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

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.