Designing Biological Micro-Sensors with Chiral Nematic Liquid Crystal Droplets

Biosensing using liquid crystals has a tremendous potential by coupling the high degree of sensitivity of their alignment to theirsurroundings with clear optical feedback. Many existing set-ups use birefringence of nematic liquid crystals, which severelylimits straightforward and frugal implementation into a sensing platform due to the sophisticated optical set-ups required. In thiswork, we instead utilize chiral nematic liquid crystal micro-droplets, which show strongly reflected structural colour, as sensingplatforms for surface active agents. We systematically quantify the optical response of closely related biological amphiphilesand find unique optical signatures for each species. We detect signatures across a wide range of concentrations (fromμMto mM), with fast response times (from seconds to minutes). The striking optical response is a function of the adsorptionof surfactants in a non-homogeneous manner and the topology of the liquid crystal orientation at the interface requiring ascattering, multidomain structure, which we observe to be different between molecules. We show lab-on-a-chip capability of ourmethod by drying droplets in high-density two-dimensional arrays and simply hydrating the chip to detect dissolved analytes.Finally, we show proof-of-principlein vivobiosensing in the intestinal tracts of live zebrafish larvae, demonstrating CLC dropletsshow a clear and differential optical response between healthy and inflamed tissues. Our unique approach has great potentialin developing on-site detection platforms and detecting biological amphiphiles in living organisms.