Sensing of cocaine using polarized optical microscopy by exploiting the conformational changes of an aptamer at the water/liquid crystal interface.

Liquid crystals (LCs) have the ability to transduce and amplify a molecular stimulus into optical signals due to their elastic and birefringence properties. An aptamer-based LC sensor for cocaine is described here. 3-Morpholinopropanesulfonic acid with amphipathic structure was used to establish recognition sites at a water/LC interface for the detection of cocaine. The cocaine-binding aptamer is formed at the interface. The conformation of the aptamer undergoes a change on binding cocaine, and this triggers the LCs anchoring transition from homeotropic to planar. Binding can also be detected by polarized optical microscopy. The fluorescence spectroscopy and circular dichroism results are used to prove that the conformation of aptamer changed from a hairpin structure to a special three-way junction structure on binding of cocaine at the interface. The assay works in the 1 nM to 10 μM cocaine concentration range and is specific. Graphical abstract Schematic representation of aptamer-based liquid crystal (LC) biosensor for the detection of cocaine. In this interface biosensing system, after the aptamer binding with cocaine, the conformation of aptamer at the aqueous/LC interface was changed from a hairpin structure to a special three-way junction structure. This triggered the Liquid crystals (LCs) anchoring transition from homeotropic to planar and the sign-on optical signal could be obtained by polarizing optical microscope (POM) in real-time.