Detecting enzymatic reactions in penicillinase via liquid crystal microdroplet-based pH sensor

We report a liquid crystal (LC) microdroplet-based pH sensor and explore its application for detecting enzymatic reactions in penicillinase. Nematic LC 4-cyano-4'-pentylbiphenyl (5CB) microdroplet, doped with 4'-pentyl-biphenyl-4-carboxylic acid (PBA), is produced in phosphate buffer solution (PBS) with a tapered capillary microtube connected to a syringe pump. The PBA-doped 5CB microdroplets exhibit configuration transitions from bipolar to radial as PBS pH values increase from 5.7 to 6.0. These pH-induced changes in optical appearance are experimentally verified and attributed to the deprotonation of PBA at the aqueous/LC interface. We also examine the feasibility of the PBA-doped 5CB microdroplets as optical microcavities of whispering gallery modes (WGMs). Typical WGM lasing emission is observed in micrometer-sized microdroplets under a pulse laser pump. The WGM spectrum from the microdroplet responds to pH variations, reflecting as a spectral redshift as the pH decreases. The PBA-doped 5CB microdroplet can be successfully exploited accordingly for detecting enzymatic reactions in penicillinase. We demonstrate that the detectable shifts in WGM spectra associated with the microdroplet configuration transitions can serve as an indicator of the H + released from the enzymatic reaction.