Supramolecular design of CO₂-responsive lipid nanomaterials

Stimuli-responsive materials can innovate in various fields, including food and pharmaceutical sciences. Their response to a specific stimulus can be utilized to release loaded bioactive molecules or sense their presence. The biocompatibility and abundance of CO₂ in the environment make it an exciting stimulus for such applications. We hypothesize the formation of CO₂-responsive self-assemblies of oleyl-amidine in water. Their integration into glycerol-monooleate-based (GMO) dispersions is further thought to form CO₂-switchable liquid crystalline nanoparticles. The switch from an non-charged acetamidine surfactant to its cationic amidinium form triggers curvature changes that ultimately induces phase transitions.

The CO₂-switchable lipid (E)-N,N-dimethyl-N-((Z)-octadec-9-en1-yl)acetimidamide (OAm) is synthesized and formulated into emulsions and dispersed liquid crystals with GMO. The supramolecular structure and its response to CO₂ are characterized using small angle X-ray scattering, dynamic light scattering, ζ-potential measurements and cryogenic transmission electron microscopy.

Depending on the composition, OAm is discovered to self-assemble into a variety of CO₂-responsive lyotropic liquid crystalline structures that can be dispersed in excess water. CO₂-triggered colloidal transformations from unstructured OAm-in-water emulsions to direct micelles; dispersed inverse hexagonal phase to direct rod-like micelles, and sponge phase to vesicles are discovered. These structural changes are driven by the reaction of OAm's amidine headgroup with CO₂. The results provide a fundamental understanding of CO₂-triggered functional nanomaterials and may guide their future design into delivery platforms and biosensors.