Fluorescence-based ion sensing in lipid membranes: a simple method of sensing in aqueous medium with enhanced efficiency

Detection of ions in chemical, biological and environmental samples has gathered tremendous momentum considering the beneficial as well as adverse effects of the ions. Generally, most of the ions are beneficial up to an optimum concentration, beyond which they are toxic to human health. However, most of the fluorescence-based ion sensors are only active in non-aqueous solution because of the low solubility of the sensor molecules in aqueous buffer medium. In the present work, we have demonstrated that encapsulation of an aqueous insoluble thiocarbonohydrazone-locked salicylidene-based macrocyclic ligand in 1-palmitoyl-2-oleoyl- sn-glycero -3-phosphocholine (POPC) membranes allows the selective detection of Zn 2+ in aqueous medium with approximately 3-fold enhanced efficiency compared to its efficiency in DMSO medium. We have further modulated the charge of the membrane surface by adding various concentrations of a negatively charged lipid, 1-palmitoyl-2-oleoyl- sn-glycero -3-phosphoglycerol (POPG), and showed that negative surface charge further enhances the Zn 2+ sensing efficiency up to approximately 6-fold. This strategy opens up a new avenue of utilizing organic sensors to detect vital ions in aqueous medium. Our strategy provides an opportunity to tune the hydrophobic molecular sensors to sense ions in aqueous solutions with enhanced efficiency by incorporating them in lipid membranes.