Design of liquid crystal-aqueous interface for detection of calcium ions using protein as recognition probe

The advancement of liquid crystal (LC)-based biosensing platform for understanding the interactions of adsorbed proteins with lipid-designed LC-aqueous interface has significant scientific interest due to its point-of-care diagnostic applications and fundamental biological research. The ordering transition of LCs at 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) decorated LC-aqueous interface was examined to observe the biosensing of a physiologically important Ca2+ signalling protein recombinant human calmodulin (CaM). Self-assembled POPC molecules promote homeotropic anchoring of LC at the LC-aqueous interface with dark optical appearance under crossed polarizers. Interestingly, the optical state of LC changed to planar (bright optical appearance) when CaM was in the proximity of lipid adsorbed LC-aqueous interface, indicating label-free detection of CaM at those interfaces. It was further observed that the POPC decorated LC-aqueous interface is specific for the detection of CaM protein among other amphiphiles (LPA, DLPC, CTAB and SDS) decorated LC-aqueous interfaces tested. The sensitivity of the POPC decorated LC-aqueous interface towards the detection of CaM was observed in the nanomolar regime (~150 nM). CaM is a Ca2+ binding protein; hence, the CaM-POPC decorated LC-aqueous interface was further utilized for the detection of Ca2+. Interaction of CaM with Ca2+ increases the α-helical content of CaM, which results in the reorganization of POPC molecules at the LC-aqueous interface and leads to the homeotropic orientation of LC with a dark appearance under crossed polarizer. Compared to other spectroscopic/biological techniques, the LC-based system is easy to handle due to its simple label-free optical data, high sensitivity, cost-effectivity and low sample requirement.