Selective fluorescence detection of divalent and trivalent metal ions with functionalized lipid membranes

A series of functionalized lipid membranes were shown to respond selectively to Fe³⁺, Cr³⁺, Cu²⁺, and Fe ²⁺ with a distinctive change in bilayer fluorescence. The optical response is due to a change in the aggregational state of functionalized, pyrene-labeled lipids as they recognize and bind to the metal ion. Two new lipids are introduced with 2,2′-bipyridine (PSBiPy) and N-methylamide (PSMA) headgroups. These functionalized lipids, along with two other lipids having hydroxyl (PSOH) and iminodiacetic acid (PSIDA) headgroups, were incorporated into bilayers of distearylphosphatidylcholine (DSPC) producing materials that exhibited an excellent range of sensitivity for the metal ions at low pH. At neutral pH the metal ion sensitivities were lost for PSOH and PSMA while the sensitivity and selectivity for Cu²⁺ increased for PSBiPy. In addition to optical sensing, the bilayers also can undergo restructuring of the membrane architecture. Dynamic light scattering and transmission electron microscope images show that for the majority of the bilayers, metal ion addition caused only aggregation of the vesicles. However, for the PSBiPy/DSPC membranes extraordinary square-shaped assemblies of planar bilayers formed in the presence of 100 μM Cu²⁺.