in real-time, especially in an aqueous solution with a high buffer capacity. 16 4'-pentyl-biphenyl-4-carboxylic acid (PBA), which contains a pH-functional group and has a similar structure to the nematic LC, 4-cyano-4'-pentylbiphenyl (5CB), was doped into 5CB. When a small quantity of H + was generated from the enzymatic hydrolysis of penicillin G in PBS (pH=7.0), the optical appearance of the acid-doped 5CB experienced a dark-to-bright shift, indicating an orientational transition of LCs, which was attributed to the protonation of PBA at the aqueous/LC interface.…”

“…[10][11][12] Due to its highly cooperative and long-range anchoring transitions, which propagate from aqueous/LC interfaces, LCs can be used to amplify and transduce molecular and biomolecular events into optical outputs visible by the naked eye. [13][14][15] Studies on highly sensitive LC-based biosensors based on the orientational transition of LCs involve pH indication, 16 enzymatic reactions, [16][17][18] DNA hybridation 19 and protein binding events. 20 In terms of pH sensors, amphiphiles containing pH-sensitive functional groups are usually employed to detect pH shifts related to ordering transitions of LCs.…”

“…During the hydrolysis of substrates, the orientation of the LCs was disturbed at the aqueous/LC interface, which influenced the optical appearance. 16 Since the PBA-doped 5CB showed an orientational transition when an aqueous mixture of urease and urea was transferred into the optical cell, we hypothesized that changes in optical appearance might also be observed when the urease-modified gold grid filled with the acid-doped 5CB was immersed into an aqueous solution containing urea. In order to prepare the urease-decorated grid, 11-mercaptoundecanoic acid (MUA) was used to modify the gold surface.…”