Eu-DTPA-PE modulates temperature-dependent birefringence in polymer-free monoolein-based lipid particles

Lanthanide-chelating lipids offer a route to responsive soft submicron lipid-particle materials, yet how their interfacial properties encode membrane curvature and temperature-dependent optical behavior remain poorly understood. Here, polymer-free monoolein/DOPC submicron lipid particles doped with Eu-DTPA-PE, with the monoolein : (DOPC + Eu-DTPA-PE) molar ratio fixed at 17 : 3, over Eu loadings of 1-12.5 mol% are examined using temperature-dependent polarized light microscopy (PLM), dynamic light scattering (DLS), and Eu³⁺ fluorescence to relate interfacial structure to curvature-linked changes in mesoscopic organization. In the absence of chelating lipids, MO/DOPC submicron lipid particles display an isotropic optical response at low temperature that reproducibly develops strong birefringence upon heating, behavior consistent with but not uniquely diagnostic of a transition from a cubic-like to a more anisotropic nonlamellar state. Introducing low Eu-DTPA levels (1-2.5 mol%) retains predominantly isotropic-appearing fields over a similar or modestly expanded lower-temperature range, consistent with altered interfacial interactions without requiring polymeric stabilizers. At higher Eu-DTPA-PE contents (≥5-12.5 mol%), the PLM micrographs exhibit increasingly extensive birefringent textures across the temperature series, although fixed-threshold image analysis identifies formulation-specific abrupt, broad, or nonpersistent changes rather than a single monotonic loading-dependent birefringence-onset sequence. DLS showed that the formulations remained submicron, colloidally stable lipid-particle dispersions across the full Eu-DTPA-PE loading series. Steady-state Eu³⁺ fluorescence revealed substantially enhanced emission for Eu-DTPA-PE lipid-particle formulations relative to aqueous EuCl₃, indicating a formulation-dependent Eu³⁺ emission response. The increased luminescence is consistent with a changed local Eu³⁺ environment and altered nonradiative quenching within the lipid assemblies; however, steady-state emission alone does not independently establish local hydration or coordination geometry. For the 10 and 12.5 mol% Eu-DTPA-PE (18 : 0) formulations, integrated Eu³⁺ emission normalized to ICP-MS-determined Eu concentration was similar, indicating that the greater raw emission at 12.5 mol% was largely explained by its greater Eu content. Together with earlier Gd-DTPA monoolein submicron lipid particles, these results establish lanthanide-chelating lipids as a modular platform in which chelator-lipid composition can modulate temperature-dependent birefringence and Eu³⁺ photophysical behavior in polymer-free monoolein-based submicron lipid particles.