Detection of heavy metal ions using whispering gallery mode lasing in functionalized liquid crystal microdroplets
Reconfigurable liquid crystal microresonators enable the optical detection of heavy metal ions at ultra-low concentration in liquids. Microresonators such as microspheres, microtoroids, or microrings are known for their excellent performance for optically monitored sensing. Usual sensor platforms are based on solid microresonators immersed in a liquid matrix. These microresonators support whispering gallery resonances, which shift with minute changes in the local refractive index around the microresonator. Tracking these resonances revealed the presence of biomolecules or nanoparticles at very low concentration in the matrix. In contrast, by this approach it is challenging to detect ions, which are smaller and thus have a weaker impact on the local refractive index. To bypass this limitation, Rui Duan and coworkers have considered a different sensing approach. They use liquid microresonators whose structure gets reconfigured in the presence of heavy metal ions. The microresonators, which support narrow whispering gallery resonances, are based on a mixture of liquid crystal and stearic acid. In the presence of ions, the stearic acid molecules get deprotonated. This induces a reorientation of the liquid crystal molecules that translates into a resonance shift. Resonance tracking yields a limit of detection below 100 pM for Cu(II) ions.