Direct response of liquid crystal to pH changes for wide range pH sensing via the whispering gallery mode.

pH sensing is essential in various fields including healthcare, environmental monitoring, food safety, and agriculture. In this Letter, we propose a method for a wide-range pH measurement based on the direct response of liquid crystal (LC) to pH variations via the whispering gallery mode (WGM). The liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) is selected for its excellent optoelectronic properties and the birefringence of 5CB microdroplets serving as spherical resonators. This sensor achieves wide-range pH sensing without any intermediaries. Polarized optical microscopy (POM) and WGM provide a qualitative morphological analysis and accurate quantitative spectral measurements. Under alkaline conditions (pH 7.6 to 13.3), POM images of 5CB microdroplets show a complete transition from bipolar to radial structures, with an average sensitivity of 1.78 nm/pH of redshift in WGM spectra. In acidic conditions (pH 6.65 to 1.5), structural changes observed by POM are minor, with a sensitivity of 1.06 nm/pH of blueshift in WGM spectra. This difference is attributed to the varying refractive indices of HCl and NaOH solutions. This method offers a reference for theoretical studies on pH and LC interactions and holds promise for the development of LC-based pH sensors.