Real-time, quantitative and sensitive detection of urea by whispering gallery mode lasing in liquid crystal microdroplet

In this study, we demonstrate a novel nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) microdroplet-based biosensor for real-time, quantitative and sensitive detection of urea by whispering gallery mode (WGM) lasing technology. The single stearic acid-doped 5CB microdroplet is taken as both optical resonator and sensing reactor. To the best of our knowledge, this work is the first report of urea detection with WGM lasing. The enzymatic reaction between urea and urease produces hydroxide ions. Deprotonation and self-assembly of stearic acid occur at the aqueous/LC interface with increased pH value, promoting the reorientation of LC molecules. The detectable shift of WGM lasing spectra related to the configuration transition of LC microdroplets can be used as an indicator of enzymatic reaction, and this allows detection of urea molecules by the sensor at a concentration of only 0.1 mM. The proposed sensor also demonstrated the ability to detect urea in real urine samples. Compared with conventional POM observations, monitoring and identification of lasing spectra from LC microdroplets as a versatile method allow for more quantitative and sensitive detection of analyte reactions and can be expected to replace POM imaging technology for current LC-based biosensor systems.